Lambeth college removable partial dentures
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Lambeth college removable partial dentures Lambeth college removable partial dentures Document Transcript

  • E I I I 1* I I (DENTAL TECHNOLOGY) lN DIPLOMA sc|ENCE NATIONAL BTEC : li^ t: DENTURES PARTIAIREMOVABLE GUIDE A MODULE l;il: l'lid li4 i-, iI d i" I . **| I 1. BridgeCentre Coliege Tourer Lambeth
  • ENVOY This handbook has been produced introdure some of the very important aspects of the designand construction removablepartialdentures. of The student who is newto this subjectwill, it is hoped,acquirethe theoretical toolswith which to beginthe processof design,constructionand discussionabout removablepartial dentures with fellow studentsand the dental lecturersat Lambeth College,Tower Bridge Centre We must remember, however, that the information containedhereinwas not given to us on tabletsof stone,but has been developedover many years by surgeonsand technicians in conjunction with the, often forgotten,long suffering patient. Designingpartial denturesis a process of applying theoreticalprinciples,backed up by technical expertise, meetthe requirements the individual to patient. of This moduleguide is intendedto be practicaland useful and will, it is hoped, increasethe knowledgenecessaryto make successful partial dentures and at the same time, provide muchfood for thought. M. WelchCertEd. Lecturer Dental in Technology Lambeth College TowerBridge Centre TooleyStreet London SE12JR
  • t I I I I ACKNOWLEDGEMENTS I i r -r I i -i I I -a i I I I J I .J i{ J J J f I t I -J -J I I # I I i I I I I wishto thankthe following lecturers Lambeth at College, TowerBridge Centre theirinput for and inspiration, Comwallfor notes,the discussions Jeff and consequently ideas that the grew.Rowland Gardner his computerknowledge. for ChrisGrigson continued for support, ideasand for editing this latesteditionof the 'Handbook'. KarenHorton providing for notes, 'Neddy'for more nbtes and last, but not least, Robert Woods for ideas, handoirts and practical suggestions timeswhentheyweresorelyneeded. at
  • 1 CONTENTS REMOVABLE PARTIAL DENTURE PROSTHETICS lndications contraindications the provision partial and for of dentures. Page 1 Classification partial of dentures based the nature partial on of denture support. Page3 Classifi cation partial of dentures. Page 5 Castsurveying partial and denture design. Page 7 Principles partial of denture design. Page 14 Principles mucosa of bomepartialdenture design. Page 19 Principles toothbomepartial of denture design. Page 20 Theapplication the principles partial of of denture design the tooth/mucosa to bourne case. Page24 principles consider the tooth/mucosa Design to for bourne case. page26 Partialdenture retention" page31 Majorconnectors. page3g procedure designing General partial for dentures. page41 Procedures the production acrylicresin/metallic for of complete partial and dentures. Page 44 Principles the lostwax process. of Page 46 Fumaces. Page 47 Metalmelting systems. Page 48 Casting forces. Page 51 prcedures metallic Finishing partial for denture bases. Page 53 Bibliography. Page 55
  • Removable PartialDentures Definition of a Removable Partial Denture A partialdentureis an appliance, removableby the patient,for the replacement one or more of naturalteeth in the mandibleor maxilla in which one or more natural teeth remain. In this Handbook, term partialdenture imply one which is removable. the will Reasons for the provision of a partiat denture . To restoremasticatoryefficiency . To restoreand improve aesthetics . To improve impairedspeech o To preventthe collapseof the dental arch and over-eruption teeth of r To maintain verticaldimension the These reasonswill be expandedduring discussionsessions. Harmful effects of poor partial denture design Poor partial denture design will cause damage to teeth, encouragecaries and damage the tissuessupporting denture,includingthe gingival margins.This occurs becausethe partial the denture:6 fits too closelyinto the gingivalmargincausingphysicat injury. 6 Allows food to pack down between the denture and the teeth. Food packed under pressure againstgingivalmarginswill force them away from the teeth. lf this is allowedto remain it will cause inflammation. Such damage to the gingival margins, if left untreated progresses, to involvethe deepersupporting tissuesof teeth leadingto periodontal diseaseand endingwith the loss of teeth. @ lt can also causetraumatic damageto the palateknownas hyperplasia. Denturehyperplasia is an abnormalincreasein the cell arrangementof a tissue which can result in a flap or roll of hyperplastic tissue resultingfrom chronic initation by a denture. Cases best left without dentures The decision not to fit a prosthesiswhen natural teeth are missing is usually a difficult one to make. lf such a course contemplated the dental surgeonwill carry out an examinationof the mouthto ensurethat no harmfuleffects result from the decisionnot to restorethe missingteeth. As a generalguide the followingquestionsshould be answeredand if these are mainly in the negative, then the probability that the patientwould be betteroff withouta denture. is o ls mastication inefficient with the remainingnaturalteeth? . ls appearance and speechadverselyaffected? I ls the occlusion of the teeth such that there is a possibility of over-eruption or tooth drift occuning? o Wll the benefitsof the prosthesis outweighthe possibledamage it may cause to the tissues? ' Doesthe patientmaintain carefuloral hygieneor likelyto followhygieneinstruction? a ' Are temporalmandibular joint symptomslikelyto developas the resultof a disturbed occlusion followingthe loss of teeth? . ls the patientsmental attitudeto denturessatisfactory?
  • ' Doesthe patientsmedical historysupportthe fitting of a partial prosthesis? "The ansWer to this /asf guesfion may be the sole deciding factor regarding the fitting of a partiat denture. A prosthesis may be contra indicated by a history of frequent and severe epiteptic attacks". FennLidlowand Gimson. Partial dentures fall into four categories El Fixedpartial dentures Theseare the besttype of denture available because they only fill the denture spaceie. the edentulous space.However some anatomical contouris lost due to tooth preparation and replacement retainers. by The basicpremise thatthe patient is with minimaltoothlossand goodoral hygiene can be fitted with bridgesthat restorelost anatomical contours,ie. caseswhere the second premolaris jaw. missing either in upper lower or However, mainareas the where fixedpartial dentures triumph overany othercategory three are fold, the first being aesthetiqs and the secondthe restoration occlusion as completea of to functional levelas possible. thirdis thatlargeareas mucosa notusedfor support The of are thus reducing amount tissuecovered connectorsthe of by [1 Acrylicremovable partial dentures Acrylicpartialdentures, matterhow well they are constructed, no shouldalwaysbe viewedas beingtemporary, used mainlyfor transitional periodsor when the long term prognosis is uncertain to poorpatient due response. Patients supplied all acrylic are partial with dentures familiarise to themwithwearing eating and with dentures. Partialdentures this type damagetissuestructures to pooiOesign of due anO construction techniques, however, theyare stillthe mostcommon produced typeof denture and fitted today, simply because can be easilymodified they whenexlractions the remaining of teeth become necessary. tr Metallic partial removable dentures Metallicpartialdenturesare designedto replaceand preserveexistinganatomical oral structures. Partialdenturesconstructed cobalt chromiumalloy can be designedand in construcled components suiteveryindividual with to case. Thistypeof partial dentures idealfor all categories partial are of dentitions. Theyare the only altemative fixedpartial to dentures whichprolong dental health. Patients arefitted who withthese dentures usually are those withgoodoralhygiene sound and remaining natural dentitions. E Complex partial removable dentures By their complex nature,thesepartialdentures outside remit of thismodule. are the
  • Classification PartialDentureSupport of Partialdenturesare designedand constructed restorelost anatomical to structures and to maintain existing ones,however, damagemay be done by poorlysupported dentures teeth to andtissues. Partial dentures classified theirmeansof support. are by Dentures whichderiveall theirsupport from basaltissues classified mucosa borne. are as Dentures whichderivetheir supportfrom the remaining naturalteeth are classified tooth as borne. Dentures which are supported a combination both teeth and mucosaare classified by of as tooth/mucosa borne. Fig- 1. Shols a case where a denturemay be entirelymucosa bome Fig. 2. Shovtrs case where a denturemay be entirelytooth borne. a Fig. 3. Shorc a case where a denturemay be entirelytootVmucosabome
  • Transmission Masticatory of Stresseson PartialDentures During masticationvarious stresses, or loads are transmittedto the underlyingmucosa (in the case of the mucosa bome denture),to the teeth and periodontalmembrane (in the case of the wholly tooth bome denture) and a combination of the aforementionedstructuresin the case of the toothltissuebome denture. Fig.4. Shons how the masticatory stress is transmittedvia teeth to the underlying bone. The greatestload to which teeth are subjectedare applied along their long axes approximatelyat rightanglesto the occlusal plane. These loads vary with the muscle activity, type of food and the occlusalsurface area of natural and artificialteethand are divided into two functions: ' Maslicationis the stressor load appliedto a denture and the supportingtissuesduring chewing. tissueswhen the teeth are held " Clenchedloadingis appliedto a denture and its supporting togetherunder pressureby the musclesof mastication.
  • Classification Paftial Dentures of quotea classification Osbourneand Lammie(1974) basedon support a denture a whole. of as Class I Mucosa-borne Thedenture entirely is supported the mucosa by andunderlying bone. Class ll Tooth-bome All the support the partial for denture comesfromthe teeth. Class lll Combination mucosa-bome of and tooth-borne Somepartsof the denture supported the teethandsomeby the mucosa. are by Beckeftand Wilson'sclassification Beckett and Wilsonbasedtheir classification an earlierone by Bailynand felt that the on following must be considered when decidingthe proportionate amountof supportprovidedby teethandtissue(thewordtissuemeansmucosa andunderlying bone) o Thequality abutment of support. o Themagnitude the occlusal of support. I The harmony the occlusion. of o Thequality the mucosa residual of and ridge. Theybelieved everyeffortshouldbe madeto avoidtissuesupport that alone. Class I Boundedsaddle Abutment teethqualified support denture; mucosa usedfor support. to the the not Class ll Free-endsaddle (a)tooth-and-tissue borne (b)tissue-bome Classlll Bounded saddle Abutment teethnotso wellqualified support denture in Class to the as l. Kennedy's Classifi cation partialedentulous Kennedy.classified archesby relating edentulous the spacesto the remaining standing ieeth. Fig. 5. CLASS | - Bilateral free end saddlespcsteriorto the standingteeth
  • Fig. 6. CLASS ll - Unilateral free end saddleposteriorto the standingteeth. Fig. 7. CLASS lll - Boundedunilateral saddlehavingstandingteeth at eitherend Fig. 8. CLASS lV - Saddleanteriorto the standingteeth. All classes except Class lV are subdivided into modifications,each modification denoting an additional bounded saddlearea. In additionthe followingpointswere noted: . The most posterioredentulous area determinesthe classification. o The size (number)of modificationsis not important. o lf a third molar is missing,and not to be replacedit is not consideredin the classification. Many classifications partial dentures and their designs are available to clinicians and of technicians,however, even now there is no universally accepted classification.Classifications describe either the various tooth and saddle positions, like those of Kennedy's,or else look closelyat the amount of availablesupportfor partial dentures,such as Beckett and Wilson's. Kennedy'sclassification usefulwhen describingthe basic outlinesof any particularcase and in is forming a common partial denture language amongst colleagues,however, it does not take into accountthe available support upon which the success of any partial denture depends. For this reason a support classifrcation preferable. is 6
  • CastSurveying and PartialDentureDesign Cast surveying is an important sequence in the design and construction of partial dentures. Before any decisions can be made about the component parts of the design, casts of the patient's mouth must be surveyedusing an instrument which enablesthe technicianand dentist to analyseany undercutareas of the soft and/or hard tissue in relationto the path of insertionand removal of the denture.Survey lines drawn on the labial and buccal aspects of teeth may show areaswhereclaspscan be positioned, linesdrawn on the mesial,distalor lingualareas may also show areas of undercut. The Gast Surveyor The cast surveyoris an instrumentthat holds a graphite rod above a horizontaltable upon which the cast to be surveyedcan be mounted and locked in various positions.The table may be tilted either antereo/posterior laterally. The marking rod can be moved up and down, but always or remainsverticalto ensurethat the angle it makes with the cast is constantfor any tilt. The graphite rod marks survey lines at the greatest horizontalwidth of the tooth being surveyed (area of maximum convexity). Survey Lines The survey line (fi9.9) is a line which is marked on a tooth by the graphite rod at the greatest width of a tooth in relationship the common path of displacement, to the path of insertion to or and removal.Survey lines divide teeth into two sections,the area below the line (towardsthe gingival margin),possessundercutsthat lie in relationto the path in which the cast is being surveyed,whilst areas above the survey line (towardsthe occlusal surface), do not lie in undercuts relationship that path. in to Fig, 9. A surveyline
  • Paths of Displacement Paths of displacement are all possible paths along which all or part of the denture may be duringfunction. displaced Common path of disptacement The most common path of displacement for partial dentures is at right angles to occlusal surfacesduring function,ie sticky food tends to dislodge denturesalong this path when the jaws are opened. Initiallythe cast is surveyedwith a marking rod parallelto this common path of plane. displacement, with the surveyofstable in a horizontal eg Path of insertion and removal The path of insertion and removalis the angulatedpath along which the dentureis insertedand removedfrom the mouth. This path must be decided before the denture is designed.The cast is analysed, usingan analysing rod, to decide the best path of insertion and removal by tilting the surveyortableto variouspositions. Once decidedthe table is securedby the lockingdevice and the path of insertion clearly marked on the sides of the cast. A second survey line is drawn around the teeth and mucosal surfaces of the cast in relation to this path. The colour of the marking rod should be differentfrom that used to mark the survey lines in relationto the common path of displacement. Undercut The definition of an undercut is, according to the BSI Glossary of Terms Relating to Dentistry (1983) "that area of a tooth, or soft fissue surface which is beyond the survey line when viewed from a particulardirection"(fi9. 9). Undercutscan be eliminated,or createdwhilst surveying and can be describedas true or false undercuts.True undercutsare those in relationto the common path of displacement and provide resistance removal along this path. False undercutsare to those undercutsproducedby tilting the cast. They provide resistanceto removal of the denture alongthe path in whichthe cast has been surveyed,but not in the common path of displacement. Once the path of insertion and removalfor the denturehas been chosen,the retentiveportionof the clasparms shouldlie in relation both the path of insertion to and removal of the dentureand to the commonpath of displacement. Fig. 10. Soft and toothtissue undercutsviewedfrom a horizontaldirection. Guiding surfaces Guidingsurfacesof.teethguide removable partial denturestowardstheir final and exacl positions in the mouth. These surfacesideallyshould be parallelto each other, howeverthis is seldom possiblebecausethe surfacesof teeth are curved. Guiding surfacesare ne@ssarybecausethey decreasethe numberof pathsalong which a denture may be displaced. The influencingfactor when choosinga path of insertionand removal of the denture is the locationof these surfaces on a cast of the mouth by the analysingrod of the surveyor.
  • Object of surveying Objectivesfor surveyingare many and varied for whichever type of case the dental surgeon and technicianmay be faced with, howeverthere exists a common theme for all situations.Surveying ensures that no rigid portions of a partial denture lie in undercuts in relation to the path ol insertion and removalof the denture, although they may lie in undercuts relativeto the numerous possible pathsof displacement. The flexibleends of clasp arms must lie in undercuts relation in to all possiblepaths of insertion, removal or displacement, since they are the mechanismsby which partial dentures are held secure in the mouth (ng.11). The most likely path oi displacement,as mentioned before, is at right angles to the occlusal surfaces of the teeth, thereforeit is imperativeto ensurethat the tips of the clasp arms and if possiblethe denture base itself, resist displacementalong this path, but in so doing we must rememberthat patients need to be able to insert and remove the denture easily so the flexibility of the clasp arms must providethe only resistance insertionand removal. to Fig. 11. Shoainga claspand its relationship a surveyline_ to To summarize, objectives surveyingmay be listedas follows the for o To mark the bulbousparts of the teeth and soft tissues. o To identifyareason teeth and alveolarridgesof a cast relativeto any given path of insertion, removalor displacement partial of dentures. I To locatetooth surfacesthat may help to guide partialdenturesto their exact positions the in mouth. o To find undercuts that can be used for retentionand to measurethe amount of horizontal undercut the teethselected clasping. on for . To aid the designand exact location the clasp arms. of o To block out unwantedundercuts(areasof interference)on the cast. o To help decide where the extreme margins of the denture base should lie in relationto the survey lines on the natural teeth, which helps to ensure that no unsightly gaps are apparentbetweenthe teeth and the denture base to affect aesthetics. o To assistthe planning fixed partialdentures(bridges) of beforethe preparation the teeth. of r To decidethe location and parallelism precision of attachments. . To prepareparallelsurfaces, surfacesat specifiedangles by millingon abutmentretainers or and bars,orto drill parallelholes.
  • A Surveying Procedure ! Securethe cast on the movabletable so that the occlusalplane is parallelto the base of the surveyorand lock the table in position.Doing this ensuresthat the graphitemarker is at right plane(fig. 12). anglesto the oc,clusal Fig. 12. Graphitemarkerheld at right anglesto the occlusalplane. ! Check the graphite marker to make sure it has a straight,vertical edge free from wear, loosen the holderand insertintothe mobileverticaltoolholderthensecurethe graphitemarkerby lighily tuming the screw thread of the holder. Be careful at this stage because it is very easy to Oieai< surveying leads. I Bring the graphitemarker into contactwith all surfacesof the teeth by moving it around the cast makingsurethat the tip of the markeris touchingthe cast. Linesare markedon the teeth by the markerat theirwidestpartsbecausethis is the only point at which it touchesin relationto the common path of displacement with the tip of the marker outliningthe horizontal exent of the undercutrelativeto that path. These survey lines indicate areas of the teeth that can be used for retention, whichare those belowthe survey lines,or towardsthe gingivalmargins. D Once the teeth and the soft tissue undercutshave been surveyedin relationto the common pathof displacement, replace graphitemarkerwith the analysing and beginthe procedure the rod to establish path of insertion the and removalfor the denture. I Beforedecidingthe most suitablepath of insertion and removalfor the denturethe answersto two questionsmust be found: What are the positions and anglesof the guidingsurfaces? ls a labialflangerequired? D To establish the positions and angles of the guiding surfacesloosenthe screwthat locksthe tiltingtable of the surveyorso that it can be tilted easily but will stay in the position which it is to tilted. Movethe analysing to the saddleson the right side of the cast and positionit so that it rod is in line with the distal surfaceof the tooth anteriorto the saddle (ng. 6 ). Then move the rod until it touchesthe mesial surface of the tooth distal to the saddle. lf lhere is a difference betweenthe angles that the teeth present to the rod then tilt the cast until the anglesmesially and distallyare equal. Fig. 13. The analysingrod establishing any available guide surfaces. 10
  • tr Repeat the same to the saddle on the opposite side of the cast, then place the analysing rod on the lingualsurface of each abutmenttooth in tum, tilting the cast until the angles presentedto the rod are equal as before. Whilst carrying out this proceduresurfacesthat are parallel may be foundmesially, distally,buccally lingually or that will guidethe partialdentureinto place. Continuecheckingthe parallelsurfacesuntil an angle of tilt for the table is found where the maximumnumberof parallelsurfacesare in contactwith the analysingrod. Once this has been achieved lock the table in positionas this is the most suitable path of insertionand removal for the denture.lf teeth are tilted and ill positioned, minimum of two guidingsurfacesshould be a found. n Mark the vertical walls of the cast by using the analysing rod as a ruler in conjunctionwith a wax knife, by scribingat least three vertical lines in different areas,this enables relocationof the cast in exactlythe same positionafter it has been removed from the surveyingplatform. I Guidingsurfacescan usuallybe found for posteriorsaddlesof partialdentures, but if a labial flange is requiredthen it may be neressary to have an oblique path of insertionand removal to restorelost alveolarand gingival tissues (fig. 14). However, oblique paths of insertioncan cause problemsif posteriorsaddles are also present because areas of undercutare increasedcreating zonesof displacement (fig. 15) and lack of retention. Fig. 14. Restoringanatomical structuresin anteriorregionsusing an obliquepath of insertion Fig. 15. Shov,rs how zones of displacement createdin posteriorregionswhen posterior are titts are chosen. The saddlearea createdis wedge shaped,which is easily displacedalong a greaternumber of paths.Another problemariseswhen teeth are divergent (fig. 16) becausethis also creates a zone of displacement from which the denture can be displacedat any angle betweenthe diverging lines. 11
  • I lzoN€ I I of DISPIACI^4ENIr, F8. 16. Divergent teeth createa zone of displacement. ! The final path of insertion and removalwill be the anteroposterior and lateralpositionof the cast in relation to the vertical arm of the surveyor that meets the criteria of eslablishingguide surfaces,areas of retention,freedom from interferenceand aesthetics. ! The next stage is to establishthe type and position of clasps (direct retainers)that are to be used. Let us assumethat we have surveyed an upper cast that has two boundedposteriorsaddle areas each with the second premolar and first molar missing. The clasps chosen will occupy a position alongand belowthe survey line,the part of the claspthat is belowthe survey line is the part that supplies the direct retention our partialdenture(ng. 11), to establish for the positionof the retentive of the claspa surveyingtool called an undercut tip gauge is used.Undercut gauges measurethe amount of horizontal undercutavailablebelowthe survey line for clasps,they are gradedin three sizes,0.25mm,0.50mm and 0.75mm and are selectedaccording the flexibility to of the metal used to make the clasp. For instancea 0.25mm for cobalt-chromium, 0.50mm for cast gold and 0.75 for wroughtgold, nickel chrome and stainlesssteel wire. These gauges are fitted to the arm of the surveyor (fig 17) to ensure that both the side of the rod and the edge of the disc touch the tooth simultaneously. The edge of the disc then indicatesthe horizontal undercut and the position the clasptip. of Fig. 17. Showingthe use of undercutgauges. The pointof contact thenmarked is usinga different pointfrom coloured marker lhe furthest at the adjacent tooth/saddle undercut area,this ensures that the clasparm is madeas long and flexible possible. as Having established position the retentive the of areas, decision a aboutthe typeof clasp be usedcanbe made,ie occlusally to (single approaching gingivaly or approaching, armor roachclasps). U The undercut areasare theneliminated (blocked out)withwax,whichis thentrimmed using thewaxtrimmer attached the mobile to (fig vertical holder 18). tool 12
  • Fig. 18. The trimmer is used lo trim the wax to achieveparallelsurfaces 13
  • Principles PartialDentureDesign of Thereis little doubt a conectlydesignedpartialdenturepreserves existingoral structures by spreading masticatory overas widean areaof the supporting the load tissues possible. as obviously this is easier withan upper thana lowersimplybecause thereis a greater surface area availablelncreasing masticatory particularly efficiency, when premolars molarsare replaced or to restore occlusion and maintain workingand balancing contacts wherepossible, increase will the chances success anypartial of for denture. Comparatively patients few request partialdenture a due to the inability masticate to food,this is because modemdietof softcooked the food meansthat people survivewithout needfor can the teeth.Neverthe lessmostpeople prefersometeethto masticate normal foods. Aesthetics generally patients are a main concemwhenteethare lost.The mostcommonreason for the request a partial of denture for the restoration appearance, is patients quitehappily of will tolerate lossof posterior the teethbut as soon as an anterior toothis iost a changeof attitude occurstowardsreplacement. The loss of any tooth is liable to be followedby migrationof sunounding teeth resulting closureof the spaceand malposed in teeth. partialdentures are essential preserve maximum to the width of theseedentulous spacesso that a replacement of the conectsizecanbe inserted. Speech may be impaired the lossof upperor loweranterior by teeth,this is because labiodental sounds suchas F and V require contact between lowerlip andthe incisal the edgesof the upper anleriors. Dentalsounds suchas TH, CH, J, Z, require intactupperand lowerarches. Restoration of anterior posterior and arches almostcertainly will phonetics. resultin improved Whenteethdrift it causes themto losecontactwith the teethadjacent them.Foodcan pack to between teethwithconsequent damage gumsand an increase the actionof caries. to in Lossof occlusion oftencauseover eruptionand if this is grossmay meanthe loss of that will tooth.Two or three teeth extracted from differentparts of th-emouth,if not replaced with a prosthesis, would in a few years lead to a completecollapseof the dental arches and a derangement occlusion maybecnme of that traumatic justior anyteethinvolved, alsofor not but thetemporo joint. mandibular Driftingteeth With a partialdentureno driftingis possibleif the edentulous spacesare adequately filled by contacling artificial teethor denture surfaces. OverEruption When, through extraction toothonejaw is deprived an opponent occlusion, a of in changes take placein its supporting tissues. boneand periodontal The membrane no longer are stimulated by the physiological masticatory stresses consequently atrophic and an reaction takls place. Atrophy A termin medicine usedto describe slateof wasting a due to someinterference the function with of healthy nutrition. Partialdenturedesignattemptsto: @ Preserve existing structures prevent oral and trauma. O Restoremasticatory phonetic. and functionsby replacinglost occlusion and conecting the sounds madeduring speech. O Restore aesthetics, support facialappearance. lip and @ Preventarch collapse, individual tooth movement, over-eruption opposing of teeth and splinting teeth. of 't4
  • @ Spread masticatorystressesover the supportingtissueswithin their physiological limits. g Be stablein the mouthduringnormalfunction. Functions of the component parts of partial dentures Saddles That part of the denture which replaces lost alveolar tissue and normally contains the replacementteeth, it transmits masticatory load and retains retention devices. These may be describedas being, free end or bounded saddles or trssue borne or tooth borne. Connectors That part of a denturewhich combinescomponentsto form an integralunit. o rigidlyconnectthe saddles. o rigidlyconnectthe retainers . ensureeven load distributionbetweenthe saddles. r brace againstlateral movement. e providei ndirectretention(anterio-posterior). Occlusal supports Means by which vertical occlousalload is transmittedto the alveolar bone. Usually through metallicsupportspositioned the naturaldentitionwhich transmitmasticatory on load throughthe abutmentteeth. (occlusalrests) Retainers Meansby which a partialdentureis retainedin the mouth. This is normallyachievedby flexible metal extensions encircling the abutmentteeth to enter undercutareas,these extensions being calledclasps. ldeal physical properties of partial denture components Saddles $ Cood stimulation natural of tissues. $ Wett toleratedby the oral mucosa- non toxic, non initant. $ Dimensionally stable. S ruotpermeableto oral fluids. $ Maintains surfacepolish. a $ Goodthermalconductivity. Gonnectors Shouldhave the same properties the saddlematerials, as and in addition o TheYmust be rigid.A connecting bar that is flexible will allow saddlesto move under stress resulting masticatory in forces not being transmittedverticallyto the supporting tissue or being distributed evenlyover that supporting tissue. o TheYmust be strong. In practicea connectorwhich is rigid is also sufficientlystrong. o TheYmust be unobtrusive. Particularly the patientstongue.This meansthat they must not to be bulkyand caretaken in their positioning. I TheYmust not be food traps or encroachon areas of tissue which are best left uncovered. 15
  • Peripheral Outlines and Basic Contours of partial Dentures Extensionsof borderareas,or peripheries(fig, 19) for partial denturesmust take into accountthe structureand function of tissues in the immediate vicinity. Muscles of masticationwhich lie in sulcus areas must be avoided as must frenal attachments,if the denture is to stay in position becauseduring masticationand speakingthese structuresare active and mobile, any extension onto these structures will resultin displacementof the denture and eventuallyulcers. RAMUS F JAW O MED PT MASSE M BUCCINATOR MODIOLUS L A B I A LF Fig. 19. Shouring peripheral the extension a lower free end saddle. of As a generalrule denturebasesshould cover the maximum area possibte and extend into the labial and buccal sulcii. The contoursof the flanges should be slightly concave to allow the cheeksand lips to rest againstthem insteadof applyingpressure, which resultsin the dentures being forced out of position.Buccal and lingual flanges provide some form of bracingagainst lateralforces, claspsprovide bracing,against lateralforces and retentionby resistingvertical and rotationalforces tlcob Po"c Fig. 20. An exampleof how short extensions offer litile resistanceto lateralforces. 16
  • / The lengthof saddleareasdepends the amountof natural on teethlhat remainin the arch form. Free end saddleareasshouldend at the hamularnotcharea for uppersand at the retromolar pad area for lowers.For bounded saddlesthe most anteriorextension shouldfit as closelyas possible to the distal aspect of the abulment. The most posteriorextensionshould fit as closely as possibleto the mesial aspect of the last standing abutment tooth so that retention ulilizing undercuts gainedand rotation is reduced. Where collettsare necessary they should be on or above the survey line and be relievedin the gingival margin area eilher prior to constructionof the denture or after to prevent physical damageto the margins,knownas 'gum stripping'. Collettsshouldjoin naturalteeth at an angle of less than 90o so that they merge into the naturalcontourand deflect food away from the teeth (fis.21). Fig. 21 . Colletts should be constructed to deflect food awav from the tooth Acrylic resin as a connector for paftial dentures Acrylicresin connectors have to be fairly large or they will break.Sometimesthey have to cover all available soft tissueto achievethe rigidityand strength necessary a connector. for The saddlesof this tissueborne denture,$g 22) are joined by a large acrylicconnectorwhich also providessupportand retenlion. Fig. 2. A large acrylic conneclor covering a large surface area of mucosa. It is not always necessary cover as much tissue with an acrylic connector(fig. 23). This lo denture will be worn more for appearancethan for mastication,it needs less retention and support beqausethe acrylic connector is subjectedlo less force, therefore it can be rnuch smaller. 17
  • I Fig. 23, Denturesfor aestheticpurposesonly can havesmallerconnectors. The size and design of partial dentures is based on the assessment of the following ' The coveragenecessary give slrength,this dependson the number to and positionof artificial teeth and whether or not occlusionis to be restored. . Any extra coverageneededfor support and retention. o The need to keepgingivalmarginsclearto avoid unnecessary lissuedamage. 18
  • Principles mucosaborne partialdenturedesign of Replacement teeth are set in relation the articulated to castsand the remaining naturalteeth. once centric jaw relationshiphas been establishedin the clinic with either wax squash bites or occlusal record rims. The casts are articulatedso that the artificialteeth can be "set up,'to the remaining naturaldentition. The relationship the upperand lower naturaldentitionmay differ from the classicskeletaljaw of relationships that allows intercuspation when setting complete dentures. lf this is the case then certain modificationsto occlusal surfaces will be ne@ssary to provide the patient with an occlusionthat is both functionaland stressfree. To establishthe maximum amount of contact, the largest surface area, spot grinding of artificial tooth surfaces is necessary.Before this procedure is commenced, any'opposing natural tooth surfacesare coveredwith thin metal foil, which is burnished(adapted)ctoseiyto protectthe cast from potentialdamage. The teeth are then positioned one by one to conformto the occlusalplane,lengthand arch form of both upper and lower casts. The articulator is then closed down with a piece of articulating paper betweenthe upper and lower casts, once contact has been made, open up the articulatoi and look at the occlusal surfacesof the artificialteeth. Marksleft by the articulating paperwill be apparenton the surface,colouredeither red or blue, these areas are then grounb with a small metal bur, usually a rose head, and the articulatoris then closed again to check that the occlusion established is and that the verticaldimensionis correct.This ii done by ensuring that the incisalpin is touchingthe incisaltable, however,if this is not the case then the procedure is repeated untilit does beingcarefulnot to over grind the artificial tooth. This technique establishes centricocclusionin centricjaw relationship, the technician now has to check both the lateraland protrusivejaw movements to ensurefreedom from interferenceand where possibleworking contactby tooth surfaces.This is done by moving the articulatorfrom side to side with articulating paperin positionbetweenthe teethwith contaaingareas being groundto establish workingand balancing contacts, this may not howeverbe possible with all partialdenturecasesdue to the canineguidance,or lift. 19
  • Principlesof tooth borne partial denture design partial A typical removable denture havethe following will components I Majorconnector o Minor connectors . Rests o Directretainers o Indirect retainers o Reciprocal bracing or components o Saddle areas Partialdentures should be designedto resist displacing forces in a vertical,horizontat and anteroposterior direction. This is achieved throughthe use of components providesupport that andretention the partial for denture, whichprevent movement anyof these in diiections. partsthat resistverticaldisplacement The component (movement towards and awayfrom the mucosa), occlusal are restsandclasps. Horizontal displacing forces are resistedby the bracing sectionsof retentiveclasp arms, reciprocal clasp armsandmajorandminorconnectors. Anterior-posterior displacing movements resistedby occlusalrests,claspsand major and are minor connectors. Occlusal tooth bornesupportfor the partialdenturemust be provided some kind of rest or by placedon exi9ing abutment teeth.Theseshouldalwaysbe placedupontoothsurfaces which havebeenprepared receive to them.Restsmay be placeduponthe prepared occlusal surface of a premolar molartooth,usually the marginal or in ridgearea gg. 24),or uponthe prepared lingual palatal or surface an anterior of toothcapable withstanding forces of the applied, upon or a prepared incisal surface. ftct Et Fig. 24. Occlusalrest posilion. Occlusalsupportis sometimesobtainedby using a tooth surfacewhich is inclinedocclusallyor incisally,from its greatest convexity. Rest! tnat ire so placed upon an unpreparedsurface are subjectto slippage alongtooth inclinesproducing orthodontic forceswith the possibility moving of the abutment teeth. This is an infringement one of the basic rules providedfor rest design which is that a resf of should be so desrgned to transmit any occlusal force directly along the long axis of the supporting tooth. 20
  • Fig. 25. Inclinedsurfaces,if unprepared subject to slippage. are A second ruleis thata restmustbe placedso that it will prevent movement the partial of denture in a cervical (downward) direction. Occlusal restsmusttherefore rigid and receivepositive be support from the abutment tooth or teethif the required amount stabilityis to exist and be maintained. of Occlusal rests,in addition to distributing occlusal load,serveotherpurposes. Restsact to maintain occlusal the relationship withthe opposing teethby preventing settling the partial of denture. the sametime setilingof At the partialdentureagainst gingivaltissuesis prevented thus avoiding any impingement or damage the gingival to tissues adjacent the abutment to teeth.By preventing movement a in cervicaldirection, positionof the retentiveportionof the clasp arm is maintained its the in intended relationship thetoothundercut. with portion a clasparm,whenin its terminal The retentive of position passive in should remain and contactwith the toothso that it is readyto resistany verticaldislodging force. Thus when a dislodging forceis applied clasparm becomes the activeto resistverticaldisplacement. When partialdentures madewith claspassemblies do not contain are that occlusal restsas an integral partsomesettling the denture of takes place,whichmeansthat the clasparm is standing away from the tooth, consequently some verticaldisplacement possiblebeforethe retainercan is become functional. Rests, whenincorporated retainers prevent into suchsettling thereby helping to maintain vertical the stability partialdentures. of Partial denture components used to provide occlusal support Occlusal Rests Theseare metallic extensions whichfit into prepared areasof teethto provide occlusal support by resisting vertical displacement the denture of towards softtissues, the theyresistlateral forces andactas indirect retainers resisting by movement thedenture anteroposterior of in directions. Cingulumand Incisalrests Cingulum restsare smallmetallic extensions whichfit intoprepared areasof anterior teethon either lingual palatal the or aspects. Incisalrestsare small metallic extensions whichfit into prepared areason the incisaledgesof anterior teeth A modified formof incisal is the'embrasure rest hook', whichis placed the embrasure in between twoteethandcontinues the incisal over edgeontothe labial surfaces. In addition the above components, 'Kennedy to the bar' is a continuous lingualor palatal cingulum rest,whichshould havea reit preparation eitherendto be effective. at Onlays Thesecomponent oftenregarded enlarged are as occlusal rests,whichare usedto maintain, or re-establish vertical the dimension orderto restore original in the function occlusion. Onlays of apeoftennecessary whenocclusal surfaces groundawayover the yearsby mastication are or bruxism, theyalsoprotect occlusal the surface from further wear. 21
  • lntemalRests Such restsare seatswhichare incorporated the surfaces cast retainers into of formingparallel slotsor dovetails utilizebothocclusal to support and horizontal stability. c-) t rl J '2 m -t ffi tYl lA, I f I t '- a. Fig. 26. The varioustypesof rests. clinical modificationsto tooth surfaces that provide occlusal support Planning supportof tooth bornepartialdentures often necessitate grindingof tooth the will the surfaces the dentalsurgeon providespacefor occlusal by to rests,the use of restswill prevent complete closure centric in relationship cuspscontact if them. Fig. 27. Cusps of opposingteeth may haveto be modifiedas well. Rests seats should not be cut very deep through enamel, if deeper rests seats are required restorations should be placed on abutment teeth into which rests seats may be cut. On.the occlusal surfaces of posterior teeth they should be dish shaped with vertical walls and be positioned eitherthe distal or mesial marginal ridges.The positionfor rest seats on anterior at teeth will be eitheron the cingulumor incisalareas and take the form of narrownotcheswhich are flat and at right angles to the long axis of the teeth. The floor of occlusal rest seats on posteriorteeth should be inclinedslightly towardsthe centre of the tooth. Rest seats on posterior teeth should form an angle of less than 90o with the vertical minor connectorfrom which it (ng. 28). originates It shouldbe notedthat anglesgreaterihan gOofail to transmitthe resulting occlusalforcesatong the long axis of abutment teeth because the force is not channelledtowards the centre of the tooth. 22
  • ^A tr, C N Fig. 28. Occlusalrests shouldform an angleof less than 90' with the minor connectorto transmitocclusalforces alongthe long axis of the supportingtooth. Anterior teethcan be usedto provideocclusal supportif the lingual palatal or surface slopesare gradual rather thanperpendicular, whichhelpsto prevent slippage, with canines beingprefered to incisors occlusal for support because they have a largerrootsurface areaandgreater alveolar provide bonesupport.Cingulum greater restson canines support than incisalrestsbecause they are placedcloserto the centreof rotationof abutments therefore reducing tendency tip the to abutments. Cingulum restsare aesthetically moreacceptable patients to thanincisal rests. Poorly designed indirect retainers resting toothsurfaces on may resultin of @ movement the toothawayfromthe component loading a tooth resulting traumafollowedby inflammation the periodontal of in @ excessive of membrane, looseningof the tooth and loss of bone support resultingin the loss of the tooth. 23
  • TheApplication the Principlesof PartiatDentureDesignto the of tooth/mucosaborne case Tooth/mucosa borne denture support is derived from the remainingabutmentteeth and edentulous areas. Partial dentures this kindof support haveocclusal with will and incisalrestsincorporated the into designto allowthe transmission masticatory of forcesthroughthe abutmentteeth, however, some of the mastication force will be transmitted throughthe mucosa.The most common example wherethistypeof actionoccurs the Kennedy of are Classes & ll. I Possiblemovements the partialdenture of Thereare at leastthree possible movements a distalextension partialdenture(ie bi-lateral of freeendsaddles). Thefirstis rotation aboutan axisformedby the two principal occlusal rests. Thisaxis,whichis known the fulcrumline,actsas a centreof rotation the baseof the distal as as extension movestoward supporting the tissues duringthe application an occlusal of load. When the occlusal load is released, fulcrumline then shiflsto any anteriorly the plecedre$s as the basemovesawayfromthe supporting tissues whichallowsverticaldislodging forcesto become effective. Thesedislodging forcesare the verticalpull of food between opposing toothsurfaces, effect the of moving border tissues the effectof gravityagainst maxillary and a denture. Assuming that direct retainers functional are and that occlusalrests remainseated,it will be rotationof the prosthesis that occursratherthan total displacement. This dislodging force is resisted onedirection the tissues the residual in by of ridge,(theaccuracy the fit of the denture of base) with resistance the opposite in direction beingprovided the actionof the occlusal by rests serving indirect as retainers. The secondmovement rotation is abouta longitudinal axis formedby the crestof the residual ridge.Resistance provided is initially the rigidityof the majorconnector its ability resist by and to torquewith the occlusalrests contributing the stabilityof the partialdentureas indirect to retention only. lf the major connector not rigid this rotationabout a longitudinal is axis either appliesunduestressto the sides of the supporting ridgeor causeshorizontal shiftingof the denture base. Thethirdmovement rotation is perpendicular located aboutan imaginary axis nearthe centreof the dentalarch. This movement o@ursduringfunctional occlusal stresses from a diagonal and horizontal direction and is bestresisted stabilizing bracing by or components suchas reciprocal clasparmsand minorconnectors whichmakecontact withvertical toothsurfaces. Stabilizing part of any partialdenture components an essential are designregardless the of manner support of and the type of directretention employed. Thesebracing components, on eachside of the arch,act to stabilize partialdentureagainstany horizontal the forcesapplied fromopposite sides. Rigidconnectors mustbe employed achieve effect. to this Movement tooth/mucosa of bomepartial dentures result the breakdown supporting will in of bony tissues abutment of teeth,if the majorpartof the masticatory fallson the saddle. load This is because compressibility tissueincreases posteriorly of pad towards regionof the retromolar the area.On application a vertical of forcethe abutment toothmay be slightly displaced rigid if (occlusally approaching) claspsare used.The alveolar tissues displaced a greater are to extent, resulting rotation in arounda fulcrumaxis provided the occlusal (rests). by support The amount of displacement compression and depends uponthe condition the mucoperiosteum covers of that the bone(thealveolar ridge). Increasing verticalload will causebone resorption the breakdown cells underpressure by of (fibroclasts). The denturebase must cover the maximumarea possibleto give maximum support, thusspreading masticatory the load. 24
  • After resorptionhas taken place, if no attempt to reline or rebase is made, further resorptionis likely to take place as the downwarddisplacementcontinues. When rigid clasp arms embrace a tooth vertical loading will tend to rotate the tooth distally, the centre of movement being locatedin the area betweenlhe apex and the gingival margin.Tensile forces act on the bony socket in the distal area of the gingival margin and mesiallynear the apex. Such forces acting upon abutment teeth result in a breakdown of the periodontaland bony supportproducinga loosetooth. Vertical loadingwill also result in some movement of the clasp arms over the enamel surface of abutmentteeth, but this dependson . The thicknessof the flexible portion of the clasp arm. o The modulusof elasticityof the clasp arm material. . The degree of retentiveundercutengaged. Clasps constructedin alloys with a low modulus of elasticity will allow greater movement. lt therefore follows that wrought clasps will allow greater movement, as they are usually round in cross sectionallowinggreaterflexibility in horizontaland verticaldirections. To summarisethe effects of clasping,the more rigid the clasp the greater is the torque ratio on abutment teeth with less load applied to the mucrsa and alveolar ridge. Flexible clasp arms effect less torque on abutmentteeth but allow greater mucosaland laterai loading. 25
  • bournecase to Designprinciples considerfor the tooth/mucosa Reduction of load The vertical load acting on the saddle area during masticationcan be lessenedby reducingthe surfacearea of the oerlusaltable. This is achieved when caninesand premolarsare used instead of molars, using teeth with a narrow bucco-lingualwidth and sometimes leaving a tooth off the saddlearea (usuallythe last molar). This is most usual when the bite is heavy and possiblyclose to the saddle area or the saddle is long or the bone supportis poor. Reducingmasticatoryload will also reduce the amount of lateral loadingthat acts on the saddle, so helping to preservethe ridge form and the supportingtissues of the abutment teeth, this is achievedfurther by the use of flat cuspedor shallow cuspedteeth. of Distribution the masticatoryload over a wide area can be achieved by the coned peripheral extensionof the saddle areas of partial dentures.Under extendedsaddleareas have the effect of concentratingthis load on a very small area, particularly if a well defined residual ridge is present. of Anteriorplacement occlusalsupports and surfaces premolars of Although prefened the sitesfor occlusalrests may be the occlusal the simplybecause premolar molars may be neecssary include reston a canineor incisor it to a or molar missing. is is rest. retainer an auxiliary A canine much or Anterior teethmaybe utilized support indirect to an prefened restseatscan be are multiple over an incisor this purpose. for When canines missing, incisor. incisors rather thana single spread overthe remaining crownto the inclination the tooth and ratioof the lengthof the clinical Rootform,rootlength, of before decidingthe site and form of rests placed on alveolarsupportmust be considered to they are placednearer the centre restsare prefened incisalrestsbecause to incisors. Lingual are Lingual rests or to of rotation abutments havelesstendency tip teethdistally mesialy. of and moreacceptable the patient. to alsoaesthetically of on are the restsshouldbe positioned the mesialaspect the Whenpremolars present, occlusal of ridge.The distribution forces in rest seat cut into the marginal occlusal surfac,e, a prepared as support occlusal by abutment teethandthe ridgecan be alteredsignificantly placing between possible in an increase the load on the ridge,but proportionately far forwardas This resultsin of less on the abutments, giving a more even distribution stress in an anteroposterior so direction. to of Combined actionof restsand claspsreversethe direction rotation a forwardmovement whichis resisted contac{ anterior by with teeth. of Distribution load betweenteeth and ridges and clasps saddles. between of Thiscanbe achieved varying nature the connection by the Stressbreaking Insleadof having a rigid connection betweenclasp and free end saddlesome degree of is the movement allowedbetween two. The stressesacting upon the dentureare distributed differently. as unit unit whichallows movement between saddle andthe retaining is known a the Anydevice breaker. stress the covering into downwards the softtissues Whenvertical loadis applied saddleis displaced the the greater existsbetween a rigidconnection edentulous ridgeto a extentthan is the casewhen retainer, occlusal rest and the saddle.This meansthe mucosaand alveolarbone has to overthe whole increased vertical withstand load,but alsothatthe loadis muchmorewidespread thanconcentrated the freeend. at ridgerather 26
  • ln generalstressbreakerscan be divided into two groups e Those having a movablejoint betweenthe direct retainerand the saddle ie hinges,sleeves. cylindersand ball and socketdevices. These are also termed precisionattachments. . Those having a flexible connectionbetween the direct retainerand the saddle, ie wrought wire connectoror a divided (cast)major connector. The resultof stressbreakingaction is Vertical load - a greater proportionof load is assignedto the edentulousridge and less to the abutments. Lateral load - greater lateral stress is placed on the alveolar bone and less on the abutment teeth. Stress Breaker Flexibility The flexibilityof a stress breakergovems the amount and distributionof load betweenthe ridge and the standing teeth but will dependupon o The metallurgical conditionof the connector(cast or wrought). o The lengthand positionof the connector. . The crosssectional dimensions and shape of the connector. Advantages of the stress breaking principle O Horizontal forcesactingon abutmentteeth are minimized,the alveolarsupportof these teeth is preserved. O Stress-breakers distributethe load between the teeth and the mucosa so that the mucosa takesa greaterproportion the load when the periodontal of cnndition poor. is O With carefulselection the type of flexibleconnectorit may be possible obtaina balance for to of stress betweenridge and the abutment. @ Sptintingof weak teeth by the denture is made possible,despitethe movementof a distal extensionbase. pressure the denturebases massagethe mucosa,thus providingphysiological O Intermittent of stimulation, which prevents bone resorption and eliminates needfor relining. the O Stressbrokendesignsare best when the remainingteeth are of poor prognosis and cannot providesupportfor conventional partialdentures. Disadvantages of the stress breaking principle @ me dentureis more expensiveto construct. @ A concentration forces on the edentulousridge results in its speedierresorption. of @ tne more complicatedappliance lessens the tolerance of the patient. Spaces between components sometimes are openduringfunctionleadingto food trapsand tonguenipping. 6 tt a lightand flexibleconnector used it can be easily bent and distortduringcleaning. is @ lf rebasingis not undertaken when necessarythen marked resorptionwill occur. @ Repairand maintenance any stressbreakeris difficult,costly,and frequently of required. . 27
  • Fig. 29. A flexibleconnectorfor a free end saddle Advantages and disadvantages of rigid designs Advantages a rigiddesign of g Technically framework easierand lesscosflyto make. the is O Equitable distribution stressbetween of abutments the residual and ridge(s) possible is with a rigid design. O tne needfor relining rigid prosthesis less frequent the is sincethe residual ridgedoes not haveto carry functional unaided. the load O Indirect retainers otherrigidcomponents and may act to preventrotational movement the of dentureand will providehorizontal stabilization that is not possible when stressbreakers are used. O By reducing number flexibleor movingpartsthere is less dangerof distortion the of by careless handling the partof the patient. on g Moving parts being absent appliance moreeasily the is keptclean. Disadvantages a rigiddesign of @ Objectionable torquemay be applied the abutment to teeth if abutment retainers not are passive" 6 nigiO continuous clasping maybe hazardous whenstress breakers notused. are @ Intracoronal retainers may not be usedat all without stress-breakers, because theyare locked within abutment tipping the and forces wouldbe transmitted directly the abutment to tooth.The onlyexception whenmultiple is splinting abutments, of coupled witha minimum occlusion of on thedistal extension base, actsto reduce abutment torque. @ tne useof wrought-wire retentive particularly clasparms presents sometechnical difficulties when high-fusing chromealloys are used. Wroughtwire may be crystallized improper by application heatduring of casting soldering or operations, resulting earlyfracture. may also in lt be easily distorted careless by handling, leading excessive insufficient to or retention, ultimate or fracture to repeated due adjustment. @ lf reliningis not done when needed, the abutment tooth may be loosened and suffer permanent periodontal damagebecause the repeated of application torqueand tipping of sil.resses. 28
  • Type of clasps used in conjunction with rigid connectors The type of clasps used in conjunctionwith rigid connectorshas markedly different effects on load distributionlf gingivallyapproachingclasps are chosen,a conditionexistswhich is similar in principleto stressbreakingaction ie Roach back action (half r or L). The effectivenessof the.stress breaking action of bar clasps is increasedsimply by increasing the length of the bar further into the sulcus, or attachingthe clasp to the minor connectoras far from the abutmentas possibleand by the type of material used, ie wroughtor cast alloy. With gingivally approachingclasps a phenomena known as 'trip action' exists. This increases retention of free end saddle designs and may be defined as 'fhe ftictional resrsfance fo movement, as opposed to deformation resr'sfance'.On attempted removal the angle between the tooth and clasp increasesand the clasp beds into the tooth surface,thus resistingdisplacement. A combinationof rigid connectionand occlusaly approachingclasps is the opposite extreme to stress breaking. In this situation the maximum load is placed on the abutment tooth and the minimum on the ridge. However,if wrought gold occlusalyapproachingclaspsare provided with rigid connectionthen the added flexibility means that a reductionof stress occurson the tooth but increaseson the ridge,the oppositebeing so for cobalVchrome. Altematives to cast components for metallic denture base connectors Connectorsfor the partial denture may also be formed by using wrought stainlesssteel, these connectors beingthe bar type ie labial,lingualor palatalbars. Usingwroughtconnectors has the followingadvantagesover cast connectors;More flexibility exists Oetween clasp unit and the tfre saddle,the cross sectionalshape of the material allows flexibilityin universaldirectionsand Iastly,the structureof the wrought alloy used makes it less likely to fracture.However,there are also disadvantages wrought connectorsin comparisonwith cast conneclors.Cold working of to alloys can produce work hardeningof connectors,which leads to fatigue and eventual fracture due to the compression of the structure, soldering can anneal wiought components, thus reducingflexibility,elasticityand the ability to retain shape or resistdeformltion. Physical properties required for connectors The physical propertiesrequired for connectors used in stress broken and non-stress broken designsare similarto that of cast or wrought clasp units and cast major connectors. The following applies to stress broken connectors They shouldhave a high proportional limit to resistpermanent deformation, they shouldpossess a low modulusof elasticity allowfor flexiblemovementduringfunction. to The following applies to non stress broken connectors The major connector non stressbrokendesignsshould have a high modulusof elasticity for for rigidityin thin sections and a high proportional limit to avoid permanent deformation. 29
  • lmpression Techniquesfor Removable partiat Dentures Functional lmpression techniques ln the mid 1950's McClean recognised denture that bearing areashadtwoforms(anatomical and functional). Anatomical formbeing denture bearing areasfree fromocclusal masticatory or loading. Functionalform beingonewhichis underpressure, loadduringfunctions mastication or of and occlusion McClean's methodof taking impressions involvedthe use of occlusalrecordrims lined with paste. impression The patient bitingtogether providethe functional to loading the mucosa on of the edentulous areas. overallimpression the recordrims in situwasthentakenand sent An with to the laboratory. Mucco-displacement impression techniq ue A mucco-displacement impression technique seeksto produce impression the denture an of bearing areaswhilstundersimulated functional loading. The type of materialused for this technique zinc oxide/eugenol is paste,which displacesand compresses soft tissue in a the similarmanner that of a denturebase,thus givinga more accurate to cast upon whichto construct denture any basefor tooth/mucosa bornepartial dentures. aim of thistechnique The is thesameas McClean's. Technique 1. Special traysaremadefor secondary impressions using studycastspreviously the taken. The typeof traythat is constructed a dualpurpose. has The tray will haveboth alginateand zinc oxideeugenolpaste used in it to producea secondary cast. 2. Firstthe saddleareasare relievedby using0.5 mm wax spacer. 3. A 2 mm wax spaceris laiddown over the remainingnatural teeth on the cast. 4. Locatingstops cut into two different areas of the occlusalsurfaces. 5. The tray is constructed the usualmannerand sent to the surgery, in wherethe dentisttakes an impression the saddle areas using zinc oxide eugenol paste,this materialis viscous under of pressureand will flow to all sectionsof the saddle area. Once set any excess is removed in the abutment areasand the next stageusingalginateis begun. 6. Firstthe locating stops are removedand the alginate(ineversible hydrocolloid) loaded into is the tray and the impression completed. Other impression materialsthat can be used are silicone rubber base and tissue conditioners such as ViscoGel for the saddleareas. Applegate technique Anotheralternative methodis the alteredcast techniquefirstdescribed Applegate. by 30
  • Partialdentureretention Retentionof removablepartial dentures is achieved by means of direct retainersof one type or another. A direct retaineris any unit of a removable prosthesisthat engagesan abutmenttooth so as to resistdisplacement that prosthesisaway from the basal tissues. of This can be accomplishedby frictional means, by engaging a depressionin the abutment tooth, or by engaginga horizontal tooth undercutlying below the survey line. Direct retainer types Basically therearetwotypesof directretainers, theseare Precisionattachments Theseare eitherprefabricated, made by the technician provideretention or to and/orsupport. They can be elilra coronal, that is lying outsidethe naturalcontours the teeth, or intra of coronal,that is lyinginside natural the contours the teeth. of Extracoronal retainers haveedensions withhinges and railswhichengage slotsandslides. lntracoronal retainers engage verticalwallsbuilt intothe crownof the restored abutment toothto create frictional resistance movement removal. to and Clasps Othertypesof extracoronalretainers engageextemalsurfaces abutment of teeth in an area cervical the greatest to convexity in a depression or created that purpose.The mostcommon for extracoronalretainer the flexible is retentiveclasparm- Ratherthan creatingfrictionalresistance to removal, flexiblearm is forcedto deformtherebygenerating a resistance removal. to Clasp retention baseduponthe resistance metalto deformation.A clasp,if it is to be retentive is of must be placedin an undercut area of the tooth whereit is forcedto deformwhen a vertical dislodging forec,suchas occlusally directed force is applied and is this resistance to deformation whichgenerates necessary the retention, is only proportional the amountof but to flexibility the metalfrom of whichthe clasparm is constructed. Clasps should beara passive relationship the teethexcept to whena dislodging forceis applied, any activerelationship claspto tooth,that is to say a claspwithoutocclusal of supportor reciprocal action resultin orthodontic will movement or damage the periodontal and to tissues. Claspunitsresistocclusally directed forcesby the inclusion an occlusal of rest,whichprevents movement a downwards in direction towards mucosa, the resistance lateral to forcesis provided bythe broaderpartof theclasparm, orreciprocal clasparms, is known bracing. and as Teeth,if purposes they are to be usedfor retentive must have a heightof contour cervical whichthe to surface converges, the gingival eg margin. All teeth,when surveyed, possessa heightof contour,or an area of greatestconvexity will (width), areasof cervicalconvergence but may not exist when a tooth or teeth are viewedin relation a givenpathof placement to because, areasof mnvergence change withthe tilt of the platform. cast on the surveyor Sometimes areasof convergence too closeto the gingival are margins and are therefore available the placement retentive not for clasps. areas of Conversely of occlusal convergence may be utilisedfor the placement non-retentive,.reciprocating, or of slabilisi components. ng Flexible components the onlycomponents are whichmay be placedbelowsurveylinesbecause if rigidcomponents wereso placed, undercut the areaswouldbecnmeareasof interference to placement removal and rather thanareasof retention. degree toothundercut location The of and available retention the partialprosthesis therefore for of is relative the pathof placement and to removal. 31
  • .!:'1r : ,llt .. ': a.::. . . Factors determining the amount of retention generated by a clasp. Size and positionof undercutareas. . How far into the angle of cervicalcrnvergenc€ (undercut)the clasp terminal is placed. r Flexibilityof the clasp arm, which is the producl of . lts length, measuredfrom its origin to its terminal end. - lts relativediameter,regardless its cross sectionalformof . lts cross sectionalshape,that is whetherit is round, half round or some other shape. . The materialfrom which the dasp is made, ie cast gold alloy, cast chrome alloy,wrought gold alloy,orwroughtnickelchromealloy. Uniformity of retention : The placement of clasp arms into areas of convergencewill be determined by the size of the angle of convergence,uniformityof retentiondepends upon locating the clasp terminal not in relation to the height contour but in relationshipto the angle of cervical follows then that all retentiveclasp arms should be located in the same degree of undercuton each of the abutment teeth, howeverclasp arm positionswill be different for each tooth due to dissimilar contoursThe degree of undercutused for retentionof partial dentureswill vary with the choice of alloy for the retentivecomponent,ie 0.25mm for cobalt chrome alloys, 0-50mm & 0.75mm for gold alloys and wrought alloys. Factors affecting the flexibility of the clasp arm Length The longer the clail arm the more flexible it will be, the length of a circumferentialclasp is measuredfrom the point at which a uniformtaper begins.Retentivecircumferentialclaspsshould have an arm that is tapered uniformly from ils point of origin continuous along its full length to the terminalend. Clasp arrn cross section The greater the cross sedion of a clasp arm the less flexible it will Oe, if the taper is completely uniform (fig. 30), then the average cross seclion should be at a point midway between its origin and its terminal end. howeverif its taper is not uniform a point of flexure and thereforea point of weakness will exist that will then be the determining factor in its flexibilily regardlessof the average cross seciion of its entirelength. l 'J l*€tat rt,n! o2 tctt * -* t*"* Fig. 30. The uniform taper for clasp arrm. 32
  • Cross sectionalform. Flexibilitycan exist in any form of materials used to make clasps whether they be half round or round. The only universallyflexible form is the round form, which is practically impossible to obtain by casting and polishing.This flexibility is limited to only one direction in the case of the half round form. Cast claspsare essentiallyhalf round in form and flex away from abutments in horizontalplanes only, this ability of the alloys used to cast clasps allows for adjustmentof the arm, howeveredgewiseadjustmentand flexing is limited (verticaldirection). lt is for this reason that retentivecast clasp arms, when used for tooth borne partial dentures are more acceptable becausethey will be called upon to flex only during placementand removal of the prosthesis. Tooth tissue bome cases ideallyshould have clasps which have a round cross sectionalform. This is best illustratedby the example of a distal e)ilensionbase which has a retentiveclasp arm on an abutment adjacent to the free end saddle. A clasp in this situation must be capable of flexing not just during placementand removal but also during functional movement of the dislal extensionbase, in short it must possess either universal flexibility to avoid the transmissionof tipping forces to the abutment tooth or be capable of disengagingthe undercut when vertical forces directed against the denture are towards the residual ridge. Round clasp forms are the only circumferential clasp form that may safely be used to engagea tooth undercuton the side of an abutmenttooth adjacentto a distal extensionbase. Clasparm materials, and wrought cast physical properties Theideal required analloyused construct of to clasps should be o A highproportional to resistpermanent limit deformation. o A low modulusof elasticity that the clasp is flexibleenoughto allow withdrawal so over undercuts without toothor claspbeingoverstressed. the Clasparm assemblies be madefrom cast alloyssuchas gold and cobaltchromium well can as as preformed wire of nickelchrome,stainless steel and gold (wrought clasps). previously As stated action a claspdepends the of uponthe resistance the alloyto deformation. alloywith of An a lowmodulus elasticity of flexeseasily, whilstonewith a highmodulus stiff. is The elasticlimit,whichis the pointat whichpermanent deformation takesplace,needsto be as highas possible prevent to suchpermanent deformation the clasparm during gold of function, alloy,nickel chrome andstainless steelclaspsare placed 0.50mm 0.75mm at & because their limitsare simplylowerand greater flexibility can be achieved, theseclaspsare usually made frompreformed wire. The amountof horizontal undercut usedfor retentive clasparms has to be proportionate the to modulous elasticity, cobaltchromealloyclasparm tips are placedat 0.25mmbecause of ie this is theamount deflection canbe achieved of that physical within alloys the limits. Type lV gold alloysshouldbe heat treatedin orderto provideadequate stiffness. Cast cobalt chromium alloys cannot usually hardened heattreatment be by unless theycontain MolyMenum. A disadvantage cobalt of chromium alloysis thatthe grainsizeis large, whichmeans thatthere may onlybe two or threegrains across wholethickness an alloyclasp,and the of therefore is it y easif distorted broken. presence 0.25- O.5o/o molybdenum the alloyreduces or The of of in the grainsizeandincreases strength thinsections, alsoproduces solidsolution the of it a hardening effect.Nickelincreases ductility cobaltchromium the of alloys,but decreases theirstrength. The methodof coolingafter castingalso influences strength the and structure cobaltchromium of alloys. 33
  • Clasp design and influencingfactors Clasps are extra coronal retainersand fall into two main groups Occlusally-approaching clasps, (supra bulge)The clasparm approaches undercut the the of toothfromits occlusalsurface. Theseclasps alsotermedcircumferential. are Gingivally-approaching clasps(infrabulge)The clasparm approaches undercut the the of toothfromthe gingival surface. Theseclaspsmay alsobe termedbarclasps. The following general principles of clasp design should be observed S RigiO portions the clasparm shouldlie on or abovethe survey line. of S Retentiveflexible tips of clasp arms should lie below the survey line to provide retention. $ Cast clasparms shouldbe made as long as possible. $ Claspsor collettsshould be on oppositesides of the teeth to provide reciprocalaction. $ Where ever possible claspassemblies should be bilaterally opposed, that is, retentionon one side of the arch shouldbe complimented retentionon the opposite by side. Clasp assemblies should consist of . One or more minorconnectors from whichthe clasp arms originate. o A principlerest. . A retentiveclasp arm engaginga measuredtooth undercutwith a retentiveterminal. o A non-retentive clasp arm on the oppositeside of the tooth for reciprocation and stabilization againsthorizontal movementof the denture.Rigidityof this clasparm is essential its purpose. to TENTION BRACING parts of clasps. Fig. 31. Component Poordesign andchoice direct of retainers may result in S partlat dentures rotating around toothusedfor retention the causing gradual a loosening the of tooth dueto boneloss. I Placing claspsin undercuts are incorrectly that measured may causetraumato the toothand fracture clasp. the 34
  • It rl i I I I* I I Blatterfein's classification of buccal and lingual survey lines Blatterfein derived a classification survey lines accordingto their positionon the tooth, the idea of being that this classification would prove helpful and practicalwhen deciding on clasp design. What Blatterfeindid initiallywas to divide teeth in half vertically by using the long axis and once divided then gave names to the divisions, he called such divisions on teeth nearest to a saddle the Near Zone and the divisionfurthest away from the saddlethe Far Zone. Classification surveylines of Medium Survey Line Situated approximatelyacross the centre of a tooth it will curve in a slightly occluso-gingival direction from the near to the far zone. The type of clasp most used in this situation is the circumferential occlusaly approaching clasp arm but this does not mean that gingivally approachingclasp arms cannot be used. Clasp arms of either type can be made of cast cobalt chromeor gold alloysas well as preformedwire. DiagonalSurvey Line Situated approximately the occlusal near surface linetravelsdiagonally the across toothfrom the the nearzonetowards far zonefinishing or very closeto the gingival the at margin. Suchsurvey lines found upper are on canines premolars either and in maxilla mandible. or Bothtypesof retentive clasparmscan be utilisedon teethwhichhavethe diagonal surveyline, listedbelow someof the specialist are designs employed. o Thereverse actionor hairpin claspandthe ringclaspwhichare bothcircumferential clasps. . The bar clasp, whichis a gingivally approaching claspand mostcommonly called.aRoach clasp. I Allthese designs haveadvantages disadvantages, example, reverse and for the action hairpin or clasphasthe disadvantage havingno application of wherea shortclinical crownexistsbecause insufficient space available accommodate double is to the clasparm,alsoas the diagonal is line foundmainly teeththatareconspicuous on (whilst palient smiling) double the is the clasparm is easily seen. Ringclasps encircle toothon threesides,firstlyit follows lingual the the surface abovethe survey line,thenflowsinto the embrasure between toothto be clasped the neighbour and its adjacent andthencrosses buccal the surface fromthe far zoneto the nearzone. Clasps this type may of require extensive modification toothsurfaces. to Gingival approaching clasps over may be prefened because the obvious of aesthetic advantages portionof the claspcan be L or T shaped. the occlusally approaching. retentive The The L type part hasa portion whichliesin the undercut bracing areabelow survey andan additional the line whichfiesabovethe surveyline whilstthe T type has a retentive element whichlies entirely withinthe undercut area. The High SurveyLine Found muchcloser theocclusal to surface on thanthe gingival margin bothnearandfar zones in teeth thathavea small degree convexity because toothis lingually of or the inclined. Occlusally approaching clasps shouldencompass if threesurfaces theyarethe casttypeand one if a wrought clasp used.Sucha wrought is the clasparm is positioned immediately below survey gold wire is line havinga greaterlengthof the arm engaging the undercut area (wrought prefened stainless to steelbecauseit is more flexible).Bracingeffectsof this clasp are poor becauseinsufficient space existd betweenthe survey line and the occlusal surface to accommodate rigidportion a castclaspsinceit couldinterfere the occlusion. the of with surface Wherehighsurveylinesresultfrom an inclined toothit will be foundthaton the opposite area,the thereis littleor no undercut, thesecasesa clasparm that encircles non-undercut in the nearproximal surface the surface and with the highsurveyline is oftenthe type used. The first portion the arm lieson the non-undercut of area or surfaceabovethe surveyline and is thicker 35
  • thanthe rest of the ensure rigidity,the portion contactingthe proximalsurface is variable positionwise lying in or out of the undercut,the portion which lies below the survey line is resilient alongits lengthgradually tapering the terminalend. to This clasp supplies unilateralretentionwith a considerablebracing effect on the non-retentive side, and will be attached to the denture by a thick strut joining together with the thicker extremityof the clasp arm. When this sfruf rs buccally placed,as is frequentty the case with tower dentures fhe c/asp r's designated a REVERSE BACK ACTION CLASP. Whenthe sfruf is palatallyorlinguallyplaceditisdesrpnafedaBACKACflON CLASP. Ring clasps possesssimilar action becausethey encircle the tooth with the terminal point being in the near zone of the undercutarea, this clasp is used on single slandingmolar teeth which have a severe tilt and consequentlya high survey line. This clasp is attached in a more direct fashion to the denture but should always have a buccal or palatal reinforcingarm lying against the mucosato lend extra strengthand rigidityfor the bracingseclion of the claip arm. The Low Survey Line Low survey lines are found on the buccalor lingualaspectsnear to the gingivalmargins,they occur in this positionwhen a tooth is conical in form or when there is a marked ihclinationin one directionor another(a high survey line appearson the oppositesurfaceif inclination a tooth of has occurred). Claspinga tooth with this type of survey line has obvious disadvantagesbecausethere is very little undercut available for a retentive clasp arm to be effective in retaining a denture, also a clasp in this positionis almostcertainlygoingto be near the gingivalmarginwhich could lead to traumaof the mucosaand oral hygieneproblems. Tooth surfaceswith these low survey lines can always bear a bracing arm as with the ring and reverse back action clasps, however if additionalretentionis requiredthree alternativemethods are available obtainit. to The first is the use of the extendedarm claspwhich is similar to the circumferential clasp arm but covers two teeth, it remains above the survey line on the first tooth and then crosses over into the undercutarea of the adjacenttooth. This increases the splintingaction and dislributionof lateralloadingovertwo teeth. Secondlyeven if no suitableundercuts exist buccallyor linguallycertainlyone will be found on the near zone aproximalsurfaceallowingthe use of a DeVan clasp.The DeVan clasp is a bar clasp with a very small retentivehead that lies below this approximalsurvey line arising from very low down near the peripheryof the saddle area. This clasp has reciprocalaction achieved by a lingual or palatal strut which is in contact with the lingual or palatal far proximal surfaces ending in an occlusalrest,the primary occlusalrest being positionedon the near proximal part of the ocrlusal surface,however,little or no bracingeffect is achievedby this clasp. Thirdly an abutment may be crowned so that suitable contours can be incorporatedinto the restoration clasp positioning. for 36
  • Efficiency of occlusally and gingivally approaching clasp arms Retention Gingivally approaching claspsprovidebetterretention due to the phenomena knownas ,trip action', this actionoccursas the retentive of the clasparm wedgesagainst tooth as it is tip the displaced to the flexible due nature the extended of clasparm. Bracing Occlusally approaching clasparms have a rigid portion which lies in contaclwith the tooth surface a non undercut in areawhichprovides greaterbracing a effectagainst horizontal forces thanthegingivally approaching arm. clasp Tolerance Gingivallyapproaching clasps are not toleratedso well as occtusally approaching clasps particularly a largetissueundercut present if is because means it that the flexiblebar clasparm hasto be positioned a largegap between andthe mucosa. with it Aesthetics Gingivally approaching clasps maybe lessconspicuous portion the mouth towards anterior the of particularly a canine to be usedfor clasping. if has Indirect retention Assuming claspunits havebeenusedfor retention, displacing forces occur will around clasps the during function. drawing linethrough teethcarrying claspunitson a horizontal By a the plane the (ng.32),the axisor fulcrum lineaboutwhichthe denture movecan be is possible will thattheremay be morethanone fulcrum linefor the samepartial denture. Indirect retention is achieved placing by components, as occlusal such rests, the partial of denture rightangles at to thefulcrum axis. The efficiency indirect of retention increased moreanterior is the thesecomponents fromthe lie fulcrumaxis,because they resistrotation the saddleareasawayfromthe mucosa of around the directretainers. When components placedcloseto free end saddleareasindirectrelention achievedby are is resisting rotation the saddle of towards mucosa the round directretainers. the Thequality indirect of retention depends the position the fulcrum on, of line,the distance the of indirect retainer fromthefulcrum axisandthe overall surface areacovered thedenture by base. 37
  • t' -_-/ )t , )f I ll Fig. 32. lllustrations shoring the fulcrumaxes. Components which give indirectretentionare Occlusal rests, onlays, cingulum and incisal rests,embrasure hooks, Cummer arms,Kennedy barsandmajorandminorconnectors. Poorlydesigned indirect retainers resting toothsurfaces on may resultin movement the tooth of away from the component. Excessiveloading of a tooth results in trauma followed by inflammation the periodontal of membrane and loosening the toothcausedby a reduction of in bonesupport resulting eventually the lossof thetooth. in 38
  • MajorConnectors Types and Functions "A maior c:annector the unit of a partial denture that connects the partsof the prosthesis/ocated is on one side of the arch with those on the opposite. W.L. McCracken. " The functionof a major connectoris to rigidlyjoin all the other components is that partof the dentureto which all othercomponent partsare joined. A major connectormust be rigid so that any load applied to it, is distributedover the whole of the denture bearing area. Connectorsthat are rigid will resist torque (a tuming force) which may otherwiseact as leverageon abutmentteeth. Connectors shouldbe placedawayfrom movingtissueand not put pressure upon gingivaltissue. Bony or soft tissue prominences,should be avoided, however,this may not always be possible and it will be necessaryto relieve these areas before the denture is constructed.The connector shouldalso be positioned as not to interferewith the tongue. so Maxillary connectors fall into the following categories o Palatalplate r Palatalbar. e Combination Anteriorand Posterior of pattem). bars(skeleton o Palatalstrap. Mandibular connectors o Lingualbar Best constructed a half pear profile in I Lingualplate Covering the lingualalveolarwall form sulcusto the tooth cingulumof anteriorteeth of finishing abovethe surveyline on otherteeth.Somewriterssuggest that lingualplatedesignsare in effect an extension the lingualbar. of o Buccalor labialbar The exception usinga lingualbar or plate is that of a buccalor labialbar where the teeth are to so lingually inclinedas to preventthe use of a lingualconnector. However, thesedesignsshould only be used as a last resort. Minorconnectors Arising fromthe majorconnector, minor joinsthe majorconnector otherparts the connector to of thepartial denture. joins Where minorconnector the claspunitto the majorconneclor saddle, alsohasthe a and its function contacting guiding of the surface so will position clasp and the unitaccurately. Wherea minorconnector required join an indirect is to retainer, an auxiliary to a major ie rest connector, suchas a lingual bar,the minorconnector should followthe interdental embrasure ratherthan coverthe convexsurfaceof the tooth.This is to reduceany wedging effecton the part teeththe deepest of the embrasure mustbe relieved whensurveying cast. the On a cast metallic partialdenlurebasethat part of the framework the saddlethat formsthe of retention the acrylic to resin(saddle baseand teeth)is alsoa minorconnector. Thesemay be described eitheropenlattice as workor mesh,whichis spaced the fromthe master castallowing denture be more easilyrelinedand is ideal for free end saddles to is wheretissueresorption likely be greater to thanthatof bounded saddles. The othertypeis described platewith nail as 39
  • head retentionand is used more often on short boundedsaddleswhich are unlikelyto need relining. Minorconnectors shouldhave sufficient bulk to be rigid withoutinterfering with the placementof artificial teeth. Connectors As previously mentioned, connector the componentof a partialdenturewhich connectsthe a is saddles allthe remaining to parts.To fulfil this functiona connector component must be rigid and capable of transmittingor equalisingstress over the whole supportingtissues (teeih and mucosa). Functions of major connectors $ Rigiotyconnecls saddles, distributing load over as wide an area as poasible. S RigiOty connectsthe retainerspreventing any torque actionwhich could act as leverageon individual abutment teeth. $ Directsocclusal stressto all the abutment teeth. S Stabilizes dentureby providing bracingactionagainstlateralmovement. the a s Indirect retention preventing by tippingin an antenor-posterior plane. Criteria construction connectors for of S Coveras wide an area as possible distribute-load. to S Compromiseof bulk and thicknessto strength of material used. Thin sections must be sufficiently rigidto lransferload and resistfracture(more of a problemwhen constructing acrylic partialdenturebases). S A bar or skeletontype of connectorshould ideally be a minimum of 6mm away from the gingival tissues.This preventsfood from collectingbetweenthe teeth and the denture base whichcausesinitationand possible periodontal problems.By leavingthe gingivaeexposedthere is an unrestricted bloodsupplyand stimulation tissuethat maintains healthymucosa to a $ Platetype connectors, whichcovergingivaltissuemust avoid pressure whichis liableto cause traumaor,damage. relieving gingivalmarginspriorto construction problemof damage By the the will be reduced.In the case of a conectly designedand supportedpartialdenture,where the patienthas moderate oral hygiene, problemshouldoccur. no The greatest damageoccurswhen the gingivalmarginsare not relievedand the partialdenlure is of the tissue bome type. Often these are constructedin acrytic resin, (some incorporate stainless steel lingualbars)and are knownas a "gum strippers". However, this is not the limit to potential damageto oral tissue.Lack of verticalsupportmay also lead to accelerated bone loss (atrophy) and tissueresorption the localised in tissueareas. NB. See handoulfor information regarding various advantages and disadvantages individual of connector types. 40
  • General Procedure designingpartialdentures for Patientswho are about to be fitted with a partial denture should be free from caries, have good oral hygieneand have no uncontrolled periodontal disease.There are three groups ot partialty edentulouspatientswho will have dentures fitted they are either, tooth/bome, or tooth/mucosi bome, or totallymucosaborne. These patients,are sub divided into two further groups ' Patientswho have sufficientteeth to indicatethe intercuspalpositionof occlusion. e Those with so many teeth missingthat the intercuspalpositionis not indicated. Initially surgeontakes impressions, the usuallyin alginate, the upperand lowerjaws to provide of study casts. When a sufficient number of teeth are present a wax record is taken to record the intercuspalposition.Where insufficientteeth are present it may be necessaryto delay the taking of this recorduntil wax occlusalrecord rims can be made. The impressions pouredin ClassI stone,tdmmed,then mountedon an adjustable are articulator. (Recordrims are made at this stage if there are not enoughteeth presentto assume the correct occlusalrelationship). Special impressiontrays can now be made for the secondaryimpressionsto provide the master cast, (the cast on which the metallic denture is made). Survey the study casts in relation to the commonpath of displacement (blackcarbon)and in relation the path of insertion to and removal (red marker). Once these stagesare completethe dentalsurgeonexaminesthe occlusionin the intercuspalposition and decides what modifications,occlusal or otherwiseare ne@ssary before proceeding with the designsequence and takingthe secondary impressions. Design sequence The following designsequence but one of the varioussequences is that are available, has been it chosenas a generalstartingpointfor thosewho are new to designing partialdentures. Plan the support of the denture Decidethe positionand outlineof occlusaland incisalrestsand outlinethe saddles.ln the first instanceyou are decidingwhich of the oral structures,both hard and soft tissues,are capable of supporting dentureagainstthe forcesof mastication.Later,whenthe retention plannedthe the is occlusal rest positions can be checked to ensure they provide indirect retention, that is they should resist rotationof the saddles,away from the mucosa, round the fulcrum axes through the clasp arm tips. Plan the saddle connectors In general bilaterally similar saddles (tooth borne) should be connectedrigidly,and bilaterally dissimilar saddles, which are a combination tooth borne and tooth/mucosa of lhe same arch form, can be connectednon rigidly.Some rigid and flexibleconnectorsare shown, along with various rigid connectorsfor maxillary and mandibulardentureswith bi-lateralfree end saddles(fig. 33). 41
  • Designs of removable panial upp€r dentures ro itlusrrare various types of connecrors and to show the placement of clasps and rests' I' II. III. Rnteiior, middle and posterior rigid saddle connecrors. Iv. v. vI, srress-broken designs- Sectionsof various types of rigid connectorsfor mandibular dentures with bilareral free-end saddles.A, ungual bar' B, goliiouous clasp and -f"?r. .lingual bar. D. L6;i ' plare covering gingival margins. E, plate on teeth onlv' 9' l-abial bar. c, l-abial -and F*r H, Linguat plate and swing-lock :1H* Fig. 33- Variousdesignfor connectors. 42
  • Plan the retention The type and positionof retainersmust be decided. When precisionattachmentsare uSedthey shouldprovidestress-breaking placed at the ends of mucosabome saddles,but may be rigiOit if used for retainingtooth borne dentures. Wherever possible,retention by extra coronal ctasps should be assistedby rigid parts of the denture base lying in undercutsrelative to the common path of displacement. Where the black and red survey lines are at different positions,flexible tips of clasp arms should be in appropriateundercutsin relationto both lines. Where designsincorporate flexiblesaddle connectors,three or even four retainersare necessary to hold the tooth bome partsof the denture in positionwhen the mucosa bome saddles are under load and the connectorsbend. The effectivenessof the indirect retentiondepends chiefly on the direct retentionof the clasps, but also on the distance of the occlusal rests from the axis of rotation. Thus, although the occlusal rest positionsare decided early in the designing procedure,these positionsshould be reviewedlaterwhen indirectretentionis being looked at in the overall assessmentof the design. When the planning is completed the exact design is drawn on the laboratory card and the materialtobe used is recorded iegold,cobaltchromealloys,wroughtnickelchromeor stainless steel, together with the dimensions and types of various components (clasps, rests and connectors). casesin whichthe occlusionis not indicated, designshouldonly be tentative In the untilthe occlusal relationship been verified. has The next stage is canied out in the clinic, occlusal conections,any restorations,guide surface preparation and rest seat preparations must be completedbefore the secondaryimpressionsare taken. These impressions are usuallytaken in alginate,or other elastic materialusing special trays. Next the intercuspal positionis recordedwith a wax wafer, or when this is not indicatedthe use of wax occlusal record rims constructedon study casts prior to the secondary impression stage is recommended. A facebow record is then taken. Next, the shade and any unusual characteristics the standingteeth are noted on the laboratoryworksheet. In cases where the of designhas only beententative,mountthe primarycasts on the articulator beforeconfirming the design. Once the secondary impressionshave been received in the laboratory,they are cast using a Class ll stone. The best methodto use is the two stage pouringtechnique.This is where the anatomical sectionis pouredfirst (withoutinverting)followedby basingthe anatomicalsection with plasterof Paris. Casts producedfrom these impressionsneed to be treated with the utmost care,they shouldnot be rubbedor damagedin any way. When the definitive cast is produced,it is surveyed and any undercutsthat are not requiredfor retention relation the path of insertion in to and removalare blockedout usingwax. Shelvesare waxed in position indicate to wherethe clasp arms shouldlie. Free end saddleareasthat needto be relievedhave wax saddles laid down on them, (this is necessaryfor saddle area relief). Borderseal linesare then scribedon the cast. The mastercast is now duplicated, using reversiblehydrocolloid, the wax pattem is 'waxed and patternis completeit is sprued and investedin a refractorymaterialand cast. up'. Once the wax 43
  • :.: Procedures the productionof acrylic resin/metallic for complete and partialdentures Modificationsto flasking techniques for acrylic/metallicdenture bases The open method is recommendedfor free end distal extensiondentures, but where there are individual bounded saddle areas a method known as the hooded technique can be used. This technique requires the complete coverage of teeth, flanges if present and any metal work to leaveonly the palatalor lingualaspects wax showing. of Once boilingout has been completedall that can be seen is the taggingand the tooth'slingualsurface.This techniquehas the advantage of securingsingle teeth in the conect position,however it does pose other problems,eg it may 6e difficult to pack the acrylic resin through to any labial flange for single ieeth due to eithei the presen@of retentiontaggingor the lack of space. Continuedpacking may fill the mould but may lead to excessive pressurebuild up in the mould, resulting in breakage and movement of the tooth beforethe mould can be completelyclosed. The hoodedtechniquecan be combinedwith the open technique,this has the advantageof allowingfree end saddle areaswith limited space to be open whilst isolatedanteriorteeth can be hooded. This makes the flasking process much simpler because attempting to use the open methodon several isolatedteeth is besetwith all the problemsconcemingfracture of the mould, creatingundercutsand movementof teeth. Siliconeor plastercasts may be used where small saddle areas are involved may be processed usingauto-polymerising resin. Flaskingedentulous cobaltchrome basesemploysthe use of 'capping', which means covering the metal palateslightlyabove the metal acrylic finishingline with the investingplasterto prevent movement of the metal base. Trial packing presents a problem due to acrylic joining with the metal retentiontagging.The Stellon flaskingtechnique,which employs wlx as a separating medium prior to topping, allows a thickness of flash without a considerable increase in the verticaldimension, thus removing the needto trial pack (although this is advisable). This problem also appliesto distal extensionsaddleareas for partialdentures. procedures Other processing includeinjectionmouldingand modernday pour techniquesusing specialistacrylic resins. Types of faults that can occur The faultsthat occurduringthe conversion the wax saddleareasto acrylicresinare not much of different from those that occur when processingall acrylic resin complete or partial dentures, however there are significant differences the flaskingand deflasking in techniques. Flaskingprocedures should leave land areas free from undercuts,the presenceof any undercuts or lack of separating mediumwill make openingof the flasksto boil out difficultand is tikelyto resultin the breaking or fractureof the mould. up When boilingout wax, insufficient scaldingmay leave tracesof wax which resultsin incomplete acrylicsaddlesand chemicalunionwith acrylicteeth. Teeth may become loose after boiling out due to excess wax on their surfaces and may move out of the correct position.Failureto replace them properlywill result in teeth embedded in the saddleareas and acrylic resin replacingthe void left to form a pink tooth that has to be replaced with a new tooth using self cure. Fine edgesof investing plasterat the junctionbetweenacrylicand metal finishinglines are likely to breakoff duringthe packingprocedure, thus, all featheredges must be trimmedto avoid the inclusion plasterin the acrylicresin. of 44
  • I I i I I I "J Heat soaking of flasks prior to boiling out should only soften the wax enough to open the flask withotd damage, usually 5-7 minutes in boilirg water. Loqger periods result in the impregnation of wax inlo the cast, which inhibits the separating medium and alfiowsthe acrylic resin to adhere to the cast surfaces. Once processed the acrylic will be dirty' and need to have a lot of time spent cleaning and trimming to remove the investing plaster. Excessive use of cold mould seal results in areas called 'ptddles'which can be incorporated into the acrylic resin, which have to be removed after processingby'bodging'with cold cure. I I I I J I I i I I I -J I I I J I I -J I I I I I -J i .J I I _i I I 45 -J I I
  • Principlesof the lost wax process Producingmetallic castingsfrom pattems is a techniquewhich has been practicedfor thousands of years, from early Egyptiantimes to the presenttechniques.Through ihe centuriesthere has been certain constant faciors in the equation, the persisient use oia pattern formed from a materialsuch as wax, resin or wood, the use of a refractorymaterialto form the mould, the use of heat to raisethe temperatureof the mould and also heatingthe metal to a molten state so that it could be poured into the mould, replacingthe void left by the removal of the pattem, thus formingthe metalliccasting. The cunent technologydates from 1907,when W. H. Taggart introduceda castingtechniqueand associatedmachineryto an AmericanOrthodonticgroup. Sincethat time, the technology and accompanying machineryhave been specialised to easily so produce metallic partial dentures for prosthetics,gold restorationsfor conservationtechnology and silver cap splintsfor maxillo-facial technology. The castings produced are extremely accurate, so long as care is taken from the impression stage through to the finished article This means foilowing manufacturers instructions and possession a thoroughworkingknowledgeof techniques. of Castingsneed to be as accurate as possible,however, increasesor decreasesin size of + or 5o/o for inlays is acceptable;the average linear change being as litile as +/- 2 microns during construction (the average human hairwidth is 40 microns). During the constructionof wax pattems, dimensionalchanges are common place. This is becausethe technique of wax adaption to casts or dies allows shrinkage and ti"reinjection of stresses.Thermal contractionmay not occur immediatelyduring the cooling of wax from a liquid to a solid state because the material cools in layers before the bulk of material solidifies,this helps to reducethermal contractionbut producesintemal stresseswhich, when released,cause dimensionalchange.Cold carving can inject stressesonce the pattem has been formed. Relief of stress can be caused by flaming wax patterns once adaptationis complete. Allowing wax pattemsto stand over a long period of time on dies or casts will also give rise to stress relief. Wax paftems ' can be produced the direct method, which meansin the mouth. by o The lift off or withdrawal method,as in the construction of inlays,or postsand cores,which are then cast in the laboratory. o An indirect method which means constructinga refractorydie or cast upon which the wax pattem is made, and then investedin the same refractorymaterialand cast. Metallicpartial dentures Castpartial dentures produced are by'waxingup'on a duplicate refractory cast.Thewax pattern is laid downon a refractory cast madefrom the same material with whichthe mouldis to be formedSprues, channels allow moltenmetalto enterthe mouldare addedand then or to 'ring'for support). covered a refractory in investment material (using metalor plastic a The ring is then heatedin a fumace,which bums out the wax and bringsthe mould to the casting temperature. is determined thetypeof metalused. This by Castsand mouldsare constructed from refractory materials whichare virtuallyunaffected by heat over the short period of time ne@ssaryto constructmetallic denture bases. The temperatures usedduringthe castingprocedure can exceed1O00oc. reachthese casting To temperatures investment mouldsare heatedin fumaces. Currneily, most popular the method typeof fumaceis electrically powered therefore particular and attention shouldbe paidto safety. i 46
  • Furnaces Fumaces consistof the followingcomponentparts The muffle Usually madeof fireclay, whichretains heatand actsas an insulator allowing hightemperatures to be sustained controlled and duringinvestment priorto casting. mouldheating HeatingElements Thesecan be eitherelectrical gas coilsor integral jets fashioned the muffle. into TemperatureControl Maintained a thermocouple,ensures conecttimeltemperature cycle. by it the rise Sensor A second thermocouple Thermocouples rely on an electrical cunent rrvhich occurs between tvro different metals when there is a difference in temperature experienced al the joint of these tv'/o metals- Millivolt meters are used to measure this effect. These are usually modifyto displaytemperature, and mountedon the front of furnaces. Flue Necessary exclude to gasesfrom investment dangerous materials duringheating cycles.These flues are connected edraction systemswhich convey the gases away from the main to laboratory. Extraction systems can be turbo chargedie air is suckedin from the base of the fumaceandexpelled loxicgases with through fan assisted a flue. 47
  • MetalMeltingSystems Metal melting systems_are used in conjunction with casting machinesthat throw or propel an alloy, which is molten, into an investmentmould to reproducea castingthat can be used as an oral appliance. Some-ofthe metalmeltingsystemsare integralwith castingmachinesand othersare separate, however,it must be said that these lattersystems are becoming increasingly obsolete. o 0 Flame systems r NaturalGas and Compressed Air e Gas and Oxygen . Oxygen and Acetylene I Electrical alloy melting systems r Induction heating coil . Resistanceheating Flamesystems E ruaturatGas Compressed and Air Rubber or compositepipes supply metal torches with compressedair and gas whicl-ris mixed togetherto form a combustiblemixture producinga flame used to melt dental alloys with a range below1000oC. silverand gold. ie The two gases are fed along two separatetubes to a multijetted nozzle. The blow pipe has two separatecontrolvalves, one for each gas which mixes them togetherin the correct proportions. Control of this mixture is all important,if too little air is added to the gas then a generally reducingflame is obtained,but it will be lacking in heat due to incomplete gas combustion. On the other hand,if too much air is addedto the gas the blast of cold air has the effect of cooling the surfaceof the alloy being melted.A properlyadjustedgas/airflame consistsof a dark blue innercone,a lightbluecentreconeand a purpleblue outersheath. The inner cone is composedmainly of a mixture containing unburntgases and compressed air which is relativelycool and is strongly oxidising. This part of the flame should under no circumstances directedat the alloy duringheating,or when it is in the moltenstate,because be inclusionof gases into the metal contribules porosityas the metal cools. to The light blue centrecone is an area of almost completecombustionand consequenily the is hottestpart of the flame. This part of the flame is knownas the reducing zone, it is this area of the flame that shouldbe in contactwith the alloy duringthe meltingprocedure ensure rapid to heatingand protection from oxidation. The purpleoutersheathconsists the products combustion burninggasesand ambientair, of of ie it is coolerthan the outerzone and oxidising effect. in Flamesusingnaturalgasand compressed reachan approximate air lemperature 19O0oC of with a maximummelting temperature just over half this figure. of Procedure for use at" Connections gas and air pipesmusl be checked,turn on the gas at the gas tap, tum gas on at for torch and ignite.The compressed is then turned on and adjustedat the torch end to obtain the air desiredflame. Once adjusted the reducing zone of the flame is playedon the metalto quicklyreachthe alloys melting point, this preventsboth oxidationand the burningof base metals such as zinc and copper. 48
  • Duringthe heatingprocedure,move the flame over the metal to prevent local overheating,being careful at the same time not to under heat. A dull red appearancewhilst heating inoicatei inconect use of the flame - oxidation.The conect heating,using the reducingrone, sho*s a 1.eJ shiny appearance, followedby the molten state. Once the metal has reachedits molten or liquidousstate the phenomenaknown as spinning occurs, it is at this point that the metal is cast into the investmentmould and the flame removed, the air is tumed off first followedby the gas at the torch and then at the main supply. 6 Gas and Oxygen Flames produced with this system are hotter than those produced with gas/air mixures. The flame temperature approximately is 2200oC,which means it is capableoi melting alloys up to 12000c. The oxygen is suppliedcompressedinside tanks or bottles,which have a control valve to allow the rate of oxygen flow to be adjustedalong the feeder pipes to the torch. The gas and oxygen are mixed togetherto producea flame possessing same configurationas the naturalgas ind the compressedair system. Procedure for use a,n The torch is made up of a single jet nozzle with a single gas control valve. Oxygen flow to the torch is cnntrolledby a valve on top of the oxygen botile. B.efolethe casting procedureis initiated connectionsto the gas and oxygen supplies must be checkedto ensurethat no leaksare possible once the supplyii tumed on ie that connections are tight and not loose. After these routinechecks have been carried out, tum on the gas and ignite. The oxygen is then turned on and a pressureof 10 - 15 psi is set, at this point the gas is adjustedto produce a light bluejet 2.5 cm long accompanied a genflehissingsound. by Meltingthe alloy is best achievedby usingthe area of the flame that is just outsidethe tight btue zone, almostbrushing metal as it is heatedin the crucible. the When the metal melts it becomes sphericalin shapeand has a shiny appearance, is at this point that the metal is cast into the it mould. After castingthe oxygen is tumed off and allowed to clear the pipeline before the gas is turned off. 0 O'" Oxygen and Acetylene This flame system is used on alloyswith a melting range of 13OO cobalt chrome base metal alloys.Temperatures 3500oCare obtainedusingthis mixtureof gasesand has the of overwhelming advantageof being cheaper to use than electricalmelting syltems. The main disadvantage that the properties the alloy can be affectedby the gaieous make up of the is of flame duringthe meltingprocedure. Mufti-jetnozzle blowpipesare used, as singlejet nozzles producea scatteringaffect, so causing turbulenceand the possibility trapping gases in the alloy. Overheatingcan be caused by of holdingthe flame too close to the alloy, best resultsare achievedwhen the flame is held 3-4 inchesawayfrom the alloy beingmelted. When meltingalloysin a crucible the enclosing effect of the crucible walls mean that not only is the heat contained,but so are the mixture of gasses. Both sulphur and carbon are preseni in acetylene.These form carbides which makes the alloy very brittle, with a subsequent loss of flexibility especiallyin clasp arms,. Conosion resistance is also lost if a carbonisingflame is used. Castingproceduresusingthis flame system must always be canied out using colouredprotective goggleseven by onlookersfor two reasons,the first is that whilst the metal i! being heated a red halo effect sunoundsthe crucibleand alloy making it virtuallyimpossible see wnetheror not to 49
  • the metal has melted, secondly the amount of ultra-violet light emitted from the torch is damagingto the cell structureotine eyes in much the same way as is observingthe sun without protectiveglasses. az" E 0. Electrical alloy melting systems a Induction heating This system employs the use of a coil of copper tube through which water passes. The water preventsthe coil itself melting during use. A crucible is ptaLo within the coil, containing the metal to be cast' A high frequenCycunent is passed through the coil inducing a rapidly fluctuatingmagneticfield within in aitoy. The alteination of the cunent is approximatety600 1000 times per second has the effect of inducing heat *itnin in" alloy because the electrons in the atomic lattice are subjected to a magnetic force which is ever changing (polarity being reversed)'Energy created by this altemating magnetic neu causes a breakdownin the electrostatic bondingforce.between-the bondingeiectrons,valent electrons and the nuclei,eventually leading a changein stateof the alloyfrim a solid to to a liquid. Metalswhich have a high electrical resistance cobalt chrome,will melt much quicker eg than a metalwhichhas a low electrical resistance goto,when usingthis system.when meltinggold, eg a carbon crucible must be used, not onty lo-inciease the speecl of the melt, but to reduce oxidationby creatinga carbon atmospherewhich excludes oxygen during the melting procedure and preventsporosity. Procedure for use 4/^ 1' Raisethe heatingcoil and checkthe alignment with the guidingpin and tock into position 2' Switch on the water_flow throughthe coil and allow 3 - 4 minutes before the crucible is positioned insidethe coil. 3' Place the crucible into the coil, crucibles can be pre-heatedto prolong life and prevent cracking, however,this may have a detrimental effect on the coppercoil due lo removal of the protectivelacquer. 4. charge the crucible with alloy ensuring each ingot is in contactwith another. 5' Switchthe machineto pre-heat 30 secondsand for then to full heat.wait for the ingotsto gtow r red and tum switch back to the pre_heat position. 6' .Liftlid and positionthe investmentmould from the fumace into the cradle,close the lid and switchback on. 7. Mew alloymeltingthrougha coloured visor. 8' when the alloy is moltem,removethe coil and activatethe motor driven centrifuge(in most . machines this is done automatically). f Resistance heating Metals to be melted are containedin a graphite crucible which is surroundedby a muffle containing heatingelementsas the temperatureis raised.A carbon monoxide/carbon dloxide atmosphere produced is which prevents oxidation the alloy. of 50
  • CastingForces . Centrifugalforce. o Steam pressure. o Combinations vacuum, gravity and compressedair pressures. of Castingdental alloys has, over the last thirty years, led to the developmentof a variety of casting machinesusing differenttypes of force to cast the metal into the refractorymould. Beforethe advent of specialisedalloys such as cobalt chromium,gold and silver alloys were cast using either centrifugalforce or steam pressure in a very crude but effective way. The basic techniqueused to cast gold alloys for example requireda hand-swinger which was a metal ring attachedby a chain to a handle.The technicianwould melt the alloy in the top of the casting rin! using a gas and air blow torch (the air being supplied by bellows which were pumped OVtne technicianwith a spare foot). Once the alloy had reachedthe molten state the whole assemblywas then s:wungin a circle to producethe castingforce necessaryto fill the mould, happilywe have today no.need to depend upon such methodsno matter how successfulthey were. Centrifugal force Centrifugalforce is stilt the most common type of force used in precisiondental casting. One of the many advantagesis that centrifugal offers a constantforce for a given mass of alloy and the force is applied directly to the molten alloy and not dissipatedaway from the alloy during the castingprocedure. The centrifugalforce is directly proportional the mass of the alloy and the length of the casting to arm, with the initial accelerationof the arm being the most important factor determining the castingforce. The arm of the casting machine can be rotated by a spring mechanism or driven by an electricalmotor, which provides rapid accelerationof the crucibie,tne molten alloy and the refractorymould. lnertia provided by this rapid movement throws the molten alloy from the crucible into the preheated refractorymould,filling the space left by the burnt out wax pattern as quickly as possible (approximately one twentiethof a second). Castingmachinearms are eitherhorizontal verticaland are containedin drums or cabinetsto or increasethe safety factor. The castingarm usually consistsof one or two arms rolating around a centralpivot,at one end of the arm a crucibleis mountedin which the alloy is meltedalongwith a plate or cradle to align the casting.At the other end the arm acts as a balance with adjustable weightson a thread, so allowingeven distributionof weight and castingforces. Procedure for use 1. Placethe cold refractory mould,the crucibleand the alloy in positionon the castingmachine arm and adjustthe balanceby tuming the weightsalong the castingarm until the arm swings aroundfreely,this can be done manuallyin most cases. 2- Once the mould and crucible have been heated to the requiredtemperature,the casting machine and casting arm are prepared.This is done by either switching on electrical casting machines tensioning springin the casefor manualcastingarms. or the 3. Cruciblesare then chargedwith the alloy which is to be melted and the refractorymould positioned line with the crucible. in 4. Meltthe alloy.Once the alloy is molten,the heat sourceis removedand the arm released. The centrifugalforce is applied, thus throwing the alloy from the crucible dowri the sprue holes to complete castingas it moves aroundin a is assumedthat castingis.complete the the in first quartertum of the centrifugalarm. 51
  • Steampressure Steam pressureis a castingforce employedwhen using Solbrig casting machinesto cast gold or is an old fashioned technique, neverthe lessvery effectivein producinglounO but castings. The steam pressure is provided by a moistened pad mounted on a casting arm, which is positionedover a refractorymould containingthe molten alloy (melted by a gaslair torch). Once the pad comes into contact with heat radiating from the metal and mould, steam pressure is generatedforcing the alloy down the sprue holes to completethe casting. When using this systemthe sprue holes need to be both thin and long with a shallowsprue former to allow the alloyto be meltedthoroughly avoid any pre-casting the prosthesis to of and chillingof the alloy. combinations of vacuum, gravity and compressed air pressures These are usually small electricalunits which can be mounted on a work bench without fixings because they have no rapidlymovingparts.All components housedin a self contained are unit. Minimal amounts of alloy can be used because the need for a button after casting is unnecessary due to the constantapplicationof pressureduring and after the casting procedure. This ensures that virtuallyno shrinkage the castingoccurs. of The casting procedure is canied out in a carbon crucible which produces a CO and CO. protective gas mixture which effectively excludes oxidation of the alloy. Metal melting temperaturesare pre-setto avoid any overheatingof precious alloys. The pre-heatedmould is placedin the castingchamberof the machinewhich also containsthe crucibleand alloy. The chamberis then sealedand evacuated removingall air. Duringthis processthe alloy is meltedto the corect castingtemperature. once this is reached, the castingchamberis tumed 180" by meansof a handle,aliowing the alloy to run into the mould underthe influenceof gravity,accompanied the introduction compiessedair to complete by of the casting. 52
  • . i.l Finishing proceduresfor metallic partial denture bases Cobalt chromium After casting and removal from the investment mould the casting will have to be cleaned to remove all traces of the refractorymaterialand the oxide layer,which forms on the surface of the completed casting. This is easilyrecognisable a dark greencoveringall over the surface. as Oxide is removedby passing silicaorquartz beadsoverthe surfacethrougha fine nozzleat 100 psi,a technique knownas sandblasting. Sandblastingunits are containedin large cabinets with a compressedair supply to provide the force of up to 100 psi and extractionfacilities to ensure that none of the very fine grains used during blasting are allowed to escape into the atmosphere.Such liberation of fine grains is a considerable health hazard. After cleaningthe sprues may now be removed using a Carborundumtype cut off disc, rotating at a minimum of 10,000rpm. Discs should always be used in conjunctionwith a metal centre to prevent break up during use. Other safety precautionsincludethe use of a protectiveglass visor on high speed grinders,wearinggoggles of the high impact type, a dust respiratordesigned to remove fine dust particlesand dust extraction at source ie integral bench extraction. Note that Carborundum dust is a carcinogen. It may be necessaryto place the low speed revolving disc, mounted on a mandrill, against an abrasivematerial,this has the effect of forming a uniformcirculardisc which runs true. Water will be requiredto cool down the casting whilst trimming. A rapid build up of heat is generated by the grinding operation which removes sprues and other imperfections. Large imperfectionsmay be removedwith a cut off disc. Small casting imperfections, such as pimples found on the fitting surface and general trimming to shape, may be canied out using mounted silicon carbide stones. Particularattention should be paid to the manufacturersrecommended maximumgrinding speed,in compliance with the abrasive wheel regulations. After grinding and shaping the fit surface of the casting is electrolyticallybrightened (this is sometimesrefened to as electrolyticpolishing.The principlesof electrolysisare used to remove a fine layer of metal so giving the appearanceof a polishedsurface. Once electrolytic polishing is complete, the non fitting surface is smoothed by using rubber bondedabrasivewheelsand banels, startingwith the coarsestand finishingwith the finest grade. Emery cloth used in a sandpaper mandrillor in cylinderformat can also be used to achieve a very smoothsurfacepriorto polishing. Beforepolishing castingis embedded a base of plasterof Paristo preventdistortion any the in of thin sections and injuryto the technician (eitherbumingdue to heat build up or the penetration of claspstothe fleshof fingersorhandsif the castingcannotbe held firmly).The polishing motoris used at high speed with a metal centred black bristle brush dressedwith the polishingcompound 'Rosa'.Once a dull polishedsurface is achieved,'Grun' (chromiumoxide) is used. The final polishbeing achievedby using a polishingcompound,such as whiting (mixed with water or in block form) in conjunction with a soft wool mop. Under no circumstances should'rouge'(ironoxide) be used to polishcobalt chromiumcastings, as it contaminates alloy by combining the with the carbonmolecules the grain boundaries at thus weakeningthe structureand reducingthe corrosionresistance(it goes rusty!). Polishing of gold Followingthe removal of sprues and casting imperfectionsthe casting is shaping using various stonesand diamonds. The next stage is to smooththe surfacewith rubber bondedabrasivewheels'whichproducea dull sheen.The final gloss is achievedusing a series of brushesand mops (calico and/or lambswool) with rouge (iron oxide either in fine powderor block form). 53
  • ' * Polishing stainless steel The surface of the alloy is prepared in a similar manner as for cobalt chrome alloys. once the rubber bonded abrasive wheels have produced a smooth surface, chrome oxide polishing oompound on metal centred bristle brushes, followed by calico or lambswool mops, will produce gloss finish. Rouge should not be used as a polishingagent for the same reason as given 9 nig! for cobalt chrome alloys. -- I I I I I I ti 54
  • BIBLIOGRAPHY All the books listed beloware worth viewing,however,be ararare that some are now a bit dated and include procedure more suitablefor cast gold and/orwroughtalloys.Those markedwith a * are particularly useful as they are currentpublications. (1966)Essenfia/s Removable Applegate of PartialDenture'Proshfes,s. Mosby Bates(1978)PaftialDentureConstruction Wright *Bates,Hugget& Safford(1991)PartialDentureConstruc'tion Wright *Davenport, Basker, Heath Ralph(1988) C,olour & A Atlas Removabte of PartiatDentures. Wolfe Dykema, Cunningham Johnson & (1969) Modern Practice Removable in Partiat Pro sthodo ntics.Saunders Fanell(1971 PartialDenture Designing. Kimpton ) Fenn,Liddelow Gimson (1961) & Clinicat DentalProsthetics. Staptes Grant, (1983) Introductionto Johnson An Removabte PartialDenture Prosthetics. Churchill Livingstone Henderson Steffel(1977)McKracken's & PartialDentureProsthodonfibs. Mosby (1 Kennedy 951)PartialDentureConstructionKimpton Lammie Laird(1986) & Osbourne Lammie's & PaftialDentures. Blackwell Neill& Walters (1983) Partial Dentures. Blackwell Rudd,Morrow Rhoads (1986)DentalLaboratory & Procedures Volume Removable 3 Paftial Dentures. Mosby Singer Schon(1973) & Partial Dentures. Quintessence *Stratton Wierect (1988) Atlasof Removabte & An Partiat DentureDesign. Quintessence *Watt& McGregor (1984) Designing PartialDentures. Wright 55
  • Anterior and posterior palatal bar comhnation is the most rigid of palatal major connectors and, when properl,"-designed, is neither ob- i I i'ctionable to patient nor harmful to adjacent tis- Anterior and posterior palatalbrrr-t1pe major connectors. Anterior bar is a ffat sbrap located as lar posteriorly a-spossible to avoid rugae cor-erage and tongue interierence. Anierior border of this bar should be loc.itedjust posterior to a rugae crest or in the vallel betw'eenhvo crests. Posterior bar is half-oval (approrimatel;-6 gauge) and is located as far possible, yet entirely on hard palate. lt should be located at right angles to midline rather tlran on a diagonal. .nterior-posterior b.rr-hpe major connector for a Class IV parti:ijlr edentulous arch. Positir:esupport for anterior portion of rnajorconby nector is fi-rrnished a "splint b.rr" joining the lbur crowned premolars. Notict that lateral portions of major connector are pl.rcedat least 6 mm Fom lingu,rl gingivalcrevices. This single palatal sbap-gpe major con_ nector ma;- be used fior tooth-borne restorations of bilateral edentulorx areas of short span. It may also be used in a tooth-borne unilateral edenfulous situation *itb provision for cross-arch attachment by either estracomnal retainers or internal attachments. !'ilth of palatal strap should be confined within the borudaries of supporting rests. Palatal major connector cor-ering two thirds of palate. Anterior border follor+s valleys bet*'een rugae anci does not extend :rnterior to indirect retainers on first premolars. Posterior border is located at junction of handand soft palates but does not extend onto soft palate. In a bilateral distal extension situation as illustrated, indirect retainers are a must to aid in resisting horizonta.l rotation of the restoration. . Full palatal major connector. Posterior border terminates at junction of hard and soft palates. Anterior portion is in the form o[a palatal linguoplate irnd is supported bl positire lingual rest seatson canines.Locationo[finishing lines is most important in this $pe of major connector' Unless the-"-relatively follow the line of the arch and are located just lingual to an imaginary line contacting the lingual surfacesof the lost natural teeth, alteration ofnatural palatal contour should be anticipated with its atten&nt detrimental e[fect on speech.
  • LI. - , L ,'L D.t I INDIctTro!,is FoR uSE: The lingual bar should be used for mandibular removable partial dentures wherein sufEcient spac'e exists between the slightiy elevated alveolar lingual sulc"-'-and the lingual glnglval tissues to place a rigid bar. Sagitt;rj sectioo shou'ing half-pear shape of a hnsual"bar. The inferior bortier of the bar *.. .o,rnd"d after the tsamework was cast to avoid impingement of lingual tissueswhen the denture .otrt"i'eten slightly in frrnction. A taper of the". b.,rder"of the bar to soft tissues above will minimize interference to the tongue and will be more acceptable to tbe patient than would a dissimilar contour. Lingual bar and continuous bar retainer major connector. Upper portion of this majorconnector is lrrcatedon cingula of anterior teeth and is ertended superiorlv to interproximal c'ontact pornts. Rerlrrirementof positive support bv rest se.itj. Jt le.rst;rs lar .rnteriorll as the clnines. is cntrcal. ote that superior border of bar portion nrust be placed objectionablv close to gingrral m.irqins if sufficient rigidity is to be obtained. Ths hpe of major connector is definitely a food trap and is often much more objectionable to patient than a linguoplate from the standpoint of anno)rnce. Linguoplate is useC *'hen ;pace betr*'een bounding connectors is better filled in th:rn Ieft open. Such an apron does not s€lae to rgpla9e but, instead, is added to fundatf,*" "onn""tors mental denture design to aake m:1br connector risid. Sagitt.iJsecrxln th:;r,gh a lingrroplate clemonstrating basic half-per-shaped inferior border with a metirllic aXrrotr errending superiorll . -h;:dged" ote that gingival crerrtf, is b1 linguo-' plate to avoid impingeoent of gngival tissue. The linguoplate receives pcsitive *rpport Fom a linguai rest (in a properF.. prep;-ed rest s€ai) oD the canine and a mesiocdusal rast on the Srst pre. molar. Extension of tie lingucglate to height of contour on premolar '*as actr*plished to enclose a rather large triangr-ria-:interprorimal space in'ttts<-:. lerior to contrct point c:rnine anri premolar. Such spacesma] often i< bi'idged to eliminate obvious food trap: rnd to provide a more bvgienic oral environrnent Diastemx are n,-,1 bnd5:-i br the major connector. Frarne*ork rn::st bt supported bv rests on caninespl.rcedin p:operl. prepared rest seats. Rigidity of major crrrrnect.ri LSnot nec€ssarilv c.ompromised,but surlt a ct-rtour definitel;' lends itself to food being retaine3 around teeth and under apron ertensior..r
  • r- tvr ,t4Y' ,,,1(t,5 '-F'.',i (-_ >' #r-=-# $/r 3--r Half-round palatal bar must be bulkv to obtain rigidity required for cross-arch distribuion of stress. Its bulk may' make it objectionable to p:rtient's tongue. U-s-haped palatal connector is probably the- least ngd t]?e of ma'dllari. ma.lorconnector anC should be used onll' when'a lurge inooerable palatal toms prer-ents use of palatal;r"o!" ;; ; com bi nahon an terior-pos terior palatal r tr"Ip-ryp" oesrgne€l tramef,orlL e ' the " A, tar'illary major connector in form of a palatal linguoplate with prorision tor attaching a full-toverage acq'lic resin denture base. Bl Completed remot'able partial denture with acrylic t.iin brt". Palatd linguop'late is supported bv rests occupving lingual rest seats preoared in'eold restoraiionson carrines'This t'pe partial denture is particularlv appliif ."*otJl" cable where ili residual ridges have undergone extreme vertical resorption and (2) terminal |butments have sufferd some bone loss and splinting ior cannot be accomplished some reiison' ,1 {r i;