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Lab steps in rpd
- 1. LAB PROCEDURES IN RPD Presented by Dr Vijyanta Suman Post graduate student 1
- 2. CONTENTS 2 • Preparation of master cast • Duplication of master cast • Wax pattern & Spruing • Investing & burn-out • Casting of framework • Finishing & Polishing • Making Record Bases • Making a Stone Occlusal Template • Arrangement • Processing • Review of literature • Conclusion • References 2
- 3. INTRODUCTION 3 • Metal partial denture framework fabrication, as an important constituent part of a future partial denture, holds a very important place in the fabrication process. • It is necessary to point out that metals being considerably more resistant to fractures than acrylic resins, enable fabrication of partial dentures that contribute to a better and faster adjustment of the patient to new conditions in the oral cavity. 3
- 4. Laboratory steps 4 • Preparation of master cast • Duplication • Wax pattern • Spruing, Investing & burn-out • Casting • Finishing & Polishing • Making Record Bases • Occlusion Rims • Making a Stone Occlusal Template from a Functional Occlusal Record 4
- 5. • Arranging Posterior Teeth to an Opposing Cast or Template Posterior Tooth Forms Arranging Teeth to an Occluding Surface • Waxing and Investing the Removable Partial Denture • Processing the Denture • Remounting and Occlusal Correction to an Occlusal Template • Polishing the Denture McCracken’s Removable partial prosthodontics.13th edition. 2015 5
- 6. PREPARATION OF MASTER CAST 6 • Design transfer from diagnostic cast • Beading of maxillary major connector • Spraying • Block out • Relief 6
- 7. Impressions with work authorization forms (1) The signature and license number of the dentist (2) The date the authorization was signed (3) The name and address of the patient (4) A description of the service or material ordered 7 Stewart’s clinical removable prosthodontics.4th edition.p314 7
- 8. Retripoding the master casts 8 Kenneth D Rudd, Robert M morrowDental laboratory procedures- Removable partial denture vol 3; 2nd edition. 8
- 9. Kenneth D Rudd, Robert M morrowDental laboratory procedures- Removable partial denture vol 3; 2nd edition. MARKING THE HEIGHT OF CONTOUR 9 9
- 10. Design transfer from diagnostic cast 9 Stewart’s clinical removable prosthodontics.4th edition.p314 10
- 11. Beading of the maxillary major connector • Scraping the outline of the major connector into the master cast. • Adepth of half the diameter of a no.2 round bur. • Positive contact against palatal tissues. • Reduce the packing of food particles underneath major connector. • 3-4mm away from marginal gingiva. 10 11
- 12. SPRAYING • Treating the master cast with a surface sealer (acetone, diethyl phthalate, and cellulose acetate) using a brush or an aerosol spray to seal the cast and protect the design during the other steps of cast preparation and duplication. • Sealer application should be done in front of a suction vent because inhaling aerosol is potentially dangerous 12
- 13. Block out • Elimination of undesirable undercuts on the master cast with wax is commonly referred to as block out. • Both hard and soft tissue areas will require block out. 13
- 14. 14 Blockout wax can be shaped with hand instruments to provide a slight ledge just apical to the clasp terminus This ledge guides the placement of the wax or plastic pattern and ensures that the clasp tip is accurately positioned in the desired undercut the body and shoulder areas commonly require blockout Types of block out: • Parallel block out • Shaped block out • Arbitrary block out
- 15. Parallel block out • Proximal tooth surfaces used as guiding planes. • Beneath all minor connectors. • Tissue undercuts to be crossed by rigid connectors. • Tissue undercuts to be crossed by origin of bar clasps. • Deep interproximal spaces to be covered by minor connectors or linguoplates. • Beneath bar clasp arms to gingival crevice. 15
- 16. Shaped block out • On buccal and lingual surfaces to locate plastic or wax patterns for clasp arms 16
- 17. Arbitrary block out • All gingival crevices, deep palatal clefts • Gross tissue undercuts • Tissue undercuts distal to cast framework. • Labial and buccal tooth and tissue undercuts not involved in denture design • Soft wax, clay or mortite can be used 17
- 18. Relief • Beneath lingual bar connectors or the bar portion of the linguoplates when indicated • Areas in which major connectors will contact thin tissue, such as areas frequently found on lingual or mandibular ridges and elevated palatal Raphes • Beneath framework extensions onto ridge areas for attachment of resin bases • Margin of the relief wax forms the internal finish line of the framework. • The finish line should be sharply defined • finish line should be placed 1.5 mm from the neighbouring abutment 18
- 19. Tissue stop • Asmall square of 2x2 mm of relief wax is removed at the distal end of the saddle. • It provides stability of the framework during acrylic resin processing. 19
- 20. • Some removable partial denture alloys are sprued with an overjet sprue former and reservoir • Small tapered cylinder (wax, plastic or metal) on the master cast in the exact position that main sprue will occupy in the refractory cast 20
- 21. Duplication of the master cast Duplication with irreversible hydrocolloid Ticonium flask – simple design 21
- 22. Dental laboratory procedures vol 3;Rudd KD 2nd edition Duplicating procedure 22
- 23. Stewart’s clinical removable partial prosthodontics. Rodney D. phoenix.4th edition Reversible Hydrocolloid/Agar : complex polysacharide extracted from sea weed Primarily consists of 13-17% colloidal suspension of agar in water Borates- 0.2-0.5% Potassium sulphate- 1-2% Wax- 0.5 - 1% Thixotrophic material – 0.3- 0.5% Water- around 84% 23
- 24. 24 • The master cast must he thoroughly soaked in clear slurry water (minimum 30 minutes) • At least 1/4 inch (0.63 cm) clearance in all directions • Center the cast on the base & secure it with three small pieces of modeling clay • Fill Outer rim of flask base and seat body then place reservoir ring • Modelling clay balls adjacent to vent holes Kenneth D Rudd, Robert M morrow.Dental laboratory procedures- Removable partial denture vol 3; 2nd edition.
- 25. 25 • Place chopped agar on top and water at bottom then place on fire, boil, 1 hr 30 min for liquefaction of agar, allow cooling for 1 hr (until 57-600c) then pour on flask • Place on tray under cool running tap water (min 45 minutes) Kenneth D Rudd, Robert M morrowDental laboratory procedures- Removable partial denture.vol 3; 2nd edition.
- 26. McCracken’s Removable partial prosthodontics.13th edition. 2015 26
- 27. Robert G. Craig & John M. Powers : Restorative dental materials (10th edition) Gypsum bonded • mixed with water • composed of a mixture of silica (SiO2) and calcium sulphate hemihydrates (gypsum product) • Three types of gypsum bonded investments can be identified as follows: -Type 1 thermal expansion type; for casting inlays and crowns. -Type 2 hygroscopic expansion type; for casting inlays and crowns. -Type 3 casting complete and partial dentures. 27
- 28. Robert G. Craig & John M. Powers : Restorative dental materials (10th edition) Phosphate bonded • powder contains silica, magnesium oxide and ammonium phosphate • Can be mixed with water or colloidal silica • Two types of phosphate-bonded investment can be identified as follows: -Type 1- for inlays, crowns and other fixed restorations. -Type 2- for partial dentures and other cast removable restorations 28
- 29. Robert G. Craig & John M. Powers : Restorative dental materials (10th edition) Silica bonded • consist of powdered quartz or cristobalite which is bonded together with silica gel or ethyl silicate. • On heating, the silica gel turns into silica resulting in a completed mould is a tightly packed mass of silica particles. • Use for high fusing alloys. 29
- 30. • In rpd fabrication, a cast made from refractory material serves as the foundation for waxing and casting procedures • Refractory material is introduced in the colloid mold • Mold then placed in humidor, after it sets, cut colloid & retrieve • Drying oven-93°C for 30 to 60 minutes • trim to within 6 mm of the proposed design 30 Stewart’s clinical removable prosthodontics.4th edition.p336
- 31. the cast is positioned on its end to allow excess wax to run off Refractory cast is dipped into beeswax at 138°C to 149°C for 15 seconds 31 Kenneth D Rudd, Robert M morrowDental laboratory procedures- Removable partial denture vol 3; 2nd edition.
- 32. • Before the waxing begins, design must once again transferred from master cast to refractory cast • Master cast is evaluated and the outline of framework design is transferred precisely to refractory cast. Design transfer 32 Stewart’s clinical removable prosthodontics.4th edition.p337
- 33. Waxing technique • Pre-made wax patterns are used for waxing the framework. • Tacky liquid (mixture of acetone and plastic pattern scraps) is painted on the design outline with a fine brush and allowed to dry for a few seconds. • Patterns are carefully adapted to the cast • A pattern adapter or a soft rubber pencil eraser shaped into a wedge is useful for adaptation • Exercise extreme care not to stretch the pattern 33 McCracken’s Removable partial prosthodontics.13th edition. 2015
- 34. • Once the patterns are in place on the refractory cast, they must be joined together with wax • The areas where the resin retentive elements joins the major connector must be reinforced • For final contouring, carvers with miniature rounded blades are most apt to be used 34
- 35. Spruing • Spruing is attaching wax or plastic form to the wax pattern, to provide an entrance to the mold space and to serve as a reservoir of metal during casting procedure Rules: • The sprues should be large enough: 8- to 12-gauge round wax is usually used for multiple spruing • The sprues should lead into the mold cavity as directly as possible • Sprues should be attached to the wax pattern at its bulkier sections. 35 McCracken’s Removable partial prosthodontics.13th edition. 2015
- 36. Types of spruing • There are two basic types of sprues, multiple & single. • Asingle sprue is preferred for complete denture metal bases. • The majority of partial denture castings require multiple spruing. 36 McCracken’s Removable partial prosthodontics.13th edition. 2015
- 37. Multiple spruing • Top spruing: Acone-shaped plastic sprue is attached to the central sprue from which auxiliary sprues run to each corner of the wax pattern • 8- to 12-gauge round wax shapes used for main sprues and 12- to 18-gauge round wax shapes for auxiliary sprues • Use a few sprues of large diameter rather than several smaller sprues • Keep all sprues as short and direct as possible 37 McCracken’s Removable partial prosthodontics.13th edition. 2015
- 38. Inverted spruing • In which the base of the refractory cast should have a 7 mm diameter hole in its center. • Acone-shaped plastic sprue is placed into the hole which extends about 10mm on the pattern side. • 6 guage half round leads are used for spruing • Main and auxiliary sprues should be attached 7 mm below the central sprue. • All the sprues have to be attached at the same level to the central sprue to have equal distribution. • All sprues should be short and direct. • All junctions should be reinforced and rounded. • Auxiliary sprues are attached between the main sprue and the thick sections of the wax pattern. 38
- 39. ERRORS IN SPRUING 39 Kenneth D Rudd, Robert M morrowDental laboratory procedures- Removable partial denture. vol 3; 2nd edition.
- 40. Grp A:Ribbon-sprue design Kennedy Class III Mod I Grp D: Round with reservoir- sprue design Grp B: Square-sprue design Grp C: Round-sprue design Evaluation of the fit on master die Viswambaran M, Agarwal S. The effect of four sprue shapes on the quality of cobalt-chromium cast removable partial denture frame-works. Contemporary Clinical Dentistry. 2013;4(2):132. Occlusal Radiograph showing internal defect 40
- 41. PURPOSES OF THE INVESTMENT • Provides the strength necessary to hold the forces exerted by the entering stream of molten metal • provides a smooth surface for the mold cavity • Provides an avenue of escape for most of the gases entrapped • Provides necessary compensation for the dimensional changes of the alloy from the molten to the solid 41 McCracken’s Removable partial prosthodontics.13th edition. 2015
- 42. Two part mold: • The investment for a removable partial denture casting consists of two parts: the investment cast on which the pattern is formed, and the outer investment surrounding the cast and pattern • Refractory cast is soaked in a slurry water. • Investment material is measured and mixed. • A paint on layer about 3-4mm is painted on waxed refractory cast and ensure that no voids are present. • As the layer reaches its initial set, the second part of investing procedure begins. • The cast is placed in an investing ring, having a moistened ring liner 3 mm short of the crucible end of the ring. • This ring liner permits the escape of gases and mold expansion. 42
- 43. Burn out • The time and temperature required to eliminate wax from the mold cavity is specific to the refractory-alloy system that is being used. • Each system is developed to provide mold expansion that closely matches the anticipated shrinkage of the alloy as it solidifies.. • Purposes: • It drives off moisture in the mold. • It vaporizes & eliminates the wax pattern & sprues leaving a cavity in the mold. • It expands the mold to compensate for contraction of the metal on cooling. 43
- 44. Burn out furnaces • Can be either electric or gas and must be vented to allow the escape of noxious fumes. • Industrial gas type furnaces can hold 25 casting molds & small electric furnaces with a capacity of only 1 or two molds. • Modern furnaces are controlled electronically to permit time/temperature relationship to be set exactly to the alloy manufacturer’s specifications. semiautomatic fully programmable controls manual 44
- 45. Steps of burn out • For the investment to heat uniformly, it should be moist at the start of the burnout cycle. Steam will then carry the heat into the investment during the early stages of the burnout • Also because of the water trapped in the pores of the investment it reduces the absorption of wax • The mold should be placed in the oven with the sprue hole down • Burnout should be started with a cold oven, or nearly so • The temperature of the oven should be increased slowly and should be maintained for the period to a temperature recommended by the manufacturer. 45 McCracken’s Removable partial prosthodontics.13th edition. 2015
- 46. Casting • Molten metal is forced into the pre-heated mold • Centrifugal force or air pressure is used • Metal is melted with a gas-oxygen blow torch or by an electric muffle surrounding the metal • Induction casting is the method of choice for base metal alloys • Titanium is melted using an argon arc melting and casted in a vacuum pressure chamber 46
- 47. 47 • The placement of the alloy ingot into an uncontaminated crucible begins the casting sequence • Mold is removed from the furnace and placed in the casting arm’s holding mechanism • Modern induction casting machines are normally programmed to cast when the alloy has reached the desired temperature
- 48. Recovery of the casting • The mold is removed from the machine and allowed to cool • At the appropriate time the mold is broken by tapping it with a wooden mallet to break off the outer layer of investment. • The remaining investment is then removed by sand blasting using aluminium oxide particles. • If gold casting alloys are used then pickling is done to remove the base metal deposits, oxidation and other contaminants 48
- 49. Errors during burnout and casting 49
- 50. Finishing & Polishing Following several rules for finishing the casting are important: • High speeds are preferable to low speeds • The wheels or points and the speed of their rotation should do the cutting • A definite sequence for finishing should be adopted and followed for every framework • Clean polishing wheels should be used • Each finishing operation should completely removes all scratches left by the preceding one 50 McCracken’s Removable partial prosthodontics.13th edition. 2015
- 51. Rubber wheels, rubber points, Rubber wheels, knife edge wheels Finishing the framework: Armamentarium Point is formed on truing stone or heatless stone 51
- 52. Sprue stumps removed with heatless stone or separating disk Inaccessible areas-carbide bur or small mounted stone Major connector finished with abrasive stones Separating disk to cut sprues 52
- 53. Inverted cone stone used with sharp edge in undercut Flat surface against finish line Remove nodules from latticework Felt cylinder used with Tripoli on surfaces that contact teeth Felt wheel to polish tissue side and tongue side of major connector 53 53
- 54. Knife edge wheels for finish lines Rag wheel with Tripoli to polish rest of framework Scrub casting to remove tripoli. Chloroform on cotton can be used to remove last traces of tripoli 54
- 55. ELECTROPOLISHING • Electropolishing, a form of electrolytic stripping, is usually the first step in polishing the framework • In this process, atoms of metal from rough projections on the framework go into solution before those in smooth areas do • This results in a very consistent, satin-like surface • The polishing occurs in a bath of 85% orthophosphoric acid, which is heated to 49° to 60°C • The anode is attached to the cast metal framework and cast is immersed in the solution. • Each square inch of metal surface area requires 2 amperes of current for 6 minutes. 55 The Ti-Lectro Polisher
- 56. Fitting the framework • The technician carefully seats the framework on a duplicate master cast and attempts to identify areas that prevent seating • Special powdered sprays and liquid disclosing media are commercially available to aid in this process • The seating and spot grinding continues until the rests completely contact the cast. • Technician must carefully relieve the cast in the area of the clasp tip undercuts to allow the clasps to pass the height of contour . 56 56
- 57. Rubber wheeling and final polish • Once the casting seats completely on the master cast without rocking, proceed with rubber wheeling and polishing. • Each alloy manufacturer sells a specific variety of polishing compound that best suits alloy. • Ultrasonic cleaning is commonly used to remove all traces of polishing materials. 57
- 58. 58
- 59. Sectioning and resoldering the framework • Sectioned segments must be satisfactorily related in the mouth and then transferred with either a plaster index or a resin matrix to the laboratory • Technician carefully pours a soldering cast against the properly related segments • Foil is placed between the cast and the components to be soldered • Electrosoldering tips are placed on each side of the joint to heat the solder 59
- 60. • Fluxes containing fluoride must be used in this procedure. • The electrosoldering machine is basically a step-up transformer with two terminals, one copper and one carbon • carbon tip is placed on the solder and the copper tip on the framework • Laser welding represents a fourth method for joining a wire to the framework (Nd:YAG) • Twin flex or any wrought wire clasp is soldered to framework 60
- 61. Adjusting occlusion • Preliminary adjustments to the occlusion of a removable partial denture framework should be accomplished in the laboratory • The clinician should provide an opposing cast and suitable interocclusal records • Occlusal and incisal rests are sometimes made excessively thick to ensure complete casting • Once cast, these areas are adjusted to return to the proper occlusal vertical dimension • Once the framework has been adjusted to the minimum required dimensions, the interfering part of the opposing cast should be relieved and clearly marked with a red pencil • In this way, a potential problem area is called to the attention of the clinician. • This approach allows the clinician the option of performing the final framework adjustment personally, reducing the opposing tooth, or both. 61
- 62. MAKING RECORD BASES • Well fitting, maximum contact • Block out of undercuts • Relief • Separating media • Wet with monomer • polymer 62
- 63. OCCLUSION RIMS • Impression plaster or bite registration paste used in conjunction with wax rims to record static occlusal relations 63
- 64. 64 WAXING AND INVESTING THE REMOVABLE PARTIAL DEN • The only difference is the waxing of and around exposed parts of the metal framework • waxing is merely butted to the finishing line with a little excess to allow for finishing • When waxing to polished metal parts that do not possess a finishing line is done, wax should be left 1.5 to 2.0 mm thick so that the acrylic-resin will have some bulk at its junction with the polished metal
- 65. 65
- 66. 66 Bolouri A. Hilger T. C. & Gowrylok M. D. Modified flasking technique for removable partial dentures. The Journal of Prosthetic Dentistry. 1975;34(2):221–223. • Trim the artificial stone teeth on the waxed-up master cast with a sharp knife to eliminate undercuts • Cover the exposed parts of the metal framework with plaster to minimize damage to the cast and framework. • Apply a suitable separating medium, such as petroleum jelly, to the cast and plaster after the plaster has set. • Flask the prepared master cast, and boil out, pack, and cure in the conventional manner
- 68. Guilin Y, Nan L, Yousheng L, Yining W. The effects of different types of investments on the alpha- case layer of titanium castings. The Journal of Prosthetic Dentistry. 2007;97(3):157-164 68 • To evaluate the effect of 3 types of investments on the microstructure, composition, and microhardness of the a-case layer on titanium castings • Fifteen wax columns with a diameter of 5 mm and a length of 40 mm were divided into 3 groups of 5 patterns each • SEM and electron probe microanalysis (EPMA) • The a-case consisted of 3 layers: the oxide layer, alloy layer, and hardening layer. the oxide & alloy layer were called the reaction layer. • The thickness of the reaction layer for titanium castings using SiO2-, Al2O3-, and MgO-based investments was approximately 80 mm, 50 mm, and 14 mm respectively. The surface microhardness of titanium castings made with SiO2-based investments was the highest, and that with MgO-based investments was the lowest.
- 69. Unrestored condition of patient. Use of tool in FreeForm. Completed virtual build. Williams R, Bibb R, Eggbeer D, Collis J. Use of CAD/CAM technology to fabricate a removable partial denture framework. The Journal of Prosthetic Dentistry. 2006;96(2):96-99. 69
- 70. Screen capture of virtual RPD and supports prepared for building (adequate supports had to be created using software) Framework emerging from selective laser melting(SLM). SLM Co/Cr RPD fitted to cast. The framework showed an accuracy of fit judged to be at least comparable to the results obtained by traditional casting methods. 70
- 71. Conclusion 71
- 72. References • McCracken’s Removable partial prosthodontics.13th edition. 2015 • Stewart’s clinical removable prosthodontics.4th edition • Dental laboratory procedures-Removable partial dentures. Morrow, Rhudd.Vol-3 • Robert G. Craig & John M. Powers : Restorative dental materials 10th edition • Viswambaran M,Agarwal S. The effect of four sprue shapes on the quality of cobalt- chromium cast removable partial denture frame-works. Contemporary Clinical Dentistry. 2013;4(2):132 72
- 73. • Guilin Y,Nan L, Yousheng L, Yining W.The effects of different types of investments on the alpha-case layer of titanium castings. The Journal of Prosthetic Dentistry. 2007;97(3):157-164. • Williams R, Bibb R, Eggbeer D, Collis J. Use of CAD/CAM technology to fabricate a removable partial denture framework. The Journal of Prosthetic Dentistry. 2006;96(2):96-99. • Bolouri A. Hilger T. C. & Gowrylok M. D. Modified flasking technique for removable partial dentures. The Journal of Prosthetic Dentistry. 1975;34(2):221–223. 73
- 74. 74
Editor's Notes
- Although most dentists do not fabricate their own removable partial denture castings, it is essential that they have an understanding of the dental laboratory procedures involved. This enables them to design the removable partial denture framework, complete a laboratory work authorization that communicates the desired design and authorizes its fabrication, and evaluate the quality of the framework
- For successful construction of a removable partial denture, the dentist must provide dental laboratory personnel with the following: a written work authorization describing the desired prosthesis a properly surveyed diagnostic cast with an appropriate removable partial denture design a properly articulated master cast that provides an accurate reproduction of existing hard and soft tissue contours. Anything less will compromise quality and adversely impact patient care
- A written request for dental laboratory services is termed a work authorization and is equivalent to a prescription. If the work authorization is used correctly, it represents an excellent line of communication between dentist and laboratory. The dentist should supply dental laboratory personnel with the information needed to produce the requested prosthesis In addition to providing instructions to the laboratory, the written work authorization serves as a legal document in the event of a lawsuit
- the dental laboratory technician must transfer the prescribed design from the diagnostic cast to the master cast. The preferred method for design transfer is described in the following sections. Retripoding During the survey and design process, the clinician determines a specific path of insertion and removal for the partial denture. This path is recorded by placing tripod marks on the diagnostic cast. In turn, the clinician prepares both hard and soft tissues of the mouth in harmony with this path. The technician must ensure that all framework fabrication steps are based upon this path of insertion and removal.
- With the master cast properly oriented on the survey table, the technician uses a dental surveyor and carbon marker to indicate the heights of contour for the teeth and soft tissues
- Without removing the master cast from the survey table or changing the tilt, the technician transfers the design from the diagnostic cast to the master cast Areas of special consideration, such as undercut depths for retentive clasp arms, gauges of wrought-wire clasps, cast clasps’ pattern sizes, and other critical items should be transferred to the master cast with extreme care and clearly marked to avoid confusion.
- The bead line is approximately 0.5 mm deep and becomes less distinct as it approaches the gingival margins Bead lines are not used in conjunction with mandibular major connectors because these connectors rest on thin gingival tissues that cannot tolerate the associated pressures.
- The sealer is a mixture of acetone, diethyl phthalate, and cellulose acetate Careful application is required to prevent a buildup of material, which could create false contours and affect the accuracy of the associated prosthesis. Following application of sealer, the cast must be allowed to dry for a minimum of 5 minutes.
- Most formulas employ a mixture of hard baseplate wax, gutta percha, sticky wax, and a colorant for visual contrast against the dental cast. It is very important to use the following sequence when mixing: 1. Melted gutta-percha in a pasty consistency* 2. Sticky wax 3. Kaolin T. Baseplate wax 5. Coloring ( red lipstick or carpenter blue chalk) Blockout wax is normally kept fluid in an electrically heated pot. The molten wax can be applied to the dental cast using a spatula. Wax is placed apical to the height of contour and is not applied in areas where intimate metal contact is desired. Wax from occlusal or incisal to the height of contour must be removed, a slight excess of blockout wax is placed into all undercut areas, removed by surveyor
- Areas associated with retentive clasp tips are critical to the success of the prosthesis and warrant special consideration. A soft tissue undercut is blocked out under the approach arm of an infrabulge clasp for the maxillary right first premolar. A ledge is present at the disto-facial aspect of the first molar. This ledge aids in placement of the circumferential clasp’s retentive terminus. Stewart mentions tapered parallel and arbitrary blockout
- Normally the surface of the blockout instrument is held perpendicular to the surveyor base. This results in a 0-degree block out (ie, a block- out that is parallel to the path of insertion). Other blades are available and offer blockouts that range from 2 to 6 degrees from the path of insertion. All tooth-supported removable partial dentures should be blocked out parallel to the path of insertion. Tooth tissue supported removable partial dentures may be blocked out in a similar manner, or they may be given a tapered block out to allow increased freedom of movement in function.
- Areas of undercut that are not directly involved with framework fabrication also must be addressed These areas should be recontoured to minimize distortions that may occur during cast duplication Duplicating materials such as reversible hydrocolloids can rebound from relatively small undercuts (< 3 mm), but distort or tear when larger undercuts are present. To prevent distortion and tearing, areas of gross undercut should be eliminated. Because these areas do not directly affect framework construction, blockout can be somewhat arbitrary. Deep palatal clefts below max major connectors to be filled with wax, polished in flame or gauze
- Removable partial denture frameworks usually display intimate contact with the teeth and soft tissues. However, there are certain areas where contact is not desirable Any addition of wax intended to make the framework stand away from the master cast is termed relief The retentive latticework must be raised above the edentulous area to allow resin to completely surround the longitudinal and transverse struts. For practical purposes, 1 mm of space should be created between these struts and the surface of the master cast. This allows resin to encompass the lattice work A properly formed internal finish line permits formation of a butt joint between the framework and the acrylic resin denture base This distance ensures that the porous resin will not contact the marginal gingivae. One thickness of baseplate wax makes an ideal relief pad for latticework and mesh components An adhesive wax sheet (arrows) is sealed at the finish line, approximately 1.5 mm distal to the premolar abutment.
- Relief for small or inoperable tori may be provided using a thin, uniform wax covering. Both the work authorization and the diagnostic cast design should indicate the location and amount of relief needed. When providing relief for tori, the wax borders should be blended with contours of the cast. No sharp demarcation should be visible. In a distal extension prosthesis, the use of relief produces a minor connector that is supported at only one end. As a result, the minor connector may bend when a load is applied. Since considerable force is applied during the packing and processing of acrylic resin, the probability of bending is increased during these procedures. To prevent bending, a small area at the free end of the minor connector should contact the master cast (Fig 2-58). This portion of the minor connector is termed a cast stop. A cast stop is created by removing a small square of relief wax (2 × 2 mm) where the posterior strut of the minor connector crosses the center of the ridge (Fig 2-59).This depression is incorporated into the refractory cast. During the waxing process, this depression is filled with wax. Upon casting the framework and returning it to the master cast, the cast stop should contact the surface of the master cast
- Duplication is done in stone of the original or corrected master cast to preserve the original: use -fitting a removable partial denture framework without danger of fracture or abrading the surface of the original master cast., for processing a temporary prosthesis Another purpose of duplicating a cast is to allow an investment cast to be formed for framework fabrication. duplication materials and techniques are alloy specific. Eg low melting alloys use reversible hydrocolloid while high melting alloys use glycerine based colloids for duplication Alginate irreversible hydrocolloid may also be used by increasing the volume of water to three times the volume used for a regular impression
- Careful preparation of the master cast for production of this investment cast involves consideration of the defined path of insertion, heights of contour, and retentive and stabilization areas designed into the mouth preparations. A duplicating flask will he used to support the cast and confine the hydrocolloid, since it aids in controlling shrinkage. It is a simple design, consisting of three pieces: the base, the body, and the reservoir ring. Two holes in the top surface of the body permit air to escape as the duplicating material fills the flask.
- Agar hydrocolloid is prepared by grinding or chopping the colloid into small pieces and heating it until it liquefies at 212° F (100° C). The melted colloid can he stored for a few days in a hydrocol- loid storage unit at 135° to 140° F. (57° to 60° C). The material may he reused a number of times to make duplicate casts. After each use, it should he washed under cool water to remove all traces ol stone and investment, cut into small pieces, and stored in a closed container to prevent loss of water. If the ma- terial is permitted to dry out, it cannot he used again.
- A duplicating flask should contain the fluid material to facilitate its cooling, to support the mold while it is being filled with the cast material, and to facilitate removal of the cast from the mold without permanent deformation or damage to the mold if wax or clay blockout is present, the temperature of the duplicating material must not be any higher than that recom- mended by the manufacturer, to prevent melting and distortion of the blockout material.
- When the color in the entire cast has changed If a cast is completely submerged in water, it will require 5 to 8 hours to soak all the way through. These will be used to close the holes when the duplicating material has filled the flask, and the air has escaped. Agar hydrocolloid cannot be heated in a pan directly over a heat source because it will burn and cannot be used. When agar is kept in storage unit, it is ready to use any time of day.
- 1. Silicone Duplicating material with a vacuum curing unit in the background, which is used to ensure a dense mold. 2. Duplicating mold following removal of the relieved and blockout cast. 3. Close up of the refractory cast (A) with initial wax added to rests and the posterior bead line (B) 4. Example of a completed maxillary waxed framework on a refractory cast with the sprue access positioned. A, From occlusal. B, From anterior.
- Refractory materials (investments) must be measured and mixed according to manufacturer's instructions if mold expansion is to match the shrinkage of the associated alloy Refractory materials used for chrome cobalt alloys, titanium, and titanium alloys are termed high-heat investments and may be heated to temperatures approximating 1,037°C (1,900°F). These high heat investments are phosphate bonded and usually require a special, silica-containing liquid to mix with the refractory powder. Trimming within 6mm is an essential step in ensuring the escape of gases during the casting process.
- The refractory cast is dipped in hot beeswax to ensure a smooth, dense surface and to eliminate the need for soaking the cast prior to the investment process
- Care is taken to draw with a minimum of pressure so the surface of the refractory cast is not damaged. The position of individual clasp tips is the most critical part of the transfer. If appropriate ledges were created during blockout procedures, the placement of retentive clasp tips is much easier and more precise
- The availability of prefabricated plastic patterns has dramatically reduced the amount of freehand waxing that must be performed during framework construction. The most helpful of these patterns are the clasp arms. As might be expected, pattern choices are greatly influenced by clasp length. A retentive clasp that is longer than 10 mm can be thicker than a shorter clasp, while maintaining the same degree of flexibility. When the clasp is shorter, it must be thinner to ensure adequate flexibility. Manufacturers also market prefabricated patterns for the construction of major connectors. The clinician has the responsibility of evaluating the connector and informing the laboratory if design changes are required to improve rigidity. Although this increases the likelihood that the pattern will stick to the cast, too much tacky liquid will result in a flash of metal around the casting. This excess metal increases the amount of finishing required and may render the cast- ing unusable.
- A wax with a formulation similar to blue inlay wax is indicated because it sets hard and polishes easily. This wax is also used to seal the margins of the major connector and to create the minor connectors and rests. HYLIN carver
- Sprues have the purpose of leading the molten metal from the crucible into the mold cavity. Sprues have the further purpose of providing a reservoir of molten metal from which the casting may draw during solidification Sprues should leave the crucible from a common point Type IV gold and a number of high-heat, chrome-cobalt alloys are cast using a sprue that approaches from above and gives rise to a number of smaller, accessory sprues
- With removable partial dentures, the use of a single sprue is limited to those maxillary frameworks in which—because of the presence of a palatal plate—it is impossible to locate multiple sprues centrally. One disadvantage of using a single sprue for large castings is that an extra-long investment ring must be used.
- Reinforce all junctions with additional wax to prevent constrictions in the sprue channel and to prevent V- shaped sections of investment that might break away and be carried into the casting. Any area that is separated from the bulk of the framework by a long span of latticework or meshwork requires a secondary sprue former.
- Brumfield has listed factors that influence the excellence of a dental casting Sprue: Size, length, configuration, points of attachment, and manner of attachment of the sprues Investment: Choice of investment Location of the pattern in the mold Mixing water: Amount, temperature, and impurities Spatulation of the investment during mixing Restraint offered to expansion of the investment caused by the investment ring
- 40 refractory casts were made, 10 wax patterns for each sprue design were waxed up, invested with phosphate bonded investment material and castings done with induction casting machine by the same operator under standardized protocols. the defects in Groups C and D was significantly lower than Group A and Group B.
- so that the final casting will require as little finishing as possible Cast gold alloy: casting shrinkage of the alloy varies from 1% to 1.74% (the highest figure being the shrinkage of pure gold). Generally, the higher the percentage of gold in the alloy, the greater is the con- traction of the casting on solidifying. Chromium cobalt alloy: Expansion to offset casting shrinkage is accomplished primarily through thermal expansion of the mold and must be sufficient to offset their greater casting shrinkage, which is on the order of 2.3%. For this reason, the casting ring is usu- ally removed after the mold has hardened to allow for the greater mold expansion necessary with these alloys. Investment for co-cr is less porous so more chances of gas entrapment so venting is important, spru attachment careful
- The position of the sprue former is marked on the outside of the mold to ensure proper orientation in the casting machine.
- Modern induction casting machines are normally programmed to cast when the alloy has reached the desired temperature.
- Some alloy manufacturers recommend removal of the investment ring before the mold is placed in the furnace for wax elimination. Other manufacturers recommend leaving the ring in place. In either situation, the sprue former is removed and the entrance of the mold is carefully inspected. Just before it is placed in the furnace, the mold should be placed in the casting machine to balance the weight against the weight of the mold. At this time, the mold should be properly oriented to the machine and its crucible, and a scratch line should be made at the top for later repositioning of the hot mold. More time must be allowed for plastic patterns, particularly palatal anatomic replica patterns. It is important that the peak temperature recommended by the manufacturer not be exceeded during the burnout period.
- If little force is used, the mold is not completely filled before the metal begins to freeze. If excess force is used,turbulence may result in the entrapment of gases in the casting. Induction casting is based on the electric currents in a metal core induced from a magnetic field. An electronic sensor directly above the crucible meas- ures the temperature of the alloy throughout the heating process (Fig 11-42).This optical sensor activates the cast- ing mechanism at the temperature selected by the opera- tor. Casting machines have the capability to revolve at up to 600 rpm.
- in every case the mold must be balanced by placing a counterweight on the casting arm. Some alloys require a heat-soak period of up to 3 seconds, during which time the alloy is kept at the cast- ing temperature by a rapid cycling of the alternating current. In addition, some machines allow the operator to deter- mine the exact moment to cast.Technicians who cast using this method require both experience and a means to view the molten metal.
- Divesting of the framework involves bulk removal of the investment The framework is divested with aluminum oxide Chromium-cobalt alloys are divested but not cleaned by pickling. chromium- cobalt castings are electropolished; this is a controlled deplating process
- A cast clasp arm should not be indiscriminately polished and then bent into place. The waxing should have been done in such a manner that a minimum of finishing is necessary and the intended relationship of the clasp to the abutment is maintained Excessive pressure heats the work, crushes the abrasive particles, causes the wheels to clog and glaze, and slows the cutting. each successive finishing step uses a finer abrasive and therefore cuts more slowly and requires more time to accomplish.
- Some authorities hold that the sprues should not be removed from the casting until most of the polishing is completed. Although it is true that this policy may prevent accidental distortion, it is difficult to adhere to and is therefore somewhat impractical. An experienced technician uses coarser disks and stones and then proceeds to finer grinding agents Excessive pressure heats the work, crushes the abrasive particles, causes the wheels to clog and glaze, and slows the cutting. each successive finishing step uses a finer abrasive and therefore cuts more slowly and requires more time to accomplish.
- Rag and felt wheels are used on high-speed lathes to apply polishing agents
- Because the retentive clasp tip engages undercut areas, they will be among the first areas to bind. Therefore, the technician carefully relieves the cast in these areas
- Framework must be fitted to oral cavity before denture bases and prosthetic teeth are added. If the framework cannot be seated, the clinician must: decide to have new framework made or attempt to correct fit of the existing framework by sectioning and soldering.
- a short distance from the soldering area, the electric circuit is completed through the framework and the electric energy, dissipated as heat (resistance), melts the solder In the twin-flex technique, a Ticonium 19-gauge wrought wire is adapted into a measured undercut area on the proximal surface of an abutment on the master cast, in refractory cast it appears as a channel, after casting it is soldered to framework Wrought wire clasp arms: 18- and 19-gauge nickel-chromium-cobalt wires are currently the most popular The most dependable results are achievable by solder- ing the wire clasp to the framework after the framework is completed. If the clasp is soldered at the rest–minor connector junction, the heat of the soldering operation will adversely affect the physical characteristics of the wire. As a result, the best method of attaching a wrought-wire clasp to the framework is by soldering it on the lattice- work, well away from the area where it will be required to flex
- Those areas are most often undercut and require blockout of the distolingual and retromylohyoid areas of the mandibular cast, the distobuccal and labial aspects of the maxillary cast 1. Cast under- cuts are blocked out with wax, and the cast is coated with separator. 2. Peripheral extent of the base (the same as peripheral extent of the prosthesis captured during border molding and an impression-making procedure for this altered cast) is outlined with rope wax to contain resin. This process is repeated selectively until a uniform layer has been built up that is just thick enough so that none of the underlying cast or relief may be seen. The tissue side of the record base (intaglio) should possess similar accuracy and stabil- ity as are seen with the completed prosthesis.
- accuracy of the base supporting a maxillomandibular record must be ensured before the function of occlusion rims is considered. Modeling plastic (compound) has several advantages and may be used rather than wax for occlusion rims. It may be softened uniformly by flaming, yet when chilled it becomes rigid and sufficiently accurate
- All of these features should be considered against the specific oral environment in which the prosthesis will be placed. A finished and polished prosthesis that demonstrates esthetic features not found in the patient’s mouth may be considered objectionable
- In investing a removable partial denture for processing an acrylic-resin base, it must be remembered that the denture cast must be recovered from the flask intact for remount- ing.
- The formation mechanism for the a-case layer is not clear. This a-case layer may possibly alter the mechanical properties of cast titanium
- Once the dental cast was scanned, virtual surveying and design of the framework on a 3-dimensional com- puter model was accomplished. A rapid prototype machine was used for direct fabrication of the alloy framework. b/l distal extension case: lingual plate, infrabulge clasps
- The purpose of the sup- ports is to provide a firm base for the part to be built onto while separating the part from the substrate plate. In addition, the supports conduct heat away from the material as it melts and solidifies during the build pro- cess.
- Although the clinician’s direct involvement with construction phases of the removable partial denture framework is impractical, each dentist should be aware of the techniques and processes used in prosthesis fabrication. A good working relationship between the dentist and the dental laboratory technician permits the construction of quality prostheses and the delivery of quality care.