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Intra & extra coronal restoration resistance form  /certified fixed orthodontic courses by Indian dental academy

Intra & extra coronal restoration resistance form /certified fixed orthodontic courses by Indian dental academy



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    Intra & extra coronal restoration resistance form  /certified fixed orthodontic courses by Indian dental academy Intra & extra coronal restoration resistance form /certified fixed orthodontic courses by Indian dental academy Presentation Transcript

    • Resistance form for intracoronal and extracoronal Dental restorations INDIAN DENTAL ACADEMY Leader in Continuing Dental Education www.indiandentalacademy.com www.indiandentalacademy.com
    • • Introduction. building restoration is similar to building any mechanical structure, in that the stress pattern of the available foundation and contemplated sytucture must be predetermined.The fundamental concept form of resistance form is based on reaction with the restoration and remaining tooth structure to the occlusal loading. www.indiandentalacademy.com
    • • Def.Resistance form may be defined as the shape and placement of preparation wall that best enables both the restoration and the tooth structure to withstand, without fracture, masticatory forces delivered principally in the ong axis of the tooth. www.indiandentalacademy.com
    • • Sress patterns of teeth. Anterior teeth.a.The incisal edges espicially if they are square are subject to tensile and shear stress in normal occlusion. Masive stresses are present in edge to edge occlusion.b.The slopes of the cuspids will bear concentrated stresses (three types) espicially if the cuspid is protector of occlusion.(canine protected occlusion).c. The distal surface of the cuspid exhibits unique stress pattern as a result of anterior component of force concentrating compressive loading at the junction of anterior and posterior of dental arch and microlateral movement of cuspidsduring excrusive www.indiandentalacademy.com
    • Movements. These factors leads to tremendous stress concentration with resultant abrasive activity.D. The lingual concavity in the upper anterior teeth bears substatntial compressive stress during centric and shear stress during protrusive movements.e. The incisal edges of the lower anterior teeth are subjected to compressive stress in centric and shear stresis in protrusive movements.2. Srress patterns in posterior tooth.a. Functional cusp tips bears compressive stresses.b. Marginal and crossing ridges bears tremendous compressive and shear stresses. www.indiandentalacademy.com
    • c. During function compressive stress is on the occluding contacting side and tensionon non contactiing side. The junction of crown and root bears tremendous shear stress during lateral excursion.Some Applied Mechanical Properties Of Teeth.Compressive strength of enamel supported by vital Dentin 36-42000 psi.When enamel looses support of dentin it looses more than 85% of its strength.Compressive strength of dentin os 40-50,000 psiNon-vital dentin looses 40-50% its strength. www.indiandentalacademy.com
    • • Resistance form is of two types.• Primary resistance form. Primary resistance form is established during the initial tooth preparation stage. Principles.1. Use box shape with relatively flat floor.2. Restrict extension of the external walls to allow strong cusp and ridge areas with sufficient dentin support.3. Slight roundening of the ine angles4. Cap weak cusps in extensive preparations.5. To provide enough thickness of restorative materials to prevent its fracture under load. www.indiandentalacademy.com
    • • Factors affecting the resistance resistance form.• A. Remaining tooth structure. The amount of remaining tooth structure also affect the need and type of resistance form. Very large tooth even though extensively involved with caries or defects require less resistance form consideration, espicially in regards to cusp capping because th eremaining tooth structure is still bulky and strong enough to resist fracture.b. Type of restorative material. Amalgam requires minimum of 1.5mm thickness for sufficient strength, cast metal requires 1mm and ceramics 2mm. www.indiandentalacademy.com
    • • Amalgam.• Class I Preparation.• 1.Should have a Mortoise shape.• Features.• Seat of the restoration is at right angle to the direction of stresses.• It prevents buccal and lingual splitting• 2. If caries penetrates deeply removing the undermining caries would lead to conical preparation. (fig)• 3. when th epreparation walls comes in cantact with marginal ridge , the wall should be divergent pulpo- occlusally. www.indiandentalacademy.com
    • • 4.when ever preparation wall comes in contact with a crossing ridge make the wall perpendicular to the pulpal floor.• 5. The width of the preparation should be ¼ to 1/5 the intercuspal distance(Not less than 1.5mm). distance(Not less than 1.5mm).• 6.The crossing ridges should be preseved ie. No effort should be made to them in preparartion unless dictated by caries. Loss or involvement of such crossing ridges is often more detrimental to the tooth than loss or involvement of marginal ridge.• 7.whenever preparation comes very close to the cuspl tips thinning them to the extent that they cannot be self resistance, it is advisable to flatten these cusps(Cusp www.indiandentalacademy.com capping) creatng a table with a minimum
    • • 8. when it is required to cross marginal ridge, faciolly or lingually, the following rules must be observed• i.preserve the width being crossed. It should be 1.5mm or dictated by cariogenic factors.• ii. If small cusp is being isolated from the rest of the tooth (weakening it) elevate the pulpal floor at the crossing isthmus is advisable..• 9. all cavosurface angles should be right angles to create butt joint with amalgam.• 10. all line and point angles should be rounded. www.indiandentalacademy.com
    • • Class II .Occlusal loading and its effects.a. A small cusp contact the fossa away from the restored proximal surface in a proximoocclusal restoration I centric closure.(Fig) www.indiandentalacademy.com
    • • 2. a large cusp contacts the fossa adjecent to the restored proximal surface in a proximo-occlusal relation at centric closure. www.indiandentalacademy.com
    • 3. Occluding cuspal elements contacts facial or lingual tooth structures surrounding a proximo-Mocclusal during centric and excursion movements. www.indiandentalacademy.com
    • 4. Ocludin cuspal elements contacts the facial or lingual parts of restoration surrouded the tooth structure during the centric. www.indiandentalacademy.com
    • 5.Contacts at the facial or lingual parts of restoration which completely replaces the facial or lingual parts. www.indiandentalacademy.com
    • • 6. contacts at the marginal ridges. www.indiandentalacademy.com
    • Design Features for the mechanical integrity of the restoration.a. Isthmus. The junction between the occlusal part of the restoration and proximal, facial or lingual parts, potentially deleterious tensile stresses occur.Studies reveals three thingsI. Flucrum of bending ocurs at the axiopulpal line angleII. Stresses incresae closer to the restotation away from fulcrumIII. Tensile stresses predominate at the marginal ridge area. www.indiandentalacademy.com
    • Materials tend to fail therefore starting from the surface near the marginal ridge and proceeding internally.These problems can be solved by applying three engineering principles. www.indiandentalacademy.com
    • • The advantages of this design are1. Increases accessibility to th eproximal surface2. Sharp junctions are avoided ,which would otherwise concentrate stresses in the restoration.3. It increases the bulk of the amalgam Other features for the isthmus area.The pulpal and gingival floor should be perfectly flat in order to resist forces at the most advantageous angulatrion.b. Every prt of the preparation (Facial, occlusal, lingual, proximal) should be self retenrive. If every part of the restoration is locked independently there will be minimum stresses at the junction of one part with anotherie.www.indiandentalacademy.com Isthmi.
    • Margins.Four design features.1.Butt joint2.Leave no frial enamel at cavosurface margin3. Remove flashes of amalgam on tooth surface adjecent to the enamel margins.4. As practically a s possible the interface between the amalgam and tooth structure should not be at occluding contact area.Cusp and Axial angles.1. Bulk of amalgam in all three dimensions should be 1.5mm.2. Each portion should be independently immobilised with retention modes. www.indiandentalacademy.com
    • 3. Amalgam should be seated on a flat floor or table in this area.Design Features for protection of physiomechanical integrity of tooth structure.1..Isthmus.The width of the preparation should not exceed more than 1/4 to 1/5 the intercusapl distance. If it is exceeded, it results in iuncresaed internal stresses in the tooth, reduces tooth resistance to fracture2.If the class II preparation includes the occlusal surface all the basic features in the prepareation should be included.3.Cuspa and axial angles.As in class I preparation the ideal length to width ratio www.indiandentalacademy.com
    • Should be less than 1:1 (Mesiodistally :buccolingually), if the ratio is more than 2:1 then this part of the cuspal wall should be shortened untill there is a maximum ratio of 1:1 this is done in form of flat table to accommodate 1.5mm thickness of amalgam.Facial and lingual retentive grooves should not br deep as to undermine enamel.Margins.Facial and lingual walls of the occlusal surface approach the proximal surface, they should meet at right angle. www.indiandentalacademy.com
    • Class III1. Anterior component of forces concentrates at the junction between the anterior segment of the arh and the premolar region.the cuspids stand at this corner where these forces will first be receieved and distributed2. The incisal slopes of cuspids in normal excursion mechanism will be the last to disocclude during lateral excursion so the canines carries maximum occlusal load for the longest time.3. If the incisal margin is located far enough onto the distal slope, restoration will be directly loaded vertically and horizontally a situation that necessiates maximum strength. www.indiandentalacademy.com
    • To overcome these forces, the tooth preparation on the distal of the cuspid should have1. Bulkiest walls possible2. Minimal incisal extension. Tooth coloured materials have no chance of withstanding these types of forces. If esthetics is of great concern here due to extensive labial extension, an amalgam restoration can be built with a windiw like cavity on the labial side which can be later restored with tooth colouredmaterial. www.indiandentalacademy.com
    • • Class V .• 1. the margins should not be placed on the cementum as it lacks self resistance.• 2.in cases the lesion is close to the bifurcation and trifurcation areas, special attention should be given to the gingival margins and walls. Gingival floor should be flat, one planed, and with no retentive grooves , this allows more bulk to the tooth structure.• 3. to protect the critical axial angles of the tooth the mesial and distal walls should diverge axiofacially or lingually. www.indiandentalacademy.com
    • • Pin retained amalgam restorations.• Mechanical aspects of pin retaine dfrestorations.• Stressing capabilities of pins.• Stresses are always inudced in dentin as a result os pin insertion.if the stresses exceeds th elastic limit of the dentin permanent deformation occurs.• Types of pins.• 1.Smaller the diameter of the pin compared to pin hole lesser willl be the stress.• 2. Diameter of pins.• Greater the diameter of pin greaterr will be the stress. www.indiandentalacademy.com
    • 3. pin depth and dentinal engagement. greater the depth of pin channel greater will be the stresses4. Bulk of dentin. the greater the bulk of dentin pulpally or towards the surface of pin is, the less will be the stresses per unit volume.5. Type of dentin6. Inter pin distance.7. Loose pins.8. Ratio of the depth of the pin into the dentin to that pritruding into the preparation.(2:1) www.indiandentalacademy.com
    • • Retentive Features in the remaining portion of the tooth.• Pins and restorative material..• 1. Pin will not increase the compressive strength of restorative material. They will only help in retaining it.Infact there will be a drop in compressive strength and tensile strength. www.indiandentalacademy.com
    • Cast metal restorations.cast metal restorations are alternative to amalgam when higher strength is needed. Their main advantages are high conpressive and tensile strengths.They are basically indicated to support the weakened tooth like large restorations, endodontically treated teeth, teeth at risk of fracture. www.indiandentalacademy.com
    • • Resistance features.• Flat pulpal floor• Incluson of weakened tooth structure• Preservation of cusps and marginal ridges• Reduction of cuspa when indicated.• Minimm thickness of the metal can be 1.mm.• 30 degree marginal bevel provides burnishable metal which protects the enamel margins.• Resistance form can be incresaed by conservig the tooth structure.• a. enameloplasty• b. marginal bevel can be used to include (eliminate) terminal end of the fissures in outline form. www.indiandentalacademy.com
    • if majhor facial and lingual extension is required to remove undermined enamel capping the weakened cuspal structure is indicated. www.indiandentalacademy.com
    • • Tooth coloured restorations.• Ideally restorations made of ooth coloured materials should not be loaded directly ,ie., there should be intervening tooth structure between occluding tooth and restorations.• Composite resins are only non cast materials that can combine undermine enamel and cuspal elements to sound tooth structure there by improving resistance form.• Consrevative composite restorations. this typr of design is used for smallpits and fissures.It allows for restoration of lesion or defect with minimal removal; removal of tooth structure. It is termed as conseravtivewww.indiandentalacademy.com composite restoration(CCR).
    • Resistance features.The design is primarily conventional design.The thickness of th erestorative material should be minimum1.75mm.The roundness of the lineangles should be more exagerrated trhen amalgam because of brittle nature of materials.Walls directly loaded i.e., in contact with opposing teeth in centric and non centric occlosion should be formed by enamel supported by dentin.In areas where extensive caries undermines the cusps it is advisable to skirt these areas. It is applicable for functional cusps rather than non functional cusps.Knife edge enamel should be trimmed and flattened. www.indiandentalacademy.com
    • In high stressed areas in centric an eccentric contacts it is essential to create sufficient eeduction of tooth structure for bulk of restoartion.Of all the surrounding walls in class II, III andiV preparation, the gingival wall is most important in shaping the resistance form. It should be as pronounced as anatomically possible The margins should not be placed close tro the important anatomicx struictures like incisal angle, marginal ridge and axialangle, this will leave the tooth bulk espicially dentinal support to these structures. www.indiandentalacademy.com
    • • The modofied preparation for composites conserve more tooth structure because retention is obtained primarily by micromechanical adhesion to the surrounding enamel and underlying dentin, rather than by preparation of retention grooves or coves in dentin. www.indiandentalacademy.com
    • • Effect of polymerization shrinkage on resistance form. Moder n composites undergo polymerisation shrinkage from 2.6% to 7.1%.During polymerization resin composite may pull away from least retentive margin resulting in gap formation.Tensile forces developed in enamel margin can result in margination degration mastication.Contraction forces on cusps can result in cuspal deformation, enamel cracks and crazes and ultimately decreased fracture resistance of cusps.Prevention.Incremental palcement.Beta quartz inserts www.indiandentalacademy.comAuto cured resin has less polymerisation shrinkage.
    • • Two step or soft start polymerisation. Initial irradiation150 mW/cm2 followed by high level irradiation 650 mW/cm2. www.indiandentalacademy.com
    • Extracoronal resistance form. Cusp capping.It is a method of encompassing a cusp with a restorative material to increase the resistance form of the tooth.Indications.If the preparatiion outline extends 2/3 the distance from the central groove to the cusp tip.Cusp cappng provides1. Protection of the weakened tooth structure.2. Remove occlusal margin from a region subjected to heavy stress and wear.Materials usedAmalgamCast gold. www.indiandentalacademy.com
    • • Cuspal reduction for capping for amalgam is doneby reducing the cusp 2mm and making a flat table.• For cast metal it is done by placing occlusal bevel , the Table and the counter becvel. www.indiandentalacademy.com
    • • Capping of margianl ridge.• It is done when the marginal rige is weakened and the embrasure is pronounced• It is similar to capping a cusp but on a klesser scale.• C apping the ocluso proximo facial or lingual corners of preparation.• It is done to protect the thinned corners due to over preparation or wide preparation in ovoid tooth.• Over the facial or lingual extension a bevel extension is added ending in a knife edge or chamfer finishing line. www.indiandentalacademy.com
    • • Peripheral sleeve.• This feature add resistance for amalgam preparation design.• It is 2mm axial and 11mm cervicall deep prepared feature. www.indiandentalacademy.com
    • Onlay.It is partly intracoronal and pertly extracoronal restoration which has cuspal proection as main feature.Mainly indicated for cuspal protection,when the width of the lesion has exceeded ½ the intercuspal distance. The cusps are protected by dong cusp capping on the functional cusp and <Shoeing< on the non functional cusps.Resistance features.Secondary Flare. www.indiandentalacademy.com
    • It is indicated when the surface extension are required to include facial lingual defects beyond the line angles of the tooth.Surface extension is necessary to encompass an axial angle for reinforcement and support.Secondry bevel is is in the form of partial bevel. It involves enamel only, with its maximum depth at the main cavity preparation. It ends at the facial and lingual surface with knife edge finishing line. www.indiandentalacademy.com
    • • Skirt.• For cast metal.• It is more extensive surface extension than secondary flare. It is imparts retention and retention to a missing or shortened facial or lingual wall.• It is used• Features.• It includes a part of the facial and lingual surfaces near the axial angle to a depth of 0.5 to 2mm.it ends with a chamfer or a hollow ground bevel.• It should be terminated in the facial or lingual groove. www.indiandentalacademy.com
    • • For composites.• In areas where extensive caries undermines the cusps it is advisable to skirt these areas. It is applicable for functional cusps rather than non functional cusps www.indiandentalacademy.com
    • Collar.This type of surface extension is most involving surface wise and depth wise.i could be a. cuspal collar. (ivolving one cusp ) b. Tooth collat. (Involving the entire facial or lingual surface)The help in retention and resistance when the entire cusp is lost.A collar can accommodate both metal and porcelin b.ulk in PFM restoration.Features.It has axial depth of 1.5 to 2mm.It ends gingivally in bevelled shoulder finishing line.There should be a definite line angle at the shoulder and axial collar. www.indiandentalacademy.com
    • • STEP 3 PRIMARY RETENTION• Definition:• Is that shape (or) form of the prepared cavity that resists displacement (or) removal of the restoration from tipping (or) lifting forces.• According to their locations, retention means can be classified as• 1. Intracoronal (With in the tooth)• 2.Extracoronal (On preparation surface• replacing external surface ) www.indiandentalacademy.com
    • • PRINCIPLES MEANS OF RETENTION :• 1.Frictional retention, depends on 4 factors:• a)Surface area of contact between tooth structure and restorative material. Greater surface area produces a greater frictional component of retention.• b)More opposing walls / surfaces involved greater retention and a more stable restoration within the preparation.• c)Parallelism & non– parallelism, a higher degree of parallelism between opposing walls. Higher convergence of walls – in intracoronal preparation. Higher divergence of walls – in extra coronal preparation, provides a greater locking ability of the tooth to the restorative material.• d)Proximity : bringing the material closer to tooth structure during insertion will increase the frictional retention. www.indiandentalacademy.com
    • • 2.Elastic deformation of dentin :• Changing position of dentinal walls & floors microscopically by using condensation energy within the dentin’s proportional limit, can add more gripping action by the tooth on the restorative material. This occurs when the dentin regains its original position while the restorative material remains rigid thereby completely obliterating any remaining space in the cavity preparation.• 3.Inverted truncated cones (or) undercuts.• 4.Dovetail : This is a purposeful modification in outline form in some cases but usually extension for prevention will create a dovetail outline.• 5.Mechanical interlocking with cements counteracts pull due to sticky foods. www.indiandentalacademy.com
    • Means of retention for materials.Amalgam.Primary Retention features.1. Mechanical locking into the surface irregularities2. Preparation of vertical walls which converge occlusally3. Special retentive features such as locks, grooves pins steps and amalgapins4. Bonding of the amalgam (Optional) Retention locks.They are placed in the axiogifacial and axiolingual line angle(S p 706) www.indiandentalacademy.com
    • • Groove extensions.• Occlusal dove tail.• Secondry retention form.• Slot.• A slot is a retention groove in dentin whose length is in a horizontal plone• Slot retention may be used in conjunction with pin retention or as an alternative to it .• Outwaire et el reported pin retained amalgam has more tendency to slip whereas slippage does not occur in slots.• Preparation.• Using a No. 33 ½ bur a slot is placed in the gingival floor 0.5mm www.indiandentalacademy.com axial of the DEJ.
    • • Slots in the gingival floor may be used to provide additional retention in an extensive proximal box. Slot dimension will depend on the size of the proximal box.• Normal Dimensions of the slot• 0.5mm to 1mm- Deep gingivally• 2 mm to 3 mm - Length faciolingually• 0.2mm to 0.3mm - Inside DE Junction. www.indiandentalacademy.com
    • • Groove.• In classII, III and Class V• Features.• Prepared b y ¼(0.5mmdiameter) round bur.• Groove is prepared by placing the bur at the line angle cutting half the depth of bur i.e.,o.25mm..• Cove.25 mm into dentin.• They are`palced inj cl III amalgam• They are palced at axiogingivofacial and axiogingivolingual point angles. . www.indiandentalacademy.com
    • • Retention of amalgam in special situations.• In Box type preparation. When restoring a small, cavitated, proximal lesion in tooth, a proximal box preparation is recommended. To maximize retention, preparations with facial and lingual walls that almost oppose each other are advised.• To compensate for the lack of an occlusal preparation, the proximal retentive locks should have a 0.5mm depth at the gingival point angle, tapering to a depth of 0.3mm at occlusal surface• In slot type preparationsentive grooves are given at the occlusoaxial and gingivoaxial line angles.• www.indiandentalacademy.com
    • • Cusp capping.• Cusp reduction significantly decreases retention form caused by loss of height of vertical walls. When additional retention is indicated, slots and pot holes can be prepared along the gingival floor, 0.2mm pulpally from the ‘DE’ junction. www.indiandentalacademy.com
    • • Pin retained amalgam restorations.• DEFINITION• A Pin retained restoration may be defined as any restoration requiring the placement of one or more pins in the dentin to provide adequate retention form. www.indiandentalacademy.com
    • • INDICATIONS• For restoration of mutilated and badly broken down teeth especially in young patients where the gingival lines are still high, where massive tooth preparations necessitated by cast restorations are contraindicated, pins are required for auxiliary retention.• In badly broken down teeth, prior to endodontic or orthodontic treatment, pin retained restorations are placed as a transitional restoration to act as a build up for rubber dam application or band attachment.• As a foundation for partial or full veneer cast restoration or metal ceramic restorations, thereby saving considerable amount of tooth structure by eliminating the need to remove undercuts.• As a provisional restoration in teeth with questionable prognosis endodontically or periodontally, until a definitive prognosis is established.• In preparations where adequate retention form cannot be established with slots, locks or undercuts, pins are used as a means of providing auxiliary retention. www.indiandentalacademy.com
    • CEMENTED PINS FRICTIONAL GRIP THREADED PIN • .Pin channel is larger in diameter than 1. Pin channel is slightly narrower 1. Pin diameter is narrower than pin. in diameter than pin. that of the pin.Pin Channel Diameter 2. Pin Channel Diameter 2. Pin channel diameter- 0.020 0.021 0.027- 0.021 3. Pin diameter 0.021Pin Diameter 0.022 0.018- 0.025 It is 2-3 times more retentive than 0.013- 0.020 cemented pin. 3. Pin diameter 0.031 0.023 0.020 0.015 It is 3-6 times more retentive than cemented pin. www.indiandentalacademy.com
    • • FACTORS AFFECTING THE RETENTION OF THE PIN IN DENTIN AND AMALGAM• Type of pin: In order of retentiveness, self-threading pin is most retentive, friction lock pin is intermediate and cemented pin is the least retentive.• Surface characteristics: Retention of the pin in amalgam is influenced by the number and depth of elevations in the pin. Therefore, self-threading pins are most retentive.• Orientation of the pins: Retention of the pins is increased by placing them in a non-parallel manner.• Number of pins: Within limits, increasing the number of pins increases the retention in dentin and amalgam.• Pins placed closer than 2 mm in dentin to each other in one tooth will result in• Crazing of dentin and increased potential for fracture.• Decrease in the amount of available dentin between the pins.• Strength of amalgam restoration decreases. www.indiandentalacademy.com
    • • Diameter of the pins: Within limits, as the diameter of the pin increases the retention in dentin and amalgam increases.• Extension into dentin and amalgam: Pin extension into dentin and amalgam greater than 2 mm is unnecessary for pin retention and contraindicated to preserve the strength of the dentin and amalgam. www.indiandentalacademy.com
    • • Amalgam Foundations.• It is an initial restoartion of broken down tooth . The tooth is restored so that it will provide resistance and retention fprms needed for indirect cast restoration.• It also provide resistance form against forces that otherwise might fracture tooth structure.• It does not depend primarily on coronal; tooth structure instead it rely mainly on secondary preparation retention features(Pins, slot,coves and`proximal retention locks.) www.indiandentalacademy.com
    • • CHAMBER RETENTION :-• A Nagyar et al has described technique for developing foundations in multi rooted endodontically treated teeth. It is recommended only• Dimension to the pulp chamber is adequate to provide retention and bulk of amalgam.• Dentin thickness in the region of pulp chamber is adequate to provide rigidity and strength to the tooth.• Extension into the root canal space 2 to 4mm in recommend when pulp chamber height its 2mm or less• When pulp chamber height is more than 6mm no adavantage is gained from extending into root canal. www.indiandentalacademy.com
    • • Amalaga pin• These are circular`chambers that are cut in the dentin to provide rewsistance and retention of the restoration. These are also called Amalgam inserts.• They provide retention similar to self thresded pins.• It is 1.5 to 2mm in depth and 0.8mm in diameter. www.indiandentalacademy.com
    • • Cast metal restoration.• – Principle retentive feature is close parallelism of longitudinal walls. Small range for divergence (2-5 degrees / wall) from the line of draw will enhance retention form (concept of taper)• Since exact parallelism create technical problems in processingl,a slight taper is desired.• The average taper is 2-5 degrees.• The taper can be altered by following afctore• 1.length of the preparation. www.indiandentalacademy.com
    • • 3. need for retention.• Bevels.• Grooves.• These are indicated when th eprepaartion walls are short.• Groove extensions. Well defined line angles are also important in obtaining resistance and retention form. www.indiandentalacademy.com
    • Secondary modes of retention :1) Luting cement : fills gap between inlay and tooth giving a physio chemical bonding. Physical – Zinc phosphate, chemical : glass inomer + polycorboxylate. The exposed cement dissolves in the oral cavity and so should not be considered as main retentive factor. www.indiandentalacademy.com
    • 2) Grooves : Place two grooves, one on bucco axial and other on lingual axial line angle with #165 bur. It is totally placed on dentine. Depth of groove should be 0.3mm at the expense of buccal and lingual walls and never at expense of axial walls. www.indiandentalacademy.com
    • 3)Reverse Bevel : It is given on gingival seat. This bevel has generally three planes, i) reverse bevel plane where inclination is on gingivoaxial plane which prevents proximal displacement. ii) secondly, flat plane made of dentine. iii) thirdly ,plane which is sloping away from the axial wall made of enamel and dentine, www.indiandentalacademy.com this helps in
    • 4) Internal box: made on the pulpal floor, which improves the retention by 4-5 times. this is on the uninvolved side. it should not have sharp line and point angles and definite walls. This prevents micromovement of the inlay. Internal box should always be reciprocated with a reverse bevel or groove to avoid micromovement. www.indiandentalacademy.com
    • 5) External box : these are box shaped preparations opening to axial tooth surface .they can be proximal, facial or lingual. They can be either stepped occlusally or gingivally. www.indiandentalacademy.com
    • 6) Roughening of pulpal floor : at specific areas of tooth preparation, esp. in pulpal floor, is done for more retentive and laterally locking. irregularities should have no frail or undermined enamel. Creating different levels out of flat, dished up, gingival or pulpal floors could change a mechanically negative situation into a positive one. www.indiandentalacademy.com
    • • 7) Precementation grooves : after casting make grooves on the walls of the inlay and/or grooves on the cavity wall exactly opposite. This will house with solid mass of cement which helps in good retention www.indiandentalacademy.com
    • 8)Electrolytic etching of inlay : This gives hinge like projections. Procedure : Protect proximal and occlusal surface with sticky wax and keep it in the electrolyte solution of 0.5 normal nitric acid. Inlay is kept in anode and metal with increased EMF as cathode.(316 stainless steel ) www.indiandentalacademy.com
    • current causes microporosities on fitting surface which enables the luting cement to flow into it for better retention. This process takes place for around 10 to 15 mins. www.indiandentalacademy.com
    • It is usually advantageous to use a diamond stone for preparing the tooth for a composite restoration. This results in a roughened prepared surface, which increases the surface for bonding.• The modified preparation for composites conserve more tooth structure because retention is obtained primarily by micromechanical adhesion to the surrounding enamel and underlying dentin, rather than by preparation of retention grooves or coves in dentin. www.indiandentalacademy.com
    • ADVANTAGES OF ENAMEL BEVEL The ends of the rods are more effectively etched than otherwise only the sides of the enamel rods are exposed. Increase in etched surface area results in a stronger enamel to resin bond, which increases retention of the restoration and reduces marginal leakage and marginal discoloration. Incorporation of cavosurface bevel may enable the restoration to blend more esthetically with the coloration of the surrounding tooth structure. www.indiandentalacademy.com
    • Even recognizing these advantages, bevels are not usually placed on the occlusal surfaces of posterior teeth or other areas of potential heavy contact because a conventional preparation design already produces end on etching of the enamel rods by virtue of the enamel rod direction on occlusal surfaces• Bevels are not placed on proximal margins if such beveling results in excessive extension of the cavosurface margins.• Therefore this design is rarely used for posterior composite restorations. www.indiandentalacademy.com
    • • For restoration of large carious lesions, wider bevels or flares and retention grooves, coves or locks may be indicated (similar to amalgam) in addition to the retention afforded by the adhesive procedures.• Class III• The boxlike design is considered a part of retention form.the external walls are retentive because of opposing wall parallelism or slight undercuts.• Retention groove may help in minimizing the potential negative effects of polymerization shrinkage and increae retention.. It is palced .25 bb bisecting external wall and axial wall. It is not indicated in bevelled conventional preparation. www.indiandentalacademy.com
    • • Coves are prepared with no.1/4 bur at the axioincisal point angle with the bur oriented in a similar angle,0.2 mm inside the DEJ and 0.25mm deep. www.indiandentalacademy.com
    • • Cleass IV.• Gingival and incisal retentive undercuts may be indicated in large class IV preparations in which rounded undercuts are placed in the dentin along line angles and into point angles wherever possible. www.indiandentalacademy.com
    • • Retention form is provided primarily by the micromechanical bonding of the composite to the enamel and dentin.• Additional retention may be obtained by increasing the width of the enamel bevels or placing retention undercuts.• If retention undercuts are deemed necessary prepare a gingival retention groove using a no ¼ round bur. It is prepared 0.2mm inside the DEJ at a depth of 0.25 mm and at angle bisecting the junction of the axial wall and gingival wall. www.indiandentalacademy.com
    • • Class V.If retention grooves are necessary they are prepared with a no.1/4 bur along the full length of the gingivoaxial and incisoaxial line angles.• For bevelled conventionalGroove retention usually is not indicated when the periphery of the tooth preparation is located in enamel. Many of these larger preparations will be a combination of beveled enamel margins and 90-degree root-surface (non enamel) margins, with the root surface areas having groove retention .• www.indiandentalacademy.com