Class II Cavity Preparation for Amalgam and Variations
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CONTENTS
INTRODUCTION DEFINITION OF CLASS II CAVITY PRINCIPLES IN CLASS II CAVITY PREPARATION STAGES AND STEPS OF CLASS II CAVITY PREPARATION MODIFICATIONS IN CLASS II CAVITY PREPARATION CONCLUSIONS REFERENCES
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Class II Cavity Preparation for Amalgam and Variations Introduction: Since the advent of restorative dentistry, management and treatment of posterior proximal caries lesion has posed great challenge to dentistry, scientific ability, manual dexterity and knowledge of art. In Lieu of this fact, I welcome you to this seminar. Definition: Class II restoration is one involving mesial or distal surface of a posterior tooth. Principles in Class II cavity preparation: Axial wall, pulpal and gingival floors should meet each other at right angle. Axial wall do not follow the bell-shaped contour of crown but are parallel with long axis is of the tooth. The pulpal and gingival floors are flat and parallel to occlusal plane (Triangular to forces of mastication). Cavo-surface margins at gingival floor right angle to surface of enamel or cementum. Uniform peripheral thickness of tooth substance should be removed.
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Stages and steps of class II cavity preparation: Initial tooth preparation stage: Step 1 – Outline form and initial depth. Step 2 – Primary resistance form. Step 3 – Primary retention form. Step 4 – Convenience form. Final tooth preparation stage: Step 5 – removal of any remaining infected dentin/ old restorative material. Step 6 – Pulp protection. Step 7 – Secondary resistance and retention form. Step 5 – Finishing the external walls. Step 9 – Final procedures; cleaning inspecting sealing. Initial tooth preparation stage: Outline form and initial depth: Establishing the outline form means, placing preparation margins in positions they will occupy in final preparation. First prepare an initial depth of 0.2 to 0.8mm pulpal to DEJ or normal root surface position. There are certain principles which should follow while establishing the out line.
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General principles: 1. Extend preparation to sound tooth structure. 2. Avoid terminating or margins on cuspal heights or ridge crests. 3. Extend margins to allow sufficient access for proper manipulative procedures. 4. Restrict the axial wall pulpal depth of proximal preparation to a maximum of 0.2-0.8mm into dentin. 5. Gingival margins in proximal contact area should extend apically to a minimum clearance of 0.5mm between gingival margins of adjacent tooth. 6. The facial and lingual margin in proximal area is usually extended into their respective embrasures. Factors which dictate outline form: 1. Proportional size of the caries cones in enamel and dentin and their relative size to that of un cleansable area. -
Caries cones in enamel (In forward decay) are smaller than that of size of uncleansable area, the governing factor out line form is the extent of uncleansable area.
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Caries cones in dentin (In backward decay) are larger than that of size of uncleansable area, the governing factor out line form is the extent of dentinal caries.
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Caries cones are smaller than that of size of uncleansable area.
Caries cones are larger than that of size of uncleansable area
2. Extension for convenience or access. The occlusal part should include in the cavity preparation. 3. Location and condition of gingiva. -
0.5-1mm apical to crest of gingival in young and patient with high caries index.
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In the case of gingival recession the gingival margin should located supragingivally.
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At the crest of free gingiva in normal cases.
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In pronounced interdental col the gingival seat should be established in 3 planes.
0.5-1mm apical to crest of gingiva young and patient with high caries index
At the crest of gingiva
In gingival recession
Gingival margin in two planes in the case of pronounced interdental col
4. Condition of marginal ridge. When marginal ridge is not undermined separate occlusal and proximal cavities should be prepared. 5. Convexity of proximal surface.
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More convex tooth surface – wider embrasure area with small contact area requires less extension.
6. Location and extent of contact areas and their relationship to marginal ridges, embrasures and gingiva. 7) Masticatory load – Increased masticatory load – Decreased extension. 7. Generalized plaque index is directly proportional to extent of cavity preparation. 8. Esthetics can modify the cavity out line form in Class II cavity preparation by Decreased facial extension i.e. mesiofacial extension of premolars and molars. Making margins curved, and parallel to adjacent facial surface. 9. Malalignment and rotated teeth. Occlusal outline form: Similar to class I cavity preparation start at the pit nearest to involved surface. Bur position viewed from proximal surface (No. 245 bur). It should parallel to long axis of tooth with slight lingual tilt. Maintain proper depth during initial entry 1.5mm from central fissure, 2mm from prepared walls. Maintaining same bur position and proper depth extend preparation to opposite pit through central fissures. Isthmus width should not be more than ¼ of intercuspal distance.
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Pulpal floor should have uniform depth with slight rise and fall of DEJ. Dovetail should prepared in opposite pit area to provide adequate retention (if fissures are present).
Reverse curve: Develops while making mesio-facial enamel wall perpendicular to enamel rods. Lingually reverse curve is very slight often unnecessary because of large embrasure form.
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Importance: To get butt joint at mesiofacial in origin for prevention of unnecessary tooth destruction. Proximal box preparation: Components are: -
Gingival seat.
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Facial and lingual wall.
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Axial wall.
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Isthmus.
Objectives of proximal box preparation: To include all the caries, faults or existing restoration. Create 90° cavosurface margin. Establish ideally not more than 0.5mm clearance with adjacent facial, lingual and gingival wall.
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Initial procedure for preparation of proximal box is by giving proximal ditch cut. 2/3 of dentin (0.5 – 0.6mm). 1/3 of enamel (0.2-0.3mm). Bur should positioned along the long axis is of tooth pressure is directed gingivally and lightly toward the proximal surface which involved. Bur moved facio lingually (Pendulum motion). Facial and lingual walls: The facial and lingual walls flared or diverged gingivally.The final position of facial and lingual walls depending upon -
Extent of caries.
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Position of self cleansing area.
Proximally the facial and lingual walls positioned in self cleansing area with clearance of 0.2-0.3mm from adjacent tooth. This clearance is mainly for convenience form.
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The proximal walls should join the outer tooth surface in right angle. Facial and lingual walls should be diverged or flared because of convexity of tooth surface. Ideally the proximal flare should perpendicular to external tooth surface. (Fig A) -
Insufficient proximal flare leaving unsupported enamel rods.(Fig B)
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Excessive proximal flare it is difficult to condensation, resulting in weakened margins of amalgam restoration.(Fig C) Ideal proximal flare (proximal flare should
perpendicular to external tooth surface) Insufficient proximal flare leaving unsupported enamel rods
Excessive proximal flare it is difficult to condensation resulting in week end margins of amalgam restoration
Gingival seat: It should be placed 1-2mm below contact i.e. supra gingival. Sub gingival in young patient and patient with high caries index.
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Shape: Flat and perpendicular to long axis of tooth, so masticatory forces are transmitted to periodontium. A slight apical incline of enamel wall results when gingival seat is near to cervical portion of tooth. There is 0.5 mm clearance should be the there from adjacent teeth. In caries susceptible individual it is 0.75mm. Width: In premolars it is 0.6-0.8mm. Molars 0.8-1mm When placing gingival margin in cementum, the round tooth pick or triangular wedge is placed in gingival embrasure to protect underlying gingiva. Axial wall: Placement in dentin it should be parallel to long axis of tooth because - For convenience form. - To provide additional bulk of amalgam. - Retentive grooves can easily placed in dentin. Shape: Follow the contour of tooth (convex) or straight for increased resistance and retention. Length of axial wall from pulpal floor is 0.4-0.6mm Width is not uniform triangular shape apex towards occlusal and base towards gingival seat. Isthmus: Isthmus is junction between occlusal portion and proximal facial and lingual part of preparation.
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Width should be ¼ of inter-cuspal distance. Increased width Increased loading area Decreased tooth volume Decreased resistance of tooth to structural deformation. Therefore its width places an important role mechanical integrity of restoration. There some other features of cavity design which helps to maintain the integrity of restoration are: -
Rounding of Axio-pulpal ling angle.
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Increasing depth of cavity near isthmus area.
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Pulpal and gingival floor near isthmus should be perfectly flat.
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By slanting the axial wall.
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Every part of cavity preparation should be self rentive.
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Remove all discontinues in the preparation i.e. scratches, grooves.
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By eliminating the occlusal prematurites in the restoration.
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Primary resistance form: Shape and placement of preparation walls that best enable tooth structure and restoration to withstand without fracture masticatory forces delivered principally in the long axis of tooth. Factors affecting resistance form: -
Occlusal contact on tooth and restoration. Tooth with increasing occlusal contact and masticatory load needs more retention i.e. posterior teeth.
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Amount of remaining tooth structure.
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Type of restorative material i.e. thickness of restorative material.
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Mechanical properties of tooth i.e. compressive strength, tensile strength, modulus of elasticity. 15
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Principles of resistance form based on physical properties of various part of tooth.
Features: A) Occlusal surface: -
Flat pulpal floor (fig 6-29).
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Divergence of walls towards marginal ridge.
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Perpendicular walls towards crossing ridge.
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Facial and lingual walls should be perpendicular to isthmus.
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Occlusal ledging.
Other features of resistance: 1. Relatively flat floors i.e. pulpal floor, gingival seat. 2. Box shape cavity: To provide definite line angles and point angles. 3. Inclusion of weakened tooth structure. 4. Preservation of cusps and ridges. It is provided by following features -
Reverse curve.
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Preparation wall in relation to marginal ridge diverge occlusally.
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Preparation wall in relation to crossing ridge converge occlusally.
5. Rounded internal ling angles. 6. Adequate thickness of restorative material. 7. Cusp capping if indicated. B) Isthmus: -
Width Âź to 1/5th of intercuspal distance.
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Rounded axio-pulpal line angles.
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Obtuse axio-pulpal line angles.
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Round the axial wall in bucco-lingual direction i.e. convex.
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C) Margins: -
Butt joint at amalgam tooth interface.
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Amalgam tooth interface should not be placed at occluding contact areas.
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Facial and lingual margins should meet proximal surface at right angle.
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Beveled gingival margins.
Primary retention form: “That shape or form of the conventional preparation that resist the displacement or removal of the restoration from tipping or lifting forces.’’ Features: 1. Occlusal convergence (Fig. 6-30): Facial and lingual walls both in occlusal and proximal portion converge occlusally.
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2. Dovetail: Prevent tipping movement of restoration. 3. Rounding line angles and point angles. Rounding of axio-pulpal line angle. -
Avoids stress concentration.
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Decreased tensile stress for unit area.
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Provide greater bulk to restoration.
4. Preserve critical areas of tooth: Strong cusps and ridges. 5. Minimal exposure of restoration to occluding loading. 6. Cusp capping if indicated. 7. To bond the material to tooth structure: By adhesives provide micromechanical retention.
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Convenience Form It is that shape or form of the preparation that provides for adequate observation, accessibility and ease of operation in preparing and restoring the tooth.
Methods: During Cavity preparation: Mechanical methods: -
Tooth separation.
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Gingival retraction.
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Tissue recontouring.
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Surgical repositioning.
Advantages: -
Access to lesion.
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Access to condensing instrument.
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Allows for proper finishing.
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Periodic assessment and home care.
Final tooth preparation stage: Removal of any remaining enamel pit or fissure infected dentin, old restorative material if indicated.
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It is defined as removal of any remaining pit or fissure infected dentine, and / or old restorative material is the elimination of any infected carious tooth structure or faulty restorative material left in the tooth after initial tooth preparation. Soft dentinal caries removed by discoid type of spoon excavator. Hard caries removed by round carbide bur with slow speed with air/water coolants. Removal of defective enamel or infected dentin should not compromise resistance form of preparation. Any old restorative material should not removed unless it is: -
Symptomatic.
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Showing evidence of recurrent caries.
Pulp Protection: Indications: Pulp irritation is needed because to protect the pulp from various types of insults. These insults due to -
Frictional resistance and heat of rotary instruments.
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Ingredients of restorative material.
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Thermal changes in oral cavity.
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Galvanic shock.
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Increased depth of cavity preparation.
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Methods of pulp protection: -
Never cut dry – A rise of 11°F pulp temperature cause pulpal damage.
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Cavity liners – Calcium hydroxide; Zinc oxide eugenol.
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Cavity bases: Zinc phosphate; Zinc poly carboxylate; GIC.
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Cavity varnish.
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Bonding agents.
Secondary resistance and retention form: Retention locks: These are vertically oriented at axiofacial and axio-lingual line angles. They should be placed 0.2 mm inside the DEJ.
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They prevent lateral displacement and increases fracture strength of restoration. Indications: Extensive proximal preparation, preparation with short walls. Four characteristics of retention locks: Position: Axiofacial and axio-lingual line angle. Depth: 0.5mm at the level of gingival floor. Translation: No. 167L bur positioned parallel to DEJ so that it should bisect the axiofacial or axiolingual line angles. Occluso-gingival direction: Amount of tilt given to bur during preparation of retention locks. It dictates the occlusal height of the lock given a constant depth.
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Four types of retention locks: 1. Gingival floor to occlusal surface. 2. Occlusal to axio-pulpal line angle. 3. From axio-pulpal line angle occlusal surface. 4. Gingival floor to axio-pulpal angle conventional type.
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Slots: Horizontally placed retentive grooves in dentin primarily on gingival floors. Indication: In crowns with short length and preparations with short walls. The 0.5mm width and 0.6mm depth.
Pins: -
Small threaded structure placed in pin holes.
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Placed in gingival seat / axial angles.
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Depth of pin hole is 2mm in dentin and 0.2mm in restoration.
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Used when adequate retention form can not be achieved by locks undercuts only.
Pot holes (Reciprocating box): Small holes which made in pulpal floor which prevent tipping of restoration and help in retention. Amalga pins: Shavell in 1980; depth is 1.5mm (0.5mm dentin).
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Occlusal offsets and cleats: Occlusal offsets are horizontal grooves placed in buccolingual direction prevents lingual displacement of restoration. Occlusal cleats are placed in bulbous part of crown provide good anchorage.
Finishing the external walls of tooth preparation: Objectives: -
Good marginal seal.
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Smooth marginal junction.
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Provide strength to tooth and restoration.
No unsupported enamel should be there in margins. No occlusal cavosurface bevel is required. This occlusal Cavo surface margin should have 90째-100째 angulation. If it is less than 90째F fracture of amalgam.
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The 20째 gingival margin bevel should be given to get full length of enamel rods. If it is below CEJ no bevel is indicated (Fig. 17-56).
Cleaning / inspecting / sealing: Objectives: -
Debris removal.
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Moisture elimination.
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Disinfection.
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Air/water jets.
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Dry cotton pellets.
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Cavity cleansers.
Methods:
Citric acid. Ascorbic acid. Acetic acid. -
EDTA for removal of smear layer.
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Modifications in Class II cavity preparation: Slot preparation: In older patients with gingival recession assuming that contact does not need restoration. Tooth preparation is usually facial or lingual.
Initial depth is 0.75mm-0.8mm at gingival aspect if enamel is present. In occlusal aspect the depth is 1-1.25mm (0.5mm inside the DEJ). The infected caries is not removed at this stage. It should removed at end with round bur. Prepare retention grooves with Âź bur in occlusoaxial and gingivo axial line angles (0.2mm inside DEJ).
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External walls should have 90째 Cavo surface angle.
Class II cavity preparation for extended carious lesion: Outline is determined by extent of caries, already present restoration with secondary caries. Gingival seat may be extended toward root surface. The width of gingival seat for premolar 1mm and molar 1-1.5mm. -
Depth of axial wall premolar 1.2mm; molar 1.8mm.
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Width of axial wall is not altered.
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Facial and lingual walls proximally more wider than initial class II preparation.
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Facio-gingival and linguo-gingival line angles are sharp.
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The width of isthmus is 1/3 of inter-cuspal distance.
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To prevent any injury to gingiva place triangular wedge in inter-dental area.
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The axial grooves are placed on facial and lingual walls as a secondary retention to prevent lateral displacement.
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If deep caries is there in axial wall it should be removed by spoon excavator and application of appropriate cavity liners.
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Mesio-occlusal distal cavities (MOD cavity): This modification is indicated in case where both proximal surfaces involved. Here both proximal surfaces sharing one pulpal floor. Cavo-surface margin design should have 90° configuration. Secondary retention features should included in cavity preparation.
Cusp-capping: When outline form crosses stress bearing area. Rule for cusp capping: Extension from primary groove toward cusp tip is more than ½ distances then cusp capping are considered. The Primary reason for cusp capping is loss of dentin support by caries. But in the case of severe attrition cusp capping can be considered. Here 1/3 of length of cusp is removed. The common error we do while cusp capping is to remove affected cusp portion retaining small portion of unaffected portion of cusp and place vertical wall of preparation on down-hill slope of preparation. It leads to disintegration of inner end enamel rods and no dentin support for rods results in loss of enamel.
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It can be avoided by placing the vertical wall on up-hill slope of cusp only terminal portion of enamel rods. So that there will be an inward rest on dentin so retain their nourishment and support. Carious lesion crosses cusp tip then preparation should extend all the way out to the edge of the tooth. The preparation should not extend on distolingual cusp. Box preparation (conservative design): Indications: -
Decay restricted to proximal surface and sound intact crossing ridge and inclined planes.
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Less crown height.
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The restoration which subjected less occlusal forces.
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There is no occlusal extension facial and lingual walls converge occlusally. The gingival floor located in three planes. Rentive locks are placed facial or lingual walls. They are placed 0.5mm into DEJ. The axial wall slanted pulpally more. So that more amalgam can be placed inside the cavity.
Sloping pulpal floor: This variation mainly because of difference in morphology between mandibular 1st and 2nd premolars. In 1st premolar i.e. Disto-occlusal cavity.
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Features:
- Sloping pulpal floor. -
Gingival margin should be parallel to soft tissue contour.
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Y shaped occlusal anatomy with mesial marginal ridge higher level than distal obtuse angle to get equal thickness of amalgam pulpal floor and axial wall.
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Strong transverse ridge should be retained.
Joining of restoration: It is permissible to repair or replace a defective portion of an existing restoration if the remaining portion of restoration retains adequate resistance and retention. Adjoining restorations on the occlusal surface occur more often in molars because the dovetail of the new restoration can
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usually be prepared with out eliminating the dovetail of existing restoration. The intersecting margins of two restorations should be right angle to each other as much as possible. Outline and margin of old restoration should not altered. Mercury rich amalgam is placed close to first restoration.
. Rotated teeth:
When tooth rotated 90 degrees proximal box is displaced
facially or lingually. In these cases the preparation may require isthmus that includes cuspal eminence also.
Unusual outline: Outline should satisfy the restoration requirement of tooth not necessarily to the classic example of Class II preparation.
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If occlusal fissures which are separated 0.5 or more of sound tooth structure they are treated with two individual restorations.
Abutment tooth for RPD: Require more facial, lingual and axial extensions (Tooth supported RPD).Entire rest seat should be placed amalgam with depth of 0.5mm.In tooth and tissue supported RPD no alteration in outline form is indicated.
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Tunnel tooth preparation: Conservative tooth preparation which occlusal and proximal lesions are joined by means of tunnel under the involved marginal ridge. In this way marginal ridge remain intact. These cavities usually restored by glass ionomer cement. In assessing this technique the adequacy of preparation access may be a controversial.
Extended margins: Cervical areas are caries prone because these areas difficult to clean i.e. old Class II restoration. When the margin ends on cementum, it may necessitate the alteration in cavity design. Lack of access in these places precludes the use of hand cutting instruments rather than the burs. A sharp chisel or hatchet is the most effective way to establish and execute a good gingival floor and margin.
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Combination of Class II cavity Class V: Where proximal caries is not active and not restricted to areas underneath the contact point but extend to facial or lingual embrasures as well. For eg lower first molars. Here the margin can not be terminated with in decalcified enamel, yet it is counterproductive to move the entire wall outward to include a small finger like extension of caries. It is permissible to curve and indent the facial wall to include this portion, remembering of course, that this indentation must be readily accessible for condensation with amalgam after matrix band has been placed. Moreover inclusion of the defect must be cleanly cut to blend with adjacent enamel ,and above all the cavosurface margin ca not be flared but must be at a 90° angulation. If caries involve only one side (facial or lingual) than cavity outline is “Lshape” both sides “inverted T shape”.
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Lesion on proximal surface with very limited access on
facial or
lingual surface: Indications: - Small to medium sized proximal lesions with intact marginal ridge and contact point -
Restoration which subjected to normal displacing forces.
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Patients with good oral hygiene.
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The lesion does not involve contact area
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The gingival embrasure is not accessible, exposed or pronounced enough to facilitate proximal instrumentation with out cutting a facial or lingual access cavity.
Two types of cavity designs:
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Facial or lingual access cavity i.
With dovetail.
ii.
Without dovetail.
Gaining access: Facial or lingual depends on -Extent of decay -Condition of axial line angle. Most involved side should chosen for the access cavity. Location of margins: Gingival margin: - Crest of free gingiva. - In gingival embrasure Occlusal margin: In gingival embrasure just below the contact area. Facial or lingual margin on non access side located in corresponding embrasure just short of axial line angle.
Access side: The preparation margin should cross the axial line angle. But it should not extend more than Âź facial or lingual surfaces. If occlusal or gingival margins are located in cementum or dentin it should have two planes inner dentinal grooved plane outer dentinal plane. If the occlusal margin is located on enamel, it is always at the gingival third of
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surface, so the occlusal wall will be one planed following the direction of the enamel rods. Class II when two or more surfaces of an endodontically treated tooth that does not require post retention Indications: a. The remaining tooth structure, after endodontics, can support and retain an amalgam restoration. b. The tooth has a sufficient pulp chamber to accommodate retaining, self-resisting amalgam bulk, i.e., minimum 2mm thickness in three dimensions. c. The post-endodontic pulp chamber has at least two opposing intact walls. d. The tooth contains sufficiently large root canals to accommodate retaining, resisting amalgam bulk at its (their) occlusal one-thirds (i.e., minimum 1.5mm thickness). e. A foundation is needed for a reinforcing restoration (Cast restoration). f. There is sufficient remaining tooth structure to permit the preparation of flat planes at right angles to occluding forces. g. There has been successful root canal therapy leaving an intact sub pulpal floor. h. The tooth does not show any signs of cracking or crazing. General shape: 40
Excavate from the entire pulp chamber any residual root canal filling materials or debris. Bare dentin should be exposed on the surrounding walls and subpulpal floor. Large root canals that can accommodate an amalgam thickness of 1.5mm should have the root canal filling removed to a 3-4 mm depth. If possible, “square up� surrounding wall provided this action will not perforate to the surface, furcation, or thin tooth structure to the extent of making these areas non-resistant. Establish as pronounced gingival floors as possible. If they are in dentin, they will be one planed; and if on enamel, they will be two planed. Any external boxes of retention should not perforate to the pulp chamber or cause thinning in the intervening walls. Flat portions of the tooth preparation receiving forces at a right angle to their inclination should be strategically distributed, in order to be maximally concentrated in number and dimensions at locations of maximal loading.
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CONCLUSION
“Form ever follows function�
Hence while restoring Class II lesions we must strive towards achieving proper form of the restoration. So that it can fulfill its function of providing longevity and stability to the natural dentition without encroaching physiological limits of pulp, periodontium and occlusion.
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REFERENCES: 1. Art and Science of Operative Dentistry, 3rd & 4th EditionSturdevant 2. Silver Amalgam in Clinical Practice, 2nd edition- I. D. Gainsford 3. Operative Dentistry, Modern Theory and Practice- M. A. Marzouk.
4. Principles
and
Practice
of
Operative
Dentistry-
Charbeneau
5. Textbook of Operative Dentistry- Vimal Sikri
6. Textbook of Operative Dentistry- Baum & Phillips
7. Operative dentistry by Gilmore
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G.T.