04.principles of tooth preparation

Principles of ToothPreparation

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PURPOSE OF TOOTH PREPARATION

• Restoration is usually required to repair a diseased, injured or defective tooth structure.

• The restoration helps in maintaining proper form, function and esthetics.

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TYPES OF RESTORATION

• Tooth restoration may be classified as intracoronal, when it is placed within a preparation made in the crown of a tooth

• extracoronal, when it is placed outside the tooth as in the case of a crown.

• Intracoronal restoration is placed directly into the tooth preparation while extracoronal restoration uses an indirect technique.

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Factors to be considered before restoration of a tooth

• Tooth factorsPrimary or permanentOcclusal stressesQuality of tooth (hypoplasia)Location of toothType of toothType of tooth preparation.

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• General patient factorsPatient’s exposure to florideAgeXerostomiaSocioeconomic statusDietCaries statusGeneral healthPresence of any parafunctional habit.

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Factors related to clinician and the restoration to be used

Type of restoration

Physical properties of the restoration

Whether moisture control can be achieved or not

Technical experties.

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TERMINOLOGY OF TOOTH PREPARATION

• Simple Tooth PreparationA tooth preparation involving only one tooth surface is termed simple preparation for example occlusal preparation.

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• Compound Tooth PreparationA tooth preparation involving two surfaces is termed compound tooth preparation for example mesioocclusal or disto-occlusalpreparation.

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• Complex Tooth PreparationA tooth preparation involving more than two surfaces is called as complex tooth preparation for example MOD preparation.

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Tooth Preparation Walls

• Internal WallIt is a wall in the preparation, which is not extended to the external tooth surface

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• External WallAn external wall is a wall in the prepared tooth that extends to the external tooth surface. External wall takes the name of the tooth surface towards which it is situated.

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• Pulpal WallA pulpal wall is an internal wall that is towards the pulp and covering the pulp.

• It may be both vertical and perpendicular to the long axis of tooth.

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• Axial WallIt is an internal wall which is parallel to the long axis of the tooth

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• FloorFloor is a prepared wall which is usually flat and perpendicular to the occlusal forces directed occlusogingivally,for example, pulpal and gingival walls

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Cavosurface Angle Margin/Tooth Preparation Margin

• Cavosurface angle is formed by the junction of a prepared tooth surface wall and external surface of the tooth.

• The acute junction is referred to as preparation margin or cavosurfacemargin.

• The cavosurface angle may differ with the location of tooth and enamel rod direction of the prepared walls and also differ according to the type of restorative material to be used.

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• Line AngleIt is a junction of two surfaces of different orientationsalong the line and its name is derived from the involvedsurfaces.

• Point AngleIt is a junction of three plane surfaces or three line anglesof different orientation and its name is derived from itsinvolved surfaces or line angles.

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Steps in tooth preparation

Stage I: Initial tooth preparation steps • Outline form and initial depth• Primary resistance form• Primary retention form• Convenience form.

Stage II: Final tooth preparation steps• Removal of any remaining enamel pit or fisure, infected dentin and/or old

restorative material, if indicated• Pulp protectin, if indicated• Secondary resistance and retention form• Procedures for finishing the external walls of the tooth preparation• Final procedures: Cleaning, inspectig and sealing• Under special conditions these sequences are changed.

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• Outline form defines the external boundaries of the preparations.

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Following principles are kept in mind while preparing anoutline form:

• • Removal of all weakened and friable tooth structure

• • Removal of all undermined enamel

• • Incorporate all faults in preparation

• • Place all margins of preparation in a position to afford good finishing of the restoration.

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Features for establishing a proper outline form are

• Preserving cuspal strength

• Preserving strength of marginal ridge

• Minimizing the buccolingual extensions

• If distance between two faults is less than 0.5 mm, connect them

• Limiting the depth of preparation 0.2 to 0.8 mm into dentin

• Using enameloplasty wherever indicated

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Primary Resistance Form

• Definition:

Primary resistance form is that shape and placement of preparation walls to best enables both the tooth and restoration to withstand, without fracture the stresses of masticatory forces delivered principally alongthe long axis of the tooth.

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Factors affecting resistance form

• Amount of occlusal stresses

• Type of restoration used

• Amount of remaining tooth structure.

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Primary Retention Form

• Primary retention form is that form, shapeand configuration of the tooth preparation that resists the displacement or removal of restoration from the preparation under lifting and tipping masticatory forces.

• Usually, resistance and retention forms are obtainedby providing same features, hence they are sometimesdescribed together. The retention form is affected by the type of the restorative material used

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Factors affecting retention form

• Amount of the masticatory stresses falling on the restoration

• Thickness of the restoration

• Total surface area of the restoration exposed to the masticatory forces

• The amount of remaining tooth structure

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Retention form for differentrestorations

• Amalgam:

• Retention is increased in amalgam restoration by the following:

• – Providing occlusal convergence (about 2–5%) of thedentinal walls towards the tooth surface

• – Giving slight undercut in dentin near the pulpal wall

• – Conserving the marginal ridges

• – Providing occlusal dovetail

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Cast metals:

• Retention is increased in cast restorations by the following:

• – Close parallelism of the opposing walls with slightocclusal divergence of two to five degrees

• – Making occlusal dovetail to prevent tilting of restoration in class II preparations

• – Use of secondary retention in the form of coves, skirts and dentin slot

• – Give reverse bevel in class I compound, class II, andMOD preparations to prevent tipping movements

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Composites:

• In composites, retention is increased by:

• – Micromechanical bonding between the etched and primed prepared tooth structure and the composite resin

• – Providing enamel bevels.

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Direct filling gold:

• Elastic compression of dentin andstarting point in dentin provide retention in direct gold fillings by proper condensation.

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Convenience Form

• The convenience form is that form which facilitates and provides adequate visibility, accessibility and ease of operation during preparation and restoration of the tooth.

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Features of convenience form

• Suffcient extension of distal, mesial, facial or lingual walls to gain adequate access to the deeper portion of the preparation.

• The cavosurface margin of the preparation should be related to the selected restorative material for the purpose of convenience to marginal adaptation.

• In class II preparations access is made through occlusal surface for convenience form.

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Features of convenience form

• Proximal clearance is provided from the adjoining tooth during class II tooth preparation.

• To make Class II tunnel preparation, for convenience, the proximal caries in posterior teeth is approached through a tunnel initiating from the occlusal surface and ending on carious lesion on the proximal surface without cutting the marginal ridge.

• In tooth preparation for cast gold restorations occlusaldivergence is one of the feature of convenience form.

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Final Stages of Tooth Preparation

• After initial stages of the preparation, the prepared tooth should be carefully examined

• The remaining carious portion should be removed onlyafter the initial tooth preparation has been completed. Itprovides two advantages:

• 1. It allows optimal visibility and convenience form forremoval of remaining carious lesion.

• 2. Completion of the initial preparation permits immediate placement of a base and the restoration.

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• Removal of Any Remaining Enamel Pit or Fissure, Infected Dentin and/or Old Restorative Material, if Indicated

• After the establishment of external and internal outlineform, if any of the remaining carious tooth structure ordefective restorative material is left in tooth, it is to beremoved in this stage.

• Infected dentin must be removed even if it leads toexposure of pulp which is treated accordingly

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Pulp Protection

• When remaining dentin thickness is less, pulpalinjury can occur because of heat production, high speed burs with less effective coolants, irritating restorative materials, galvanic currents due torestoration of dissimilar metals, excessive masticatory forces transmitted through restorative materials to the dentin and ingress of microorganisms and their noxious products through microleakage.

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Bases• cements used on pulpal and axial walls in thickness of

about 0.5 to 2 mm beneath the permanent restorations.

• They provide thermal, galvanic, chemical and mechanical protection to the pulp.

• Commonly used restorative materials as base are

• zinc phosphate cements,

• Glass ionomers,

• polycarboxylate cements,

• zinc oxide eugenol,

• calcium hydroxide cement. 55

Secondary Resistance and Retention Forms

• This step is needed in complex and compound tooth preparations where added preparation features are used to improve the resistance and retention form of the prepared tooth.

• These are as follows:

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Grooves and coves:

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Slots or internal boxes:

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Locks

Pins:

• Different types of pins of various shapes and sizes are used to provide additional retention.

• They can be used in all types of restorations like amalgam, composite and castrestorations.

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Skirts:

• Skirts are prepared for providing additional retention in cast restorations.

• They increase the total surface area of the preparation.

• Skirts can be prepared on one or all four sides of the preparation depending upon the required retention

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Amalgam pins

• Amalgam pins are vertical posts of amalgam anchored in dentin.

• Dentin chamber is prepared by using inverted cone bur on gingival floor 0.5 mm in dentin with 1 to 2 mm depth and 0.5 to 1 mm width.

• Amalgam pins increase the retention and resistance of complete restoration.

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Beveled enamel margins:

• Beveling of the preparation, margins increases the surface area and thus the retentionin composite restorations.

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Enamel wall etching:

• Etching results in microscopic roughness, which increases the surface area and thushelps in enhancing the micromechanical retention.

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• Dentin conditioning (etching and priming): Etching and priming of the dentin surface done in some restorative materials increases the retention.

• Adhesive luting cements:

Adhesive luting cements increase the retention of indirect restorations

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04.principles of tooth preparation

Health & Medicine

Transcript of 04.principles of tooth preparation