Principles of tooth preparation Dr. Hsu Zenn Yew Dr Jasmina Qamaruz Zaman Department of Operative Dentistry 11/7/2012
Definition
Tooth preparation is the mechanical alteration of defective, injured or diseased tooth to receive a restorative material that reestablishes a healthy state for the tooth; √ √ √
√
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normal form & function esthetic (corrections where indicated) Longevity Comfort
Indications 1.
To provide restoration for carious teeth (seen clinically or radiographically)
2.
To restore form and function (eg: Malformed teeth – due to fracture, tooth wear, congenital malformations Amelogenesis imperfecta
Fractured tooth
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Indications 3.
To improve esthetics/appearance (eg: diastema, discoloration)
4.
Replace defective restorations (secondary caries, improper proximal contacts, overhang, open margins, poor esthetics, bulk fracture)
Diastema Defective amalgam margins
Fluorosis
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Biological •Maintain vitality of the tooth and protect the pulp •Eliminate defects and diseased part of tooth •Be as conservative as possible •Prepare the tooth so that the tooth and restoration remain intact under masticatory Esthetic forces
Mechanical Produce a functional restoration
•Retention form •Resistance form •Convenience form •Finishing •Debridement
Produce an esthetic result •Minimum display of metallic restoration •Adequate esthetic material thickness
Objectives of Tooth Preparation
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Terminology Buccal
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Distal
Mesial
Lingual
Terminology buccoaxiogingival
buccoaxial LINGUAL
Point angle = point where 3 point angles meet
BUCCAL Buccal wall
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Line angle = line between 2 cavity walls
Margin = angle between the prepared cavity and outer surface of the tooth (aka cavosurface angle cavosurface margin)
Cavo-surface margin
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Cavo-surface angle
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MESIAL
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DISTAL
Principles of tooth preparation
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History: Traditional cavity preparation GV Black (1836-1915) Described 6 stages of cavity preparation EXTENSION FOR PREVENTION – the restoration is extended into sound fissures to prevent caries from occurring: destructive to the tooth.
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Reasons for Extension for Prevention
Prevent recurrence of decay in the surface of the enamel next to the restoration Self-cleaning benefit of the embrasures with saliva and fluids of the diet. Properties of amalgam
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adequate strength in bulk but brittle in small sections does not adhere to tooth structure
Features of cavities with Extension for Prevention
Occlusal surface extends through pits and fissures
Proximal line angles extended Bucally and lingually through embrassures Cervically below the gingival margin
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Features of cavities with Extension for Prevention ď Ž
Margins of the restoration placed on line angles of the tooth ď Ž
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Mesiobuccal, distobuccal, mesiolingual, distolingual line angles of the tooth
GV Black’s principles of cavity preparations Steps: 1. Outline form 2. Resistance form 3. Retention form 4. Convenience form 5. Removal of remaining caries 6. Defining and finishing enamel margins 7. Debridement of the cavity (Cavity toilet) 11/7/2012
GV Black’s principles of tooth preparations
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Very precise preparations Flat walls Flat floors Uniform depth Applied to Amalgam Cast Gold
Mechanical
Biological •Prevent recurrence of decay in the surface of the enamel next to the restoration •Self-cleaning benefit of the embrasures with saliva and fluids of the diet
1. 2. 3. 4. 5. 6.
7.
Outline form Resistance form Retention form Convenience form Removal of remaining caries Defining and finishing enamel margins Debridement of the cavity (Cavity toilet)
??Esthetic
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Extension for prevention
Reasons why extension for prevention is no longer relevant.
Natural and fluoride-induced remineralization
Effect of calcium and phosphate from saliva Fluoride introduction through water, toothpastes, mouth rinses, restorative materials, ect.
Advancements in instrumentation
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Smaller and more precise instruments
Reasons why extension for prevention is no longer relevant.
Introduction of new restorative materials
Composite resin requirements v. conventional amalgam
Design of more conservative preparations
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Slot preparations and PRR’s
Modern Restorative Concept ď Ž 1. 2. 3. 4.
5.
Minimal Intervention Reduces cariogenic bacteria Early lesion Uses preventive measures Early lesions remineralised Minimal cavity preparation on cavities / Conservative preps Repair of defective restorations.
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Arrested lesion
Why conservative preparations? d.
Preservation of Tooth Structure Preserve tooth structure Enamel doesn’t regenerate Minimize pulpal insult Tooth retains strength Less effect on surrounding teeth/tissues Minimizes esthetic problems
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How to achieve conservative preps? 1.
2.
3.
Minimal extension of preparation Supragingival margins
Rounded internal line angles
Modified tooth preparation design Modified cavity preparation
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GV Black’s
Modified preparations Applied to CR, GIC and other bonded materials non uniform depths shallower preps marginal angles of 90 degrees or more enamel bevels (for CR) less need for retention and resistance form preparation features (eg: locks, slots, pins and grooves) 11/7/2012
Traditional Black’s cavity design
Modified tooth preparation design
Fundamental concepts for tooth preparation Applies for all tooth preparation (conventional and modified): No weak tooth structure left (e.g undermined enamel) 2. The fault, defect or caries is removed 3. The remaining tooth structure is left as strong as possible 4. The underlying pulp tissue is protected 5. The restorative material is retained in a strong, esthetic (in some cases)and functional manner. 11/7/2012 1.
Steps for cavity preparation 1. 2. 3.
4.
Access and outline form Caries removal Cavity preparation - Resistance form, Retention form, Convenience form, Finishing Cavity debridement
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ACCESS AND OUTLINE FORM
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Access and Outline form
The outline form is the shape that the cavosurface angle will assume after the cavity has been prepared.
Determined by: Extent of caries
It is obtained by removing all enamel undermined by caries.
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Access and Outline form ď Ž 1. 2. 3.
Principles: All weakened enamel must be removed All faults should be included Margins ideally placed in areas which is easy Undermined/ to finish. weakened enamel
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Access and Outline form Specifications:
Must be able to see the DEJ Extend preparation to sound tooth structure
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Enamel DEJ
Maintain limited pulpal or axial depth Caries in dentin is left at this stage
Caries spread laterally at the DEJ
Access and Outline form Access cavity needs to be wide enough to enable you to see all the carious dentine esp at the DEJ
Enamel wall / margin must rest on sound dentine
Incorrect
correct
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Access and Outline form ď Ž 1.
2.
3.
Features: Preserve cusp strength – avoid terminating margins on cusp heights or ridge crests Preserve marginal ridge strength (maintain at least 1.5mm, may be slightly less for bonded restorations) Minimize bucco-lingual extensions
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Access and Outline form ď Ž
Features:
4.
Apply fissure widening (enamel biopsy/ enameloplasty) where necessary Restricting depth into dentine
5.
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6.
Connecting 2 close (<0.5mm apart) tooth preparations
Access and Outline form
Features:
7.
Gingival margins of proximal cavities extended apically to provide clearance of 0.5mm between gingival margin and adjacent tooth.
8.
Enhance placement of matrix, insertion of material, finishing of restorative material and defining gingival margin of prep)
Buccal and lingual margins of proximal cavities extended into embrasures to provide clearance.
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However, if patient’s caries risk is low and oral hygiene is good, it is permissible to leave the buccal and lingual margins in contact with adjacent tooth)
CARIES EXCAVATION
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Caries excavation
Determined by: Location and size of demineralized area/zone Proximity of carious lesion to pulp
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Caries excavation ď Ž 1. 2.
Principles: All infected caries must be removed All decalcifications at cavosurface margin must be removed
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Caries excavation Infected dentin Characterized by irreversibly denatured collagen Bacteria present Not able to be remineralized Must remove during tooth preparation
Affected dentin Reversibly denatured collagen Capable of remineralization Not to be removed during tooth preparation *Fusayama T. Two layers of carious dentin: diagnosis and treatment, Oper Dent 4:63-70, (1979).
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Caries excavation Caries â&#x20AC;&#x201C; soft to explorer, may be yellow, orange or brown stain. If hard (calcified) leave stain.
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Leave calcified (hard) dentin. Colour does not matter.
BREAK
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CAVITY PREPARATION Retention form •Resistance form •Convenience form •Finishing •Debridement •
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RESISTANCE FORM
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Resistance form ď Ž
Definition: Resistance form is the design of a cavity which enables the remaining tooth substance and the restorative material to withstand masticatory stress.
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Resistance form ď Ž ď&#x192;&#x2DC;
Determined by: Biological and physical properties of enamel and dentine
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Resistance form
Determined by: Biological and physical properties of restorative material
Magnitude and direction of occlusal forces
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Resistance form ď Ž
Create adequate resistance form
ď Ž
Principles:
1.
Preserve adequate bulk of tooth (or protect it) Provide adequate bulk of restorative material (Amalgam need at least 1.5mm. This principle is less important for CR and GIC/RMGIC) Create shapes that resist or diffuse stress.
2.
3.
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Resistance form ď Ž
Features:
1.
Relatively flat floors Box shape Rounded internal line angles Preservation of cusps and marginal ridges (ie restrict extension of prep) Inclusion of weakened tooth structure Adequate thickness of restorative material Reduction of cusps for capping when indicated.
2.
3. 4.
5. 6.
7.
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Resistance form
Create adequate resistance form A
Features:
1.
Box shape with relatively flat floor. Help tooth resist occlusal loading because it is at right angles to the force of mastication.
Flat floor prevent restoration movement B. Rounded floor – may allow non bonded restoration to rock/ rotate and dislodge or wedge tooth apart – tooth fracture. However, sound tooth tissue should, not be removed simply to obtain a flat pulpal floor. Place a base 11/7/2012 A.
B
Resistance form ď Ž
Features
2.
Rounding of internal line angles to reduce stress concentration in line angles. External outline form should also be rounded for the same reason. Stress concentrated in sharp line angles
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Resistance form
Features:
3.
Restrict extension of preparation (as small as possible). • Allow strong cusps and ridges to remain with sufficient dentine support. • A marginal ridge which is too weak during prep of occlusal cavity - Class II cavity may have to be prepared instead, so as to eliminate the weak marginal ridge. • Especially where the ridge is only of enamel thickness and unsupported by sound dentine.
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Resistance form
Features:
4.
Cap weak cusps Rule for cusp capping: a. If cavity margin is less than ½ the distance form the primary groove to cusp tip – NO CAPPING b. If cavity margin is ½ - 2/3 distance from primary groove to cusp tip – CONSIDER TO CAP c. If extension > 2/3 distance from pimary groove to cusp tip – CUSP CAPPING
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Resistance form Note fracture line on distal surface. This further weakens already weaken cusps. Prevent by cusp capping.
THE RESULT
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Resistance form 5.
Features: Enough thickness of restorative material to prevent fracture under occlusal load.
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Min thickness amalgam = 1.5mm Min thickness CR = 1mm (Bonded restorations (eg: CR material to tooth structure may increase strength of weakened tooth structure and reduce fracture)
RETENTION FORM
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Retention form ď Ž
ď Ž
Definition: Shape of preparation that resists displacement or removal of restoration by tipping or lifting forces. Retention from is less critical for adhesive materials (GIC / CR).
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Occlusal
proximal
Retention form
Determined by:
General shape of prep Surface area and position of axial walls Magnitude and direction of (para)functional forces Type of material used
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Retention form ď Ž ď&#x192;&#x2DC;
Principle: Create a shape that provides macro or micromechanical locks or frictional resistance to dislodgement of the restoration along any path of withdrawal.
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Retention form Features: ď Ž Related to restorative material used: Amalgam Macromechanical interlocking 1. Undercut (external walls that converge occlusally). Must not overdoâ&#x20AC;&#x201C; can leave unsupported enamel rods on cavosurface margin
X
X 2. Box form Friction at walls 11/7/2012
Retention form Feature: Amalgam 3. Retentive groove / slots
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Retention form Feature: Amalgam 4. Pins
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Retention form Features: Amalgam 5. Dovetail Extension of the occlusal portion of a Class II cavity into carious pits and fissures which resists the displacement of the Class II cavity in proximal direction.
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Retention form
Feature:
Composite resin – Macromechanical and micromechanical interlocking – between bonding resin and tooth. Micromechanical porocities created by acid etching procedure Macromechanical interlocking 11/7/2012
Retention form Feature: Glass ionomer cementâ&#x20AC;&#x201C; chemical adhesion by material to enamel and dentine. Macromechanical interlocking ď Ž
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Retention form ď Ž
Features: Feature
How does it work?
Walls oppose and parallel
Friction
Wall oppose and converge (undercut)
Macro lock
Floors flat and parallel occlusion
Anti-slip
Walls 90Âş or less to floors
Anti-torque and shear
Adequate surface area of retention walls
Friction
Dovetails
Macro lock
Accessory retention (Pins, amalgapins, grooves, bonding)
Macro and micro lock
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ď Ž
After you have created the cavity to receive the material, check again the preparation prior to finishing, cleaning the cavity and insertion of the restorative material.
ď Ž
Consider convenience form
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CONVENIENCE FORM
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Convenience form ď Ž
Definition: Shape or form of the preparation that allows adequate observation, accessibility and and ease of operation in preparing and restoring the tooth.
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Convenience form ď Ž
Features: 1.
2.
3.
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Cavity is wide enough to admit instruments used in operative procedure. Margins of restoration placed in areas accessible for proper finishing. Outline wide enough to allow visualization of the DEJ and other areas of the cavity.
FINISHING
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Finishing ď Ž 1.
Determined by: Biological and physical properties of enamel and dentine at tooth restoration interface - Direction of enamel rods - Support for enamel rods at DEJ - Location of the margin -Degree of smoothness/roughness required.
2.
Biophysical properties of restorative material at tooth restoration interface. - Type of restorative material to be placed
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Finishing
Principles: Margin of tooth and restoration must be fracture resistant Margin of restoration can be properly finished
Objectives 1. Create best marginal seal between tooth and restoration 11/7/2012
Finishing Features: 1. Create smooth and accessible margins 2. Margin shape approprite for material (see following slides) 3. Unsupported enamel removed 4. Preparation outline must be smooth curves , line angles rounded off
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Finishing Type of Cavosurface Margins. ď Ž
Amalgam â&#x20AC;&#x201C; - All unsupported enamel must be removed. - Margins must be smooth. - Bevel - ? Gingival cavosurface margin
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Finishing Type of Cavosurface Margins. ď Ž
Composite resin â&#x20AC;&#x201C; bevel in some areas - Buccal enamel (eg Class III cavity): Acceptable to leave unsupported enamel in the anterior teeth. - Unsupported enamel in the posterior teeth, (esp on
occlusal margin) must be removed.
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Finishing Type of Cavosurface Margins.
Composite resin – bevel in some areas
Function: 1. Improves retention – increase surface area, enamel rods are cut
perpendicularly – better etching pattern compared to enamel rods
cut longitudinally.
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Finishing Cavosurface Margins. ď Ž
Composite resin â&#x20AC;&#x201C; bevel in some areas Function:
2. Adjacent minor defects may be included with the bevel 3. Esthetics may be improved by creating a gradual transition between restoration to tooth
4. Marginal seal may be enhanced. 11/7/2012
CAVITY DEBRIDEMENT
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Cavity debridement
Wash debris away with water spray.
The cavity is dried with air spray.
Examine the cavity from all aspects to check for defects.
Errors must be corrected.
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Cavity debridement ď Ž ď&#x192;&#x2DC;
Feature: The preparation is clean and dry but not dessicated.
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References
Studervant’s ART AND SCIENCE OF OPERATIVE DENTISTRY pg: 283-319
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