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PRINCIPLES OF CAVITY PREPARATION

Department of Pediatric & Preventive Dentistry

Presented By: Rahul Mishra,

Junior Resident-I, UP Dental College & Research Centre,

Lucknow.

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DEFINITION OF CAVITY PREPARATION

Cavity preparation is the mechanical alternation of a tooth to receive a restorative material, which will return the tooth to proper anatomical form, function, and esthetics. The procedure of the preparing the tooth is the removal of the defective or friable tooth structure. Any remaining infected or friable tooth structure may result of further carious progression, sensitivity or pain or fracture of the tooth and / restoration.

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Cavity preparation is the mechanical alternation of defective, injured or diseased tooth in order to best receive a restorative material that will reestablish a healthy state for the tooth including esthetic correction when indicated, along with normal form and function.

The reason of the need for restoration as follow:

• To restore the integrity of the tooth surface.• To restore the function of the tooth.• To restore the appearance of the tooth.• To remove the diseased tissue from the tooth.

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OBJECTIVES OF CAVITY PREPARATION

• To remove diseased tissue as necessary and at the same time provides the protection to the pulp.• To locate the margins of the restoration as conservative as possible.• To ensure the cavity form, it should not be under the force of mastication of the tooth.• To allow the functional placement of the restorative material.

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PRINCIPLES OF CAVITY PREPARATION

• Gain access to caries.• Removal of all carious lesions.• Cut away all significantly unsupported enamel.

• Extended margins so that they are accessible for instrumentation and Cleaning.

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CLASSIFICATION(G.V. BLACK CLASSIFICATION)

Black suggested that it was necessary to

•Remove additional tooth structure to gain access and visibility .•Remove all trace of demineralized enamel and dentin from the floor, walls and margins of the cavity.•Make room for the insertion of the restorative material in sufficient bulk to provide strength.•Provide mechanical interlocking retentive designs.•Extend the cavity to self-cleansing areas to avoid recurrent caries.

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CLASS IThe lesions involving the occlusal

surfaces of molars and premolars, the occlusal 2/3 of buccal and lingual surfaces of molars, and the palatal pits in anterior teeth.CLASS II

The lesions involving the proximal surfaces of the posterior teeth with access established from the occlusal surface.CLASS III

The lesions involving the proximal surfaces of anterior teeth which may or may not involve a labial or a lingual extension & not involving incisal edge.

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CLASS IV The lesions involving all proximal

surfaces of anterior teeth which involves the incisal edge.

CLASS V The lesions involving the cervical third

of all teeth, including the proximal surface of posterior teeth where the marginal ridge is not included in the cavity preparation.

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ACCORDING TO SITE INVOLVED

Site 1 . Pits, fissures and enamel defects on occlusal surfaces of posterior teeth or other smooth surfaces.Site 2 . Approximal enamel in relation to areas in contact with adjacent teeth.Site 3 . The cervical one third of the crown or, following gingival recession, the exposed root surface.

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ACCORDING TO THE SIZE AND EXTENT OF THE LESION AT THE TIME OF IDENTIFICATION

Size 0 . The earliest lesion that can be identified as the initial stages of demineralisation. This needs to be recorded but will be treated by eliminating the cause and should therefore not require further treatment,Size 1 . Minimal surface cavitation with involvement of dentine just beyond treatment by remineralisation alone. Some form of restoration is required to restore the smooth surface and prevent further plaque accumulation,Size 2 . Moderate involvement of dentine following cavity preparation. Remaining enamel is sound, well supported by dentine and not likely to fail under normal occlusal load. The remaining tooth is sufficiently strong to support the restoration,

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Size 3 . The lesion is enlarged beyond moderate. Remaining tooth structure is weakened to the extent that cusps or incisal edges are split, or are likely to fail if left exposed to occlusal load. The cavity needs to be further enlarged so that the restoration can be designed to provide support to the remaining tooth structure,

Size 4 . Extensive caries or bulk loss of tooth structure e.g. loss of a complete cusp or incisal edge has already occurred.

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DESIGN AND PREPARATION OF CAVITIES

•The design and preparation of cavities are based on Black’s principles that have been determined and re-applied with importance directed towards protection of tooth in preparation rather than only on the material.• Each diseased tooth has an individual cavity form determined by caries involvement, morphology of tooth and its location in oral cavity – leading to new conservative cavity designs.

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STEPS IN THE CAVITY PREPARATION(Given by G V Black)

Obtaining Outline FormObtaining Primary Resistance FormObtaining Primary Retention FormObtaining Convenience FormRemoval of Remaining Carious DentinObtaining Secondary Resistance & Retention FormProviding Pulp ProtectionFinishing of Enamel Walls & MarginsPerforming the Toilet of the Cavity

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ARMAMENTARIUM

• Basic Instruments – mouth mirror, explorer, tweezer, etc.• Hand Instruments – Excavators, enamel hatchet, monoangle or biangle chisels, Gingival Marginal Trimmer.• Rotary Instruments –

• Burs No.-carbide burs 55, 56, 57.• Ultraspeed and conventional speed contrangle handpiece.

• Safety glasses.

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These are pit and fissure type cavities that involve the occlusal surfaces of molars and premolars, the occlusal 2/3 of buccal and lingual surfaces of molars, and the palatal pits in anterior teeth.

Definition:

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These are self-cleansable areas. However, they may get involved by caries due to their inherent defective structure. These areas are retentive for food and thus invite caries.

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1.A small surface opening which may remain unnoticed until the lesion becomes of a considerable size.

2.A conical spread in both enamel and dentin, with the bases of cones at the Dentinoenamel Junction.

3.It is rapid burrowing at the dentinoenamel junction. These lesions may involve one or more surfaces and hence a simple or compound cavity should be prepared.

These lesions are clinically characterized by:

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Designing the Outline Form

The outline form of a routine class I cavity should describe a symmetrical design running in sweeping curves along all pits, fissures, and angular grooves between the cusps and with a minimum width.

Simple occlusal cavities

Marginal ridge walls should be 1/2 distance from mesial and distal pit to the crest of each marginal ridge and in

a direction parallel to these ridges.

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The mesial and distal wall should have a slant or slight divergence from the pulpal floor outward to avoid undermining the marginal ridges.

PERPENDICULAR IN MESIAL-DISTAL DIRECTION

•Pulpal Floor mesio-distally is flat and perpendicular to the long axis of the tooth

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In a bucco-lingual direction, the cavity is extended just sufficient to eliminate the defective and susceptible tissues. The lingual and the buccal wall should be parallel to the respective tooth surface.

•INTERNAL FORM

•Buccal and Lingual Walls are Parallel to each other and to the Long Axis of the Crown (Provides retention) •Buccal, Lingual, and Proximal Walls meet Pulpal Floor at sharp angle •Buccal and Lingual Walls meet Proximal in smooth, rounded form.

BUCCO-LINGUAL VIEW

LONG AXIS OF

THE TOOTH

LONG AXIS OF

THE CROWN

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It must be reemphasized that the outline form for class 1 cavities should be very conservative since they involve cleansable areas.

It is governed only by the extent of caries in both enamel and dentin and the amount of extension or need to eliminate pits and fissures to secure smooth margins.

Isthmus just wide enough to accept instrumentation

1/4th intercuspal distance

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CORRECT OUTLINE FORM

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The resistance form here consists chiefly of a pulpal wall parallel to the occlusal plane with dentin walls at right angles to it., i.e. Boxing the preparation.

The form of this cavity provides automatically for effective retention and, therefore, no special retentive features are required.

Obtaining the Resistance and Retention Forms

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In small size cavities, the carious dentin should have been removed during making the cavity extensions.

In moderately deep and deep cavities, the carious dentin is peeled off carefully at the sides using large spoon excavators, and then scooped out in few and large pieces.

Only light pressure in a direction parallel to that of the pulp is utilized. This is continued until a sound dentin floor is reached.

Removal of remaining Carious Dentin

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The enamel walls of the cavity should be finished free from any loose, short, or undermined enamel, and trimmed to meet the tooth surface at a right cavo-surface angle.

This may be done by sharp and regular edged chisels and hatchets, plane fissure burs, stones, or sand-paper discs.

All sharp corners in enamel must be rounded, as they may contain short enamel rods.

Planning of Enamel Walls

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A sharp explorer is then used to check the details of the prepared cavity and to loosen the tooth debris which are then blasted out with warm air.

Performing of the toilet of the cavity

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The outline form is performed by first gaining access through the enamel to the carious dentin floor of the cavity followed by making the necessary cavity extensions.

Procedure

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• In case of initial carious lesions, access is obtained by employing a small round bur #330.

• In big carious lesions, access is obtained easily by breaking down the undermined enamel overlying the carious dentin, using a suitable size chisel.

• In either case, access is started at the most defective area of enamel, i.e., a carious pit or fissure.

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The bur is held at a right angle to the involved surface of the tooth and light pressure in an in-and-out direction is exerted. Cutting is continued until the Dentinoenamel Junction is reached.

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The necessary cavity extensions through pits, fissures, and deep developmental grooves are made using a #330 round bur held atRight Angle To The Surface Of The Crown.

The bur is rotated, and carefully introduced through the opening just obtained, so that its weak corners do not touch the enamel and get dulled.

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With the bur seated in the cavity just below the dentinoenamel junction 1/2-1 mm. Gentle pressure is applied in the direction of required extension.

During cutting, the bur should be kept moving in-and-out of the cavity and at right angle to the tooth surface. In this way, the bur will undermine and lift the cut enamel, and at the same time unclog itself.

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Provision of ample resistance and adequate retention through boxing of the preparation could be obtained.

This is obtained by using a #56 fissure bur held perpendicular to the surface of the tooth. All the line angle in dentin must be squared up by help of the HOE Excavators.

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•Deepest or most carious pit entered with a punch cut using No. ½ round bur or No.245 inverted cone bur oriented perpendicular to long axis of tooth.

•Depending on cuspal incline, depth of prepared external walls is 1.5 – 2 mm and 1.5mm pulpal depth measured at central fissure.

•Desired pulpal depth – 0.1 to 0.2 mm into dentin.

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•Maintaining depth, cavity extended to include all defective supplemental and developmental grooves (No.57 plain St. fissures carbide bur).

•Isthmus width of 1/4th intercuspal distance so that it does not reduces the strength of tooth (Diameter of bur should be considered).

•If fissure extends farther into marginal ridge, slight occlusal divergence is given, to prevent undermining of marginal ridge & to provide dentinal support.

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•Pulpal floor remains at initial ideal depth, relatively flat, in dentin and provides a strong stable seat for restoration.

•Enameloplasty done on terminal ends of shallow fissures to conserve tooth structure.

•Final tooth preparation includes removal of remaining defective enamel / infected dentin, pulp protection and finishing of external walls accomplished with hand instruments.

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•The outline of these cavities usually describes a triangle with its base facing the gingival wall and its sides forming the mesial and distal walls.

•The gingival wall is placed at or slightly occlusal to the height of contour of the tooth.

Buccal Pit Cavities

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OUTLINE FORM FOR PIT RESTORATIONS

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All walls are extended just enough to eliminate defective enamel and dentin.

The enamel walls are planed in the direction of enamel rods and perpendicular to the axial wall.

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Hoe excavators are used to smooth the axial wall and make it parallel with the external surface of the tooth.

It should be re-emphasize that the shape of the cavity will be governed by the extension of caries, accordingly the outline of these cavities may be a rounded or oval in shape.

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Buccal and Lingual Extensions

In case of occluso-buccal and occluso-lingual cavities extensions are made through the fissures and towards the respective surfaces.

The cutting is done in dentin at the dentinoenamel junction using a #56 bur until the occlusal ridge is undermined and removed.

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If the caries is still gingival to the level of the pulpal seat, a step is indicated: a #330 or 56 bur is used to cut the dentin at the dentinoenamel junction, applying pressure in a gingival direction and at the same time moving the bur mesio-distally.

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The enamel thus undermined, is broken down with chisels.

Retentive grooves are then made in dentin along the axio-mesial and axio-distal line angles. The cavity walls and margins are finished as previously described.

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In case of deeply-seated caries, where removal of the carious dentin will leave a round cavity floor, flattening of which to obtain the required resistance form, will expose the pulp.

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The following technique is used:

a)The cavity floor is covered with a sub base followed by a base or base alone which fills it to the routine cavity depth.

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b)A ledge is cut on the expense of the buccal and lingual side walls of the cavity for obtaining the required resistance in sound dentin.

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I. OUTLINE FORM – Smooth flowing, regular curves.

Angular irregularities in the outline are susceptible to fracture during condensation – a smooth flowing outline is easier to visualize and carve following condensation.

PrinciplesRationale

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II. EXTENSIONS (Extension for Prevention) Conservation of tooth structure is the basis for all cavity preparations in order to preserve the strength of the tooth.However, sufficient extension of cavity preparations is necessary to ensure access (convenience form) for instrumentation, removal of defective tooth structure, insertion and finish of the restorative material, and maintenance of the restoration (prevention).

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A. Extensions consist of:a. Caries and

decalcificationsb. Enamel unsupported by

sound dentinc. Pits and fossaed. Major fissures and

groovese. Existing restorations

eliminates defective tooth structure and eliminates areas (pits, fissures, etc.) which are susceptible to recurrent caries and facilities oral hygiene procedures (extension for prevention).

PrinciplesRationale

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B. Bucco – lingual extension

1. Extend fully in areas of buccal and lingual grooves to terminate on smooth surfaces.

to allow a smooth tooth-restoration margin to be created (easier to finish and keep clean).

PrinciplesRationale

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2. Extend minimally in areas of triangular ridges (optimal isthmus width is ¼ intercuspal distance or less) terminating on smooth surfaces.

to preserve the strength and function of the cusps while eliminating susceptible grooves or defective tooth structure (must be wide enough to allow condensation).

PrinciplesRationale

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C. Mesio-distal extension

1. Stop short of the marginal ridge crest

Marginal ridge walls should be 1/2 distance from mesial and distal pit to the crest of each marginal.

2. Parallel to the contour of the marginal ridge.

to preserve strength of marginal ridges.

to preserve a uniform bulk (strength) to the mariginal ridges.

PrinciplesRationale

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3. Groove extensions are kept narrow (mesio-distally) where possible terminating on smooth tooth structure.

to preserve strength of cusps while eliminating susceptible grooves and/or defective tooth structure (must be at least as wide as the narrowest condenser).

PrinciplesRationale

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4. If marginal ridge is unsupported or very thin it should be included, resulting in a Class II preparation.

If not included the marginal ridge may fracture. (amalgam will be stronger than the unsupported enamel)

PrinciplesRationale

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A. Depth = 0.1-0.2mm into dentin (approx. 2 mm measured at triangular ridges).

Minimum depth is required to provide sufficient bulk to prevent fracture and retain the amalgam.

PrinciplesRationale

III. RESISTANCE/ RETENTION FORM

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B. Pulpal floor

1. Smooth and flat

2. Parallel to the occlusal plane

Uniform thickness of restorative material.

resists occlusal stress (resistance form) and forces of condensation.

PrinciplesRationale

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C. Buccal and lingual walls

1. Smooth and curved mesio-distally.

2. Smooth and straight pulpo-occlusally.

Facilitates adaptation of amalgam and elimination of weak tooth structure.

PrinciplesRationale

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3. Converge slightly pulpo-occlusally in areas of triangular ridges (60).

To provide mechanical lock or retention to the occlusal portion and create bulk at the margins.

PrinciplesRationale

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4.Diverge slightly pulpo-occlusally in buccal and lingual groove extensions (60).

protects buccal and lingual surfaces from being undermined (RESISTANCE FORM).

PrinciplesRationale

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D. Mesial and distal wall

1. Smooth and straightfacilitates adaptation of amalgam and elimination of weak tooth structure.

PrinciplesRationale

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2. Diverges slightly pulpo-occlusally (forms an obtuse angle with pulpal floor).

protects marginal ridge from being undermined or weakened (enamel must be supported by dentin)

PrinciplesRationale

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IV. CAVITY FINISH

A. Pulpo-occlusal line angle is well defined (no point angles are present) and follows general configuration of cavosurface outline.

increases retention of the amalgam restoration and preparation is more easily visualized.

PrinciplesRationale

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B. Cavosurface margins

1. Sharp (well defined)

2. Sound (well supported)

easier to visualize and carve.

provides marginal integrity.

PrinciplesRationale

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C. Cleanliness – cavity is free of debris and moisture.

facilitates adaptation of amalgam to the cavity and improves the physical properties of the restoration by elimination of void or foreign material.

PrinciplesRationale

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V. TISSUE RESERVATION

A. Rubber dam is intact

B. Adjacent tooth structure and restorations are intact

C. Adjacent soft tissue (periodontium) is intact

preserves isolation, eliminates moisture.

conservation of tooth structure.

prevention of post-operative pain and inflammation.

PrinciplesRationale

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REFERENCES

1. ART & SCIENCE OF OPERATIVE DENTISTRY- STURDEVANT.

2. OPERATIVE DENTISTRY- WEINE3. OPERATIVE DENTISTRY- MARZOUK4. DENTISTRY FOR THE CHILD AND ADOLESCENT-

MCDONALD.5. ESTHETICS IN DENTISTRY- GOLDSTEIN.6. CLASSIFICATION & CAVITY PREPARATION FOR

CARIOUS LESION- G J MOUNT & W R HUME. 7. MINIMALLY INVASIVE DENTISTRY- JADA, Vol. 134,

January 20038. CARIES PREVENTION CURRENT STRATEGIES- NEW

DIRECTIONS- JADA, Vol. 127, October 1996