Outline Form 2a1p4j

OUTLINE FORM:

G .V. Black has described the outline form as being “the form of the area of the tooth surface to be included within the outline or enamel margins of the finished cavity”. It is the placement of cavity margins in the positions they will occupy in the final preparation, except for finishing enamel walls and margins and preparing an initial depth of 0.2 – 0.8mm pulpally of DEJ or normal root surface position. Two outline forms can be appreciated 1.

External outline form-which dictates the external perimeter of the outline form.

2.

Internal outline form-which dictates the inner dimension and detail of the cavity.

The factors that determine outline form are the following:

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Extent of the carious lesion.

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Margins should be paced in easily cleansable areas.

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Extend the cavity margins into fissures that cannot be eliminated by appropriate enameloplasty.

Extend the cavity margin until sound tooth structures obtained and no uned enamel remains. Average depth of the cavity should be 0.5 mm into dentin.

PIT AND FISSURE CAVITIES

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The general factors mentioned above are to be followed. In addition the following are considered:

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Avoid terminating the margin on extreme eminences such as cusp heights and ridge crests.

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In case of conservative cavity preparation shallow supplemental grooves and fissure crossing lingual or facial

Circumventing of cusps should be followed resulting in a smooth free flowing outline form. ridge can be eliminated by enameloplasty.

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When two pit and fissure cavities have been separated by less than 0.5 mm of sound tooth structure, they should be ed to eliminate a weak enamel wall between them.

PROXIMAL SMOOTH SURFACE CAVITIES Occlusal cavity outline is same as above. Gingival margins According to G.V.Black’s concept the gingival margin should be placed 0.5 to 1.0 mm apical to the crest of healthy free gingival. Because this area was thought to be sterile due to the alkalinity of crevicular fluid and less chances of food impaction in this area, moreover the knife edge relationship of the healthy free gingival to the adjacent tooth surface will discourage food accumulation on the adjacent restored surface occlusal to the sulcus for considerable periods during and after food ingestion. But, Dr. John Me Call has shown that mechanical causes, systemic conditions and traumatic occlusion change the alkalinity of the crevicular fluid to an acidic form making it more prone to aciduric environment. Emphasis is made to place the margin occlusal to or just clear of the margin of the gingival crest. Facial margin and lingual margin The following factors govern them:

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Flare and mesiodistal width of embrasures.

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Wider and more flared the faciolingual embrasures are, less are the chances of food accumulation and therefore requires less extension facially and lingually.

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Caries index and oral hygiene.

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Faciolingual extension of the cavity in the corresponding embrasure is directly proportional to the caries index.

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The more ideal the relationship between the adjacent and opposing teeth is, the better is the cleansability of the

Occlusal and masticatory forces. facial and lingual embrasures. Therefore, less extension is required.

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Age of patient and tooth structure and attrition of areas.

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An older tooth has higher fluoride content, is more resistant to caries and requires less extension.

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More convex restorations lead to wider embrasures and therefore less extension and better self cleansable

Creation of a more convex restoration. areas.

Gingival margin is placed as in class II cavities. In young patients the margin is covered by the gingival whereas in patients with gingival recession the margins are kept supragingival. The mesiodistal extension is obtained considering the occluso-gingival convexity, occlusion and masticatory forces, caries index, and age of the patient. Occlusal extension should not include the height of contour of facial and lingual surfaces. MODIFICATIONS DUE TO THE TYPE OF RESTORATIVE MATERIAL AMALGAM: With the introduction of modern alloys, the modern concept of cavity preparation can be compared to the conventional G V Black’s design which are less time consuming, conserving and with decreased chances of failure. CONVENTIONAL OCCLUSAL PORTION- extension for prevention, mortise shaped with definite line angles and point angles. PROXIMAL PORTION- either box type or truncated cone. ISTHMUS-will not exceed 1/3rd intercuspal distance, atleast 1.5mm. SWEEPING CURVES-given to the proximal and occlusal walls where they meet. ACCESSORY RETENTION- proximal grooves are not so critical. MODERN OCCLUSAL PORTION:

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More conservative approach. Here G V Black’s concept of extension for prevention does not apply.

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Going around the cusps to conserve tooth structure.

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Using enameloplasty on the terminal ends of shallow fissures to conserve tooth structure.

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Margins of the facial and lingual class I involve the entire facial and lingual groove to avoid feather edged

Not extending the facial and lingual more than midway between the central grooves and cusp tips.

marginal amalgam. PROXIMAL PORTION: Only a unilateral inverted truncated cone on functional side of marginal angles to preserve tooth structure. The gingival margin should be located occlusal to the height of contour. ISTHMUS: Not to exceed 1/4th intercuspal distance. SWEEPING CURVES: Universal sweeping curve is exaggerated and more bulk is accommodated. ACCESSORY RETENTION: Proximal grooves are used more often due to limited occlusal width.

TOOTH COLORED RESTORATIONS Emphasis is more on conservative tooth preparation. G.V.Black’s concept of “extension for prevention” does not apply. With the introduction of acid etching phenomena by Buonocore, coupled with the use of synthetic resins has revolutionized the concept of cavity design. Butt t marginal configuration and placement of floor in dentin for retention were the characteristic features of conventional method. The modern concept is to include all faulty areas and conserve more tooth structure. Floor is not routinely placed in dentin. It depends on the caries extent and depth. Bevel is given on the enamel cavosurface margin. As in the conventional types, when a butt t is made, a white halo is seen after the restoration is complete. This can be avoided by beveling the margin. The ends of the enamel rods are more effectively etched than when only the sides are exposed to the etchant. In case of composite resins the area should be maintained, left untouched as this material cannot maintain integrity, these materials can accommodate undermined enamel as long as it is not carious, discolored directly loaded in centric or eccentric or cracked. CAST GOLD RESTORATION The main characteristic feature in these restorations is taper and bevel incorporation in the outline form. The gingival to occlusal divergence of these cavity walls may range from 2 to 5 degree taper on each wall. The taper should be minimum in shallow cavities for better retention and resistance. The taper should be more in deep cavities for proper seating of the restoration. The taper prevents the undisturbed withdrawal of the wax pattern and subsequent seating of the casting. Resistance form may be defined as that shape and form of cavity walls that best enable both the restoration and the tooth to withstand occlusal forces without fracture. Fundamental principles involved are: 1.

Box shape or mortise shaped with flat floor, which helps the tooth to resist occlusal loading by virtue of being at right angles to the forces of mastication.

2.

Slightly curved than acute line angles decrease the stress concentration of stresses and hence reduce the incidence of fracture.

3.

Conservation of strong cusps and ridges with sufficient dentin . Weakened areas should be included in cavity preparation to prevent fractures (capping of the weakened cusps).

4.

To provide enough thickness of restorative material to prevent fracture under load.

5.

Slight roundening of the line angles to prevent stress concentration.

STRESS PATTERNS OF TEETH According to Gabel application of mechanical principles to the design of restorations will help conceive favorable stress patterns for the teeth and the restorations. These principles vary according to the type of restoration and cavity. TYPE OF RESTORATION: The minimal thickness of amalgam and cast gold to resist fracture is approximately 1.5mm, though a little more depth is required for amalgam to achieve the requisite bulk. However in composite and glass ionomer, the depth is not the criteria for achieving resistance form. Porcelain also requires a depth of 2mm for inlays and 1.5mm for crowns. TYPE OF CAVITY: CLASS I: A flat pulpal floor is appropriate. In case of deep caries where a rounded pulpal floor may result, the stress is doubled in the deepest portion of the cavity. Fractures in these rezsstorations are due to insufficient dentinal thickness in the

center. Bending stresses are proportional to square of depth. Therefore for large restorations depth should be increased with increase in diameter. CLASS II: A proximo-occlusal inlay restoration acts like a curved beam of cantilever type. Due to differences in modulus of elasticity of dentin and the material there will be displacement of the restoration in the gingival seat area with the axiopulpal line angle as axis of restoration. This is prevented by a lock in the form of groove pins, etc in the gingival floor. In M.O.D. cavity axio-pulpal line angle should be more rounded. CLASS III: Due to the thickness of incisal edge the cavity is extended lingually as close to the incisal edge as possible. CLASS V: The functional cusp and functional fossa relationship dictates the stress pattern. EFFECT OF GROOVES: Grooves provide resistance to a certain degree. Courdadee and Jimmerman have shown that localized areas of stress are produced in tooth tissue by provision of supplemental intracoronal retention in the form of pins. Primary Resistance Form Definition: Primary Resistance form is that shape and placement of the cavity walls to best enable both the tooth and restoration to withstand, without fracture the stresses of Masticatory forces delivered principally along the long axis of the tooth. Resistance form is the design of a cavity in such a way that the remaining tooth substance and the restorative material can withstand masticatory stress. To achieve this ,the prepared cavity should possess the following 6 attributes discussed below: 1.

Flat Floor

2.

Adequate bulk of the restorative material

3.

Absence of weak cusps or marginal ridges

4.

Occlusal cavity margins in areas not subjected to excessive occlusal trauma . In practice one-quarter (1/4) of the intercuspal width

5.

Flat floor at right angles to the line of stress

6.

Walls of the cavity parallel to the direction of the stress

1) Flat floor: A flat pulpal floor should be given while the cavity is being prepared to avoid unwanted stresses and forces on the pulpal floor.

Flat Pulpal Floor

2) Adequate bulk of the restorative material: Bulk of the material should be 1.5-2mm the prepared cavity should be deep enough to take adequate bulk of the restorative material capable of withstanding masticatory stress. The bulk required will depend on the flexural strength of restorative material. In the case of amalgam it is estimated that a minimum of 1.5-2mm thickness of the restorative material is required to withstand masticatory stress. 3) Absence of weak cusps or marginal ridges: After cavity preparation, the tooth should not be left with any weak cusps or marginal ridges. Any weak cusp must be removed and restored with a metallic restorative material, such as silver amalgam or dental gold. If a marginal ridge is found to be too weak in the cause of an occlusal cavity preparation, a Class II cavity may have to be prepared instead, so as to eliminate the weak marginal ridge. This is particularly indicated where the ridge is only of enamel thickness and uned by sound dentine 4) Occlusal cavity margins in areas not subjected to excessive occlusal trauma . In practice one-quarter (1/4) of the intercuspal width The cavity should be designed that the occlusal margins of the cavity are in areas not subjected to excessive occlusal trauma, otherwise the enamel wall of the cavity and/or the margins of the restorative material may fracture. In practice, this may be achieved by placing an occlusal margins of a cavity about one-quarter (1/4) of the intercuspal

distance. Note, that efforts should always be made to conserve sound tooth tissue.

5) Flat floor at right angles to the line of stress The floor of prepared cavity should be flat and right angles to the line of occlusal stress, which is usually in the direction of long axis of the tooth. Sound tooth tissue should, however, not be removed simply to obtain a flat pulpal floor of prepared cavity 6) Walls of the cavity parallel to the direction of the stress To achieve this , the walls of the cavity are prepared parallel to the corresponding tooth surfaces. Fundamental principles involved to Obtain Primary Resistance Form are: 1.

Box shape or mortise shaped with flat floor, which helps the tooth to resist occlusal loading by virtue of being at right angles to the forces of mastication.

2.

Slightly curved than acute line angles decrease the stress concentration of stresses and hence reduce the incidence of fracture.

3.

Conservation of strong cusps and ridges with sufficient dentin . Weakened areas should be included in cavity preparation to prevent fractures (capping of the weakened cusps).

To provide enough thickness of restorative material to prevent fracture under load. RETENTION FORM: Retention form is that form of cavity that best permits the restoration to resist displacement through tipping or lifting forces, especially masticatory loading. PRINCIPAL MEANS OF GAINING RETENTION:

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Inverted truncated cones or undercuts.

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Elastic deformation of dentin e.g. gold foil.

Dovetail. Friction – which depends on surface area, opposing walls or surfaces involved, parallelism or non-parallelism, proximity of material to walls.

OTHER MEANS: 1.

Grooves: Mainly indicated in cast restorations, they are prepared in the walls of the proximal box inside the DEJ. The depth is equal to the width. They are placed at the axiobuccal and axiolingual line angles, putting

more pressure on the buccal and lingual walls rather than on the axial wall. They are prepared parallel or slightly diverging occlusally for the proper withdrawal of the pattern. 2.

Internal boxes

3.

Posts

4.

Pins: Provide extra retention and can be used with amalgam, composite and cast restorations.

5.

Triangular areas (gold foil)

6.

Etching: Provides micromechanical retention by increasing the surface area for retention. This is mainly indicated in bonded restorations.

7.

Slots: They are prepared in dentin to increase the surface area of the preparation and have more convergent walls. These are 1 – 1.5mm deep box type preparations and can be given 9in occlusal wall or gingival wall or both. Usually they are given all along the width of the occlusal / gingival wall. Each of it has four walls that aids in retention.

8.

Locks: They are given in proximal box of class II cavity and are indicated mainly for amalgam restorations. They are given in dentin either in the walls of the proximal box or in the occlusal box at the line angles which the pulpal wall of the cavity makes with the occlusal wall. These are 0.2 to 0.3 mm wide and 0.5mm deep into dentin.

9.

Skirts: Mainly indicated in cast restorations, they are extensions of the proximal box at the line angles of the tooth or even away from it. The margins of the restorations are kept on healthy tooth structure and bevelled. The enveloping of the walls increases the surface area and aids in retention.

Retention techniques in Different Restorative materials: AMALGAM: Retention is enhanced by 1.

Parallel walls and flat pulpal floor or gingival floors

2.

Occlusal convergence of walls (axial retention) in class of class II.

3.

Occlusal convergence and dovetail

4.

Proximal retention in the from of axiofacial and axiolingual locks

5.

Slots in gingival floor.

CAST GOLD RESTORATIONS Axial retention in the form of cement locking and friction in micro-irregularities. This is enhanced by 1.

Parallelism: A slight divergence of the walls 2 degrees-5 degrees can be given for proper withdrawal of the pattern. In case the available height of the walls is less, the divergence should be kept minimum. At least one half of the walls should be kept parallel and the rest can be diverged.

2.

Increase in area.

3.

Occlusal extension is mandatory since it prevents tilting of the restoration.

4.

Increase in strength of cementing media.

5.

Sharp line angles except axiopulpal line angle.

6.

Lateral retention by dovetail and pinholes and pot holes.

TOOTH COLORED RESTORATIONS Retention is achieved by 1.

Acid conditioning.

2.

Retentive cavity preparation.

3.

Physico-chemical retention.

4.

Posts.

DIRECT GOLD Retention is established by 1.

Elastic compression developed in dentin because of condensation.

2.

In classIII-undercut at point angles.

3.

In class IV-grooves along occlusopulpal and gingivopulpal line angles.

4. 5.

Slight roundening of the line angles to prevent stress concentration.

CONVENIENCE FORM Definition: Convenience form is defined as that form of cavity preparation that allows adequate observation, accessibility and case of operation in preparing and restoring the cavity. Modification in tooth preparation for convenience form:

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Occlusal step in classII.

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Occlusal divergence of cavity walls in cast restorations.

Labial/lingual access for classIII/classIV.

INSTRUMENT MODIFICATION:

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Contra angling

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Bayoneting Addition of angles to the shank of the instrument.

SEPARATION:

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Wedging of teeth.

OF ENAMEL RODS:

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All enamel walls should be ed by sound dentin.

CAVOSURFACE ANGLES FOR DIFFERENT RESTORATIVE MATERIALS 1.

Amalgam-90 degree cavosurface

2.

Inlay- beveled cavosurface (20-40 degrees)

3.

The margins should always should be always located on self-cleansing areas. They should be in smooth curves.

INSTRUMENTATION Low speed with tungsten carbide burs are preferred for finishing cavity walls and the margins, as there is lessening of tactile sense and rapid removal of tooth structure with high speed.

REMOVAL OF REMAINING CARIOUS DENTIN OR OLD RESTORATIONS: It is the elimination of any infected carious tooth structure or faulty restorations left in the tooth after initial cavity preparation. Caries left in the pulpal/ axial floor is excavated thoroughly making the cavity deeper. 0.75- 1 mm of dentin should cover the pulp. All the infected/ soft dentin should be removed. Old restorative material should be removed if: 1.

The material may negatively affect the esthetic result of the new material.

2.

It weakens the needed retention.

3.

Secondary caries is present.

4.

Tooth pulp is symptomatic.

5.

Periphery of the old restorative material is not intact.

PULP PROTECTION: Though not taken in principles of cavity preparation it is one of the important steps before final restoration. Pulpal injury can result due to: 1.

Heat generated by injudicious cutting.

2.

Restorative material with good thermal conductivity.

3.

Chemical from the restorative materials.

4.

Galvanic currents

5.

Microleakage.

Liners and bases are used fro pulp protection. Liners are volatile or aqueous suspensions or dispersion of zinc oxide or calcium hydroxide that can be applied to the cavity surface in a relatively thin film. Bases are those cements, which are applied, in thicker dimensions beneath permanent restorations to provide for mechanical, chemical and thermal protection of the pulp.

FINISHING OF THE CAVITY WALLS: It is the further development, when indicated, of a specific cavosurface design and degree of smoothness that produces the maximum effectiveness of the restorative material being used. OBJECTIVES: 1.

To create the best marginal seal possible between the restorative material and the tooth structure.

2.

Afford a smooth marginal junction.

3.

Provide maximum strength of both tooth and the restorative material at and near the margin.

Factors to be considered: 1.

Direction of enamel rods.

2.

of enamel rods at DEJ and cavity margins.

3.

Type of restorative material to be used.

4.

Location of the margin.

5.

Degree of smoothness desired.

6.

The strongest enamel margin is that margin which is composed of full length of enamel rods that are ed on the cavity side by short enamel rods, all of which extend to sound dentin.

7.

Line angles formed by the junction of enamel rods should be rounded whether acute or obtuse.

FEATURES: There are two primary features related to the finishing of enamel walls: 1. The design of cavosurface angle: For amalgam the cavosurface angle should be 90 degrees due to low edge strength of amalgam. However when extending the facial and lingual walls in treating extensive occlusal caries, tilting the bur is often indicated to conservatively extend the margin and provide a 90 – 100 degrees cavosurface angle. For cast gold/ metal/ composite restorations bevelling of external walls is done. Margins of gold foil restorations form a cavosurface angle much less

obtuse than for gold / metal castings and composites. The bevel of the cavity margin in preparation for castings should produce a cavosurface angle of 30 – 40 degrees marginal metal. Providing a 30 degrees bevelled metal will provide with a sliding lapp type fit that definitely improves adaptation of metal to tooth at this margin. 2. The degree of smoothness of wall: The advent of high-speed cutting procedures ahs produced two pertinent factors related to finishing enamel walls (i) lessening of tactile sense (ii) rapid removal of tooth structure. Plain cut fissure burs produce the finest surface. The prepared wall of inlay/ onlay requires a very smooth surface to permit undistorted impressions and close adaptation of the casting to enamel margins. In amalgam, goldfoil, composite, a very smooth surface is not desired as it decreases the retention.

TOILET OF THE CAVITY: This is the act of freeing the preparation walls and margins from objects that may interfere with proper adaptability and behavior of the restorative material. It is accomplished by 1.

Removal of all enamel and dentin chips due to excavation and grinding with warm water.

2.

Drying with air syringe.

3.

Sterilization.

Englander et al have shown that silver nitrate and alcohol cause irreparable pulp damage if these are allowed to enter into the dentinal tubules. Shay, Allen et al have shown that ZnOE, Ca(OH)2and fluoride content in some restorative material s show certain amount of protection even on unsterilized condition of cavities. SMEAR LAYER Later based on research debridement comprised of cleaning the cavity with warm water so as to protect the smear layer formed. Smear layer prevents penetration of bacteria and their products further into the pulp dentin complex through the dentinal tubules. Further treatment with caustic solution damages smear layer.