Steps in teeth cavity preparation -Part 5 Final finishing
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. Support 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.
The strongest enamel margin is that margin which is composed of full length of enamel rods that are supported on the cavity side by short enamel rods, all of which extend to sound dentin.
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
• Removal of all enamel and dentin chips due to excavation and grinding with warm water.
• Drying with air syringe.
• 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.
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