Definition of cavity:
Is defined as a defect in enamel, dentin or cementum resulting from the pathological processes, mostly the dental caries. Other processes such as abrasion, erosion etc. can also cause such defects.
Cavity preparation:
It is defined as the mechanical and surgical alteration of a defective, injured or diseased tooth in order to best receive a restorative material which will re establish a healthy state for the tooth including aesthetic corrections where indicated along with the normal form and function.
Cavity preparation is the performance of those dental surgical procedures required to expose the carious lesion, permit of removal of affected tissue and so shape the remaining dentin and enamel as to receive a restoration to its original form and function give it strength and prevent re occurrence of decay in the same surface.
History:
Cavity preparation techniques have been known for many centuries. During the ninth century itself cavities have been prepared for dental inlays.
During the early 19th century, cutting was done mainly by hand-operated instruments. Later drills operated by bowstring, Archemedian drives were used. Dr. Samuel Fastlich has suggested that the Mayas used tube drills and bow drills.
During the late 19th century pedal driven engine using flexible cables, hand piece and burs came into existence. Page-Chayas hand piece, the first belt driven angle hand piece to operate successfully at speeds over 100,000rpm was introduced in 1955. In 1957 the first clinically acceptable air turbine hand piece Borden airotor handpiece came into play. Then Air motor, Electric micro motor, straight handpiece, right angled handpiece and contra-angled handpiece were introduced. At the latest, now air abrasion and lasers are being used.
Hamelton Jameson for the first time in the 19th century emphasized the need for organized cavity preparation. He suggested the following steps:
• Removal of soft and infected dentin using hand instruments.
• Sterilization of dentinal surface.
• Retention form using dentinal buttons.
In 1930’s G.V. Black, gave the systemic approach to cavity preparation with special emphasis on retention form. Black’s principles of cavity preparation were specially meant for metallic restorations. Several operators, with the intention of improving aesthetics, had modified some of the original suggestions of Black.
Dr. Charles E. Woodbury suggested that the labial margin of the preparation to be in harmony with the lines of refraction of the labial surface. Henry A. True suggested another inconspicuous type of preparation using a special “slant technique”.
Introduction of new aesthetic restorative materials, particularly during the last two decades have changed the conventional ideas of cavity preparation, probably this is the only area wherein Black’s general principles are becoming obsolute. Conservative cavity designing is done by limiting the preparation of the cavity only to the areas affected.
Classification of cavities:
Based on the type of treatment and the areas involved, Black gave his classification.
CLASS I
All pit and fissure cavities.
IA Cavities on the occlusal surface of molars and premolars.
IB Cavities on the occlusal 2/3rd of the facial and lingual surface of molars.
IC Cavities on the lingual surface of maxillary incisors.
CLASS II
Cavities on the proximal surface of molars and premolars.
CLASS III
Cavities on the proximal surfaces of anterior teeth that do not involve the incisal edge.
CLASS IV
Cavities on the proximal surface of anterior teeth that involve the incisal angle.
CLASS V
Cavities on the gingival third of the facial and lingual surface of all the teeth.
CLASS VI
Cavities on the incisal edge of anterior teeth or the occlusal cusps of posterior teeth.
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Following are some of the objectives that one should keep in mind while preparing a cavity
1.To remove all defects and give the necessary protection to the pulp.
2.To locate the margins of the restoration as conservatively as possible.
3.To form the cavity so that under the force of mastication the tooth or restoration or both will not fracture and restoration will not be displaced.
4.To allow for the aesthetic and functional placement of a restorative material.
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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 support.
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 axio-pulpal 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.
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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
• Inverted truncated cones or undercuts.
• Dovetail.
• Elastic deformation of dentin e.g. gold foil.
• Friction – which depends on surface area, opposing walls or surfaces involved, parallelism or non-parallelism, proximity of material to walls.
OTHER MEANS:
• 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.
• Internal boxes
• Posts
• Pins
Provide extra retention and can be used with amalgam, composite and cast restorations.
• Triangular areas (gold foil)
• Etching:
Provides micromechanical retention by increasing the surface area for retention. This is mainly indicated in bonded restorations.
• 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.
• 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.
• 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.
AMALGAM:
Retention is enhanced by
• Parallel walls and flat pulpal floor or gingival floors
• Occlusal convergence of walls (axial retention) in class of class II.
• Occlusal convergence and dovetail
• Proximal retention in the from of axiofacial and axiolingual locks
• Slots in gingival floor.
CAST GOLD RESTORATIONS
Axial retention in the form of cement locking and friction in micro-irregularities.
This is enhanced by
• 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.
• Increase in area.
Occlusal extension is mandatory since it prevents tilting of the restoration.
• Increase in strength of cementing media.
• Sharp line angles except axiopulpal line angle.
• Lateral retention by dovetail and pinholes and pot holes.
TOOTH COLORED RESTORATIONS
Retention is achieved by
• Acid conditioning.
• Retentive cavity preparation.
• Physico-chemical retention.
• Posts.
DIRECT GOLD
Retention is established by
• Elastic compression developed in dentin because of condensation.
• In classIII-undercut at point angles.
• In class IV-grooves along occlusopulpal and gingivopulpal line angles.
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CONVENIENCE FORM
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:
Occlusal step in classII.
Labial/lingual access for classIII/classIV.
Occlusal divergence of cavity walls in cast restorations.
INSTRUMENT MODIFICATION:
Contra angling
Bayoneting
Addition of angles to the shank of the instrument.
SEPARATION:
Wedging of teeth.
SUPPORT OF ENAMEL RODS:
ALL enamel walls should be supported by sound dentin.
CAVOSURFACE ANGLES FOR DIFFERENT RESTORATIVE MATERIALS
Amalgam-90 degree cavosurface
Inlay- beveled cavosurface (20-40 degrees)
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.
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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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