Fundamentals of Cavity Preparation PDF

CHAPTER (4) FUNDAMENTALS OF CAVITY PREPARATION INTRODUCTION There are a number of general and fundamental principles, which must be realized in preparing cavities for the reception of restorative materials. I- Biological Principles These principles are concerned w...

CHAPTER (4) FUNDAMENTALS OF CAVITY PREPARATION INTRODUCTION There are a number of general and fundamental principles, which must be realized in preparing cavities for the reception of restorative materials. I- Biological Principles These principles are concerned with the health and integrity of the remaining tooth tissues. They include the following: 1.Dentin-Pulp protection. 2.Protection of gingival and periodontal tissues. 3.Prevention of caries recurrence. 1. Dentin-Pulp Protection The pulp should be protected against traumatic, thermal or chemical irritation. The following procedures should be done to avoid: I- Mechanical or traumatic irritation: 1- Avoid unnecessary cutting of enamel, to avoid gross weakening of the remaining tooth substance. 2- Avoid over cutting of dentin and unnecessary deepening of the cavity. A routine depth of cavity is 0.5-1 mm beyond D.E.J. 3- Avoid cutting across the recessional lines of the pulp chamber, it should be in parallel direction. 4- Avoid unnecessary pressure in scooping out soft dentin. N.B. Gentle pressure is applied using sharp excavators. 24 II-Thermal irritation: Minimize heat generation by using sharp, small, unclogged rotary tools at the correct speed ranges, minimal pressure with small area of cutting. Use copious amounts of coolant. III- Chemical irritation: Avoid using caustics or chemical irritants in performing the toilet of the cavity. No cavity sterilization should be done. IV- Bacterial irritation: Use of sterilized instruments. Proper isolation of the operative field. 2. Prevention of Caries Recurrence 1. Removal of all carious enamel and dentin. 2. Proper extension of cavity margins to include all susceptible areas as pits and fissures. 3. Removal of all undermined enamel. 4. The C.S.A. should be compatible with the physical characteristics of the R.M. 3. Protection of Gingival Tissues 1. Controlled cutting to avoid instrument slippage and injury either hand or rotary. 2.Avoid using irritating chemicals. 3.Proper matricing or wedging during cavity preparation to protect adjacent teeth. 25  II- Mechanical Principles Definition: Steps followed during cavity preparation for preservation of the structural integrity of both the tooth and restoration by decreasing the tensile stresses to be within the physical tolerance of these structures. III- Esthetic Principles 1. Margins must be free of any opaque, discolored and pitted enamel. 2. Conservation of maximum amount of tooth structure. Steps of cavity preparation According to G.V. Black, the mechanical steps of cavity preparation are: 1. Outline form. 2. Resistance and retention forms. 3. Convenience form. 4. Removal of remaining carious dentin. 5. Finishing of the enamel walls and margins. 6. Performing of the toilet of the cavity. 1- Outline Form Definition: It refers to the external shape or form of boundaries of the completed cavity. General factors affecting outline forms 1. Carious enamel, should be included in the cavity outline. 2. The marginal enamel rods should be supported by sound dentin. All undermined enamel should be eliminated. 26 3. Pits and fissures are defective, non-remineralizable and retentive areas for food stagnation. Non-carious fissures treated by enameloplasty and Pit and fissure sealants. 4.Cavities, closely approached, should be united into a single cavity to avoid leaving week tooth structure (less than 0.5 mm). 5. The outline form must be in the form of harmonious sweeping curves in order to prevent recurrent caries, avoid stress concentration areas and provide better esthetics. >0.5 mm 6. Oral hygiene: the better the oral hygiene, the lesser will be the extension. 7. Patient age: the older the patient, the lesser will be the extension due to tooth attrition. 8. Severely weakened cusps and ridges should be reduced. 9. CSA should accommodate with the selected restorative material. 27 Special factors affecting outline form Occlusal contact: As much as possible cavity margins be placed at contact free areas. Occlusal anatomy: We should follow the carious pits and fissures. In some cases the anatomy of the occlusal surface is different e.g. wisdom teeth. Outline Forms of Different Class I Cavities 1- Simple Class I: Bucco-lingual extension: ¼ - ⅓ intercuspal distance. Mesio-distally: just beyond the triangular fossa. 2- Compound Class I: Buccal or lingual extension is prepared with a step: if caries is extended beyond the level of the pulpal floor. Buccal or lingual extension is prepared without a step: if caries is stopped at the same level of the pulpal floor. 3- Class I Buccal or palatal pit: Cavity limited to the buccal or the palatal pit without connection to the occlusal cavity. Cavity outline is triangular with its base directed gingivally or determined according to caries extent. Outline Form of Class II Cavities 1- Site and size of contact: Labial, lingual and gingival walls should extend sufficiently to free margins of the cavity (flaring) to facilitate restoration finishing and oral hygiene. 28 2- Width of the embrasure The isthmus outline will follow one of the Ingerham's lines according to the width of the contact area. These lines may be: A) Straight: for cases with open contact between teeth or missing adjacent tooth. B)Uniform or universal curve: In case of narrow contact with wide embrasure (lingual surface) C)Reverse curve: In case of broad contact with narrow embrasure (Buccal surface). Portions of Class II cavities 1- Occlusal portion: - Same as Class I 2- Isthmus portion: - It should be slightly wider than the occlusal and narrower than the proximal. - Follow one of the Ingerham's lines 3- Proximal portion: Extension of buccal, lingual and gingival seat of proximal box to remove all carious tissues and to free the contact. Buccal and lingual walls of the proximal box are parallel to the corresponding surfaces of the teeth. The axial depth is 1.5-2 mm from the external tooth surface to provide enough bulk. Other forms of Class II cavities 1- Simple Class II - In case of absence of adjacent teeth or wide embrasure 29 2- Simple Box In case of proximal caries with sound occlusal surface. 3- Class II without step Class II could be prepared without a step in some clinical cases. This requires the following: No Contact to be freed as in case of missing adjacent teeth or open contact. Proximal caries is totally removed at the same level of the pulpal floor. 2- Obtaining of Resistance and Retention Forms Definition: ❑ Resistance form refers to the form given to the prepared cavity to resist the functional stresses of mastication without fracture of tooth structure and/or restoration. ❑ Retention form refers to the form given to the prepared cavity to prevent displacement of the restoration. Factors Affecting Resistance Form 1. Occlusal loading - Compressive stresses. - Tensile stresses. - Shear stresses. 2. Cavity design a) Walls direction: Should be either parallel or perpendicular to the long axis of the tooth to decrease the analysis of force into destructive tensile components. b) Depth and width: That gives bulk to the restorative material to withstand the occlusal forces. N.B.: Isthmus width should be slightly wider than occlusal portion for brittle amalgam restorations. 30  c) Conservation: Maximum conservation of remaining sound tooth structure to avoid their weakening and fracture. d) Pulpal floor: Should be flat and smooth to avoid stress concentration and to provide equal distribution of occlusal stresses. It also prevents rotation of the restoration under stresses which subsequently results in tooth fracture e) Line angles: Should be rounded without sharpness to avoid stress concentration. f) Amount of retention: Adequate amount of retention for each part of the cavity increases the stability of the restoration under stresses. g) Cavosurface angle (CSA): Should be given the correct inclination according to: a) Type of restoration. b) Direction of enamel rods. For amalgam and composite [CSA 90°], for gold inlay CSA [135 °], to protect enamel rods. Reverse curve: It is formed in the buccal wall of the proximal box (narrow embrasure) to finish the C.S.A 90º to avoid fracture of the restoration. 31  3. Physical properties of restorative materials a) Ductile materials: ▪ Strong and withstand compressive and tensile stresses. ▪ It can be finished into thin margins (beveled CSA). ▪ It protects and support weak tooth structure, e.g Gold. b) Brittle materials: ▪ Very sensitive to tensile stresses. ▪ Need bulk & cannot be finished into thin margin (90ᵒ CSA). ▪ Avoid placement of the margins in areas of high tensile stresses, e.g. Amalgam and Composite. Retention form The retention form refers to the form given to the prepared cavity to prevent displacement of the restoration. I-Chemical retention: e.g. Glass ionomer II-Mechanical retention: Micro-mechanical retention: it uses acid etching and bonding e.g. resin composite. Macro-mechanical retention: added features to the cavity to enhance retention. 32 Types of Macro-Mechanical Retention - Axial retention: It prevents displacement of restoration in occlusal direction. - Lateral retention: It prevents displacement of restoration in proximal or lateral direction. Types of retentive features 1- Undercuts: occlusal convergence of cavity walls prepared for direct restorations (angle 5- 15°). 2-Frictional retention between parallel opposing cavity walls. It depends on the degree of parallelism of opposing walls, their length and roughness. 3-Gripping action of dentin due to its visco-elasticity. 4- Buccal and lingual extension in molars: (Occlusal lock) 5-Dove-tail: prepared in Class II cavities to provide lateral retention. 6- Proximal axial grooves: Prepared in the proximal axial line angles. It is prepared on the expense of buccal and lingual walls rather than axial wall. It provides lateral retention. It starts from the gingival seat up to the level of the pulpal floor. Or fade to the occlusal margin. Prepared using tapered fissure bur. 33 7- Pin holes with pins: They are made from cast or wrought metal. They may be parallel or non-parallel, vertical or horizontal, threaded or cemented. 8-Reverse gingival bevel: Prepared at the gingival floor with gold inlays. It prevents proximal displacement and rotation around axio-pulpal line angles. 9- Cement, or luting agents 10- Inter-radicular Posts: They are made from wrought or cast metal and placed in the root canals. 11- Dentin Ledge It is formation of flat shelves in dentin floor in case of rounded pulpal floor to prevent rotation of restoration. A minimum of 3 dentin ledges should be prepared. 34 Box or Modified Box Form (Mortise Form) 1-Walls are either parallel or perpendicular to the long axis of the tooth. 2-Walls and floors are flat, smooth and plane. 3-Proper convenience to improve manipulation of restoration. 4-Adequate retention by friction and gripping action of dentin. 5- Additional retention could be made easily. 6- Provides bulk to restoration. 7-Prevents wedging action of the restoration on the tooth. 3- Convenience Form It is form given to the prepared cavity making it more conveniently seen, approached and restored. These features include: 1.Accentuation of point and line angles. 2.Slight extension of cavity outline to facilitate insertion of the restorative material. 3.Use of smaller specially designed instruments to prepare surfaces, which are difficult to reach. 4.Mechanical methods as application of tooth separators, gingival retracting cords and rubber dam. 5. Occlusal access to proximal caries. 4- Removal of Remaining Carious Dentin It is the process of removing decay and decalcified dentin after completion of cavity preparation. After inspection, either there is no residual caries present or there is. The routine cavity depth: 1. Pulpal depth: 0.5 - 1 mm beyond the DEJ or 1.5-2 mm the whole cavity depth. 2. Axial depth: 0.5-1 mm beyond DEJ in premolars. 35 Cavity must extend deeper to DEJ in order to: 1. Avoid cutting at this sensitive area i.e. DEJ. 2. To detect lateral spread of caries at DEJ. 3. To provide sufficient bulk of the restoration. 4. To add retentive features if needed. Conditions may be found at routine cavity depth: a) Hard sound dentin: The best condition. b) Hard discolored dentin: it is left as it is considered sound dentin but discolored due to chromogenic bacterial products. In anterior teeth it must be removed as it affects esthetic. c) Soft dentin: it must be removed in order not to extend to involve the pulp. Instruments used to remove caries: A. Rotary instruments, e.g. large round bur with low speed and without pressure. B. Excavators. The direction of cutting should be parallel to the pulp horns starting from the cavity periphery to the center with scooping motion. How can we remove soft carious dentin? Dentin is peeled off from the periphery to the center of the cavity parallel to the recessional lines. Removed using sharp excavators with minimum amount of pressure. Shelves in dentin are prepared for the seat of the restoration called dentin ledges. Direct or indirect pulp capping are done using ca(OH)2. 5. Finishing of Enamel Walls Objectives: ✓ To give the CSA its correct angulation according to the restorative materials used. ✓ To remove any undermined enamel. 36  ✓ To produce smooth enamel walls that increases adaptation of the restoration. ✓ Roundation of line and point angles. ✓ Make the cavity outline in a sweeping curve. Finishing instruments: 1- Hand instruments: Used to cleave undermined enamel and loose enamel rods as hatchets, chisels and gingival marginal trimmer. 2- Rotary instruments: Cutting fissure bur or diamond stone. According to NOY'S rules for keeping strong enamel wall at margins: 1. Enamel wall must rest on sound dentin. 2.Enamel rods which form the CSA must have their inner ends resting on sound dentin and the outer ends are covered by the R.M. 3-The gingival CSA must be trimmed or beveled to avoid its fracture during condensing the R.M. Beveling of enamel wall: Beveling means increasing CSA angulation more than 90°. Functions of bevel in resin composite: Increase surface area available for bonding. Color degradation for esthetic purposes. 37 Functions of bevel in cast gold restorations: Protection of weak enamel rods. Facilitate burnishing of ductile materials. Protection of cement line from solubility. Protection of weak cusps counter bevel. Add retention reverse gingival bevel. 6. Toilet of the Cavity Definition: The process of removing all debris from the prepared cavity e.g. cut chips, blood, saliva and bacteria. Objectives: Increasing adaptation of restoration to cavity walls. Prevents contamination of the restorative material. Enables the operator to examine properly all steps. Conservation: There is a shift to substitute the concept of extension for prevention by newly introduced philosophy based on conservation prevention rather than extension. This trend has evolved due to: 1- Understanding of caries process. 2-Development of adhesive restorative materials. 3-Modern instruments and techniques. Adverse effects of extension for prevention: 1. Weakening of the sound tooth structure. 2. Increased irritation to the pulp. 3. Increased liability for gingival and periodontal problems in compound cavities. 4. Increased liability to recurrent caries. 38