Fundamentals of Cavity Preparation PDF

2. Primary Resistance Form The design features in CP that allow both tooth and restoration to resist functional stresses of mastication without fracture The shape and placement of walls to enable the remaining tooth structure and restoration to withstand masticatory forces without fracture Princ...

2. Primary Resistance Form The design features in CP that allow both tooth and restoration to resist functional stresses of mastication without fracture The shape and placement of walls to enable the remaining tooth structure and restoration to withstand masticatory forces without fracture Principles 1. box shaped cavity (chamber) preparation with flat horizontal floor at right angles to forces of mastication (directed in the long axis of the tooth) helps the tooth to resist occ loading by virtue of being at right angles to these forces  irregular Smooth the occ force concentrated on deepest areas which will cause fracture of the tooth structure the occ forces concentrated on the elevated areas cause fracture of the restoration Dentin seat (ledge) Infected carious dentin extending beyond ideal pulpal wall position Incorrect lowering of pulpal wall to include infected carious dentin Correct extension B and L beyond infected carious dentin 2. Restrict the extension (conservative extension ) of the external walls to allow strong cusp and ridge areas to remain with sufficient dentin support 3. Rounding the internal & external line angles to reduce stress concentrations in tooth structure 4. Capping weak cusps in extensive tooth preparations and include weakened tooth substance within the restoration to prevent fracture of the tooth by forces both in the long axis and obliquely (laterally) directed 5. Provide enough thickness of restorative material to prevent its fracture under load 6. Bond the restorative material to tooth structure when appropriate to work as 1 unit Factors affecting resistance form: 1. Occlusal contact on both the restoration and the remaining tooth structure the greater the occlusal force and contacts, the greater is the potential for future fracture (the further posterior the tooth, the greater is the effective masticatory force because the tooth is closer to the condyle head). The amount of remaining tooth structure also In very large teeth, even though severely involved with caries or defects, may require less resistance form consideration, because the remaining tooth structure is still bulky and strong enough to resist fracture 2. The type of restorative material used:  Amalgam requires a minimum occlusal thickness of 1.5 mm for adequate strength  Cast metal requires less thickness to resist fracture but should still have a dimension of at least 1 mm in areas of occlusal contact  Ceramics require a minimum dimension of 2 mm to resist bulk fracture  The dimensional needs of composite are more dependent on the occlusal wear potential of the restored area. In posterior teeth, the thickness requirement is greater than for anterior teeth 3. Bonding a restoration to the tooth enhances the resistance to fracture Bonding amalgam, composite, or ceramic to prepared tooth structure increases the strength of the remaining unprepared tooth, thereby reducing the potential for fracture 3. Primary Retention Form: Is the form given to CP to prevent displacement or removal of the restoration from its place by tipping forces Mechanisms of Retention: I. Chemical Retention  Is surface interaction of an adhesive (restoration) and solid substrate (tooth)  Glass Ionomer Cement II. Macro-mechanical Retention depends on certain retaining features that prepared in the remaining tooth substance to anchor restoration mechanically  III. Micro-mechanical Retention It depends on an intermediary joint that interlock the restoration to tooth. Acid etching of E &D→ demineralization Resin impregnation→ resin tags→ micromechanical interlocking Types of mechanical retention:  According to direction  Axial  Lateral  According to location  Coronal (B, L, G, …)  Radicular (in endodontic treated teeth)  According to case (amount of remaining tooth structure)  Primary  Secondary Axial Retention  Prevent displacement of restoration parallel to long axis of the tooth (in occlusal or apical direction)  For direct restorations:  Occlusal convergence  Undercuts  For indirect restoration:  Frictional parallelism of cavity walls increased by: degree of parallelism () walls length of parallel walls degree of roughness of walls Lateral Retention  Prevent displacement in any other direction than the axial (in lateral direction) proximal, buccal, lingual (Dove-tail lock)  Class V the external walls diverge outwardly to provide strong enamel margins, and retention coves or grooves are prepared in the dentinal walls to provide retention form Factors affecting selection of retentive features:  Size of CP ↑size of a cavity, ↓remaining tooth structure → ↑ need for retentive features  Number of missing walls: ↑ number of missing walls→ multiple displacement directions → ↑ need for retentive feature  Site of CP and occlusal stresses: subjected to high stresses as compound & complex cavities → additional means of retention  Type of restoration: Amalgam, RC, indirect affect type of retention and retentive features  Pulp vitality: In endodontic treated teeth, radicular retention gained through post insertion Retention and Resistance forms ??? WHICH IS MORE IMPORTANT Retention and Resistance forms can be prepared at the same time? 4. Convenience Form:  Is shape or form of CP that provides adequate observation, accessibility, ease of restoring the tooth  obtaining this form may necessitate extension of D, M, F, L walls to gain adequate access to the deeper portion of the preparation Extending proximal preparations beyond proximal contacts preparations wider than proximal contacts for better access to finish the preparation walls and the restorative material and to place a matrix (clearance) palatal access in class III occ divergence of vertical walls of tooth preparations for Class II cast restorations clearance is mandatory to: finish the preparation walls, make accurate impression, try-in, insertion, removal of excess cement, finishing and polishing of casting margins Teeth with minimal carious lesion most of carious dentin removed during initial CP