Designing tooth preparations for optimal success

The following guidelines are proposed when preparing teeth for complete coverage crowns and fixed partial dentures: The TOC (angle of convergence between opposing prepared axial surfaces) should range between 10° and 20°. However, posterior teeth are frequently prepared with greater convergence angles as are fixed partial denture abutments. When the TOC angles exceed the recommended levels, the tooth preparation should be modified to include auxiliary features such as grooves or boxes. Three millimeters should be the minimal occlusocervical/incisocervical (OC/IC) dimension of incisors and premolars when they are prepared within the recommended TOC range of 10° to 20°. The minimal OC dimension of molars should be 4 mm when prepared with 10° to 20° TOC. When the OC dimension is less than the recommended dimension, the tooth preparation should be modified to include auxiliary features such as grooves or boxes. The ratio of the OC/IC dimension to the FL dimension should be 0.4 or higher for all teeth. When this ratio is not present, as on large diameter molars, the tooth preparation should be modified to include auxiliary features such as grooves or boxes. Teeth should be prepared in a manner that preserves the facioproximal and linguoproximal corners whenever possible because circumferential irregularities enhance resistance form. When prepared teeth lack "corners" and are round after tooth preparation, they should be modified to include auxiliary features such as grooves or boxes. When auxiliary features are placed into teeth, the preferred locations are the proximal surfaces. The facial and lingual surfaces are secondary locations to be used when the addition of proximal features leaves the tooth in a state of questionable resistance form. Proximal grooves/boxes should routinely be used when mandibular molars are prepared for fixed partial dentures because mandibular molars often are prepared with the greatest TOC and frequently have limited OC dimension and large FL dimensions that lead to unfavorable ratios. For the purpose of optimizing periodontal health, finish lines should be located supragingivally when the condition of the tooth and esthetic requirements permit such a location. When subgingival finish lines are required, they should not be extended to the epithelial attachment. For all-metal crowns, 0.3-mm-deep chamfer finish lines should be used. The axial and occlusal reduction depths for all-metal crowns should be at least 0.5 mm and 1.0 mm, respectively. For metal ceramic crowns, finish line selection should be based on formation ease, personal preference, esthetic requirements, and the type of crown being fabricated rather than on expectations of enhanced marginal fit with one type of finish line compared with the others. Many teeth, because of available tooth structure thickness external to the pulp, cannot be reduced facially to depths that exceed 1 mm. Two millimeters of occlusal/incisal reduction can be achieved even on young teeth because sufficient tooth structure thickness is present incisal/occlusal to the pulp. Shoulder finish lines are recommended for all-ceramic crowns when they are not bonded to the underlying tooth. However, shoulder and chamfer finish lines can be used with all-ceramic crowns that are bonded to the prepared tooth using a resin cement and acid etching. Finish line and facial reduction depths >1 mm are not required when using a semi-translucent type of all-ceramic crown but are beneficial with more opaceous porcelain systems are used or when the tooth structure is discolored. Incisal/occlusal reduction depths of 2 mm are achievable due to the available thickness of tooth structure. Rounded line angles on tooth preparations for all-ceramic crowns decrease the stress placed on the crowns and thereby increase crown longevity. With crowns that contain metal (all-metal and metal-ceramic crowns), line angles are rounded to facilitate pouring impressions and investing wax patterns without trapping air bubbles and to facilitate the removal of casting nodules. Tooth preparation smoothness seems to enhance restoration fit, but its effect on retention appears to be related to the type of cement used. Surface roughness generally increased retention with zinc phosphate cement, but no definitive relationship has been established when crowns are cemented with adhesive cements (eg, polycarboxylate, glass ionomer, resin). It therefore seems appropriate to recommend that teeth be prepared with a reasonable degree of surface smoothness.