Reading

Factors to consider in designing the metal substructure

(Summary)

Porcelain in dental restorations should be thin yet strong, ideally 1.0–1.5mm thick and supported by metal of 0.3–0.5mm. Uniform thickness and full metal backing help reduce stress and improve durability, while partial coverage can cause stress at the metal-porcelain junction, increasing fracture risk. Metal surfaces must be smooth, convex, and free of sharp angles to support the porcelain properly.

For aesthetics and strength, porcelain should cover visible areas like proximal contacts, while metal is better placed lingually. Porcelain should not overlap metal at occlusal junctions, as thin edges break easily. Avoid placing the porcelain-metal junction near the incisal edge to maintain translucency and reduce fracture risk. Bridge connectors should be 2.5mm (anterior) or 3.0mm (posterior) in depth and thickness to prevent flexing and ensure stability.

Reference

• McLean J.W. 1980. The Science and Art of Dental Ceramics. Vol II. Quintessence Publishing Co. Inc Tokyo
  
  

PROFESSIONAL READING (ONLINE ARTICLE)

TOOTH PREPARATION

Tooth preparation primarily involves the use of the aerator, with diamond burs being ideal for enamel and either diamond or tungsten carbide burs used for amalgam and dentine. The specific shape of the bur must correspond with the intended tooth contour and margin design—such as square-ended burs for shoulder margins or appropriately shaped burs for chamfers. When working on the concave palatal surface of upper incisors, a large diameter diamond bur that matches the tooth’s contour is recommended for better control. The finishing phase, although more time-consuming than the bulk reduction, is essential to refine the shape, eliminate undercuts, round off sharp angles, and ensure the margin is well-positioned in relation to the gingiva. A slightly rough surface is preferred at the end to enhance retention, so polishing is generally unnecessary.

To ensure precision and conserve tooth structure, each surface should be prepared individually. This methodical approach helps regulate the amount of reduction applied to each section. The finishing stage is especially critical as it determines the accuracy of fit, affects wax pattern removal, and influences the success of cementation. If preparation is rushed or the surface left too rough with deep score marks, complications can arise during the restorative process. Overall, proper selection and use of instruments, guided by the shape and needs of the tooth, are vital for effective preparation and long-term success of crowns and restorations.

PREPARING TEETH FOR COMPLETE POSTERIOR CROWNS
The occlusal reduction in crown preparation should mirror the natural contours of the original tooth to ensure uniform crown thickness and minimize unnecessary removal of tooth tissue. For worn-down teeth, a flatter preparation may be acceptable, while in other cases, replicating the cusp anatomy is beneficial. A dome-ended, parallel-sided diamond bur is particularly useful for this step, as it helps shape both the cuspal inclines and rounded fissure patterns effectively. The instrument's shape facilitates controlled cutting and consistent depth, which is essential for optimal crown fit and function.

Axial reduction varies between the buccal/lingual and mesial/distal surfaces. Buccal and lingual surfaces can be prepared using parallel-sided or tapered diamonds shaped to create the desired margin. Instrument taper must sometimes be adapted due to tooth curvature, existing restorations, or core build-ups. A larger bur diameter helps prevent vertical ridges. The mesial and distal surfaces pose more challenges, especially when adjacent teeth are present. These areas require careful handling to avoid damaging neighboring teeth—an unfortunately common occurrence. After preparation, adjacent teeth must be checked for damage and treated if necessary. The shape of the margin—shoulder, chamfer, or knife-edge—is determined by the bur's end shape and should be formed during the initial bulk preparation for better efficiency and precision.