Mathematical derivation of the minimally acceptable all-porcelain margin angle.

There is limited research defining the minimally acceptable porcelain margin angle required to withstand the stresses incurred during seating a restoration. A mathematical model was derived to calculate this critical porcelain margin angle. The factors involved in margin fracture were determined to be the tensile strength of the porcelain, the axial reduction, the diameter of the tooth, the porcelain margin angle, and the seating force. As the seating force increases, the porcelain margin angle required to prevent failure also increases. Increasing either the axial reduction or the diameter of the tooth allowed the minimum porcelain margin angle to decrease.

Evaluation of margin angles of collarless metal ceramic restorations.

This study was designed to measure the porcelain margin angles of completed collarless metal ceramic restorations and determine if these angles correspond to the most frequently recommended porcelain margin angle of 90 degrees. A sample of 99 metal ceramic restorations with porcelain labial margins were evaluated. A 1.0 mm slice taken from the midfacial impression of each restoration was evaluated by tooth location with a stereomaster microscope and the angle of the labial porcelain margin was calculated. The comparison of margin angles by tooth location showed no statistical difference between the groups. Comparison of the average margin angle with the most suggested 90-degree angle was analyzed with a One-Sample t-test and found to be statistically significant (p = 0.000). Although the majority of dental schools teach the shoulder preparation for collarless metal ceramic restorations, only 43% of the restorations measured fell within 10 degrees of this standard.