The influence of auxiliary features on the resistance form of short molars prepared for complete cast crowns

Abstract

Several factors exist which result in crown preparations that are less than ideal. In these situations, the clinician should find a practical way to overcome the lack of resistance of the compromised tooth preparation. The purpose of this study was to evaluate the effect of different auxiliary features on the resistance form of crowns with reduced axial wall height and increased total occlusal convergence. An Ivorine tooth was prepared on a milling machine with 22 degrees of total occlusal convergence (TOC), 3.0 mm of occlusocervical height, and a chamfer finish line (Group Ctrl). The crown preparation was subsequently modified to include proximal grooves (Group Grv), and reduced TOC from 22 to 4 degrees in the cervical 1.5 mm (Group Rdc). Ten standardized metal dies were fabricated for each group. Cobalt-chromium copings were fabricated for all specimens. The metal copings were cemented onto their corresponding metal dies with zinc phosphate cement. The resistance of each specimen was evaluated when force was applied at a 45-degree angle to the long axis of the die with a universal testing machine in a buccal to lingual direction. The maximum force (newtons) was applied before coping dislodgment was measured. Data from the 3 groups were compared with a 1-way ANOVA (α=.05) and a Post Hoc Bonferroni test. The mean (SD) force needed to dislodge crowns with proximal grooves was 156.75 (30.96) N and for those with reduced TOC, it was 221.06 (27.02) N, Both were effective in increasing the resistance form of the original shape (P=.002 and P<.001 respectively). The reduced TOC increased the resistance significantly more than the proximal grooves (P<.001). Within the limitations of this in vitro study, crown preparation modifications enhanced the resistance form; however, the reduction in cervical TOC proved to be more effective than proximal grooves.