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

Purpose: To evaluated the effect of different auxiliary features on the resistance form of crowns with reduced axial wall height and increased total occlusal convergence.Materials and Methods: An ivorine tooth was prepared on a milling machine with 22-degree of total occlusal convergence (TOC), 3.0mm of occlusocervical height, and a chamfer finish line. The crown preparation was subsequently modified to include proximal grooves (Group B) and reduced TOC from 22 to 4 degrees in the cervical 1.5mm (Group C). Ten standardized metal dies were fabricated in each group. Metal 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 in a buccal to lingual direction. The maximum force (in Newtons) applied before coping dislodgment was measured using a universal testing machine. Data from the three groups were compared using ANOVA (p=0.05). Results: Proximal grooves and reduced TOC were both effective in increasing the resistance form of the original shape. The reduced TOC could increase the resistance more than proximal grooves.Conclusion.: Within the limitations of this in vitro study, crown preparation modifications enhanced the resistance form; however reduction in cervical TOC proved to be more effective than proximal grooves.