[Selection of optimal occlusal thickness and shoulder thickness in mandibular first molar all-ceramic crown: a three-dimensional finite element analysis].

Objective: To investigate the effect of different occlusal thickness and shoulder finish line depth on stress distribution of all-ceramic crowns and to select optimal occlusal thickness and shoulder finish line depth using continuous variation of parameters.

Methods: This analysis was performed using mandibular first molar finite element model. The range of occlusal thickness was set from 1 mm to 3 mm, and that of shoulder finish line depth was from 0.4 mm to 1.2 mm. Load of 225 N was applied perpendicularly to the occlusal surface of the tooth at all buccal cusps to simulate functional occlusal force. The maximum equivalent stresses in crown, cement layer, abutment, periodontal ligament, alveolar bone were calculated, and the sensitivities of stresses to the variables were also evaluated.

Results: The maximum equivalent stresses in crown, cement layer and abutment decreased as occlusal thickness and shoulder finish line depth were increased, while no obvious change were found in maximum equivalent stresses in periodontal ligament and alveolar bone. When occlusal thickness exceeded 1.87 mm and shoulder finish line depth exceeded 0.66 mm, the tangent slope rate of the maximum equivalent stress response curves ranged from -1 to 1. Data indicated that occlusal thickness played a more important role in reducing maximum equivalent stress in cement layer than finish line depth did, and shoulder finish line depth was a more effective parameter in reducing maximum equivalent stress in crown and abutment than occlusal thickness was.

Conclusions: Occlusal thickness exceeding 1.87 mm and shoulder finish line depth exceeding 0.66 mm are optimal design for ceramic crown on mandibular first molar from biomechanical point of view.