Effect of preparation design on stress in MOD restored premolars

Objective: The aim of the study was to determine the optimal shapes of MOD ceramic restorations in premolars in order to minimize the potentially damaging effects of stress on teeth structures and restorations. Material and methods: The study was performed on an upper first premolar, using a finite element analysis. Starting from an 3D model of an unrestored tooth created after laser scanning, twenty-two 3D models of maxillary first premolars, with the following designs of MOD ceramic restorations were generated: eleven inlays with butt joint margins, eleven onlays with butt joint margins, both with different tapers (between 0 and 10 degree). The model of the inlay restored tooth was divided into 21337 solid elements connected at 35214 nodes, and the inlay into 5990 elements connected at 10809 nodes. The model of the onlay restored tooth was divided into 17671 solid elements connected at 29625 nodes, and the onlay into 14087 elements connected at 23609 nodes. Computational simulation of an occlusal load of 200 N was conducted, and stresses occurring in the ceramic restorations, and teeth structures were calculated using Ansys finite element analysis software. Results: Occlusal load on an MOD restored tooth produces stress surrounding the contact areas. In the teeth restored with ceramic MOD inlays, the von Mises equivalent stress values were higher than in the intact tooth. The stresses were significant lower in all cases when an onlay was used for the MOD restoration compared to those when an inlay was used. The taper of the preparation has no influence on the stress values for all the studied cases. Conclusion: The MOD restoration on premolars has to distribute the load over a wide surface. Covering the occlusal surface with the restoration, using an MOD onlay, may prevent failure and produce a lower stress values than an MOD inlay.