Corono-radicular reconstruction of pulpless teeth: a mechanical study using finite element analysis.

STATEMENT OF PROBLEM Following endodontic therapy, teeth need to be protected, particularly in the cervical region, where the majority of fractures occur. The likelihood of a fracture depends on the condition of the crown and the type of reconstruction performed. PURPOSE This simulation study was designed to compare the effect of different corono-radicular reconstruction methods on stress transmission to dental tissues. MATERIAL AND METHODS The study software performed stress analysis of complex structures by finite element analysis. Seven 3-dimensional models were created, each representing a tooth embedded in a bony medium. The following parameters affecting corono-radicular restoration were studied: 2 levels of coronal destruction, core materials, post materials when present, and absence of post. The 2 levels of coronal tissue loss were (1) total tissue loss of the coronal dentin and (2) partial tissue loss of the coronal dentin with 2-mm surviving dentin walls. Teeth with 2 different levels of tissue loss (first study parameter) were reconstructed by 4 different techniques: nickel chromium (NiCr) cast post and core, NiCr post and composite core combination, carbon fiber post and composite core combination, and composite restoration without post. A NiCr crown covered each of the models and received a 30 degrees oblique occlusal load at a constant intensity of 100 N. The software computed the stresses (local tensile stress inducing cracks and compressive stress) for each of the models, comparing maximum intensity observed, localization, and concentration. RESULTS Whatever the type of stress (tensile or compressive), the greatest stress was observed in the cervical region, regardless of the model. Only tensile stresses potentially responsible for fractures were compared. Cervical tensile stresses exceeded 230 Pa in the absence of a ferrule and were less than 140 Pa when a ferrule was present. In the absence of a ferrule, the NiCr composite/post combination generated greater cervical stress (254 Pa) than the cast post and core (235 Pa). Results with a ferrule showed 92 Pa for the NiCr composite/post combination and 90.5 Pa for the cast post and core. In the presence of a ferrule, the tensile stress intensities generated by the composite restoration with no root canal post (139 Pa) were 51% greater than those generated by the NiCr/composite combination and approximately 26% greater than those generated by the composite/carbon combination. CONCLUSION Within the limitations of this study, it was confirmed that all simulated reconstructed teeth were more subject to stress in the cervical region. The absence of a cervical ferrule was found to be a determining negative factor, giving rise to considerably higher stress levels. When no ferrule was present, the NiCr post/composite combination generated greater cervical stress than cast post and cores. Nevertheless, the peripheral ferrule seemed to cancel the mechanical effect of the reconstruction material on the intensity of the stresses. With a ferrule, the choice of reconstruction material had no impact on the level of cervical stress. The root canal post, the purpose of which is to protect the cervical region, was also shown to be beneficial even with sufficient residual coronal dentin. In the presence of a root canal post, cervical stress levels were lower than when no root canal post was present. Moreover, the higher the elasticity modulus, the lower the stress levels.