Anisotropic elasticity of cortical and cancellous bone in the posterior mandible increases peri-implant stress and strain under oblique loading

The aim of this study was to compare implant-bone interface stresses and periimplant principal strains in anisotropic versus isotropic three-dimensional finite element models of an osseointegrated implant in the posterior mandible. We obtained anisotropic (transversely isotropic) elastic constants for mandibular bone and derived equivalent isotropic constants by averaging over all possible spatial orientations. A finite element model was constructed using ten-node tetrahedral p-elements, providing curved edges where necessary and increasing the accuracy of the results in regions of high stress gradients. Perfect bonding was assumed at the implant-bone interface. An oblique load was applied at the coronal aspect of the crown with 100 N vertical and 20 N bucco-to-lingual components. Implant-bone interface stresses exceeded reported bond strengths and principal strains reached yield strain levels in the cortical crest. Anisotropy increased what were already high levels of stress and strain in the isotropic case by 20 to 30% in the cortical crest. In cancellous bone, anisotropy increased what were relatively low levels of interface stress in the isotropic case by three- to four-fold to exceed bond strength levels. Anisotropy has subtle, yet significant effects on interface stresses and peri-implant strains and careful consideration should be given to its use in finite element studies of dental implants.