Relative Contribution of Trabecular and Cortical Bone to Primary Implant Stability: An In Vitro Model Study

The specific aim of this study was to examine the relative contributions to the implant insertion torque value (ITV) by cortical and trabecular components of an in vitro bone model. Simulated bone blocks of polyurethane were used with 2 densities of foam (0.08 g/cm(3) to mimic trabecular bone and 0.64 g/cm(3) to mimic cortical bone). We have developed a new platform technology to collect data that enables quantitative evaluation of ITV at different implant locations. Seven groups were used to model varying thicknesses of cortical bone over a lower-quality trabecular bone that have clinical significance: a solid 0.08 g/cm(3) block; 1 mm, 2 mm, and 3 mm thick 0.64 g/cm(3) sheets with no underlayer; and 1 mm, 2 mm, and 3 mm thick 0.64 g/cm(3) sheets laminated on top of a 4 cm thick 0.08 g/cm(3) block. The ITVs were recorded as a function of insertion displacement distance. Relative contributions of ITV ranged from 3% to 18% from trabecular bone, and 62% to 74% from cortical bone depending on the thickness of the cortical layer. Inserting an implant into 2-mm and 3-mm cortical layers laminated atop trabecular blocks had a synergistic effect on ITVs. Finally, an implant with a reverse bevel design near the abutment showed final average torque values that were 14% to 34% less than their maximum torque values. This work provides basic quantitative information for clinicians to understand the influence of composite layers of bone in relation to mechanical torque resistances during implant insertion in order to obtain desired primary implant stability.