Peri-implant bone microstructure determines dynamic implant cut-out.

Dynamic implant cut-out is a frequent complication associated with surgical fracture fixation. In this in vitro study, we investigated the influence of the local trabecular bone microstructure on the rate and path of implant migration. Dynamic hip screws were implanted into six human femoral head specimens with a wide range of bone volume fractions. The specimens were subjected to image-guided failure assessment using physiological dynamic hip loading. Mechanical testing was used intermittently with high-resolution computed tomography scanning. A high correlation was found between the bone volume fraction and implant migration (R(2)=0.95). Profiles of the bone-implant interface were computed based on the positions of the screw and the femoral head. With a larger interface, the implant migration rate was smaller. The bone-implant interface was significantly smaller on the approximated screw migration path than if it had been on a straight line in loading direction. We thus hypothesize that implants migrate on a path of least resistance. This would indicate a relevant mechanism for targeted surgical intervention.

Time-lapsed imaging of implant fixation failure in human femoral heads.

The failure mechanisms of bone-implant constructs are still incompletely understood, because the role of the peri-implant bone in implant stability is unclear. We hypothesized that implant fixation failure is preceded by substantial peri-implant bone failure. A new device was developed that combines mechanical testing of large bone-implant constructs with high-resolution peripheral quantitative computed tomography, following the principles of image-guided failure assessment (IGFA). In this study, we investigated the push-in failure behavior of dynamic hip screws (DHS) implanted in human cadaveric femoral heads. For the first time the fixation failure of a clinically used implant in human trabecular bone could be experimentally visualized at the microstructural level. The ultimate force was highly correlated with the peri-implant bone volume fraction (R(2)=0.85). We demonstrated that primary fixation failure of DHS implants was accompanied by trabecular bone failure in the immediate peri-implant bone region only. Such experimental data are crucial to enhance the understanding on the quality of the bone-implant interface and of the trabecular bone in the process of implant fixation failure. We believe that this newly developed device will be beneficial for the development of new implant designs, especially for use in osteoporotic bone.