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.