Development of anti-migration implants for fixation of osteoporotic fractures of the proximal femur and humerus

Abstract

The development of novel anti-migration implants for internal fixation of osteoporotic fractures is presented, based on two basic studies conducted into the mechanics of implant migration in trabecular bone that yielded insights and computational tools novel to the discipline of orthopaedic research. In the first study, the effect of implant tip design on axial migration was explored using a novel fracture mechanics model based on the Griffith Criterion and material compaction. Three tip designs were considered based on typical 5 mm locking screws: flat and conical tip designs, as well as a novel polymer tip. Tip designs were inserted into polyurethane foam (0.16 g/cc) to a depth of 10 mm at a rate of 2 mm/min. At maximum depth, polymer tips required the greatest force for axial migration (248.24 N, 95% CI: 238.1–258.4 N), followed by conical tips (143.46 N, 95% CI: 142.1–144.9 N), and flat tips (113.88 N, 95% CI: 112.2–115.5 N). Crack formation and friction were found to be negligible mechanisms of energy absorption; material compaction was the dominant mechanism under axial loading, regardless of tip design. Compacted material cross-sectional area was a strong determinant of force (Pearson Coefficient=0.902, p<0.001). Implant tips designed to maximize the cross-sectional area of compacted material – such as the elastomeric tip – may be useful in reducing excessive implant migration under axial loads in trabecular bone. In the second study, a novel computational model of implant migration in trabecular bone was developed using smoothed-particle hydrodynamics (SPH). Six human cadaveric specimens from the proximal femurs of female donors (age 75-90) measuring 10×10×20 mm were penetrated under axial loading to a depth of 10 mm with 5 mm diameter cylindrical indenters bearing either flat or sharp/conical tip designs. SPH models were constructed based on microCT scans of the specimens prior to penetration and the experiments were repeated in silico. Peak forces varied between 92.0–365.0 N in the experiments, and 115.5–352.2 N in the SPH simulations. The concordance correlation coefficient between experimental and simulated pairs was 0.888, with a 95% CI of 0.8832–0.8926, a Pearson ρ (precision) value of 0.9396, and a bias correction factor Cb (accuracy) value of 0.945. Patterns of bone compaction were qualitatively similar. This is the first known study to show that simulations based on SPH can produce accurate predictions of trabecular bone penetration that are useful for characterizing implant performance under high-strain loading conditions. Insights from these basic research studies were applied to the design of an initial pair of novel implants for femoral and humeral fractures with polymer (polycarbonate urethane elastomer) tip designs. Pilot studies showed the superiority of these implants in resisting migration in porous bone. These prototype implants were biomechanically tested according to standard methods from the literature, and a pilot biocompatibility study of in rat femurs was performed. Safety and efficacy of the implants was suitably demonstrated to anticipate ready approval of such devices, or improved future designs based on similar principles, in the major regulatory jurisdictions (e.g. US FDA, CE, CFDA).