Bone Health Deterioration in Transfemoral Prosthetic Users: An Analytical Biomechanical Explanation

There is a five-decade recorded history indicating that persons with transfemoral amputation experience bone loss in their amputated femur at levels seen in bedridden and post-menopausal individuals, irrespective of age or mobility levels. We used computer simulation to recreate the mechanical environment created by the mechanical design of a prosthetic device in the surviving femur of individuals with transfemoral amputations. Finite element models of gait instances were developed from the hip joint computerized tomography scan of a subject along with a coupled ischial containment prosthetic socket fitted as per standard clinical guidelines. Accompanying mirror models, assembled similarly but without the prosthetic socket were used for stimulus comparison. Simulation showed that more than 90% of the trabecular bone volume in the amputated femur with an ischial containment socket registered compressive strain magnitudes below 300με. These strain magnitudes are below the threshold for bone maintenance as per mechanotransduction theory (i.e., they lie within the disuse window). Only 50% of the bone was in the disuse window for the mirror model for the gait instances considered. These results are consistent with reported in vivo evidence which shows that transfemoral prosthesis users may lose bone mass irrespective of age or mobility levels when using traditional socket designs. Clinically, this study shows that prosthetic sockets that support load through the ischium alter the kinetic chain and preclude application of mechanical stimulus that sustains healthy levels of bone mass in the proximal femur. The study also shows that femur length, prosthetic alignment and tissue tone influence this stimulus.