Biomechanical Stability of Intraarticular Fractures of the Distal Femur Fixed by Intramedullary Nailing

Intramedullary (IM) nailing is a common fixation procedure for fractures of the distal femur (thighbone). These injuries are commonly associated with older individuals, and aging population demographics present the need to investigate IM nailing procedures for distal femur fracture fixation in osteoporotic bone. Accordingly, the goal of this study was to develop a mechanical testing fixture and protocol to assess the stability of distal femur fractures treated by IM nailing, including fracture models with and without metaphyseal comminution, and further investigate synthetic surrogate bone materials to best mimic osteoporotic bone for mechanical testing. This work describes the design and development of a testing fixture and loading protocol that mimicked the anatomic and mechanical loading scenarios of the femur while additionally providing a non-interfering bearing surface during mechanical testing. Results from axial loading tests indicated that for intraarticular distal femur fractures with moderate bone quality, a total subsidence (or permanent deformation resulting in fixation collapse) of 4-5 mm may occur. The potential implications of this subsidence may be relevant for construct selection in a clinical setting. Separately, ASTM-standard materials tests were carried out on materials identified as potential candidates to serve as homogeneous substitutes for metaphyseal bone. Polyurethane foam blocks of three different densities were tested and the results compared with reported relationships between age and mechanical properties of bone in the distal femur. These findings suggested that outside of severe osteoporosis, a 15 lb/ft3 density foam may most accurately represent the clinically relevant bone for future testing utilizing synthetic materials. Further investigation into in vivo bone quality of distal femur fracture patients may support improved synthetic material selection.