Intramedullary nail deflection correlates to mismatch of a 1.5 m vs 1.0 m femoral nail bow design: a biomechanical human cadaveric study.

Abstract

Intramedullary nailing is the standard treatment method of femoral shaft fractures with generally satisfying results. However, recent clinical studies reported a possible radius of curvature (RoC) mismatch between treated femora and existing intramedullary implants. The aim of this biomechanical study was to compare the anatomical fit of two femoral nail designs with a 100 cm and 150 cm RoC and examine the implant deflection as a surrogate measure for the fit within the medullary canal. Seven paired fresh-frozen human cadaveric femora were investigated by insertion of either a long Trochanteric Femoral Nail - ADVANCED Proximal Femoral Nailing System (TFNA) or a long Proximal Femoral Nail Antirotation System (PFNA) with RoCs of the nails 100 cm and 150 cm, respectively. Nails stiffness was measured before insertion via three-point bending. Nail anatomical fit and deflection were analysed using computed tomography scans before insertion and then at full insertion before and after axial rotation of the nail for alignment of the proximal locking element axis with the femoral neck axis and the centre of the femoral head. Bending stiffness of the two nail types was similar (P = 0.308). A significantly higher deflection of PFNA versus TFNA was detected at full insertion with axial rotational nail alignment when proximal nail end registration was applied (P = 0.043), and trend wise higher when both ends registration was implemented (P = 0.063). Although the bending stiffness of both intramedullary nails was similar, the larger RoC of the PFNA resulted in significantly bigger deflection in comparison to TFNA at full insertion before and after final axial rotation of the nail for alignment of the proximal locking element axis with the femoral neck axis and the centre of the femoral head. This suggests that an implant with a 100 cm RoC reflects better the femoral anatomy.