Biomechanical validation of a new nail-plate for the repair of stable proximal femoral fractures.

The mechanical performance and the stress shielding effects of the Howmedica Spherolock MKII implant were evaluated. Three types of stable proximal femoral fractures were created in composite femurs and fixed with the nail-plate. The femurs were loaded to failure, and the yield strength, stiffness, and failure modes were noted. Additional intact composite femurs were fitted proximally with strain gauges, and the strain was examined under load. The femurs then underwent intertrochanteric fracture, plating, and re-testing. Comparative analysis showed that while the Spherolock system is less strong and stiff than other commonly used implants, it provides excellent and uniform load transfer across the fracture site. Varus rotation of the femoral head under load tended to open the fracture gap and localize the resultant load medially. Stress shielding of the calcar was less than 25% of the applied load even in the worst case. This very low stress shielding behavior was attributed mainly to the implant's low stiffness.