Subsidence in two uncemented femoral stems: an in vitro study

Many aspects of the performance of different implant designs remain as open questions in total hip arthroplasty. Despite the increased survivorship of each hip replacement, the amount of bone removed during surgery remains an important factor because of the potential need for revision surgery. Given that a smaller implant will have less surface area over which to transfer load, constructs that preserve more bone stock may be susceptible to mechanical complications related to the fixation of the implant in the femur. To assess mechanical fixation, this study compared the fiber metal taper and Mayo conservative hip stems in subsidence, frontal plane rotation and failure load. After dual-energy x-ray absorptiometry scans, pairs of cadaveric femurs received implants of each type and were loaded for 10,000 cycles. The subsidence and rotation were measured. Finally, specimens were loaded to failure. The subsidence and rotation after cyclic loading were -0.73 mm and 0.1°, respectively, for the Mayo implants and -0.87 and 0.52°, respectively, for the fiber metal taper implants, but no significant differences between implant types were found. There was also no significant relationship to bone mineral density. A power analysis revealed that 914 specimens would have been required to achieve a power of 0.8.