Assessment of femoral neck fracture risk for a novel proximal epiphyseal hip prosthesis

Background: This study addresses the risk of femoral neck fracture associated with resurfacing hip prostheses. A novel cemented Proximal Epiphyseal Replacement (PER) featuring a short curved stem was investigated. Methods: Seven pairs of femurs were in vitro tested. One femur of each pair was randomly assigned for PER implantation. The contralateral femur was tested intact. All femurs were loaded to failure in a validated, physiological configuration. High-speed videos (10,000–12,000 frames/s) were acquired to identify the location of fracture initiation. For comparison, data were included from Birmingham Hip Resurfacing previously tested in an identical fashion (N=3). Findings: Relative to the contralateral intact femurs, the failure load of the PER and Birmingham implants was 15.4% higher and 10.0% lower, respectively. In six of the seven PER implants, fracture initiation (neck or inter- trochanteric) was similar to the contralateral intact femurs, suggesting comparable stress distribution. Conversely, fracture initiation in the Birmingham implants occurred at the lateral prosthesis rim, which differed substantially from the intact femurs. No correlation existed between bone quality and strengthening/ weakening effect of the PER (failure load of implant as a percentage of intact: R^2=0.067). Conversely, Birmingham implantation weakened the femurs with lower density (R^2=0.92). Therefore, unlike most resurfacing prostheses, the PER seems suitable also for osteoporotic subjects. Interpretation: This study seems to confirm that resurfacing with a Birmingham Hip tends to reduce the strength of the proximal femur. The opposite seemed to happen with the PER, which slightly reduced the risk of neck fracture, also in low-quality bones.