Abstract
Purpose
The Birmingham Mid-Head Resection (BMHR) is a bone-conserving, short-stem alternative to hip resurfacing for patients with compromised femoral head anatomy. It is unclear, however, if an uncemented, metaphyseal fixed stem confers a mechanical advantage to that of a traditional hip resurfacing in which the femoral prosthesis is cemented to the prepared femoral head. Thus, we aimed to determine if a metaphyseal fixed, bone preserving femoral component provided superior mechanical strength in resisting neck fracture compared to a conventional hip resurfacing arthroplasty.
Method
Sixteen matched pairs of human cadaveric femurs were divided evenly between specimens receiving a traditional epiphyseal fixed hip resurfacing arthroplasty (BHR) and those receiving a metaphyseal fixed BMHR. Pre-preparation scaled digital radiographs were taken of all specimens to determine anatomical parameters as well as planned stem-shaft angles and implant sizes. A minimum of 10 degrees of relative valgus alignment was planned for all implants and the planned stem-shaft angles and implant sizes were equal between femur pairs. Prior to preparation, bone mineral density scans of the femurs were obtained. Prepared specimens were potted, positioned in single-leg stance and tested to failure using a mechanical testing machine. Load-displacement curves were used to calculate construct stiffness, failure energy and ultimate failure load.
Results
Human cadaveric femur pairs were well matched for anatomic parameters and BMD with no statistically significant differences in neck-shaft angle (p=0.110), neck width (p=0.173), femoral offset (p=0.224) or neck BMD (p=0.525). There was a statistically significant difference between failure loads for femurs prepared with a BHR and those prepared with a BMHR (p<0.001). Femurs prepared with a BHR (7012 N, SD 2619) failed at an average of 1578 N (SD 865) greater than paired femora prepared with a BMHR (5434 N, SD 2297), representing a 24% increase in failure load. Both construct stiffness and failure energy were not statistically different between groups (p>0.065). Transcervical vertical shear fractures accounted for 19 of 32 failures, the remaining 11 were subcapital fractures. There were no fractures observed at the base of the femoral neck for either implant.
Conclusion
A metaphyseal fixed, bone conserving femoral implant does not provide superior mechanical strength nor increased resistance to femoral neck fracture compared to a conventional hip resurfacing arthroplasty.
