Background: Early clinical results of hip resurfacing arthroplasty have led to the recommendation to achieve a neck-shaft angle of 140° when inserting the femoral component. In addition, the idea of adhering to an absolute angle when inserting instrumentation in hips with excessive anatomic varus or valgus neck-shaft angles has raised concern. A biomechanical analysis was completed in order to determine if the achieved valgus orientation of the femoral component reduced the risk of periprosthetic fracture.
Methods: Twenty fresh-frozen cadaveric femora were blindly assigned to be implanted with a neutral or valgus-oriented hip-resurfacing femoral component. Bone mineral density scans were acquired for all femora. All specimens were loaded axially to failure at a rate of 0.21 mm per second. Radiographs of the specimens were measured in order to determine the relative valgus orientation of the femoral components and the change in offset.
Results: There was a significant increase in the ultimate failure load for the valgus-oriented components. While the bone density scans revealed that the bone mineral densities measured in the neutral and valgus-oriented femoral components were almost identical, the ultimate failure load was found to be significantly increased for the valgus-oriented components (6955 N) compared with the neutral-oriented components (5254 N). For the valgus-oriented femoral components, two had failure at the subcapital level, seven had vertical shear fractures, and one had an anterior shear fracture. For the neutral-oriented components, five subcapital fractures and five vertical shear failures were observed.
Conclusions: The study suggests that a valgus orientation decreases the risk of periprosthetic femoral neck fracture following hip resurfacing. It also brings into question the use of an absolute angle for all patients. Obtaining the maximum possible valgus angle, while avoiding notching, may in fact provide the optimum protection from periprosthetic femoral neck fractures.