Less range of motion with resurfacing arthroplasty than with total hip arthroplasty: in vitro examination of 8 designs.

Background and Purpose: Hip resurfacing arthroplasty has had resurgence recently and is now a popular option for younger, active patients. We used an in vitro model to assess range of motion and impingement profile for hip resurfacing and compared them to those for conventional total hip arthroplasty.
Methods: 8 different hip replacement designs were implanted into adult composite femurs and pelvises. These were mounted onto a 3-dimensional compass allowing all motions, with the degrees and impingement recorded. The designs tested were the Conserve Plus Hip Resurfacing System, Depuy ASR, Birmingham Hip Resurfacing System, Charnley, McKee-Farrar metal-on-metal, SROM Hip metal-on-metal, SROM Hip metal-on-polyethylene, Prodigy metal-on-metal, and also a native intact composite femur and pelvic articulation. Femoral stems were tested at 0 and 20 degrees of anteversion.
Results: Conventional hip arthroplasty exhibited statistically significantly greater range of motion than resurfacing arthroplasty. Resurfacing showed neck impingement in 29/30 motions. Conventional arthroplasties showed femoral neck impingement in 41/100 motions.
Interpretation: In situ range of motion of resurfacing arthroplasty was less than that of conventional total hip arthroplasty. Resurfacing systems impinged almost entirely on the femoral neck, while conventional hip arthroplasties had a varied impingement profile. Our findings raise concern for early neck-on-cup impingement, which may cause component loosening and femoral neck fracture, both of which are observed after hip resurfacing.