When it comes to hip resurfacing arthroplasty (HRA), what are the biomechanical implication of varying femoral version (including femoral retroversion) with respect to impingement and force transmission across the joint?
Also, asked a team of researchers from Canada and Iran, how do differing acetabular cup positions affect impingement and joint reaction forces? The study which attempted to find answers to those important questions, “Computational modelling of hip resurfacing arthroplasty investigating the effect of femoral version on hip biomechanics,” appears in the May 27, 2021 edition of PLOS ONE.
Co-author Kelly Johnston, M.D. with the Department of Surgery, Section of Orthopaedic Surgery, at the University of Calgary in Canada told OTW, “This is a novel paper that specifically looks at how femoral neck anteversion affects a hip resurfacing arthroplasty. Although resurfacing arthroplasty has lost popularity, many centers around the world continue to offer this operation to young male patients with excellent results. The concern around metal-on-metal articulation and its sequelae along with femoral neck fracture remain the main concerns of those centers that no longer offer it as an option.”
The researchers built 60 models using finite element analysis in order to determine the effect of different femoral versions (between 30° of anteversion to 30° of retroversion) on varying functional loads (0°of hip flexion, 45°of hip flexion, and 90° of hip flexion).
“All models were tested with the acetabular cup in four different positions,” wrote the authors. “(1) 40°/15° (inclination/version), (2) 40°/25°, (3) 50°/15°, and (4) 50°/25°. Differences in range of motion due to presence of impingement, joint contact pressure, and joint contact area with different femoral versions and acetabular cup positions were calculated.”
Thumbs Down for 30° Retroversion
The researchers discovered that impingement was most significant with the femur in 30° of retroversion, irrespective of acetabular cup position. “Anterior hip impingement occurred earlier during hip flexion as the femur was progressively retroverted,” wrote the authors. “Impingement was reduced in all models by increasing acetabular cup inclination and anteversion, yet this consequentially led to higher contact pressures. At 90° of hip flexion, contact pressures and contact areas were inversely related and showed most notable change with 30° of femoral retroversion. In this model, the contact area migrated towards the anterior implant-bone interface along the femoral neck.”
“The results of this paper are important,” said Dr. Johnston to OTW, because it identifies that reduced femoral neck anteversion, which is a common contributing cause of hip impingement and subsequent hip osteoarthritis in young male patients, may adversely affect the performance of a hip resurfacing arthroplasty.”
Surgery primarily for young males?
“The clinical implications are that hip resurfacing may be best reserved for young male patients with normal femoral neck anteversion,” explained Dr. Johnston to OTW. “This paper essentially refines the indications for hip resurfacing as surgeons have limited ability to change femoral neck anteversion when performing a hip resurfacing arthroplasty. Changing the acetabular cup position to compensate for reduced femoral neck anteversion can reduce the abnormal forces when the hip is loaded in flexion but unfortunately, this cup position change leads to abnormal forces when the hip is loaded in extension. We feel that femoral neck anteversion should be determined pre-operatively to better select patients for hip resurfacing arthroplasty.”
