Theoretical relationships between component design, patient bone geometry and range-of-motion post hip resurfacing

Clinical studies indicate that range of motion until prosthetic impingement is important in understanding unexplained failures of hip resurfacings, yet the underlying biomechanical principles have received little attention. This study investigates the mathematical relationships between component design, position, patient bone geometry and range of motion in hip resurfaced prostheses. Variations in range of motion and impingement-free safe-zones for cup position were calculated using an established method of vector analysis that facilitated parametric analysis in a time efficient manner. The alpha angle, defined as the angle between the centreline of the femoral neck and the waist of the femoral head/neck junction, was used to represent the natural femoral neck. Range of motion and impingement-free safe-zones were inversely proportional to the alpha angle and cup inclusion angle. The size of the safe-zone was most sensitive to the alpha angle with a 6 degrees reduction, decreasing the range of cup positions without impingement by 80-100%. Lowering the upper limit of cup inclination from 55 degrees to 45 degrees reduced the range of cup positions that allow impingement-free motion by 47-94%. No common safe-zone was observed for the range of component sizes and positions investigated. This offers an explanation to why clinic studies have failed to associate outcome with standardised positioning criteria.