True Angle between Anterior-Posterior Impingement in Total Hip Arthroplasty: Why Common Constrained Cups have to Fail

Background: Dislocation after total hip arthroplasty is a common complication and partially based on impingement. Questions/purposes: (1) The purpose of this study was to investigate the angle between anterior and posterior impingement in current hip cup designs. (2) Furthermore, the aim was to design a model of an acetabular cup with sparing gaps that match to impact areas. Methods: The range of motion was simulated with Maple R8 software using standard parameters in hip arthroplasty. Afterwards, a preliminary model for an optimized acetabular cup was designed in order to avoid impingement and dislocation. Results: (1) Anterior and posterior areas of impingement were not opposite but twisted by an angle of 108.3°. (2) The two main trajectories of motion were identified and areas with corresponding reductions and elevations were appropriate modified. The improvement resulted in a “bidirectional total hip prosthesis” with a combination of a snapfit acetabular cup and a reduced cup profile. Conclusion: The improvements of the described hip prosthesis are based on a simulation and are most likely to prevent impingement and subsequent dislocation. In addition, simulation with standard implantation parameters resulted in a rotational asymmetric implant design. Clinical relevance: Our data provide evidence that conventional hip cup designs fail to prevent impingement due to (1) The incorrect assumption of diagonally arranged impingement areas and the diagonal arranged sparing gaps (2) The sparing gap design itself that technically is not reducing the rim of the cup but instead only has an elevated coverage relative to the center of motion.