Impingement During Dislocation-Prone Activities in Acetabular Cups with Lipped and Standard Liners: A Geometric Model

Background/Objectives: Asymmetric acetabular liners used in Total Hip Arthroplasty (THA) are clinically successful in reducing revision associated with dislocation. The performance of an asymmetric liner is dependent on its positioning, although few studies have directly investigated the effects of variables such as cup orientation or extended rim position on outcomes such as impingement. Therefore, the aim of this study was to compare the incidence of impingement in THA with a standard and lipped liner. Methods: A THA was virtually implanted into a previously developed geometric model. Acetabular cups were placed in a range of clinically relevant orientations, and the model was moved through seven joint motions associated with dislocation-prone Activities of Daily Living (ADLs). The occurrence of implant–implant, implant–bone, and bone–bone impingement was assessed for the standard and lipped constructs across a total of 6,237 unique combinations of cup orientation and lipped liner apex rotations. Results/Conclusions: The model predicted that with both standard and lipped constructs, there were no component placement positions which avoided impingement for all ADLs. The number of impingement incidents was minimised when components were positioned at the higher end of the manufacturer’s recommendations for cup inclination and anteversion. The use of a lipped liner increased the frequency of impingement events compared with the standard construct. However, for anterior impingement leading to posterior dislocation, this increase may be minimised by rotating the lip apex to a posterior-inferior position. Anterior lip apex placement tended to minimise the increase in the less critical posterior impingement.