Bone-on-bone versus hardware impingement in total hips: a biomechanical study.

Dislocation remains a serious concern for total hip arthroplasty (THA). Impingement, typically between the implant femoral neck and the acetabular cup, remains the most common dislocation impetus. Wear reductions from recent bearing technology advancements have encouraged introduction of substantially increased femoral head diameters. However, there is some evidence that range of motion with larger head sizes is limited by bone-on-bone, rather than hardware, impingement. While all impingement events are of course undesirable, currently little is known biomechanically if these two impingement modes differ in terms of generation of potentially deleterious stress concentrations or with regard to dislocation resistance. Finite element (FE) analysis was therefore used to parametrically investigate the role of head diameter on the local biomechanics of bone-on-bone versus component-on-component impingement events. Of several dislocation-prone patient motion challenges considered, only squatting consistently resulted in bone-on-bone (as opposed to hardware) impingement. Implant stress concentrations arising from hardware impingement during squatting were greater than those from bony impingement, for all head sizes considered. Additionally, dislocation resistance was substantially greater for instances of bony impingement versus hardware-only impingement. These findings suggest that hardware impingement may still be a/the the predominant mode of impingement even with the use of larger femoral heads, for sub-optimally positioned cups. Additionally, the data indicate that, should impingement occur, impingements between the implant neck and cup are (1) more likely to dislocate, and (2) have a greater propensity for causing damage to the implant compared to impingement events involving bony members.