Adverse Wear in MOM Hip-Arthroplasty Related to the Production of Metal Fragments at Impingement Sites

Metal on metal (MOM) bearings were reintroduced as resurfacing arthroplasty (RA) for the younger, more active patient and were later incorporated into total hip arthroplasty (THA). Early results were encouraging. However, recent publications identified adverse tissue responses to metal debris, such that the majority of MOM designs have been abandoned due to the increase in cobalt-chromium (CoCr) debris and associated metal ions. Reports of MOM THA cited risks that included acetabular cups with high-inclination angles, i.e. “edge-loading”, and “trunnionosis”. Hip impingement was also a cited risk in one MOM study, with “type-IV” wear noted to be a sliding/impaction type of wear, characterized by deep scratches. Sliding/impaction wear mechanisms produced at impingement are not well represented in current MOM literature. Therefore, our objective in this review was to consolidate evidence for impingement risks. We hypothesize that hip impingement and subluxation with metal-backed acetabular cups can trigger wear mechanisms that result in, 1) femoral-neck notching, 2) release of large metal particles, 3) production of uniquely large scratches, defined as “microgrooves” on heads and cups, 4) formation of “polar” and “basal” microgrooves precisely aligning with cup profiles during impingement, and 5) equatorial microgrooves relate to soft-tissue sites of impingement. Relevant risk scenarios were evaluated and included hip impingement in both sitting and standing postures, head subluxation, wear patterns defining in-vivo component positions, and evidence for circulating metal fragments. The study relied on mapping of wear patterns to deduce in-vivo positioning of devices and relied on surrogate femoral stems of the same brand to simulate neck-cup impingement. EOS imaging techniques were used to analyze functional-sitting and functional-standing postures and prove existence of hip impingement sites in patients. The study identified 8-risk scenarios for wear damage on MOM bearings. The microgrooves on femoral-heads crossing the main-wear area (polar) and non-wear regions (basal) aligned well with cuprim profiles at impingement sites. This may represent the first description of such large scratches (40 - 300 μm wide) we termed microgrooves, that formed on femoral heads at sites representative of prosthetic impingement. As an abrasive wear process, similar to the formation of femoral-neck notches, these would have been acquired over millions of gait cycles. The pitting and linear microgrooves crossing the non-wear areas of heads (basal) represented the ingress sites of circulating metal particles. Similar micro-grooves were evident in acetabular cups, also signifying 3rd-body abrasion by large metal particles. Hip impingement and head subluxation were implicated by the unequivocal evidence of 3rd-body abrasive wear as the triggering events producing large metal fragments. One caveat regarding retrieval studies is that such damage may be only representative of failed MOM devices. This study demonstrated that emerging technologies such as EOSTM x-ray analyses can reveal subtle changes in implant positioning using patient shifts in functional postures (sitting, standing, hyper-extension, etc.), and thereby assess impingement/subluxation risks in the clinical setting before failure occurs.