Pseudotumours following metal-on-metal hip resurfacing arthroplasty
Keywords: pseudotumours
hip resurfacing arthroplasty

Metal-on-metal hip resurfacing arthroplasty (MoMHRA) has undergone a recent resurgence as an alternative treatment option for young and active patients with significant hip osteoarthritis. Despite the satisfactory short-term implant survivorship, abnormal periprosthetic soft-tissue masses relating to the hip joint (pseudotumours) are being increasingly reported. These were found to be locally destructive, requiring revision surgery in a high proportion (75%) of patients, the outcome of which is poor. However, there is currently no definitive evidence of potential mechanisms involved in the occurrence of such destructive soft tissue masses. The broad aims of this thesis were: 1) to determine the extent of this emerging clinical complication (pseudotumour) with a high revision burden; and 2) to determine whether pseudotumour results from a local biological reaction to an increased wear debris burden generated by excessive MoMHRA implant wear. A clinical study investigated the prevalence of, and association between, pseudotumours and serum metal ion levels in 158 MoMHRA patients (201 hips). The overall prevalence of pseudotumour was found to be 4%, considerably higher than previously reported. The study also established that pseudotumour only occurred with elevated metal ion levels, suggesting the underlying cause is increased wear. It was postulated that pseudotumours are likely to be a biological reaction to the large amount of metal debris generated in vivo due to excessive wear. Two potential biological reactions mediating the occurrence of pseudotumours were then investigated. An in vitro clinical study demonstrated that systemic hypersensitivity type IV reaction, mediated by lymphocyte reactivity to these metals, is not the dominant biological response involved in pseudotumours. A further in vitro experiment demonstrated that metal particle-induced cytotoxicity is likely to be an important factor leading to pseudotumours. Subsequent research focused, firstly, on assessing the magnitude of bearing surface wear that occurs in pseudotumour patients, and secondly on elucidating the potential wear mechanism responsible for the increased implant wear in these patients. A retrieval study demonstrated that MoMHRA implants revised due to pseudotumour were associated with significantly greater linear wear rates. This combined with the metal ion data confirmed that pseudotumour is associated with increased wear at the MoM articulation. An edge-loading wear pattern was always found in the pseudotumour group. An in vivo study was then developed to investigate whether the edge-loading occurs during functional activities. Edge-loading in the pseudotumour group was found to occur with up to 4-fold increase in duration and up to 7-fold increase in force. This in vivo finding supported the in vitro retrieval study finding of an edge-loading wear pattern in the pseudotumour group, suggesting that edge-loading during functional activities is an important in vivo mechanism responsible for localised high wear and subsequent elevation of metal ion levels in MoMHRA patients with pseudotumours. It is concluded that pseudotumour is likely to result from a local biological reaction to increased metal debris load, generated by excessive MoMHRA implant wear due to edge-loading. In susceptible patients, the dose-dependent cytotoxicity of wear debris leads to subsequent necrosis of periprosthetic soft tissues. Clinicians need to be aware of pseudotumours and surgeons should undertake MoMHRA with great care in the knowledge that even with satisfactory component positioning, the problem can occur. In evaluating MoMHRA patients with unexplained symptoms with normal plain radiographs, further investigation with soft-tissue imaging modalities such as ultrasound or MRI is recommended.