The John Charnley Award A Study of Implant: Failure in Metal-on-Metal Surface Arthroplasties

The reintroduction of surface arthroplasty of the hip with metal-on-metal bearings has the potential to eliminate or substantially reduce long-term wear-induced osteolysis as the major cause of failure. To determine important modes of failure, implant retrieval analysis was done on 98 failed surface arthroplasty components from different manufacturers. Analysis involved sectioning the components, measuring cement mantle thickness and the depth of penetration, histopathology, and measurement of the bearing wear. A finite element model was constructed to examine cement thermal necrosis. Femoral neck fracture and femoral loosening were the main causes of failure. The finite element model showed thermal necrosis could occur when cysts were filled with cement. Histologic observations verified necrosis of interfacial bone, although adaptive remodeling was commonly seen. The amount of cement varied considerably with implant type, and failure mode and was greater in loosened components. Although implant failure is multifactorial, these observations should be a cause for concern in current cementing techniques in which controlling mantle thickness and extent of penetration may be difficult. Optimizing cement technique to avoid leaving the component proud, and to avoid extensive cementation of the femoral head, may be important in reducing some modes of failure.

With the application of metal-on-metal bearings, surface arthroplasty is again being performed in a growing number of centers worldwide. Although relatively few procedures have been performed in the United States, thousands of surface arthroplasty components have been implanted in Europe and Australia. We anticipate the problems faced by the first generation of metal-polyethylene surface arthroplasties, primarily related to debris-induced osteolysis caused by polyethylene wear,^3,5,6,23 can be overcome by the current generation of low wearing metal-on-metal surface arthroplasties.

Short-term clinical followup reports of metal-on-metal surface arthroplasties have been encouraging,^4,15-17 although femoral neck fractures^4,6,33,34 and femoral loosening² have been identified as causes of failure. Risk factors in surface arthroplasty highlight the importance of patient selection criteria and good bone quality for implant survival.¹¹ Currently, the role of femoral head vascularity in implant durability is controversial; some surgeons are concerned the posterior surgical approach sacrifices the important extraosseous blood supply to the femoral head,^8,10,32 whereas others believe adequate blood supply will be provided intraosseously.²¹ Although the reduced wear of metal-on-metal bearings is well recognized, there have been concerns that heat-treating the components after casting can lead to higher wear, possibly sufficient to cause osteolysis.¹⁵ The unknown long-term consequences of metal wear debris are also a concern.^26,28 Despite these concerns, the conservative nature of surface arthroplasty and the restoration of a high degree of function, including the ability to return to sports, make this surgery appealing to young, active patients. In our experience, patients are willing to travel long distances to specialty, high-volume centers, often at their own expense, for this surgical option.

With the introduction of any new device there will often be a learning curve as surgeons gain experience and understand the limitations and the factors involved in clinical success and failure. This occurred with the introduction of cementless fixation and it will likely be no different for surface arthroplasty of the hip, which is recognized as a more technically demanding operation than standard total hip replacement. Before the orthopaedic community moves forward with the widespread use of surface arthroplasty, it is paramount we identify mechanisms of failure not fully understood in the previous metal-polyethylene surface arthroplasty experience.

Despite many procedures performed over the past several years, failure analysis reports of revised metal-on-metal surface arthroplasty specimens are limited. We examined the failure mechanisms in metal-on-metal surface arthroplasty components submitted to our laboratories over the past 8 years. We compared the findings with failure modes identified in the metal-polyethylene surface arthroplasty era to examine the influence of removing the problem of wear-debris induced osteolysis as a primary mode of failure. The primary goal of these analyses was to understand which failures may be preventable through optimized patient selection and surgical techniques before the widespread reintroduction of surface arthroplasty. Second, we looked for new complications that were unique to this generation of metal-on-metal surface arthroplasty.