There is growing recognition that metal-on-metal hip arthroplasties may fail early from novel mechanisms of failure in addition to the conventional mechanisms well described for all bearing surfaces. In reviewing all metal-on-metal hip arthroplasties revised at our institution over the past 3 years, our purpose was to document and describe the clinical and intraoperative findings in patients with early failures that appear to be unique to these particular implants.
We recognize limitations to our study. First, the study was not designed to provide information about the incidence or prevalence of the failure mechanisms that we encountered. We have attempted to put this failure mechanism into an overall clinical context by reviewing all revisions with a metal-on-metal bearing. However, as the majority of patients were referred to our institution, we do not know the denominator when it comes to the number of metal-on-metal implants that had been used in our referral area during the time of this study. Second, many available diagnostic modalities were not routinely or consistently employed in the workup of these patients. It is important to note that there is no clear consensus for the diagnosis of an adverse metal reaction, and individual laboratory and imaging studies may be useful (but not mandatory) adjuncts in the evaluation and treatment of these patients. Serum and joint ion levels reportedly correlate with in vivo wear and metal debris-related failures [6
] and may find routine use in clinical practice when the implications of these levels are better defined. Advanced imaging (MRI, CT, or ultrasound) was obtained in select patients on an individual basis when concern existed for pseudotumor or soft tissue destruction, but we do not have extensive cross sectional imaging data for all patients. Patch testing is unreliable and clinical validation of the accuracy of in vitro lymphocyte testing is lacking at the present time [10
]. Third, retrieval analysis was not performed in these patients, limiting our ability to comment on the role of wear of the bearing surfaces in the development of adverse immune responses.
The clinical presentation of patients with an adverse metal reaction was varied and often nonspecific. The majority of patients were women, consistent with previous reports in the literature [17
], although this phenomenon did not completely exclude males. One universal complaint in these patients was the feeling that they had either never fully recovered from the procedure or had experienced a new pain sensation since the hip replacement was performed. No patients reported being fully satisfied after the operation, which differs from previous reports that suggest early satisfaction with later development of pain [13
]. Stiffness and groin pain was typical when first arising from a chair or getting out of a car, nonspecific complaints that have also been reported with aseptic loosening of large-diameter metal-on-metal acetabular monoblock components [14
]. Several patients presented with features concerning for infection, including fevers, elevated inflammatory markers, and an elevated nucleated cell count on aspiration. On average, patients underwent revision around two years after the index procedure, although we noted substantial variability. This is consistent with previous reports suggesting that metal-related complications may take several years to progress to reoperation [13
The evaluation of patients with a painful metal-on-metal joint can be difficult with an expanded differential diagnosis including metal hypersensitivity and wear. In our experience, the presumptive reason for early failure in patients with an adverse reaction to metal included infection, component malposition and soft tissue irritation, and fibrous fixation of the acetabular component without osseointegration. Our approach to the diagnosis of adverse metal reaction requires exclusion of other well-defined causes of failure. It appears that the inflammatory reaction to the metal debris may increase serum markers and mimic a low grade infection with systemic symptoms [16
]. As such, inflammatory markers do not appear to adequately distinguish between an adverse metal reaction and infection, and all patients with abnormal values undergo aspiration with cultures. A high nucleated cell count was present in one of our patients; we therefore consider a positive culture off antibiotics to be more reliable. Furthermore, while component malposition has been suggested to play a role in patients with metal hypersensitivity [13
], only a minority of our patients had excessive abduction or anteversion. Early osteolysis has been also associated with metal hypersensitivity [19
], although this finding was not prevalent in our experience and seen in only one of our patients. The technetium Tc-99 m white blood cell/colloid study, which labels polymorphonuclear cells but not lymphocytes, may have some intuitive appeal but is of unknown clinical utility [3
We did not identify any pathognomonic intraoperative findings. Effusion was universal, metallosis was common, and the synovium often appeared shaggy. Obvious macroscopic wear and damage of the components was seen in three patients, suggesting that wear may play a role in the pathophysiology of the adverse metal response, although the majority of implants did not show any macroscopic damage. It is clear from our experience and others that encountering purulent-appearing fluid intraoperatively is not necessarily indicative of infection [7
]. Reports exist of two-stage revision for presumed infection with subsequent tissue pathology suggestive of a metal reaction and negative cultures [7
]. Frozen specimen analysis was used in all patients to exclude acute inflammation. In our series, the treatment for all patients with clinical, intraoperative, and frozen specimen findings consistent with metal hypersensitivity was single stage revision to a metal-on-polyethylene bearing couple.
The majority of patients had histologic findings consistent with a chronic inflammatory response. We observed perivascular lymphocytic infiltration similar to previously reported histological findings and consistent with ALVAL [5
]. Two patients did not appear to have an inflammatory component, but rather had extensive necrosis. These findings are consistent with previous reports suggesting a role for both cytotoxicity and delayed hypersensitivity in the pathogenesis of these reactions [15
]. While our experience corroborates previous findings, histopathologic testing continues to have unknown sensitivity and specificity for metal hypersensitivity. The prevalence of these histologic changes in well-functioning metal-on-metal hip arthroplasties is unknown.
The clinical response to revision procedures with a diagnosis of metal hypersensitivity has not been extensively reported. Revision surgery for pseudotumors with massive soft-tissue destruction has a poor reported outcome with substantial challenges [9
]. Grammatopolous et al. experienced a 50% incidence of major complications and described lower Oxford hip scores and UCLA activity scores following revision of a failed hip resurfacing in patients pseudotumors compared to matched controls undergoing total hip arthroplasty [9
]. Our patients have generally reported pain relief with revision of the bearing surface to a polyethylene insert. While the preoperative limp was seen to improve in many patients, one patient with major damage to the abductor musculature had a persistent severe limp. The amount of soft tissue destruction appeared to generally correlate with subsequent functional outcome. Following revision, there was one case of deep infection, one of femoral component loosening, one with radiolucencies around the acetabular component in an asymptomatic patient, and one case of persistent disabling pain.
While the main focus of our paper is to describe our clinical experience with adverse reactions to metal debris, review of our entire metal-on-metal revision population also suggests a substantial number of early failures of large-diameter metal-on-metal articulations are related to early acetabular component loosening [12
]. Technical issues with a steep learning curve have been suggested as a potential reason for these failures [12
], although other studies have suggested implant design features may be responsible [14
]. Our study is not designed to answer these questions. However, the solid stiff cobalt-chrome shells may be technically more challenging to implant correctly compared with traditional titanium acetabular components and do not allow for supplemental screw fixation.
The majority of revisions of metal-on-metal articulations at our institution were not specific or unique to this bearing couple. However, an adverse local reaction to metal must be considered when evaluating a failed metal-on-metal bearing surface. Consideration of this possibility, along with recognition of the varied clinical and intraoperative findings, should help guide treatment decisions.