Metal-on-metal (MOM) THAs have been performed in selected patients over the past 15 years, based on the theoretical advantage of improved wear and the ability to use large-diameter femoral head components to minimize dislocation . Despite these potential benefits, many MOM implants have not fared as well as other bearings [2, 8, 9, 17, 25, 27]. Adverse reaction to metal debris (ARMD) from either aseptic lymphocytic vasculitis-associated lesions (ALVALs) or metallosis can precipitate implant failure, in addition to more universal problems, including infection, dislocation, fracture, and component wear [1, 3-5, 7, 11-13, 24, 26]. An increasing body of evidence suggests that these implants have higher rates of complications and failure than ceramic or polyethylene alternatives [9, 15, 25]. As such, revision of MOM arthroplasty has become increasingly common.
Currently, few studies demonstrate how these patients fare after their revision surgery [18, 24]. In this study, we posed the following questions: (1) What is the short-term survivorship of revision THA in patients whose index THAs employed an MOM bearing? (2) What are the causes of failure of revision THA (that is, what are the indications for repeat revisions, when they occur) in these patients? And (3) what are the most common complications after revisions of MOM THAs?
Patients and Methods
After obtaining institutional review board approval, we reviewed all patients undergoing revision hip arthroplasty at our institution between January 2005 and May 2010. We identified 58 patients who had revision of a primary MOM THA. Revisions from hip resurfacing and septic revisions were excluded, leaving 37 patients (37 hips) for review. Twenty-four patients were women and 13 were men, with an average age of 55 years (range, 29-76 years) at the time of their primary MOM implant surgery. The time between index MOM THA and revision was available in 34 patients and averaged 27 months (range, 0.75-110 months). All of the revisions were performed for aseptic modalities of failure. The reasons for revision (diagnosed as described below) included pain with loose components (n = 19, 51%), ARMD (n = 8, 22%), periprosthetic fracture (n = 4, 11%), impingement (n = 3, 8%), dislocation (n = 2, 5%), and pain of unknown origin (n = 1, 3%). Patients were followed for a minimum of 24 months (mean, 33 months; range, 24-81 months) postrevision. Eleven of the 37 patients have not been seen at our institution within the past 24 months as our routine joint arthroplasty followup is at 2 and 5 years postoperatively.
Our study cohort included only aseptic revision cases after review of preoperative and postoperative infection parameters. The indications for revision surgery were determined to be aseptic by the operating surgeon and medical team through a combination of physical examination, radiographs, and laboratory studies. Furthermore, all cases had negative microbiology cultures from intraoperative tissue specimens and/or synovial fluid aspirates, which we use as our gold standard in the diagnosis of infection.
At the time of revision, the previous surgical approach was utilized when possible. A thorough débridement of soft tissue and cystic masses was done, with special attention at removing necrotic tissue wherever present. The implants were revised; however, the femoral stem was commonly not revised and translated into an anterior or posterior soft tissue pocket to allow exposure of the acetabulum. The acetabular component was removed with an explant device. Reaming was carried out to achieve bleeding bone within a hemispherical cavity. Implantation of a press-fit uncemented hemispherical component was performed and supplemented with screw fixation. A highly crosslinked polyethylene liner was employed in addition to a ceramic head if possible. Ceramic heads with a titanium sleeve were utilized on a used trunnion, that is, if the stem was not revised. A 36- or 40-mm head was used most commonly, but the goal was to enhance stability by using a larger femoral head size when possible.
Radiographic parameters were assessed in all patients preoperatively and at last followup (Table 1). Pre- and postrevision complications were defined as follows. Pain with loose components applied to all patients with components displaying positional change, complete radiolucent lines about the implant, or that were found to be grossly loose intraoperatively. Infection was defined as a positive intraoperative tissue culture. ARMD was determined by pathologic findings of ALVALs or with the presence of a pseudotumor with well-fixed implants. Fracture was the primary diagnosis for patients with evidence of fracture on radiograph and intraoperatively. Dislocation was applied to patients with a history of recurrent dislocation and instability as the primary reason for revision. Impingement was the categorization for patients who experienced pain or limited ROM consistent with prosthetic-bone impingement on physical examination, radiographs, and intraoperative findings. Pain of unknown origin was used for one patient who had pain in the absence of any identifiable cause listed above.
A Kaplan-Meier curve was generated to analyze postrevision survivorship. All analyses were performed using SAS® Version 9.2 (SAS Institute, Inc, Cary, NC, USA) and R Version 2.14 (R Development Core Team, Vienna, Austria).
Survivorship free from further revision for any cause was 95% at 24 months and 92% at most recent clinical followup, which ranged from 24 to 81 months postrevision depending on the patient (Fig. 1). At latest followup, three hips (8.1%) required a reoperation for infection. The three patients who developed infection after the index revision had initially been revised for ARMD (n = 2) and dislocation (n = 1). Intraoperative cultures were positive for coagulase-negative Staphylococcus in two and Group G Streptococcus in one. Apart from the repeat revisions for infection, no other postrevision complications were observed in this patient cohort.
Many recent studies have demonstrated that some MOM THAs have inferior implant survivorship in comparison to other bearing surfaces [9, 15, 25]. In light of the early failure of many MOM implants, more practices are seeing patients who meet indications for revision surgery for failures related to this bearing surface, but there are few reports on survivorship and causes of failure after revision of MOM THAs [18, 24]. We therefore reported our experience from one institution and answered the following questions: (1) What is the short-term survivorship of revision THA in patients whose index THAs employed an MOM bearing? (2) What are the causes of failure of revision THA (that is, what are the indications for repeat revisions, when they occur) in these patients? And (3) what are the most common complications after revisions of MOM THAs?
This study is limited in some critical areas. First, our relatively small sample size does not allow for statistically significant conclusions to be drawn, in addition to limiting generalizability. When considered in combination with the short-term followup, it is important to realize that perhaps a greater variety of postrevision complications could be observed with a larger patient cohort and extended followup period. However, despite the small sample size, the risk of infection was surprisingly high, and we therefore felt compelled to report on it. Second, our study did not include a contemporaneous control group or a statistically valid matched historical control group. We do note, though, that compared to an earlier published series (including patients from 1969 to 1996) that did not include MOM THAs , the frequency of infection in the present series appears high. It is also possible that ARMD was underreported in this series as we only included those with ALVALs from histology or frank intraoperative pseudotumors or metallosis.
The survivorship of our patients was 95% at 2 years and 92% at most recent clinical followup (24-81 months). These rates are concerning particularly when considering the short-term followup in this cohort. Importantly, 11 of the 37 patients have not been seen at our institution in the past 24 months, and given that they could have been revised elsewhere during this time period, the concerning survivorship data in this series only represent a best-case scenario.
The singular cause of repeat revision observed in our study cohort at this early followup period was infection, at 8.1% (three of 37 patients). This is much higher than what we have seen over sustained periods of time at our institution. In a publication by Hanssen and Rand , 3.2% of 7161 revision THAs performed between 1969 and 1996 at our institution were complicated by infection. This represents a time period before MOM was performed and thus serves as an internal control to compare infection rates with the current study. With a larger sample size and longer-term followup, we hope to determine whether this apparent finding indeed is valid. Nevertheless, the rate of infection at this time point is concerning and demands current attention and further investigation.
Beyond the three patients who were rerevised for a failure due to infection, no other postrevision complications were observed in this series. The complication profile observed at our center differs from the findings of a smaller series of 32 patients reported on by Munro et al.  in which they observed major complications in 38% of patients where dislocation was prominent at 28%. Dislocation is certainly a concern in patients being revised from an index MOM implant as it often necessitates downsizing the femoral head component. The risk of this complication can be further amplified in cases where ARMD or infection leads to significant soft tissue damage in failure of the primary implant. This was mitigated in our series as we only selected aseptic failures and was amplified in the study by Munro et al.  as their study cohort involved many patients being revised for a pseudotumor. It is however common practice at our institution to use larger head sizes to prevent postoperative dislocation.
While still being susceptible to complications universal to all hip arthroplasties, MOM implants also pose the challenge of ARMD. The exact molecular and cellular mechanisms of ARMD are complex and poorly understood, but some authors have suggested the primary etiology is a delayed Type IV hypersensitivity reaction to metal debris [16, 19-21]. What is clear is that ARMD has a broad spectrum of clinical presentations that can be difficult to differentiate from other modalities of hip pain. Complications can include soft tissue damage, osteolysis, pseudotumor formation, fluid collections or cysts, and vascular pathology. In addition, it has been shown to mask or be present in conjunction with infection [10, 14]. Although we cannot establish a cause-and-effect relationship in our study, it is interesting to note that two of the three postrevision infection complications were in hips initially revised for ARMD. Perhaps this aggressive inflammatory process is locally predisposing to a greater infection risk through immune system modification, remaining particulate debris, or bacterial seeding of damaged tissue. This observation is not unprecedented, as Petrie et al.  found that patients with metal-backed patella components in TKA were at an increased risk of postoperative infection.
Patients who present with a failed MOM THA may be at higher risk for infection when they undergo revision THA. The experience from this series should raise the index of suspicion for surgeons in equivocal cases of infection after revision of a MOM THA. As revision of primary MOM THA becomes more commonplace, it will be important to continue monitoring the postrevision outcomes of these patients and providing treatment options to mitigate complications with their revision implants.
The authors thank Youlonda Loechler who manages the joint registry at our institution for her contribution to identifying patients in the study cohort.
1. Barrett WP, Kindsfater KA, Lesko JP. Large-diameter modular metal-on-metal total hip arthroplasty: incidence of revision for adverse reaction to metallic debris. J Arthroplasty.
2. Cohen D. Revision rates for metal on metal hip joints are double that of other materials. BMJ.
3. Evans EM, Freeman MA, Miller AJ, Vernon-Roberts B. Metal sensitivity as a cause of bone necrosis and loosening of the prosthesis in total joint replacement. J Bone Joint Surg Br.
4. Grammatopolous G, Pandit H, Kwon YM, Gundle R, McLardy-Smith P, Beard DJ, Murray DW, Gill HS. Hip resurfacings revised for inflammatory pseudotumour have a poor outcome. J Bone Joint Surg Br.
5. Gruber FW, Bock A, Trattnig S, Lintner F, Ritschl P. Cystic lesion of the groin due to metallosis: a rare long-term complication of metal-on-metal total hip arthroplasty. J Arthroplasty.
6. Hanssen AD, Rand JA. Evaluation and treatment of infection at the site of a total hip or knee arthroplasty. Instr Course Lect.
7. Huber M, Reinisch G, Trettenhahn G, Zweymuller K, Lintner F. Presence of corrosion products and hypersensitivity-associated reactions in periprosthetic tissue after aseptic loosening of total hip replacements with metal bearing surfaces. Acta Biomater.
8. Huo MH, Stockton KG, Mont MA, Bucholz RW. What's new in total hip arthroplasty? J Bone Joint Surg Am.
9. Jameson SS, Baker PN, Mason J, Porter ML, Deehan DJ, Reed MR. Independent predictors of revision following metal-on-metal hip resurfacing: a retrospective cohort study using National Joint Registry data. J Bone Joint Surg Br.
10. Judd KT, Noiseux N. Concomitant infection and local metal reaction in patients undergoing revision of metal on metal total hip arthroplasty. Iowa Orthop J.
11. Korovessis P, Petsinis G, Repanti M, Repantis T. Metallosis after contemporary metal-on-metal total hip arthroplasty: five to nine-year follow-up. J Bone Joint Surg Am.
12. Kwon YM, Thomas P, Summer B, Pandit H, Taylor A, Beard D, Murray DW, Gill HS. Lymphocyte proliferation responses in patients with pseudotumors following metal-on-metal hip resurfacing arthroplasty. J Orthop Res.
13. Maezawa K, Nozawa M, Matsuda K, Sugimoto M, Shitoto K, Kurosawa H. Serum chromium levels before and after revision surgery for loosened metal-on-metal total hip arthroplasty. J Arthroplasty.
14. Mikhael MM, Hanssen AD, Sierra RJ. Failure of metal-on-metal total hip arthroplasty mimicking hip infection: a report of two cases. J Bone Joint Surg Am.
15. Milosev I, Kovac S, Trebse R, Levasic V, Pisot V. Comparison of ten-year survivorship of hip prostheses with use of conventional polyethylene, metal-on-metal, or ceramic-on-ceramic bearings. J Bone Joint Surg Am.
16. Milosev I, Trebse R, Kovac S, Cor A, Pisot V. Survivorship and retrieval analysis of Sikomet metal-on-metal total hip replacements at a mean of seven years. J Bone Joint Surg Am.
17. Molli RG, Lombardi AV Jr, Berend KR, Adams JB, Sneller MA. Metal-on-metal vs metal-on-improved polyethylene bearings in total hip arthroplasty. J Arthroplasty.
18. Munro JT, Masri BA, Duncan CP, Garbuz DS. High Complication Rate After Revision of Large-head Metal-on-metal Total Hip Arthroplasty. Clin Orthop Relat Res.
2013 April 10 [Epub ahead of print].
19. Pandit H, Glyn-Jones S, McLardy-Smith P, Gundle R, Whitwell D, Gibbons CL, Ostlere S, Athanasou N, Gill HS, Murray DW. Pseudotumours associated with metal-on-metal hip resurfacings. J Bone Joint Surg Br.
20. Pandit H, Vlychou M, Whitwell D, Crook D, Luqmani R, Ostlere S, Murray DW, Athanasou NA. Necrotic granulomatous pseudotumours in bilateral resurfacing hip arthroplasties: evidence for a type IV immune response. Virchows Arch.
21. Park YS, Moon YW, Lim SJ, Yang JM, Ahn G, Choi YL. Early osteolysis following second-generation metal-on-metal hip replacement. J Bone Joint Surg Am.
22. Petrie RS, Hanssen AD, Osmon DR, Ilstrup D. Metal-backed patellar component failure in total knee arthroplasty: a possible risk for late infection. Am J Orthop (Belle Mead NJ).
23. Quesada MJ, Marker DR, Mont MA. Metal-on-metal hip resurfacing: advantages and disadvantages. J Arthroplasty.
24. Rajpura A, Porter ML, Gambhir AK, Freemont AJ, Board TN. Clinical experience of revision of metal on metal hip arthroplasty for aseptic lymphocyte dominated vasculitis associated lesions (ALVAL). Hip Int.
25. Sehatzadeh S, Kaulback K, Levin L. Metal-on-metal hip resurfacing arthroplasty: an analysis of safety and revision rates. Ont Health Technol Assess Ser.
26. Willert HG, Buchhorn GH, Fayyazi A, Flury R, Windler M, Koster G, Lohmann CH. Metal-on-metal bearings and hypersensitivity in patients with artificial hip joints: a clinical and histomorphological study. J Bone Joint Surg Am.
27. Zijlstra WP, Cheung J, Sietsma MS, Raay JJ, Deutman R. No superiority of cemented metal-on-metal vs metal-on-polyethylene THA at 5-year follow-up. Orthopedics.