Metal-on-metal (MOM) hip articulations were reintroduced several years ago but have recently been subjected to intense discussions due to reported re-operations and adverse tissue reactions.

The recent generation of MOM articulations developed showed initially superior resistance to wear and raised hope of less re-operations, specifically in younger patients with increased activity and longer life expectancy [1]. In addition, MOM articulations could provide superior range of motion because of better resemblance to hip anatomy, a normalized gait pattern as well as better stability with reduced dislocations and decreased impingement symptoms [2, 3]. MOM was on this background reintroduced and several short and middle term studies of the MOM articulation showed promising results with few re-operations [4-11]. Recently though, information on adverse tissue reactions and high re-operation rates have emerged [12-25]. This leads to the issue of a Medical Device Alert by the Regulatory Agency in the UK in 2010 and the recall of certain articulations from the marked [26, 27].

Several MOM articulations are still produced and sold for implantation but with conflicting research data accumulating regarding adverse reactions, as well as evidence of high re-operation rates, it is imperative to gather further data on the follow up results of patients receiving MOM articulations.

The Conserve^® MOM articulations are still sold and in use. The Conserve^® MOM resurfacing articulations have been thoroughly investigated but the Conserve^® MOM TOTAL hip articulation (MOM THA) only sparse and with conflicting results.[12, 28-36]. To disclose any irregularities in the use of this prosthesis, further investigation of patients receiving the Conserve^® TOTAL hip articulation, is required.

The aim of this study was to evaluate the implantation of the MOM THA from a single manufacture (Wright Medical Technology, Inc. (WMT), Arlington, TN). Patients receiving this articulation had at time of retrospective evaluation their journals reviewed to identify re-operation rates and adverse reactions around the implanted articulation.

The mean follow-up period was 53 month (SD: 11.5 and range: 7-76) and at the time of retrospectively evaluation 20 hips where found to have required re-operation. Different reasons for re-operations were diagnosed: aseptic cup loosening, (n = 7), pain (n = 5), infection (n = 2), aceptic loosening of the stem (n = 1), dislocation (n = 1), ectopic bone (n = 1), elongation of leg (n = 1) and postoperative bleeding (n = 1) (Table 2).

Revisions had a total failure rate of 18,5% (CI: 0.12 – 0.26) where men presented with a 19.4% (n =13/67, CI: 0.1 – 0.29) re-operation rate and women with a 17.1% (n=7/41, CI: 0.056 – 0.29) re-operation rate. No statistical significance difference was detected between the two groups (p-value: 0.76). The mean age at insertion of the MOM THA was 51.4 years and the overall mean time to revision was 18.7 months.

Two adverse reactions were reported by the surgeons in the operating theatre and described as located in the area around the implanted component. One was a pseudotumor previously diagnosed using MRI and the other, a suspected pseudotumor found during operation with no previously diagnosis from ultrasound or MRI. No histology analyses were conducted to exclude or document adverse tissue reactions.

MRI results were available from 8 patients and pseudotumors were diagnosed in 4 of these. One pseudotumor was classified as a type I with a cystic wall thickness of 3 mm. A second was diagnosed as a type II and a third described as small with no further classification. The fourth was characterized as “widely spread” but not with further details given. Only the “widely spread” pseudotumor was at retrospective evaluation re-operated. The other three hips presenting with pseudotumors are all being followed closely at the out patients clinic. Evaluation of the radiographs documented no hips with sign of adverse reactions.

Self reported pain was observed in 3 cases (n =3/108) a side from the people previously receiving revision. These patients were enrolled to receive extended clinical control and were to be followed on a more frequent basis in the out patients clinic.

Our results originate from two hospital centres where senior hip surgeons apart from two cases performed the operations. Apart from three re-operations, the re-operations were performed by one senior surgeon at The Department of Orthopeadic Surgery at The Regional Hospital in Viborg. Three re-operations were conducted in Thy and one at other hospitals. We had full access to all the patient’s journals.

The Kaplan-Meier survivorship analysis conducted estimated a survivorship of the Conserve^® MOM THA to 96% at 1 year, 93% at 3 year and 88 % at year 8. End point was defined as re-operation of the MOM THA (Fig. 2).

Fig. (2). Kaplan-Meier survivorship of the implanted THA. Re-operation was defied as failure.

MOM THA was reintroduced several years ago and has recently been subjected to intense debate. Promising results were initially shown using the MOM THA but recently studies have disputed this and accumulating research have been reported presenting high re-operation numbers and adverse reactions. In present study, we report on a high re-operation rate of 18,4% at a mean follow up of 53 months using the Conserve^® MOM THA.

There are several limitations correlated with this retrospective study. Loss to follow up of patients and sectional biases in election of patients to receive the MOM articulation. The suggested correlation with the metal sensitivity in the literature and the debris of the MOM THA was not apparent before years after initiation of usage of the Conserve^® MOM THA. Therefore, we have inconsistencies in blood ion sample collection, making analysis of these results impossible. X-rays of the pelvis was not conducted on a regulatory basis before re-operation and we have therefore not been able to investigate the inclination and the anteversion of the prosthesis effect on articulation failure.

Short and middle term studies have investigated MOM THA, from different manufactures, with conflicting results. Several studies have produced promising results with low re-operation rates ranging from zero to a few percent while other have identified high re-operation rates between 15-30% [4-9]. The Conserve^® resurfacing articulation has been intensively investigated and produced remarkable results with survival percentages reaching 90% in middle and short-term studies [28-34, 36]. The Conserve^® TOTAL MOM THA has only, to our knowledge, been investigated in few studies. A one year follow up study found no re-operations of the articulation while another study by Levy YD et al. reported on the short-term outcome of 78 consecutive implanted MOM THA with a mean follow up period of 1.3 ±1 years [12, 35]. This study showed a re-operation rate of 15.4%. Our study of insertion of 108 Conserve^® TOTAL MOM THA on 92 patients showed a re-operation rate of 18,4% divided on 17,1% for women and 19,4% for men with a mean follow up period of 53 month. The Kaplan Maier survivorship analysis predicted a survivorship of the Conserve^® MOM THA on 88 % at 8 years (Fig. 2). No statistical significant difference regarding re-operation rates in women or male patients was found. This is not in accordance with the literature on MOM THA where studies have shown significant higher re-operation rates in women [36, 40].

Our results correlate with the high revision rates previously found in other studies and our study is largely comparably to these. Introduction of a new implant and the “learning curve” associated with this could explain some of the re-operations but can surly not account for the total amount of re-operations. Table 2 lists several reasons of re-operations but our study failed to provide a control group and on this background it is difficult to conclude on these specific reasons of articulation failure. The high re-operation rates encountered and the subsequently failure to appropriately treat patients, raised concern about the use of the MOM THA and subsequently lead to the termination of implantation of the MOM THA at the two orthopaedic departments.

Several different failures of the articulations were documented as cause of re-operation in our study with aseptic loosening and pain listed as the most abundant failures (Table 2). In MOM THA clinical failures and pain have been associated with reported adverse reactions in tissue around the articulation [12, 20-22]. The definition of the adverse reactions in MOM hip articulations is subject to an on going discussion. No consensus has been reached but several terms are currently in use. These include: Metallosis [20], pseudotumors [21], aseptic lymphocytic vasculitis-associated lessions (ALVAL) [22], adverse reactions to metal debris (ARMD) [23] and adverse local tissue reactions (ALTR) [24]. These all describe different aspects or stages in the development of soft and bone tissue reactions around the hip. A study reported on pseudotumors in 28 % of hips after an average follow up of 41 months using the Birmingham Hip Resurfacing Prothesis [25]. Another investigation of the Conserve^® TOTAL MOM THA showed pseudotumors diagnosed in 40% of the re-operations at a mean follow up of 1,3 years [12]. In present study, two pseudotumors were suspected during surgery but no tissue was sent to histology test. No other adverse tissues reactions were identified during surgery. Pseudotumors were diagnosed around 4 articulations of our patients after MRI scanning’s. Of these articulations, one was re-operated at the time of retrospective evaluation. The additional 3 were followed closely in the out patient clinic. We have on this background not substantiated any link between adverse reactions and re-operations in our study.

Groin pain has in the literature been correlated with adverse reactions around MOM THA and has in addition been mentioned as a first predictor of the Conserve^® TOTAL MOM THA failure [12, 41]. We found, in accordance with this, pain preoperative in all MOM THA re-operated. At the time of retrospective evaluation, additional three patients presented with groin pain at time of follow up.

In conclusion, we found a re-operation rate of 18.4% in present study. Only few of these re-operations were correlated with adverse reactions and did not substantiate a sure link between these two. We can on the basis of present data, not firmly conclude on the reasons of articulation failure but because of the high revision rate encountered the departments stopped using the MOM articulation. To further disclose and determine re-operations and possible adverse reactions related to this articulation further investigation of this MOM THA is necessary.

Research design and collection of data: Thomas Jakobsen, Niels Krarup, Hardy Christoffersen and Simon Limbrecht Mogensen

Wrote or contributed to the writing of the manuscript: Simon Limbrecht Mogensen, Thomas Jakobsen and Niels Krarup.

The authors declare that they have no competing interests.

The authors greatly appreciate the help by Inger Holm in collecting MRI results.