Oliver Marin‐Peña MD1, Olga Pidgaiska MD2, and Pedro Reis‐Campos3 1 Hip Unit, Orthopedic and Traumatology Department, University Hospital Infanta Leonor, Madrid, Spain 2 Orthopaedics Arthrology and Arthroplasty Department Sytenko Institute of Spine and Joint Pathology, Kharkiv, Ukraine 3 Orthopedic and Traumatology Department, Hospital Vila Franca de Xira, Portugal The indications for MoM‐THA are currently limited due to a loss of confidence in certain devices. In analyzing survival rate of MoM hip implants, it is important to differentiate between HR and THA. While some MoM devices associated with recalls have caused concern, other MoM‐THA devices continue to have an acceptable rate of success. In young patients, MoM‐HR is believed to provide slightly better functional outcomes but slightly worse survival rate in the long‐term compared with ceramic‐on‐ceramic (CoC). Two studies that have analyzed 28 mm femoral head MoM‐THA found better outcomes compared to 36 mm heads, with a survivorship greater than 90% survivorship at 15.11,12 National database registries report failure rates with MoM‐THA to be two‐ to threefold higher than THA with non‐MoM bearings. In a meta‐analysis, MoM was found to have an all‐cause revision rate that was higher than CoC.1 Similarly, in a cohort of 6215 MoM‐THA patients versus 7360 CoC THA patients, the revision rate in MoM was higher than CoC bearing cohort. A recent systematic review which included 40 randomized controlled trials (RCTs) confirmed the same conclusion of a lower survival rate of MoM‐THA.3 Higuchi et al. compared MoM and CoC hip arthroplasties and concluded that the incidence of osteolysis was lower in CoC, but that the survival rate was similar in both groups.3 Another long‐term study revealed that patients younger than 50 years of age with MoM HR maintained substantial improvements in health and function beyond 10 years after the surgery.10 The Nordic Arthroplasty Register Association analyzed 32 678 cementless stemmed THA. At six‐year follow‐up, the revision rate was significantly higher for MoM compared to metal‐on‐polyethylene (MoP) stemmed THA. In contrast, the prevalence of revision due to dislocation was lower for MoM‐THA.4 The Australian Joint Registry demonstrated 5‐ and 10‐year revision rates of 3.3 and 7.4%5 with stemmed MoM‐THA. Seppanen et al. reported an 86% 10‐year survival rate of MoM‐HR from the Finish registry when all type of centers were analyzed.7 However, excellent survival rates were reported from certain single centers as high as 97% at 10 years.8 In patients younger than 45 years of age, survival rate was similar between HR and conventional THA, and HR patients were able to return to a moderate or high activity level.9 Furthermore, recent systematic review has suggested some clinical potential advantages of HR against THA1 but slightly higher revision with lower complications rates.2 At the 6th Advanced Hip Resurfacing Course, 67% of surgeons suggested completely abandoning stemmed MoM‐THA with large diameter heads.6 Younger women in combination with larger head size were associated with increased revisions. Moreover, it was described that reoperations were more frequent and occurred earlier for MoM in this high‐risk group.2 However, they recommended that MoM‐HR should not be abandoned and should be viewed separately from stemmed MoM‐THA with a large diameter head.6 Metal ion levels of chromium (Cr) and cobalt (Co) in patients with MoM‐THA could be increased during follow‐up and this issue could be related with THA malfunction and potential complications. The authors have in their experience seen asymptomatic young patients with an MoM‐THA and minimal radiological changes during routine follow‐up visits who have incidentally been noted to have elevated metal ion levels. Optimal monitoring of metal ion levels is unclear. After MoM hip arthroplasty, patients with blood metal ions levels below international thresholds have a lower risk of adverse reactions to metallic debris.16 However, some authors have reported that blood metal ions levels are not correlated with intraoperative tissue damage, presence of pseudotumor, or pseudotumor size.28,29 Another study reported that the synovial fluid metal ions levels were also not correlated with histological severity in MoM hip arthroplasty revisions.17 Moreover, it’s known that the interpretation of the blood metal ions levels can be difficult in patients with systemic renal disease, other metallic implants, or bilateral MoM implants. Thus, the analysis of the blood metal ion levels should be used as complementary information but not as an isolated parameter to establish the need for revision surgery.18 MacNair et al. found poor correlation between blood metal ion levels and the occurrence of adverse reaction to metal debris (ARMD) on magnetic resonance imaging (MRI) and recommended that the decision to revise implant should be based on imaging and not on blood metal ion levels.23 Malek et al. demonstrated positive MRI findings combined with high metal ions levels increased detection of a malfunctioning MoM‐THA implant.24 Langton et al. recommended measuring ion values even in asymptomatic patients due to silent osteolysis when Co blood concentration greater than 20 μg/L was present.25 Ohtsuru et al. recently reported a positive correlation in cup inclination and metal ion levels.19 However, another study concluded that the acetabular inclination angle was not a meaningful determinant of higher metal ion levels.20 Furthermore, RCT data have reported no correlation between acetabular inclination and metal ion levels.13 Another study analyzed malfunctioning MoM‐THA with a mean cup inclination of 45.6° and concluded that there is no relationship between cup inclination and metal ion levels.34 The 6th Advanced Hip Resurfacing Course established 40° of inclination (±10°) and 15° of anteversion (±10°) as acceptable limits for acetabular positioning.6 De Haan et al. proposed that metal ion levels increased when cup inclination was >55° compared with <55°. Indeed, functional arc of cover and component design were mentioned as important risk factors.31 In contrast, a prospective study with unilateral MoM HR concluded that metal ion levels positively correlated with the three‐dimensional orientation of the acetabular component and gender but not body mass index (BMI), femoral head size, or hip type.14 Threshold of seven parts per billion (ppb) had 89% specificity, but only 52% sensitivity for detecting a failed MoM hip prosthesis. At a threshold of 4.97 μg/L sensitivity was 63%, and specificity was 86%.14
24 Metal‐on‐Metal Hip Arthroplasty
Clinical scenario
Top three questions
Question 1: In young, active patients undergoing MoM‐HR, is the revision rate higher than those undergoing metal‐on‐metal total hip arthroplasty (MoM‐THA)?
Rationale
Clinical comment
Available literature and quality of the evidence
Findings
Resolution of clinical scenario
Question 2: In patients who have undergone MoM‐HR, does monitoring metal ion levels, compared to no active monitoring, affect outcomes or revision rates?
Rationale
Clinical comment
Available literature and quality of the evidence
Findings
Metal ions levels and adverse tissue reaction
Metal ion levels and imaging findings
Metal ion levels and component malpositioning
Metal ion level threshold
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