Where Are We Now?
In this retrospective study of the recalled Articular Surface Replacement (ASR) hip system (DePuy Orthopaedics, Warsaw, IN, USA), Galea and colleagues  identified the risk factors associated with revision and provided guidelines for managing patients with unrevised ASR hip systems.
The authors set out to determine whether the presence of symptoms as measured by patient-reported outcome measures or an ion level threshold correlated with metal artifact reduction sequence (MARS)-MRI abnormalities. A total of 327 patients received MARS-MRIs regardless of clinical presentation, which was a strength of the study. The authors found that while symptoms (defined as a Harris hip score below 75, or a VAS pain score of 4 or more) seemed to correlate with MRI abnormalities in ASR THAs, they did not correlate in ASR hip resurfacings. Additionally, they found that a cobalt ion level threshold of 3.2 parts per billion (ppb) for metal-on-metal (MoM) THAs and 2.9 ppb for hip resurfacings were associated with an increased risk of MRI abnormalities. Synovial thickness also seemed to correlate with symptoms in patients with MRI abnormalities.
Many MoM implants remain in service, and while the majority continue to function well, identifying malfunctioning bearings is still a concern. Fortunately, clinicians are aware of the many issues surrounding these implants, which now more broadly include modular-neck implants and traditional metal-on-polyethylene implants, where mechanically assisted crevice corrosion has been found at the trunnion head interface [7, 9]. Attempts have been made to provide an algorithmic approach for the management of MoM hips  by dividing patients into low-, moderate-, and high-risk based on symptoms, clinical exam, radiographs, implant type, ion levels, or cross-sectional imaging.
Where Do We Need To Go?
A few caveats must be considered when applying such algorithms. It is essential to rule out all intrinsic or extrinsic reasons for hip pain before implicating the bearing as the reason for symptoms. Additionally, overreliance on a single investigative tool used in the clinical decision-making process should be avoided. While all clinicians are looking for a silver diagnostic bullet, unfortunately, one does not currently exist. Vigilance is essential in evaluating patients with MoM hip arthroplasties. Recognizing and diagnosing symptoms early will promptly facilitate suitable treatment options before any adverse biological reactions.
The biggest challenge is how to treat the asymptomatic patient with a MoM bearing. As this paper, and others like it point out [2, 4, 8], adverse local tissue reactions (ALTRs) reflected as MRI abnormalities can occur without symptoms. Hence, a lack of symptoms does not absolve the clinician of his or her surveillance responsibilities. When presented with a symptomatic patient with a MoM implant, the decision to proceed with further ion testing or cross-sectional imaging is clear, as long as other intrinsic or extrinsic sources have been ruled out. However, in asymptomatic patients, the presence of abductor weakness, mechanical symptoms, a malpositioned acetabular component, or a recalled implant should stimulate the immediate need for screening ion levels or cross-sectional imaging regardless of symptoms.
While one would assume that symptoms should correlate with MRI abnormalities, this is not always the case. Symptomatic patients certainly can have MRI lesions; however, multiple studies have shown MRI abnormalities in asymptomatic patients with resurfacing MoM implants [6, 12]. A report from our center found a 31% prevalence of MRI abnormalities in asymptomatic MoM patients with well-positioned acetabular components. A total of 24 out of 26 (92%), and 22 out of 26 (85) patients had chromium and cobalt levels less than seven parts per billion, respectively .
Cystic MRI abnormalities in asymptomatic patients with MoM implants can diminish over time , and MARS-MRI abnormalities can be seen in approximately one-third of asymptomatic patients with metal-on-polyethylene implants . Therefore, an algorithmic approach using the presence or absence of symptoms to determine a decision tree pathway should be viewed with skepticism.
Additionally, while Galea and colleagues suggest a cobalt threshold of approximately three parts per billion; their sensitivity in predicting an associated MRI abnormality, which reflects diagnostic accuracy, is low (68%). Two different centers found that the sensitivity of metal ion levels as a predictive diagnostic test is also low, and both studies could not find a direct correlation between ion levels and the tissue damage observed at the time of revision surgery [5, 11]. It appears that, as with patients with metal-on-polyethylene bearings, patients with MoM implants may have a variable response to metallic debris and variable thresholds for when cobalt and chromium ion levels lead to ALTRs
Unanswered questions remain regarding how to evaluate patients with MoM implants. It is unclear how often asymptomatic patients need to be monitored—should such followup include merely a clinical or radiographic examination? Should routine ion levels and cross-sectional imaging be performed instead? Additionally, the natural history of MRI abnormalities noted on routine followup should be clarified. If a minor MRI abnormality is encountered initially, such as a small, thin-walled cystic lesion that does not warrant surgical treatment, the timing of a followup MRI remains unclear. The natural history of such lesions has surprisingly shown that cystic MRI abnormalities can diminish with time in asymptomatic patients . Consequently, clinicians should determine whether a revision is warranted based on a single diagnostic test. However, more studies of this nature need to be done to verify what type of lesions decrease in size, increase, or remain the same.
How Do We Get There?
Centers with a large number of patients with MoM implants under surveillance should conduct longitudinal studies to help answer these salient questions. While Galea and colleagues had 327 patients with yearly MRIs and clinical exams, they had a total of 721 patients enrolled initially . If the authors had MRIs on all of their patients, then there would no concerns regarding potential bias.
Longitudinal multicenter studies collating ion levels may help determine whether these levels progress linearly, plateau over time, or diminish. This may help determine ion surveillance guidelines. Likewise, serial MRIs over time in patients with specific MRI abnormalities may help distinguish which patients can be observed and which need revision surgery by documenting the natural history of specific lesions.
Finally, there is still a need for a serologic test that could accurately determine the presence or absence of tissue necrosis in the hip at an early stage before large scale damage is noted on MRI. If one could predict necrosis before extensive tissue destruction became clear or on cross-sectional imaging, it would be invaluable to clinicians and patients alike. Research concerning biomarkers specific for tissue necrosis are promising, but this technology is currently not commercially available.
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