Additional proximal femoral modularity at the neck-stem junction in total hip arthroplasty facilitates anatomic reconstruction of the hip center, allowing adjustments of femoral anteversion, leg length, and offset. However, these implants have been associated with complications arising from fretting and corrosion at their modular neck-stem junction, such as adverse local tissue reactions (pseudotumors)12,22,23. Authors of previous studies have reported the relationship between serum metal-ion levels and imaging findings associated with modular stem-neck junction corrosion11,21,33-38. Although multiple studies have examined the prevalence of pseudotumors in asymptomatic patients with MoM total hip arthroplasty or hip resurfacing arthroplasty25-31, reported data on asymptomatic pseudotumors related to taper corrosion in dual-taper modular total hip replacements are limited. In a study that included 83 hips treated with Rejuvenate modular total hip arthroplasty, Meftah et al.34 reported a prevalence of symptomatic pseudotumors of 43% (10 pseudotumors in 23 hips revised because of pain) at a mean of 2.7 years (range, 12 to 40 months). However, as MARS-MRI was performed only for patients with elevated serum metal-ion levels or symptoms related to the hip, they did not report the prevalence of pseudotumors in the remaining 58 asymptomatic patients in their study. Walsh et al.39 suggested that psoas tendinopathy with partial or complete rupture seen on MRI should be considered a sign of adverse local tissue reaction in symptomatic patients with modular hip implants. However, despite a relatively large number of patients treated with dual-taper modular total hip arthroplasty in their study, they did not describe the patients’ symptomology.
We evaluated one of the largest consecutive series of patients with a dual-taper modular stem screened with MARS-MRI for the presence of an adverse local tissue reaction (pseudotumor). At a mean of 22 months postoperatively, the overall pseudotumor prevalence was 36%. The pseudotumors observed were often large and resembled previously reported adverse local tissue reactions described in association with MoM implants. In addition, our study identified a 15% prevalence of pseudotumors in asymptomatic patients, which accounted for approximately one-third of all pseudotumors detected. Interestingly, asymptomatic patients with a pseudotumor had functional outcomes that were similar to those of patients who did not demonstrate a pseudotumor on MARS-MRI. This finding suggests that the absence of pain or other symptoms should not reduce the physician’s index of suspicion for possible taper-corrosion-related pseudotumor formation in patients with a dual-taper modular stem.
The patients with a pseudotumor had a significantly higher serum cobalt level and cobalt/chromium ratio regardless of symptoms. Elevation in serum metal-ion levels is likely secondary to a combination of fretting and crevice corrosion at the modular neck-body interface22. Our findings are supported by previous studies that demonstrated similar findings in patients with taper corrosion in a dual-taper modular implant, with serum cobalt levels exceeding serum chromium levels17,21,22,38,40. It has been hypothesized that, in contrast to MoM bearing-surface wear, the predominant ion release at modular taper junctions is due to a chemical corrosion process that involves chromium precipitating as chromium orthophosphate and more soluble cobalt dissipating as free ions, resulting in a preferential elevation of serum cobalt and an increased cobalt/chromium ratio17,22,41. Although the precise mechanism remains unknown, the occurrence of pseudotumors after dual-taper total hip arthroplasty with a cobalt-chromium modular neck implant is likely to be associated with excessive taper corrosion at the neck-stem modular junction, which results in preferential elevation of cobalt ion concentrations in vivo. This is supported by retrieval analysis studies5,21,23,34 that demonstrated taper geometry, taper contact area, and taper surface roughness as important contributing factors to taper junction fretting and corrosion. Moreover, in vitro results have demonstrated dose-dependent cytotoxicity with predominantly cobalt, rather than chromium, nanoparticles42.
As it remains unclear whether asymptomatic patients with a pseudotumor will develop symptoms with time, further study is necessary to evaluate the natural history of asymptomatic pseudotumors associated with dual-taper modular stems. Our study suggests that asymptomatic patients should be clinically evaluated. There should be a low threshold for measuring chromium and cobalt ion levels. As both asymptomatic and symptomatic patients with elevated ion levels are more likely to have a pseudotumor, MARS-MRI is indicated for patients with elevated metal-ion levels.
The results of the current study need to be interpreted in light of its potential limitations. First, there was a small number of patients with an asymptomatic pseudotumor. However, our study is one of the largest single-surgeon, single-implant, consecutive cohort studies that has allowed estimation of the prevalence of pseudotumors in patients with a dual-taper modular stem. Second, there may have been a reporting bias that led to patients reporting worsening symptoms after the recall. In addition to the 21 patients with a symptomatic pseudotumor, there were 2 patients who reported symptoms but did not have a pseudotumor. Assuming that these 2 symptomatic patients without a pseudotumor were affected by reporting bias, this would have resulted in a 14.5% prevalence of asymptomatic pseudotumor (9 of 62) instead of the 15% prevalence reported in our study. Third, as periprosthetic tissues can be obscured by metal artifacts when MARS-MRI is used, pseudotumors may have been missed. However, a previously published MARS-MRI protocol32 was used, and all scans were interpreted by an experienced musculoskeletal radiologist. Fourth, measurements of serum metal levels can be confounded by renal impairment and cobalt ions from other sources. However, none of the patients had renal impairment and, as the bearing surface was ceramic-on-highly crossed-linked polyethylene in all patients, none had an additional head-neck taper junction as a potential source of cobalt and chromium corrosion. Fifth, we did not attempt to correlate the findings of our histological analysis and those of a retrieval analyses. Finally, this study included only a single type of implant; thus, generalizability of our results to other types of dual-taper modular stems may be limited.
In conclusion, the prevalence of asymptomatic taper-corrosion-related pseudotumors seen on MARS-MRI in this study demonstrated that the absence of symptoms does not exclude the presence of adverse local tissue reactions. Elevated cobalt levels and cobalt/chromium ratios were associated with the presence of pseudotumors in asymptomatic and symptomatic patients. Cross-sectional imaging such as MARS-MRI is indicated for patients with elevated metal-ion levels. A longitudinal study is required to determine whether asymptomatic patients with a taper-corrosion-related pseudotumor will become symptomatic with time.
Investigation performed at the Center for Metal-on-Metal Hip Replacement Evaluation and Treatment, Massachusetts General Hospital, Harvard Medical School, Boston, Massachusetts
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