As the number of primary hip arthroplasties is expected to increase substantially over the next 15 years, the need for noninvasive, accurate diagnosis in the assessment of adverse conditions following arthroplasty placement is essential . The recent recalls of metal on metal (MOM) total hip replacements, as well as some modular nonmetal on metal designs, combined with the British MHRA press release in 2012 and the 2011 FDA 522 Post Market Surveillance Call establishes a basis for the potential use of cross-sectional imaging in the evaluation of these patients.
The Hospital for Special Surgery (HSS) MRI Department’s previous interest in the evaluation of arthroplasty was founded in the detection of osteolysis, given that the condition was one of the more common problems encountered with the commonly-utilized metal on polyethylene (MOP) compounds. We have previously demonstrated that MRI was more sensitive than radiographs in detecting osteolysis, and further that it was the most accurate means by which to quantify and locate osteolysis following total hip arthroplasties, when compared to conventional radiographs or optimized CT evaluation [2, 3, 4].
Although the cellular basis of both osteolysis and local adverse tissue reaction is still not well established, we recognize that osteolysis is an inflammatory upregulation of osteoclasts and downregulation of osteoblasts, thereby leading to aseptic loosening .
In MOM constructs, there appear to be both cell-mediated and foreign body reactions resulting in a delayed type hypersensitivity reaction resulting in local tissue necrosis, termed by investigators from the United Kingdom as “pseudotumors”. Foreign body reaction or metallic debris may be seen in the absence of tissue necrosis. This is often noted in patients with abnormal radiographic measurements, leading to increased loads upon the implant and metallic wear.
More recently, tribocorrosion due to a combination of corrosion from electrochemical reaction between slightly different metallic composites, and wear due to mechanical degradation due to offset geometry, has been seen around modular head-neck junctions, modular stems or at the trunnion. Given this latter reaction, we recognize that adverse tissue reactions may occur in all bearing constructs, and not just those composed of MOM designs. Thus, early detection of adverse tissue reactions is essential. The findings from the UK [6, 7] demonstrate the utility of imaging in diagnosing these conditions.
Adverse tissue reactions can occur in both symptomatic and asymptomatic cohorts; Hart et al. noted that the prevalence of “pseudotumors” was not significantly different in painful versus asymptomatic well-functioning arthroplasties . The finding of a high prevalence of adverse tissue reactions in asymptomatic MOM arthroplasties has also been noted by additional investigators [8, 9].
In our own recently published study of 73 hips in 68 patients with resurfacing arthroplasty, there was a similar volume of synovitis in symptomatic and asymptomatic cohorts . Of note, in the patients who underwent revision, the mean volume of synovitis was higher in those patients who also had a documented adverse tissue reaction at revision. While previous groups have devised observational MR classification systems, our group has attempted to correlate MRI findings to both clinically and biologically relevant outcomes.
In a recently published study, we noted that the combination of several MR variables, most notably the presence of solid synovial deposits and maximum synovial thickness, were highly correlated to both the intensity of the ALVAL score, a histologic score obtained on tissue at the time of the revision, as well as the degree of intraoperative tissue damage present . The sensitivity and specificity for MRI was higher than that which has been reported for metal ion levels . Increasingly, metal ion levels have shown to be poorly predictive of the presence and extent of adverse tissue reaction .
While patients may have pain that is explained by physical examination or radiographic findings, an important group of patients have “unexplained pain”. In a further study correlating MR findings to wear analysis, our group noted that articular wear rates were lower in the unexplained pain group than in the control group, and that synovial thickness on MRI was statistically higher in the unexplained pain group and highly predictive of adverse histologic score at the time of revision .
We are continuing to study the effects of tribocorrosion as well as the effects of reactions around MOM implants. Our recently awarded R01 (NIH 1 R01 AR064840) research grant will provide us with a very effective means to longitudinally assess different types of implants. This grant will support an extraordinarily collaborative effort between HSS scientists looking at wear patterns using retrieval analysis in Dr. Timothy Wright’s lab, the arthroplasty service under the direction of Dr. Douglas Padgett, and the MRI lab under the co-direction of both myself and Dr. Matthew Koff. The diagnosis of adverse tissue reactions is made dramatically easier by the advent of newer pulse sequences aimed at reducing susceptibility artifact associated with the metallic components of the arthroplasty. Our collaborations with General Electric Healthcare have been very efficacious in that regard, with the development of the new “MAVRIC” pulse sequence [14, 15].
Overall, the evaluation of adverse reactions around implants reflects a cooperative effort between the Departments of Radiology and Imaging, Biomechanics, Pathology and Laboratory Medicine, and the Arthroplasty Service. Additional collaborators, including Dr. Edward Purdue in the Osteolysis Research Laboratory, will provide insight into cellular mechanisms which produce these adverse reactions.
The unique multidisciplinary efforts of HSS scientists, clinician scientists and clinicians will set the stage for an important road to discovery in the biology of this process.
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