Pseudotumors may be 10 times more common in patients after metal-on-metal (MoM) hip resurfacing and total hip replacement (THR) than thought, suggest Dutch researchers.
The study, published in the UK edition of the Journal of Bone and Joint Surgery, reveals a "substantially higher" rate of periarticular soft-tissue mass development in MoM patients than the 1-4% of cases previously identified.
"Because most revision cases were identified only after an intensive screening protocol, we recommend close monitoring of patients with MoM THR," say C Verheyen (Isala Klinieken, JW Zwolle) and co-workers.
The team examined for pseudotumors in 119 patients who underwent MoM THR with large-diameter femoral heads between 2005 and 2007 using ion levels, radiographs, and computed tomography. Patients with pseudotumor symptoms, such as pain, swelling, clicking, or subluxation, were examined using magnetic resonance imaging and ultrasound-guided biopsy.
Pseudotumors were defined as a semi-solid or cystic periprosthetic mass of 2 cm in diameter or larger other than malignancy, scar tissue, or bursa.
An average of 3.6 years after surgery, CT images for 109 hips in 108 patients were available, revealing pseudotumors in 42 (39%) patients. A further four tumors were found in the contralateral hip in patients who had bilateral arthroplasty, two of whom had pseudotumors in both hips.
In multivariate analysis, serum cobalt ion levels above 5 µg/mL were the only significant predictive factor for pseudotumors. Patients with elevated cobalt were four times more likely to develop pseudotumors than those with lower levels, after adjusting for age, gender, surgical characteristics, allergy, and other factors.
Thirteen patients underwent revision surgery, revealing significant bone loss, accompanied by extensive metallosis, periarticular swelling, and necrosis, or a large cystic or semi-solid tumor, the researchers report. These patients were given a polyethylene acetabular component and a smaller diameter metal head.
In addition, ultrasound-guided biopsy samples from 24 patients showed mixed histiocytes, perivascular aggregates, necrosis, and metal debris.
Verheyen et al observe that patients with and without pseudotumors did not significantly differ with regard to clinical scores assessing THR outcome, reflecting the slow and insidious development of the masses.
"When symptoms of swelling, pain in the groin and clicking become sufficient to warrant revision, a sizeable pseudotumour with extensive local-tissue destruction may already be present," they warn.
They therefore conclude that "the high rate of pseudotumour formation and the accompanying local soft-tissue destruction at the time of revision found in our patients is a cause for great concern"
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