High prevalence of pseudotumors in patients with a Birmingham Hip Resurfacing prosthesis: a prospective cohort study of one hundred and twenty-nine patients.

BACKGROUND: Recently, concern has emerged about pseudotumors (lesions that are neither malignant nor infective in the soft tissues surrounding total hip arthroplasty components) after hip arthroplasties with metal-on-metal bearings. Patients treated in our hospital for degenerative arthritis of the hip with a Birmingham Hip Resurfacing (BHR) prosthesis were invited to return for follow-up evaluation. The prevalence and clinical relevance of pseudotumors were investigated. Risk factors for pseudotumor formation were sought.
METHODS: A single-center cross-sectional prospective cohort study was conducted and included all patients who received a BHR from 2005 to 2010 in Martini Hospital, Groningen, The Netherlands. Data were collected on patient and surgical characteristics, clinical hip outcome scores (Harris hip score and Oxford score), serum metal ion levels (cobalt and chromium), and radiographs. A computed tomographic scan (without metal suppression) was made. In patients who had a revision, tissue samples were histologically examined.
RESULTS: Originally, there were 129 patients with 149 BHRs. Four patients (six hips; 4%) were lost to follow-up. Our final cohort consisted of 125 patients (143 hips). From this final cohort, eleven patients (twelve hips) had a revision, and three of them (three hips) had the revision before the present study was conducted. Seven patients (eight hips; 5.6%) had a revision because of a symptomatic pseudotumor. Survival analysis showed an implant survival rate of 87.5% at five years (failure was defined as a revision for any reason). A pseudotumor was found on computed tomography in thirty-nine patients (forty hips; 28%). Of those patients, ten (eleven hips; 28%) had complaints involving groin pain and discomfort, a noticeable mass, or paresthesia. Symptomatic pseudotumors were significantly larger than asymptomatic pseudotumors (a mean volume of 53.3 cm3 compared with 16.3 cm3; p = 0.05). A serum cobalt level of >85 nmol/L was a predictor for pseudotumor formation (odds ratio, 4.9).
CONCLUSIONS: Pseudotumor formation occurred in 28% of hips after an average follow-up of forty-one months. Most pseudotumors (72.5%) were asymptomatic. Larger pseudotumors were associated with more complaints. Survival analysis showed an implant survival of 87.5% at five years. Failure occurred in 5.6% (eight) of 143 hips because of a symptomatic pseudotumor.