Posttraumatic hip bulging mimicking an abscess in a patient with hip prosthesis: the role of ultrasonography.

A 48-yr old female patient, with stage 4 rheumatoid arthritis, who had undergone multiple joint prostheses including four arthroplasties of the left hip, presented for a sudden-onset large bulge on the left thigh, after a minor local injury. Orthopedic examination and radiography excluded fractures. Ultrasonography revealed a large mixed hypo- and hyperechoic collection,with no Doppler signal, but with comet tail artifacts. Repeated cultures from the collection were negative. Fluid analysis revealed increased quantities of titanium and cobalt. The sudden-onset deformity and fluid collection were in favor of a shear lesion (Morel-Lavallée). The new collection communicated with a previously asymptomatic periprosthetic aseptic abscess, mimicking an infection. To our knowledge, Morel-Lavallée shear lesions have not been described in patients with hip prostheses. Their presence may add to the difficulties of ruling out silent infections in such patients. Ultrasonography is a very effective method for the diagnosis and follow-up of collections in this setting.