Detection of infection in loosened hip prostheses: efficacy of sonography

The purpose of this study was to determine the efficacy of sonography in the detection of infection in loosened hip prostheses. The normal capsular morphology in 15 asymptomatic patients with total hip replacements was studied sonographically. Sonograms were then obtained in 33 patients who had pain in the hip after arthroplasty and radiologic findings of loosening of the prosthesis. These patients subsequently underwent aspiration and arthrography of the hip. Six of the 33 symptomatic patients proved to have prosthetic joint infection. On sonograms, the normal pseudocapsule is adherent to the proximal part of the anterior femoral cortex, and the capsule-to-bone distance is less than 3.2 mm (average, 2.6 mm). No hips with a capsule-to-bone distance less than 3.2 mm were infected. Sonograms in the six patients with infection showed marked intraarticular effusion with a mean capsule-to-bone distance of 10.2 mm. Five of these six had extracapsular fluid collections. Two patients with hip dislocations and four with aseptic loosening of the prosthesis had capsular distension on sonograms and cultures of aspirated material that showed no growth. Sonography can be used to diagnose infection around loosened hip prostheses. All patients who had an intraarticular effusion with extraarticular extension seen on sonograms had infection.