How can a prosthetic infection be separated from aseptic loosening?
Infection should be suspected in any patient with progressive pain in a prosthetic joint. Fever may or may not be present, but ESR and CRP are frequently elevated (75% and 88%, respectively). Synovial fluid will be inflammatory. Radiographs may show >2 mm lucency between bone and cement but does not separate infection from aseptic loosening. MRI, CT scan, and positron emission tomography-fluorodeoxyglucose scans are of limited value due to artifact from the prosthesis. The most accurate imaging study is combined bone marrow scan/indium scan showing increased uptake around the prosthesis (bone scan should normally be negative 6 months after surgery) with an accuracy of 88% to 98%. Most commonly, radiographs are combined with a diagnostic joint aspiration to make the diagnosis. Positive cultures of synovial fluid or tissue are needed to confirm the diagnosis of infection. Granulocytes in biopsies of periprosthetic tissue are supportive diagnostically.