Review Question - QID 213019

The patient is presenting with increasing knee pain for several years consistent with either septic or aseptic loosening of the prosthesis. Serum and synovial labs are not diagnostic for an infection, but there is a positive synovial alpha-defensin suggesting the presence of a chronic prosthetic joint infection.

Prosthetic joint infections are diagnostic challenges as there is no single confirmatory test. Rather, diagnosis is composed of a conglomerate of physical and laboratory findings as laid forth by the Musculoskeletal Infection Society criteria. Diagnosis can be made by either the presence of one major criterion or four minor criteria. Synovial alpha-defensin is a new assay that tests for the presence of an antimicrobial peptide that is part of the innate immune system. Recent studies have suggested a high sensitivity and specificity of this test for prosthetic joint infections, even with prior antibiotic administration. Treatment of chronic periprosthetic joint infections (infection >4 weeks) involves two-stage revision arthroplasty with culture-specific antibiotics for at least six weeks. Reimplantation of a prosthesis is done with infection eradication is confirmed.

Shahi et al. performed a retrospective diagnostic study looking at whether prior antibiotic administration affected synovial alpha-defensin levels. The authors found that alpha-defensin was not affected by prior antibiotic administration. The authors concluded that since many patients with PJI will present with prior antibiotic administration, alpha-defensin may be an ideal diagnostic adjunctive test.

Frangiamore et al. performed a prospective cohort study on the sensitivity and specificity of alpha-defensins in diagnosing prosthetic joint infections. The authors found that alpha-defensin has a sensitivity and specificity of 100% and 98%, respectively, for diagnosing PJI in single-stage and first-stage revisions. The authors concluded that alpha-defensin has the potential as a useful adjunct in diagnosing PJI.

Figures A and B demonstrate AP and lateral radiographs of the right knee with loosening of the tibial and femoral components. Illustration A depicts the MSIS criteria for the diagnosis of PJI.

Incorrect Answers:
Answers 1-3: Revision arthroplasty without addressing the infection that is present will lead to failure. A two-stage revision is needed given the chronicity of the infection.
Answer 4: One-stage revision for prosthetic joint infections has been found to be effective in select cases, however, two-stage revision remains the gold-standard for addressing chronic infections.