In the workup of a painful total knee replacement at 5 years, aspiration yields fluid with a white cell count of 6,500 cells/µL with 80% polymorphonuclear cells. The erythrocyte sedimentation rate is 42 mm/h and CRP is 18 mg/L. According to the MSIS (Musculoskeletal Infection Society) criteria, this represents:
- A Definitely aseptic failure — mechanical loosening
- B Periprosthetic joint infection (PJI) ✓
- C Crystal arthropathy causing synovitis
- D Inflammatory arthritis flare
Correct answer: B. Periprosthetic joint infection (PJI)
Explanation
MSIS 2013 criteria diagnose PJI when at least one major criterion is met (two positive cultures of the same organism, or sinus tract) or three of five minor criteria are fulfilled. Minor criteria include elevated ESR/CRP, elevated synovial WBC (>3,000 cells/µL for chronic PJI), elevated synovial PMN% (>65%), positive histology, and a single positive culture. Here, synovial WBC 6,500 (above threshold), PMN% 80% (above threshold), plus raised ESR and CRP fulfil PJI criteria. Mechanical loosening typically yields a non-inflammatory aspirate.
Reference: Maheshwari Essential Orthopaedics, 6th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.