To describe the ultrasound abnormalities seen in septic arthritis and to assess their associations with clinical, biological, and radiological outcomes.

We prospectively included 34 patients with septic arthritis of a native joint (knee, n=19; shoulder, n=6; hip, n=4; ankle, n=3; or wrist, n=2). Ultrasonography was performed to record synovial-membrane thickness and vascularity, joint effusion, and abnormalities of adjacent soft tissues, at baseline then 4days, 2weeks, and 3months later. Motion-range limitation of the affected joint was evaluated after 3months. Radiography was performed at inclusion and after 3months.

Mean age was 63.7±17.6years. After 3months, 20 (58.8%) patients had motion-range limitation with worsening of the total radiological score (P<0.001). The proportion of patients with synovitis was very high initially (96.4% at baseline, 96.3% after 4days, and 100% after 2weeks) then diminished to 77.8% after 3months (P=0.051). Synovial-membrane thickness was significantly higher after 4days and 2weeks compared to baseline (median, +17.3% and +20%, respectively; P=0.015) and was significantly lower after 3months compared to the earlier time points (median, −31.5%, P=0.015). A positive Doppler signal was common at baseline (n=18, 64.3%) then significantly less so after 3months (n=7, 25.9%; P=0.04). An unchanged or higher Doppler grade after 2weeks compared to baseline was associated with motion-range limitation at last follow-up (P=0.033).

We report the first study on ultrasound evidence of synovitis, joint effusion, and soft tissue alterations at baseline and over time in patients with septic arthritis. Persistent synovitis and joint effusion 3months after starting antibiotic therapy was not associated with treatment failure. However, Doppler signal changes over the first 2weeks were associated with the 3-month functional outcome.