Reliability of ultrasound-detected effusion-synovitis in knee osteoarthritis

Objective

There is increasing use of ultrasound to identify effusion and synovitis, which are proxies for inflammation, in knee osteoarthritis (OA). Ultrasound appears to be a valid modality for assessing effusion and synovitis when compared to MRI, but its reliability has been questioned. We aimed to assess the agreement of ultrasound-identified effusion and synovitis between two readers.

Design

We included 30 unique knees from a cohort with symptomatic and radiographic knee OA. A single ultrasonographer performed ultrasound of the suprapatellar recess. Two readers with different levels of experience read the images. We specified maximal effusion depth first as continuous (mm) and then as binary, dichotomized at ≥4 mm (present) vs. <4 mm (absent). We graded synovitis in the suprapatellar recess (0–3) based on the Outcome Measures in Rheumatology (OMERACT) guidelines. We used proportions to determine inter-rater agreement for continuous variables and Cohen's Kappa or weighted Cohen's Kappa for agreement of binary and ordinal variables.

Results

The mean age of the participants was 66 years. Seventeen (57%) were female. For maximal effusion depth the readers differed by ≤ 2 mm 80% of the time. For the presence of effusion (binary), the Cohen's Kappa for the two readers was 0.69 (95% CI 0.40–0.97). Synovitis grade (0–3) for the two readers had a weighted Cohen's Kappa of 0.76 (95% CI 0.58–0.94).

Conclusion

We found moderate to strong inter-reader agreement of ultrasound identified effusion-synovitis in knee OA between two readers with differing ultrasound experience. Ultrasound may be a viable bedside tool for identifying the subset of OA patients with effusion or synovitis.