Validation of a semi-quantitative ultrasonography score for knee synovitis in juvenile idiopathic arthritis and its association with clinical findings

Ultrasonography is an important tool for assessing synovitis in juvenile idiopathic arthritis (JIA), but the reproducibility of semi-quantitative scores and their relationship with clinical findings still need clarification. We aimed to evaluate the intra- and inter-observer reproducibility of a previously proposed semi-quantitative ultrasonographic score for synovitis in JIA and to investigate the association between gray-scale (B-mode) and power Doppler (PD) grades and clinical joint findings. In this retrospective study, 68 consecutive patients (1–15 years; mean age 9.1 years, SD 3.9) with a diagnosis of JIA who underwent knee ultrasonography between January 2017 and March 2020 were included. Clinical data (joint pain, swelling, increased temperature, and limitation of movement) were recorded. Two musculoskeletal radiologists independently graded synovitis in B-mode and PD using a four-grade semi-quantitative score. Intra- and inter-observer agreement were assessed with weighted kappa coefficients. The association between B-mode and PD grades and the presence of at least one clinical sign was analyzed using a log-binomial regression model with random effects (α = 0.05). Intra-observer agreement for synovitis grading was almost perfect for both B-mode (κ = 0.90; 95% CI 0.86–0.95) and PD (κ = 0.84; 95% CI 0.77–0.92). Inter-observer agreement was substantial for B-mode (κ = 0.74; 95% CI 0.66–0.81) and moderate-to-substantial for PD (κ = 0.63; 95% CI 0.53–0.73). Ultrasonography detected synovitis in joints considered clinically negative in 19.2% of examinations. Compared with higher B-mode grades, joints with grade 0 synovitis showed a significantly lower prevalence of at least one clinical finding, and similar patterns were observed for PD grades 0 versus 1–2. The semi-quantitative ultrasonographic score for synovitis in JIA demonstrated almost perfect intra-observer and substantial inter-observer reproducibility. Ultrasonography identified subclinical synovitis beyond physical examination, and higher B-mode and PD grades were associated with a higher prevalence of clinical joint abnormalities, supporting its role in the assessment of disease activity in JIA.