Hip Joint Synovial Cavity Thickness in Early Juvenile Idiopathic Arthritis Without Effusion: A Cross-Sectional Ultrasound Study

<b>Background</b>: The clinical meaning of hip joint synovial cavity thickness (HJSCT) on ultrasound (US) in juvenile idiopathic arthritis (JIA) without effusion is uncertain. <b>Methods</b>: In this cross-sectional study, we analyzed 369 children (187 JIA; 182 controls) undergoing hip US at a referral center in Kraków, Poland. JIA examinations were performed upon initial referral, early in the care pathway. We excluded patients with hip effusion and pre-existing inflammatory, traumatic or degenerative hip pathology. HJSCT was defined as the distance from the outer capsule margin to the femoral neck cortex. We used a Toshiba Aplio 400 system with a 12 MHz probe to measure and derive mean bilateral HJSCT. Bilateral concordance was assessed. Iterative multivariable linear regression modeling was used to compare groups, adjusting for non-linear age effects (natural splines) and WHO height-for-age z-scores (HAZ). <b>Results</b>: Left-right HJSCT agreement was high (ICC 0.947; mean difference 0.03 mm; 95% limits of agreement -0.64-0.70). In unadjusted analysis, mean (SD) HJSCT was similar in JIA versus controls: 5.83 (1.09) vs. 5.95 (0.99) mm, respectively (<i>p</i> = 0.25). In the final model (adj. R2 0.656), HJSCT was strongly associated with age (non-linear, <i>p</i> < 0.001) but not significantly associated with HAZ (β = 0.04; <i>p</i> = 0.11) or JIA status (β = 0.07; <i>p</i> = 0.30). Predicted HJSCT showed a steep increment in childhood and plateau in adolescence. <b>Conclusions</b>: In children without hip effusion, HJSCT mainly reflects physiological growth and does not differ significantly between early JIA patients and healthy controls. These findings suggest that capsular thickening is not a reliable standalone marker for early disease in the absence of effusion.