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

Background

The clinical meaning of hip joint synovial cavity thickness (HJSCT) on ultrasound (US) in juvenile idiopathic arthritis (JIA) without effusion is uncertain.

Methods

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).

Results

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 (p = 0.25). In the final model (adj. R2 0.656), HJSCT was strongly associated with age (non-linear, p < 0.001) but not significantly associated with HAZ (β = 0.04; p = 0.11) or JIA status (β = 0.07; p = 0.30). Predicted HJSCT showed a steep increment in childhood and plateau in adolescence.

Conclusions

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.