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

## TL;DR

This study finds that hip joint synovial cavity thickness in children without fluid buildup is similar in those with early juvenile arthritis and healthy controls, suggesting it's not a reliable disease marker.

## Contribution

The study provides new evidence that synovial cavity thickness is not a distinct marker for early juvenile idiopathic arthritis when no effusion is present.

## Key findings

- HJSCT showed high left-right agreement and was strongly associated with age but not with JIA status.
- Predicted HJSCT increases rapidly in childhood and stabilizes in adolescence, indicating a physiological pattern.
- HJSCT did not significantly differ between JIA patients and healthy controls in adjusted models.

## Abstract

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.