Abstract
BACKGROUND: Recurrence of transient synovitis of the hip (TSH) is often distressing but difficult to predict. Ultrasonography of the hip is essential for evaluating and monitoring hip effusion in patients with TSH. This study aimed to identify ultrasonographic features of the hip in children with recurrent TSH and identify factors associated with recurrence. METHODS: Medical records and hip ultrasound data, including age, weight, anterior capsule distance (ACD) in both hips, and ultrasound signs from children diagnosed with TSH at a single center between September 2015 and August 2023, were retrospectively reviewed. The ACD ratio was calculated based on the ratio of the ACD of the affected side to that of the unaffected side in children with unilateral TSH. Children with TSH were divided into recurrent and non-recurrent groups. Spearman's correlation analysis was performed for age and ACD in both hips. Univariable and multivariable logistic regression analyses were performed to identify factors associated with recurrent TSH. RESULTS: Data from 668 children with TSH were included. Of 515 cases of unilateral TSH, 448 were non-recurrent and 67 were recurrent. There were 153 bilateral cases, including 130 in the non-recurrent group and 23 in the recurrent group. There were no significant differences in demographic characteristics between the non-recurrent and recurrent groups (P > 0.05). The first hip ultrasound examination of children with unilateral TSH revealed that the ACD of the affected side in the recurrence group was greater than that in the non-recurrence group(P = 0.03), although there was no significant difference in ultrasound signs(P > 0.05). There was a significant positive correlation between ACD and age in children with TSH (R = 0.47; P < 0.05). Factors significantly associated with the recurrence of TSH included ACD of the affected side (odds ratio (OR) 1.14 [95% confidence interval (CI) 1.01-1.30; P = 0.04) and ACD ratio (OR 2.51 [95% CI 1.29-4.88]; P = 0.01). CONCLUSION: Among children with unilateral or bilateral TSH, the ACD in both hips increased with age. Higher ACD and ACD ratio were risk factors for recurrence. As ACD increased with age in children with TSH, using the ACD ratio can reduce the effect of age on ACD when assessing the risk for recurrence of TSH. These results provide valuable information for understanding the demographic and hip ultrasound characteristics of TSH, and reveal the association between ACD and age in children with TSH, which aids in the assessment of TSH prognosis. Further studies are needed to better understand the clinical relevance of recurrent factors in TSH.