Measuring the Impact: MRI Response of Sacroiliac Joints to TNF Inhibitors in Youth with Axial Disease

Objective To evaluate the timeline for resolution of sacroiliac joint (SIJ) inflammation, changes in structural lesions, and their correlation with patient‐reported outcomes in youth with axial juvenile spondyloarthritis (axJSpA) initiating TNF inhibitor (TNFi). Methods This prospective, multicenter study included youth aged 8–18 with a clinical diagnosis of axJSpA starting TNFi. Assessments were conducted at baseline and 12 weeks, including clinical evaluation, MRI, and patient‐reported outcomes. Participants with persistent SIJ inflammation at 12 weeks were reassessed at 24 weeks. Three blinded reviewers evaluated MRIs using SPARCC inflammation (SIS) and structural (SSS) scores. Results Of 75 enrolled participants, 73 completed baseline visits, and 62 had MRI‐confirmed axJSpA. Fifty‐seven completed a 12‐week follow‐up; 89% (51/57) showed SIS improvement, and 63% (36/57) achieved inflammation resolution (SIS <2). Median SIS change from baseline to 12 weeks was ‐8 (IQR: ‐18 to ‐3). Among those with persistent inflammation at 12 weeks (n=26), 85% reported at least moderate clinical improvement. At 24 weeks, 56% (14/25) had ongoing inflammation. 84% of SIS improvement occurred within the first 12 weeks. In patients with ≥2 scans, structural lesion scores decreased/increased/stayed the same from baseline to the 12‐week scan for erosions (58%/25%/18%), sclerosis (21%/9%/70%), fat metaplasia (0%/30%/70%), and backfill (4%/28%/68%). Conclusions Most participants showed early imaging response to TNFi, with most improvement occurring within 12 weeks. Despite residual inflammation persisting in nearly half of patients, most reported symptom improvement, underscoring both the rapid impact of TNFi and the heterogeneity of treatment effects. image