Zone-specific analysis of radiographic pelvic enthesitis in axial spondyloarthritis: identification and associations with mobility, function, and disease activity

Objectives: To evaluate the frequency of radiographic pelvic enthesitis in patients with axial spondyloarthritis (axSpA), focusing on zone-specific entheseal sites and their associations with disease activity, mobility, and functional outcomes. Methods: This prospective cross-sectional study included 100 patients with radiographic axial spondyloarthritis (r-axSpA) and 100 patients with non-radiographic axial spondyloarthritis (nr-axSpA), who were consecutively admitted to a tertiary hospital’s rheumatology outpatient clinic. Pelvic enthesitis was evaluated in three zones using anteroposterior pelvic X-rays, guided by the recently introduced Radiographic Enthesis Index (REI). Disease activity, spinal mobility, and functionality were assessed using ASDAS-CRP, BASDAI, BASMI, and BASFI. Results: Radiographic enthesitis in the pelvic region was found in 128 patients (64% of the total), comprising 72 (72%) in the r-axSpA group and 56 (56%) in the nr-axSpA group (p = 0.018). Zone 3 (ischiopubic ramus) enthesitis was the most common (59.5%) and was significantly correlated with impaired spinal mobility (BASMI), reduced lumbar lateral flexion, cervical rotation, and modified Schober test scores (p < 0.001). Zone 2 (pubic symphysis) enthesitis was associated with higher disease activity (BASDAI, p = 0.026). Patients with pelvic enthesitis also experienced longer symptom duration (p < 0.001). Conclusions: Pelvic enthesitis detected by AP pelvic radiography is frequent and clinically significant in axSpA. Our findings highlight novel associations with increased disease activity (particularly at the Zone 2) and reduced spinal mobility (notably at the Zone 3).