Enthesitis-related arthritis (ERA), a distinct subtype of juvenile idiopathic arthritis (JIA) related to HLA-B27 and peripheral and axial involvement, presents with insidious onset of arthritis and/or enthesitis. However, there is a lack of data concerning axial new bone formation in patients transitioning into adulthood. To evaluate the axial radiographic structural damage (axRxSD), encompassing the sacroiliac joints (SIJ), hips, and spine, in ERA patients across various age groups. A cross-sectional cohort study was conducted with patients aged up to 35 years. Specific tools were used for measuring disease activity (BASDAI, ASDAS), function (BASFI, HAQ-S), mobility (BASMI), clinical enthesitis (MASES), ultrasound evaluation (MASEI), and axRxSD, including mSASSS for spine, Kellgren-Lawrence for hips and modified New York criteria for SIJ. A total of 26 patients were included, of whom 76.9% were males, with a mean age at diagnosis and assessment of 11.9 and 19.7 years, respectively. HLA-B27 positivity was found in 58.3%. Current active arthritis and enthesitis were present in 19.2% and 23%, respectively, with mean MASEI score of 12 (IQR 6-17). Peripheral joint limitation was observed in 50%, despite a BASMI score of 2.2 and 16% occurrence of abnormal FABER test. Most patients were in remission or low disease activity [ASDAS-ESR = 1.2 (0.6-2.3)
ASDAS-CRP = 1.55 (0.6-2.4)]. Modified New York criteria were fulfilled by 73.1% of patients and 15.4% had radiographic hip involvement. Spine involvement, measured by mSASSS, was low (IQR 0-4.2), with only two patients exhibiting syndesmophytes. There was no statistical association between any imaging methods and clinical, laboratory, and ultrasound variables, including scores for activity, functionality, and mobility. Significant association was found only between axRxSD and BASMI. Our results showed high frequency of SIJ ankylosis alongside lower radiographic involvement in the spine and hips, suggesting a distinct structural damage phenotype. The early recognition of this outcome and the use of immunobiological therapy may mitigate syndesmophyte occurrence over time.