Objectives : To determine which anatomical sites and which ultrasonographic entheseal lesions are best able to discriminate between spondyloarthritis (SpA) patients and healthy controls (HC).

Methods : We included patients with psoriatic arthritis (PsA) and axial SpA (axSpA), from six Swiss hospital outpatient clinics, as well as HC. Participants completed quality of life and physical activity questionnaires and underwent a clinical examination of both joints and entheses, followed by a detailed musculoskeletal ultrasound examination including nine entheseal sites bilaterally. Entheses were scored according to the Outcome Measures in Rheumatology criteria, with an additional evaluation of bursae and power Doppler (PD) in the 2–5 mm zone.

Results : Overall, 121 participants were included, including 41 with PsA (mean age in years (SD), percentage male: 54.5±11.0, 63.4%), 39 with axSpA (45.1±10.0, 51.3%) and 41 HC (43.9±10.9, 56.1%), with a total of 2178 entheses evaluated. The PsA and axSpA groups showed no significant differences regarding inflammatory markers or disease activity scores.

In the univariable analysis, all ultrasonographic lesions at the enthesis showed a significant association with SpA vs HC. Only B-mode inflammatory lesions (OR=1.38, p=0.034) and active enthesitis (OR=4.45, p=0.030) retained this association in multivariable analyses. While 4/9 entheses were associated with SpA in univariable analyses, only the distal patellar ligament insertion remained significantly associated with SpA (OR=1.74, p=0.039) in multivariable analyses.

Conclusion : To distinguish SpA patients from controls, the sonographic scoring system used should account not only for the presence of specific entheseal lesions (structural and inflammatory) but also for the individual site affected.