The goal of this prospective study was to determine the prevalence of shoulder abnormalities on magnetic resonance imaging (MRI) in patients with ankylosing spondylitis (AS) who have normal shoulder X-ray examinations and no clinical shoulder abnormalities using a case-control study.

Fifty-three patients with AS according to the SpondyloArthritis international Society (ASAS) criteria were enrolled in the study. Fifty-three patients with no AS served as control subjects. Shoulder MRI examinations of patients in the two groups were analyzed and results were compared.

In the patient group, 26/53 patients (49.1%) demonstrated one or two of the defined pathological shoulder MRI findings, whereas 5/53 patients (9.4%) had similar findings in the control group. In the patient group, 11/53 patients (20.8%) had enthesal bone marrow edema, 19/53 patients (35.8%) had increased synovial fluid, 8/53 patients (15.1%) had tendinitis, and 2/53 patients (3.8%) had bursitis. There was statistically significant difference between the patient and control groups in terms of prevalence of enthesal bone marrow edema, increase in synovial fluid, and tendinitis.

Shoulder involvement is often overlooked in AS. Knowledge of the early-stage findings of the shoulder involvement due to AS is important to establish an early diagnosis and select treatment options.