Ankylosing spondylitis

35 year old male who presented with low back pain. As part of his workup, radiographs of the pelvis were obtained. The red arrows demonstrate sclerosis along the iliac side of both sacroiliac joints, with slightly asymmetry and greater involvement of the right sacroiliac joint. The second image is windowed for better visualization of the sacroiliac joints. No discrete erosions are identified. Differential considerations for sacroilitis include ankylosing spondylitis, psoriatic arthritis, reactive arthritis, or inflammatory bowel associated spondyloarthropathy. Given the bilateral findings in this young male patient, findings are most consistent with ankylosing spondylitis. Additional findings which can be seen in the lumbar spine in more advanced disease include ossification of the anterior longitudinal ligament and vertebral body ankylosis.