Patellar tendon enthesitis as initial manifestation of ankylosing spondylitis: an atypical presentation

A 19-year-old man presented to the orthopaedic clinic because of a 3-month history of left anterior knee pain. There was apparently no history of trauma or other comorbidities. He was not involved in athletic activities. On physical examination, there was tenderness at the tibial tubercle. He underwent anteroposterior (AP) and lateral radiographs of the left knee, which showed a focal lucency at the tibial tubercle and soft tissue thickening at the distal patellar tendon (figure 1). He further underwent MRI of the knee and AP radiograph of the pelvis. MRI showed features of Osgood-Schlatter disease (OSD) (figure 2). The pelvic radiograph taken to evaluate referred knee pain revealed bilateral sacroiliitis (figure 3). Subsequent detailed history and clinical examination including …