Radiographs at baseline and 2-year follow-up of a 61-year-old woman with osteoarthritis. At two time points, radiographs were taken with 5°, 10°, and 15° angulation of the knee. (a) Anteroposterior (AP) radiograph taken at 5° angulation shows medial joint space narrowing (OARSI grade 2 and Kellgren-Lawrence grade 3). (b) However, AP radiograph taken at 10° angulation shows OARSI grade 3 joint space narrowing (Kellgren-Lawrence grade 4). (c) Similarly, at follow-up, AP radiograph taken at 5° angulation shows OARSI grade 2 joint space narrowing (Kellgren-Lawrence grade 3) whereas that taken at 15° angulation (d) shows OARSI grade 3 joint space narrowing (Kellgren-Lawrence grade 4). If images (b) and (c) are compared longitudinally, one observes a paradoxical 'joint space widening'. This example highlights the importance of technique to ascertain the reproducibility of the same angulation of the knee in longitudinal studies. OARSI, Osteoarthritis Research Society International.