We took posteroanterior weight-bearing radiographs, both with the joint fully extended and in 30° of flexion, in a consecutive series of 50 knees in 37 patients referred for the primary assessment of pain and/or stiffness. These radiographs were reported ‘blind’ both by an orthopaedic surgeon and a radiologist. Direct measurement of the joint space, together with grading of the severity of erosion according to the Ahlback criteria, was undertaken. Any other abnormality present was also documented.

The radiographs of the knees in 30° of flexion consistently showed more advanced erosion in both the medial (p = 0.001) and the lateral (p = 0.0001) tibiofemoral compartments, when compared with those of knees in full extension. The Ahlback classification of 25 joints was altered, in some cases by several grades, by the flexed position of the joint. In every case in which another abnormality was identified on the radiograph in full extension, it was also noted on that of the knee in 30° of flexion. In a further four cases, additional pathology could only be seen in the flexed knee. Every patient was able to complete the radiological examination without difficulty. Our study supports the adoption of a weight-bearing view in 30° of flexion as the standard posteroanterior radiograph for the assessment of tibiofemoral osteoarthritis in patients over 50 years of age.