Effect of alignment of the medial tibial plateau and x-ray beam on apparent progression of osteoarthritis in the standing anteroposterior knee radiograph

Objective. Previous studies of knee osteoarthritis (OA) have yielded variab le estimates of the rate of joint space narrowing (JSN) in the standing ant eroposterior (AP) radiograph, due largely to longitudinal changes in the al ignment of the medial tibial plateau (MTP) and x-ray beam. To characterize this bias, we examined serial radiographs of subjects with knee OA in popul ation-based and clinical OA cohorts from 3 locations in the United States a nd the United Kingdom. Methods. Radiographic features of knee OA (e.g., osteophytosis, JSN) and MT P alignment in 428 OA knees were evaluated by consensus of 2 readers. Align ment was considered satisfactory if the anterior and posterior margins of t he MTP were superimposed within I mm. Readers were blinded to subject ident ity, and films were read in random order. The minimum medial joint space wi dth was also measured manually (standard error of repeated measurements 0.2 0 mm) in serial knee images. Results. Only 14% of serial radiographs exhibited alignment of the MTP in b oth images. In OA knees with satisfactory alignment in both images, the mea n rate of JSN over 2-3 years (0.26 mm/year) was significantly larger (P = 0 .004) than that in OA knees with misalignment in 1 or both radiographs and was 86% more rapid than the mean JSN in all OA knees. Moreover, the within- group standard deviation of JSN was significantly smaller among knees with reproduced alignment of the MTP than in knees in which misalignment occurre d in 1 or both images (P = 0.006). Conclusion. Poor standardization of knee positioning in serial standing AP radiographs in previous studies of OA progression has obscured the rate and variability of articular cartilage loss in subjects with knee OA. True JSN (i.e., JSN that is not attributable to longitudinal changes in the alignme nt of the MTP with the x-ray beam in serial radiographic examinations) may occur more rapidly, and with less between-subject variability, than that pr eviously thought to be characteristic of knee OA.