Alignment of the medial tibial plateau affects the rate of joint space narrowing in the osteoarthritic knee.

Summary Objective To determine, in serial fixed-flexion (FF) radiographs of subjects with knee osteoarthritis (KOA), the importance of, and basis for, the effect of alignment of the medial tibial plateau (MTP), as determined by the inter-margin distance (IMD), on joint space narrowing (JSN). Methods Baseline and 12-month X-rays of 590 knees with Kellgren and Lawrence grade (KLG) 2/3 OA from the public-release dataset of the Osteoarthritis Initiative (OAI) were assigned to subgroups based upon IMD at baseline (IMD BL ) and the difference between IMD BL and IMD 12mos . Relationships of JSN to IMD BL and to the difference between IMD BL and IMD 12mos were evaluated. Results In all 590 knees, mean JSN was 0.13±0.51mm ( P BL in the 12-month film were, in general, poor. JSN was significantly ( P =0.012) more rapid in Subgroup A (IMD≤1.70mm at both time points) than in Subgroup B (both IMDs>1.70mm): 0.15±0.43; 0.08±0.47. Within Subgroup B we identified a subset, Subgroup B1, in which, although alignment was poor at both time points, the large IMD BL was, by chance, highly reproduced by IMD 12mos (difference between the two IMDs=0.01±0.27mm, NS). JSN in Subgroup B1 was 0.06±0.41mm and did not differ from that in other knees of Subgroup B ( P =0.87). The standardized response mean (SRM) in all 590 knees and Subgroups A, B and B1 was 0.25, 0.34, 0.17 and 0.06, respectively. Independent of IMD BL , JSN correlated significantly with the difference between the IMDs in the two radiographs ( r =0.17, P =0.0001). Conclusion Skewed MTP alignment in serial films and poor replication of IMD BL in the follow-up exam affect JSN measurement. The magnitude of change in joint space width (JSW) related to the poor quality of alignment that is common with the FF view jeopardizes accurate evaluation of JSN.