Cross-sectional comparison of extended anteroposterior and posteroanterior fixed flexion positioning to assess radiographic osteoarthritis at the knee: The Johnston County Osteoarthritis Project




To compare values for Kellgren/Lawrence (K/L) scale grade, joint space narrowing (JSN), and osteophytes in anteroposterior (AP) extended and fixed flexion posteroanterior (PA) radiographs obtained during a single clinic visit (the first followup of the Johnston County Osteoarthritis Project).


All films (n = 1,664 bilateral knees) were read by an experienced musculoskeletal radiologist. For each subject, AP and PA fixed flexion films were read in one sitting. K/L scale grades (range 0–4) and JSN and osteophytes (ranges 0–3) were assessed using standard atlases. Descriptive statistics were calculated for demographic and clinical variables. AP and PA fixed flexion results were compared by contingency table methods to obtain frequencies for K/L scale, JSN, and osteophyte grades using percent agreement and kappa coefficients. Results from the right and left knees were similar; data for the right knee are presented.


There was substantial agreement between AP and PA fixed flexion reads for radiographic osteoarthritis, defined as a K/L scale grade ≥2 (89% agreement; κ = 0.73, 95% confidence interval 0.69–0.76). Substantial agreement was also seen for tibial osteophytes and medial JSN; slightly lower kappa values were observed for femoral osteophytes and lateral JSN.


The requirements of large observational cohort studies are different than those of clinical trials, and sensitivity is less of an issue because of longer followup times. In cohort studies such as the Johnston County Osteoarthritis Project, there is substantial agreement by K/L scale grade for AP and PA fixed flexion radiographs, allowing incorporation of older films in longitudinal analyses.