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.


Longitudinal cohort studies of osteoarthritis (OA) have been essential to our understanding of OA, providing insights regarding risk factors and modification, natural history of the disease, and population variations (1–3). Because these studies are expensive and time consuming to conduct, there is a great need for readily available, cost-effective technologies to identify and to follow OA over many years. In contrast, clinical trials require a very sensitive method to show a difference between drug and placebo in the shortest possible time. Although most studies of radiographic techniques have focused on improving sensitivity for clinical trials, longitudinal studies provide challenges to researchers as well.

Conventional radiography, which is relatively inexpensive and widely available, remains the most widely used and accepted method of establishing the diagnosis of radiographic OA. Anteroposterior (AP) weight-bearing radiographs are commonly obtained in clinical practice and are often used as a criterion for study entry (4), whereas other techniques, often utilizing posteroanterior (PA) views and fluoroscopic positioning to maximize sensitivity to change, are used for followup (5–7). Because fluoroscopy adds expense, time, and the need for trained technologists, several investigators have proposed non–fluoroscopy-based methods to achieve reproducible, reliable measures of radiographic OA incidence and progression (8, 9). One such technique, utilizing “fixed flexion,” has shown reproducibility comparable with fluoroscopically-guided radiographs (8, 10).

The Johnston County Osteoarthritis Project is a large population-based cohort study in rural North Carolina that has been collecting data since the early 1990s. Baseline radiographs, obtained from 1991 to 1997, included weight-bearing AP extended radiographs of the knees. Because of subsequent evidence suggesting superiority of PA views, the protocol was changed at the first followup time point (1999–2004), when both the AP view and a PA fixed flexion view using the SynaFlexer positioning device (CCBR-Synarc) were obtained. The aim of the current analysis was to compare the AP and PA fixed flexion radiographs, obtained at the same clinic visit and read together, by Kellgren/Lawrence (K/L) scale grade, overall categorization of radiographic OA, semiquantitative joint space narrowing (JSN), and osteophytes. Our hypothesis was that the AP and PA fixed flexion views would show strong agreement for K/L scale grading, with lesser agreement, as suggested in prior studies, for JSN and osteophytes (5–7).

Materials and Methods

This cross-sectional sample includes individuals enrolled in the Johnston County Osteoarthritis Project, an ongoing, biracial, population-based study in rural North Carolina. Details of this study have been reported elsewhere (11), and it has been approved by the Institutional Review Boards of the Centers for Disease Control and Prevention and the University of North Carolina. Briefly, this study involved civilian adults ages ≥45 years who resided in Johnston County, North Carolina, who were recruited by probability sampling, with oversampling of African Americans. The sample for the current analysis included individuals who participated in the first followup of the study, conducted between 1999 and 2004 (n = 1,733); 69 individuals were excluded from the current analysis because of a lack of complete radiographic data for both knees, leaving 1,664 for the current analysis. There were 37 right knees and 21 left knees with total joint replacements, leaving 1,627 right knees and 1,643 left knees for analysis of K/L scale grades. Because JSN and osteophyte reads were added later, these measures were available on a subset of films (n = 518). All of the participants underwent knee radiography using 2 methods.

AP films were obtained with the participant standing and the knees in full extension, and with the horizontal x-ray beam centered at the level of the superior patellae. PA fixed flexion views were obtained with the participant standing on a positioning guide (SynaFlexer), keeping the feet in 5 degrees of external rotation with the knees flexed until the knees and anterior thighs were in contact with the cassette holder, providing approximately 20 degrees of fixed knee flexion. The x-ray beam in the PA fixed flexion views was centered on the joint line posteriorly and angulated caudally 10 degrees (8, 10).

All of the films were read by a single experienced musculoskeletal radiologist (JBR) previously shown to have high intra- and interrater reliability (κ = 0.89 and 0.86, respectively) (2). Each participant's AP and PA fixed flexion films were read simultaneously in one sitting. K/L scale grades were assessed using a standard atlas as previously described (12). JSN was graded for both the medial and lateral tibiofemoral joint compartments using the Burnett atlas (range 0–3); osteophytes were graded for medial and lateral compartments and for femoral and tibial aspects, ranging from 0–3 in each site (13).

Descriptive statistics were calculated for demographic and clinical variables, including race, sex, age, and body mass index (BMI). AP and PA fixed flexion results were compared by contingency table methods to obtain frequencies for K/L scale, JSN, and osteophyte grades. The crude percent agreement was calculated, and kappa coefficients were determined to assess agreement exceeding chance alone (14). When using the range of K/L scale grades (0–4) or JSN and osteophytes (0–3), weighted kappa was used to grant partial credit to results differing by one or more categories of agreement (as opposed to no credit for simple kappa). Unweighted kappa was used to compare dichotomous K/L scale grades (K/L scale grade <2 compared with K/L scale grade ≥2). Since the results were very similar for the right and left knee, only data from the right knee are presented.


The overall sample consisted of 65% women and 73% whites, with a mean ± SD BMI of 30.2 ± 6.2 kg/m2 and a mean ± SD age of 66.0 ± 9.9 years. The smaller subsample for JSN and osteophytes was slightly older (mean ± SD age 67.1 ± 9.4 years) with a greater proportion of whites (81.2%), but was otherwise similar to the overall sample.

Sixty-seven percent of the right knees (1,099 of 1,627) were categorized as not having radiographic OA (K/L scale grade <2) by both methods. A K/L scale grade ≥2 was identified in 21.6% of knees (352 of 1,627) by both methods. Overall agreement as to the presence or absence of radiographic OA for the 2 methods was 89.2% (κ = 0.73, 95% confidence interval [95% CI] 0.69–0.76), indicating substantial agreement for the 2 methods (Table 1). For the 10.9% of knees in which the 2 methods differed, 4.9% of knees (79 of 1,627) were graded as K/L scale grade <2 in the PA fixed flexion view but ≥2 in the AP view, whereas 6.0% (97 of 1,627) were graded as K/L scale grade <2 in the AP view and ≥2 in the PA fixed flexion view.

Table 1. Agreement by crude percentage and kappa statistics for radiographic grades on anteroposterior and posteroanterior fixed flexion radiographs*
 No.Agreement, %Weighted κ95% CI
  • *

    95% CI = 95% confidence interval; K/L = Kellgren/Lawrence; JSN = joint space narrowing.

  • Except for K/L scale grade as a 2-category outcome, where the unweighted kappa is provided. A kappa value <0 indicates agreement less than expected by chance, 0.01–0.20 indicates slight agreement, 0.21–0.40 indicates fair agreement, 0.41–0.60 indicates moderate agreement, 0.61–0.80 indicates substantial agreement, and 0.81–0.99 indicates almost perfect agreement (16).

K/L scale grade (<2 vs. ≥2)1,62789.20.730.69–0.76
K/L scale grade (range 0–4)1,62766.60.690.66–0.71
Medial tibial osteophytes51782.60.680.62–0.74
Lateral tibial osteophytes51887.50.720.65–0.79
Medial femoral osteophytes51891.70.420.28–0.56
Lateral femoral osteophytes51895.90.620.45–0.79
Medial JSN51777.80.680.63–0.74
Lateral JSN51894.00.650.54–0.76

For comparison by individual K/L scale grades between the 2 views, a slightly greater percentage of knees was given a K/L scale grade of 0 on the AP view compared with the PA view (41.5% and 39.5%, respectively) (Figure 1). A K/L scale grade of 1 was assigned to a similar percentage of knees on both views (AP 32.0%, PA 32.9%). More knees were given a K/L scale grade of 2 on the AP (14.5%) compared with the PA (11.4%) view, whereas more knees were graded as having a K/L scale grade of 3 or 4 by the PA view (10.5% or 5.7%, respectively) than the AP view (8.7% or 3.3%, respectively). The weighted kappa for individual K/L scale grades was 0.69 (95% CI 0.66–0.71) (Table 1).

Figure 1.

Comparison of Kellgren-Lawrence (K-L) scale grades by anteroposterior (AP) extended and posteroanterior fixed flexion (PA-FF) knee radiographs. The percentage of knees in each K-L scale grade (range 0–4) by AP and PA-FF reads is shown.

Agreement between AP and PA fixed flexion views was substantial for medial and lateral tibial osteophytes (Table 1). Medial tibial osteophytes received the same grade on both views in 82.6% of knees, whereas another 17% of knees were assigned grades within one level on the 2 views (i.e., osteophytes = 0 on one view and osteophytes = 1 on the other view). A trivial number of knees (0.2%) differed by more than one grade between views (i.e., osteophytes = 0 on one view and osteophytes = 2 on the other view). Lower kappa values and wider 95% CIs were noted for the less frequent medial and lateral femoral osteophytes. Agreement was high for medial JSN, with a weighted kappa of 0.68; 77.8% of knees were assigned the same JSN grade on both AP and PA fixed flexion views, and another 21.1% of knees were within one grade (i.e., JSN = 0 on one view and JSN = 1 on the other), whereas only 1.2% of knees were assigned grades that differed by more than one level (i.e., JSN = 0 on one view and JSN = 2 on the other). Lateral JSN was less frequent and had slightly lower kappa statistics with wider 95% CIs (Table 1).


In this analysis of more than 1,600 AP and PA fixed flexion knee radiographs taken at a single clinic visit and read together by a single, experienced musculoskeletal radiologist, we found substantial agreement between the 2 views for categorizing radiographic OA (K/L scale grade <2 versus K/L scale grade ≥2), for individual K/L scale grades, and for the most commonly identified radiographic features, tibial osteophytes and medial JSN. This level of agreement by kappa rivals that of inter- and intrareader reliability of the K/L scale grade itself in other large studies (1, 3). Therefore, for the purposes of categorizing joints by affected status (radiographic OA versus not radiographic OA), the results from these views are comparable. This result suggests that for broad comparisons of radiographic OA prevalence in a longitudinal cohort study such as the Johnston County Osteoarthritis Project, both AP and PA fixed flexion views can be incorporated into analyses spanning multiple time points. However, for specific analyses of individual radiographic features, the baseline AP films are not as readily comparable with PA fixed flexion views.

We were unable to identify any prior large head-to-head studies of AP compared with PA views of the knee for K/L scale grade because the focus has generally been on joint space width. There are, however, studies showing the superiority of weight-bearing compared with non–weight-bearing views (15) and of flexion views compared with extended views in either projection (5, 6) for the detection of JSN. In the current analysis, the AP and PA fixed flexion films were read simultaneously by a single experienced reader, which although a strength of our study, may limit the generalizability of our findings to other studies. Because the films were read in this manner rather than independently, it is not possible to determine whether one view is superior to the other in the current analysis. However, a few observations can be made. There was substantial agreement between the views for osteophytes, despite concerns that uncontrolled rotation in AP views may obscure osteophytes. Most of the observed difference in grades for the K/L scale and JSN were in the most severe categories, in which PA fixed flexion views tended to be graded higher than AP views. This suggests that AP views underestimate JSN, which is in agreement with the literature (5–7).

Other radiographic techniques may improve the sensitivity and reproducibility of measurements by improving the alignment of the tibial plateau. Compared with AP extended views, fluoroscopically positioned semiflexed PA knee radiographs produce higher semiquantitative JSN scores and smaller mean joint space width measurements, and have a greater sensitivity to change over 1 year (6, 7). Although the use of fluoroscopically-positioned films has demonstrated the greatest sensitivity to change (6, 9), this technique requires costly equipment and trained technologists, making it logistically challenging for large, longitudinal OA studies (8). Therefore, it is likely that standardized single views such as the PA fixed flexion discussed herein will continue to be utilized for both clinical and research purposes.

Analyses of AP extended weight-bearing knee radiographs, commonly used in clinical practice and screening for trial inclusion, agree substantially with PA fixed flexion knee radiographs for semiquantitative grades (K/L scale, osteophytes, and JSN) in this cross-sectional study.


All authors were involved in drafting the article or revising it critically for important intellectual content, and all authors approved the final version to be submitted for publication. Dr. Nelson had full access to all of the data in the study and takes responsibility for the integrity of the data and the accuracy of the data analysis.

Study conception and design. Nelson, Renner, Shreffler, Jordan.

Acquisition of data. Renner, Jordan.

Analysis and interpretation of data. Nelson, Renner, Shi, Shreffler, Schwartz, Jordan.