Boston University Arthritis Center,
1 Department of Radiology at the Boston VA Medical Center, Boston, MA,
2 Center for Health Outcomes and Economic Research of the HSR&D Field Program, VAMC Bedford, MA, and Boston University School of Public Health, Boston, MA, USA
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Abstract |
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Results. Of the different views, skylines had to be excluded most often because the image of the patellofemoral joint was technically unsatisfactory. In the remaining knees, adding either a lateral or a skyline view to an anteroposterior view yielded roughly equal and high sensitivity (9497%) when compared with the gold standard of a positive X-ray on any of the three views.
Conclusion. As long as at least an anteroposterior view and one image of the patellofemoral joint is obtained (either skyline or lateral), few cases with radiographic disease will be missed. For clinical or epidemiological studies the lateral view may be easier to acquire with high quality than the skyline view.
KEY WORDS: Knee, Osteoarthritis, Imaging, Epidemiological methods.
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Introduction |
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Studies have suggested that the skyline view is preferable to the lateral view because patellofemoral narrowing is read more reproducibly on skyline views [1]. The reproducibility of one feature in one location should not be the sole basis for the selection of images. Rather, the appropriate choice of an image depends, in large part, on its ability to detect disease when disease is present, an especially acute issue in osteoarthritis, given the putative lack of sensitivity of plain radiographs. Obviously, the optimal detection of abnormalities could be achieved by obtaining all three (or even more) images of each knee, but this is often not practicable.
It can be difficult to obtain images of the weight-bearing patellofemoral joint in a consistent manner without a highly trained technician. For clinical evaluation and population studies, it is important to choose views that maximally detect more radiographic OA and yield technically satisfactory films.
The sensitivity (number detected positive by a test/number with disease) of combinations of views for detecting known OA has not been studied previously. To determine the sensitivity of different combinations of X-ray views for detecting radiographic OA, we evaluated the relative performance of PA, skyline and lateral views in a group of persons with knee OA who had evidence of osteoarthritis on at least one radiographic view, to determine how often OA was missed when only one or two views were obtained and to evaluate which combination of views should be recommended for clinical practice and for studies. We obtained AP, skyline and lateral radiographs on both knees of elderly subjects with symptomatic knee OA. We then compared the AP view alone and the combinations of AP plus skyline views and AP plus lateral views to determine which combination of views identified the greatest number of OA cases.
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Methods |
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X-rays
Three X-ray views (PA, lateral and skyline) were obtained on both knees for all subjects. Initial protocols called for VA subjects to obtain X-rays at the VA institution of usual care (four separate locations). The subjects received weight-bearing AP (using the Framingham OA study protocol [3]) and supine skyline and weight-bearing lateral films using standard technologist-selected techniques.
Due to the poor quality of films at the multiple centres, the study was carried out at a single centre with highly trained staff. The AP and skyline protocols were changed to weight-bearing fluoroscopy-based PA and weight-bearing skyline films using the BucklandWright protocols [1]. We continued to require a weight-bearing lateral X-ray but instituted the Framingham X-ray protocol, which specified that the knee be imaged in 30° of flexion (confirmed by goniometry).
X-ray scoring
All X-rays were scored by a single reader (DRG). The Framingham atlas [4] was used to score AP (and PA) and lateral films, and the Altman atlas [5] was used for skyline films. Intra-reader reliability for scores of individual radiographic features was high ( = 0.60.9).
Analyses
Our analyses of sensitivity were based on knees (not individual subjects) and included symptomatic and asymptomatic knees. A knee was eligible for inclusion in the analysis if all three X-ray views were of acceptable quality and there was a definite osteophyte on any of the views. A film was defined as unreadable or as technically unsatisfactory when neither osteophytes nor joint-space narrowing could be scored accurately. Occasionally, osteophytes were scored, but not narrowing, and these knees were included for definitions of OA that are osteophyte-based.
Defining radiographic OA
Because sensitivity may change depending on how one defines X-ray OA and several different definitions have been proposed, we used the following definitions of symptomatic knee OA: (1) knee symptoms plus a definite osteophyte of grade 1 (03 scale); (2) knee symptoms plus a definite osteophyte of grade
2 (03 scale); (3) knee symptoms plus either an osteophyte of grade
2 or joint-space narrowing of grade
2 (03 scale) with at least one other feature (an osteophyte grade 1, sclerosis or cyst). The first definition emanates from the American College of Rheumatology criteria, which require the presence of a definite osteophyte of any size. The two latter definitions have recently been proposed and validated by Felson et al. [4] as a definition of radiographic knee OA incorporating multiple views.
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Results |
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Subjects
There were 258 men, with a mean age of 67.1 yr, who contributed 410 knees, and 134 women, with a mean age of 64.8 yr, who contributed 250 knees. The men had a lower body mass index than the women (Table 1).
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Sensitivity
Using a definition of radiographic knee OA as any definite osteophyte, the sensitivity of AP (or PA) alone was 73%. The combinations of AP plus skyline and AP plus lateral views had identical sensitivities of 9697% for OA (Table 2). When OA was defined as symptoms plus a grade-2 osteophyte, the two film combinations were also quite similar in sensitivity. AP plus skyline views had 93% sensitivity and AP plus lateral views 94% (Table 3
). We then used the definition of radiographic disease that defines disease as present when either a moderate-sized osteophyte (
grade 2) or
grade-2 joint-space narrowing plus another radiographic feature (smaller osteophyte, cyst, sclerosis) is present. Using this definition (Table 4
), we found similar results, with the sensitivity of AP and skyline views almost identical to the sensitivity of the AP plus lateral combination (each roughly 95%).
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Twenty-one knees were characterized as having OA by posterior osteophytes seen on the lateral view only, and accordingly would not have been identified as having OA based on PA and skyline views (Table 5). Those who would be missed if no skyline view or no lateral view were obtained were similar in age and to others, but were more likely to be men than the others with OA.
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Discussion |
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Our findings have implications for both clinical practice and studies of knee osteoarthritis. Radiology suites in clinical practice settings are similar in their proficiency to those of our unstandardized films, a situation in which skyline films were often unreadable. The main reasons were positioning problems including incorrect flexion of the knee, an abnormally rotated view, and the patellofemoral joint space being cut off the edge of the film. For lateral films, excessive rotation was the most frequent technical problem, leading to the rejection of films as technically unsatisfactory (see Methods for definition).
Results of enquiries among X-ray technologists in three X-rays suites in Boston suggest that a knee series' in most locations includes AP and lateral but not skyline films, and that skyline views are obtained infrequently. Even the technique used to obtain skyline views varies from technologist to technologist and from patient to patient. The technical difficulty of obtaining high-quality skyline vs lateral views, the infrequency with which technologists take skyline views and the variability of technique all combine to create problems in quality. For clinical practice, skyline views may not provide optimal information unless the technologists are specifically trained to take them.
Our adoption of a weight-bearing skyline technique [9] provided high-quality images, but this method is unfamiliar to almost all X-ray technologists. With a weight-bearing view, joint-space narrowing may be more detectable at a physiological angle of flexion (3040°). A smaller-scale study will need to devote resources and attention to the training of technologists for the skyline view, as will studies whose goals are to measure patellofemoral narrowing.
Knee OA affects more women than men. One important limitation of our study was the preponderance of male subjects. This is because much of our sample was derived from the VA. As noted earlier, gender-specific analyses yielded the same results, suggesting that, overall, results would be the same in a sample with a larger percentage of women.
We cannot address larger questions about the usefulness of radiographs in knee OA. However, if the goal of a clinical referral or of a study is to detect the presence of radiographic OA, a combination of an AP and a lateral view may be preferable to the combination of AP and skyline views for the reasons we have outlined.
The implications of our findings for studies aimed at identifying those with OA include the recommendation, for multicentre studies which involve a large number of X-ray technologists, that lateral films may be preferred to skyline films. Our results suggest that lateral views will detect a similar proportion of those with OA as skyline views taken under optimal conditions.
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Acknowledgments |
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Notes |
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References |
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