Abstract
Objective
Computerized measurement of changes in joint space width (JSW) on serial radiographs of the knee in the semiflexed, anteroposterior (SFโAP) view has been used recently as a primary outcome measure in clinical trials of diseaseโmodifying osteoarthritis drugs (DMOADs). In the use of fluoroscopy to achieve reproducible alignment of the medial tibial plateau and xโray beam, the SFโAP radiographic protocol affords greater sensitivity in the detection of joint space narrowing (JSN) than that achieved by conventional radiographic positioning techniques. However, the utility of the SFโAP view is compromised by the variation in xโray penetration in each examination, which may confound the correction of the automated measurement of JSW for the radiographic magnification inherent in an AP view of the knee. A recent DMOAD trial using the SFโAP protocol showed an improbable increase in JSW of โฅ0.50 mm (i.e., greater than the measurement error). The present report provides an analysis of this problem, and the study aim was to demonstrate that substitution of the automated estimates of JSW with precise manual measurements can markedly reduce the problem attributable to radiographic magnification.
Methods
SFโAP radiographs were obtained at baseline and at 16 months and 30 months thereafter from subjects enrolled in a 6โcenter DMOAD trial. For each examination, a 6.35โmm steel ball was affixed to the skin over the head of the fibula to permit estimation of the percentage of radiographic magnification (%Mag) and correction of JSW measurements. Measurements of the minimum interbone distance (IBD) in the medial tibiofemoral compartment and the %Mag were obtained by an automated method (edge detection) and manually. Combinations of automated and manual measurements of the IBD and %Mag in estimates of magnificationโcorrected JSW were compared with respect to their reproducibility, agreement, and sensitivity to JSN.
Results
With fully automated measurements, variations in xโray penetration in analog radiographs and edge enhancement in digital radiographs resulted in the computer โseeingโ a metal ball whose diameter was artifactually reduced, resulting in an inflated measurement of JSW. Use of manual measurement of the IBD and %Mag largely eliminated these problems and reduced, from 16% to 2%, the frequency of knees exhibiting an increase in JSW โฅ0.50 mm. In 14 of the 15 knees in which a significant increase in JSW was noted with the manual method, this increase in JSW could be explained by the development of significant lateral compartment narrowing during the study or poor alignment of the medial plateau.
Conclusion
Although automated and manual methods of JSW measurement of the knee in the SFโAP view possess comparable intrareader reproducibility, the manual method is less susceptible to technical factors that affect the correction of raw JSW estimates for radiographic magnification. Until we can identify practical, effective solutions to these technical problems, use of any radiographic protocol involving AP imaging of the knee in a DMOAD trial must be viewed with caution.