Pitfalls in the accurate measurement of joint space narrowing in semiflexed, anteroposterior radiographic imaging of the knee.

Objective: Computerized measurement of changes in joint space width (JSW) on serial radiographs of the knee in the semiflexed, anteroposterior (SF-AP) view has been used recently as a primary outcome measure in clinical trials of disease-modifying osteoarthritis drugs (DMOADs). In the use of fluoroscopy to achieve reproducible alignment of the medial tibial plateau and x-ray beam, the SF-AP 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 SF-AP view is compromised by the variation in x-ray 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 SF-AP protocol showed an improbable increase in JSW of > or =0.50 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: SF-AP radiographs were obtained at baseline and at 16 months and 30 months thereafter from subjects enrolled in a 6-center DMOAD trial. For each examination, a 6.35-mm 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 magnification-corrected JSW were compared with respect to their reproducibility, agreement, and sensitivity to JSN.

Results: With fully automated measurements, variations in x-ray penetration in analog radiographs and edge enhancement in digital radiographs resulted in the computer "seeing" 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 > or =0.50 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 SF-AP 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.