Our data confirm results from a previous study that examined longitudinal radiographs with a 36 month follow-up time and also used the Synaflexer positioning frame and the fixed flexion protocol(17
). In that study, JSW(x) also showed greater sensitivity to detect change than mJSW in some regions and there was a trend toward improved disease sensitivity in the more central portion of the joint and for more diseased knees. In our current study, we examine a one year follow-up from a different cohort of OA subjects and provide a direct comparison between radiographic JSW and cartilage morphometry measures from MRI. demonstrates that the measure of JSW at x = 0.275 (SRM = −0.32) performs better than all cartilage morphometry measures for all subjects with the exception of Th(MF) (SRM = −0.34) and V(MF) (SRM = −0.39). For KL = 2 and 3 subjects, JSW at x = 0.250 (SRM = −0.34) performs better than all cartilage morphometry measures with the exception of V(MF) (SRM = −0.42).
Our results suggest that radiography may be a valuable modality for clinical trials of knee OA given the general availability of radiology facilities for image acquisition, and the expense required for MRI image acquisition and evaluation. Our study presents the results of a head-to-head comparison between quantitative MRI and radiography measures of OA progression. For KL = 2 or 3, a population that is likely to be used for clinical trials, the difference between the most responsive measures (0.32 and 0.42) is modest. These data imply that an equally powered radiography study would require approximately 1.5 times as many subjects as for a study that used MRI since the minimum sample size to see a statistically significant effect is inversely related to the square of the SRM(25
). The expense of additional subjects would most likely offset by the decrease in imaging costs.
Although located in a generally less weight bearing portion of the joint, increased performance for the more central portion (higher x for the medial compartment) may be explained by the difficultly associated with visualizing the margins in the more damaged portion of the joint. While the best region for longitudinal assessment may not be located at the exact site of the joint damage, JSW(x) at the optimal location may provide an indirect, but more precise, measure of the joint damage that occurs less centrally.
The measures of lateral compartment JSW progression suggest that different JSW metrics could be used to monitor progression depending on the knee alignment to maximize the responsiveness. For example, it may be optimal to use JSW at x = 0.250 for varus knees while JSW at x = 0.725 (lateral compartment) may be the preferred location for valgus knees. Both these sub-groups show improved responsiveness over measures of JSW for the entire set and for all MRI measures. It should also be noted that it was sometimes necessary to use the tibial rims for the delineation of the lateral compartment tibial margin in cases where the plateau could not be visualized.
We found a relatively weak correlation between the radiographic and MRI metrics. While a stronger correlation might indicate an improved understanding of knee OA progression, the results also suggest that each method potentially probes independent OA changes. There are unknown factors affecting structural progression that each method fails to detect. Once these effects are better understood, there is reason to hope that both methods can be further improved. One possible explanation for the low correlation may be that, while radiography was performed with weight-bearing joints, the MRI acquisitions are collected with the subject supine. In addition, the leg is fully extended during the MRI scan while the radiograph is acquired with the knee flexed. A study using an open MRI system where individuals can be imaged while standing would probe this question.
This study has several limitations. While the total number of subjects is substantial, the number of individuals in the valgus group is only 16. Future studies should use larger numbers of valgus aligned knees to confirm these results. We made no attempt to correct for a any possible rotation of the femur which could affect the consistency of the coordinate system; in theory, the use of the positioning frame should minimize this effect. Our study used images from the OAI that were obtained using a fixed-flexion protocol (20
). Previous work has demonstrated that different knee positioning protocols may produce dissimilar results(26
). One study which compared the fixed flexion to a fluoroscopically guided protocol using the same subjects found increased reproducibility and responsiveness of mJSW using the Lyon schuss protocol and a similar software approach(26
). We cannot necessarily generalize our conclusion for other protocols aimed at aligning the tibial rims on the radiograph such as fluoroscopically guided knee radiography(6