Joint Space Width (JSW) has been the gold standard to assess loss of cartilage in knee OA. Here we describe a novel quantitative measure of joint space width, standardized JSW (stdJSW). We assess the performance of this quantitative metric for joint space widthat tracking Joint Space Narrowing OARSI grade (JSN) changes and provide reference values for different joint space narrowing OARSI grades and their annual change.
We collected 18.934 individual knee images from the OAI study, from the follow-up visits up to month 48 (baseline plus 4 follow-up exams). Absolute JSW measurements and JSN readings were collected from the OAI study. Standardized JSW and 12-month changes were calculated for each knee. For each JSN grade and 12-month grade change, the distribution of JSW loss was calculated both for standardized JSW as well as absolute JSW measurements retrieved from the OAI study. Area under the curve of the ROC curves was calculated for the performance of both absolute and standardized JSW at discriminating between different JSN grades. Standardized response mean (SRM) was used to compare the responsiveness of the two measures to change in JSN grade.
The areas under the ROC curve for stdJSW at discriminating between successive JSN grades were AUCstdJSW= 0.87, 0.95, and 0.96, for JSN>0, JSN>1 and JSN>2, respectively, whereas these were AUCfJSW= 0.79, 0.90, 0.98 for absolute JSW. We find that standardized JSW is significantly more responsive than absolute JSW, as measured by the Standardized Response Mean. Furthermore, we present reference values for standardized JSW stratified by base JSN grade and 12-month JSN change.
Our results show that standardized JSW is a better choice to track changes in JSN and to discriminate between JSN grades. Furthermore, our results show that standardized JSW cancels part of the normal variation in JSWs that comes from height variation.