A novel quantitative metric for joint space width: data from the Osteoarthritis Initiative (OAI)

Joint space width (JSW) has been the gold standard to assess loss of cartilage in knee osteoarthritis (OA). Here we describe a novel quantitative measure of joint space width: standardized JSW (stdJSW). We assess the performance of this quantitative metric for JSW at tracking Osteoarthritis Research Society International (OARSI) joint space narrowing grade (JSN) changes and provide reference values for different JSN grades and their annual change.

We collected 18,934 individual knee images along with JSW and JSN readings from baseline up to month 48 (4 follow-ups) from the OAI study. Standardized JSW and 12-month JSN grade changes were calculated for each knee. For each JSN grade and 12-month grade change, the distribution of JSW loss was calculated for JSW and stdJSW. Area under the ROC curves was calculated on discrimination between different JSN grades for JSW and stdJSW. Standardized response mean (SRM) was used to compare the responsiveness of the two measures to changes in JSN grade.

The areas under the receiver operating characteristic (ROC) curve (AUC) for stdJSW at discriminating between successive JSN grades were AUC_stdJSW = 0.87, 0.95, and 0.96, for JSN>0, JSN>1 and JSN>2, respectively, whereas these were AUC_fJSW = 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 SRM.

Our results show that stdJSW outperforms absolute JSW at discriminating and tracking changes in JSN and further that this effect is in part because stdJSW cancels JSW variations attributed to patient height variations.