[en] OBJECTIVE: To assess the clinical relevance of mean and minimum femorotibial joint space narrowing (JSN) for predicting future osteoarthritis related surgery in patients with knee osteoarthritis. METHODS: 126 subjects with primary knee osteoarthritis were followed prospectively for a mean eight years. Minimum and mean joint space width (JSW) were assessed from standard x rays at baseline and after a follow up of three years. The rate of knee osteoarthritis related surgery was recorded for the following five years. RESULTS: After a mean follow up of eight years, 16 patients (12.7%) had received osteoarthritis related joint surgery. The areas under the curves (AUC) resulting from the receiver operating characteristic curve analyses for predicting osteoarthritis surgery were 0.73 (p=0.006) for minimum JSN and 0.55 (p=0.54) for mean JSN. The cut off for minimum JSN maximising sensitivity and specificity for predicting future surgery was a change of 0.7 mm or more in minimum joint space width over a period of three years. However, no meaningful differences were observed for cut off values between 0.5 and 0.8 mm The relative risk (adjusted for age, body mass index, baseline symptoms, and baseline JSW) of experiencing osteoarthritis related surgery during the eight year of follow up was 5.15 (95% confidence interval, 1.70 to 15.60) (p=0.004) in patients with a minimum joint space narrowing of 0.7 mm or more during the first three years of the study. CONCLUSIONS: A cut off of 0.5 to 0.8 mm in minimum JSN, measured on standard x rays, reflects a clinically relevant progression in patients with knee osteoarthritis.