Rapid Structural Disease Progression in Knee Osteoarthritis Reply to Letter to the Editor.

We appreciate the letter from Drs. Riddle and Dumenci on our analysis assessing trajectories of joint space width (JSW) progression in knee osteoarthritis (OA). The authors raise good points about the analytic approach and in interpreting the results in general. The authors raise a question about the interpretation of early vs. late progression in the context of a cohort study enrolling subjects with prevalent knee OA. We describe the so-called "horse-racing effect" in the discussion section of our manuscript and gladly expand here.