The validity of different definitions of radiographic worsening for longitudinal studies of knee osteoarthritis

Several definitions have been used to characterize radiographic worsening o f knee osteoarthritis in longitudinal studies, yet a valid definition with maximal power to detect differences between groups is not known. We used se rial radiographs from the Framingham Osteoarthritis Study to compare five d ichotomous definitions according to construct validity (strength of associa tion) and discriminant power (power to reject null hypotheses of no differe nce) for 1) known risk factors for knee osteoarthritis, and 2) development of new knee pain. For risk factors: definitions that included scores for os teophytes (bone spurs) showed good construct validity and discriminant powe r, a definition using the Kellgren and Lawrence grade of overall knee osteo arthritis was conservative with good construct validity but low discriminan t power: a definition based solely on ordinal assessment of joint space nar rowing had weak construct validity and low discriminant power. AII definiti ons had comparably strong associations with the development of new knee pai n. similar associations with new knee pain were found when the analysis was confined to either knees with no osteoarthritis at baseline or knees with prevalent osteoarthritis, with increased standard errors for prevalent oste oarthritis. Use of any of these definitions, other than joint space narrowi ng alone, would permit detection of associations with most known risk facto rs. Definitions incorporating both osteophytes and joint space narrowing of fer the most precise estimation of the association of risk factors with dis ease worsening. (C) 2001 Elsevier Science Inc. All rights reserved.