OBJECTIVES--The natural history of knee osteoarthritis (OA) is poorly understood. The principal aim was to assess the rate of contralateral knee OA in middle aged women in the general population with existing unilateral disease and to identify the major factors that influence this rate. METHODS--Fifty eight women aged (45-64) from a general population study cohort were identified with unilateral knee OA diagnosed radiologically (Kellgren and Lawrence 2+) (K&L). Follow up AP films were obtained at 24 months and compared with the baseline for K&L grade and individual features of osteophytes and joint space. RESULTS--Twenty women (34%) developed incident disease in the contralateral knee (based on K&L 2+ or osteophyte changes) and 22.4% (n = 13) of women progressed radiologically in the index joint. Obesity at baseline was the most important factor related to incident disease, 47% of women in the top BMI tertile developed OA, compared with 10% in the lowest tertile: relative risk 4.69 (063-34.75). No clear effect was seen for age, physical activity, trauma or presence of hand OA. CONCLUSIONS--Over one third of middle aged women with unilateral disease will progress to bilateral knee OA within two years and a fifth will progress in the index joint. Obesity is a strong and important risk factor in the primary and secondary prevention of OA. These natural history data provide a useful estimate for planning therapeutic intervention trials.
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