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This study aims to investigate the effects of exercise on patellar cartilage using T2 relaxation time mapping of magnetic resonance imaging in postmenopausal women with mild patellofemoral joint osteoarthritis (OA).Eighty postmenopausal women (mean age, 58 (SD, 4.2) yr) with mild knee OA were randomized to either a supervised progressive impact exercise program three times a week for 12 months (n = 40) or a nonintervention control group (n = 40). Biochemical properties of cartilage were estimated using T2 relaxation time mapping, a parameter sensitive to collagen integrity, collagen orientation, and tissue hydration. Leg muscle strength and power, aerobic capacity, and self-rated assessment with the Knee Injury and Osteoarthritis Outcome Score were also measured.After intervention, full-thickness patellar cartilage T2 values had medium-size effect (d = 0.59; 95% confidence interval, 0.16 to 0.97; P = 0.018); the change difference was 7% greater in the exercise group compared with the control group. In the deep half of tissue, the significant exercise effect size was medium (d = 0.56; 95% confidence interval, 0.13 to 0.99; P = 0.013); the change difference was 8% greater in the exercise group compared with controls. Furthermore, significant medium-size T2 effects were found in the total lateral segment, lateral deep, and lateral superficial zones in favor of the exercise group. Extension force was 11% greater (d = 0.63, P = 0.006) and maximal aerobic capacity was 4% greater (d = 0.55, P = 0.028) in the exercise group than in controls. No changes in Knee Injury and Osteoarthritis Outcome Score emerged between the groups.Progressively implemented high-impact and intensive exercise creates enough stimuli and exerts favorable effects on patellar cartilage quality and physical function in postmenopausal women with mild knee OA.