Background Obesity is one of the most important risk factors for knee pain in the elderly . Cross sectional studies suggest that body fat mass is associated with knee pain and body lean mass may be protective [2, 3]. Few cohort studies have investigated the association between obesity, body composition and change in knee pain.
Objectives To describe the longitudinal relationship between obesity, body composition and change in knee pain.
Methods Our study is a prospective, population-based study with 1099 subjects (female 51.1%) aged 50 to 79 at baseline. Knee pain was assessed by the Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC). Consistent knee pain was defined as knee pain at all 3 time-points. Anthropometrics were measured and body mass index (BMI) was calculated. Body fat, trunk fat and lean mass were measured by dual energy x-ray absorptiometry (DXA).
Results 875 participants made follow-up at 2.6 years and 767 at 5.1 years. Compared to those without knee pain at baseline, participants with knee pain had higher BMI (28.6 vs 27.1 kg/m2), body fat mass (34.9% vs 33.0%) and trunk fat mass (34.6% vs 32.4%), but lower body lean mass (62.2% vs 64.0%).
After adjustment for age, gender height and radiographic OA, BMI and body fat mass at baseline were associated with the risk of consistent knee pain over 5.1 years in this elderly cohort. With one standard deviation (SD) increase in BMI and body fat mass, the risk increased by 13% and 89%, respectively. In contrast, the risk of consistent knee pain decreased by 48% for one SD increase in body lean mass percentage.
All obesity indicators at baseline showed significant associations with incident and worsening knee pain over 5.1 years. BMI was the strongest predictor of increased knee pain in all WOMAC subscales. Body fat mass and trunk fat mass were more consistently associated with increase in non-weight-bearing pain than weight-bearing pain ( Figure 1a and Figure 1b). Similarly, lean mass was more consistently associated with lower risk of increased non-weight-bearing pain than weight-bearing pain.
In a linear mixed-effect model, for every increase of SD in BMI, knee pain score was 1.07 (95% CI 0.62∼1.51) higher. Having more total body fat mass and trunk fat mass were associated with a higher total WOMAC score (beta=0.70 and 0.51, respectively). Consistently, body lean mass showed an inverse association with total knee pain (beta=-0.67, 95% CI -1.18∼-0.16).
Conclusions Obesity, proportionately more body fat mass and less lean mass, is an important risk factor for incident and worsening of knee pain in the elderly. BMI is the most consistent predictor of knee pain and body fat mass is correlated with non-weight-bearing knee pain. A proportionately greater amount of lean mass may have beneficial effects in reducing knee pain.
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Zhai G, Blizzard L, Srikanth V, et al. Correlates of knee pain in older adults: Tasmanian Older Adult Cohort Study. Arthritis Rheum 2006;55(2):264-71.
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Acknowledgements Special thanks go to the participants who made this study possible. The role of Tasmania Older Adult Cohort staff and volunteers in collecting the data is gratefully acknowledged.
Disclosure of Interest None declared