In longitudinal studies, obesity has been shown to be a major risk factor for knee osteoarthritis (OA) (Felson, 1988), but the cause of this increased prevalence is not yet clear. A common hypothesis is that obesity increases joint loads due to increased body mass, causing the articular cartilage to experience higher load and degenerate more quickly (Griffin, 2005). However, it has been shown that in healthy, normal-weight subjects, knee cartilage thickness increases in proportion to the loads applied during ambulation (Andriacchi, 2004). It is not known whether this same relationship also holds true for obese people who do not have OA. Because it is difficult to measure joint loads directly in vivo, the external adduction moment can be used as a surrogate measure of the relative load distribution between the medial and lateral compartments of the knee (Andriacchi 2004). If cartilage responds positively to load, a higher adduction moment will be correlated with thicker cartilage on the medial side and thinner cartilage in the lateral compartment. Similarly, average cartilage thickness should be proportional to body mass index. Therefore the purpose of this study was to examine the following hypotheses in a group of healthy-weight, overweight, and obese individuals: 1. Average cartilage thickness in both compartments is proportional to body mass index (BMI). 2. Average cartilage thickness in the lateral compartment is inversely proportional to the knee adduction moment. 3. Average cartilage thickness in the medial compartment is proportional to the knee adduction moment. 4. The ratio of medial to lateral cartilage thickness is proportional to the adduction moment.

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