Obesity alters the in vivo mechanical response and biochemical properties of cartilage as measured by MRI

BACKGROUND: Obesity is a primary risk factor for the development of knee osteoarthritis (OA). However, there remains a lack of in vivo data on the influence of obesity on knee cartilage mechanics and composition. The purpose of this study was to determine the relationship between obesity and tibiofemoral cartilage properties. METHODS: Magnetic resonance images (3T) of cartilage geometry (double-echo steady-state) and T1rho relaxation of the knee were obtained in healthy subjects with a normal (n = 8) or high (n = 7) body mass index (BMI) before and immediately after treadmill walking. Subjects had no history of lower limb injury or surgery. Bone and cartilage surfaces were segmented and three-dimensional models were created to measure cartilage thickness and strain. T1rho relaxation times were measured before exercise in both the tibial and femoral cartilage in order to characterize biochemical composition. Body fat composition was also measured. RESULTS: Subjects with a high BMI exhibited significantly increased tibiofemoral cartilage strain and T1rho relaxation times (P <0.05). Tibial pre-exercise cartilage thickness was also affected by BMI (P <0.05). Correlational analyses revealed that pre-exercise tibial cartilage thickness decreased with increasing BMI (R(2) = 0.43, P <0.01) and body fat percentage (R(2) = 0.58, P <0.01). Tibial and femoral cartilage strain increased with increasing BMI (R(2) = 0.45, P <0.01; R(2) = 0.51, P <0.01, respectively) and increasing body fat percentage (R(2) = 0.40, P <0.05; R(2) = 0.38, P <0.05, respectively). Additionally, tibial T1rho was positively correlated with BMI (R(2) = 0.39, P <0.05) and body fat percentage (R(2) = 0.47, P <0.01). CONCLUSIONS: Strains and T1rho relaxation times in the tibiofemoral cartilage were increased in high BMI subjects compared with normal BMI subjects. Additionally, pre-exercise tibial cartilage thickness decreased with obesity. Reduced proteoglycan content may be indicative of pre-symptomatic osteoarthritic degeneration, resulting in reduced cartilage thickness and increased deformation of cartilage in response to loading.