Obesity and spinal loads; a combined MR imaging and subject-specific modeling investigation.

Epidemiological studies have identified obesity asa possible risk factor for low back disorders. Biomechanical models can help test such hypothesis and shed light on the mechanism involved. A novel subject-specific musculoskeletal-modelling approach is introduced to estimate spinal loads during static activities in five healthy obese (BMI>30kg/m2) and five normal-weight (20<BMI<25kg/m2) individuals. Subjects underwent T1 through S1 MR imaging thereby measuring cross-sectional-area (CSA) and moment arms of trunk muscles together with mass and center of mass (CoM) of T1-L5 segments. MR-based subject-specific models estimated spinal loads using a kinematics/optimization-driven approach. Average CSAs of muscles, moment arms of abdominal muscles, mass and sagittal moment arm of CoM of T1-L5 segments were larger in obese individuals (p<0.05 except for the moment arm of CoMs) but moment arms of their back muscles were similar to those of normal-weight individuals (p>0.05). Heavier subjects did not necessarily have larger muscle moment arms (e.g., they were larger in 64kg (BMI=20.7kg/m2) subject than 78kg (BMI=24.6kg/m2) subject) or greater T1-L5 trunk weight (e.g., the 97kg (BMI=31kg/m2) subject had similar trunk weight as 109kg (BMI=33.3kg/m2) subject). Obese individuals had in average greater spinal loads than normal-weight ones but heavier subjects did not necessarily have greater spinal loads (117kg (BMI=40.0kg/m2) subject had rather similar L5-S1 compression as 105kg (BMI=34.7kg/m2) subject). Predicted L4-L5 intradiscal pressures for the normal-weight subjects ranged close to the measured values (R2=0.85-0.92). Obese individuals did not necessarily have greater IDPs than normal-weight ones.