The effects of body mass index and age on cross-sectional properties of the femoral neck

Research on the relationship between body mass index (BMI) and cross-sectional geometry of long bone diaphyses demonstrates that strength properties are significantly greater in obese versus normal BMI individuals. However, articular dimensions do not differ appreciably. If femoral head size remains constant, we hypothesize that the femoral neck remodels to accommodate greater loads associated with increased BMI. High-resolution CT scans (n=170 males) were divided into three BMI groups (normal, overweight, and obese) and two age groups (21-50 and >50). OsiriX software was used to obtain a cross-sectional slice at the waist of the femoral neck. Cortical area (CA), total cross-sectional area (TA), percent cortical area (%CA), circularity index (I_max/I_min), section modulus (Z_pol), and second moment of area (J) were measured with ImageJ software. The effects of age and BMI were evaluated statistically. Pairwise comparisons in the younger group only detected significant differences between normal and obese males in the circularity index (P=0.022). The older cohort showed significant differences in CA (P<0.001), %CA (P=0.004), Z_pol (P=0.007), and J (P<0.001) between normal and obese groups. This study shows that the effects of obesity on the cross-sectional geometry of the femoral neck are more pronounced in older males relative to younger males. Older males with increased BMI have greater cortical area and bone strength in the femoral neck relative to younger males, thus making the femoral neck less susceptible to fractures in obese individuals.