The Effect of Age on Fat and Bone Properties along the Vertebral Spine 2013-01-1244

The human body changes as it becomes older. The automotive safety community has been interested in understanding the effect of aging on restraint performance. Recent research has been focused on assessing the structural and material changes associated with age. In this study, structural tissue distribution was determined using the computed tomography (CT) scan data of more than 19,000 patients, aged 16 and up. The data consisted of subcutaneous fat cross-sectional area, visceral fat cross-sectional area, and trabecular bone density taken at each vertebral level. The data was quantified as a function of five age groups with the youngest group defined as 16-29 years old and the oldest group as 75 and up. An additional analysis stratified on gender was carried out.

Overall, visceral fat increased with age. Compared to the 16-29 group, the visceral fat measured at the L1 level was 1.97 in the 30-44 group, 2.55 in the 45-59 group, 3.33 in the 60-74 group and 3.21 times greater in the 75+ group. Subcutaneous fat also increased with age up to the 60-74-year-old group. The subcutaneous fat measured at L1 level was 1.34 in the 30-44 group, 1.39 in the 45-59 group, 1.38 in the 60-74 group and 1.09 times greater in the 75+ group than in the 16-29 group. A significant association between trabecular bone density and age was found. Trabecular bone density in Hounsfield units (HU) decreased as a function of age, by 2.57*age + 0.0056*age₂ for females and 2.57*age + 0.0082*age₂ for males.

Gender differences were also observed. Females had 1.43 times more subcutaneous fat and 1.10 times higher trabecular bone density than males at L1, while males had 1.88 times more visceral fat than females. Females gained more subcutaneous fat at L1 as they increased in age up to the 45-59 group and then progressively lost subcutaneous fat. Males and females gained more visceral fat at L1 as they aged up to the 60-74 group. Both consistently lost bone density at L1 as they grew older.

The data was also analyzed for a male subgroup approximating the height and weight of a 50th percentile male dummy. Visceral fat increased with age while trabecular bone density decreased.

There was an overall-trend for an increase in subcutaneous fat with age. The results obtained in this study provide insight on the quantified effect of bone and fat distribution as a function of age, gender, and vertebral level. Fat and bone distribution needs to be considered in human mathematical models used to develop safety countermeasures for the older population.