Relative contribution of vertebral body and posterior arch in female and male lumbar spine peak bone mass

Peak bone mass (PBM) is an important reference value in the diagnosis of osteoporosis. It is usually established by determining the areal bone mineral density (BMD in g/cm²) for a given site of the skeleton in young healthy adults. This measurement takes into account both the thickness and the integrated mineral density of the bone scanned. It should therefore be a major determinant of the resistance to mechanical stress. However, in lumbar spine the mean BMD as determined by dual-energy either isotopic or X-ray (DXA) absorptiometry in antero-posterior (ap) view was repeatedly found not to be different between male and female young healthy adults despite the greater volume of lumbar vertebral bodies in males. A greater contribution of the posterior vertebral arch to areal BMD-ap in females than in males could account for such an apparent discrepancy. In order to clarify this issue we have determined in 65 (32 male and 33 female) young healthy adults aged 20–35 years the relative contribution of the vertebral body (VB) and posterior vertebral arch (VA) to BMD and bone mineral content (BMC) of L2–3 measured by both antero-posterior and lateral (lat) scanning using DXA. In young healthy adults mean BMC in antero-posterior view was found not to be significantly different from the total BMC determined by lateral scanning including both VB and VA. This allowed us then to calculate the VA BMC by substracting VB BMC-lat from BMC-ap. The results indicated that the mean value for males was significantly greater than that for females for BMC-ap (male/female ratio (mean±SEM): 1.16±0.05,p<0.01), BMC-lat (1.38±0.07,p<0.001) and VB BMD-lat (1.16±0.04,p<0.001). In sharp contrast, no sex difference was found in BMD-ap (male/female ratio: 0.99±0.03) and VA BMC (male/female ratio: 0.97±0.06). VA BMC represented 44% and 53% (p<0.001) of BMC-ap in males and females, respectively. Furthermore, in neither sex was any correlation between VA BMC and VB BMC found. In summary, this study indicates that the relative contribution of the posterior vertebral arch to the bone mineral content of L2–3 is significantly smaller in males than in females. This difference could partly explain the absence of a sex difference in areal BMD as measured in antero-posterior view. In agreement with lumbar anthropomorphometric data this study further shows that the sex difference in vertebral body size, an important component in mechanical resistance, is expressed when areal BMD is measured in lateral but not in antero-posterior scanning. Finally, the data analysis underlines the quantitative importance of the vertebral arch in the value of areal BMD as measured by DXA in the classical antero-posterior view, and demonstrates the absence of a significant quantitative relationship between the bone mineral content of the vertebral body and that of the posterior vertebral arch.