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 bo ne mineral density (BMD in g/cm(2)) for a given site of the skeleton i n young healthy adults. This measurement takes into account both the t hickness and the integrated mineral density of the bone scanned. It sh ould therefore be a major determinant of the resistance to mechanical stress. However, in lumbar spine the mean BMD as determined by dual-en ergy either isotopic or X-ray (DXA) absorptiometry in antero-posterior tap) view was repeatedly found not to be different between male and f emale young healthy adults despite the greater volume of lumbar verteb ral 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 determ ined in 65 (32 male and 33 female) young healthy adults aged 20-35 yea rs the relative contribution of the vertebral body (VB) and posterior vertebral arch (VA) to BMD and bone mineral content (BMC) of L2-3 meas ured 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 indicat ed that the mean value for males was significantly greater than that f or 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-a p (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 ma les and females, respectively. Furthermore, in neither sex was any cor relation between VA BMC and VB BMC found. In summary, this study indic ates that the relative contribution of the posterior vertebral arch to the bone mineral content of L2-3 is significantly smaller in males th an in females. This difference could partly explain the absence of a s ex difference in areal BMD as measured in antero-posterior view. In ag reement with lumbar anthropomorphometric data this study further shows that the sex difference in vertebral body size, an important componen t in mechanical resistance, is expressed when areal BMD is measured in lateral but not in antero-posterior scanning. Finally, the data analy sis underlines the quantitative importance of the vertebral arch in th e value of areal BMD as measured by DXA in the classical antero-poster ior view, and demonstrates the absence of a significant quantitative r elationship between the bone mineral content of the vertebral body and that of the posterior vertebral arch.