PEAK BONE MASS

Peak bone mass, which can be defined as the amount of bony tissue pres ent at the end of the skeletal maturation, is an important determinant of osteoporotic fracture risk. Measurement of bone mass development. The bone mass of a given part of the skeleton is directly dependent up on both its volume or size and the density of the mineralized tissue c ontained within the periosteal envelope. The techniques of single-1 an d dual-energy photon or X-ray absorptiometry measure the so-called 'ar eal' or 'surface' bone mineral density (BMD), a variable which has bee n shown to be directly related to bone strength. Bone mass gain during puberty. During puberty the gender difference in bone mass becomes ex pressed. This difference appears to be essentially due to a more prolo nged bone maturation period in males than in females, with a larger in crease in bone size and cortical thickness. Puberty affects bone size much more than the volumetric mineral density. There is no significant sex difference in the volumetric trabecular density at the end of pub ertal maturation. During puberty, the accumulation rate in areal BMD a t both the lumbar spine and femoral neck levels increases to four- to sixfold over a 3- and 4-year period in females and males, respectively . Change in bone mass accumulation rate is less marked in long bone di aphyses. There is an asynchrony between the gain in statural height an d bone mass growth. This phenomenon may be responsible for the occurre nce of a transient period of a relative increase in bone fragility tha t may account for the pattern of fracture incidence during adolescence . Variance in peak bone mass. At the beginning of the third decade the re is a large variability in the normal values of areal BMD in the axi al and appendicular skeleton. This large variance, which is observed a t sites particularly susceptible to osteoporotic fractures such as lum bar spine and femoral neck, is barely reduced after correction for sta tural height, and does not appear to increase substantially during adu lt life. The height-independent broad variance in bone mass develops d uring puberty at sites such as lumbar spine and femoral neck, where th e accretion rate is markedly increased. Time of peak bone mass attainm ent. Despite the fact that a majority of studies did not indicate that bone mass continues to accumulate significantly during the third and fourth decades, it has been generally accepted that peak bone mass at any skeletal site is attained in both sexes during the mid-thirties. H owever, recent studies indicate that in healthy Caucasian females with apparently adequate intakes of energy and calcium, bone mass accumula tion can virtually be completed before the end of the second decade, f or both lumbar spine and femoral neck. It is possible that both geneti c and environmental factors could influence the time of peak bone mass achievement. Determinants of peaks bone mass. Several variables, more or less independent, are supposed to influence bone mass accumulation during growth; heredity, sex, dietary components, endocrine factors, mechanical forces, and exposure to risk factors. Quantitatively, the m ost prominent factor appears to be the genetic determinant, as estimat ed by studies comparing monozygotic and dizygotic twins. That heredity is not to be the only determinant of peak bone mass is of practical i nterest, since environmental factors can be modified. With respect to nutrition, the quantitative importance of calcium intake in bone mass accumulation during growth, particularly at sites prone to osteoporoti c fractures, remains to be clearly determined. The same can be said fo r the impact of physical activity. Finally, the crucial years when the se external factors will be particularly effective on bone mass accumu lation remain to be determined by longitudinal prospective studies in order to produce credible and well targeted recommendations for the se tting up of osteoporosis prevention programs aimed at maximizing peak bone mass.