Measured and predicted bone mineral content in healthy boys and girls aged 6–18 years: adjustment for body size and puberty

Abstract

Dual‐energy X‐ray absorptiometry (DEXA) is a rapid and precise technique for the assessment of bone mineralization in children. Interpretation of the results in growing children is complex as results are influenced by age, body size (height and weight) and puberty. Conventionally, bone mineral data derived from DEXA have been presented as an areal density [BMD; bone mineral content (BMC, g)/projected bone area (BA, cm2)], yet this fails to account for changes in BMC that result from changes in age, body size or pubertal development. Measurement of BMC and BA of the whole body, lumbar spine and left hip were made in 58 healthy boys and girls using DEXA. The relationship between BMC and BA was curvilinear, with the best fit being that of a power model (BMD = BMC/BAλ, where λ is the exponent to which BA is raised in order to remove its influence on BMC). The value of λ changed when measures of body size and puberty were taken into account (e.g. for lumbar spine from 1.66 to 1.49). Predictive formulae for BMC were produced using regression analysis and based on the variables of age, body size and pubertal development. This provides a method for interpreting the measured BMC which is independent of such variables and a constant reference range for children aged 6–18 y.

Importance of Adjusting Dual-energy X-Ray Output for Body Size

“…The conclusion that poor bone health was common among the patients might have been reached on the basis of these findings, particularly with a background of recent evidence that skeletal morbidity is common among patients with ALL, and more general concerns over poor bone health in patients with ALL. [1][2][3][4][5] However, such a conclusion would be inappropriate since, when BMD was corrected for body size, the prevalence of low BMC at lumbar spine and for the whole body was very much lower. Corrected data suggested an alternative conclusion in relation to the bone health of these patients.…”

“…Although adjustment of BMD for body size is recommended widely, 4,6,9 there is currently no consensus as to how best to make the adjustment of DXA-derived BMD data for body size. For the present study we used the approach described by Warner et al, 4 based on calculation of bone mineral content (BMC) Z scores, adjusted for age, sex, and bone size. This approach provides a percentage predicted bone area for age and a percentage predicted BMC for bone area.…”

“…This approach provides a percentage predicted bone area for age and a percentage predicted BMC for bone area. 4 Moreover, by using linear interpolation, these data give predicted and percentage predicted bone area for age and sex. 4 As indicators of possible concern over bone health we used the cut-offs for BMD and BMC Z scores of À1.0 and À2.0.…”

“…In some specialties it has been well known for some time that there is a need to correct DXA-derived bone mineral density (BMD) data for body size, particularly in children and adolescents with chronic disease, including malignancy. 4,6 Such corrections are not universal in the literature in other specialties though, and a number of recent reports on bone health in patients with childhood cancer have been based on BMD data with no formal correction for body size. 7,8 For example, a recent study of BMD in patients with neuroblastoma used BMD data, without formal correction for body size, to conclude that impaired bone health was very common.…”

Radiology of Osteoporosis

“…Another approach for correcting DXA for size involves the estimation of a 'size adjusted, BMC', which is calculated using a regression, or a multivariate, statistical model (Prentice et al 1994) to adjust BMC for cofounders, such as projected bone area, overall body height and weight (surrogates for bone size) and Tanner stages of sexual develop-ment (Warner et al 1998). 'Size adjusted BMC' is often used in research studies, for example when comparing BMC in a group of children with a disease to healthy controls.…”