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Letters to the Editor |
Endocrine Unit, Department of Pediatrics University of Pisa I-56125 Pisa, Italy
The data of Baroncelli and colleagues (1) reinforce the need to relate bone mass to bone volume in the growing skeleton. They have demonstrated that growth hormone deficiency separately reduces both the size of the bones and the amount of bone within the periosteal envelope, but this important conclusion is obscured by confusing terminology. "True" bone density refers to bone as a substance or material, whereas "apparent" bone density refers to a whole bone as an organ (2, 3). The phrase "apparent true bone density," which appears several times in the text and in the editorial flyer on the cover, is an absurdity. Also, the accumulation of cancellous and cortical bone during growth should not be referred to as "mineralization," which is the accumulation of mineral by osteoid. This process is unaffected by growth hormone deficiency, which causes not "undermineralization" but reduced net bone formation on the endosteal envelope.
We appreciate the comments of Dr. M. Parfitt about our recent study (1), and we agree with them. Our study shows that anthropometric variables and bone size affect lumbar bone mineral content (BMC) and bone mineral density (BMD), measured by dual energy x-ray absorptiometry (DEXA), in children with growth hormone (GH) deficiency. In order to reduce the influence of bone size on BMD measurement, mathematical models to correct the BMD values on the basis of the estimated lumbar bone volume have been proposed (2, 3, 4). However, there is no doubt that BMDarea and BMDvolume are surrogate of the true bone density. This is also evident in the terminology we and others use, e.g. "bone mineral apparent density" (3, 5), "apparent volumetric bone mineral density" (2), "volumetric bone mineral density" (6, 7), "volumetric bone mineral apparent density" (8, 9), or "apparent true bone density" (1) to indicate the correction of BMC for bone volume. The term "apparent" reinforces that density is not a real or true density but a bone mineral mass within a region, not all of which is mineral (10). Thus, BMDvolume reflects only an estimation of true bone density, but it is not the true bone density. Indeed, as recently stated in an editorial by Seeman (11), the terms "apparent" and "real" are almost universally dropped from the literature for brevity and convenience, though at the price of understanding that the size of the bone influences the reported measurement of bone density.
During growth, volumetric apparent BMD depends not only on the amount of cortical and trabecular bone contained within the periosteal surface of the bone and its true BMD, but also on the growth of the external size related to the accrual of bone taking place within the growing bone (11). Thus, in children with GH deficiency a reduced BMDvolume may reflect a decrease in mineral accrual or in bone size. Additionally, the reduced bone size did not completely account for the reduction in BMDarea and BMDvolume, suggesting that children with GH deficiency may have a reduced amount of bone within the periostial envelope (1). However, on the basis of the methodological problems related to the measurement of BMD by DEXA, in the conclusion of our article (1) we clearly stated that "further studies are needed to define whether true bone density is affected in children with GH deficiency."
Although we agree with Dr. Parfitt that "mineralization" reflects the accumulation of mineral by osteoid, many authors employed the terms "bone mineralization" (12, 13, 14, 15), "skeletal mineralization" (16), or "bone mineral status" (12, 15) to indicate the mass of mineral in a skeletal region (not all of which is bone) measured by noninvasive methods such as single- or dual-photon absorptiometry or DEXA, and expressed as BMD. Thus, in our article (1) the term "undermineralization" was used synonymously with reduced BMD corrected for the estimated bone volume (BMDvolume) in children with GH deficiency.
We thank Dr. Parfitt for giving us the opportunity to clarify some methodological problems related both to DEXA measurement in children and to the terminology usually employed to report the results obtained with this technique. In this regard, unanimous agreement on the terminology used to define "bone mineral" measured by DEXA would avoid confounding messages.
Footnotes
Address correspondence to: Giampiero I. Baroncelli, Endocrine Unit, Department of Pediatrics, University of Pisa, Via Roma 67, I-56125 Pisa, Italy.
Received December 23, 1998.
References
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