Bone, 13, 283-284, (1992) Printed in the USA. All rights reserved.

8756-3282192 $5.00 + .OO Copyright 0 1992 Pergamon Press Ltd.

Letter to the Editor

To the Editor:

Concerning the article by Rilegsegger et al. (1991), I have some disagreement with their method and results of compact bone density measurement in the radius by quantitative computed to- mography. True compact bone density is the weight of (wet) bone per unit volume, and this can only be determined by re- moving the bone, determining its volume, and weighing it. All that the radiologic methods are capable of doing is to measure what may be called “bone mineral density” (i.e., the amount of mineral per volume), since only the element calcium (and to a minor degree phosphorus) have effective atomic numbers suffi- ciently higher than those of the other components of bone (water, ground substance, collagen, etc.) to be detectable. The bone mineral density should, thus, equal the chemically determined ash content of bone per unit volume of bone tissue.

The values of “compact bone density” by Riiegsegger et al. (1991) of approximately 1.85 g/cm3 are quite different from those of several chemical and radiological studies. Thus, Arnold (1960) found the compact bone density of human femur ranging from 1.14-l .21 mg of bone ash per cubic centimeter. Gong et al. (1964) determined the ash content of femoral and tibia1 compact bone in the steer (average 1.18 g/cm3), the dog (1.17 g/cm3), and the monkey (1.23 g/cm3). The quantitative microradiographic studies by Jowsey (1964) in other animals reported compact bone mineral density of approximately 1. l-l .3 g/cm3.

All these findings are similar to x-ray densitometric studies by Doyle ( 1970) for normal ulna (approximately 1.15 g/cm3) and Meema and Meema (1978) for normal radius (approximately 1.18 g/cm3). Both these studies, contrary to the finding of Rileg- segger et al. (1991), also showed significant loss of compact bone mineral density in normal women after age 50. This dis- crepancy in age-relationships can be explained by the fact that cortical thinning by endosteal resorption is often preceded by the appearance of juxtaendosteal resorption spaces (Meema & Meema 1988), which apparently is one of the causes of decreas- ing the average cortical bone mineral density in normal post- menopausal women, the other reasons being larger haversian canals (“incompletely closed osteons”), and a lower degree of mineralization of many osteons with aging (Jowsey 1960). Rileg- segger et al. (1991), however, have determined the mineral con- tent in the central zone of the cortex where the described intra- cortical changes may be minimal or less marked. This would explain the apparent diagnostic insensitivity of the compact bone study by their method for diagnosis of osteoporotic conditions, and the much better sensitivity for hypetparathyroidism where intracortical porosity tends to develop throughout the cortical thickness. Such intracortical porosity can, however, be diag- nosed in the metacarpal bones by much simpler means than quantitative computed tomography (Meema 1991).

References

Arnold. 1. S. Quantitation of mineralization of bone as au organ and tissue in osteopomsis. Clin. Or&p. &I. Res. 1’1:16?-175; 1960

Doyle, F. H. Basic technical rquitements for measurement of bone mineral by an x-ray source. Whedon, D. G.; Cameron, 1. R.; eds. Proceedings of Confer-

ence: Progress in Methods of Bone Mineral Measurement. 1968, February 15-17: Bethesda, MD: U.S. Department of Health, Education and Welfare; 1970.

Gong, 1. K.; Arnold, J. S.; Cohn, S. H. Composition of trabecular and conical bone. Anat. Rec. 149~325-332: 1964.

Jowsey, 1. Age changes in human bone. Ck Orrhop. Rel. Rrs. 17:210-218; 1960.

Jowsey, 1. Variation in bone mineralization with age and disease. Frost, H. M., ed. Bone biodynamics. London: J. A. Churchill; 1964.

Meema, H. E. Radiologic study of endosteal. intmcortical and periosteal surfaces

of hand bones in metabolic bone diseases. Hand Clinics 7:37-51; 1991.

Meema, H. E.; Meema, S. Compact bone miuet-al density of the normal human radius. ACIU Radial. Oncol. 17:342-352; 1978.

Mcema. H. E.; Meema, S. Longitudinal microradioscopic comparisons in en- dosteal and juxtaendosteal bone loss in premenopausai and postmenopausal women and in those with end-stage renal failure. Bone 8:34>350; 1988.

Riiegsegger, P.; Durand, E. P.; Dambacher, M. A. Differential effects of aging and disease on trabecular and compact bone density in the radius. Bone 125%

105; 1991.

Professor H. Erik Meema, M.D., F.R.C.P.(C) University of Toronto

Toronto Western Hospital 399 Bathurst St.

Toronto, Ontario Canada M5T 2S8

Author’s Response

We see no disagreement between our paper and the facts stated by Dr. Meema. Our viewpoint, however, is considerably differ- ent .

Traditionally, true compact bone density was determined by removing the bone, determining its volume, and weighing it. This procedure, however, can’t be applied to patients and volunteers. We, therefore, developed a noninvasive proce- dure based on computed tomography (CT) examinations. With CT we determine the mean linear attenuation coefficient p in a volume of interest. All material (not only bone min- eral) contribute to p,. Then p is converted to density. If the volume of interest is set inside compact bone, then the density of compact bone (CBD) results. As reported in our paper, CBD contains all bone components (not only bone mineral). It describes true bone density. The value of 1.85 g/cm3 (our result for true compact bone density) is consistent with the value of 1.18 g/cm3 (bone mineral density) given by Dr. Meenla. Our CBD is the density of the central zone of compact bone. Changes in CBD are caused by changes in porosity and min- eralization of the bulk material. Naturally, it is not sensitive to cortical thinning because the volume of interest is pur- posely selected inside the compact bone region. Cortical thin- ning is determined separately (see next paragraph). The quantitative evaluation of high resolution CT images en- ables us to analyze bone changes in great detail. It is no longer necessary to stick to one lump parameter (as BMC or

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