Roentgenographic Estimation of the Mineral Content of Bone

It is well known that adult bone is not merely an inert structural material but is a living tissue with a mineral content which fluctuates under the influence of endocrine and enzyme activity. Hevesy (1) showed, by the use of radiophosphorus, that there is active metabolism in bone. He found, for example, that within fifty days 29 per cent of the mineral constituents were renewed in the femoral and the tibial epiphyses of rabbits. In pregnancy there may be a transfer of calcium and phosphorus from the bones and teeth of the mother to the tissues of her child. Over 60 per cent of the skeletal calcium of the newborn child is laid down during the last two months of prenatal life. The trabeculae of the bone constitute a calcium store which is available for maintenance of the calcium requirements of other tissues when the ingested supply is insufficient. The ratio of calcium to phosphorus remains essentially constant. The mineral content of bone may fluctuate even in health, but particularly in certain cases of endocrine imbalance, such as in hyperparathyroidism. It is of considerable interest, therefore, to be able to estimate, to a fair degree of accuracy, this mineral content. The problem divides itself into two categories: (a) the absolute determination of the mineral content, so as to make possible a comparison of the value with corresponding normal values, and (b) the comparison of successive determinations of the same patient, so that the progress of the bone changes may be followed. The area of bone best suited for an absolute determination of the mineral content is probably the central portion of the femur (Fig. 1), because here we have a thickness of cortex which can be measured in millimeters. The mineral content per millimeter of cortex gives a measure of the concentration of calcium and phosphorus in the bone. The cortex is so dense, however, that a change in mineral content, as a result of therapy for example, may not show up as soon as it will in the cancellous bone. Therefore, for comparisons of successive determinations, an area of cancellous bone may be chosen, as this will be more quickly sensitive to changes in calcium metabolism. Other bones might have been measured. Bywaters (2), for example, used the heads of the metacarpals. This is a good area for comparative purposes, between successive examinations, as the soft tissue here is quite thin. The bone, however, is somewhat cancellous, and thus it is difficult to estimate the absolute amount that has been traversed by the x-ray in the area being measured. It is desirable to use for measurement a bone which has a fairly smooth surface and an area large enough to be measured with a densitometer. The size of the area depends upon the type of densitometer employed. We use an area 4 mm. in diameter (3). The calcaneus might be used, as it offers a fairly flat area, and the soft tissue there is quite thin.