A METHOD FOR DETERMINATION OF BONE-SALT CONTENT OF CORTICAL BONE.

Abstract

In recent years there has been an increasing demand for practical clinical methods of determining the mineral content of bone in vivo. The subject has been extensively reviewed by Garn (3) and others (2, 5, 8–10, 14). The basic difference between the present method and those of previous authors is that the mineral density of pure cortical bone is determined in grams per cubic centimeter. Such an approach has previously been suggested by Henny (4). Radiologic determination of the mineral content of cancellous bone has limited value for two reasons: (a) the cancellous bone is always surrounded by a cortex of variable thickness and (b) there is a wide normal variation in trabecular bone texture. Arnold (1) and Vose (19) reported a normal range of approximately 150 ± 50 mg./c.c. of bone tissue in vertebral bodies and femoral condyles respectively. This large normal variation of ±33 per cent would render the determination of bone salt in cancellous bone relatively unsuitable for diagnostic purposes and is one of the reasons why early diagnosis of spinal osteoporosis is difficult. In the shafts of the extremities, cortical bone exists in virtually pure form. Microradiographic studies of cortical bone in man (7) have indicated that its structural appearance is homogeneous from the third to the fifth decade of life. In the sixth decade, an increase appears in the porosity of the cortical bone as well as evidence of increased bone resorption. The mineral content of cortical bone reported by Arnold (1) and obtained by chemical analysis is 1,140 to 1,210 mg./c.c. Since these figures are based on only 3 normal cases, the full normal range is probably greater. On the other hand, Robinson and his co-workers' figures (15) of 900 to 1,400 mg./c.c. may include abnormal cases. The present work presents a radiographic method for determining the mineral content of cortical bone and indicates the usefulness of such a method in the clinical investigation of metabolic bone disease. Method The Basic Principles 1. Theoretical Development. Most methods of bone density measurement, including the present one, are based on the comparison of the density produced by a standard stepwedge to the density produced by a bone simultaneously exposed on a radiograph (Fig. 1). The photographic densities of the wedge measured by a densitometer are plotted against the wedge thickness, and a calibration curve is obtained (Fig. 2). The density of a given area of bone can then be expressed in terms of the stepwedge. The amount of energy transmitted by any substance exposed to diagnostic radiation is related to the atomic number of the substance as follows (16): As will be shown in Tables I and II, there are distinct advantages when the effective atomic number of the standard and of hydroxyapatite are similar.