Sexual Dimorphism in Skeletal Size, Density, and Strength

Abstract

Where is the wisdom we have lost in knowledge?Where is the knowledge we have lost in information? From The Rock by T. S. Eliot Techniques are not ends in themselves, they are only as good as the answers they provide. The answers are only as good as the questions, and the questions are only as good as the insights generated from observations made using the technique. Problems arise when the technique takes over, like HAL in Stanley Kubrick’s 2001: A Space Odyssey, driving inquiry because of the accessibility of its “ON” button, but misdirecting inquiry by producing flawed inferences due to the technique’s unrecognized limitations (1, 2). Although the bone densitometer has a central role in the diagnosis, prevention, and treatment of osteoporosis in clinical practice, it has taken over in clinical research (“I’m sorry Dave, … . . I can’t do that… . ”), and its printout, the bone “density” measurement, has become an end in itself, producing conceptual errors because of its technical limitations. The use of densitometry has led to several misconceptions: 1) volumetric bone mineral density (BMD) increases during growth. It does not. Growth builds a bigger, not more dense, skeleton; 2) peak volumetric BMD is higher in men than women. It is not. Bone size is greater; 3) women lose more trabecular bone from the spine than men. No so, there are qualitative, not quantitative, differences; 4) cortical bone loss is less in men than women during aging. This is not so; the amount of bone resorbed from the endocortical (inner) surface of the cortical shell is similar in men and women. The amount of bone formed on the periosteal (outer) surface during aging is greater in men so that “net” cortical bone loss is less in men; and 5) bone mass is reduced in patients with fractures. It certainly is, but the deficit is exaggerated in patients with smaller bones than controls and underestimated in patients with larger bones than controls. These erroneous notions are produced by uncritical use of densitometry. Bone consists of the mineralized matrix fashioned into a cortical shell and a trabecular network of plates within a marrow cavity. Only the mineralized mass of bone is seen by the photons so that the degree of attenuation of the photons during their transmission through the bone is a measure of the bone mineral content (BMC) of the whole bone, not all of which is mineralized tissue. The image produced during scanning is like a silhouette, just a two-dimension image of the three-dimensional structure, an “areal” projection of the bone in the coronal plane (Fig. 1A). The length and width of the scanned bone is known, but not its depth. Because the depth of the bone is not seen, a bone with greater depth will attenuate more photons and will be reported as being more dense. The bone is not more dense, it does not have a greater amount of bone mineral distributed (as thicker trabeculae, more trabeculae, a thicker cortex) within the periosteal envelope, the bone is just larger; there is more mineral, but this mineral is distributed in the larger volume of the whole bone. a, Photons are attenuated during transmission, producing an attenuation wave profile proportional to the mass of mineralized bone in the scanning path. The attenuation produced by each passage is summated to derive the BMC subtended by the projected area of the bone in the coronal plane. [Reproduced with permission from E. Seeman.]. b: A, Age-related bone loss reduces the amount of bone within the periosteal envelope so that the photon attenuation profile is reduced (more are transmitted through the porous bone); B, Age-related periosteal apposition offsets bone loss, the photons are more greatly attenuated even though the same bone loss has occurred inside the bone. [Reproduced with permission from E. Seeman.] a, Photons are attenuated during transmission, producing an attenuation wave profile proportional to the mass of mineralized bone in the scanning path. The attenuation produced by each passage is summated to derive the BMC subtended by the projected area of the bone in the coronal plane. [Reproduced with permission from E. Seeman.]. b: A, Age-related bone loss reduces the amount of bone within the periosteal envelope so that the photon attenuation profile is reduced (more are transmitted through the porous bone); B, Age-related periosteal apposition offsets bone loss, the photons are more greatly attenuated even though the same bone loss has occurred inside the bone. [Reproduced with permission from E. Seeman.] The measurement obtained is the BMC per unit projected bone area of the bone in the coronal plane, or an areal BMC (g/cm2). “Areal” is deleted, and“ content” is replaced by density, so BMC per unit projected area is called “bone mineral density” or “BMD.” The problems that follow from this poor choice of language affect the interpretation of every aspect of the pathophysiology of skeletal growth, aging, and drug therapy in which densitometry is used.