OSTEOPOROSIS is as universal an alteration of aging bone as is osteoarthritis of aging joints. Together these comprise the main symptom-producing manifestations of skeletal senility. Although osteoporosis appears to be physiological in aging bone, its degree is very variable from person to person. PATHOLOGICAL PHYSIOLOGY Pathologically, osteoporotic bone is characterized by fewer and less thick trabeculae. Functionally such bone is more susceptible to compression and fracture, but until fracture occurs there is preservation of bone contour. So far as is now known, there is no biochemical difference between the thin trabeculae and those of normal presenile bone; the protein matrix is the same, as is the bone salt, which is deposited in the matrix in an apatite crystal lattice. The ratio of minerals to matrix is probably the same in osteoporotic as in normal bone. The skeleton is normally in a state of dynamic equilibrium, with bone resorption and bone