The most common bone disease is osteoporosis (OSS-tee-oh-pore-OH-sis), literally, "porous bones." This is a disease in which the bones lose mass and become increasingly brittle and subject to fractures. It involves loss of both organic matrix and minerals, and it affects spongy bone in particular, since this is the most metabolically active type (fig. 7.20). The highest incidence of osteoporosis is among elderly white women, where it is closely linked to age and menopause. Osteoporosis also affects men (white males somewhat more than black), although less severely than it does women. It rarely affects black women.
Fractures are the most serious consequence of osteoporosis. They occur especially in the hip, wrist, and vertebral column and under stresses as slight as sitting down too quickly. Hip fractures usually occur at the neck of the femur, while wrist fractures occur at the distal end of the radius and ulna (Colles fracture). Among the elderly, hip fractures lead to fatal complications such as pneumonia in 12% to 20% of cases; they involve a long, costly recovery for half of those who survive. As the weight-bearing bodies of the vertebrae lose spongy bone, they become compressed like marshmallows. Consequently, many people lose height after middle age, and in some women, the spine becomes deformed into a "widow's hump," or kyphosis (fig. 7.21).
Postmenopausal white women are at greatest risk for osteoporosis because women have less bone mass than men to begin with, begin losing it earlier (starting around age 35), and lose it faster than men do. By age 70, the average white woman has lost 30% of her bone mass, and some have lost as much as 50%. Young black women develop more bone mass than whites. Although they, too, lose bone mass after menopause, the loss usually does not reach the threshold for osteoporosis and pathologic fractures. In men, bone loss begins around age 60 and seldom exceeds 25%.
Estrogen stimulates osteoblasts and bone deposition, but the ovaries stop producing estrogen after menopause. Ironically, osteoporosis also occurs among young female runners and dancers in spite of their vigorous exercise. Their percentage of body fat is so low that they stop ovulating and the ovaries secrete unusually low levels of estrogen.
Treatments for osteoporosis are aimed at slowing the net rate of bone resorption. These include estrogen replacement, drugs to enhance estrogen sensitivity, and drugs that inhibit osteoclasts. Therapies to stimulate bone deposition, such as calcitonin nasal spray and small intermittent doses of parathyroid hormone, are still under investigation. Milk and other calcium sources and moderate exercise can also slow the progress of osteoporosis, but only slightly.
As is so often true, an ounce of prevention is worth a pound of cure. The time to minimize the risk for osteoporosis is between the ages of 25 and 40, when the skeleton is building to its maximum mass. The more bone mass a person has going into middle age, the less he or she will be affected by osteoporosis later. Ample exercise and calcium intake (850-1,000 mg/day) are the best preventive measures.
Although osteoporosis has become a major public health problem because of the increasing age of the population, it is not limited to the elderly. Disuse osteoporosis can occur at any age as a result of immobilization or inadequate weight-bearing exercise. In the absence of preventive exercise, astronauts on prolonged microgravity missions have experienced disuse osteoporosis. Other risk factors include smoking, low calcium and protein intake, vitamin C deficiency, and diabetes mellitus.
Chapter 7 Bone Tissue 239
Saladin: Anatomy & I 7. Bone Tissue I Text I I © The McGraw-Hill
Physiology: The Unity of Companies, 2003 Form and Function, Third Edition
240 Part Two Support and Movement
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