BIOLOGY Volume 2 - A Guide to General Biology - 2004

15. HEALTH AND DISEASE

15.7. Aging

15.7.2. Changes in the Musculoskeletal System

The human musculoskeletal, or locomotor, system comprises the Skeleton (bones, joints, and ligaments), Muscles, and tendons. Some age-related changes in The Musculoskeletal System can be attributed to alterations in Collagen fibers, one of the primary components of both bone-Muscle and Connective Tissue. Tendons consist almost entirely of collagen. Muscles contain less collagen, but with advancing age, muscle fibers are increasingly replaced by collagen (which is why meat from older animals is so tough). As the body ages, the collagen fibers themselves change Structure—becoming thicker and less elastic due to an increase in the number of cross-links between protein molecules. As a result, the Tissues of which they are a part, such as bones, become increasingly brittle.

Bone Changes

With age, bones become thinner, less dense, and less elastic. This is partly because, after roughly the age of 35, the resorption (removal) of calcium from bones outpaces its deposition in Bone tissue. While this affects everyone, in some individuals it is particularly pronounced and leads to a condition known as Osteoporosis.

Osteoporosis affects the entire skeleton, but is especially prevalent in the hip bones, forearm, and vertebrae. Even minor impacts (such as a fall on the street) can result in fractures. Furthermore, in the case of the vertebrae, compression fractures can occur spontaneously without any external impact, caused simply by the load of the body's own weight. These types of damage, along with the flattening of intervertebral Cartilage discs due to loss of elasticity, cause elderly individuals to "shrink" in height and their posture to deteriorate (resulting in a hunched back). Osteoporosis is particularly common in older women: after the age of 60, one in four women suffers from it, whereas in men it is four times less frequent. This is because, prior to menopause (the age-related cessation of menstruation), bone strength is maintained by estrogens, levels of which subsequently decline in the body. Estrogens act as antagonists to the parathyroid hormone, or parathormone, which stimulates an increase in Blood calcium concentration. This occurs through the "leaching" of calcium from the bones; consequently, a deficiency in Female Sex Hormones leads to decreased bone strength, thereby increasing the likelihood of fractures in elderly women (Fig. 15.24).

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Fig. 15.24. Scanning electron micrograph clearly showing the fragile and spongy structure of a fractured Femur in osteoporosis. Dark spots indicate areas of mineral resorption ("leaching"), which has led to a reduction in bone strength.

For most postmenopausal women, hormone replacement therapy is recommended to prevent osteoporosis. They are prescribed estrogens either orally or in the form of subcutaneous implants. This reduces the risk of osteoporosis to levels typical of men. It should be noted, however, that estrogen use increases the risk of intravascular blood clotting, though this can be mitigated by concurrent progesterone administration. A balanced, calcium-rich diet and regular Physical Exercise also help maintain bone strength. The risk of osteoporosis is increased by smoking and alcohol consumption.

This condition poses a major challenge for healthcare systems. Estimates suggest that in the UK alone, £640 million is spent annually on treating individuals with osteoporosis-related fractures. Only a quarter of patients with hip fractures (the most common type associated with osteoporosis) make a full recovery, and many remain permanently bedridden.

Changes in Joints and Osteoarthritis

Age-related changes are most pronounced in weight-bearing joints (particularly the hip and knee), though all other joints also typically become stiff and painful.

As we age, the cartilage covering the bone surfaces within joints gradually wears away. In advanced cases, it disappears entirely, causing the bones in the joints to grind directly against each other. This causes wear and tear, while simultaneously prompting the bones to react by thickening and growing at the surfaces subjected to extra load. Consequently, joint movement becomes restricted and painful. This condition was formerly called osteoarthritis, but recently the term osteoarthritis (Fig. 15.25) has been preferred since the suffix "-itis" traditionally denotes inflammation, which is absent in this case.

Another degenerative joint disease affecting the elderly is rheumatoid Arthritis. It is autoimmune in nature, meaning the body's immune system attacks its own tissues. During flare-ups, affected joints become hot and swollen—a typical inflammatory response.

Fig. 15.25. A severe case of osteoarthritis in a man's left knee. The image clearly shows Swelling and deformation of the affected knee joint.

Muscle Changes

With age, muscle fibers are gradually replaced by connective tissue rich in collagen fibers. As a result, muscles weaken and lean body mass decreases. Because the myocardium is also Muscle tissue, The Heart weakens as well.