10.15 : Bone Disorders

Aging and its effect on bone remodeling is the most common cause of bone disorders. In young and healthy people, bone deposition and resorption happen at an equal rate to maintain optimal bone health.

Bone deposition is also affected by the levels of sex hormones like estrogen and testosterone that promote osteoblast activity and bone matrix synthesis. When the level of these hormones decreases due to aging, it causes a reduction in bone deposition. As a result, bone resorption by osteoclasts outpaces bone deposition by osteoblasts, lessening bone strength in old age.

This age-related decrease in bone health is more severe in women. The decline in estrogen levels after menopause and comparatively less bone mass in women than in men makes women more susceptible to bone mass reduction disorders like osteoporosis. In females, loss of bone mass due to demineralization starts after age 30 and speeds up around age 45. The rate of bone mass reduction is about 8% every ten years, resulting in up to 30% cumulative bone loss in women by age 70. Meanwhile, in males, demineralization starts only after age 60, losing only 3% of bone mass every ten years.

Another effect of aging is reduced protein synthesis and associated decrease in the bone matrix, which is mainly composed of collagen fibers. As a result, there is a reduction in tensile strength and bones become more brittle and susceptible to fractures. Sometimes, a decline in the production of growth hormones with age can also contribute to the reduction in collagen fiber synthesis, making the bones brittle.