9.4: Age Changes in Bones

The matrix and the cartilage in bones seem to undergo most age changes; the functional capacity of bone cells and bone marrow seems to remain largely unchanged regardless of age. Some bone cells may function slower with age, though this appears to be due to changes in the control signals they receive. If the bone cells are stimulated, as when a fracture occurs, they resume rapid functioning. Exceptions are osteoblasts in the endosteum, which covers the inner surfaces of bones (Figure 9.4) These osteoblasts have an age-related decrease in sensitivity to stimulation by vitamin D. Sensitivity may decline by 60 percent by age 50. This change contributes to age-related thinning of bones.

Bone Matrix

Age changes in bone matrix are complex and are far from being understood. This is partly due to the variety of factors that influence bone matrix. The factors include genes; amount of exercise; nutrition; levels of hormones; amount of exposure of skin to sunlight; levels of chemicals in the blood; and the functioning of skin, intestines, and kidneys.

Proteins and Minerals

With aging, the balance between the amount of protein and the amount of minerals in bone matrix shifts in favor of the minerals. Therefore, bones become more rigid, brittle, and likely to fracture.

Quantity

The quantity of bone matrix decreases with aging because matrix formation becomes slower than matrix removal. This decline may begin in some individuals at age 20, and by age 30 most people are losing bone matrix. It seems that by age 35 everyone has begun to lose bone at a substantial rate (Figure 9.12).

Figure 9.12 Age changes in bone mass. (Copyright 2020: Augustine G. DiGiovanna, Ph.D., Salisbury University, Maryland. Used with permission.)

Structure

Figure 9.6 Age changes in bone tissue (a) Trabecular bone and (b) Cortical bone. (Copyright 2020: Augustine G. DiGiovanna, Ph.D., Salisbury University, Maryland. Used with permission.)

At first, only trabecular bone is removed. In this type of bone, all the trabeculae become thinner and weaker (Figure 9.6a). Trabeculae can become thicker and stronger again if osteoblasts are stimulated to replace the missing matrix. This often occurs when a person who has been sedentary begins to exercise. However during aging, some trabeculae disappear completely and cannot be replaced. The weakening of the bone at that spot is permanent. Also, as the matrix joining trabeculae dissolves, some trabeculae become disconnected from the others and can no longer contribute significantly to bone strength.

The decline in cortical bone is not detected until about age 40, and the loss is quite slow until age 45. The rate of loss then begins to increase significantly, though it remains approximately half the rate of trabecular bone loss. The loss of cortical bone takes place on the inside of the bone only, and the process proceeds outward toward the surface. Therefore, the layer of cortical bone becomes thinner while the overall width and length of the bone remain the same.

During the removal and replacement of cortical bone matrix, old mature osteons are gradually dissolved and shrink while new osteons are formed next to them (Figure 9.6b) Small portions of the old osteons often remain behind. As years pass, the number of osteon remnants and the number of new osteons increase. As a result, the number of points of fusion among the osteons increases, causing the matrix to become weaker.

At first the new osteons that form fill all the space left by the old ones. However, as a person gets older, the new osteons fail to fill these spaces completely and the number of gaps between the osteons increases. This change in structure also weakens the matrix.

Effects of Menopause

Most experts agree that the rates of loss of trabecular and cortical bone matrix in women are increased by menopause, the time when menstrual cycles cease. Menopause usually takes place between ages 45 and 55. As it occurs, the production of the hormone estrogen by the ovaries is greatly reduced. Actually, since estrogen production probably drops gradually in the years just before menopause, the effects of declining estrogen begin before menopause.

The combined effects of aging and menopause result in a loss of 15 percent to 20 percent of trabecular bone in the 10 years after menopause. This is two times to three times faster than the rate of loss in women before menopause or the rate of loss in men. As a result, very old women may have only half the amount of trabecular bone they had at age 25. Men have lost only two-thirds as much trabecular bone during the same period.

Cortical bone loss also accelerates because of menopause, with a loss of 10 percent to 15 percent of the cortical bone in the decade after menopause. This is a threefold to fourfold increase over the rate of loss before menopause. The rate of loss eventually slows down so that by age 70 it has dropped to the same rate found in men of that age. As with trabecular bone, a very elderly woman probably has less than half the amount of cortical bone she had during her twenties. Again, bone loss in a man of the same age is one-third less.

Therefore, because of menopause, a very elderly woman can expect to have considerably less bone material than does a man of the same age. This difference in the amount of bone material is usually made greater because men generally have more bone matrix than do women when bone loss begins.

Variability in Loss

Trabecular bone loss begins earlier and occurs faster than does cortical bone loss. Since some parts of the skeleton have a higher proportion of trabecular bone, different regions have different rates of decreases in matrix. We will examine two important examples.

First, vertebrae are composed mostly of trabecular bone. Therefore, they begin to lose bone sooner and lose more bone than do bones in the arms and legs, which contain mostly cortical bone. Because of this, there is a higher incidence of vertebral fractures among the elderly, especially elderly women.

The second example involves the bone in the thigh called the femur (Figure 9.1). The upper part of the femur, which joins with the pelvis at the hip, contains a high percentage of trabecular bone, while the long central shaft is made up almost entirely of cortical bone. Therefore, the upper end loses bone earlier and faster than does the shaft. This contributes to the higher rate of hip fractures as age increases.

Consequences

Although age-related alterations in the composition, quantity, and structure of bone matrix result in weakening of the bones, the degree of weakening normally is not great enough to reduce substantially the reserve capacity of the bone matrix. Unless a very heavy load or strong force is applied, the bones are able to provide support and protection for the body as long as a person lives. Of course, large forces from accidents and severe falls cause fractures more frequently as a person ages. Since women end up with less bone matrix than men do, they are at higher risk for fractures.

Fractures are painful, hinder or prevent normal activities, and can lead to serious complications such as infection. Treatment of fractures is often quite expensive. Though elderly individuals who develop a fracture face the same problems, the adverse effects multiply with increasing age because healing of a fracture proceeds slower as one gets older. Slower healing can mean prolonged immobility, which increases the risk of complications, such as bedsores, blood clots, and pneumonia. Prolonged immobility also leads to faster loss of matrix, which in turn increases the risk of developing another fracture.

To assure that a normal skeleton serves a person well, it is important to compensate for the weakened condition of the aging skeleton. One way to do this is to avoid abusing the skeleton. Since falling is among the most common causes of skeletal abuse resulting in fractures among older people, reducing falls is of prime importance. A second way to assist the skeleton is to reduce the loss of bone matrix.

Minimizing Loss of Matrix

Though the cause of bone matrix loss with aging is not known, much has been discovered about factors that modify the rate of loss. Many of these factors are easily regulated; therefore, much can be done to reduce the loss of bone matrix. In so doing, individuals can make significant contributions to the strength of the skeleton and its ability to serve them.

The condition of a person's bone matrix depends on how well it is treated throughout life. Much more benefit can be derived from taking steps to assist in building and maintaining bone matrix through early and late adulthood rather than just in old age, after a considerable amount of matrix has been lost. While the loss of bone matrix can be slowed at any age, little of the bone matrix and bone strength that are lost early in adulthood can be replaced later in life.

Numerous steps can be taken to build a large reserve of bone matrix before age 35 and minimize decreases in bone matrix at any age (Table 9.1). Each of the steps in the table contributes to one or more of the following: (1) maintaining high calcium levels, (2) stimulating matrix production, and (3) inhibiting matrix removal. Of course, elderly individuals and persons with known diseases should seek qualified professional advice before changing their normal activities.