Factors Contributing to Bone Loss


Researchers and clinicians who study osteoporosis have known for some time that weight-bearing exercise contributes to the development and maintenance of bone mass (e.g., Dalsky et al., 1988; Krall, 1994; Nelson et al., 1994). Conversely, studies as far back as 1892 by Wolff have shown that bone is negatively influenced by reduction of its load-carrying role. In fact, without gravitational or mechanical loading of the skeleton, there is a rapid and marked loss of bone.

Wolff’s theory that bones become stronger in response to increased exercise is still accepted today (Drinkwater, 1994). Living bones adapt themselves, both in size and internal structure, to the mechanical forces applied to them, and the amount and strength of the bone are directly linked to the amount of activity that forces the bones to bear weight and move against resistance (Simkin, 1990).

Weight-bearing activity can be thought of as any activity that is done while upright, requiring the bones to fully support the body’s weight against gravity (Bonnick, 1994). Impact-loading, weight-bearing activity, therefore, involves some impact or force being transmitted to the skeleton during weight bearing. Examples of impact-loading, weight-bearing exercise include: walking, jogging, stair climbing, dancing, weight training and cross-country skiing. Activities that involve less impact and less weight-bearing force include swimming and bicycling.

While weight bearing and impact loading stimulate the development of healthy bones, it must be remembered that for exercise to be effective, the mechanical stress placed on the bone by an activity must exceed the level to which the bone has adapted (i.e., short periods of intense loading can produce more new bone than long-term routine loading) (Frost, 1990). However, long-term routine loading is important in maintaining bone density. And although bone responds to mechanical loading, it is easier to lose bone through inactivity than to gain more through changes in functional loading. When weight-bearing exercise is not continued, bone mass reverts to pre-training levels (Dalsky, 1988; Drinkwater, 1995).

People who cannot perform weight-bearing exercise may be especially at risk for bone loss. Prolonged bed rest (following fractures, surgery, spinal cord injuries, illness, stroke, or complications of pregnancy) or immobilization of some part of the body often result in significant bone loss. Exposure to reduced gravity during space travel has also been found to have a direct negative effect on bone. In fact, space travel has provided significant research data on the subject of weightlessness, immobility, and bone loss.

People who must stay in bed or immobile are not weightless, but their bones bear much less weight than when they are vertical. To understand this phenomenon, one must first have a basic understanding of bone metabolism.

Bone Metabolism and Disuse

Bone is a dynamic structure that is continuously remodeling itself through a closely balanced process of resorption and formation. During resorption, old bone tissue is broken down and removed by special cells called osteoclasts. Then bone formation begins and new bone tissue is laid down–by cells called osteoblasts–to replace the old.

There appears to be an acute increase in both bone resorption and bone formation during periods of bed rest and immobilization, although there is a higher relative increase in bone resorption, which leads to a net loss of bone mineral in the weight-bearing bones. Over several months the rates of bone resorption and bone formation gradually decrease, and the bones reach a new equilibrium, or “steady state,” in response to the reduced load (Sinaki, 1995).

The precise mechanisms that cause the change in bone metabolism are being studied, although it is possible that the absence of weight-bearing alters bone cell function (Mundy, 1995). Other researchers speculate that bed rest triggers an increased recruitment of osteoclasts that continues until the end of the bed-rest period (Uebelhart, 1995).

When body weight is removed from the bones, the parts of the skeleton most affected are the lower extremities; those least affected are the upper extremities and the skull. This is because the higher a certain bone is positioned in the skeleton, the less body mass that bone must carry. Hence, the lower extremities and the spine are classified as weight-bearing bones, and the upper extremities as non-weight-bearing bones.

During bed rest, the body mass that usually presses on the bones in a top-to-bottom direction is loading the bones in a lateral direction, distributed over a larger area. This makes the bones experience considerably lower stress, resulting in a change of bone metabolism. Immobilization through casts and similar devices is usually accompanied by a reduction in loading of the bone, as well as a decrease in the force applied to bones by muscles. In space, there is a total removal of body weight from the skeleton due to micro-gravity (Hangartner, 1995).

Trabecular and Cortical Bone

There are two types of bone in the body: cortical and trabecular. Cortical bone is dense and compact, and comprises 85 percent of the bone in the body. Trabecular bone has a spongy, honeycomb-like structure, and makes up the remaining 15 percent. The rate of remodeling is much faster in trabecular bone (e.g., the spine) than in cortical bone (e.g., the long bones and the hip) because remodeling takes place on the surface of bones, and trabecular bone tends to have greater surface area (Mundy, 1995).

Bone Loss Magnitude

The pattern of calcium imbalance and bone loss due to disuse is similar in prolonged bed rest, immobilization, spinal cord injury, and space travel. Urinary calcium increases within days of the onset of disuse, and the body’s calcium balance may become negative, reaching a peak at about five weeks (Hangartner, 1995). However, there are differences in magnitude. In bed rest, the average urinary calcium loss at the peak is about -150 mg per day, which corresponds to 0.5 percent of total body calcium (Deitrick, 1948; Donaldson, 1970; Hangartner, 1995). Losses in bone density are greatest in weight-bearing bones with a large proportion of trabecular bone, such as the heel bone. The amount of bone loss in the spine is smaller and occurs later; in some studies, no significant bone loss was detectable in the spine (Hangartner, 1995; LeBlanc, 1987).

Studies of patients whose limbs were immobilized have shown that, if a weight-bearing bone is involved, immobilization leads to bone loss in that limb. The bone loss is more significant in trabecular bone than in cortical bone (Janes, 1993). Fortunately, these studies also suggest that there is a good chance to fully recover the lost bone if the immobilization period is limited to 5 to 10 weeks (Hangartner, 1995).

Spinal cord patients have the longest experience with disuse osteoporosis. In these patients, there is an immediate increase in urinary calcium, leading to a negative calcium balance of about -100 mg per day. The calcium balance usually reverts back to normal within 6-18 months, but by that time about one-third of cortical and one-half of trabecular bone may have been lost (Chantraine, 1979; Hangartner, 1994; Minaire, 1974).

Studies of bone atrophy during space travel indicate that urinary calcium levels increase immediately and the negative calcium balance peaks at about -200 mg per day (NASA, 1990). The calcium balance remained negative in flights up to 84 days. The most significant bone loss occurs in weight-bearing parts of the skeleton (NASA, 1990).

Minimizing Bone Loss Caused by Disuse

In general, healthy people who undergo periods of bed rest or immobilization can regain bone density through the resumption of weight-bearing activities. It is not yet known whether bone lost in space travel is fully recovered upon return. The greatest concern is for patients who can never resume weight-bearing activities, because they typically do not regain lost bone density.

Numerous researchers have tested methods to minimize bone loss during the period of disuse. Methods studied include dietary changes, pharmaceutical agents, weight-bearing and strength training exercises when possible, and functional electrical stimulation (FES) of muscles.

Dietary changes, such as increased intake of calcium and/or vitamin D, have not proven effective at minimizing disuse bone loss (Sinaki, 1995). Research into the pharmacologic treatment of disuse osteoporosis has shown that several of the bisphosphonates may prove helpful in minimizing bone loss during periods of weightlessness or immobility.

There is some uncertainty as to whether physical activity can minimize bone loss during periods of disuse. Some early studies indicated that the stress on bones from any muscular activity (even in a supine position) can be beneficial (Wyse & Pattee, 1954; Abramson & Delagi, 1961). However, more recent research suggests that weight-bearing activity–through tilt-table exercises or periods of standing–is necessary to minimize disuse bone loss (Kaplan, 1981). Studies of physical countermeasures in space travel tend to support this latter conclusion (NASA, 1990).

Several studies have tried using FES with spinal cord-injured patients. Although some of these studies showed no positive effects, others showed that the rate of bone loss was less than expected. This illustrates the importance of assessing bone density data relative to expected losses rather than as absolute values. Even if an intervention does not fully halt or reverse bone loss, slowing down the loss may be a very positive result (Hangartner, 1995).

The search continues for new ways to minimize the bone loss that results during periods of disuse. As more accurate and sensitive techniques are developed to assess bone and connective tissue metabolism, more information will be available regarding bone loss in paralyzed and/or immobilized individuals. These techniques will definitely be helpful in orienting new therapeutic trials with drugs and/or procedures intended to correct the loss of bone density resulting from bed rest, immobilization, or weightlessness.

The Bottom Line

  • A lifetime of weight-bearing exercise is important for everyone, to build and maintain bone mass, improve balance and coordination, and promote overall good health.
  • Weight-bearing exercise should be resumed and maintained after a prolonged period of bed rest or immobilization to reverse bone loss during disuse.
  • Those who cannot resume weight-bearing exercise are at significant risk for osteoporosis. Researchers are investigating alternative ways to protect bone mass among this population. Until scientific studies yield definitive results, the best advice is to reduce or eliminate other risk factors for osteoporosis.

References

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