What is Osteoporosis?

Osteoporosis is a condition in which bone mass, and therefore bone strength, is decreased. This results in a greatly increased risk of fracture. Primary osteoporosis is osteoporosis which occurs due to normal, predictable changes within the body during the aging process. Secondary osteoporosis occurs as a result of some other specific disease process which produces osteoporosis as one of its symptoms.

The basics of bone formation

To understand osteoporosis, it is helpful to understand the basics of bone formation. Bone is formed on a protein base (collagen) by the deposition of minerals, particularly calcium. This laying down of bone is carded out by specialized cells called osteoblasts. The formation of new bone occurs most effectively along lines of stress/weight that are experienced by the bone.

Other cells, osteoclasts, are responsible for resorbing (taking up) bone. These cells actually digest already-formed bone.

This active resorption-formation cycle within bone occurs throughout life, so that old bone is always being replaced by new bone. When the resorption phase is accelerated, or the formation phase is slowed, less calcified bone exists. This is the state which results in the weakened bone structure present in osteoporosis.

Why osteoporosis occurs

A decrease in the rate of bone mineralization is a predictable effect of aging. For example, in infancy, the turnover rate of calcium in bone is 100%; by adulthood, this turnover rate falls to only 18% per year.

Women are particularly prone to osteoporosis because of several factors. Women have less bone mass than men to begin with, so the threshold level at which osteoporosis may cause fractures is reached more quickly. It is believed that the bone formation phase is encouraged in some way by the presence of estrogen. In women, estrogen production drops off drastically following menopause (the cessation of the menstrual period). This change in the chemical environment within the bodies of older women apparently results in a decrease in the bone formation phase. With bone resorption continuing at its normal pace, but without the normal pace of bone replacement occurring, bone mass decreases.

Because the pattern of bone formation occurs in response to weight/stresses borne by the bone, disuse osteoporosis occurs in individuals who are on bed rest for prolonged periods of time, as well as in individuals experiencing the relative weightlessness of space flight.

Other causes of osteoporosis include many diseases which alter the hormonal/chemical environment of the body, including thyroid disease, disease of the parathyroid (a gland responsible for calcium levels within the body), gastrointestinal diseases (which can alter the ability of the body to absorb calcium in the diet), diseases which decrease the amount of estrogen produced, and certain liver diseases.

Alcohol and some drugs can also affect calcium levels in the body, thus producing osteoporosis. Some of these drugs include thyroid medications, steroid preparations, anti-seizure medications, and certain chemotherapy (anti-cancer) agents.

Congenital diseases (diseases present at birth) of connective tissue (a group of tissues of the body which includes bone) can cause abnormalities of bone structure, and therefore osteoporosis. Such diseases include osteogenesis imperfecta (brittle bone disease) and Marfan's syndrome.

Symptoms of osteoporosis

Symptoms of osteoporosis occur primarily due to the results of bone fractures. The most common locations for such fractures are those bones that normally have the highest turnover rate of resorption-formation. The wrist is one such location, and a characteristic fracture of the wrist due to osteoporosis is known as a Colle's fracture.

The vertebrae normally also have a high bone turnover rate, and osteoporosis frequently manifests itself by compression fractures of the vertebrae. These fractures can occur after seemingly normal activity, including sneezing or bending/twisting to pick up a relatively light object. This can be asymptomatic for the patient, or can result in back pain. Either way, the patient's vertebrae are compressed down on themselves, and the patient actually loses height. The hunchback appearance of many elderly women (sometimes referred to as dowager's or widow's hump) is due to this effect of osteoporosis on the vertebrae.

The hip (specifically the bone called the femur) is another extremely common location for an osteoporotic fracture. In fact, while it was initially thought that an individual falling resulted in a broken femur, it now believed that some femur fractures occur somewhat spontaneously, and the already broken hip then causes the individual to fall.

Diagnosis

Ideally, diagnosis of osteoporosis should be made prior to the occurrence of symptom-causing fractures. Various radiologic techniques are available to measure the density (solidity) of bone, and include x ray and CT (computed tomography) examinations of the spine, femur, and wrist bones.

In the case of osteoporosis that is not due to normal aging, but is secondary to another disease process, other laboratory examination may be necessary. Calcium blood level, thyroid, liver, and parathyroid function may need to be evaluated. Other diseases that cause secondary osteoporosis (such as gastrointestinal disease) are usually evident due to other symptomatology.

Treatment

Treatment of secondary osteoporosis varies depending on the actual disease process which has produced the osteoporosis, and may include adjustments to thyroid medication, dietary supplementation with calcium or vitamin D (which is involved in the ability of the intestine to absorb calcium in the diet), or other treatment of the primary disease.

Treatment of primary osteoporosis in the elderly involves adequate intake of calcium and vitamin D, as well as regular exercise. Recommendations for calcium supplementation suggest taking 1500 mg per day alone, or 1000 mg per day in conjunction with estrogen replacement therapy. Exercise is helpful both to strengthen muscle and to increase weight-bearing activity (remember that bone formation occurs most effectively along lines of stress and weight-bearing).

A current area of interest in the study of osteoporosis prevention is the role of estrogen replacment therapy. Bone loss can be decreased in elderly women by estrogen replacement therapy, ideally beginning during the first years of menopause. Such estrogen replacement therapy, however, has been called into question for a number of reasons. Estrogen given alone, for example, has been shown to increase the rate of endometrial cancer. For this reason, most hormone replacement regiments couple estrogen with progesterone, which reduces the risk of endometrial cancer. Unfortunately, some studies have also pointed to estrogen (with or without progesterone) as potentially causing an increased risk of breast cancer development. Several important studies are underway to investigate this association.

In the past, the majority of treatment for osteoporosis occurred in the form of treatment of the fractures resulting from the disease. Now, however, some exciting new therapies exist. Calcitonin is normally produced by the thyroid gland, and works to lower blood calcium levels and prevent bone resporption. A calcitonin product is available for treatment of osteoporosis. It is given either as a nose spray, or as an injection. Its effects simulate those of naturally produced calcitonin, resulting in slower progression of bone loss. Alendronate is a biophosphonate which is taken orally, and which slows bone breakdown. Raloxifene is a type of Selective Estrogen Receptor Modulator. This class of drugs have estrogen-like effects on the body, including on the bone and heart. They are believed, however, to have less pronounced effects on uterine lining (endometrial cancer development) and breast tissue (breast cancer development).

Treatment of actual symptoms of osteoporosis include pain medications and heat for vertebral compressions, simple casts for uncomplicated fractures, or hip replacement surgery for more complicated hip fractures.

The importance of osteoporosis in terms of the misery it causes and its economic impact is underscored by these statistics. About one-third of all women over the age of 70 experience hip fracture. Of those elderly people who fracture a hip, about 15% die of complications secondary to that hip fracture. A large percentage of those who survive are unable to return to their previous level of activity, and many times a hip fracture precipitates a move from self-care to a supervised living situation or nursing home. The yearly cost of osteoporotic injury in the United States is greater than $10 billion.

See also Skeletal system.

Further Reading

Andreoli, Thomas E., et al. Cecil Essentials of Medicine. Philadelphia: W. B. Saunders Company, 1993.

Berkow, Robert, and Andrew J. Fletcher. The Merck Manual of Diagnosis and Therapy. Rahway, NJ: Merck Research Laboratories, 1992.

Ganong, William F. Review of Medical Physiology. Norwalk, CT: Appleton & Lange, 1993.

Rosalyn Carson-DeWitt

Ostrich see Ratites

KEY TERMS

Estrogen--A hormone present in both males and females. It is present in greatly larger quantities in females, however, and is responsible for many of those physical characteristics which appear during female sexual maturation.

Fracture--A break in a bone.

Menopause--The time in a woman's life when the chemical environment of her body changes, resulting in decreased estrogen production (among other things) and the cessation of her menstrual period.

Osteoblasts--Those cells which are responsible for the building of new bone.

Osteoclasts--Those cells which are responsible for the taking up/digestion of old bone.

Record Number: A30434444

SOURCE:
Gale Encyclopedia of Science.  June 2001 pNA.

Full Text: COPYRIGHT 2001 Gale Group