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Post-Polio Health (ISSN 1066-5331)

Vol. 9, No. 3, Summer 1993

Read selected articles from this issue ...

Osteoporosis
Daria A. Trojan, MD, Department of Neurology, Neurological Institute & Hospital, McGill University, Montreal, Quebec, Canada

Vaccine Injury Compensation: A Battle Won

Readers Respond

Update ... On Polio Outbreak; On Scoliosis; On L-Carnitine; From National Center for Health Statistics; From Coalition

An Untold Vaccine Story

Oral Polio Vaccine (OPV)

Readers Write


Osteoporosis

Daria A. Trojan, MD, Department of Neurology, Neurological Institute & Hospital, McGill University, Montreal, Quebec, Canada

Osteoporosis refers to a decreased density (mass per unit volume) of normally mineralized bone which is below that needed to maintain the skeletal function of adequate mechanical support. Fractures are the most important complication of osteoporosis, with associated pain, deformities and loss of function.

More than 1.5 million Americans have fractures attributable to osteoporosis each year. A reduction in osteoporosis-related fractures can be achieved by a reduction in falls, and by prevention and treatment of bone loss. This essay will be primarily concerned with methods of preventing and treating bone loss as a result of osteoporosis.

Survivors of paralytic poliomyelitis may have a greater risk for osteoporosis-related fractures. Weakness from poliomyelitis may have caused a chronic reduction in or complete absence of weight-bearing activity, with a resultant reduction in bone mass. (This residual weakness from past paralytic polio may also predispose post-polio individuals to more frequent falls.) In addition, a long period of immobilization at the time of acute poliomyelitis or after a surgical procedure could have produced significant bone demineralization. Studies show that as much as 30-40% of bone mass may be lost after six months of complete immobilization. However, restoration of normal activity may result in reversal of disuse-related bone loss.

For these reasons, maneuvers to prevent a further loss of bone mass and to decrease the risk of falls may be necessary to prevent osteoporosis-related fractures in post-polio individuals. Unfortunately, few studies have assessed the effect of specific therapies on osteoporosis resulting from disuse and various disease processes.

Living bone is never metabolically at rest; it is continually undergoing a process of remodeling through new bone formation and bone resorption (to "dissolve"). This is accomplished through the action of specialized bone cells called osteoblasts and osteoclasts. Osteoblasts are bone cells which produce new bone, whereas osteoclasts are bone cells which resorb or destroy bone. Usually there is a balance between the process of bone resorption and formation. Osteoporosis can occur either with a relative increase in bone resorption or a decrease in bone formation.

The structure of bone is not uniform throughout the skeleton. There are two types of bone: trabecular and cortical. Trabecular bone is concentrated in the spine at the end of long bones (such as at the hip). Because osteoporosis involves primarily a loss of trabecular bone, areas with predominantly trabecular bone are at greatest risk for osteoporosis-related fractures.

The two most common types of osteoporosis are post-menopausal osteoporosis and age-related osteoporosis. Both occur as part of the normal aging process. Bone mass normally increases until the age of 30. After a short period of stabilization, age-related bone loss begins. In women, the rate of bone loss increases to 2-3% a year following menopause. However, after 8-10 years, this rate of bone loss returns to baseline levels. Over their lifetimes, women lose about 50% of the trabecular bone and 30% of the cortical bone, and men lose about 30% and 20%, respectively. Thus, the degree of peak bone mass attained in early adulthood together with the rate of subsequent bone loss will influence the probability of developing osteoporosis later on in life.

Postmenopausal osteoporosis involves primarily an increased bone resorption, while age-related osteoporosis involves primarily decreased bone formation. Other less common causes of osteoporosis need to be considered in a patient with bone loss. These rare forms can include hereditary causes of osteoporosis, endocrinological abnormalities, diet-related osteoporosis, drug-induced osteoporosis (from drugs such as glucocorticoids, methotrexate and some anticonvulsants), disuse osteoporosis (e.g., from immobilization due to illness or neurological causes), disease-related osteoporosis, and idiopathic osteoporosis (of unknown cause).

Many factors which can influence the attainment and maintenance of peak bone mass have been identified. Race is important in the development of osteoporosis. Blacks have a greater bone mass than whites and Orientals at all ages, and thus a lower prevalence of osteoporosis. Osteoporosis occurs less frequently in persons from southern Europe than in those of northern European descent.

Continued ...

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