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Osteoporosis Information

by Michele F. Bellantoni, M.D

Is it a Disease?

Current Definition

While the loss of bone mass is an expected part of aging, it has consequences for successful aging. The medical literature defines osteoporosis as a disease characterized by abnormalities in the amount and architectural arrangement of bone tissue that leads to impaired skeletal strength and an undue susceptibility to fractures(ref 1). The World Health Organization has proposed a clinical definition of osteoporosis based on epidemiological data that link low bone mass with increased fracture risk. In study populations of Caucasian postmenopausal women, a bone mineral density that was lower than 2.5 standard deviations (SD) of normal peak bone mass was associated with a fracture prevalence of 50&037;, meaning that 50% of women with bone mass at this level had at least one bone fracture(ref 2). Based on these data, the WHO defined osteoporosis as bone mineral density 2.5 or more SD below peak bone mass, osteopenia as bone mass between 1.0 and 2.5 SD below peak, and normal as 1.0 SD below normal peak bone mass or higher. However, the WHO criteria apply only to Caucasian, postmenopausal women, and not men, premenopausal women, or women of ethnicity other than Caucasian. We have yet to classify clinically significant low bone mass in this populations.

How common is osteoporosis?

Using the WHO criteria, 30% of Caucasian postmenopausal women in the US have osteoporosis, and 54% have osteopenia. The prevalence of low bone mass increases with age. Using the WHO definition of osteoporosis, the prevalence in the US of osteoporosis in Caucasian postmenopausal women based on the lowest bone mass at any site is estimated to be 14% of women aged 50-59 years, 22% of women aged 60-69 years, 39% women aged 70-79 years, and 70% women aged 80 years or greater(ref 3).

Factors Influencing Bone Mass

Peak bone mass occurs for both men and women by the early thirties. Genetic factors play the greatest role in determining peak bone mass, but there are clinically significant contributions from nutrition, drug exposures, endocrine health following puberty, and weight-bearing status (ref 4). For example, most teenagers and young adults do not receive the Recommended Daily Allowance (RDA) for calcium of 1200 mg. Smoking and excessive alcohol use contribute to low bone mass. Systemic glucocorticoid use of 7.5 mg daily or greater impairs bone formation. Phenytoin and other anti-seizure medications impair vitamin D metabolism. Oligomenorrhea and amenorrhea cause accelerated bone loss, as do hyperthyroidism or over-replacement of thyroxine supplementation such that the serum TSH is suppressed. Immobility is associated with thinning of the bone from lack of weight-bearing forces.

The menopausal transition is associated with bone loss that can exceed 4% per year and extend for 10 years or more(ref 4). There is individual variation as to the rate and duration of bone loss. It appears that body fat, a non-ovarian source of circulating estrogens, influences the rate of bone loss; with higher amounts of body fat protecting against menopausal bone loss. Studies of African American women have shown that although on average they have higher peak bone mass than Caucasian women, they experience comparable rates of menopausal bone loss that are clinically significant for lean African American women.

Bone loss in women continues into older age, as the Study of Osteoporotic Fractures showed clinically significant bone loss occurring in women 65 years of age and older (ref 5). Factors contributing to this bone loss include inadequate intake of calcium and vitamin D, lack of weight-bearing exercise, and possibly age-related changes in endocrine functions beyond those of estrogen deficiency.

On average, men achieve higher peak bone mass than women, and they do not experience as dramatic a change in reproductive function with aging as do menopausal women. However, levels of circulating testosterone as well as those of growth hormone and adrenal androgens decrease with normal healthy aging. These age-related endocrine changes combined with nutritional and lifestyle changes result in a gradual loss of bone mass with normal aging in men. Accelerated bone loss occurs with abrupt loss of testosterone production such as that experienced during the treatment of prostate cancer.

Does Bone Mass Predict Fracture Rate?

For every one standard deviation below peak bone mass the risk of vertebral fracture is two times that of normal bone mass, and for the hip, the risk is 2.5 times.(ref 6) Low bone mass is a modifiable risk factor for fracture analogous to hypercholesterolemia or hypertension for myocardial infarction and stroke.

The clinical consequence of low bone mass is fracture. Pain and immobility result from fractures of the limbs and spine. Multiple vertebral fractures result in irreversible spinal deformity and chronic pain syndromes. However, hip fractures result in institutionalization and excess mortality. One year mortality according to age at hip fracture are estimated to be roughly 20% in individuals less than age 70 years; 30% for ages 70-79.9 years, and almost 40% ages 80-89.9.(ref 7) In summary, bone loss is a natural consequence of aging that if untreated, results in loss of independence and quality of life. Yet, preventive and treatment strategies have been developed (see below), thus making bone health an appropriate part of preventive health care in the primary care setting.

Do Previous Fractures Predict Future Fractures?

One vertebral fracture is associated with a 5-fold increase in risk for subsequent vertebral fractures and a 2-fold increase in hip fracture. Two or more vertebral fractures increase the risk of subsequent vertebral fracture by 12-fold.(ref 8) In our experience, almost half of frail elders admitted to an inpatient rehabilitation unit following a fracture had experienced a previous fracture.(ref 9) The majority of the earlier fractures were of minimal or short-term impact on functional state; whereas the more recent fracture greatly impacted physical function. Yet, none of the earlier fractures resulted in an effective treatment program that may have prevented the more recent fracture.

Do Falls Increase Risk for Fracture?

The great majority of all fractures in older women result from falls.(ref 10) Fall risk factors include leg weakness, impaired gait, and balance dysfunction. These can occur from global physical deconditioning as well as specific syndromes such as stroke, osteoarthritis of selective joints, and medical conditions such as polypharmacy, use of psychoactive medications, and orthostasis.(ref 11)

The occurrence of fracture during a fall is determined by the intensity of the trauma and by bone strength. Risk factors identified for injurious falls include fear of falling, reduced knee extension strength, and poor distance visual acuity.(ref 12) A simple test that predicts risk of injurious falls is the one-leg balance test – the ability to stand unassisted for 5 seconds on one leg.(ref 13) Inability to perform this task increased relative risk of injurious falls by 2.1 (confidence intervals 1.04-4.3).

Resources of Patient Education

References

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