Bone Mass Density

Bone mass density refers to the absolute amount of bone as measured by bone mineral density (BMD) testing. BMD generally correlates with bone strength and its ability to bear weight. The BMD is measured with a dual energy X-ray absorptiometry test (DEXA scan). By measuring BMD, it is possible to predict fracture risk in the same manner that measuring blood pressure can help predict the risk of stroke.

Reduced bone mass is the most common clinical skeletal disorder. An age-related decline in bone mass begins around age 35 and accelerates in women after menopause. Loss of bone mass is due to osteopenia or, if more severe, to osteoporosis. Bone density in any adult that is between 1 and 2.5 standard deviations below the mean is defined as osteopenia, and a value more than 2.5 standard deviations below the mean is defined as osteoporosis and is associated with skeletal fragility; this level is considered to represent the fracture threshold. These values are referred to as T-scores (number of standard deviations above or below the mean value in young adults of the same sex and race).

The purpose of BMD testing is to help predict the risk of future fracture and aid a decision as to whether prescription medicine therapy is needed to help reduce the risk of fracture. The risk of fractures of the hip, wrist, upper arm, and vertebrae increases for both men and women with decreased bone density. Lifestyle changes, calcium and vitamin D supplementation, and pharmacological treatments for low bone density decrease the risk of fractures.

Low bone mineral density appears to predict fractures in all ethnic groups. However, the differences in fracture risk across different ethnic groups cannot be [Page 283]explained on the basis of differences in bone mineral density alone. The risk of hip fractures is considerably lower in African-American women than in Caucasians. A number of factors may contribute to the relative protection against fracture, including higher peak bone mass and slower rate of bone loss after menopause. However, studies have reported that after adjusting for body size and other covariates, bone density–specific fracture risk is still lower in African-American women when compared to Caucasian women.

Asian women also appear to have a lower risk of fracture than Caucasian women. Bone mineral density is lower in Asian than in Caucasian women, but this is due to their smaller body habitus. In studies adjusted for other factors known to affect bone density, bone mass density in women under 70 kilograms was similar for African-American, Japanese, and Chinese women, but lower in Caucasians. Asian women were also found to have a lower bone density–specific fracture risk than Caucasian women.

At present, the National Osteoporosis Foundation and the U.S. Preventive Services Task Force, the issuers of the two most prominent guidelines about BMD testing, have recommended that all women over the age of 65 be tested, regardless of risk factors. The guidelines differ a little bit regarding younger women because of the lack of reliable scientific information. In addition, there are several medications and medical conditions that a doctor reviews to determine if a person is at an especially high risk for fracture.

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