The prevention and management of osteoporosis
Consensus statement		 Contents list

3. What is the relationship between bone density and fracture risk; are there any other useful predictors of fracture risk?

Bone density and fracture risk

There is a continuous inverse relationship between bone density and the risk of fracture, comparable to that between serum cholesterol and the risk of coronary heart disease and that between blood pressure and the risk of stroke (see Box 2).

2: Relationship between relative risk and deviation from the young normal mean for stroke and blood pressure, fracture and bone mineral density and coronary heart disease and cholesterol (adapted by Need from Marshall et al. 1996).

 

Since bone density measurements can be made at various sites with different machines and at different ages, the values are expressed more usefully in relation to population mean values. Within any age group, a one standard deviation (SD) fall in bone density multiplies the relative risk of fracture by 1.5 to 2.5 (for an explanation of this terminology see Question 7).

 

Measuring bone density

Bone densitometry measures the average density of bone mineral within the region scanned. While it may not reflect the microarchitectural changes occurring during bone loss, it is currently the best available measure of bone strength.

Single and dual energy absorptiometry and quantitative computerised tomography are techniques for measuring bone mineral density. The four sites of measurement commonly used in Australia (the forearm, spine, proximal femur and total body) all have merit, but some are more suitable for diagnostic purposes and some for longitudinal studies (see Questions 6 and 9).

 

Other tests for prediction of fracture risk

A World Health Organization Study Group in 1994 concluded that tests to predict fracture risk would be improved by the ability to assess bone mass accurately in conjunction with more specific biochemical measures of bone turnover. At present such measures remain research tools, but recent evidence indicates that they provide an independent predictor of fracture risk. Ultrasound is promising but currently remains a research tool also.

 

Other useful predictors of fracture risk

In addition to the fragility of bone, fracture risk is determined by the interaction of several other factors, including the risk of falls and other trauma, the adequacy of protective responses and the adequacy of soft tissue to absorb impact. Box 3 shows some of these factors.

3: Factors that influence the risk of fracture

In making decisions about investigation and treatment, the individual's full risk profile should be taken into account. The most clinically useful of these predictors include:
  • Age, sex and family history
    With ageing there is a decrease in bone mineral density, muscle strength, balance and other factors that increase the risk of fracture. In older individuals, fracture risk doubles every five to ten years. Among women who have a history of fracture in close family members (maternal or paternal), the risk of osteoporotic fracture is almost doubled.

  • Prior fracture
    The occurrence of one fracture increases the risk of further fractures. This is partly because those whose bones fracture have low bone mineral density, but it also indicates an increased risk beyond this of about two or three-fold.

  • Falls
    Even in a simple fall, enough energy may be generated to fracture the femur. In young people, protective responses, the strength of the bone and the quantity of soft tissue generally prevent fractures from occurring. In elderly people, other factors, such as increased body sway and reduced muscle strength, Parkinson's disease, poor vision, physical inactivity and psychotropic medications, may have an important influence on the risk of falling.

  • Low body weight
    Low body weight is associated with low bone mineral density and decreased soft tissue protection, both of which are associated with increased fracture risk.

  • Other risk factors
    There is reasonable evidence of several other risk factors for fracture: very low calcium intake, smoking, menstrual irregularity/absence and some medications, particularly corticosteroids.

Next: What diagnostic evaluations are needed in patients with presumed osteoporosis?

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©1997 Medical Journal of Australia.