How did you develop this risk assessment model?
The model was developed from data and experience acquired from the Dubbo Osteoporosis Epidemiology Study [1-8]. The Study, which began in 1989, has followed more than 2000 men and women who were at least 60 years old at the start. In addition to having their bone mineral density measured, these participants answered extensive questionnaires about their backgrounds and lifestyles. Subsequently, falls and fractures were noted, and then correlated with all the other data.

How did you define the risk factor?
After extensive analyses and consideration of more than 50 risk factors, we found that 5 factors markedly affected fracture outcome: age, bone mineral density, body weight, a history of prior fracture after the age of 50, and any falls during the past 12 months. These risk factors were then used to develop and internally validate the prognostic model [1-2].

What does “T-score” mean?
Bone mineral density can be expressed as g/cm2 or as T-score. Someone’s T-score is their bone mineral density compared to the average bone mineral density of a young healthy person of the same gender, aged between 20 and 30 years (also called “peak bone mass”). So, a T-score of -2 means that the individual’s bone mineral density is 2 standard deviations lower than peak bone mass. Any person with a T-score lower than -2.5 is considered to have “osteoporosis” [9].

What about other risk factors that are not in the assessment model?
While other risk factors such as cigarette smoking, excessive alcohol consumption, use of high dose corticosteroids, and a family history of fracture contribute to risk, they are largely reflected in the bone mineral density measurement. Moreover, because we wanted to keep the model simple, we strived to include only practical risk factors that are readily available (without difficult measurement).

If an individual is on anti-fracture treatment, is the model applicable?
Garvan’s prognostic model was developed based on data from men and women among whom 95% were not on any anti-fracture treatment. However, men and women whose T-scores are lower than -2.5 or have a pre-existing fracture, anti-fracture treatment can reduce the risk of fracture by between 35% and 50% [10].  Therefore, for individuals on anti-fracture treatment the model’s risk estimate should be lowered by 35-50%.

Why use a 5-year and 10-year risks?
We consider that 5-year and 10-year risks are easier to manage than lifetime risk. In fact, prognostic models for breast cancer [11] and coronary heart disease [12] also provide 10-year risk. The models have been used in developing practice guidelines that are well accepted by clinicians. 

Are there other fracture risk assessment models?
Yes. A number of fracture risk assessment models have been developed, although most of them focused on hip fracture or women only [13-16]. Our model is applicable to both men and women. Recently, the World Health Organization launched the FRAXTM tool [17] which is now available online. 

Will these models give different results? 
Probably. As different prognostic models are developed from different data sources and using different methods, the risk estimate from one model is not necessarily the same as that of another. However, the difference in risk estimates from different models is not likely to be large enough to be of clinical concern.

Can risk of fracture change with time?
Definitely. Bone mineral density is known to decline with advancing age, and excessive bone loss is a risk factor of fracture [18]. Therefore, the estimate fracture risk is not fixed, but is likely to elevate with age, however for most people, this increase is expected to be modest.

What is the risk threshold that is appropriate for intervention?
That threshold is partly dependent on a person’s perception of risk of fracture and should be discussed with a doctor. In broad terms, however, we consider that a 5-year risk of >10% is high, 5-10% is moderate, and <5% is low. Based on 35-50% risk reduction from ant-fracture treatment such as bisphosphonates, the cost per fracture prevented seems reasonable at a 5-year risk of 10% or 10-year risk of 20% or greater [19-20]. This threshold is also used in cardiovascular disease prevention (National Cholesterol Education Program) and has been adopted by expert osteoporosis groups [12] and recommended by panel of osteoporosis experts [21]. Given the undertreatment and underdiagnosis of osteoporosis [22], it is hoped that this prognostic model will help to improve the uptake of treatment and reduce the burden of osteoporosis in the general population.

References
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17.    Kanis JA, Johnell O, Oden A, Johansson H, McCloskey E. FRAXtrade mark and the assessment of fracture probability in men and women from the UK. Osteoporos Int 2008 Feb 22.
18.    Nguyen TV, Center JR, Eisman JA.  Femoral neck bone loss predicts fracture risk independent of baseline bone mineral density. J Bone Miner Res 2005;20(8):1349-55.
19.    Borgström F, Johnell O, Kanis JA, Jönsson B, Rehnberg C. At what hip fracture risk is it cost-effective to treat? International intervention thresholds for the treatment of osteoporosis. Osteoporos Int 2006 Oct;17(10):1459-71.
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