Assessing fracture risk in osteoporosis

Interventions for osteoporosis depend on fracture risk and the cost-effectiveness of treatment. By Dr Alun Cooper.

The WHO defines osteoporosis as: 'A progressive systemic, skeletal disease characterised by low bone mass and micro-architectural deterioration of bone tissue with a consequent increase in bone fragility and susceptibility to fracture'.1

The diagnosis of osteoporosis is based on measurement of bone mineral density (BMD), but its clinical significance is due to the resulting fractures.

Measurement of BMD by means of DXA describes four groups, but the risk of fracture varies. For example, in an 80-year-old woman with a T-score of -2.5 standard deviation (SD), the probability of a hip fracture is five times greater than that of a woman aged 50.

A normal BMD does not mean that a fracture will not occur; 96 per cent of fractures of the hip, spine, forearm and humerus occur in women without osteoporosis.2

This low sensitivity can be improved by factoring in other risk factors at least in part independent of BMD. The WHO has developed a tool (FRAX) to calculate the 10-year probability of fracture, for hip alone or for vertebral, forearm, hip and humerus combined.3

Risk factors
Age is a key risk factor. The same T-score has a different effect at different ages. The 10-year probability of hip fracture in a 50-year-old woman with a T-score of -2.5 SD is 1.7 per cent.

This rises to 11.1 per cent if the woman is aged 75.4.

Low BMI is a risk factor. Current smoking is a weak risk factor. A patient's risk of fracture is doubled if there is a history of previous fragility fracture. This is even greater for vertebral fracture in the presence of a previous spinal fracture. A history of a maternal or paternal hip fracture is a stronger risk factor than other osteoporotic fractures.

The WHO tool allows users to record that the patient is taking or has had oral steroids for more than three months at a dosage of prednisolone 5mg daily or more. An average alcohol intake of two units or less per day may be associated with a reduced risk of fracture, but three or more are associated with a dose-dependent increase in risk.

The integration of risk factors can be used for the 10-year probability of a major osteoporotic fracture; for example, for a 60-year-old woman in the UK with a BMI of 25kg/m2 who smokes and has previously had a Colles fracture, the 10-year probability of a fracture is 14 per cent. The range is 7.9-22 per cent, depending on the strength of risk factors (for example, smoking is a lesser risk factor than rheumatoid arthritis).

Measurement of BMD is indicated if it affects management decisions, as shown in the algorithm below.

Measurement of BMD

Interventions
Intervention thresholds need to be set. For the UK, thresholds have been calculated to determine the risk of hip fracture at which treatment with alendronate is cost-effective.

In women treatment is considered cost-effective from the age of 60 years and if the woman has established osteoporosis.

Treatment of a woman with a history of fragility fracture is cost-effective, whatever the BMD. The 10-year probability of a major osteoporotic fracture at which treatment is cost-effective is approximately 7.5 per cent.5 A detailed guideline for the assessment and treatment of osteoporosis and fragility fracture is expected soon.

- Dr Cooper is a GPSI in osteoporosis in Crawley, West Sussex, and a member of the FRAX guideline writing group

- This is an edited version of an article originally published in MIMS Women's Health. To subscribe visit www.hayreg.co.uk/specials

References
1. Consensus Development Conference. Diagnosis, prophylaxis and treatment of osteoporosis. Am J Med 1993; 94: 646-50.
2. Kanis JA, Johnell O, Oden A et al. Ten-year risk of osteoporotic fracture and the effect of risk factors on screening strategies. Bone 2002; 30(1): 251-8.
3. WHO Fracture Risk Assessment Tool.
4. Kanis J A, Johnell O, Oden A et al. Ten year probabilities of osteoporotic fractures according to BMD and diagnostic thresholds. Osteoporosis Int 2001; 12: 989-95.
5. Kanis J A, Burlet N, Cooper C et al; European Society for Clinical and Economic Aspects of Osteoporosis and Osteoarthritis (ESCEO). European guidance for the diagnosis and management of osteoporosis in postmenopausal women. Osteoporosis Int 2008; 19(4): 399-428.

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