The worldwide burden of osteoporosis

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

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. WHO quantifies the burden of disease by the disability incurred from it, including death. Using the concept of disability and life-years lost (DALYs), diseases can be compared. For example, the burden of COPD (about 3.5 million DALYs) and Alzheimer's disease (about 2.9 million DALYs) is greater than that of osteoporosis (about two million DALYs), but that of asthma (about 1.4 million DALYs) is smaller.

Measurement of BMD by means of DXA describes four groups (see box 1), 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. BMD is only one component of the risk. Furthermore, intervention thresholds for treatment depend on the cost-benefit ratio of treatment.

BOX 1: BMD GROUPS

Normal BMD >1 standard deviation (SD) below young adult mean (T-score [s40]-1 SD)

Low bone mass (osteopenia) BMD between 1 SD below young adult mean but less than 2.5 SD below this (T-score <-1.0 and >-2.5 SD)

Osteoporosis BMD [s40]2.5 SD below young adult mean (T-score [s39]-2.5 SD)

Severe or established osteoporosis BMD showing osteoporosis plus one or more fragility fractures.

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 that are at least in part independent of BMD. WHO has developed a tool (FRAXTM) to calculate the 10-year probability of fracture, for hip alone or for vertebral, forearm, hip and humerus combined.3

Risk factors and interventions
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 dose of prednisolone 5mg daily or more (or equivalent for other steroids). 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.

At present, population screening is not advocated in the UK and patients should be identified using a case-finding strategy. Those who have experienced a previous fragility fracture (secondary prevention) or those who have a risk factor (primary prevention) should be assessed.

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 (figure 1). Intervention thresholds need to be set. These will depend on the risk of fracture, the efficacy, the cost and side-effect profile of treatment and how much society is prepared to pay. For the UK, thresholds have been calculated to determine the risk of hip fracture at which treatment with alendronate is cost-effective. In women with osteoporosis, treatment is cost-effective from the age of 60 years and if the woman had established osteoporosis.

Fig 1: MEASUREMENT OF BMD

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 more detailed guideline for the assessment and treatment of osteoporosis and fragility fracture is being developed and is expected soon.

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

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 JA, 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 JA, 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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