Definition of osteoporosis and osteoporotic fracture

Osteoporosis is defined as a systemic skeletal disease characterised by a low bone mass and a microarchitectural deterioration of bone tissue, with a subsequent increase in bone fragility and susceptibility to fracture .

Osteoporosis behaves as a silent disease. A high percentage of the affected people are not aware they have the condition. Therefore, the burden of osteoporosis is the burden of its clinical outcome: osteoporotic fractures, which are generally recognised as low-trauma fracture resulting from low bone mineral density (BMD). The best characterised are, by order of the related disability burden, hip fracture, vertebral fracture and wrist fracture. There are other peripheral fractures related to low density or poor quality of bone mass such as proximal humeral, pelvic, rib, proximal tibia or ankle fractures.

For diagnostic and therapeutic purposes, the operational definition is based on BMD values: osteoporosis may be diagnosed in postmenopausal women and in men aged 50 and older if the T -score measured by dual-X-ray absorptiometry (DXA) at lumbar spine, total hip or femoral neck (FN) is −2.5 or less (in certain circumstances, the 33% distal radius may be used) than the mean BMD value in young females . There is a current international consensus to use the non-Hispanic, white, young female range from National Health and Nutrition Examination Survey (NHANES) III at FN as an ‘international’ reference. This approach has the advantage of giving a single location for osteoporosis definition, and it is applicable to both men and women. Therefore, it looks suitable when seeking worldwide comparisons.

However, based in the above definition, there is no burden associated with osteoporosis per se, and it becomes difficult to estimate a real prevalence of the condition in a given population. The sensitivity of the BMD measures to predict an osteoporotic fracture is good but limited. In fact, the most important proportion of the overall incident fractures occurs in subjects with osteopenia ( T -score between −1 and −2.5) because of the effect of the population distribution . Likewise, fractures can occur in subjects with normal BMD ( T score above −1.0).

As discussed later, the predictive value of the BMD to assess the fracture risk can be improved with the input of other independent risk factors for fractures, the most relevant of which is increasing age. This is a key question when assessing the burden of osteoporosis, as we can predict that the burden will be higher in those world regions with a more aged population. Gender is the second clinical risk factor that appears important in fracture risk: both osteoporosis and osteoporotic fractures are more common in women, mainly due to their lower bone mass and their longer life expectancy.

From an epidemiological approach, lifetime risks and 10-year probability for fractures are more suitable to measure the burden of osteoporosis as they reflect an absolute risk rather than a relative risk, based solely on the BMD values. Both the World Health Organization (WHO) and the International Osteoporosis Foundation (IOF) recommend the 10-year time frame to base public health analysis and interventions . For a person over 50 years living in a developed country, the lifetime risk of sustaining a fracture has been estimated at about 50% for women and 20% for men .