Osteoporosis is a multifactorial disease characterised by low bone mineral density (BMD), a disruption of the normal bone architecture, and increased risk of fracture. Common sites of osteoporotic fracture include the spine, hip, and forearm, with 1,150 people reportedly dying each month in the UK as a result of hip fracture. Worldwide the incidence of hip fractures is expected to more than triple from 1.66 million in 1990 to 6.26 million in 2050, while in the European Union an increase from 414 000 to 972 000 cases per annum is expected over the next 50 years. Osteoporosis is diagnosed clinically as a BMD of 2.5 standard deviations (SD) or more below the average for the young healthy female population. Therefore, BMD measurement is fundamental in assessing osteoporosis risk; comparable to measuring blood pressure to predict stroke and substantially better than measuring serum cholesterol to predict cardiovascular disease respectively. BMD is most commonly measured, at the hip or lumbar spine, using dual-energy x-ray absorptiometry (DXA). Measurements at other sites (primarily the heel and finger) using peripheral DXA or ultrasound is also possible, although not considered as accurate. Recently, other methods of analysis have become available, including micro- computed tomography (microCT) and quantitative computed tomography (QCT), which are able to better distinguish between trabecular and cortical bone. However, DXA is currently considered the gold standard for BMD measurement as it is accurate, precise, requires only a short scanning period, and has a limited radiation dose. The purpose of this article (the second of a four-part feature) is to define osteoporosis and its parameters, discuss the devastating impact is has on individuals and society, and explain the diagnostic methods used to quantify bone density.

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