Bone is a dynamic tissue that reflects the biological principle of adaptation of structure to function and the metabolic role of mineral homeostasis. The skeleton is made up of two types of bone: cortical (outer bone) and trabecular (the inner softer core, the more metabolically active bone). The skeleton provides the strength needed to withstand the mechanical forces of daily weight-bearing. Structurally, the long bones of the skeleton are often referred to as appendicular bones and the bones of the trunk as the axial skeleton.
During growth and development, skeletal growth occurs in a process known as bone modelling. Bone is continually broken down and rebuilt in a process known as remodelling, under the regulation of systemic hormones and local growth factors. The remodelling cycle consists of five successive events: activation, resorption, reversal, formation and termination. There are two types of bone cells: osteoclasts resorb the bone while osteoblasts form bone. Osteocytes inside bone are thought to be critical in the regulation of bone remodelling. When bone resorption exceeds formation, bone loss occurs, which, if prolonged, can lead to osteoporosis and increased risk of fracture. Further reading on bone biology and other topics related to bone health is available at the American Society for Bone and Mineral Research Bone Curriculum website (see Useful websites).
THE MEASUREMENT OF BONE MINERAL DENSITY
Bone densitometry undertaken by dual energy X-ray absorptiometry (DXA) measures the average bone mineral content (BMC) within the region scanned, known as bone mineral density (BMD) and is used to diagnose osteoporosis or low bone density. The usual sites measured are the hip and lumbar spine, which are the most common fracture sites. A whole body scan can estimate the total bone density but is more useful for body composition assessment. BMD can be measured by DXA and quantitative computed tomography (QCT). QCT, mainly used in research, provides measures of bone geometry. Both these methods involve ionising radiation, a consideration in athletes who may be undergoing other testing procedures that involve ionising radiation, such as X-rays and other computed tomography (CT) scans. DXA is also limited to a two-dimensional image and doesn’t capture bone depth, which has raised debate about its use in measuring BMD in children and how to account for size (Weaver et al. 2016).
Definitions: osteoporosis and osteopaenia
Osteoporosis is a condition of low bone mass associated with bone fragility and increased risk of fractures. The risk of developing osteoporosis and subsequent fractures is largely determined by the peak bone mass achieved in adolescence and early adulthood. Between 50% and 80% of the variability in bone density has been attributed to genetic factors (Pocock et al. 1987; Zhai et al. 2009). Various fracture risk calculators used to estimate fracture risk from clinical factors (e.g. the WHO Fracture Risk Assessment Tool or the Garvan Fracture ...