Low bone mineral density (BMD) can be defined as "a reduction of bone mineral mass per volume unit of bone tissue, in the absence of mineralization defects." The last point is particularly important regarding infants and children, to avoid confusion with rickets. Clinically significant low BMD is increasingly observed in younger patients. While this condition in adults is normally referred to as osteoporosis (1), there is no consensus about the definition of osteoporosis in children before the appearance of fragility fractures, and there are not widely accepted cut-off densitometric values, such as those proposed by WHO for adult women (2). This is due to two main reasons.
The first is the problematic interpretation of densitometric values in a growing skeleton. Bone size, shape, and mineral density are rapidly changing, and some regions can be affected by low mineral density more than others (e.g., the trunk more than lower limbs). Moreover, the growing process itself—including the onset and progression of puberty—can be influenced by the primary disease. Therefore, it is often difficult to identify an appropriate control group to define normal densitometric values. The usual comparison with sex- and age-matched controls can be inadequate in the presence of chronic diseases affecting skeletal growth and maturation, and can lead to inaccurate evaluations in terms of actual bone mass loss. The second reason is that, while BMD is a strong predictor of the fracture risk in adults (3-5), there are no comparable evidences on this point in the young, although some recent findings may support it (6,7). Presently, most researchers would consider a diagnosis of osteoporosis in the young only after at least one fracture with minimal trauma (8).
Regarding the different forms of osteoporosis, the primary forms of osteoporosis are relatively rare, while a growing number of causes of secondary osteoporosis are now known, mainly as a consequence of the improved long-term outcome of chronic diseases in the young.
Some basic information on the growth, development, and consolidation of bones are desirable as an introduction to the bone problems of adolescents affected by chronic rheumatic diseases.
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