Dual Energy XRay Absorptiometry

DXA is the most widely used technique even in children, adolescent, and young people, because of the availability of instruments, accuracy, and precision (reproducibility). It has been much studied both in vivo and

Table 1 Primary and Secondary Osteoporoses in Children and Adolescents

Primary osteoporosis Idiopatic juvenile osteoporosis Heritable disorders of connective tissue

Secondary osteoporosis Neuromuscolar disorders Cerebral palsy

Duchenne muscular dystrophy Prolonged immobilization Chronic diseases Leukemia

Systemic connective tissue diseases Cystic fibrosis

Inflammatory bowel diseases

Malabsorption syndromes (celiac disease)

Thalassemia

Primary biliary cirrhosis

Nephropathies (nephrotic syndrome)

Anorexia nervosa

Organ transplants

HIV infection

Endocrine diseases Delayed puberty Hypogonadism Turner syndrome Growth hormone deficiency Hyperthyroidism Juvenile diabetes mellitus Hyperprolactinemia Cushing syndrome Inborn errors of metabolism Protein intolerance Glycogen storage diseases Galactosemia Gaucher disease

Iatrogenic causes Glucocorticoids Methotrexate Cyclosporine Heparin Radiotherapy Anticonvulsant drugs in vitro, also in younger subjects (34). The radiation dose is low and there have been pediatric dose studies (35). The main advantage of DXA in pediatrics is the possibility to perform a separate analysis of the different skeletal regions, and of the whole skeleton, because the dual-energy X-rays overcome the problem of soft tissues surrounding bone. This also allows an evaluation of the body fat and lean mass, a particularly important aspect in pediatric patients. DXA bone mass measurement is the sum of the cortical and trabecular bone components of the studied skeletal segment. The limit of DXA is that it uses a two-dimension projection of the examined area, so that the calculated "density" is actually the mineral density per unit of bone projection area rather than per unit of bone volume.

Interpretation of DXA During Adolescence in

Chronic Rheumatic Diseases

The DXA instruments have obtained ample scientific validation and have been approved by the Food and Drug Administration in the United States of America. The manufacturers have significantly improved their software, and have developed sensitive and specific algorithms for pediatric age and for the recognition of bone and soft tissues, so that even very reduced bone masses can be measured (36). However there are intrinsic limits in the use of DXA for studying a growing skeleton. BMD as calculated by DXA is the ratio of the bone mineral content to the area of the projection of the examined bone, instead of the bone volume. This introduces an error in the presence of different bone volumes. To overcome this problem, some mathematical corrections have been proposed, both for the vertebrae and for the femur. For example, assuming that a vertebral body is a cylinder, its volume has been calculated on the basis of the height and width of the anteroposterior projection of vertebrae. The BMD corrected for this "apparent" vertebral volume has been called "apparent BMD" (BMAD, g/cm3) and can be calculated with a simple mathematical expression: BMAD = BMD * 4/(3.14 * width of the vertebral body) (37). Other corrections assume that the vertebral body is a cube, or a cylinder with an ellyptic base.

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