FEATURE PAEDIATRICS


«Up to 90% of peak bone mass is acquired by age 18 in girls and by age 20 in boys»


and dual energy x-ray absorptiometry (DXA). At present, adequate databases for children are lacking for metacarpal morphometry and quantitative ultrasound. QCT has an advantage over the other techniques because it measures bone volume and thus accurately computes volumetric density. In addition, it differentiates dense cortical bone from metabolically active trabecular bone. Its drawback is that it exposes the patient to a substantial amount of radiation (effective radiation dose 60 mSv), which limits utility for follow-up studies. DXA provides considerable advantages:


low radiation exposure (1-3 mSv versus 50mSv for adult PA chest x-ray and 8 mSv per day background radiation in the U.S.), precise results, and lower cost than QCT. However, unlike QCT, it does not directly measure true volume, and thus computes areal density, not volumetric density. Derived from the area of bone, areal density is a two-dimensional estimation of a three-


dimensional property, and the algorithms that make the conversion to density are not entirely satisfactory. Furthermore, among DXA machines,


there are important differences in software. In Hologic software, for instance, the Pediatric Option program is better at detecting the edges of bone, and thus better at measuring bone density than the standard software.


CHALLENGES IN ASSESSMENT OF BONE HEALTH IN CHILDREN The measurement of bone density in children is less standardized than in adults and requires special consideration of the effects of growth and puberty on bone mass. Interpretation of bone density in children relies on Z-scores— the number of standard deviations (SD) from the norm of a reference database of children. A Z-score of -2 SD represents low bone density, but indications for treatment and specific therapy are not yet established. In contrast, bone mass density in adults is measured in T-scores – standard deviations from peak bone mass, with a T-score of – 2.5 SD or worse representing osteoporosis, as


codified by the World Health Organization in 1994. In children,


the relationship between bone density and fracture risk is less firmly established than in adults (see figure 3). Most importantly, however, although


pediatric reference databases have improved in recent years, there are important confounding factors to consider when performing DXA, such as variations in race, gender, pubertal status and height. Reference databases should be gender- specific, because girls enter puberty earlier than boys. Racial differences may be relevant; in the U.S., bone density 


FIGURE 1: THREATS TO CHILDHOOD BONE HEALTH  Nutritional deficiencies (calcium, vitamin D, phosphorus, vitamin K, zinc)


 Decreased muscle forces/loading  Delayed pubertal maturation  Alterations in the growth hormone/IGF-1 axis


 Inflammatory cytokines (TNF-α, IL-6, IL-1β)  Medications (glucocorticoids, chemotherapy, anticonvulsants)


FIGURE 2: ASSESSING BONE DENSITY IN CHILDREN: CURRENT INDICATIONS  Recurrent fractures, especially low-impact or atraumatic


 Medical conditions or medications associated with low BMD


 “Osteopenia” on radiographs  To monitor treatment of low BMD


FIGURE 3: FACTORS TO CONSIDER WHEN PERFORMING DXA IN CHILDREN  Size of patient (height)  Skeletal maturation (bone age)  Pubertal status  Race and gender


FIGURE 4: CONCLUSION  What we know - DXA BMD correlates with fracture risk in healthy children


- Differences in hardware and software impact DXA results


- Reference data should be gender- and age- specific


- DXA results should be adjusted for poor growth


 What we think we know - DXA results should be adjusted for height Z-score


 What we need - Studies relating these measures to short- and long-term fracture risk


Arab Health Issue 3 2011 39


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