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31-05-2018 | Osteoporosis | View from the clinic | Article

Monitoring bone health with dual-energy X-ray absorptiometry in pediatric patients

Author:
Malachi McKenna

Dual-energy X-ray absorptiometry (DXA) is the key technology in adults and the young for the diagnosis of osteoporosis, estimation of fracture risk, and monitoring of change over time.

DXA acquisition and analysis

There are two sites for DXA assessment in the young: posterior-anterior lumbar spine and total body less head (TBLH) (see Figure 1), but the femur is unsuitable for DXA assessment until the patient is at least 15 years old. The primary measurements from DXA are area and bone mineral content (BMC), from which areal bone mineral density (aBMD) is derived. BMC or aBMD at either lumbar spine or TBLH are used for skeletal health assessment [1]. The absolute result (of both BMC and aBMD) is converted to a standardized score called a Z-score, which provides an estimate of the standard deviation difference from the mean for chronologic age and sex.

In children with short stature or growth delay, the results for both spine and TBLH should be adjusted for size. For the spine, either the aBMD Z-score should be adjusted for the height-for-age Z-score , or alternatively the bone mineral apparent density should be estimated [1]. For TBLH, adjust aBMD Z-score using the height-for-age Z-score, or estimate the predicted TBLH BMC for lean body mass (which is measured at the same time as TBLH) [2].

DXA image of whole body © Malachi McKenna 2018

Figure 1: DXA image of whole body

DXA normative data

In order to interpret the DXA result a reference data set, including healthy representatives from the general population, must be used that is sufficiently large to capture variability in bone measures taking into consideration sex, age, and race/ethnicity. In the USA a normative range was acquired by the Bone Mineral Density Childhood Study on the Hologic system followed by cross-calibration to GE Lunar [3]. In 2017 in the UK a comprehensive study (principally on GE Lunar systems but also on a Hologic system) established reference database that is valid for the hardware and software technological updates and is now considered as the standard of care in the UK [1]. DXA results are expressed in standard deviation units, namely Z-scores.

DXA diagnosis

Unlike in adults, a diagnosis of osteoporosis in children should not be made on the basis of DXA criteria alone. In the absence of local disease or high-energy trauma, the finding of one or more vertebral fractures indicates osteoporosis. In the absence of vertebral fractures, the diagnosis of osteoporosis is indicated by the presence of both a Z-score of –2.0 standard deviations or lower and a fracture history as follows: two or more long bone fractures by age 10 years; or three or more long bone fractures at any age up to age 19 years. T-scores (which are standardized scores based on young adult reference data) should never appear in pediatric DXA reports.

Moreover, the term “osteopenia” should not appear in pediatric DXA reports, and the term “osteoporosis” should not appear in such reports unless there is a clinically significant fracture history. “Low bone mineral mass or bone mineral density” is the preferred term for pediatric DXA reports when BMC Z-scores or areal BMD Z-scores are –2.0 SD or lower [1]. If a follow-up DXA scan is indicated, then the minimum interval between scans is 6-12 months.

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