A reference database for the Stratec XCT-2000 peripheral quantitative computed tomography (pQCT) scanner in healthy children and young adults aged 6–19 years

Abstract

Summary

We have produced paediatric reference data for forearm sites using the Stratec XCT-2000 peripheral quantitative computed tomography scanner. These data are intended for clinical and research use and will assist in the interpretation of bone mineral density and bone geometric parameters at the distal and mid-shaft radius in children and young adults aged between 6–19 years.

Introduction

Peripheral quantitative computed tomography (pQCT) provides measurements of bone mineral content (BMC), density (BMD) and bone geometry. There is a lack of reference data available for the interpretation of pQCT measurements in children and young adults. The aim of this study was to provide reference data at the distal and mid-shaft radius.

Methods

pQCT was used to measure the 4% and 50% sites of the non-dominant radius in a cohort of healthy white Caucasian children and young adults aged between 5 and 25 years. The lambda, mu, sigma (LMS) technique was used to produce gender-specific reference centile curves and LMS tables for calculating individual standard deviations scores.

Results

The study population consisted of 629 participants (380 males). Reference centile curves were produced; total and trabecular BMD for age (distal radius) and for age and height, bone area (distal and mid-shaft radius), cortical area, cortical thickness, BMC, axial moment of inertia, stress–strain index and muscle area (mid-shaft radius).

Conclusions

We present gender-specific databases for the assessment of the distal and mid-shaft radius by pQCT. These data can be used as control data for research studies and allow the clinical interpretation of pQCT measurements in children and young adults by age and height.

Appendix

Patient example

This pre-pubertal male with osteogenesis imperfecta (OI) type I had a history of low trauma fractures of the long bones from 14 months of age. He had also suffered minor fractures of the toes and fingers. At 11 years of age, he complained of non-specific back pain; subsequent radiographs revealed anterior wedge fractures of two thoracic vertebrae. He underwent bone densitometry assessment DXA and pQCT at 11.5 years of age.

For clinical assessments of all patients attending the Manchester Metabolic Bone Disease Clinic, we measure patients’ height and weight and calculate SDS age from United Kingdom reference data [35, 36]. DXA is used to measure the lumbar vertebrae (L1–L4) to provide measurements of lumbar spine (LS) bone area (square centimetre) and bone mineral content (BMC) (grams per square centimetre) [32]. LS bone mineral apparent density (BMAD) is calculated [57]. The three-stage algorithm of Molgaard [58] is used to determine whether a patient has ‘short’ (i.e. height for age), ‘narrow’ (i.e. lumbar spine bone area for height) or ‘light’ (i.e. LS BMC for LS bone area) bones at the lumbar spine; SDS are calculated [32]. pQCT is used to measure the 4% distal radius to provide measurements total and trabecular BMD, as suggested by the recent International Society for Clinical Densitometry guidelines [30]. SDS are calculated for total and trabecular BMD according to age.

Measurement	Value	SDS
Heighta for age	133 cm	−1.9
Weighta for age	32.6 kg	−0.7
Lumbar spine BMAD for age	0.18 g/cm3	−3.2
Lumbar spine bone area for height	30.33 cm2	−3.9
Lumbar spine BMC for lumbar spine bone area	9.60 g	−2.9
Total BMDa for age	192.1 mg/cm3	−3.2
Trabecular BMDa for age	116.3 mg/cm3	−2.5

Clinical interpretation of anthropometric, DXA lumbar spine and pQCT distal radius total and trabecular BMD

This patient has reduced height for age (SDS −1.9), indicating he has small bones. He also has narrow (SDS −3.9) and light (SDS −2.9) bones and reduced LS BMAD (SDS −3.2). This suggests that he has impaired lumbar spine bone growth and mineralisation, secondary to his OI [32]. At the distal radius, the total and trabecular BMD are low due to fewer and thinner trabeculae, as well as thinner cortical shell in OI [59].

We also use pQCT to provide a number of non-clinical measurements at the 50% mid-shaft radius diaphysis in order to gain a greater insight into how disease or treatment may affect bone geometry, strength and muscle. We use pQCT to provide measurements of cortical thickness, cortical BMC, the polar strength strain index (polar SSI) and axial moment of inertia (AMI) at the 50% mid-shaft radius. This is in accordance with those which the ISCD suggest be measured at a diaphyseal site [30]. In addition, we also use pQCT to measure cortical area, bone area and cross-sectional muscle area of the 50% mid-shaft radius. As height has a strong correlation with cortical bone parameters [40] and is a powerful determinant of cortical bone geometry and strength [31, 41], we calculate SDS for all variables measured at the 50% mid-shaft radius by height. In addition, we calculate SDS for cortical BMC by age.

Measurement	Value	SDS
Cortical thicknessa for height	1.7 mm	−0.9
Cortical BMCa for age	37.6 mg/mm	−2.8
Cortical BMCa for height	37.6 mg/mm	−1.8
Polar SSIa for height	70.7 mm3	−2.0
AMIa for height	193.0 mm4	−2.4
Cortical area for height	33.9 mm2	−2.3
Bone area for height	52.6 mm2	−2.3
Cross-sectional muscle area for height	1139.5 mm2	0.0

Interpretation of BMC, bone geometry, strength and muscle parameters at the 50% mid-shaft radius according to height and cortical BMC according to age

At the mid-shaft radius, adaption to higher bone material property in OI [60] results in the smaller bone area (SDS −2.3) and cortical area, which can lead to a reduction in long-bone strength (polar SSI SDS −2.0 and AMI SDS −2.4) in patients with OI [61]. Whilst this patient has reduced cortical parameters for his height, he has normal cross-sectional muscle area suggesting that bone strength of his slender long-bone diaphyses are not appropriately adapted to loading from his forearm muscles [61].

Taking into consideration the clinical, radiological and bone densitometry findings, the patient was commenced on cyclical three monthly infusions of pamidronate (3-amino-1-hydroxypropylidene-1,1-bisphosphonate) in order to reduce his risk of fractures at appendicular and axial skeletal sites.