Ultrasound for the assessment of bone quality in preterm and term infants

As 80% of intrauterine bone mineralization takes place during the last trimester of pregnancy, preterm infants should be supplemented postnatally with optimal doses of calcium, phosphate and vitamin D. Calcium and phosphate excretion in the urine may be used to monitor individual mineral requirements, but are sometimes difficult to interpret. The objective of this study was to assess the value of quantitative ultrasound (QUS) for the analysis of bone status in neonates.

All admissions to three independent tertiary neonatal intensive care units were studied. In 172 preterm and term infants with a gestational age between 23 and 42 weeks (mean 33.8±5.0) and a birth weight from 405 to 5130 g (mean 2132±1091 g) bone status was evaluated prospectively by quantitative ultrasound velocity using a standardized protocol. Infants were followed in regular intervals up to their first discharge home. While measurements were conducted in weekly intervals initially (n=55), 2-week intervals were regarded as sufficient thereafter due to limited changes in QUS values within the shorter period. Infants with a birth weight below 1500 g were followed during outpatient visits until up to 17 months of age.

The intra-individual day-to-day reproducibility was 0.62%. QUS-values from the first week of life correlated significantly with gestational age and birth weight (r=0.5 and r=0.6; P<0.001). Small-for-gestational-age infants showed lower values for QUS than appropriate-for-gestational-age infants allowing for their gestational age. Follow-up measurements correlated positively with age and weight during the week of measurement (r=0.2 and r=0.4; P=0.001). Comparing bone quality at 40 weeks of age in infants born at term versus infants born at 24 to 28 weeks, preterm infants showed significantly lower QUS than term infants (P<.0001).There was a significant correlation of QUS with serum alkaline phosphatase (P=0.003), the supplementation with calcium, phosphate and vitamin D (P< 0.001 each), as well as risk factors for a reduced bone mineralization. No correlation was found between QUS and calcium or phosphate concentration in serum or urine.

QUS is a highly reproducible, easily applicable and radiation-free technique that can be used to monitor bone quality in individual newborns. Further prospective randomized-trials are necessary to evaluate, if therapeutic interventions based on QUS are able to prevent osteopenia of prematurity.