Relationship of tibial speed of sound and lower limb length to nutrient intake in preterm infants

Background: Metabolic bone disease of prematurity is characterised by impaired postnatal mineralisation of the rapidly growing infant skeleton. Objective: To longitudinally evaluate postnatal changes in tibial speed of sound (tSOS; which reflects cortical thickness and bone mineral density) and lower limb length (LLL; a measure of tibial growth) in very low birthweight preterm infants receiving contemporary neonatal care. Methods: tSOS and LLL were measured using a quantitative ultrasound device and an electronic neonatal knemometer, respectively, in the same limb, weekly, for a median period of four weeks (3-16 weeks) in 84 preterm infants median gestation 26.8 weeks (range 23-35.2 weeks) and median birth weight 869.5 g (ange 418-1481 g)). Results: Initial tSOS and LLL were correlated with gestation (r=0.42, p<0.001; r=0.76, p<0.001 respectively) and birth weight (r=0.23, p=0.038; r=0.93, p<0.001, respectively). Postnatally, tSOS decreased (r=-0.15, p=0.01 1) whereas LLL increased (r=0.96, p<0.001) with age. The rate of postnatal change in LLL, but not in tSOS, was positively influenced by intake of calcium (p=0.03), phosphorus (p=0.01) and vitamin D (p=0.03). Conclusions: The postnatal decline in tSOS, which is probably due to cortical thinning secondary to endocortical bone loss, and increase in LLL provide new insight into the development of long bones in preterm infants.