Increase in bone volume fraction precedes architectural adaptation in growing bone

In mature trabecular bone, both density and trabecular orientation are adap ted to external mechanical loads, Few quantitative data are available on th e development of architecture and mechanical adaptation in juvenile trabecu lar bone. We studied the hypothesis that a time lag occurs between the adap tation of trabecular density and the adaptation of trabecular architecture during development. To investigate this hypothesis we used ten female pigs at 6, 23, 56, 104, and 230 weeks of age. Three-dimensional morphological an d mechanical parameters of trabecular bone samples from the vertebra and pr oximal tibia were studied using microcomputed tomography and micro-finite e lement analysis. Bath bone volume fraction and stiffness increased rapidly in the initial growth phase (from 6 weeks on), whereas the morphological an isotropy started increasing only after 23 weeks of age. In addition, the an isotropy reached its highest value much later in the development than did b one volume fraction, Hence, the alignment of trabeculae was still progressi ng at the time of peak bone mass. Therefore, our hypothesis was supported b y the time lag between the increase in trabecular density and the adaptatio n of the trabecular architecture. The rapid increase of bone volume fractio n in the initial growth phase can be explained by the enormous weight incre ase of the pigs, The trabeculae aligned at later stages when the increase i n weight, and thus the loading, was slowed considerably compared with the e arly growth stage. Hence, the trabecular architecture was more efficient in later years. We conclude that density is adapted to external load from the early phase of growth, whereas the trabecular architecture is adapted late r in the development. (C) 2001 by Elsevier Science Inc. All rights reserved .