ADAPTIVE-CHANGES IN TRABECULAR ARCHITECTURE IN RELATION TO FUNCTIONALSTRAIN PATTERNS AND DISUSE

Principal strains and their orientation, determined from in vivo and i n situ strains recorded from the lateral cortical surface of the calca neus of potoroos (a small marsupial) during treadmill exercise and ten sion applied via the Achilles tendon, were compared with the underlyin g trabecular architecture and its alignment to test Wolff's ''trajecto rial theory'' of trabecular alignment, In vivo and in situ principal c ompressive strains (-800 to -2000 mu e) were found to be aligned (mean 161 +/- 7 degrees) close to the preferred alignment (160 degrees) of underlying trabeculae within the calcaneal metaphysis [a second trabec ular arcade was closely aligned (70 degrees) with the direction (71 de grees) of principal tensile strain], This finding represents quantitat ive verification of Wolff's trajectorial theory of trabecular alignmen t. These trabecular alignments, as measured by trabecular anisotropy ( TbAn, the ratio of horizontal: vertical intercepts), remained unchange d (p > 0.05) after 8 weeks of disuse, However, trabecular bone volume fraction (BV/TV, -35%), trabecular thickness (TbTh, -25%), and trabecu lar number (TbN, -16%) were reduced for the tenotomized calcaneii rela tive to their contralateral controls (p < 0.001 to < 0.003), The reduc tion in trabecular number was associated with a corresponding increase in trabecular spacing (Tbsp, +30%), Together, these results suggest t hat once trabecular alignment is established during growth (along the directions of principal strain during locomotion), it is not altered w hen functional strains are removed.