Increased distraction rates influence precursor tissue composition withoutaffecting bone regeneration

The effect of increased distraction rate on bony tissue differentiation was studied using a paired bilateral model of rat femur lengthening. After a 6 -day latency period, one randomly selected femur for each rat was distracte d at 0.5 mm/day (normal rate) for 12 days, and the contralateral femur was distracted at 1.5 mm/day (increased rate) for 4 days. Femoral lengthening f or each side was 6.0 mm, leaving the increased rate leg with an extra 8 day s of consolidation compared with the normal rate limb. Group I rats (n = 9) were killed at day 18 postsurgery and analyzed for cartilage tissue compos ition and distribution. Group II rats (n = 7) were killed on day 36 postsur gery and analyzed by three-dimensional microcomputed tomography (MCT) for c hanges in new bone volume. Digital color analysis of slides stained with ty pe II collagen antibody showed increases in cartilaginous tissue formation on the increased rate side (1.51 mm(2) vs. 0.83 mm(2); p = 0.10). No differ ences in new bone volume were detected between increased rate limbs and the ir contralateral controls (46.13 mm(3) vs. 42.69 mm(3); p = 0.63), These fi ndings suggest that intermediate distraction rates may influence precursor tissue composition without affecting the final amount of new bone formed. B ecause damage to the tissue was not detected at either time point, these ch anges in chondrogenesis may reflect sensitivity of the pluripotential gap t issue to tension accumulation during lengthening. Future work with this in vivo model is focused on improving our understanding of the mechanisms behi nd this strain sensitivity.