Bone Abstracts

Most studies using botulinum toxin A (BTX) to achieve skeletal unloading use a study period of 3–4 weeks. The aim was to prolong this study time and to investigate the influence on the bone microstructure including the relationship between the thickness of horizontal and vertical trabeculae. Fifty-seven 16-week-old female Wistar rats were randomized into six Groups: Base; Ctrl 4w; and BTX4 4w; Ctrl 8w; BTX4 8w; BTX4+2 8w. The BTX animals were injected with 4 IU at baseline or 4 IU at baseline and 2 IU again after 4 weeks. The animals were killed after 0, 4, or 8 weeks. The distal femoral metaphysis was μCT scanned at a resolution of 6 μm and the bone strength was ascertained. After 4 weeks there was a significant loss of BV/TV (BTX4 4w: −24%) and of Tb.Th* (BTX4 4w: −11%). After 8 weeks there was a significant loss of BV/TV (BTX4 8w: −21% and BTX4+2 8w: −13%) and of Tb.Th* (BTX4 8w: −13% and BTX4+2 8w: −12%). After 4 weeks the horizontal:vertical trabecular thickness ratio Tb.Th.horz/Tb.Th.vert did not differ between immobilized and control animals, while after 8 weeks it was significantly higher in the immobilized animals (Ctrl 8w: 0.93±0.009, BTX4 8w: 0.97±0.018 and BTX4+2 8w: 0.98±0.013) indicating a more rapid thinning of the vertical trabeculae. Thus, the load-bearing vertical trabeculae at the distal femoral metaphysis are more susceptible to skeletal unloading than the horizontal trabeculae. There was a significant loss of bone strength after 4 weeks (BTX4 4w: −19%) and after 8 weeks (BTX4 8w: −22% and BTX4+2 8w: −17%).

Conclusion: No additional BTX is needed to extend the study period from 4 to 8 weeks. Horizontal trabeculae are thinned more rapidly than vertical trabeculae during disuse in rats. The experiment was approved by the Danish Animal Experiments Inspectorate.