Acta Orthopaedica 2011; 82 (5): 628–632 631 Figure 3. Effect of anti-sclerostin antibody (Scl-Ab) on trabecular bone volume in the proximal tibia, as measured by µCT. A. Bone volume fraction. B. Trabecular number. C. Trabecular Thickness. D. Trabecular separation. Botox treatment significantly reduced BV/TV, Tb.N, and Tb.Th compared to loaded controls (p < 0.001). Antibody treatment increased BV/TV both in the loaded and unloaded tibia, but only the increase in the unloaded tibia was significant (a: p = 0.03). Trabecular thickness was also significantly increased, however (b, c: p < 0.001). Scl-Ab treatment of the unloaded bone increased BV/TV by 80% (p = 0.03). There was a 25% increase in trabecular thickness (p < 0.001) with no statistically significant change in trabecular number or spacing. In weight-bearing bone, Scl-Ab treatment caused a 20% increase in trabecular thickness (p < 0.001). There was no statistically significant increase in cancellous BV/TV (Figure 3), trabecular number, or spacing. Discussion The quality of fixation of the implant may be considered to be the end result of a fracture-healing response of the kind that is prominent in cancellous bone. As in any stable metaphyseal fracture, this healing response leads to direct bone formation around the screw. This new bone is shaped like a thread, grasp- ing the screw. The strength of this new bone, which reflects the fracture healing response, directly influences the force that will be needed to pull out the screw. We have previously shown that an increase in pull-out force using anti-sclerostin antibody is related to an increase in the amount of mineralized bone surrounding the screw (Agholme et al. 2010). In the present study, Scl-Ab improved the fixation of a steel screw in the trabecular bone of normal-loaded and unloaded young male rats. Paralysis of muscles of the hind limb leads to rapid loss of bone mineral content and to changes in trabecular structure (Chappard et al. 2001, Warner et al. 2006). In this degraded bone, the Scl-Ab treatment had a positive effect. The raw data suggests that there was a greater response in loaded bone (Figure 2). However, for a correct statistical analysis, one requires log-transformation to equalize variation, and to com- pare the percentage changes rather than absolute differences. When this was performed, the interaction between loading and antibody disappeared. Scl-Ab treatment increased BV/TV in the unloaded bone. These findings are consistent with a previous study in which administration of anti-sclerostin antibody prevented bone deg- radation after immobilization of the hind limb using bandages (Tian et al. 2011). In that study, Scl-Ab was shown to increase bone formation and reduce bone resorption in unloaded bone. In our study, the region of evaluation for μCT analysis included the primary spongiosa. This is different from other studies with Scl-Ab (Li et al. 2009, Tian et al. 2011), where the primary spongiosa was excluded. Nevertheless, Scl-Ab treatment induced an increase in trabecular thickness in both loaded and unloaded bone, with minimal effects on trabecular separation and number. This result is similar to that in a previ- ous report on loaded bone (Agholme et al. 2010). The expression of sclerostin is highly upregulated by unloading (Robling et al. 2008) and mice lacking sclerostin expression are insensitive to unloading (Lin et al. 2009). We had therefore expected a proportionately stronger response to the antibody treatment in unloaded bone. However, even with a high level of sclerostin inhibition, we could not fully counter the effects of unloading. This could have been due to very strong expression of sclerostin in the unloaded bone, in comparison to normally-loaded bone, in which case the amount of antibody administered may have been inadequate to fully inhibit sclerostin. It is also possible that sclerostin is not the only mechanically responsive modulator of bone during unloading. Expression of Dickkopf-1 (Dkk1), another Wnt inhibitor, is also upregulated during unloading. However, we recently showed that blocking of Dkk1 had no effect on pull-out strength parameters in unloaded bone despite the increase in pull-out strength parameters in normally-loaded bone (Agholme et al. 2011). The present study had several limitations. We chose to only investigate the initial stages of fracture healing at a single