To investigate the properties of screwbone interface of expansive pedicle screw(EPS)in osteoporotic sheep by micro-CT and histological observation.Methods:Six female sheep with bilateral ovariectomyinduced osteoporosis were employed in this experiment.After EPS insertion in each femoral condvle, the sheep were randomly divided into two groups:3 sheep were bred for 3 months(Group A),while the other 3 were bred for 6 months(Group B). After the animals being killed,the femoral condyles with EPS were obtained,which were three-dimensionally-imaged and reconstructed by micro-CT. Histological evaluation was made thereafter.Results:The trabecular microstructure was denser at the screw-bone interface than in the distant parts in expansive section,especially within the spiral marking.In the nonexpansive section,however,there was no significant difference between the interface and the distant parts. The regions of interest(ROI)adjacent to EPS were reconstructed and analyzed by micro-CT with the same thresholds. The three-dimensional(3-D)parameters,including tissue mineral density(TMD),bone volume fraction(BVF,BV/TV),bone surface/bone volume(BS/BV) ratio, trabecular thickness(Tb. Th),and trabecular separation(Tb. Sp),were significantly better in expansive sections than non-expansive sections(P＜0.05).Histologically,newly-formed bony trabeculae crawled along the expansive fissures and into the center of EPS. The newly-formed bones,as well as the bones at the bone-screw interface,closely contacted with the EPS and constructed four compartments.Conclusions:The findings of the current study,based on micro-CT and histological evaluation, suggest that EPS can significantly provide stabilization in osteoporotic cancellous bones.