Controlling the pore size of carbonate apatite honeycomb scaffolds enhances orientation and strength of regenerated bone

To restore functions of long bones and avoid reconstruction failure, segmental defects should be quickly repaired using abundant amounts of regenerated bone with high mechanical strength and orientation along the bone axis. Although both bone volume and bone matrix orientation are important for faster restoration of long bones with segmental defects, researchers have primarily focused on the former. Artificial bone scaffolds with uniaxial channels, (e.g., honeycomb (HC) scaffolds), are considered adequate for regenerating bone oriented along the bone axis. The channel size may affect the orientation, amount, and strength of the regenerated bone. In this study, we investigated the effects of channel size in carbonate apatite HC scaffolds on the orientation of bones regenerated in segmental bone defects and determined the adequate channel size. Carbonate apatite HC scaffolds, with different channel sizes (350, 550, 730, and 890 μm in length on the side of the square aperture), were fabricated by extrusion molding of a mixture of calcium carbonate and organic binder, debinding, and subsequent phosphatization to convert the composition from calcium carbonate to carbonate apatite. No significant difference in the amounts of regenerated bones was observed for different channel sizes. However, bone along the bone axis was formed in the channels ≤550 μm in size but not in channels ≥730 μm. The HC scaffolds with a channel size of 350 μm regenerated bone with higher bending strength than those with a channel size of 890 μm. However, bone regenerated with the HC scaffolds having channel sizes of 350, 550, and 730 μm showed equal bending strength. Thus, the adequate channel size for fast regeneration of high-strength bone, oriented to the bone axis, is ≤730 μm. To the best of our knowledge, this is the first study to report the effect of channel size on bone orientation and strength. The findings of this study are relevant to the fast repair of segmental bone defects.