Reconstruction of rabbit mandibular bone defects using carbonate apatite honeycomb blocks with an interconnected porous structure.

Carbonate apatite (COAp) granules are useful as a bone substitute because they can be remodeled to new natural bone in a manner that conforms to the bone remodeling process. However, reconstructing large bone defects using COAp granules is difficult because of their granular shape. Therefore, we fabricated COAp honeycomb blocks (HCBs) with continuous unidirectional pores. We aimed to elucidate the tissue response and availability of COAp HCBs in the reconstruction of rabbit mandibular bone defects after marginal mandibulectomy. The percentages of the remaining COAp area and calcified bone area (newly formed bone) were estimated from the histological images. COAp area was 49.1 ± 4.9%, 30.3 ± 3.5%, and 25.5 ± 8.8%, whereas newly formed bone area was 3.0 ± 0.6%, 24.3 ± 3.3%, and 34.7 ± 4.8% at 4, 8, and 12 weeks, respectively, after implantation. Thus, COAp HCBs were gradually resorbed and replaced by new bone. The newly formed bone penetrated most of the pores in the COAp HCBs at 12 weeks after implantation. By contrast, the granulation tissue scarcely invaded the COAp HCBs. Some osteoclasts invaded the wall of COAp HCBs, making resorption pits. Furthermore, many osteoblasts were found on the newly formed bone, indicating ongoing bone remodeling. Blood vessels were also formed inside most of the pores in the COAp HCBs. These findings suggest that COAp HCBs have good osteoconductivity and can be used for the reconstruction of large mandibular bone defects. The COAp HCB were gradually resorbed and replaced by newly formed bone.