Reconstruction of Load-Bearing Segmental Bone Defects Using Carbonate Apatite Honeycomb Blocks.

In a globally aging society, synthetic bone blocks are in increasing demand. An ideal synthetic bone block fuses early with bone and is replaced with new bone at a suitable speed while withstanding the weight load. Herein, we report carbonate apatite honeycomb (HC) blocks with superior mechanical strength, osteoconductivity, and bioresorbability compared to a clinically used synthetic porous block (control block). Three types of HC blocks were fabricated via the debinding of HC green bodies at 600, 650, and 700 °C and subsequent phosphatization, designated as HC-600, HC-650, and HC-700, respectively. The macropores in these HC blocks uniaxially penetrated the blocks, whereas those in the control block were not interconnected. Consequently, the HC blocks exhibited higher open macroporosities (18%-20%) than the control block (2.3%). In contrast, the microporosity of the control block (46.4%) was higher than those of the HC blocks (19%-30%). The compressive strengths of the HC-600, HC-650, HC-700, and control blocks were 24.7, 43.7, 103.8, and 38.9 MPa, respectively. The HC and control blocks were implanted into load-bearing segmental bone defects of rabbit ulnae. Uniaxial HC macropores enabled faster bone ingrowth than the poorly interconnected macropores in the control block. Microporosity in the HC blocks affected bone formation and osteoclastic resorption over a period of 24 weeks. The resorption of HC-650 corresponded to new bone formation; therefore, new bone with strength equal to that of the original bone bridged the separated bones. Thus, the HC blocks achieved the reconstruction of segmental bone defects while withstanding the weight load. The findings of this study contribute to the design and development of synthetic bone blocks for reconstructing segmental defects.

Effects of Channels and Micropores in Honeycomb Scaffolds on the Reconstruction of Segmental Bone Defects.

The reconstruction of critical-sized segmental bone defects is a key challenge in orthopedics because of its intractability despite technological advancements. To overcome this challenge, scaffolds that promote rapid bone ingrowth and subsequent bone replacement are necessary. In this study, we fabricated three types of carbonate apatite honeycomb (HC) scaffolds with uniaxial channels bridging the stumps of a host bone. These HC scaffolds possessed different channel and micropore volumes. The HC scaffolds were implanted into the defects of rabbit ulnar shafts to evaluate the effects of channels and micropores on bone reconstruction. Four weeks postoperatively, the HC scaffolds with a larger channel volume promoted bone ingrowth compared to that with a larger micropore volume. In contrast, 12 weeks postoperatively, the HC scaffolds with a larger volume of the micropores rather than the channels promoted the scaffold resorption by osteoclasts and bone formation. Thus, the channels affected bone ingrowth in the early stage, and micropores affected scaffold resorption and bone formation in the middle stage. Furthermore, 12 weeks postoperatively, the HC scaffolds with large volumes of both channels and micropores formed a significantly larger amount of new bone than that attained using HC scaffolds with either large volume of channels or micropores, thereby bridging the host bone stumps. The findings of this study provide guidance for designing the pore structure of scaffolds.

Honeycomb Scaffold-Guided Bone Reconstruction of Critical-Sized Defects in Rabbit Ulnar Shafts.

Reconstruction of critical-sized defects (CSDs) in bone shafts remains a major challenge in orthopedics. Honeycomb (HC) scaffolds are considered promising as their uniaxial channels bridge the amputation stumps of bones and promote the ingrowth of bone and blood vessels (BV) into the scaffolds. In this study, the ability of the HC scaffolds, composed of the bone mineral or carbonate apatite (CAp), was evaluated by reconstructing 10, 15, and 20 mm segmental defects in the rabbit ulnar shaft. Radiographic and µ-computed tomography evaluations showed that bony calluses were formed around the scaffolds at 4 weeks post-surgery in all defects, whereas no callus bridged in the ulna without scaffolds. At 12 weeks post-surgery, the scaffolds were connected to the host bone in 10 and 15 mm defects, while a slight gap remained between the scaffold and host bone in the 20 mm defect. New bone formation and scaffold resorption progressed over 12 weeks. Histological evaluations showed that mature bones (MB) and BV were already formed at the edges of the scaffolds at 4 weeks post-surgery in 10, 15, and 20 mm defects. In the central region of the scaffold, in the 10 mm defect, MB and BV were formed at 4 weeks post-surgery. In the 15 mm defect, although BV were formed, a few MB were formed. It is concluded that CAp HC scaffolds have good potential value for the reconstruction of CSDs.