Successful Bone Healing of a Critical-Sized and Immediately Loaded Rabbit Mandibular Segmental Defect Using a Thin Hydroxyapatite-Coated Titanium Fiber Mesh Scaffold

Objective: Segmental bone defect in the mandible is a most difficult defect to repair in bone reconstructive/regenerative therapy because the mandibular bone constantly moves and is loaded immediately after surgery. Vascularized free bone flap is the most reliable treatment option; however, the procedure involves a long operation time and significant morbidity related to bone harvesting. Coating biomaterials with a thin hydroxyapatite (HA) was proven effective in enhancing bone compatibility. We developed submicron-thin HA-coated titanium fiber mesh scaffolds to reconstruct immediately loaded segmental mandibular defects and evaluated their bone compatibility in vivo.

Materials and Methods:Ti fiber mesh scaffolds with a porosity of 87% were prepared from Ti fibers 20 mm in diameter. Cubic three-dimensional (3D) scaffolds (10 x 10 mm, 5 mm thick) containing pores with a mean size of 95 mm (range 80-110 mm) were manufactured. The HA coating of Ti fiber mesh scaffolds was performed according to the molecular precursor method without affecting 3D structure of the scaffold. A 10-mm long segmental bone defect in a rabbit mandibular bone was reconstructed with non- or HA-coated scaffolds, which were removed at 9 and 21 weeks, to evaluate the survival rate of the scaffold, the mechanical strength of the bone-scaffold connection, and the bone formation around the scaffold. X-rays were taken for each sample to reveal the new bone formation using a micro-CT machine to analyze the length and the volume of newly formed bone out- and inside the scaffold. The mechanical strength of the integration between the Ti fiber mesh scaffold and mandibular bone was evaluated using the threepoint bending test. After mechanical strength evaluation, histological analysis was performed to analyze the ratio of the newly formed bone area to the scaffold as the bone formation ratio (BFR). Fisher's exact test was used in survival analysis, and ManneWhitney U-test was also used to examine differences between the non- and HA-coated groups. A P-value of <0.05 was considered statistically significant.

Results: The survival rates of connections in non- and HA-coated scaffolds at 21weeks were 58.3% and 88.9%, respectively. The survival rate of HA-coated samples was significantly higher than that of non-coated samples. In vivobone formation in HA-coated scaffolds was greater than that in non-coated scaffolds at 21 weeks. Newly formed bone in HA-coated scaffolds mostly restored bone continuity. Scanning electron microscopy identified strong integration of the bone and HA-coated scaffolds. The mechanical strength of the bone-scaffold connection was 3-fold greater in HA-coated scaffolds than that in non-coated scaffolds. Histologically, BFR of HA-coated scaffolds was significantly higher than that of non-coated scaffolds.

Conclusion: The HA-coating enabled Ti fiber mesh scaffolds to firmly connect with the bone. This enhanced osteoconductivity rapidly creates a bone- Ti fiber mesh scaffolds complex with rigidity and resistance to mechanical stress. The long-term stability of the osseointegrated scaffolds enabled successfull restoration of the segmental defect with newly formed bone even under conditions of immediately loaded critical-sized bone defect of the mandible.

References:

1. Hirota M, Shima T, Sato I, Ozawa T, Iwai T, Ametani A, Sato M, Noishiki Y, Ogawa T, Hayakawa T, Tohnai I. Development of a biointegrated mandibular reconstruction device consisting of bone compatible titanium. Biomaterials 75: 223-236, 2016.
2. Hirota M, Hayakawa T, Shima T, Ametani A, Tohnai I: High porous titanium scaffolds showed higher compatibility than lower porous beta-tricalcium phosphate scaffolds for regulating human osteoblast and osteoclast differentiation. Materials Science and Engineering C 49: 623-631, 2015.
3. Hirota M, Hayakawa T, Ametani T, Kuboki Y, Sato M, Tohani I. The effect of hydroxyapatite-coated titanium fiber web on human osteoblast functional activity. International Journal of Oral and Maxillofacial Implants 26: 245-250, 2011.