Bone Formation on the Apatite-coated Zirconia Porous Scaffolds within a Rabbit Calvarial Defect

Abstract

Previously, a strong and bioactive ceramic scaffold consisting of a porous zirconia body coated with apatite double layers (fluorapatite (FA) as an inner layer and hydroxyapatite (HA) as an outer layer) was successfully fabricated. In this contribution, the authors investigate the in vivo performance of the engineered bioceramic scaffolds using a rabbit calvarial defect model. In particular, the porosity and pore size of the scaffolds are varied in order to observe the geometrical effects of the scaffolds on their bone formation behaviors. The scaffolds supported on a zirconia framework can be produced with an extremely high porosity (~84—87%), while retaining excellent compressive strength (~7—8 MPa), which has been unachievable in the case of pure apatite scaffolds (~74% porosity with ~2MPa strength).

The experimental groups used in this study include three types of zirconia scaffolds coated with apatite; high porosity (~87%) with large pore size (~500— 700 μm): AZ-HL, high porosity (~84%) with small pore size (~150—200 μm): AZ-HS, and low porosity (~75%) with large pore size (~500—700 μm): AZ-LL, as well as one type of HA porous scaffold: low porosity (~74%) with a large pore size (~500—700 μm) for the purpose of comparison. The scaffolds prepared with dimensions of ~ 10 mm (diameter) × 1.2 mm (thickness) are grafted in rabbit calvaria defects. The histological sections are made at 4 and 12 weeks after surgery and immunohistochemical analyses are performed on the samples.

All of the specimens show a good healing response without adverse tissue reactions. Good healing is shown at 4 weeks post-surgery with the ingrowth of new bone into the macropore-channels of the scaffolds. The newly formed bone amounts to ~19.9—24.2% of the initial defect area, depending on the scaffold type, but there is no statistical significance between the scaffold groups. However, the defects without the scaffolds (control group) show a significantly lower bone formation ratio (~4.3%). At twelve weeks after surgery, the extent of new bone formation is more pronounced in all of the scaffold groups. All of the scaffold groups show significantly higher bone formation ratios (26.7—46.9%) with respect to the control without the graft. In the comparison between the scaffold groups, those with high porosities (AZ-HL and AZ-HS) exhibit significantly higher bone formation as compared to the scaffold with low porosity (AZ-LL).

Based on the present in vivo test performed within a rabbit calvaria defect model, it is concluded that the apatite-coated zirconia scaffolds show good bone forming ability and are considered to be a promising scaffolding material for bone regeneration since they possess a high level of both mechanical and biological properties.