In vivo bone biocompatibility and degradation of porous fumarate-based polymer/alumoxane nanocomposites for bone tissue engineering

Abstract

The objective of this study was to determine how the incorporation of surface-modified alumoxane nanoparticles into a biodegradable fumarate-based polymer affects in vivo bone biocompatibility (characterized by direct bone contact and bone ingrowth) and in vivo degradability. Porous scaffolds were fabricated from four materials: poly(propylene fumarate)/propylene fumarate-diacrylate (PPF/PF-DA) polymer alone; a macrocomposite consisting of PPF/PF-DA polymer with boehmite microparticles; a nanocomposite composed of PPF/PF-DA polymer and mechanically reinforcing surface-modified alumoxane nanoparticles; and a low-molecular weight PPF polymer alone (tested as a degradation control). Scaffolds were implanted in the lateral femoral condyle of adult goats for 12 weeks and evaluated by micro-computed tomography and histological analysis. For all material groups, small amounts of bone, some soft tissue, and a few inflammatory elements were observed within the pores of scaffolds, though many pores remained empty or filled with fluid only. Direct contact between scaffolds and surrounding bone tissue was also observed in all scaffold types, though less commonly. Minimal in vivo degradation occurred during the 12 weeks of implantation in all materials except the degradation control. These results demonstrate that the incorporation of alumoxane nanoparticles into porous PPF/PF-DA scaffolds does not significantly alter in vivo bone biocompatibility or degradation.

Figure 1. Micro-CT analysis

A representative micro-CT reconstructed radiograph of a cross-section of a bone specimen with two implants within it. Bar is 5 mm.

Figure 2. Histological sections: Bone formation and inflammation in pores

Histological sections from the top region of a PPF/PF-DA polymer alone sample. a) Round and pink tissue is immature bone, light pink tissue layers are fibrous tissue, and dark blue areas suggest inflamed tissue. Bar is 1 mm. b) Magnified image shows rounded pink areas of bone formation (B), dark blue areas of inflamed tissue and patches of small blue inflammatory cells (IF). Bar is 100 µm.

Figure 2. Histological sections: Bone formation and inflammation in pores

Histological sections from the top region of a PPF/PF-DA polymer alone sample. a) Round and pink tissue is immature bone, light pink tissue layers are fibrous tissue, and dark blue areas suggest inflamed tissue. Bar is 1 mm. b) Magnified image shows rounded pink areas of bone formation (B), dark blue areas of inflamed tissue and patches of small blue inflammatory cells (IF). Bar is 100 µm.

Figure 3. Histological sections: Soft tissue infiltration and bone formation in pores

Histological section from the middle region of a PPF/PF-DA polymer alone sample. Rounded pink tissue is immature bone (B), light pink tissue layers are fibrous tissue (S), and blue clusters of cells are inflammatory cells (IF). An empty or fluid filled pore is also visible (E). Bar is 100 µm.

Figure 4. Histological scoring of bone growth within scaffold pores

Histological scoring of bone ingrowth for each material group based on Table 1. Data presented as mean ± standard deviation for n = 3 – 6. No statistically significant differences were observed between material groups or regions (p > 0.05).

Figure 5. Histological sections

a) Histological section of the middle region of a PPF/PF-DA polymer alone scaffold. The white scaffold is in direct contact with the surrounding bone tissue (pink). Bar is 100 µm. b) Histological section of the top region of a macrocomposite scaffold showing a thin fibrous capsule (FC) surrounding the scaffold. The porous surface of the scaffold created a pocket where inflammatory cells (IF) accumulated. Bar is 100 µm. c) Histological section of the top region of a macrocomposite scaffold showing disorganized tissue and inflammatory cells at the scaffold-tissue interface. Bar is 100 µm.

Figure 5. Histological sections

a) Histological section of the middle region of a PPF/PF-DA polymer alone scaffold. The white scaffold is in direct contact with the surrounding bone tissue (pink). Bar is 100 µm. b) Histological section of the top region of a macrocomposite scaffold showing a thin fibrous capsule (FC) surrounding the scaffold. The porous surface of the scaffold created a pocket where inflammatory cells (IF) accumulated. Bar is 100 µm. c) Histological section of the top region of a macrocomposite scaffold showing disorganized tissue and inflammatory cells at the scaffold-tissue interface. Bar is 100 µm.

Figure 5. Histological sections

a) Histological section of the middle region of a PPF/PF-DA polymer alone scaffold. The white scaffold is in direct contact with the surrounding bone tissue (pink). Bar is 100 µm. b) Histological section of the top region of a macrocomposite scaffold showing a thin fibrous capsule (FC) surrounding the scaffold. The porous surface of the scaffold created a pocket where inflammatory cells (IF) accumulated. Bar is 100 µm. c) Histological section of the top region of a macrocomposite scaffold showing disorganized tissue and inflammatory cells at the scaffold-tissue interface. Bar is 100 µm.

Figure 6. Histological scoring of bone contact at interface

Histological scoring of bone contact at the bone-scaffold interface for each material group based on Table 1. Data presented as mean ± standard deviation for n = 3 – 6. No statistically significant differences were observed between material groups or regions (p > 0.05).

Figure 7. Histological section of in vivo degradation

Histological section from the top region of a PPF degradation control scaffold. Upper left area shows breakdown of polymer into smaller fragments and soft tissue infiltration with minimal inflammation. Bar is 250 µm.

Figure 8. Histological scoring of degradation

Histological scoring of the in vivo degradation for each material group based on Table 1. Data presented as mean ± standard deviation for n = 3 – 6. The symbol “*” signifies a statistically significant difference compared to other materials within the same region (p < 0.05). The symbol “**” indicates a statistically significant difference of a material group compared with all other material groups with all region scores were combined (p < 0.05).