C. R. Rathbone, T. Guda, B. M. Singleton, D. S. Oh, M. R. Appleford, J. L. Ong, J. C. Wenke
Highly porous hydroxyapatite (HA) scaffolds were developed as bone graft substitutes using a template coating process, characterized, and seeded with bone marrow-derived mesenchymal stem cells (BMSCs). To test the hypothesis that cell-seeded HA scaffolds improve bone regeneration, HA scaffolds without cell seeding (HA-empty), HA scaffolds with 1.5 × 104 BMSCs (HA-low), and HA scaffolds with 1.5 × 106 BMSCs (HA-high) were implanted in a 10-mm rabbit radius segmental defect model for 4 and 8 weeks. Three different fluorochromes were administered at 2, 4, and 6 weeks after implantation to identify differences in temporal bone growth patterns. It was observed from fluorescence histomorphometry analyses that an increased rate of bone infiltration occurred from 0 to 2 weeks (p < 0.05) of implantation for the HA-high group (2.9 ± 0.5 mm) as compared with HA-empty (1.8 ± 0.8 mm) and HA-low (1.3 ± 0.2 mm) groups. No significant differences in bone formation within the scaffold or callus formation was observed between all groups after 4 weeks, with a significant increase in bone regenerated for all groups from 4 to 8 weeks (28.4% across groups). Although there was no difference in bone formation within scaffolds, callus formation was significantly higher in HA-empty scaffolds (100.9 ± 14.1 mm3) when compared with HA-low (57.8 ± 7.3 mm3; p ≤ 0.003) and HA-high (69.2 ± 10.4 mm3; p ≤ 0.02) after 8 weeks. These data highlight the need for a better understanding of the parameters critical to the success of cell-seeded HA scaffolds for bone regeneration.