INTRODUCTION: β-tricalcium phosphate (β-TCP), one of the most common synthetic bone replacement products, is frequently used in pediatric craniofacial reconstruction. Although solid β-TCP can be absorbed over time, a relatively slow degradation rate predisposes this product to exposure, infection, and fracture. Our tissue engineering laboratory has successfully leveraged three-dimensional (3D) printers to manufacture 3D-printed bioactive ceramic scaffolds composed of β-TCP in an architecture that optimizes the needs of rigidity with efficient vascular ingrowth, osteogenesis, and degradation kinetics, which are further optimized when using the osteogenic agent dipyridamole. This long-term animal study of immature rabbits through the time of facial maturity reports on the new degradation kinetics profile achievable through this novel manufacturing and tissue engineering protocol.
METHODS: Twenty-two 1-month-old (immature) New Zealand White rabbits underwent creation of unilateral 10-mm calvarial defects with ipsilateral 3.5 × 3.5 mm alveolar defects. Each defect was repaired with 3D-printed bioactive ceramic scaffolds composed of 100% β-TCP and coated with 1,000 µM dipyridamole. Rabbits were sacrificed at 2 months (n = 6), 6 months (n = 8), and 18 months (n = 8). Bone regeneration and scaffold degradation were calculated using micro-CT images reconstructed in Amira software. Bone density and mechanical properties at 18 months was compared with native uninjured bone using Amira software and nanoindentation, respectively. Cranial and maxillary suture patency and bone growth were qualitatively analyzed using histology.
RESULTS: Results of 3D reconstruction are reported as a percentage of volumetric space occupied by either scaffold or bone. When comparing time points 2, 6, and 18 months, scaffolds showed significantly decreased in vivo defect occupancy in calvaria (23.6% ± 2.5%, 15.2% ± 2.2%, and 5.1% ± 2.2%; P < 0.001) and in alveoli (21.5% ± 2.2%, 6.7% ± 1.9%, and 0.2% ± 1.9%; P < 0.001), with annual degradation rates 54.6% and 90.3%, respectively. Between 2 and 18 months, significantly more bone regenerated in calvarial defects (25.8% ± 7.9% versus 55.7% ± 6.9%; P < 0.001) but was similar to native bone density (46.7% ± 6.8%; P = 0.06), and no difference was found in alveolar defects over time (28.4% ± 8.2% versus 31.4% ± 7.1%; P = 0.57) and compared to native bone (33.8% ± 3.7%; P = 0.34). Regenerated elastic modulus (E) and hardness (H) were similar to native bone in calvaria (E: 12.6 ± 1.8 GPa versus 13.2 ± 1.8 GPa, P = 0.62; H: 0.54 ± 0.06 GPa versus 0.53 ± 0.06 GPa, P = 0.81) and alveoli (E: 11.7 ± 1.5 GPa versus 11.3 ± 1.5 GPa, P = 0.71; H: 0.64 ± 0.05 GPa versus 0.66 ± 0.05 GPa, P = 0.70). Histology revealed vascularized and organized bone without suture fusion.
DISCUSSION: The degradation kinetics of β-TCP can be altered through 3D printing and addition of an osteogenic agent. Our study demonstrates an acceleration of β-TCP degradation from 1% to 3% a year to 55% to 90% a year. Absorbed β-TCP is replaced by vascularized, organized bone, with histologic and mechanical properties similar to native bone and without damage noted to the growing suture. This additive manufacturing and tissue engineering protocol has implication to future reconstruction of the craniofacial skeleton, especially as a safe and efficacious method in pediatric bone tissue engineering.