4: Bone Tissue Engineering in the Growing Skull: A Short-term Study Using Dipyridamole-loaded 3D-printed Beta-tricalcium Phosphate Scaffolds to Repair Critically Sized Calvarial Defects in a Translational Animal Model

PURPOSE: The repair of bony defects remains a challenge to reconstructive surgeons due to limitations of current bone replacement techniques. Previous studies have shown that dipyridamole-loaded 3D-printed bioceramic (DIPY-3DPBC) scaffolds composed of 100% beta-tricalcium phosphate regenerate bone across critically sized defects in multiple in vivo models. However, a critical step to translating this bone tissue engineering construct to bedside is its successful application in a pre-clinical translational model that mimics biologic similarities to humans. Pigs are considered to be an excellent species for bone tissue engineering studies due to their similarities to humans both in bone biology and wound healing. In this short-term study, our group implanted DIPY-3DPBC scaffolds in an effort to repair critically sized bony defects in a translational, skeletally immature, growing craniofacial pig model. The purpose was to evaluate the performance of these 3D-printed constructs in terms of bone growth and scaffold absorption. METHODS: Unilateral calvarial defects were created in six-week-old Göttingen minipigs (n=12). Four defects were left empty to serve as negative controls, four defects were filled with a 1000μM DIPY-3DPBC scaffold with a closed scaffold design (contains a solid barrier on the ectocortical side of the scaffold to prevent soft tissue infiltration), and four defects were filled with a 1000μM DIPY-3DPBC scaffold with an open design (no solid barrier on the ectocortical side). Animals were euthanized at 12-weeks post-operatively. Calvaria were subjected to micro-computed tomography (CT) for volumetric (3D) analysis. 3D reconstruction was performed using Amira software to quantitatively assess percent bone volume occupying the defect space (bone volume/total volume x 100%) and percent scaffold volume occupying the defect space (scaffold volume/total volume x 100%). A generalized linear mixed model (GLMM) was used to determine significance between groups. Results are presented using means with corresponding 95% confidence intervals and p-values. RESULTS: Significantly higher mean (± 95% CI) percent bone volume was observed in calvarial defects treated with the scaffolds compared to negative controls (42.6% ± 8.6% vs 4.6% ± 12.1%, p≤0.001). Defects that were filled with closed cap scaffolds had a significantly greater mean percent bone volume compared to defects filled with the open scaffolds (52.9% ± 12.1% vs 32.3% ± 12.1%, p=0.024). No significant difference was detected between percent scaffold volume between closed and open scaffolds (4.4% ± 1.6% vs 3.7% ± 1.6%, p=0.493). CONCLUSIONS: The results from this work indicate an effective bone tissue engineering scaffold design that is able to regenerate bone across critically sized calvarial defects in a short-term skeletally immature, growing translational pig model.