Development of a 'Bioengineered block graft' for Maxillofacial Reconstruction

Background: The augmentation of large and complex alveolar bone defects remains a major challenge for clinical implant therapy. Although there have been developments in the field of bone substitute materials for dental implant reconstruction, there are still challenges to be met for large bone defects, compromised patients and severe alveolar ridge resorption. One of the major current limitations is the inability to provide a sufficient blood supply in the initial phase following implantation. Insufficient vascularisation can result in nutrient limitations or deprivation which leads to suboptimal integration and cell death in the graft materials. Growth, function, and survival post-implantation are entirely dependent on ingrowth of blood vessels from the host. Therefore, additional strategies that serve to enhance vascularisation are essential for the survival of large bone augmentation. Aim/Hypothesis: In vitro pre-vascularisation involves vascularising a construct using a cell-based strategy to induce in vitro vascularisation prior to implantation. The concept has stemmed from previous success in optimising the conditions for endothelial cells to achieve formation of prevascular structures in vitro. Given a more natural 3D environment, endothelial cells will organise spontaneously to form capillary-like structure that upon implantation, these capillary-like structures can successfully connect to the host vasculature and become functional perfused vessels. Subsequently they are able to accelerate functional anastomosis with host tissue upon implantation and eventually aid the survival of the implanted tissue. Materials and Methods: Synthetic porous block graft of ß metacalcium phosphate were produced by sintering monocalcium phosphate powder pressed with poly(vinyl alcohol) PVA as the porogen. Pore size and interconnectivity of scaffold was reconstructed and analysed using µ-CT. Primary human alveolar osteoblast cells (aHOB) were employed to investigate biocompaibility and biofunctionality of the novel graft material. A co-culture model with osteoprogenitor cells and endothelial cells (EC) was optimized and applied to exploit the potential of forming, an in vitro, 3D prevascular network inside the synthetic block graft in order to facilitate vascularization and enhance bone regeneration. Results: The novel block graft was observed to have macro and micro porosity, and interconnectivity was confirmed. The pore size range was 0-400µm with highest frequency pores between 80-100µm. Physico-chemical properties of the graft were investigated and also the biological responsiveness, specifically, biocompatibility and biofunctionality was evaluated. The result demonstratedthat in vitro prevascularised could be achieved; this vasculogenic process was influenced by type of cells that were cultured with endothelial cells and the additional of exogenous VEGF. When HUVECs were co-cultured with aHOB or HUVEC monoculture with addition of VEGF, they demonstrated the ability to form networks with the subsequent formation of tube like structures. The response seen in the absence of exogenous VEGF, is thought to be due to the integral relationship between ECs and osteoblasts offering support and cross talk (direct and indirect), thus enhancing the process. This effect was not observed with coculture with messenchymal stem cells (hBMSC). It is concluded that by providing the optimised coculture condition of endothelial cells and osteoblast into 3D porous construct could be a promising strategy to prevascularised block graft for cranio-maxillofacial reconstruction. Further investigation by translates this model to in vivo study will facilitate a greater understanding the benefit of prevascularised tissue engineered bone graft for reconstruction of critical bone defect prior to implant placement.