Customizable Bone Constructs and Tunable Scaffolds for Craniofacial Tissue Engineering
Mikos, Antonios G
Doctor of Philosophy
The repair of large craniofacial defects remains a challenge, and repair with the clinical gold standard of a fibular flap can lead to donor site morbidity. In cases of tumor removal, growth factors may be contraindicated, and in trauma an infection may prevent immediate definitive reconstruction. A two-stage approach to craniofacial tissue engineering involves the implantation of a space maintainer within the defect site to optimize the site for receiving a customized bone graft grown elsewhere within the body adjacent to periosteum. The space maintainers can be loaded with antibiotics to clear infections and fabricated from novel polymers with varied mechanical properties to create a robust tissue pocket for transfer. In this dissertation, we sought to develop tissue engineering and biomaterials-based strategies for the reconstruction of large, complicated craniofacial defects. In the first specific aim, the two-stage strategy for mandibular repair is further challenged with two complex ovine models. We tested the ability of an antibiotic-loaded space maintainer to treat a mandibular infection, while simultaneously determining the effects of an untreated mandibular infection on the bone growth within the bioreactors. We showed that the antibiotic-loaded space maintainer was capable of clearing or preventing a Staphyloccus aureus mandibular infection, and that an untreated mandibular infection led to increased bone growth of more mature bone within the bioreactor chambers. We also showed that autograft-filled bioreactors led to increased new bone formation with more robust mechanical properties than commercially available cancellous bone chips. In another sheep model, we increased our defect size to the entire height of the mandible, creating a defect that was more exposed to mechanical forces than previously tested. Using fixation considered standard for a human with a similar defect, several animals experienced dehiscence and hardware failure. Radiographic analysis of the bioreactor tissue from implantation to transfer to integration within the mandible showed remodeling over time, but the tissue did not reach the same radiographic values as the unoperated contralateral side. Taken together, these two studies demonstrated that a space maintainer and bioreactor two-stage strategy is promising if fixation of the mandible is adequate to prevent hardware failure, micromotion, and mucosal dehiscence. For the second aim, we worked to commercialize our porous space maintainer through submissions to and interactions with the Food and Drug Administration (FDA). Our device was to proceed along the 510(k) pathway for a significant risk device, requiring an early feasibility study (EFS) and investigational device exemption (IDE) approval. To acquire the appropriate approvals, we developed protocols and created specimens for a battery of biocompatibility testing, proving that our device was cytocompatible, non-mutagenic, non-sensitizing, non-irritating, and non-toxic. We further worked with clinical collaborators to draft a clinical protocol for approval from the FDA to begin our EFS. Finally, the third specific aim involved the investigation of a class of novel polymers, synthesized from potentially antimicrobial monomers. These polymers were created from differing lengths of diols reacted with diacids to create polymers with tunable mechanical properties. We performed a main effects analysis on molecular weight, thermal characteristics, and mechanical properties to determine how they were impacted by the feed ratios of the polymers. We further expanded the tested ratios of the diacids and demonstrated that the mechanical properties could be varied predictably. However, the cytotoxicity also varied across groups, with high succinic acid incorporation leading to high levels of cell death. Although this platform was tunable, the effects of mechanical properties of the substrate and release of cytotoxic compounds on cell fate would be difficult to dissect. The overall goal of this thesis was to expand on the two-stage strategy of utilizing a space maintainer and bioreactor to repair craniofacial defects and to develop and investigate the applicability of a novel polymer system as a tunable tissue engineering platform. We demonstrated that antibiotic-loaded space maintainers are efficacious at clearing mandibular infection and that the two-stage approach to bone tissue engineering is promising even in larger, load-bearing defects. We worked to commercialize this technology and to create a polymer system with tunable mechanical properties. The translation of tissue engineering strategies has great potential to assist in patient treatment.
Bone tissue engineering; craniofacial defect repair; tunable scaffolds; large animal model