The Effects Of Material And Mechanics On Osteoinduction Of Calcium Phosphate Ceramic

Material properties playing important roles in the osteoinductive potential of calcium phosphate ceramic bone grafts include chemical composition, overall geometry, and porosity.The species of animal model and site of implantation also affect the osteoinduction of materials.Numerous studies have investigated possible mechanisms underlying osteoinduction by calcium phosphate ceramics, but to date mechanistic understanding of this phenomenon remains insufficient, limiting clinical use of these materials in bone defect regeneration.This thesis studied two factors that potentially affect osteoinduction of calcium phosphate ceramics: materials structure properties (e.g., shape and size of macro-pores, surface microstructure) and stress stimulation. Given the key influence of stress stimulation on bone formation,development,and homeostasis in the physiological environment,this thesis explored the possible osteoinductive mechanism between physiological environment, scaffold structure properties, and mechanical stimuli developed at the cellular level based on the theory of biomechanical regulation. Our study revealed a novel pathway to understand the correlation between the characteristics of calcium phosphate ceramics and the ability to induce bone formation. Additionally,results of this study provided experiment and theoretical bases toward optimal fabrication of calcium phosphate ceramics with controlled osteoinduction.Specifically, the following works were performed in this thesis:1) Two kinds of hydroxyapatite (HA) scaffolds with different macro-pore geometries and size distributions were fabricated and implanted into canine dorsal muscle and abdominal cavity to compare the effect of macro-pore properties on heterotopic bone formation. Possible transition pathways linking material macrostructure to osteoinductive regulatory signals were investigated by developing calculation models for 3D scaffolds and analyzing the micro-fluid dynamic environment by computational fluid dynamics (CFD) in scaffolds. The results showed that macro-pore size and geometrie had significant effects on osteoinduction of HA porous scaffolds, and both structure variates had the combined impact on osteoinduction of scaffolds. CFD simulation showed that micro-fluid dynamic environment influenced bone formation and distribution of scaffolds. Our study suggested that micro-fluid dynamic environment act as a transition pathway linking materials macrostructure properties to signals regulating osteoinduction of these materials.2) HA spheres with a range of surface microstructure (rough, specific surface area, micro-porosity) were prepared by a sol-gel route at various HA/chitin ratios. The results of cell cultures with HA spheres in vitro showed that materials surface microstructure regulated the biological behavior of bone marrow derived mesenchymal stem cells (BM-MSCs). The rough surface with abundant micro-pores favored cell proliferation at the early stage and stimulated osteogenic differentiation at the later stage. In comparison, the smooth surface with few micro-pores accelerated cell osteogenic differentiation at the early stage. In general, the rough surface with abundant micro-pores produced more positive effects on BM-MSC proliferation and differentiation, although all HA spheres exhibited good biocompatibility regardless of the surface microstructure. In vivo implantation found that scaffolds (HASAs) prepared by accumulation of HA spheres bearing different surface microstructure were compatible with the host physiological environment. Furthermore, the rough surface was more conducive to heterotopic bone formation.3) Physiological micro-vibration stress environments were constructed in vitro for evaluating the effects of micro-vibration stresses (amplitude≤ 50μm, magnitude< 1 ×g, frequency range from 1-100Hz,MV) on osteogenic differentiation and extracellular matrix mineralization of BM-MSCs cultured on HA porous scaffolds, allowing exploration of the effect of MV on the bioactivity and mechanical property of HA scaffolds, and of the relationship between MV and the osteoinduction of HA scaffolds. The results showed that MV stress environment improved the biological mineralization of HA scaffolds, contributing to an osteogenic microenvironment;MV acted as a mechanical stimulus beneficial for BM-MSCs osteogenetic differentiation on 3D porous scaffolds; MV and porous HA scaffolds functioned synergistically in regulating the expression of osteogenesis-related genes in BM-MSCs (i.e., Cbfal/Runx2, Col-I, ALP, OC),and the expression of bone characteristic morphogenic protein, ALP.4) To explore the use of the abdominal cavity as a candidate autologous bioreactor for meeting different requirements of engineering bone grafts (e.g., shape, size, volume) for repairing segment load-bearing bone defect, HA sphere-accumulated scaffolds (HASAs) were implanted in canine abdominal cavities to construct tissue-engineered bone grafts. These grafts were harvested and applied to reconstruct autologous femoral defects in dogs. Our study showed that despite limited blood supply, low stress, and lack of stem cells in the abdominal cavity, there exists a possibility of using the abdominal cavity as autologous bioreactor to construct tissue engineered bone grafts. The derived tissue engineered bone grafts were successfully applied to repair defects in the animals.