Understanding the Effects of Dopants on Calcium Phosphate Ceramics: Bone Cell Differentiation and Bone Growth in vitro and in vivo

Gary Allen Fielding

The objective of this research is to further develop the understanding of trace elements in bone biology and, specifically, their potential osteogenic effects in calcium phosphate bone substitute ceramics. Trace elements including magnesium (Mg2+), silicon (Si4+), strontium (Sr2+) and zinc (Zn2+) were incorporated into β tricalcium phosphate (β -TCP) as dopants into various form factors including dense ceramic compacts, 3D printed scaffolds and porous scaffolds fabricated by an oil emulsion method. The hypothesis of this research is that the addition of these trace element dopants into β -TCP can significantly alter its phase stability, microstructure, mechanical strength and in vitro and in vivo biocompatibility. 3D printed scaffolds containing 0.5% SiO2 and 0.25% ZnO increased the average density of pure TCP from 90.8 ± 0.8% to 94.1 ± 1.6% which resulted in an average 2.5 fold increase in compressive strength. Scaffolds that contained the smallest amount of designed porosity had compressive strengths of 5.48 ± 0.04 MPa and 10.21 ± 0.33 for pure and doped scaffolds, respectively. Doped samples demonstrated increased cellular proliferation. In vivo results in a murine model demonstrated that the presence of SiO2 and ZnO increased osteocalcin and collagen production as well as increased osteogenesis and angiogenesis over the course of 16 weeks. Further studies examining 1% SrO, 1%MgO, 0.5% SiO2 and 0.25% ZnO single dopant TCP systems on osteoblastic differentiation markers alkaline phosphatase (ALP) and runt related transcription factor 2 (Runx2) demonstrated the ability of dopants to increase cellular proliferation in the early stages of the osteoblastic lifecycle, while down regulating the Runx2 expression at later time points, allowing for faster terminal differentiation. SiO2 and ZnO dopants were also analyzed using quantitative polymerase chain reactions for the following targets in osteoblast cells: bone morphogenic protein 2 (BMP2), Runx2, receptor activator of nuclear factor kappa-B ligand (RANKL) and osteoprotegerin. At day 21, all doped samples expressed 2-4 times less BMP2, 1.5-2.5 times greater OPG and 2-4 times less RANKL when compared to pure TCP. Mg and Sr doped samples expressed 2.5 times more Runx2, Si 1.5 times more and zinc similar amounts of Runx2 to pure TCP. These results demonstrate an affinity for the production of signaling molecules that favor increased osteoblastogenesis and decreased osteoclastogenesis.

Understanding the Effects of Dopants on Calcium Phosphate Ceramics: Bone Cell Differentiation and Bone Growth in vitro and in vivo

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