Effect Of 3D Printing Of Titanium Alloy Scaffolds With Different Pore Shapes On Bone Growth

Objective:3D printing pore size and porosity of porous scaffold for plays an vital role in bone regeneration In addition,pore geometry also affects bone ingrowth.But so far the best pore geometry for bone ingrowth is unclear.This study explored the effects of 3D printed titanium alloy scaffolds with four different geometrical pores on bone growth in vivo,and provided parameters for the pore structure design of bone scaffolds in the future.Methods:Four kinds of common porous scaffolds with porosity of 65%and pore size of 650μm were designed using Unigraphics NX（DIA,Diamond unit cell;TC,Tetrahedron cell;CU,Cube;CIR,Circle）.Four kinds of porous titanium alloy scaffolds with similar porosity（65%）and pore size（650μm）and different pore geometry were prepared by Selective laser melting（SLM）technique.The compressive strength and elastic modulus of four kinds of 3D printed titanium alloy scaffolds with different geometrical pores were tested by mechanical experiments using universal testing machine of materials.In addition,through Computational fluid dynamics（CFD）analysis,the permeability velocity and flow trajectory inside the scaffold structure were calculated to simulate the flow of blood inside the scaffold.The structure of the porous titanium alloy scaffold was analyzed by micro-computed tomography（micro-CT）,and the three-dimensional reconstruction of the scaffold was performed by using Mimics21.0 software.The parameters such as pore size,strut size,porosity and surface area of the scaffold were measured.Four kinds of 3D printed titanium alloy scaffolds with different geometric shapes were implanted into the distal femur of rabbits to evaluate the growth of bone tissue in vivo.The rabbits were killed 6 weeks and 12 weeks after surgery,and the samples were collected.Micro-CT analysis and hard tissue sections were performed to detect the bone growth of titanium alloy scaffolds with different geometrical pores.Results:The 3D printed titanium alloy scaffolds manufactured by selective laser melting（SLM）have good consistency with the 3D CT reconstruction model.The pore size and strut size are controlled uniformly and well.The porous titanium alloy scaffolds with four different geometrical pores have good mechanical properties and can be used as bone tissue scaffolds.At 6 and 12 weeks,the size of new bone mass was DIA,CU,TC and CIR in descending order.BV/TV（Bone volume/Total volume）of DIA was 15.2%and 23.1%,BV/TV of CU was 13.7%and 19.1%,BV/TV of TC was 11.4%and 18.3%,and BV/TV of CIR was 9.8%and 16.9%,respectively.The BV/TV of DIA at different time points was compared with that of CU,Tc and CIR,and the difference was statistically significant（p<0.05）.The hard tissue sections at different time points showed the formation of bone tissue in the scaffolds and increased with the time of implantation of the scaffolds.The amount of bone tissue in the pore of scaffold increased gradually,the highest amount of new bone was DIA,and the lowest amount was CIR,which was consistent with the results obtained by micro-CT.The permeability of the four structures in this paper ranged from 3.8×10^-9m² to 5.9×10^-9m²,which all met the needs of bone tissue implantation.The velocity nephograms of the four scaffolds showed that the fluid velocity on the side of the DIA structure was significantly greater than the fluid velocity inside the scaffolds.This indicates that the DIA structure can accelerate the flow of fluid inside the scaffold structure and facilitate the flow of fluid to more areas of the scaffold.The velocity flow diagram of the four kinds of support structures shows that the velocity flow lines of TC,CIR and Cu are relatively simple and flow through fewer areas inside the structure.The fluid flow rate within the DIA structure is lower than that of the other three structures,and the lower velocity allows cells to attach more easily to the surface of the scaffold and to grow bone more easily in the body.The velocity of the other three structures at the junction of the struts is obviously higher than that at the pore.The faster velocity can promote the migration of cells to deeper depths,but it is not conducive to the adhesion of cells to the surface of the struts.Conclusion:Selective laser melting（SLM）can print bone scaffolds with high strength and low modulus,which has a good application prospect in orthopedics.The pore geometry has a significant effect on bone growth.DIA structure has the best effect among the four kinds of porous titanium alloy scaffolds with different geometric shapes.Through computational fluid dynamics（CFD）analysis,the permeability velocity and flow trajectory inside the scaffold were calculated.The results show that the DIA structure has the smallest fluid velocity difference inside the scaffold and the longest fluid flow trajectory inside the scaffold,which is conducive to bone growth.In this study,computational fluid dynamics analysis and in vivo bone growth experiments were combined to find that the pore geometry of porous scaffolds had a significant effect on bone growth,which provided parameters for further research on 3D printed porous bone scaffolds applicable to human body.