The Osteogenic Effects Of Porous Tantalum And Titanium Alloy Scaffolds With Different Unit Cell Structure

Critical bone defects,especially locating at the load-bearing portion of the body,are always formidable challenges.Bone repairing materials used to restore the normal function of critical bone defects are supposed to possess appropriate mechanical strength which can match that of natural bone tissues,so as to eliminate stress shielding effect and sustain the loading stress during the regeneraion of bone tissues.In addition,the biological merits of those materials are also regarded as key factors that can foster satisfactory osteointegration.As a bone impant material,titanium alloy has long been used in clinic due to its advantages including light weight,high strength and biocompatibility.On the other hand,tantalum is also an excellent implant material.Compared to titanium alloy,tantalum has higher surface energy and friction coefficient and lower microbial surface colonization rate.However,the elastic moduli of titanium alloy and tantalum are extremely higher than human bone tissues which can lead to stress shielding effects.And the mismatched elastic moduli often result in the aseptic loosening of impnats or the fragility fracture of the peripheral bone tissues.Processing the bulk titianium alloy and tantalum into porous scaffolds has been proved to be an ideal way to decrease their elastic moduli so as to match that of natural bone.Furthermore,the porous structure greatly facilitate the ingrowth of cells and bone tissues.Factors that can influence the mechanical and biological properties of porous scaffolds include pore size,porosity and unit cell structure.It is commonly accepted that pore size ranging from 300 to 500 μm and porosity ranging from 60 to 70% can satisfy the mechanical and biological demands of porous scaffold for bone repairing.Unit cell structure settles the arrangement of struts in three-dimensional space,and further influences the stress response mechanism of porous scaffolds.Based on their mechanical characteristics,unit cell structure can be divied into stretch-dominated structure and bending-dominated structure.Both diamond(Di)and rhombic dodecahedron(Do)unit cell structures are typical bending-dominated structure and commonly used in 3D printing.Finite element analysis has shown that the two unit cell structures had relatively low elastic moduli,and the compressive strength of Do structure was higher than Di structure.A numerical study has demonstrated that Di structure could provide abundant sites for the adhesion of cells and bone tissues;Do structure had better mass transport properties which facilitated the exchang of nutritions.However,the aforementioned results have not been verified via mechanical tests and biological experiments yet.And the impact of different unit cell structure on the mechanical and biological properties of porous scaffold are still equivocal.In order to clarify the influence of different unit cell structures and materials on the osteogenic effects of porous scaffolds,and to provide theoretical basis for the designing and manufacturing of bone repairing materials for load-bearing portion,a series of mechanical tests and biological experiments were then performend.Di and Do unit cell structures were used to design porous tantalum(Ta)and titanium alloy(Ti)scaffolds.Thereafter,Selective laser melting(SLM)technique was used to manufacture four groups of porous tantalum and titanium alloy scaffolds with different unit cell structure including tantalum-diamond(Ta Di)group,tantalumdodecahedron(Ta Do)group,titanium alloy-diamond(Ti Di)group and titanium alloydodecahedron(Ti Do)group.The elastic moduli and compressive strength of all the four groups can match that of huaman trabecular bone.And the Ti Do group showed the highest compressive strength among all the four groups.After the in vitro and in vivo experiments,the Ta Di group was proved to possess the most significant osteogenic effect and obtain superb osteointegration at early stage.On the other hand,Ta Do and Ti Do groups shared the similar osteogenic effects and both showed moderate osteointegration in the long term.In conclusion,different unit cell structure and material characteristics can influence the mechanical and biological properties of porous scaffolds.Among all the four kinds of porous tantalum and titanium scaffolds,Ta Di group possesses optimum osteogenic effect and moderate compressive strength and can be applied in the restoration of bone defects with inferior bone quality.Meanwhile,Ti Do group owns the highest compressive strength and medium osteogenic effect and can be applied in the portion of body with high demand of mechanical strength.