Material parameters identification of 3D printed titanium alloy prosthesis stem based on response surface method.

3D printed Titanium alloy is widely used as a material of artificial joints and its mechanical properties is a key factor for improving operation results. Because the elastic modulus of the 3 D printed titanium alloy specimen was related to the size of the metal blank. It is very difficult to identify mechanical parameters by traditional mechanics experiments. In this paper, according to the inverse analysis principle of the parameter estimation, a response surface methodology (RSM) was proposed to identify the mechanical parameters, based on finite element inverse analysis. The finite element models of femoral prosthesis stem were established in line with compression experiments. The material parameters were combined by central composite design (CCD), and the response surface (RS) models were constructed using a quadratic polynomial with cross terms and optimized using a genetic algorithm (GA). Finally, the best mechanical parameter combination of the femoral prosthesis was calculated. The calculated elastic modulus and Poisson's ratio of a 3 D printed titanium alloy femoral prosthesis stem were 109.07 GPa and 0.29, respectively, with the elastic modulus error being very small. The proposed method is effective and can be extended for the identification of mechanical parameters in other 3 D printed models.

Mechanical behavior of a titanium alloy scaffold mimicking trabecular structure.

BACKGROUND: Additively manufactured porous metallic structures have recently received great attention for bone implant applications. The morphological characteristics and mechanical behavior of 3D printed titanium alloy trabecular structure will affect the effects of artificial prosthesis replacement. However, the mechanical behavior of titanium alloy trabecular structure at present clinical usage still is lack of in-depth study from design to manufacture as well as from structure to mechanical function. METHODS: A unit cell of titanium alloy was designed to mimick trabecular structure. The controlled microarchitecture refers to a repeating array of unit-cells, composed of titanium alloy, which make up the scaffold structure. Five kinds of unit cell mimicking trabecular structure with different pore sizes and porosity were obtained by modifying the strut sizes of the cell and scaling the cell as a whole. The titanium alloy trabecular structure was fabricated by 3D printing based on Electron Beam Melting (EBM). The paper characterized the difference between the designs and fabrication of trabecular structures, as well as mechanical properties and the progressive collapse behavior and failure mechanism of the scaffold. RESULTS: The actual porosities of the EBM-produced bone trabeculae are lower than the designed, and the load capacity of a bearing is related to the porosity of the structure. The larger the porosity of the structure, the smaller the stiffness and the worse the load capacity is. The fracture interface of the trabecular structure under compression is at an angle of 45o with respect to the compressive axis direction, which conforms to Tresca yield criterion. The trabeculae-mimicked unit cell is anisotropy. Under quasi-static loading, loading speed has no effect on mechanical performance of bone trabecular specimens. There is no difference of the mechanical performance at various orientations and sites in metallic workspace. The elastic modulus of the scaffold decreases by 96%-93% and strength reduction 96%-91%, compared with titanium alloy dense metals structure. The apparent elastic modulus of the unit-cell-repeated scaffold is 0.39-0.618 GPa, which is close to that of natural bone and stress shielding can be reduced. CONCLUSION: We have systematically studied the structural design, fabrication and mechanical behavior of a 3D printed titanium alloy scaffold mimicking trabecula bone. This study will be benefit of the application of prostheses with proper structures and functions.