The current research presents a novel porous tibia implant design based on porous structure. The implant proximal portion was designed as a porous rhombic dodecahedron structure with 500 μm pore size. Finite element method (FEM) was used to assess the stem behavior under compressive loading compared to a solid stem model. CATIA V5R18 was used for modeling both rhombic dodecahedron and full solid models. Static structural analysis was carried out using ANSYS R18.1 to asses the implant designs. The results indicated enhanced clinical performance of tibial-knee implants compared to the solid titanium implant via increasing the maximum von-Mises stresses by 64% under the tibial tray in porous implant which reduce stress shielding. Also, the maximum shear stress developed in bone/implant interface was reduced by 68% combined with relieving the stress concentration under the stem tip to relieve patients' pain. Finally, porous implants provide cavities for bone ingrowth which improve implant fixation.

Background: Porous materials are recommended for orthopedic applications as they eliminate issues of interfacial instability with tissues and reduce mechanical mismatch of the young's modulus. Objective:The current research provides a finite element analysis (FEA) to investigate porous gyroid Ti6Al4V structure compared to a solid stem model for human tibial-knee implantation of total knee replacement (TKR). Material and Methods:In this study, the implant proximal portion was designed as porous gyroid Ti6Al4V structure with 500 µm pore size. CATIA V5R18 was used for modeling both gyroid and full solid models. Structural analysis was carried out using ANSYS R18.1 to evaluate the implant performance.Results: After gyroid implantation, the maximum von-Mises stress obtained under the tibial tray was increased to 10.081 MPa. Also, the maximum shear stress at the stem/bone interface was reduced to 0.7347 MPa. The stress concentration at the stem tip and the bone strain energy were also improved. The minimum factor of safety is 4.6 for the gyroid porous implant. A proof of concept model was additively manufactured successfully with pore size 577.7733 ± 34.762 µm. Conclusion:The results indicated enhanced clinical performance of the porous tibial-knee implant compared to the solid titanium implant via increasing the maximum von-Mises bone stresses and decreasing the maximum shear stress at the bone/ implant interface.

Stress shielding of the tibial component has remained one major cause of failure after total knee replacement surgery (TKR), which a relatively high stiffness of the prosthesis can cause most of the load to be transferred through the prosthesis rather than the bone. It leads to the resorption of the bone and a decrease in the bone strength and stiffness. The geometry of the tibial component has a strong effect on the bone compared to its material, so the present study investigates the optimum design of uncemented tibia tray to relieve the stress shielding by changing the dimensions of the metal tibial tray (shape optimization). The results of the optimization process for uncemented tibial component of TKR indicate a trend toward using a shorter cylindrical stem with a smaller diameter and longer metal tibial tray height to house the polymer insert compared with the initial design. This optimal shape model increases the maximum von-mises stress value five times the initial model on medial cancellous bone region. The maximum von-mises stress value of the optimal shape model is very close with that obtained using the natural bone with about 6.5% reduction in the maximum von-mises stress value on medial cancellous region. Also the maximum von-mises stress value of the optimal shape model is increased by 3% compared to the initial model on lateral cancellous bone region. Stress shielding is reduced related to increase stresses on medial and lateral regions. In addition, the maximum interface shear stress value on lateral region for the optimal shape model is decreased by 4% compared to the initial model. Aseptic loosening is decreased related to this reduction in shear stress on lateral side. This leads to reduce patient's pain and increases the implant life and stability.

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