Study of cellular femoral stem for stress shielding and interface stability.

Implant failure is due to stress shielding and interface micromotion. The application of porous structures in the femoral implant has a great effect on reducing stress shielding and improving the stability of the bone-implant interface. The performance of femoral stems with triply periodic minimal surface (TPMS) structures, IWP, and Gyroid structures was evaluated using finite element analysis. We studied the stress shielding phenomenon of the porous femoral stem based on the ability of stress transfer to the femur. The micromotion at the bone-implant interface was explored for different porous femoral stems. The effect of gradient structure design was investigated in the axial direction of the stem. These gradient designs involved a stem with an increasing volume fraction in the axial direction (IAGS) and a decreasing volume fraction along the stem (DAGS). The results showed that the axial stiffness of the stem has a direct effect on stress shielding and an inverse relation to bone-implant micromotion. The finite element analysis results inferred that bone resorption is higher in the stems with IWP structure than in Gyroid at the same volume fraction. Axially graded stems transfer higher stress to the femur than homogenous porous stems. DAGS design of IWP and Gyroid and IAGS Gyroid increased the stress on the proximal-medial of the femur. Homogeneous porous stems with high porosity (80% porosity for IWP and 70% porosity for Gyroid) and DAGS design exhibited low stress shielding and controlled bone-implant interface micromotion within an acceptable range for bone ingrowth.

On the design and properties of porous femoral stems with adjustable stiffness gradient.

There is a large gap between the elastic modulus of the existing femoral stem and the host bone. This gap can lead to long-term complications, such as aseptic loosening and, eventually, a need for revision surgery. The porous metallic biomimetic femoral stem can effectively reduce stress shielding and provide firm implant fixation through bone ingrowth. The purpose of this research is to investigate the application of different porous femoral stems in relieving bone resorption and promoting osseointegration by finite element analysis. We present an intuitive visualization method based on a diamond lattice structure to understand the relationship between pore size, porosity, bone ingrowth criteria and additive manufacturing constraints. We further obtain an admissible design space of diamond lattice structure for porosity selection. We evaluate the relative micromotion of bone-implant interface and bone volume with density loss for three femoral stems with diamond lattice-based homogenous porous structures in admissible design space. We also evaluate porous femoral stems with four different grading orientations along the axial and radial directions of the femoral stem. These include an axial graded femoral stem with a porosity increased distally (DAGS), an axial graded femoral stem with a porosity increased proximally (PAGS), a radial graded femoral stem with a porosity increased inwardly (IRGS), and a radial graded femoral stem with a porosity increased externally (ERGS). The results indicate that: (i) homogenous porous femoral stems with 40% porosity, (ii) DAGS and (iii) IRGS can maintain the relative micromotion of the bone-implant interface in the safety range for bone ingrowth. The calculated volumes of bone with density loss in the cases of DAGS and IRGS are 3.6% and 3.3%, respectively, which are nearly 74% lower than that of fully dense stems. Therefore, DAGS and IRGS have an evident advantage in promoting osseointegration and relieving bone resorption. Thus, the graded femoral stem is more promising than the homogeneous porous stem.