DESIGN OF FULLY POROUS FUNCTIONALLY GRADED TI-6AL-4V FEMORAL STEM FOR STRESS SHIELDING AND IMPLANT'S STABILITY

Abstract

The main objective of this study is to design titanium alloy femoral stems with cubic porous structures that will be able to reduce stress-shielding and promote stem stability. Stress-shielding is one of the factors that contributes toward aseptic loosening, which eventually leads to the failure of implants. These porous structure designs were accommodated into the titanium alloy femoral stem as homogeneous and functionally graded porous structures. First, the cubic cellular structures were simulated under compressive loading to measure the yield and modulus of elasticity for various porosity ranges. This allowed the selection of porosity range to design the femoral stems having stiffness in compressive loading identical to that of an intact femur bone. This was done to reduce the stress shielding effects. Based on the selected porosity range, fifteen different arrangements of radial geometrical functionally graded (FG) designs were developed with average porosities of 30, 50, and 70% respectively. The finite element models developed with physiological loads presenting three different walking speeds (1, 3, and 5 km/hrs.), where the average human body weight was assumed. Stresses at the bone Gruen zones were measured to check the percentage of stress transfer to the bone for each porous stem design and were compared with the bulk/dense stem. Micromotion for each design was measured to find the acceptable designs that enable the bone tissue ingrowth (stability of implant). It was found that stems with 70% average porosity had similar stiffness to the intact bone. Besides this, the functionally graded (FG) porous stems tend to transfer higher stress values to the bone compared to bulk/dense stems for all physiological loads associated with three studied walking speeds. Micromotion values increased as the porosity and physiological loads / walking speed increased, creating a constraint on the amount of porosity that can be introduced in the stem design. Finally, the Fatigue factor of safety of the designed stems was calculated at the studied walking speeds to find the appropriate designs for hip implants. Several FG stems designs were shortlisted as recommended candidates for hip implants.