Abstract
The current study is proposing a design envelope for porous Ti-6Al-4V alloy femoral stems to survive under fatigue loads. Numerical computational analysis of these stems with a body-centered-cube (BCC) structure is conducted in ABAQUS. Femoral stems without shell and with various outer dense shell thicknesses (0.5, 1.0, 1.5, and 2 mm) and inner cores (porosities of 90, 77, 63, 47, 30, and 18%) are analyzed. A design space (envelope) is derived by using stem stiffnesses close to that of the femur bone, maximum fatigue stresses of 0.3σys in the porous part, and endurance limits of the dense part of the stems. The Soderberg approach is successfully employed to compute the factor of safety Nf > 1.1. Fully porous stems without dense shells are concluded to fail under fatigue load. It is thus safe to use the porous stems with a shell thickness of 1.5 and 2 mm for all porosities (18–90%), 1 mm shell with 18 and 30% porosities, and 0.5 mm shell with 18% porosity. The reduction in stress shielding was achieved by 28%. Porous stems incorporated BCC structures with dense shells and beads were successfully printed.
Highlights
Cementless femoral stems are traditionally used for total hip replacement (THR)
Several designs of porous and semi-porous stems were suggested after performing static tests there is a paucity of studies that focus on the fatigue limit of porous stems in a manner that significantly differs from that of dense Ti-6Al-4V alloy
To predict the fatigue limit of BCC porous cellular structures, the stresses were obtained from 20–100% of the yield strength
Summary
Cementless femoral stems are traditionally used for total hip replacement (THR). The stems are inserted proximally in the medullary canal of the femur and firmly attached to the bone to transfer the physiological load to the bone during daily activities. Ti-6Al-4V alloy (Ti) is proven as an excellent biomaterial [1,2,3,4], that exhibits an elastic modulus of 114 GPa, which is ~6–7 times stiffer than the femur This higher stiffnesses of cementless dense Ti-6Al-4V alloy stems cause several complications such as poor bone ingrowth, stress shielding, risk of bone fracture, and even revision of surgery. Few studies have introduced new designs of porous and semi-porous femoral stems to reduce the stiffness of the stems [1, 3, 24, 25]. Several designs of porous and semi-porous stems were suggested after performing static tests there is a paucity of studies that focus on the fatigue limit of porous stems in a manner that significantly differs from that of dense Ti-6Al-4V alloy. Reducing stiffness through porous structures is a desired feature, the challenge is to ensure the design has a good fatigue limit (>5 × 106 cycles) [29]
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