Cervical Fusion Cage Lattice Structure Optimization using Computational Biomechanics

Abstract

Implants are instruments that typically inserted into a host tissue to restore any damaged physical function. In the case of a spinal implant, it usually consists of a spinal cage, pedicle screw and spinal rod, which they act together as a medical device that is implemented in the surgical treatment of patients with spinal diseases. Titanium alloy such as Ti- 6Al- 4V is a biomaterial that commonly used in the spinal implants. However, the alloy is non-degradable and may cause stress shielding effect. Hence, magnesium alloy such as ZK60 is used as a substitute for the Ti-6Al-4V due to its biodegradable and bioabsorbable characteristics. The study aims to optimize the spinal cages of Ti-6Al-4V and ZK60 by incorporating lattice alteration to the implants and to evaluate the stress distribution of optimized spinal cages under sitting condition. The spinal cages of Ti-6Al-4V and ZK60 were optimized using nTop software with Diamond lattice structures, which can provide great cell growth rates and high energy absorption capacity compared to the other lattice structure types. After that, the finite element analysis (FEA) model of the solid and optimized spinal cages were established to evaluate their stress distribution and total deformation under sitting condition. The stress in the current study showed comparable trend with the previous biomechanical and finite element analysis (FEA) study. Both current and previous study showed lower maximum stress in optimized group of spinal cages compared to the solid group, and lower maximum stress in ZK60 group than the Ti-6Al-4V group. The maximum von Mises stress in solid cages decreased by 25.63% and 26.32% for Ti- 6Al-4V and ZK60, respectively, after the lattice optimization. As for the total deformation, a larger deformation occurred in the optimized group of spinal cages than that of solid group. Maximum deformation of optimized ZK60 spinal cage was 2.09×10-6 mm while the maximum deformation of solid ZK60 was 1.86×10-6 mm. In summary, the lower maximum stress value of ZK60 in the findings showed better mechanical properties of ZK60, as the magnesium alloy has elastic modulus value that is much closer to the value of normal bone tissue than Ti-6Al-4V. Besides, the findings showed that the optimized group of spinal cages further reduced their stiffness with the application of porous structure, based on the greater total deformation in the cages.