The World Health Organization reports that back pain, especially lower back pain, is the leading cause of disability worldwide, affecting over 600 million people in 2020. While most cases are treated with physiotherapy and anti-inflammatory medication, some patients may opt for spinal fusion surgery. The intervertebral cages currently used in these surgeries pose significant risks of complications, such as loss of fixation, migration, and implant failure. Additive manufacturing (AM), also known as 3D printing, is an emerging technology for producing complex components. In this project, the laser powder bed fusion technique is used to manufacture lattice structures from a Ti-Ni shape memory alloy (SMA), which has the potential for producing lightweight and resistant cages with functional properties of superelasticity and osseointegration, possibly reducing complications.
First, the functional requirements of intervertebral cages were established and lattice structures candidates were selected. Strut-based diamond and sheet-based gyroid structures, with a pore size of 750 μm and porosity levels of 60%, 70%, and 80%, were designed and manufactured from Ti-6Al-4V alloy. The structures were characterized in terms of geometry, mechanical properties, and fluid permeability. The compressive stiffness of both structures (1.9-4.8 GPa) was comparable to that of bone, while their mechanical resistance (52- 160 MPa) was greater than that of vertebrae (3-6 MPa), thus reducing the risks of bone deterioration and implant failure. Fluid permeability (5-57 x 10-9 m2) and surface-to-volume ratios (~3) of both structures were close to those of vertebrae.
Secondly, a better understanding of the behavior of lattice structures under different loading modes was gained. The same two structures, but with porosities ranging from 50% to 80%, were tested experimentally and simulated numerically in axial tension/compression and in torsion to simulate flexion/extension, compression, and rotation of the spine. Numerical simulations overestimated the experimental behavior by about 25%, likely due to manufacturing defects, especially in most porous structures. Experimental and numerical results demonstrated that the structures had quasi-identical mechanical properties in compression and tension, but higher torsional properties than predicted by conventional limitation theories. Nevertheless, the lattice structures proved adequate and met the requirements of intervertebral cages.
Finally, lattice structures could be produced using AM of Ti-50.26%atNi SMA by using a numerical model to test and study various printing parameter sets. A volumetric energy density of 90 J/mm3 and a build rate of 10 cm3/h allowed the production of specimens with a density of 99.94% and an austenite finish temperature of Af = 26.3°C. Diamond and gyroid structures with a 60% porosity level were manufactured and compression tested. After heat treatment at 500°C for 30 minutes, diamond structures showed larger reversible deformations (7% vs 6%) and higher compliance (E 2.9 vs 3.4 GPa) and similar yield stresses (Sy ~48 MPa) compared to their gyroid equivalents. This indicates that diamond lattices are more suited for use in intervertebral cages. Finite element analysis allowed a comparison of stress distribution in both structures and the identification of stress concentration areas. Compared to the Ti-6Al-4V alloy, Ti-Ni structures were more compliant at lower porosity levels and exhibited larger reversible deformations.
Timercan, A. (Author),
Brailovski, V. (Supervisor) &
Terriault, P. (Co-supervisor),
28 Mar 2024Student thesis: Doctoral thesis › Doctorate in Engineering: Engineering