Response of 3D printed lattice-based spinal fusion cages to static and dynamic loading

Abstract

Back pain is a major economic problem that can dramatically affect the quality of life. There are many causes that can lead to back pain, but intervertebral disc (IVD) degeneration is a recognised cause. The harmful effect of whole-body vibration (WBV) environment on the human intervertebral disc has been demonstrated to have an increased relative risk of failure. One of the treatments for a degenerative disc is spinal fusion procedure, in which the human IVD is replaced by a spinal fusion cage. In this thesis, the influence of WBV on custom-made spinal fusion cages was investigated.
With the advent of additive manufacturing lattice-based scaffolds were designed as spinal fusion implants and manufactured using a Polyet 3D-printer. The influence of the unit cell design on the build quality and the quasi-static mechanical properties as well as the dynamic WBV reaction was observed. Therefore, four different unit cell types were designed: body-centered cube (BC-cube), face-centered cube (FX-cube), truncated octahedron (T-octa) and negative Poisson cube (NP-cube). The influence of 3D-printing direction and scaffold porosity on print quality and mechanical properties were also investigated.
The build quality was determined by measuring the nominal size, determining the beam diameter and using μCT. The quasi-static mechanical properties were determined with the compression test. The dynamic response to WBV was determined with a mechanical test setup that imitates a base excitation model. Two cylindrical bases were printed as one sample together with the scaffold. The bases were embedded and mounted in the testing cups. A constant sinusoidal displacement to the lower embedding cup was applied and the accelerations of the input and response were measured. The difference between the response and input was compared and observed at which frequency the sample broke or reached the highest gain. This was done for all samples and the influence of unit cell type, 3D-printing direction and porosity on the dynamic response was observed.
The direction of 3D-printing had the greatest influence on the build quality, while the type of unit cell and porosity had a less obvious influence.
The compression test provided insight into which unit cell types have a higher elastic modulus, that the 3D-printing direction perpendicular to the test direction achieved higher E values, and that higher porosity, which is desirable for implants, resulted in a lower Young's modulus.
In terms of the dynamic response, the results of this work indicate that the FX cube design is the most robust and would improve the safety of the implant when exposed to WBV.
For the 3D-printing direction, it may be concluded that the perpendicular direction, i.e. the layers are perpendicular to the test direction, can best withstand the dynamic forces. The increase in porosity from 50% to 70% showed a decrease in resistance to the dynamic loading, with scaffolds failing more often and at lower frequencies. Understanding how different designs and porosity of the spinal fusion scaffolds influence the resistance to WBV conditions will help to develop safe and resistant implants which replace degenerated natural IVD.