A Novel Motion-Preserving Lumbar Total Disc Replacement for the treatment of Degenerative Disc Disease

Abstract

Background and rationale
Total Disc Replacement (TDR) aims to replicate the rotational and translational qualities of the natural intervertebral disc (IVD), preserving motion and reducing the stress on adjacent segments [65], reducing Adjacent Segment Disease (ASD). While TDR offers a promising alternative to the current golden standard of spinal fusion, current designs exhibit limitations that compromise their effectiveness.

In this thesis, an improved design of a novel lumbar TDR design developed by Amber Implants BV was proposed and biomechanically tested in static and dynamic axial compression according to the standard ASTM F2346. Furthermore, an exploration of the performance of the design in grade 1 commercially pure titanium (CpTi) in contrast to the standard grade 23 Ti-6Al-4V was conducted to find out if CpTi is a suitable alternative to Ti-6Al-4V in orthopaedic applications for the IVD.

Experimental methods
Based on the design goal, an optimised design was proposed and compared to the original design by FEA. To test the performance of the new design against the original design, static and dynamic axial compression tests were executed according to the standard ASTMF2346. Five samples per design were tested until failure during static axial compression testing, with an acceptance
criterion of 5500 N. Dynamic axial compression was conducted with a maximum load of 3500 N (R=10), up to 2.5 million cycles, for three samples of each device.

Results
In static axial compression, all three designs passed according to the acceptance criterion, with yield loads of 5892.82 ± 140.8 for the original design, 17255.54 ± 3675.9 N for the new design in grade 23 titanium, and 7790.32 ± 1778.6 N for the new design in grade 1 titanium. The new design in grade 23 outperformed the other two designs (p<0.001). In dynamic axial compression, the new design in grade 23 was the only device to survive 2.5million cycles, without failure and a height decrease of only 0.5%. The original design failed after 60.000, 70.000 and 100.000 cycles, and the new design in grade 1 failed after 33.000 and 30.500 cycles.

Conclusion
The design goal was achieved, with varying degrees of evidence for the different facets
of the goal. In both static and dynamic axial compression, the new design in grade 23 outperformed the original design. It can also be concluded that, in the current configuration of the new design, grade 1 CpTi is not a suitable alternative for orthopaedic applications in the lumbar spine.
It must be noted that complications arose regarding the test set-up in both static and dynamic axial testing, compromising the reliability of the results.