Potential Applications of Additive Manufacturing in Intervertebral Disc Replacement Using Gyroid Structures with Various TPU Filaments

Background Disc degeneration is an increasingly common problem in modern society and is often a precursor to a herniated disc. Contributing factors include physical exertion, overuse, the natural aging process, and disease and injury. Over time, the fibrous ring of the disc develops cracks and small tears, allowing fluid from the nucleus pulposum to escape. As a result, the ability of the disc to absorb shock decreases, potentially leading to a bulging or herniated disc. In this work, previously initiated investigations are extended, and additional thermoplastic polyurethane (TPU) filaments are examined with respect to their suitability for additive manufacturing as potential disc replacement materials. Materials & Methods To remain comparable, the additive manufacturing in this work is also carried out with Fused Deposition Modeling (FDM) 3D printers and as a Ø50 mm x 10mm disc. The Gyroid was varied from 10 mm³ for the coarsest structure to 4 mm³ for the finest structure. The wall thickness of the Gyroid was also varied from 0.5 to 1.0 mm, as were the outer walls of the disc, whose wall thickness was varied from 0.4 to 0.8 mm. Four different TPU filaments (Extrudr FlexSemiSoft, GEEETECH TPU, SUNLU TPU and OVERTURE TPU) were used. This resulted in 36 different settings per filament. The 3D printed discs were analyzed using an Olympus SZ61 stereomicroscope. A tensile test according to DIN EN ISO 527-1 was performed on the 3D printed samples 5A. The aim was to investigate the difference between the different TPU filaments. To test the mechanical properties of the 3D printed discs, a uniaxial compression test was performed with at least three samples of each setting. The body was compressed to 50% of its total height and the force required was recorded as a force-deformation curve. To be comparable to a previous project, a maximum force of 4000–7500 N was used. Results Of the 36 different discs tested for each filament, only a maximum of three were within the target range of maximum force. Microscopy revealed that all wall thicknesses were within the target range with only minor variations. The tensile strengths of Geetech, SunLu, and SemiSoft were not significantly different and were in a similar range of 10-11 MPa, with Overture deviating significantly at 9 MPa. The tensile moduli exhibited a comparable distribution: 25-30 MPa for Geetech, SunLu, and SemiSoft, and 17.5 MPa for Overture. Conclusion For all of the filaments tested, it was possible to additively produce suitable discs that were within the specified range of 4000-7500 N at 50% compression. This would ensure that these discs would withstand the stresses they would be subjected to in a potential human disc replacement application. Thus, we were able to confirm the suitability of these four filaments, as well as the Gyroid structures, for use as a disc replacement.