Potential Applications of Additive Manufacturing in Intervertebral Disc Replacement Using Gyroid Structures with...

Background: Intervertebral disc degeneration is a prevalent condition and a major risk factor for disc herniation. Mechanical overload, aging, injury, and disease contribute to the annulus fibrosus' structural failure, which allows nucleus pulposus material to escape and reduces the capacity to absorb shock. This study builds on previous investigations by evaluating additional thermoplastic polyurethane (TPU) filaments as potential materials for additively manufactured intervertebral disc replacements. Materials and Methods: Disc-shaped specimens (Ø50 × 10 mm) were fabricated using fused deposition modeling (FDM). A gyroid infill structure was employed with unit cell sizes ranging from 4 to 10 mm³ and wall thicknesses between 0.5 and 1.0 mm. The outer wall thickness varied from 0.4 to 0.8 mm. Four TPU filaments (Extrudr FlexSemiSoft, GEEE-TECH TPU, SUNLU TPU, and OVERTURE TPU) were tested, resulting in 36 parameter combinations per filament. Printed discs were examined via stereomicroscopy. Tensile testing was conducted according to DIN EN ISO 527-1 using Type 5A specimens. Mechanical performance under physiological loading was assessed through uniaxial compression tests, in which samples were compressed to 50% of their height while force-deformation curves were recorded. Target forces were defined as 4000-7500 N to maintain comparability with prior studies. Results: Across all filaments, a maximum of three parameter combinations per material achieved forces within the target range. Microscopy confirmed the dimensional accuracy of wall thicknesses with minimal deviation. Tensile strength values for GEEE-TECH, SUNLU, and FlexSemiSoft were comparable (10-11 MPa), while OVERTURE showed significantly lower strength (approximately 9 MPa). Tensile modulus values followed a similar trend: 25-30 MPa for three filaments and 17.5 MPa for OVERTURE. Conclusions: All four TPU filaments could be used to fabricate discs that met the mechanical requirements for compression. These results confirm that both the tested TPU materials and gyroid structures are suitable for potential intervertebral disc replacement applications.