Q33 - Effect of Anisotropic Cage Material Properties on Intradiscal Pressure in a THUMS-Based Lumbar Spine Model

Introduction: : Lumbar interbody fusion is a common surgical intervention for treating spinal pathologies such as degenerative disc disease and instability (Calvo-Echenique, Cegonino, Chueca, & Perez-Del Palomar, 2018). Central to the success of this procedure is the biomechanical performance of the interbody cage that replaces the nucleus pulposus at the affected disc level (Mobbs, Phan, Malham, Seex, & Rao, 2015). An ideal cage material must provide sufficient segmental stability while maintaining physiological load transfer to avoid complications such as endplate subsidence or adjacent segment degeneration (Lu et al., 2022). Traditionally, cages have been manufactured from isotropic materials like PEEK or titanium (Wu et al., 2023). However, recent advances in additive manufacturing have introduced the potential for lattice structures with tunable anisotropic stiffness (Masud et al., 2025). Anisotropic lattices can provide favorable mechanics in one direction while providing biological optimization for bone fusion in another direction through geometric changes in unit cells and porosity.

This study aims to evaluate the biomechanical implications of anisotropic cage materials using a THUMS (Total HUman Model for Safety) spine model (Iwamoto, Nakahira, & Kimpara, 2015) to evaluate intradiscal pressure (IDP). IDP is a key indicator of disc health and mechanical behavior, making it a suitable parameter for assessing the performance of various cage designs under physiologically relevant loading conditions (Zahari, Latif, Rahim, Kadir, & Kamarul, 2017). By comparing intradiscal pressure (IDP) across a range of material configurations, the study aims to determine how varying stiffness distributions affects load transmission at the fusion and adjacent levels.

Materials and

Methods: : A segment of the THUMS human body model, including vertebrae L1 to L5, was used to simulate spinal biomechanics under compression as shown in Figure 1. The native nucleus pulposus at the L4–L5 level was replaced with a cage-mimicking material block in all modified models. Material properties for the cage region were derived from experiments (Masud et al., 2025). Specifically, three configurations were tested: an isotropic case (uniform stiffness in all directions), laterally stiffer anisotropic cage, and longitudinally stiffer anisotropic cage with properties in Table 1. A homogenization approach was employed to model the lattice structures. For validation and comparison, a healthy model with intact discs was included. In the healthy model, the nucleus pulposus was modeled as an isotropic elastic-plastic material with a density of 1×10⁻⁹ ton/mm³, a shear modulus of 0.00433 MPa, a bulk modulus of 21.7 MPa, and a yield stress of 0.013 MPa, reflecting its soft, gelatinous, and nearly incompressible mechanical behavior. The simulations were conducted using the LS-DYNA explicit solver. All models were subjected to a compression of 500N with follower load at the L1 vertebra, while the inferior surface of L5 was fully constrained. IDP was extracted from elements within the nucleus pulposus of each disc level from L1–L2 to L4–L5. The peak von Mises stress value was obtained and plotted to analyze segmental response to different cage stiffnesses, with a focus on IDP changes to at L3–L4 (adjacent level) and L4–L5 (fused level).

Results, Conclusions, and Discussions::

Results:
IDP values were compared across the four cage configurations: Isotropic (Ez = Ex = Ey = 92 MPa), laterally stiffer (Ez = 22 MPa; Ex = Ey = 111 MPa), longitudinally stiffer (Ez = 206.8 MPa; Ex = Ey = 57.3 MPa), and a healthy disc. As shown in Figure 2, at the adjacent L3–L4 level, all cage designs produced similar IDP values (~3.1 MPa). These values are similar to the healthy spine model, indicating that the biomechanical environment above the fused segment is largely preserved regardless of cage stiffness. The lack of significant IDP elevation at the adjacent level suggests a low risk of adjacent segment degeneration (ASD). At the fusion level (L4-L5), the longitudinally stiffer cage exhibited the highest IDP at the fused L4–L5 segment (~7.1 MPa), followed by the laterally stiffer cage (~4.4 MPa), and the isotropic cage (~4.2 MPa).

Conclusion:
Results suggest that the longitudinally stiffer cage, while demonstrating the highest IDP at the L4–L5 level, may not be mechanically ideal due to the potential for stress concentration and subsidence risk. However, the laterally stiffer and isotropic cages produced lower IDP values that more closely resembled the healthy condition, indicating a more physiologically favorable mechanical environment. Additionally, the laterally stiffer cage has smaller pores to promote bone tissue growth. Collectively, these results underscore a critical design trade-off: Achieving optimal spinal fusion requires balancing sufficient mechanical stiffness to support load bearing with structural features that promote osseointegration. Further tuning of cage anisotropic stiffness and geometry could be completed using this approach to create optimized geometries for specified patient needs.

Discussion:
Although IDP served as a key biomechanical indicator in this study, range of motion (ROM) is another clinically relevant parameter to investigate further to determine biomechanical trade-offs. In future work, modeling improvements could be addressed by adopting more anatomically and kinematically accurate models capable of realistic ROM prediction, and by exploring a range of lattice-based cage geometries. These future steps would allow for more robust evaluation of cage designs for both stress distribution and motion preservation.

Acknowledgements and/or References (Optional)::