Effect of Ligaments on Lumbar Spinal Stiffness: A Systematic Investigation Using Novel 3D-Printed Analogue Spine Models

Traditional spine biomechanical studies involve cadavers or finite element models, both of which have cost and time limitations. Validated analogue spine models have been proposed as a low-cost and high-fidelity approach in biomechanical testing. To develop them it is essential to understand the effect of underlying soft tissues on model’s biomechanical response. This chapter aims to assess the impact of intertransverse and interspinous ligaments on the rotational stiffness of lumbar spine by systematically constructing and testing an analogue spine model. A fully 3D-printed analogue lumbar spine was developed with L1-S1 vertebrae, intervertebral discs, intertransverse and interspinous ligaments. Stereolithography 3D printing was used to construct the model. The model construction involved three phases (P1, P2, P3). After each phase, model rotational stiffness was assessed by applying cyclic pure bending loads up to 5Nm in flexion–extension, lateral bending, and axial rotation, and the rotation of motion (ROM) was recorded. P1 involved assembly of vertebrae and intervertebral discs, while P2 involved addition of interspinous ligament to the P1 model. P3 concluded model construction by adding intertransverse ligaments on either side of the P2 model. The analogue spine model demonstrated a hysteretic behavior during load–unload cycles much like the human spine owing to its viscoelastic components. In flexion–extension motion, addition of ligaments significantly decreased the model ROM (\(Flexion - P1=9.9\pm 0.1^\circ \) \(P2=9.4\pm 0.1^\circ \) \(P3=8.3\pm 0.1^\circ \) and extension \(- P1=9.0\pm 0.1^\circ \) \(P2=6.7\pm 0.1^\circ \) \(P3=7.1\pm 0.1^\circ \)). In LB, there was no discernable ROM change from P1 to P2 but a decrease from P2 to P3 \((left - P1=9.0\pm 0.1^\circ \) \(P2=8.9\pm 0.1^\circ \) \(P3=8.7\pm 0.1^\circ \) and right \(- P1=9.1\pm 0.1^\circ \) \(P2=9.2\pm 0.1^\circ \) \(P3=8.6\pm 0.1^\circ \)). No trends were observed in AR as construction progressed (\(left - P1=12.8\pm 0.1^\circ \) \(P2=14.3\pm 0.2^\circ \) \(P3=12.6\pm 0.2^\circ \) and right \(- P1=12.1\pm 0.3^\circ \) \(P2=10.8\pm 0.1^\circ \) \(P3=11.4\pm 0.2^\circ \)). The model exhibited bilateral symmetry in lateral bending and axial rotation. Inclusion of interspinous and intertransverse ligaments significantly increased flexion and lateral bending stiffness, respectively, in agreement with the current understanding of lumbar biomechanics. This novel model holds potential as an effective research and educational tool to further investigate damage modes in the lumbar spine structure.