Design Improvements and Validation of a Novel Fully 3D Printed Analogue Lumbar Spine Motion Segment

Abstract

Spine biomechanical testing methods in the past few decades have not evolved beyond employing either cadaveric studies or finite element modeling techniques. However, both these approaches may have inherent cost and time limitations. Cadaveric studies are the present gold standard for spinal implant design and regulatory approval, but they introduce significant variability in measurements across patients, often requiring large sample sizes. Finite element modeling demands considerable expertise and can be computationally expensive when complex geometry and material nonlinearity are introduced. Validated analogue spine models could complement these traditional methods as a low-cost and high-fidelity alternative. A fully 3D printable L-S1 analogue spine model with ligaments is developed and validated in this research. Rotational stiffness of the model under pure bending loading in flexion-extension, Lateral Bending (LB) and Axial Rotation (AR) is evaluated and compared against historical ex vivo and in silico models. Additionally, the effect of interspinous, intertransverse ligaments and the Thoracolumbar Fascia (TLF) on spinal stiffness is evaluated by systematic construction of the model. In flexion, model Range of Motion (ROM) was 12.92 ± 0.11° (ex vivo: 16.58°, in silico: 12.96°) at 7.5Nm. In LB, average ROM was 13.67 ± 0.12° at 7.5 Nm (ex vivo: 15.21 ± 1.89°, in silico: 15.49 ± 0.23°). Similarly, in AR, average ROM was 17.69 ± 2.12° at 7.5Nm (ex vivo: 14.12 ± 0.31°, in silico: 15.91 ± 0.28°). The addition of interspinous and intertransverse ligaments increased both flexion and LB stiffnesses by approximately 5%. Addition of TLF showed increase in flexion and AR stiffnesses by 29% and 24%, respectively. This novel model can reproduce physiological ROMs with high repeatability and could be a useful open-source tool in spine biomechanics.