Effects of dynamic and rigid implantation on biomechanical characteristics of different sagittal alignment lumbar after single- or double-level spinal fixations: a finite-element modeling study

Abstract

Background Although it is critical to understand the accelerated degeneration of adjacent segments after fusion, the biomechanical properties of the spine have not been thoroughly studied after various fusion techniques. This study investigates whether four Roussouly’s sagittal alignment morphotypes have different biomechanical characteristics after different single- or double-level spinal fixations. Methods The parametric finite element (FE) models of Roussouly’s type (1–4) were developed based on the radiological data of 625 Chinese community population. The four Roussouly's type models were reassembled into four fusion models: single-level L4–5 Coflex fixation model, single-level L4–5 Fusion (pedicle screw fixation) model, double-level Coflex (L4–5) + Fusion (L5–S1) model, and double-level Fusion (L4–5) + Fusion (L4–5) model. A pure moment of 7.5 Nm was applied to simulate the physiological activities of flexion, extension, lateral bending and axial rotation. Results Both single-level and double-level spinal fixation had the greatest effect on lumbar range of motion, disc pressure, and annulus fibrosis stress in flexion, followed by lateral bending, extension, and axial rotation. In all models, the upper adjacent segment was the most influenced by the implantation and bore the most compensation from the fixed segment. For Type 2 lumbar, the L4–L5 Coflex effectively reduced the disc pressure and annulus fibrosis stress in adjacent segments compared to the L4–L5 Fusion. Similarly, the L4–L5 Coflex offered considerable advantages in preserving the biomechanical properties of adjacent segments for Type 1 lumbar. For Type 4 lumbar, the L4–L5 Coflex did not have superiority over the L4–L5 Fusion, resulting in a greater increase in range of motion at adjacent segments in flexion and extension. The difference between the two fixations was not apparent in Type 3 lumbar. Compared to the single-level Fusion, the changes in motion and mechanics of the lumbar increased after both the double-level Coflex + Fusion and Fusion + Fusion fixations, while the differences between two double-level fixation methods on adjacent segments of the four lumbar models were similar to that of the single-level fixation. Conclusion Type 3 and Type 4 lumbar have good compensatory ability and therefore allow for a wider range of surgical options, whereas surgical options for small lordotic Type 1 and Type 2 lumbar are more limited and severe.