General Session: Biomechanics - Hall F

Presented by: Z. Zhang

Author(s):

W. Liu(1), Z. Zhang(1), G. Fogel(2), Z. Liao(3), J. Zhu(3)

(1) Tsinghua University, Mechanical Engineering, Beijing, China
(2) Spinepainbegone, San Antonio, TX, United States
(3) Institute of Tsinghua University in Shenzhen, Biomechanics and Biotechnology Lab, Shenzhen, China

Abstract

Background: The interbody fusion with cage instrumentation has become common in the treatment of degenerative lumbar disease. Increased lordosis of the cage may improve sagittal balance but could be associated with endplate fracture or subsidence. The biomechanical influence of the lordotic angle of hyperlordotic interbody cages on subsidence has not been fully understood.

Purpose: We used a validated L2-L5 finite element (FE) model (Fig.1) to estimate the effects of the lordotic angles of hyperlordotic interbody cage on the biomechanics of lumbar spine.

Study Design: This 3-dimensional nonlinear finite element analysis (FEA) compared the biomechanics among intact and four surgical models with different cage lordotic angles. Range of Motion (ROM), the maximum stresses in cage and endplate, IDP, and facet joint force (FJF) were predicted.

Method: Four surgical (FE) models were constructed by inserting hyperlordotic interbody cages with different lordotic angles at the L3-L4 disc space. In all four models the surgical conditions were anterior longitudinal ligament (ALL) resected and supplemental bilateral pedicle screws. The bottom of L5 was fixed in all directions. The compressive load of 280 N and the moment of 7.5 Nm were applied to the upper surface of L2. The four motion modes including flexion, extension, bending and rotation were simulated. Biomechanical properties of four surgical models were compared by FEA.

Results: The range of motion (ROM) at surgical level increased with increasing lordotic angle of cage in flexion, extension, and bending whereas it was not significantly changed in rotation (Fig. 2A). The maximum stress in cage increased significantly with increasing lordotic angle of cage in extension and rotation, and it was the minimum with the smallest lordotic angle of cage in all motion modes (flexion, extension, bending and rotation) (Fig. 2B). The maximum stress in endplate at surgical level increased on trend with increasing lordotic angle in extension and bending, and it was the minimum with the smallest lordotic angle of cage in all motion modes except for flexion (Fig. 2C). The facet joint force (FJF) at surgical level increased significantly with increasing lordotic angle of cage in extension and rotation, whereas it was not significantly changed in flexion and bending (Fig. 2D).

Conclusion: The lordotic angle of cage significantly affects the stress in the cage and endplate, and facet contact force at surgical level. The maximum stresses in cage and endplate at surgical level were the minimum with the smallest lordotic angle of cage in all motion modes except for the endplate stress in flexion. When performing lumbar fusion with hyperlordotic interbody cage, the biomechanical properties of the lumbar spine are sensitive to the geometry of the cage instrumentation. The 15 degree cage reduces the stress in cage and endplate, which may be associated with a lower risk of fracture with subsidence.