Biomechanical effects of multifidus injury on adjacent segments after posterior lumbar interbody fusion

English Keywords:Posterior lumbar interbody fusion  Multifidus  Biomechanics  Adjacent segment  Finite element analysis

English Abstract:

  【Abstract】 Objectives: To investigate the biomechanical effects of multifidus injury on adjacent segments after posterior lumbar interbody fusion using finite element analysis. Methods: A finite element model of the lumbar spine of L1-L5 and sacral spine was constructed according to the CT image data of the lumbosacral spine of a healthy adult, and the validity was verified. The function of multifidus muscle under normal physiological conditions was simulated by establishing reference points and setting load direction and magnitude, through which the normal model(NM) was obtained. And the L4/5 posterior fusion model(PFM), the multifidus severed model(MSM), the posterior fusion and multifidus severed model(PFMSM) were established respectively. A 7.5N·m moment load was applied to the superior endplate of L1 of the NM to simulate flexion, extension, lateral bending and torsion, respectively, the rotation angles of the upper endplate of L1 during different motion conditions were recorded as 18.8°, 14.4°, 15.4° and 12.5°, respectively. Taking NM as reference,different moment loads were applied on L1 upper endplate of the other three models to simulate flexion, extension, lateral bending and torsion to achieve the same rotation angle, after that the muscle strength and the total work of multifidus, the range of motion(ROM) and the maximum Von Mises stress of the intervertebral discs of the upper and inferior adjacent segments(L3/4, L5/S1) were analyzed statistically. Results: The validity of normal lumbosacral model was demonstrated. Compared with the NM, the average multifidus muscle strength increased by 63.41%, 38.16% and 84.43% in the PFM, MSM and PFMSM, respectively. The total work of multifidus increased by 30.02%, 6.40% and 39.95% during flexion, respectively, which increased by similar percentages during the other three motion conditions. The changes in the ROM of adjacent segments during different motion conditions, compared with the NM, the PFM increased by 12.85%-40.06% in L3/4 and 11.73%-68.77% in L5/S1; the MSM increased by 13.78% and 7.51% in L3/4 during flexion and extension, decreased by 7.30% and 4.81% during lateral bending and torsion, and increased by 0.26%-20.72% in L5/S1; the PFMSM increased by 16.15%-27.86% in L3/4 and 14.38%-39.67% in L5/S1. In the aspect of the maximum Von Mises stress of the intervertebral discs, compared with the NM, the PFM increased by 15.52%-48.55% in L3/4 and 6.97%-65.13% in L5/S1, the MSM group increased by 1.05% and 0.10% during flexion and extension, decreased by 6.13% and 12.33% during lateral bending and torsion in L3/4, increased by 15.09% during flexion, and decreased by 12.91%, 4.96%, 2.18% during extension lateral bending and torsion in L5/S1; the PFMSM increased by 18.94%-30.85% in L3/4 and 20.08%-38.92% in L5/S1. Conclusions: Posterior lumbar interbody fusion and the severing of upper multifidus will cause the increase of the ROM and the Von Mises stress of the intervertebral disc in the upper and lower adjacent segments. The concurrence of posterior lumbar interbody fusion and the severing of upper multifidus have the greatest biomechanics effects on the adjacent segments, especially in flexion.