Influence of posterior pedicle screw fixation at L4–L5 level on biomechanics of the lumbar spine with and without fusion: a finite element method

Abstract

Background Posterior pedicle screw (PS) fixation, a common treatment method for widespread low-back pain problems, has many uncertain aspects including stress concentration levels, effects on adjacent segments, and relationships with physiological motions. A better understanding of how posterior PS fixation affects the biomechanics of the lumbar spine is needed. For this purpose, a finite element (FE) model of a lumbar spine with posterior PS fixation at the L4–L5 segment level was developed by partially removing facet joints (FJs) to imitate an actual surgical procedure. This FE study aimed to investigate the influence of the posterior PS fixation system on the biomechanics of the lumbar spine before and after fusion by determining which physiological motions have the most increase in posterior instrumentation (PI) stresses and FJ loading. Results It was determined that posterior PS fixation increased FJ loading by approximately 35% and 23% at the L3–L4 adjacent level with extension and lateral bending motion, respectively. This increase in FJ loading at the adjacent level could point to the possibility that adjacent segment disease has developed or progressed after posterior lumbar interbody fusion. Furthermore, analyses of peak von Mises stresses on PI showed that the maximum PI stresses of 272.1 MPa and 263.7 MPa occurred in lateral bending and flexion motion before fusion, respectively. Conclusions The effects of a posterior PS fixation system on the biomechanics of the lumbar spine before and after fusion were investigated for all physiological motions. This model could be used as a fundamental tool for further studies, providing a better understanding of the effects of posterior PS fixation by clearing up uncertain aspects.

The effects of pre-stressed rods contoured by different bending techniques on posterior instrumentation of L4-L5 lumbar spine segment: A finite element study

“…There was also no follower load in the current study. 20 There are a variety of studies in the literature with various follower load values, and PPSI and rod stresses will vary depending on the follower load value. Therefore, if a follower load had been applied here, the PPSI and rod stress values may have been higher than those observed without one.…”

“…13,16–18 Material properties were assigned for cortical bone and cancelous bone sections (Table 1). 19–21 Annulus fibrosus was achieved with membrane layers and a solid ground matrix. The annulus fiber layers were modeled with six membrane layers with mean thickness of 0.12 mm with a solid ground matrix.…”

“…25 The nucleus pulposus was described as a nearly incompressible material, as shown in Table 1. 20,26 Endplate sections of intervertebral discs were formed above and below the surfaces of the annulus and nucleus considering the endplate thickness (0.5 mm) with respect to previous studies in the literature. 13,16,18,27 Endplate material properties are also indicated in Table 1.…”

“…13,16,18,27 Endplate material properties are also indicated in Table 1. 19,20 In this study, all ligament types between lumbar vertebrae were used except for the intertransverse ligament (ITL) because the reference nonlinear stiffness dataset used here for ligaments did not include ITL data. The test data of Shirazi-Adl et al 24 and Schmidt et al 28 as indicated by Naserkhaki et al 29 were used as nonlinear stiffness data for the ligaments.…”

“…Mooney Rivlin, C10 = 0.13, C01 = 0.03, D = 0.6 Sengul et al, 20 Wang et al, 25 Annulus Fibrosus Layers Rebar Mooney Rivlin, C10 = 0.13, C01 = 0.03, D = 0.6…”