Effect of Passive Support of the Spinal Muscles on the Biomechanics of a Lumbar Finite Element Model

Finite element (FE) modeling of the passive ligamentous spine is widely used to assess various biomechanical behaviors. Currently, FE models that incorporate the vertebrae, ligaments, and the personalized geometry of the bony spine may be used in conjunction with external loads from the muscles. However, while the muscles place a load (moment) on the spine and support it simultaneously, the effect of the passive support from the adjacent spinal muscles has not been considered. This study thus aims to investigate the effect of passive support from the psoas major, quadratus lumborum, and erector muscles on the range of motion (RoM) and intradiscal pressure (IDP) of the lumbar spine. Various L2-sacrum spinal models that differed only in their muscle properties were constructed and loaded with a pure moment (2.5–15.0 Nm) alone or combined with a compressive (440 or 1000 N) follower load. The RoM and IDP of the model that excluded the effect of muscles closely matched previous FE results under the corresponding load conditions. When the muscles (40–160 kPa) were included in the FE model, the RoM at L2 was reduced by up to 6.57% under a pure moment (10 Nm). The IDP was reduced by up to 6.45% under flexion and 6.84% under extension. It was also found that the erector muscles had a greater effect than the psoas major and quadratus muscles.

Lag-Screw Osteosynthesis in Thoracolumbar Pincer Fractures

Study Design: Biomechanical. Objective: This study evaluates the biomechanical properties of lag-screws used in vertebral pincer fractures at the thoracolumbar junction. Methods: Pincer fractures were created in 18 bisegmental human specimens. The specimens were assigned to three groups depending on their treatment perspective, either bolted, with the thread positioned in the cortical or cancellous bone, or control. The specimens were mounted in a servo-hydraulic testing machine and loaded with a 500 N follower load. They were consecutively tested in 3 different conditions: intact, fractured, and bolted/control. For each condition 10 cycles in extension/flexion, torsion, and lateral bending were applied. After each tested condition, a computed tomography (CT) scan was performed. Finally, an extension/flexion fatigue loading was applied to all specimens. Results: Biomechanical results revealed a nonsignificant increase in stiffness in extension/flexion of the fractured specimens compared with the intact ones. For lateral bending and torsion, the stiffness was significantly lower. Compared with the fractured specimens, no changes in stiffness due to bolting were discovered. CT scans showed an increasing fracture gap during axial loading both in extension/flexion, torsion, and lateral bending in the control specimens. In bolted specimens, the anterior fragment was approximated, and the fracture gap nullified. This refers to both the cortical and the cancellous thread positions. Conclusion: The results of this study concerning the effect of lag-screws on pincer fractures appear promising. Though there was little effect on stiffness, CT scans reveal a bony contact in the bolted specimens, which is a requirement for bony healing.

Finite element study on effect of follower load on intersegmental rotation, facet joint force and nucleus pressure of the cervical spine

Background: The follower load is used to simulate the physiological compressive load of human spine. These compressive loads can maintain cervical spine’s mechanics stability and play a significant role in improving load-carrying capacity of the cervical spine. However, under different follower loads the biomechanical response of the cervical spine is unknown. So the aim of this study is to investigate the effect of follower load on biomechanics of the cervical spine. Results: In this study, a three-dimensional nonlinear finite element (FE) model of the cervical spine (C3-C7) was built and validated. Using this FE model of the cervical spine, we evaluated the effect of different follower loads on intersegmental rotation, facet joint force, and nucleus pressure in the cervical spine. The results indicated that with the follower load increased, the intersegmental rotation of the cervical spine in extension decreased, but the intersegmental rotation in other postures increased. The follower load increased the facet joint forces in all postures. In lateral bending (LB), the facet joint forces were only generated in the ipsilateral facet joints. In axial rotation (AR), there was a large asymmetry in the facet joint forces, and this asymmetry worsened with the follower load increased. The nucleus pressure of each segment nonlinearly increased with the follower load increased in all postures. Conclusion: An comprehensive analysis in intersegmental rotation, facet joint force and nucleus pressure under different follower loads can provide us a deeper understanding of the follower load in the human spine.

METODA COROTATIONAL BEAM 2D UNTUK ANALISIS STATIK STRUKTUR NONLINIER GEOMETRIK

Makalah ini membahas sebuah metoda corotational beam dua dimensi (CBM 2D) yang dapat digunakan untuk analisis statik struktur yang nonlinier secara geometri. Kombinasi antara formulasi corotational beam dan Euler-Bernouli beam Theory (EBT) membuat implementasi numerik metoda ini menjadi sangat sederhana dengan beban komputasi yang rendah sehingga sangat praktis untuk diaplikasikan. Akurasi dan efisiensi metoda ini terverifikasi melalui beberapa uji numerik yang dilakukan pada beberapa model uji yang terdapat pada literatur. Metoda ini pun mampu memberikan hasil yang akurat untuk kasus extensible beam dan struktur beam yang dibebani follower load. Hasil penelitian ini memverifikasi validitas, efisiensi, dan kepraktisan dari metoda yang dikembangkan.

The Path to Deliver the Most Realistic Follower Load for a Lumbar Spine in Standing Posture: A Finite Element Study

A spine is proven to be subjected to a follower load which is a compressive load of physiologic magnitude acting on the whole spine. The path of the follower load approximates the tangent to the curve of the spine in in vivo neutral standing posture. However, the specific path location of the follower load is still unclear. The aim of this study is to find out the most realistic location of the follower load path (FLP) for a lumbar spine in standing. A three-dimensional (3D) nonlinear finite element model (FEM) of lumbosacral vertebrae (L1-S1) with consideration of the calibrated material properties was established and validated by comparing with the experimental data. We show that the shape of the lumbosacral spine is strongly affected by the location of FLP. An evident nonlinear relationship between the FLP location and the kinematic response of the L1-S1 lumbosacral spine exists. The FLP at about 4 and 3 mm posterior to the curve connecting the center of the vertebral bodies delivers the most realistic location in standing for healthy people and patients having low back pains (LPBs), respectively. Moreover, the “sweeping” method introduced in this study can be applicable to all individualized FEM to determine the location of FLP.