Finite element analysis of fixation of U-shaped sacral fractures / 生物医学工程学杂志

Finite element method (FEM) was used to investigate the biomechanical properties of three types of surgical fixations of U-shaped sacral fractures. Based on a previously established and validated complete lumbar-pelvic model, three models of surgical fixations of U-shaped sacral fractures were established: ① S1S2 passed through screw (S1S2), ② L4-L5 pedicle screw + screw for wing of ilium (L4L5 + IS), and ③ L4-L5 pedicle screw + S1 passed through screw + screw for wing of ilium (L4L5 + S1 + IS). A 400 N force acting vertically downward, along with torque of 7.5 N·m in different directions (anterior flexion, posterior extension, axial rotation, and axial lateral bending), was exerted on the upper surface of L4. Comparisons were made on differences in separation of the fracture gap and maximum stress in sitting and standing positions among three fixation methods. This study showed that: for values of separation of the fracture gap produced by different operation groups in different positions, L4L5 + S1 + IS was far less than L4L5 + IS and S1S2. For internal fixators, the maximum stress value produced was: L4L5 + IS > L4L5 + S1 + IS > S1S2. For the intervertebral disc, the maximum stress value produced by S1S2 is much larger than that of L4L5 + S1 + IS and L4L5 + IS. In a comprehensive consideration, L4L5 + S1 + IS could be prioritized for fixation of U-shaped sacral fractures. The objective of this research is to compare the biomechanical differences of three different internal fixation methods for U-shaped sacral fractures, for the reference of clinical operation.

Biomechanical Effect of Laminectomy on Adjacent Segment after Lumbar Interbody Fusion / 医用生物力学

Objective To study the biomechanical influence of posterior laminectomy with varying extent on adjacent segment after lumbar interbody fusion. Methods Three finite element models of lumbar posterior fusion were developed based on the validated intact lumbar model. These models were: posterior fusion with bi-lateral incision of facet joint (Bi-TLIF），inferior partly incision of laminar (PLIF)，total laminectomy (LAM-PLIF). The range of motion (ROM), intradiscal pressure (IDP), facet joint contact force (FJF) of adjacent segment of fusion models under various loading were compared with the intact model. The follower load of 400 N under 7.5 N·m torque was exerted on superior endplate of L1 segment. The 6-DOF (degree of freedom) of sacroiliac joint surface was constrained during loading. ResultsDuring flexion, obvious biomechanical changes of superior adjacent segment (L3-4) were found in Bi-TLIF, PLIF, LAM-PLIF surgery groups. Compared with the intact model, the ROM in Bi-TLIF, PLIF, LAM-PLIF group increased by 1.0%, 9.3%, 24.5%, respectively, while IDP in the above fusion groups increased by 1.4%, 4.3%, 10.0%，respectively. These changes were not obvious in other postures. For FJF, the Bi-TLIF and PLIF group showed obvious increasing effect on L3-4 segment, while almost had no effect on L5-S1 segment. Conclusions Laminectomy increased ROM, IDP and FJF of adjacent segment (especially superior adjacent segment) after posterior lumbar fusion, which might increase the risk of adjacent segment degeneration. This biomechanical effect was more obvious with the increase in incision range of laminar. Therefore, preserving more posterior complex during decompression has a positive effect on preventing adjacent segment degeneration (ASD) following lumbar fusion surgeries.

Biomechanical effects of interbody cage height on cervical spine / 医用生物力学

Objective To investigate the biomechanical effects of interbody cage height on cervical spine during anterior cervical discectomy and fusion (ACDF) surgery,so as to provide references for selection of interbody cage.Metheds The finite element model of normal cervical spine (C2-7) was built and validated,and the cages with different height (5,6,7,8 mm) were implanted into C5-6 disc (cage 5,6,7,8 model).All the models were loaded with pure moment of 1.5 N · m to produce flexion,extension,lateral bending and axial rotation motions on cervical spine,and the effects of cage height on range of motion (ROM),facet joint stress,intervertebral pressure in cervical spine were investigated.Results The intervertebral angle at the fusion segment increased by 0.68° with per 1 mm-increase in height.The ROM in C5-6 after cage implantation was smaller than 0.44°.The influence of cage height on ROM in C4-5 was greater than that in C6-7,and the changes of ROM in non-fusion segments were smaller than 7.3％.The cage height variation had a smaller impact on the facet joint stress and intervertebral pressure.The stresses in the capsular ligament,cage and screw-plate system increased gradually with the increase of cage height,and the stresses in cage 6,7,8 models were much higher than those in cage 5 model.Conclusions For patients who need implanting fusion cage,the cage height should be 0-1 mm greater than the original intervertebral space height.

Biomechanical study of lumbar spine under different vibration conditions / 生物医学工程学杂志

We observed the effect of vibration parameters on lumbar spine under different vibration conditions using finite element analysis method in our laboratory. In this study, the CT-images of L1-L5 segments were obtained. All images were used to develop 3D geometrical model using the Mimics10. 01 (Materialise, Belgium). Then it was modified using Geomagic Studio12. 0 (Raindrop Geomagic Inc. USA). Finite element (FE) mesh model was generated by Hypermesh11. 0 (Altair Engineering, Inc. USA) and Abaqus. Abaqus was used to calculate the stress distribution of L1-L5 under different vibration conditions. It was found that in a vibration cycle, tensile stress was occurred on lumbar vertebra mainly. Stress distributed evenly and stress concentration occurred on the left rear side of the upper endplate. The stress had no obvious changes under different frequencies, but the stress was higher when amplitude was greater. In conclusion, frequency and amplitude parameters have little effect on the stress distribution in vertebra. The stress magnitude is positively correlated with the amplitude.