Influence of muscle forces on lumbar internal fixation system using the finite element analysis

doi:10.3969/j.issn.2095-4344.0277

Abstract

Abstract:

BACKGROUND: Internal fixation system is a commonly used method for lumbar spine injury. The role of lumbar muscle strength and choosing a sagittal angle are the main factors affecting the success rate of internal fixation, which should pay much attention.

OBJECTIVE: To explore the influence of muscle forces under different postures on the internal fixation system when the ploy axial pedicle screws were installed in different sagittal angles.
METHODS: A finite element model of lumbar spine (L₃and L₄) was developed based on the L₃and L₄MRI data from a healthy adult man and assembled with ploy axial pedicle screw internal fixation system in different sagittal angles. Then, three loading schemes were designed to study the distribution of stress and displacement in anteflexion, extension and lateral bending: scheme 1: loaded without gravity, following loads, or erector spinae force; scheme 2: loaded with gravity and following loads, without erector spinae force; scheme 3: loaded with gravity, following loads, and erector spinae force. The stress and displacement of each part under different conditions were compared.

RESULTS AND CONCLUSION: (1) In anteflexion, the stress was in a decent tendency in each scheme. In extension, the stress in the schemes 1 and 2 was decreased firstly and then increased, and the stress in the scheme 3 was kept on a descent tendency. In lateral bending, the stress in the scheme 1 was firstly increased and then decreased, and the stress in the schemes 2 and 3 was firstly decreased and then increased. (2) The displacement in the sagittal angles (0°-5°) was on a rise, and then began to decrease. (3) The scheme 1 revealed the lowest displacement. (4) These results indicate that the muscle force can increase the stress of the internal fixation system under each condition and the intradiscal pressure. However, the stress in each part is related to the motion of lumbar spine. Moreover, the effect of muscle force should be taken into account according to the different sagittal angles.

中国组织工程研究杂志出版内容重点：人工关节；骨植入物；脊柱；骨折；内固定；数字化骨科；组织工程

Key words: Muscle Strength, Biomechanics, Internal Fixators, Tissue Engineering

CLC Number:

R318

Zhao Peng-fei, Chen Ling, Men Yu-tao . Influence of muscle forces on lumbar internal fixation system using the finite element analysis[J]. Chinese Journal of Tissue Engineering Research, 2018, 22(23): 3654-3658.

Figures/Tables 1

2.1 前屈、后伸和侧弯姿势内固定系统的应力及椎骨的位移对前屈、后伸和侧弯姿势下的腰椎模型按照上述3种方案加载，观察内固定系统的应力和椎骨位移。前屈姿势下，矢状角为0°腰椎内固定系统的应力云图见图3，其应力主要集中在螺钉尾部连接棒与固定块的接触部位。在肌肉力的作用下，内固定系统所承受的应力最大，这与改进内固定系统的初衷一致。内固定系统在前屈、后伸和侧弯3个姿势下不同进钉角度的应力计算结果见表2。前屈姿势下，无论哪种进钉角度，3种加载方案均是方案2应力最大；而方案1，3应力相对方案2较小；不同的矢状面进钉角度对方案1，3的对应角度的应力差值影响范围在142.29-280.73 MPa之间；而且3种方案内固定系统的应力变化趋势相似。后伸姿势下，进钉角度从0°-7.5°，方案1，2应力趋势都是先上升后下降，在5°时方案1应力值达到最大497.75 MPa，方案2在5°达到小范围内的峰值为 534.2 MPa，方案3的应力值处于下降状态。当进钉角度为10°时，方案1应力值降到最低364.14 MPa，方案2应力值达到最大919.33 MPa，方案3应力值小幅上升到 592.35 MPa。侧弯姿势下，在0°-7.5°方案1的模型应力值基本处于上升趋势，在10°时应力下降。方案2，3的应力趋势在0°-7.5°都呈现下降的态势，在10°时应力上升。尽管内固定系统应力变化较大，而作用在椎骨上的应力都很小，但是椎骨上的最大位移不能忽视。而且椎骨最大位移的部位在3种加载方式下有所不同，有肌肉力加载时在椎骨前缘，在不施加肌肉力的情况下在椎骨的横突部位。为进一步探讨3种加载方案、不同进钉角度对椎骨位移的影响，将计算结果列于表3。可知无论是前屈、后伸还是侧弯，椎骨位移的变化趋势都是在0°-5°增加，之后位移随着角度的增加而减小。方案1的加载方式对3种姿势而言椎骨位移均在5°达到最大值，尤其在侧弯姿势下位移最大为 2.06 mm。方案2椎骨位移变化最为明显，特别是在前屈状态下，矢状面进钉角度为5°时达到最大值4.81 mm。方案3的椎骨位移变化较为平稳，只是在侧弯姿势下椎骨最大值位于2.5°。 2.2 侧弯姿势椎间盘的应力及位移 3种姿势下加载相同的5 N•m的力矩，只有侧弯姿势下椎间盘的应力变化最为明显，见图4。方案3应力在0°达到1.6 MPa，在2.5°时下降到1.24 MPa，当角度继续上升应力也呈上升趋势，在7.5°应力最大为1.69 MPa。选用方案2进行加载，间盘应力在0°达到1.88 MPa，之后的变化趋势与方案3类似，在7.5°应力最大为2.01 MPa。采用方案1加载时腰椎间盘在各个进钉角度之间的应力相差不大，从0°的0.56 MPa上升到7.5°的0.65 MPa，之后呈现下降的趋势。三者在装配角度为5°的时候，应力的趋势有所不同，出现一定的差异性。采用3种加载方案，椎间盘在侧弯姿势下位移随进钉角度变化折线图见图5。方案1加载时，椎间盘位移变化范围很小，基本呈一条水平线，在5°时达到最大位移量为0.96 mm。方案2加载时，在0°时椎间盘的位移量为2.18 mm，当角度增大到5°时，椎间盘位移量最大为2.21 mm，之后随角度增加，位移变化趋于稳定。方案3与方案2趋势相似，间盘位移在0°时为1.75 mm，角度上升到2.5°位移最大为 1.92 mm；随着角度继续增加，位移有下降的趋势。总之，3种加载方案下，方案1位移量最小。"

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