Influence of lumbar spine rhythms and intra-abdominal pressure on spinal loads and trunk muscle forces during upper body inclination

Improved knowledge on spinal loads and trunk muscle forces may clarify the mechanical causes of various spinal diseases and has the potential to improve the current treatment options. Using an inverse dynamic musculoskeletal model, this sensitivity analysis was aimed to investigate the influence of lumbar spine rhythms and intra-abdominal pressure on the compressive and shear forces in L4-L5 disc and the trunk muscle forces during upper body inclination.Based on in vivo data, three different spine rhythms (SRs) were used along with alternative settings (with/without) of intra-abdominal pressure (IAP). Compressive and shear forces in L4-L5 disc as well as trunk muscle forces were predicted by inverse static simulations from standing upright to 55° of intermediate trunk inclination.Alternate model settings of intra-abdominal pressure and different spine rhythms resulted in significant variation of compression (763 N) and shear forces (195 N) in the L4-L5 disc and in global (454 N) and local (156 N) trunk muscle forces at maximum flexed position. During upper body inclination, the compression forces at L4-L5 disc were mostly released by IAP and increased for larger intervertebral rotation in a lumbar spine rhythm.This study demonstrated that with various possible assumptions of lumbar spine rhythm and intra-abdominal pressure, variation in predicted loads and muscles forces increase with larger flexion. It is therefore, essential to adapt these model parameters for accurate prediction of spinal loads and trunk muscle forces.     

Trunk Hybrid Passive–Active Musculoskeletal Modeling to Determine the Detailed T12–S1 Response Under <Emphasis Type="Italic">In Vivo</Emphasis> Loads

Biomechanical models of the spine either simplify intervertebral joints (using spherical joints or deformable beams) in musculoskeletal (MS) or overlook musculature in geometrically-detailed passive finite element (FE) models. These distinct active and passive models therefore fail to determine in vivo stresses and strains within and load-sharing among the joint structures (discs, ligaments, and facets). A novel hybrid active–passive spine model is therefore developed in which estimated trunk muscle forces from a MS model for in vivo activities drive a mechanically-equivalent passive FE model to quantify in vivo T12–S1 compression/shear loads, intradiscal pressures (IDP), centers of rotation (CoR), ligament/facet forces, and annulus fiber strains. The predicted and in vivo L4–L5 IDP and L1–S1 CoRs showed satisfactory agreements. The FE model under commonly-used in vitro loading (pure moments and follower loads) predicted different kinetics from those of the hybrid model under in vivo loads (muscle exertions and gravity loads) contributing to suggest the inadequacy of such in vitro loads when simulating in vivo tasks. For an improved assessment of the injury risk, evaluation of the internal loads, and design of implants, such hybrid models should therefore be used.     

Spinal muscles can create compressive follower loads in the lumbar spine in a neutral standing posture

The ligamentous spinal column buckles under compressive loads of even less than 100N. Experimental results showed that under the follower load constraint, the ligamentous lumbar spine can sustain large compressive loads without buckling, while at the same time maintaining its flexibility reasonably well. The purpose of this study was to investigate the feasibility of follower loads produced by spinal muscles in the lumbar spine in a quiet standing posture. A three-dimensional static model of the lumbar spine incorporating 232 back muscles was developed and utilized to perform the optimization analysis in order to find the muscle forces, and compressive follower loads (CFLs) along optimum follower load paths (FLPs). The effect of increasing external loads on CFLs was also investigated. An optimum solution was found which is feasible for muscle forces producing minimum CFLs along the FLP located 11 mm posterior to the curve connecting the geometrical centers of the vertebral bodies. Activation of 30 muscles was found to create CFLs with zero joint moments in all intervertebral joints. CFLs increased with increasing external loads including FLP deviations from the optimum location. Our results demonstrate that spinal muscles can create CFLs in the lumbar spine in a neutral standing posture in vivo to sustain stability. Therefore, its application in experimental and numerical studies concerning loading conditions seems to be suitable for the attainment of realistic results.     

Finite element analysis of the spondylolysis in lumbar spine

Spondylolysis is a fracture of the bone lamina in the pars interarticularis and has a high risk of developing spondylolisthesis, as well as traction on the spinal cord and nerve root, leading to spinal disorders or low back pain when the lumbar spine is subjected to high external forces. Previous studies mostly investigated the mechanical changes of the endplate in spondylolysis. However, little attention has been focused on the entire structural changes that occur in spondylolysis. Therefore, the purpose of this study was to evaluate the biomechanical changes in posterior ligaments, disc, endplate, and pars interarticularis between the intact lumbar spine and spondylolysis. A total of three finite element models, namely the intact L2-L4 lumbar spine, lumbar spine with unilateral pars defect and with bilateral pars defect were established using a software ANSYS 6.0. A loading of 10 N.m in flexion, extension, left torsion, right torsion, left lateral bending, and right lateral bending respectively were imposed on the superior surface of the L2 body. The bottom of the L4 vertebral body was completely constrained. The finite element models estimated that the lumbar spine with a unilateral pars defect was able to maintain spinal stability as the intact lumbar spine, but the contralateral pars experienced greater stress. For the lumbar spine with a bilateral pars defect, the rotation angle, the vertebral body displacement, the disc stress, and the endplate stress, was increased more when compared to the intact lumbar spine under extension or torsion.     

斜外侧椎间融合技术的研究进展

斜外侧椎间融合术(OLIF)是目前脊柱外科新开展的一项微创椎间融合术。该术式从前侧方斜行经左下腹腹外斜肌、腹内斜肌、腹横肌的肌间隙进入腹膜外间隙,在左侧腰大肌前缘和腹部大血管鞘之间的生理间隙置入器械通道,通过置入更大的椎间融合器撑开椎间隙达到椎管和椎间孔的间接减压。OLIF适用于腰椎滑脱、腰椎管狭窄症、腰椎间盘突出合并节段不稳、退行性脊柱侧后凸畸形等情况。该术式具有创伤小、手术时间短、术中出血少、住院时间短、术后康复快、间接减压效果明显及临床疗效肯定等优点,并且能够有效避免后路椎间融合术引起的脊柱后方肌肉和韧带等软组织、骨性结构、脊髓和神经损伤。但OLIF作为一项新技术,术后出现供骨区疼痛、屈髋乏力、大腿疼痛麻木、终板骨折、融合器下沉、血管和神经损伤等并发症屡见报道。本文主要从OLIF手术的解剖结构、适应证、禁忌证、临床疗效及并发症等方面作一综述。     

Finite element modeling of the cervical spine: role of intervertebral disc under axial and eccentric loads

An anatomically accurate, three-dimensional, nonlinear finite element model of the human cervical spine was developed using computed tomography images and cryomicrotome sections. The detailed model included the cortical bone, cancellous core, endplate, lamina, pedicle, transverse processes and spinous processes of the vertebrae; the annulus fibrosus and nucleus pulposus of the intervertebral discs; the uncovertebral joints; the articular cartilage, the synovial fluid and synovial membrane of the facet joints; and the anterior and posterior longitudinal ligaments, interspinous ligaments, capsular ligaments and ligamentum flavum. The finite element model was validated with experimental results: force–displacement and localized strain responses of the vertebral body and lateral masses under pure compression, and varying eccentric anterior-compression and posterior-compression loading modes. This experimentally validated finite element model was used to study the biomechanics of the cervical spine intervertebral disc by quantifying the internal axial and shear forces resisted by the ventral, middle, and dorsal regions of the disc under the above axial and eccentric loading modes. Results indicated that higher axial forces (compared to shear forces) were transmitted through different regions of the disc under all loading modes. While the ventral region of the disc resisted higher variations in axial force, the dorsal region transmitted higher shear forces under all loading modes. These findings may offer an insight to better understand the biomechanical role of the human cervical spine intervertebral disc.     

Lumbar spinal loads vary with body height and weight

Knowledge about spinal loading is required for designing and preclinical testing of spinal implants. It is assumed that loading of the spine depends upon body weight and height, as well as on the spine level, but a direct measurement of the loading conditions throughout the spine is not yet possible. Here, computer models can allow an estimation of the forces and moments acting in the spine. The objective of the present study was to calculate spinal loads for different postures and activities at several levels of the thoracolumbar spine for various combinations of body height and weight.A validated musculoskeletal model, together with commercially available software (AnyBody Technology), were used to calculate the segmental loads acting on the centre of the upper endplate of the vertebrae T12 to L5. The body height was varied between 150 and 200 cm and the weight between 50 and 120 kg. The loads were determined for five standard static postures and three lifting tasks.The resultant forces and moments increased approximately linearly with increasing body weight. The body height had a nearly linear effect on the spinal loads, but in almost all loading cases, the effect on spinal loads was stronger for variation of body weight than of body height. Spinal loads generally increased from cranial to caudal.The presented data now allow the estimation of the spinal load during activities of daily living on a subject specific basis, if body height and weight are known.     

Evaluation of the role of ligaments, facets and disc nucleus in lower cervical spine under compression and sagittal moments using finite element method.

Cervical spinal instability due to ligamentous injury, degenerated disc and facetectomy is a subject of great controversy. There is no analytical investigation reported on the biomechanical response of cervical spine in these respects. Parametric study on the roles of ligaments, facets, and disc nucleus of human lower cervical spine (C4-C6) was conducted for the very first time using noninvasive finite element method.A three-dimensional (3D) finite element (FE) model of the human lower cervical spine, consisted of 11,187 nodes and 7730 elements modeling the bony vertebrae, articulating facets, intervertebral disc, and associated ligaments, was developed and validated against the published data under three load configurations: axial compression; flexion; and extension. The FE model was further modified accordingly to investigate the role of disc, facets and ligaments in preserving cervical spine motion segment stability in these load configurations. The passive FE model predicted the nonlinear force displacement response of the human cervical spine, with increasing stiffness at higher loads. It also predicted that ligaments, facets or disc nucleus are crucial to maintain the cervical spine stability, in terms of sagittal rotational movement or redistribution of load. FE method of analysis is an invaluable application that can supplement experimental research in understanding the clinical biomechanics of the human cervical spine.     

颈椎人工椎间盘置换有限元分析

目的　建立C4~5节段PrestigeTM-LP颈椎人工椎间盘植入后的三维有限元模型,进行手术节段的运动分析。 方法 采用对成年男性的新鲜尸体的颈椎标本进行CT三维扫描方法建立C4~5节段和PrestigeTM-LP人工间盘有限元,模拟完成C4~5人工椎间盘置换手术。测量生理加载下手术节段前屈/后伸、侧弯及轴向旋转运动角度。结果 有限元模型对颈椎的结构,包括椎体间韧带、颈椎关节突关节、钩椎关节等均进行了精确的重建,并较好地模拟手术操作进行PrestigeTM-LP人工间盘植入。运动加载后运动角度,前屈5.7°,后伸3.5°,侧弯5.0°,旋转11.3°,与文献报道结果较为接近。 结论　有限元模型具有精确度高,手术模拟真实的特点,可作为颈椎人工椎间盘生物力学研究的一种较好途径。PrestigeTM-LP颈椎人工椎间盘置换可较好地保留手术节段的运动功能。     

脊柱跟随载荷在离体生物力学应用研究中的进展

阐述跟随载荷在维持脊柱生物力学中的重要性,归纳近年来人离体脊柱标本跟随载荷模拟的各种方法及手段。通过与人体脊柱各椎体活动度、椎间盘内压等真实数据对比,从力学角度分析各类模拟手段的可行性,总结人体颈椎、胸椎、腰椎离体生物力学实验中最适合的加载载荷及扭矩,并探讨常规脊柱内固定术式对脊柱生物力学特性的影响。     

穿戴模拟重力服对微重力环境下人腰椎间盘退变影响

目的分析微重力环境下穿戴模拟重力服对人腰椎间盘的生物力学影响。方法基于健康成年志愿者CT影像,建立人L4～5脊柱有限元模型。在腰椎有限元模型空载荷以及加载4 h 400 N轴向载荷基础上,分别模拟微重力环境下无干预和穿戴模拟重力服对腰椎间盘的生物力学影响。结果微重力环境下,人腰椎间盘中心孔压、径向位移和含水量随时间推移而增大。微重力环境穿戴模拟重力服情况下,72 h后腰椎间盘中心孔压、轴向应力、径向位移和含水量与无干预组相比均有减小。结论微重力环境下,穿戴模拟重力服了在一定程度上可帮助宇航员对抗微重力造成的对脊柱的不利影响。     

关节突植骨融合联合竖脊肌离断建立邻近椎间盘退行性变动物模型的初步研究

目的研究关节突植骨融合联合竖脊肌离断的方法建立邻近椎间盘退行性变动物模型可行性。方法选用日本大耳白兔24只,3个月龄,雌雄不限,体质量（2．5±0．5）kg,随机平均分成2组。实验组切除L3-4、L4-5、L5-6。双侧关节突并原位进行植骨,同时横断L1水平双侧竖脊肌;对照组只切开皮肤即缝合。术后2个月和6个月,从形态学和影像学（三维CT重建和MRI、观察腰椎融合情况和邻近椎间盘退行性变情况。结果实验组术后2个月时,L3-6。椎体关节突处骨赘增生．腰椎活动受限,腰椎后结构发生融合;6个月时,L2-3,椎间盘高度降低,含水量减少,组织学提示纤维环排列不规整并出现裂隙．发生退行性改变.对照组未见明显变化。结论应用破坏腰椎关节突并植骨联合切断双侧竖脊肌的方法,可以建立腰椎后结构融合导致邻近椎间盘退行性变的实验动物模型．为研究邻近椎间盘退行性变的发生机制奠定研究基础。     

腰椎间盘突出症力学特征的仿真计算方法

目的建立腰椎间盘突出症力学特征的数值计算分析模型,为腰椎间盘突出症的力学机理提供一种检测评估方法。方法利用健康成人L4～5腰椎运动节段CT影像,采用Mimics 10.01医学图像处理软件和Geomagic10.0逆向工程软件分别建立L4～5腰椎运动节段的椎骨和椎间盘,并在Ansys软件中附加腰椎相关韧带及通过改变椎间盘突出后对应的材料属性,建立腰椎L4～5运动节段有限元模型,构建正常模型和腰椎间盘突出模型;运用有限元方法模拟正常椎间盘和突出椎间盘在轴向压力、前弯、侧弯、旋转和后伸5种载荷下的生物力学特征参数。结果椎间盘突出后,椎间盘的应力分布及传递载荷的能力改变,应力集中于纤维环后外侧;在相同的载荷情况下,突出的椎间盘的最大形变量比正常椎间盘的大;椎间盘突出模型的小关节突接触力比正常模型的小关节突接触力大。结论椎间盘突出后,椎间盘的承载功能下降,关节突的应力水平升高,小关节的负荷增加,从而导致腰椎稳度下降。     

Comparison of four methods to simulate swelling in poroelastic finite element models of intervertebral discs

Osmotic phenomena influence the intervertebral disc biomechanics. Their simulation is challenging and can be undertaken at different levels of complexity. Four distinct approaches to simulate the osmotic behaviour of the intervertebral disc (a fixed boundary pore pressure model, a fixed osmotic pressure gradient model in the whole disc or only in the nucleus pulposus, and a swelling model with strain-dependent osmotic pressure) were analysed. Predictions were compared using a 3D poroelastic finite element model of a L4–L5 spinal unit under three different loading conditions: free swelling for 8 h and two daily loading cycles: (i) 200 N compression for 8 h followed by 500 N compression for 16 h; (ii) 500 N for 8 h followed by 1000 N for 16 h. Overall, all swelling models calculated comparable results, with differences decreasing under greater loads. Results predicted with the fixed boundary pore pressure and the fixed osmotic pressure in the whole disc models were nearly identical. The boundary pore pressure model, however, cannot simulate differential osmotic pressures in disc regions. The swelling model offered the best potential to provide more accurate results, conditional upon availability of reliable values for the required coefficients and material properties. Possible fields of application include mechanobiology investigations and crack opening and propagation. However, the other approaches are a good compromise between the ease of implementation and the reliability of results, especially when considering higher loads or when the focus is on global results such as spinal kinematics.     

The relationship between disc degeneration and morphologic changes in the intervertebral foramen of the cervical spine: a cadaveric MRI and CT study

A cadaveric study was performed to investigate the relationship between disc degeneration and morphological changes in the intervertebral foramen of cervical spine, including the effect on the nerve root. Seven fresh frozen human cadavers were dissected from C1 to T1, preserving the ligaments, capsules, intervertebral disc and the neural structures. The specimens were scanned with MRI and then scanned through CT scan in the upright position. Direct mid-sagittal and 45 degree oblique images were obtained to measure the dimension of the intervertebral disc height, foraminal height, width, area and segmental angles. Disc degeneration was inversely correlated with disc height. There was a significant correlation between disc degeneration and foraminal width (p<0.005) and foraminal area (p< 0.05), but not with foraminal height. Disc height was correlated with foraminal width but not with height. The segmental angles were decreased more in advanced degenerated discs. There was a correlation between nerve root compression and decreased foraminal width and area (p<0.005). This information and critical dimensions of the intervertebral foramen for nerve root compression should help in the diagnosis of foraminal stenosis of the cervical spine in patients presenting with cervical spondylosis and radiculopathy.     

Sensitivity analysis of the position of the intervertebral centres of reaction in upright standing – a musculoskeletal model investigation of the lumbar spine

The loads between adjacent vertebrae can be generalised as a single spatial force acting at the intervertebral centre of reaction. The exact position in vivo is unknown. However, in rigid body musculoskeletal models that simulate upright standing, the position is generally assumed to be located at the discs’ centres of rotation. The influence of the antero-posterior position of the centre of reaction on muscle activity and joint loads remains unknown. Thus, by using an inverse dynamic model, we varied the position of the centre of reaction at L4/L5 (i), simultaneously at all lumbar levels (ii), and by optimisation at all lumbar levels (iii). Variation of the centres of reaction can considerably influence the activities of lumbar muscles and the joint forces between vertebrae. The optimisation of the position of the centre of reaction reduced the maximum lumbar muscle activity and axial joint forces at L4/L5 from 17.5% to 1.5% of the muscle strength and from 490 N to 390 N, respectively. Thus, when studying individual postures, such as for therapeutic or preventive evaluations, potential differences between the centre of reaction and the centre of rotation might influence the study results. These differences could be taken into account by sensitivity analyses.     

Effects of degenerated intervertebral discs on intersegmental rotations,intradiscal pressures,and facet joint forces of the whole lumbar spine

The effects of intervertebral disc (IVD) degeneration on biomechanics of the lumbar spine were analyzed. Finite element models of the lumbar spine with various degrees of IVD degeneration at the L4-L5 functional spinal unit (FSU) were developed and validated. With progression of degeneration, intersegmental rotation at the degenerated FSU decreased in flexion–extension and left–right lateral bending, intradiscal pressure at the adjacent FSUs increased in flexion and lateral bending, and facet joint forces at the degenerated FSU increased in lateral bending and axial rotation. These results could provide fundamental information for understanding the mechanism of injuries caused by IVD degeneration.     

肌肉加载下腰椎间盘突出的有限元研究

目的通过观察腰椎间盘突出症(lumbar disc herniation,LDH)患者肌肉加载下腰椎有限元模型的应力变化,探讨LDH患者肌肉功能对结构应力的影响。方法选取正常志愿者、LDH患者各1名,采集CT数据建立相应的正常、LDH腰椎-骨盆三维有限元模型,同时采集其步态数据驱动Any Body仿真肌骨模型,得到附着在腰椎骨盆周围肌肉的肌力及髋关节力作为加载条件,分别进行自身加载和正常模型加载LDH肌肉力,比较两种加载情况下L4、L5椎间盘及骶髂关节两侧应力变化。结果正常模型加载LDH肌肉力后,自身加载时的双峰曲线消失,代之以异常的单峰曲线,与LDH模型自身加载后的时间-应力曲线变化趋势一致。LDH患者肌肉力加载于正常模型后,L4、L5椎间盘及骶髂关节两侧应力差值较LDH模型自身加载后的应力差值减小。结论 LDH患者腰椎骨盆肌肉功能异常会引起腰椎及骶髂关节应力异常,结构失衡本身可导致应力失衡,而肌肉作为动力因素是导致结构动态应力异常的重要原因,由此可导致关节运动模式的异常。临床治疗LDH要重视对周围肌肉功能失衡的评估。     

轴向振动时节段曲度对腰椎间盘应力演化的影响

研究人体腰椎运动节段承受长期轴向振动载荷时节段曲度对腰椎间盘应力演化的影响。基于人体腰椎L4～5节段CT扫描数据,建立人体腰椎L4～5节段的有限元模型。对腰椎间盘赋予多孔材料属性,并验证有限元模型的有效性。基于有限元模型,模拟L4～5节段以3种不同节段曲度(中立位、伸展2°、弯曲2°)承受时长为1 000 s的轴向振动的过程,得到这3种节段曲度下腰椎间盘的应力演化情况。各个曲度下腰椎间盘纤维环部分的峰值轴向应力均出现在其后外侧。在受载过程中,各个曲度下纤维环峰值轴向应力均呈非线性增大,且增速不断减小,至1 000 s时已趋于稳定。1 000 s时,伸展2°下纤维环峰值轴向应力比中立位下大39%,比弯曲2°下大109%。在受载过程中,各个曲度下髓核轴向应力亦呈非线性增长,增速不断减小。1 000 s时,伸展2°下髓核的轴向应力略小于其他两种情形。当L4～5节段以伸展2°的状态受载时,腰椎间盘受到的损伤最为严重;而当其以弯曲2°的状态受载时,腰椎间盘受到的损伤最小。当长时间处于全身振动条件下时,应尽量避免使腰椎处于向后伸展的姿态,而腰椎的小幅前屈可以保护腰椎间盘。     

Influences of walking speed change on the lumbosacral joint force distribution

To more understand the influence of the walking speed on the spinal joint force distribution, a three-dimensional biomechanical model was used to estimate the spine loads during human gait with three different walking speeds. This previously developed and validated model included a dynamic external model and an internal model with forces of disc, 8 major muscles, 2 ligaments and 2 facet joints at L5/S1 level. A linear optimization method was used to solve the internal model to estimate the L5/S1 spinal joint force distribution. The results of five young male subjects showed that the mean peak L5/S1 disc compressive forces on the slow, preferred and fast speeds were 2.28, 2.53, 2.95 body weight, respectively. The peak forces happened right after the heel strike and before completely toe off. The facet joint forces were generally increased with the walking speed increase, too. To reduce the loads on the spine, the slow walking is then recommended for the patients with low back pain or after spinal surgery.