Finite Element Analysis of Interbody Cages in a Human Lumbar Spine

Abstract

An innovative surgical procedure is vertebral stabilization by interbody cages. It is currently being used to separate and stabilize vertebral bodies and to promote bony fusion of the vertebrae onto or through the cages. This surgery, at some spine levels, can be performed through a laparoscope as an outpatient procedure with low morbidity. Because the procedure is new, little structural information is available on the interbody cages. The objective of this study was to evaluate the human lumbar spine stabilized by interbody cages biomechanically. The finite element method was used to compare cage designs by considering stresses in the cage and in the bone as well as relative displacements between the cage and the adjacent bone at the interface. The biomechanical evaluation considered different bone densities and considered axial, torsional, and bending loads on the lumbar spine. Stress analysis predicts local regions of stress concentration that could be damaging to cancellous bone and will likely require a remodeling response for local damage. This study predicts relative micromotion that could cause the bone resorption and fibrous tissue formation on the contact surfaces of the cage. The geometric constraints caused by the use of two cages will reduce the relative motion and therefore be more likely to allow bone ingrowth at the posterocentral contact region. Finite element analysis suggests that cages are a promising method for separation and stabilization of the vertebral bodies.     

Computational analysis of bone remodeling during an anterior cervical fusion

The anterior cervical fusion is an established surgical procedure for spine stabilization after the removal of an intervertebral disc. However, it is not yet clear which bone graft represents the best choice and whether surgical devices can be efficient and beneficial for fusion. The aim of this work is to study the influence of the spine instrumentation on bone remodeling after a cervical spine surgery and, consequently, on the fusion process. A finite element model of the cervical spine was developed, having computed tomography images as input. Bone was modeled as a porous material characterized by the relative density at each point and the bone remodeling law was derived assuming that bone self-adapts in order to achieve the stiffest structure for the supported loads, with the total bone mass regulated by the metabolic cost of maintaining bone tissue. Apart from the analysis of healthy cervical spine, different surgical scenarios were tested: bone graft with or without a cage and the use of a stabilization plate system. Results showed that the anterior and posterior regions of the disc space are more important to stress transmission and that spinal devices reduce bone growth within bone grafts, being plate systems the most interfering elements. The material of the interbody cages plays a major role in fusion and, therefore, it should be carefully chosen.     

Biomechanical evaluation of a novel lumbosacral axial fixation device

BACKGROUND: Interbody arthrodesis is employed in the lumbar spine to eliminate painful motion and achieve stability through bony fusion. Bone grafts, metal cages, composite spacers, and growth factors are available and can be placed through traditional open techniques or minimally invasively. Whether placed anteriorly, posteriorly, or laterally, insertion of these implants necessitates compromise of the anulus--an inherently destabilizing procedure. A new axial percutaneous approach to the lumbosacral spine has been described. Using this technique, vertical access to the lumbosacral spine is achieved percutaneously via the presacral space. An implant that can be placed across a motion segment without compromise to the anulus avoids surgical destabilization and may be advantageous for interbody arthrodesis. The purpose of this study was to evaluate the in vitro biomechanical performance of the axial fixation rod, an anulus sparing, centrally placed interbody fusion implant for motion segment stabilization. METHOD OF APPROACH: Twenty-four bovine lumbar motion segments were mechanically tested using an unconstrainedflexibility protocol in sagittal and lateral bending, and torsion. Motion segments were also tested in axial compression. Each specimen was tested in an intact state, then drilled (simulating a transaxial approach to the lumbosacral spine), then with one of two axial fixation rods placed in the spine for stabilization. The range of motion, bending stiffness, and axial compressive stiffness were determined for each test condition. Results were compared to those previously reported for femoral ring allografts, bone dowels, BAK and BAK Proximity cages, Ray TFC, Brantigan ALIF and TLIF implants, the InFix Device, Danek TIBFD, single and double Harms cages, and Kaneda, Isola, and University plating systems. RESULTS: While axial drilling of specimens had little effect on stiffness and range of motion, specimens implanted with the axial fixation rod exhibited significant increases in stiffness and decreases in range of motion relative to intact state. When compared to existing anterior, posterior, and interbody instrumentation, lateral and sagittal bending stiffness of the axial fixation rod exceeded that of all other interbody devices, while stiffness in extension and axial compression were comparable to plate and rod constructs. Torsional stiffness was comparable to other interbody constructs and slightly lower than plate and rod constructs. CONCLUSIONS: For stabilization of the L5-S1 motion segment, axial placement of implants offers potential benefits relative to traditional exposures. The preliminary biomechanical data from this study indicate that the axial fixation rod compares favorably to other devices and may be suitable to reduce pathologic motion at L5-S1, thus promoting bony fusion.     

Animal Modelling of Lumbar Corpectomy and Fusion and in vivo Growth of Spine Supporting Bone by Titanium Cage Implants: An Experimental Study

##正## In this study a lumbar spinal fusion animal model is established to assess the effect of spinal fusion cage,and explore theminimum area ratio of titanium cage section to vertebral section that ensures bone healing and biomechanical property.Lumbarcorpectomy was conducted by posterolateral approach with titanium cage implantation combined with plate fixation.Titaniumcages with the same length but different diameters were used.After implantation of titanium cages,the progress of bone healingwas observed and the bone biomechanical properties were measured,including deformation and displacement in axial compression,flexion,extension,and lateral bending motion.The factors affecting the in vivo growth of spine supporting body wereanalyzed.The results show that the area ratio of titanium cage section to vertebral section should reach 1/2 to ensure the bonehealing,sufficient bone intensity and biomechanical properties.Some bone healing indicators,such as BMP,suggest that there isa relationship between the peak time and the peak value of bone formation and metabolism markers and the bone healing strength.     

腰椎Cage手术并发症分析及其防治

目的:分析后路腰椎椎间cage融合术常见的并发症并探讨对策。方法:对89例腰椎间盘突出症、下腰椎失稳症患者,经临床症状、体征和影像资料明确诊断且具有手术指征,并采用后路cage融合术治疗,对其中出现的并发症进行原因分析。结果:89例中出现并发症的有24例,包括神经症状加重、脑脊液漏、感染等近期并发症和症状缓解不明显、植骨不融合等远期并发症。术后神经症状加重5例,主要与手术适应症选择正确与否、影像学资料阅读能力高低、术者操作技能熟练程度等因素有关。结论:cage融合术是外科治疗椎间盘突出症的一种优良方法,术后神经症状加重和神经根损伤是最常见的并发症,正确把握手术适应症、熟练掌握操作技巧、提高并发症的诊断和处理能力是顺利开展该技术、提高临床疗效的关键。     

Prediction of biomechanical parameters in the lumbar spine during static sagittal plane lifting.

A combined approach involving optimization and the finite element technique was used to predict biomechanical parameters in the lumbar spine during static lifting in the sagittal plane. Forces in muscle fascicles of the lumbar region were first predicted using an optimization-based force model including the entire lumbar spine. These muscle forces as well as the distributed upper body weight and the lifted load were then applied to a three-dimensional finite element model of the thoracolumbar spine and rib cage to predict deformation, the intradiskal pressure, strains, stresses, and load transfer paths in the spine. The predicted intradiskal pressures in the L3-4 disk at the most deviated from the in vivo measurements by 8.2 percent for the four lifting cases analyzed. The lumbosacral joint flexed, while the other lumbar joints extended for all of the four lifting cases studied (rotation of a joint is the relative rotation between its two vertebral bodies). High stresses were predicted in the posterolateral regions of the endplates and at the junctions of the pedicles and vertebral bodies. High interlaminar shear stresses were found in the posterolateral regions of the lumbar disks. While the facet joints of the upper two lumbar segments did not transmit any load, the facet joints of the lower two lumbar segments experienced significant loads. The ligaments of all lumbar motion segments except the lumbosacral junction provided only marginal moments. The limitations of the current model and possible improvements are discussed.     

Mechanical study of spinal interbody implants--characteristics and limits of standardized testing]

Spinal interbody fusion has proved to be a useful procedure for the surgical stabilization of spinal segments, for which fusion cases made of metal or reinforced polymers are increasingly being used. For the mechanical testing of spinal interbody implants, a test setup has been developed on the basis of an ASTM proposal. Initially, testing of lumbar fusion cages made of CFRP (carbon fibre reinforced polymer) was carried out. The implants (UNION Cages, Medtronic Sofamor Danek), which are characterised by their radiolucency on radiography, NMR and CT scans, have a cube-shaped body with three table-tracks on the under and upper surfaces. The cages were tested at different loads. Modifications of the proposed standardized method were carried out to enable implementation of implant-oriented testing. The tested cages were shown to have adequate axial compression, shear and torsional strengths with regard to the implant body. The maximum axial compression force tolerated by the table-tracks was less than the maximal potential loading of the lumbar spine, and, with account being taken of implant design, consequences with regard to surgical technique were drawn. As dictated by the geometry of the table-tracks, parallel grooves have to be made intra-operatively in the vertebral end plates. Axial compressive loads then act on the implant body, and the table-tracks are protected from damage. To avoid in vivo failure, the tested cages should be implanted only when this specific surgical technique is employed. Using supplementary anterior or posterior instrumentation, in vivo failure of the table-tracks under physiological spinal loading is not to be expected.     

Finite Element Analysis of Minimal Invasive Transforaminal Lumbar Interbody Fusion

The purpose of our study is to develop and validate three-dimensional finite element models of transforaminal lumbar interbody fusion, and explore the most appropriate method of fixation and fusion by comparing biomechanical characteristics of different fixation method. We developed four fusion models: bilateral pedicle screws fixation with a single cage insertion model (A), bilateral pedicle screws fixation with two cages insertion model (B), unilateral pedicle screws fixation with a single cage insertion model (C), and unilateral pedicle screws fixation with two cages insertion model (D); the models were subjected to different forces including anterior bending, posterior extension, left bending, right bending, rotation, and axial compressive. The von Mises stress of the fusion segments on the pedicle screw and cages was recorded. Angular variation and stress of pedicle screw and cage were compared. There were differences of Von Mises peak stress among four models, but were within the range of maximum force. The angular variation in A, B, C, and D decreased significantly compared with normal. There was no significant difference of angular variation between A and B, and C and D. Bilateral pedicle screws fixation had more superior biomechanics than unilateral pedicle screws fixation. In conclusion, the lumbar interbody fusion models were established using varying fixation methods, and the results verified that unilateral pedicle screws fixation with a single cage could meet the stability demand in minimal invasive transforaminal interbody fusion.     

Biomechanicsl evaluation of a stand-alone interbody fusion cage based on porous TiO2/glass-ceramic on the human cervical spine]

Recently, there has been a rapid increase in the use of cervical spine interbody fusion cages, differing in design and biomaterial used, in competition to autologous iliac bone graft and bone cement (PMMA). Limited biomechanical differences in primary stability, as well as advantages and disadvantages of each cage or material have been investigated in studies, using an in vitro human cervical spine model. 20 human cervical spine specimens were tested after fusion with either a cubical stand-alone interbody fusion cage manufactured from a new porous TiO2/glass composite (Ecopore) or PMMA after discectomy. Non-destructive biomechanical testing was performed, including flexion/extension and lateral bending using a spine testing apparatus. Three-dimensional segmental range of motion (ROM) was evaluated using an ultrasound measurement system. ROM increased more in flexion/extension and lateral bending after PMMA fusion (26.5%/36.1%), then after implantation of the Ecopore-cage (8.1%/7.8%). In this first biomechanical in vitro examination of a new porous ceramic bone replacement material a) the feasibility and reproducibility of biomechanical cadaveric cervical examination and its applicability was demonstrated, b) the stability of the ceramic cage as a stand alone interbody cage was confirmed in vitro, and c) basic information and knowledge for our intended biomechanical and histological in vivo testing, after implantation of Ecopore in cervical sheep spines, were obtained.     

Investigation of different cage designs and mechano-regulation algorithms in the lumbar interbody fusion process – A finite element analysis

Lumbar interbody fusion cages are commonly used to treat painful spinal degeneration and instability by achieving bony fusion. Many different cage designs exist, however the effect of cage morphology and material properties on the fusion process remains largely unknown. This finite element model study aims to investigate the influence of different cage designs on bone fusion using two mechano-regulation algorithms of tissue formation. It could be observed that different cages play a distinct key role in the mechanical conditions within the fusion region and therefore regulate the time course of the fusion process.     

Effect of a double intervertebral cage on the mechanical behavior of the lumbar spine]

A three-dimensional nonlinear finite element model of the lumbar spine was developed. Paired threaded cages and a monosegmental internal spinal fixation device were integrated into the computer model. The model was loaded with such forces as apply during standing, as well as with pure moments in the three main anatomical planes, plus an additional preload. The latter was generated by shortening the distance between the pedicle screws on the longitudinal rod of the fixator. With the exception of torsional loading, an implant appreciably reduces the mobility in the segment concerned. At the loads studied, cages had only a minor impact on the movements and stresses in the adjacent regions, but a strong influence on the stresses in the endplates in contact with them. A preload increases these stresses dramatically. Contact conditions between vertebral body and cages also have a marked effect on the stress distribution in the corresponding vertebral endplate, especially in the case of extension loading. Owing to the preload, maximum stresses were higher for the rigid bond than when contact elements were used.     

Development of an integrated CAD–FEA system for patient-specific design of spinal cages

Spinal cages are used to create a suitable mechanical environment for interbody fusion in cases of degenerative spinal instability. Due to individual variations in bone structures and pathological conditions, patient-specific cages can provide optimal biomechanical conditions for fusion, strengthening patient recovery. Finite element analysis (FEA) is a valuable tool in the biomechanical evaluation of patient-specific cage designs, but the time- and labor-intensive process of modeling limits its clinical application. In an effort to facilitate the design and analysis of patient-specific spinal cages, an integrated CAD–FEA system (CASCaDeS, comprehensive analytical spinal cage design system) was developed. This system produces a biomechanical-based patient-specific design of spinal cages and is capable of rapid implementation of finite element modeling. By comparison with commercial software, this system was validated and proven to be both accurate and efficient. CASCaDeS can be used to design patient-specific cages with a superior biomechanical performance to commercial spinal cages.     

SDRS联合BAK技术治疗腰椎滑脱症的研究

对后方入路SDRS内固定加BAK植骨融合治疗腰椎滑脱症的临床应用研究进行初步报告,探讨此项技术的手术要点和早期临床效果。自2001年1月至2001年8月,对13例腰椎滑脱患者行后方入路椎体间BAK植骨融合、SDRS内固定。随访6个月－14个月,平均9.1个月。结合临床症状改善程度和X线片上植骨副合、复位程度综合进行疗效评定。结果：疗效优3例,良9例,差1例,优良率92.3%。结果表明从一个切口入路行椎体间BAK植骨融合、SDRS内固定术,能对前柱和后柱同时起稳定作用,有利于滑脱的复位和维持正常的腰椎前凸,符合腰椎的生物力学要求。     

单、双枚椎间融合器在单节段腰椎后路融合术中的应用效果比较

目的:比较腰椎后路融合术(PLIF)单、双枚椎间融合器(Cage)的使用对远期疗效的影响。方法:回顾我院2004-2011年间164例因腰椎退变疾病行单节段PLIF患者的临床资料,按Cage数目将其分为单Cage组(114例)与双Cage组(50例)。获得其住院资料和术后2年以上临床与影像学随访资料并比较分析。结果:平均随访时间47.5个月。两组间的年龄、性别、术前JOA评分、术后随访时间的差别无显著性(P0.05),但单Cage组失血量更低(P0.05),但随访时相对椎间隙高度低于双Cage组(P0.05);而两组间JOA评分改善率、随访时JOA评分、优良率、手术耗时、住院天数、并发症发生率的差别无显著性(P0.05)。结论:PLIF中单、双Cage的使用均能获得满意的疗效与安全性,与双Cage相比,单Cage手术失血更少,但术后远期相对椎间隙高度更低,对于这类患者应加强随访并警惕断钉等并发症的发生。     

Effect of vertebral body stiffness before and after vertebroplasty on intradiscal pressure]

Fractures of osteoporotic vertebral bodies are increasingly stabilized with bone cement. The effects of vertebral-body stiffness before and after augmentation with bone cement and of wedge-shaped vertebral body fractures on intradiscal pressure are insufficiently known. In a finite element model of the lumbar spine the elastic modulus of cancellous bone as well as the amount and the elastic modulus of bone cement were varied and the dependency of intradiscal pressure on these parameters was calculated. In addition, a wedge-shaped vertebral-body fracture was simulated. The bulge of the vertebral-body endplate and thus the intradiscal pressure depends strongly on the grade of osteoporosis in the vertebral body. The influence of amount and elastic modulus of bone cement on intradiscal pressure is small. A wedge-shaped vertebral-body fracture causes an anterior shift of upper-body centre of gravity. If this shift is not compensated, it leads to an increased flexion moment that has to be balanced by muscle forces. In addition, this shift leads to a stronger increase of intradiscal pressure than the augmentation of the vertebral body with bone cement.     

A novel method to 'exercise' rats: making rats rise to erect bipedal stance for feeding - raised cage model

We employed a novel method to exercise rats: making them rise to bipedal stance for feeding using raised cages. We studied its effects on the skeletons of 6 and 10-month-old intact or orchidectomized (ORX) rats. Body and hindlimb muscle weights, tibial BMC and periosteal cortical bone formation increased after housing in raised cages, but more so in 6-month-old animals than in 10-month-old ones. In 6-month-old orchidectomized rats, raised cages partially prevented ORX-induced bone loss by stimulating periosteal cortical bone (TX) formation and decreased bone resorption next to marrow. In 10-month-old male orchidectomized rats, raised cages also decreased the endosteal and trabecular bone resorption, but not enough to prevent completely ORX-induced net bone losses. Because the osteogenic effects of raised cages alone were only partial, we also studied the interaction between raised cage and prostaglandin E(2) (PGE(2)) in 10-month-old retired female breeders. When treated with combined raised cage and PGE(2), both cortical (TX) and trabecular bone mass of the proximal tibial metaphysis and lumbar vertebral body increased over either raised cages or PGE(2) treatment alone, that was accompanied by dramatic increased bone formation at periosteal and endosteal surfaces. Thus making rats rise to erect bipedal stance for feeding helps to prevent bone loss after orchidectomy; it amplifies the anabolic effects of PGE(2), and it provides an inexpensive, non-invasive and reliable way to increase mechanical loading of certain bones of the rat skeleton.     

Regeneration of the skeleton by recombinant human bone morphogenetic proteins

Recombinant human bone morphogenetic proteins (rhBMPs) have past a long journey in human orthopaedic surgery during the last 15 years. From the first reports of the use of rhBMPs in hostile environments such as critically-sized bone defects, avascular femoral head necrosis, unstable thoracolumbar vertebral fractures, instability between the atlas and axis due to rheumatoid arthritis; over the use for nonunions of long bones and the scaphoid, reconstructive and revision surgeries of the hip, acute fractures, allograft nonunions, congenital pseudarthrosis, and various approaches of lumbar and cervical spine fusions, rhBMPs overgrow to a safe and reliable device in the treatment of open tibial shaft fractures, nonunions of long bone fractures, anterior lumbar interbody fusion and revision posterolateral lumbar fusions. Systematic review of the published literature of rhBMPs is presented.     

A finite element model technique to determine the mechanical response of a lumbar spine segment under complex loads

This study presents a CT-based finite element model of the lumbar spine taking into account all function-related boundary conditions, such as anisotropy of mechanical properties, ligaments, contact elements, mesh size, etc. Through advanced mesh generation and employment of compound elements, the developed model is capable of assessing the mechanical response of the examined spine segment for complex loading conditions, thus providing valuable insight on stress development within the model and allowing the prediction of critical loading scenarios. The model was validated through a comparison of the calculated force-induced inclination/deformation and a correlation of these data to experimental values. The mechanical response of the examined functional spine segment was evaluated, and the effect of the loading scenario determined for both vertebral bodies as well as the connecting intervertebral disc.     

Therapeutic electric stimulation does not affect immune status in healthy individuals – a preliminary report

Background

For the treatment of low back pain, the following three scenarios of posterior lumbar interbody fusion (PLIF) were usually used, i.e., PLIF procedure with autogenous iliac bone (PAIB model), PLIF with cages made of PEEK (PCP model) or titanium (Ti) (PCT model) materiel. But the benefits or adverse effects among the three surgical scenarios were still not fully understood.

Method

Finite element analysis (FEA), as an efficient tool for the analysis of lumbar diseases, was used to establish a three-dimensional nonlinear L1-S1 FE model (intact model) with the ligaments of solid elements. Then it was modified to simulate the three scenarios of PLIF. 10?Nm moments with 400?N preload were applied to the upper L1 vertebral body under the loading conditions of extension, flexion, lateral bending and torsion, respectively.

Results

Different mechanical parameters were calculated to evaluate the differences among the three surgical models. The lowest stresses on the bone grafts and the greatest stresses on endplate were found in the PCT model. The PCP model obtained considerable stresses on the bone grafts and less stresses on ligaments. But the changes of stresses on the adjacent discs and endplate were minimal in the PAIB model.

Conclusions

The PCT model was inferior to the other two models. Both the PCP and PAIB models had their own relative merits. The findings provide theoretical basis for the choice of a suitable surgical scenario for different patients.     

Prostheses designed for vertebral body replacement

Prostheses are proposed to restore the spinal stability of patients suffering from metastatic malignant tumours in their vertebral bodies. They are designed to replace only one vertebral body and two neighbouring intervertebral discs of the spine. Experiments performed on cervical, thoracic and lumbar sections, which were obtained from fresh cadavers, have shown that the reduction in average compressive strengths of these regions due to the placement of prostheses is about 9%. This seems acceptable for those patients in performing their daily activities. The same amount of reduction has also been observed in average compressive strengths of the neighbouring healthy vertebrae due to the placement of prosthesis heads by bone cement. Developed prostheses have a number of advantages over the existing fusion constructs for the cases considered in this work.