Influence of material properties on the mechanical behaviour of the L5-S1 intervertebral disc in compression: a nonlinear finite element study

A simple finite element model of the L₅-S₁ intervertebral disc body has been constructed; it is circular and symmetrical about the sagittal plane. The annulus fibrosus of the model was idealized as an inhomogeneous composite of an isotropic ground substance, reinforced by helically oriented collagen fibres so that the model has six different structural components namely: cortical bone, cancellous bone, cartilaginous endplates, nucleus pulposus, ground substance and collagen fibres. A sensitivity analysis of the material properties of each structural component was carried out by varying those properties for one structural component at a time and evaluating the changes in the biomechanical response to compressive displacements. Experimentally available relations between the applied compressive force and the vertical displacements, the nucleus pulposus pressure increase and the disc lateral bulge were used to evaluate the biomechanical responses for each set of material properties. Results showed that both the Poisson's ratio and the Young's modulus of the ground substance play an important role in the prediction of the biomechanical response.     

Sudden loading during a dynamic lifting task: a simulation study

It is believed that nurses risk the development of back pain as a consequence of sudden loadings during tasks in which they are handling patients. Forward dynamics simulations of sudden loads (applied to the arms) during dynamic lifting tasks were performed on a two-dimensional whole-body model. Loads were in the range of -80 kg to 80 kg, with the initial load being 20 kg. Loading the arm downwards with less than that which equals a mass of 20 kg did not change the compressive forces on the spine when compared to a normal lifting motion with a 20 kg mass in the hands. However when larger loads (40 kg to 80 kg extra in the hands) were simulated, the compressive forces exceeded 13,000 N (above 3400 N is generally considered a risk factor). Loading upwards led to a decrease in the compressive forces but to a larger backwards velocity at the end of the movement. In the present study, it was possible to simulate a fast lifting motion. The results showed that when loading the arms downwards with a force that equals 40 kg or more, the spine was severely compressed. When loading in the opposite direction (unloading), the spine was not compressed more than during a normal lifting motion. In practical terms, this indicates that if a nursing aide tries to catch a patient who is falling, large compressive forces are applied to the spine.     

Biomechanical modeling of posterior instrumentation of the scoliotic spine

Scoliosis is a three-dimensional deformation of the spine that can be treated by vertebral fusion using surgical instrumentation. However, the optimal configuration of instrumentation remains controversial. Simulating the surgical maneuvers with personalized biomechanical models may provide an analytical tool to determine instrumentation configuration during the pre-operative planning. Finite element models used in surgical simulations display convergence difficulties as a result of discontinuities and stiffness differences between elements. A kinetic model using flexible mechanisms has been developed to address this problem, and this study presents its use in the simulation of Cotrel-Dubousset Horizon surgical maneuvers. The model of the spine is composed of rigid bodies corresponding to the thoracic and lumbar vertebrae, and flexible elements representing the intervertebral structures. The model was personalized to the geometry of three scoliotic patients (with a thoracic Cobb angle of 45 degrees, 49 degrees and 39 degrees ). Binary joints and kinematic constraints were used to represent the rod-implant-vertebra joints. The correction procedure was simulated using three steps: (1) Translation of hooks and screws on the first rod; (2) 90 degrees rod rotation; (3) Hooks and screws look-up on the rod. After the simulation, slight differences of 0-6 degrees were found for the thoracic spine scoliosis and the kyphosis, and of 1-8 degrees for the axial rotation of the apical vertebra and for the orientation of the plane of maximum deformity, compared to the real post-operative shape of the patient. Reaction loads at the vertebra-implant link were mostly below 1000 N, while reaction loads at the boundary conditions (representing the overall action of the surgeon) were in the range 7-470 N and maximum torque applied to the rod was 1.8 Nm. This kinetic modeling approach using flexible mechanisms provided a realistic representation of the surgical maneuvers. It may offer a tool to predict spinal geometry correction and assist in the pre-operative planning of surgical instrumentation of the scoliotic spine.     

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.     

Physical and mechanical properties of calf lumbosacral trabecular bone.

The physical and mechanical properties of calf lumbar and sacral trabecular bone were determined and compared with those of human trabecular bone. The mean tissue density (1.66 +/- 0.12 g cm-3), equivalent mineral density (169 +/- 36 mg cm-3), apparent density (453 +/- 89 mg cm-3), ash density (194 +/- 59 mg cm-3), ash content (0.6 +/- 0.05%), compressive strength (7.1 +/- 3.0 MPa) and compressive modulus (173 +/- 97 MPa) of calf trabecular bone are similar to those of young human. There were moderate, positive linear correlations between apparent density and equivalent mineral density, ash density, and compressive strength; and between compressive strength and equivalent mineral density (R2 ranging from 0.35 to 0.48, p less than 0.001). Apparent density, ash density, and equivalent mineral density did not differ significantly in different regions. In contrast to humans, the compressive strength increased from posterior, near the facet, to the anterior vertebral body. These comparisons of physical and mechanical properties, as well as anatomical comparisons by others, indicate that the calf spine is a good model of the young non-osteoporotic human spine and thus useful for the testing of spinal instrumentation.     

Patient-specific finite element model of the spine and spinal cord to assess the neurological impact of scoliosis correction: preliminary application on two cases with and without intraoperative neurological complications

Scoliosis is a 3D deformation of the spine and rib cage. For severe cases, surgery with spine instrumentation is required to restore a balanced spine curvature. This surgical procedure may represent a neurological risk for the patient, especially during corrective maneuvers. This study aimed to computationally simulate the surgical instrumentation maneuvers on a patient-specific biomechanical model of the spine and spinal cord to assess and predict potential damage to the spinal cord and spinal nerves. A detailed finite element model (FEM) of the spine and spinal cord of a healthy subject was used as reference geometry. The FEM was personalized to the geometry of the patient using a 3D biplanar radiographic reconstruction technique and 3D dual kriging. Step by step surgical instrumentation maneuvers were simulated in order to assess the neurological risk associated to each maneuver. The surgical simulation methodology implemented was divided into two parts. First, a global multi-body simulation was used to extract the 3D displacement of six vertebral landmarks, which were then introduced as boundary conditions into the personalized FEM in order to reproduce the surgical procedure. The results of the FEM simulation for two cases were compared to published values on spinal cord neurological functional threshold. The efficiency of the reported method was checked considering one patient with neurological complications detected during surgery and one control patient. This comparison study showed that the patient-specific hybrid model reproduced successfully the biomechanics of neurological injury during scoliosis correction maneuvers.     

A frontal plane model of the lumbar spine subjected to a follower load: implications for the role of muscles

Compression on the lumbar spine is 1000 N for standing and walking and is higher during lifting. Ex vivo experiments show it buckles under a vertical load of 80-100 N. Conversely, the whole lumbar spine can support physiologic compressive loads without large displacements when the load is applied along a follower path that approximates the tangent to the curve of the lumbar spine. This study utilized a two-dimensional beam-column model of the lumbar spine in the frontal plane under gravitational and active muscle loads to address the following question: Can trunk muscle activation cause the path of the internal force resultant to approximate the tangent to the spinal curve and allow the lumbar spine to support compressive loads of physiologic magnitudes? The study identified muscle activation patterns that maintained the lumbar spine model under compressive follower load, resulting in the minimization of internal shear forces and bending moments simultaneously at all lumbar levels. The internal force resultant was compressive, and the lumbar spine model, loaded in compression along the follower load path, supported compressive loads of physiologic magnitudes with minimal change in curvature in the frontal plane. Trunk muscles may coactivate to generate a follower load path and allow the ligamentous lumbar spine to support physiologic compressive loads.     

The dynamic flexion/extension properties of the lumbar spine in vitro using a novel pendulum system

The biomechanical properties of the ligamentous cadaver spine have been previously examined using a variety of experimental testing protocols. Ongoing technical challenges in the biomechanical testing of the spine include the application of physiologic compressive loads and the application of dynamic bending moments while allowing unconstrained three-dimensional motion. The purpose of this study was to report the development of a novel pendulum apparatus that addressed these challenges and to determine the effects of various axial compressive loads on the dynamic biomechanical properties of the lumbar functional spinal unit (FSU). Lumbar FSUs were tested in flexion and extension under five axial compressive loads chosen to represent physiologic loading conditions. After an initial rotation, the FSUs behaved as a dynamic, underdamped vibrating elastic system. Bending stiffness and coefficient of damping increased significantly as the compressive pendulum load increased. The apparatus described herein is a relatively simple approach to determining the dynamic bending properties of the FSU, and potentially disc arthroplasty devices. It is capable of applying physiologic compressive loads at dynamic rates without constraining the kinematics of the joints, crucial requirements for testing FSUs in vitro.     

Biomechanical analysis of the anterior cervical fusion

This paper presents a biomechanical analysis of the cervical C5–C6 functional spine unit before and after the anterior cervical discectomy and fusion. The aim of this work is to study the influence of the medical procedure and its instrumentation on range of motion and stress distribution. First, a three-dimensional finite element model of the lower cervical spine is obtained from computed tomography images using a pipeline of image processing, geometric modelling and mesh generation software. Then, a finite element study of parameters' influence on motion and a stress analysis at physiological and different post-operative scenarios were made for the basic movements of the cervical spine. It was confirmed that the results were very sensitive to intervertebral disc properties. The insertion of an anterior cervical plate influenced the stress distribution at the vertebral level as well as in the bone graft. Additionally, stress values in the graft decreased when it is used together with a cage.     

Biomechanical analysis of the anterior cervical fusion

This paper presents a biomechanical analysis of the cervical C5-C6 functional spine unit before and after the anterior cervical discectomy and fusion. The aim of this work is to study the influence of the medical procedure and its instrumentation on range of motion and stress distribution. First, a three-dimensional finite element model of the lower cervical spine is obtained from computed tomography images using a pipeline of image processing, geometric modelling and mesh generation software. Then, a finite element study of parameters' influence on motion and a stress analysis at physiological and different post-operative scenarios were made for the basic movements of the cervical spine. It was confirmed that the results were very sensitive to intervertebral disc properties. The insertion of an anterior cervical plate influenced the stress distribution at the vertebral level as well as in the bone graft. Additionally, stress values in the graft decreased when it is used together with a cage.     

Simulating the response of a standing operator to vibration stress by means of a biomechanical model

Under vibration stress the compressive forces transmitted in the joints of a standing operator are composed of nearly static and oscillating force parts. Because these forces can hardly be measured they were assessed by means of a biomechanical model. In the model, 27 rigid bodies with 103 degrees of freedom represent the segments of the human body. 106 force elements imitate the muscles of the trunk and the legs. At first, the model parameter were varied so that for the simulated sitting posture the model fits the seat-to-head transmissibility given in the literature and in ISO/CD 5982. For the standing posture, the transfer functions between the ground acceleration and the oscillating forces in the ankle, the knee, the hip, and the motion segment L3-L4 were computed. According to the moduli of these functions the forces in the ankles are higher than those in the knees or the hips and they nearly come up to the forces in the lumbar spine. Further the results of the simulation indicate that under equal vibration stress in the standing and the sitting posture the differences between the compressive forces in the lumbar spine are small.     

A three-dimensional mathematical model of the thoracolumbar fascia and an estimate of its biomechanical effect

The thoracolumbar fascia (TLF) provides a means of attachment to the lumbar spine for several muscles including the transverse abdominis, and parts of the latissimus dorsi and internal oblique muscles. Previous biomechanical models of the lumbar spine either tend to omit the TLF on the assumption that its contribution would be negligible or incorporate only part of the TLF. Here, a three-dimensional model of the posterior and middle layers of the TLF is presented to enable its action to be included in future three-dimensional models of the spine. It is used illustratively to estimate the biomechanical influence of this structure on the lumbar spine. The formulation of the model allows the lines of action of the fibres comprising the fascia to be calculated for any posture whilst ensuring that anatomical constraints are satisfied. Application of the model suggests that the TLF produces moments primarily in flexion and extension. The simulated results demonstrate that the abdominal muscles, acting via the TLF, are capable of contributing extension moments comparable to those produced by other smaller muscles associated with the lumbar spine.     

经皮椎体成形术对骨质疏松性胸腰椎骨折患者生物力学的影响研究

目的:探讨经皮椎体成形术对骨质疏松性胸腰椎骨折患者生物力学的影响。方法:选取9具冻存新鲜尸体的胸腰段脊柱开展研究,并应用随机数字表法分为观察组、对照1组和对照2组,每组各3具。观察组、对照1组均制作成骨质疏松性胸腰椎骨折模型,观察组经椎弓根注入含有对比剂的低粘度骨水泥,对照1组置入椎弓根螺钉内固定,分别于术前和术后测量两组椎体的主要生物力学指标(最大抗压强度、刚度、高度),对照2组则作为参照,仅测量一次,对三组生物力学指标检测结果进行统计分析。结果:观察组椎体术前的最大抗压强度、刚度、高度与对照1组比较差异无统计学意义(P0.05),但两组术前最大抗压强度、刚度较对照2组降低,而高度较对照2组升高(P0.05)。观察组和对照2组术后的最大抗压强度、刚度均较对照1组升高,高度较对照1组降低(P0.05),而观察组术后的最大抗压强度、刚度、高度与对照2组比较差异无统计学意义(P0.05)。结论:应用经皮椎体成形术对骨质疏松性腰椎骨折患者实施治疗,能够有效恢复患者椎体生物力学,效果确切。     

Biomechanical comparison of the effects of anterior,posterior and transforaminal lumbar interbody fusion on vibration characteristics of the human lumbar spine

Previous studies have compared the effects of different interbody fusion approaches on biomechanical responses of the lumbar spine to static loadings. However, very few have dealt with the whole body vibration (WBV) condition that is typically present in vehicles. This study was designed to determine the biomechanical differences among anterior, posterior and transforaminal lumbar interbody fusion (ALIF, PLIF and TLIF) under vertical WBV. A previously developed and validated finite element (FE) model of the intact L1–sacrum human lumbar spine was modified to simulate ALIF, PLIF and TLIF with bilateral pedicle screw fixation at L4–L5. Comparative studies on dynamic responses to the axial cyclic loading in these developed models were conducted. The results showed that at the fused L4–L5 level, dynamic responses of the von-Mises stress in L4 inferior and L5 superior endplates for the ALIF, PLIF and TLIF models were increased compared with the intact model. The endplate stresses in the TLIF model were lower than in the ALIF and PLIF models, but the TLIF generated greater stresses in the screws and rods compared with the ALIF and PLIF. At other levels, a decrease in dynamic responses of the disc bulge, annulus stress and intradiscal pressure was observed in all the fusion models compared with the intact one, but there was no obvious difference in these dynamic responses among the ALIF, PLIF and TLIF models. These findings might be useful in understanding vibration characteristics of the whole lumbar spine after different types of fusion surgery.     

Effect of annular incision type on the change in biomechanical properties in a herniated lumbar intervertebral disc

The technique used to incise the disc during discectomy may play a role in the subsequent healing and change in biomechanical stiffness of the disc. Several techniques of lumbar disc annulotomy have been described in clinical reports. The purpose of this paper was to study the influence of annulotomy technique on motion segment stiffness using a finite element model. Four incision methods (square, circular, cross, and slit) were compared. The analyses showed that each of the annular incisions produced increase in motions under axial moment loadings with circular incision producing the largest change in the corresponding rotational motion. Under shear loading mode, cross and slit-type annular incisions produced slightly larger changes in the principal motions of the disc than square and circular incisions. All other incision types considered in the current study produced negligibly small increase in motion under rest of the loading conditions. In addition to annulotomy, when nucleotomy was also included in the analyses, once again cross and slit incisions produced larger change in motion under shear loading mode as compared to the other two incision types. A comparison between the four types of annular incisions showed that cross incision produced an increase in motion larger than those produced by the other three incisions under flexion/extension and lateral moment loading and both shear force loadings. Circular incision produced the largest increase in motion under axial moment load in comparison to those produced by square, cross, and slit incisions. Sagittal plane symmetry was influenced by the incision injury to the motion segment leading to coupled motions as well as increased facet loads. From the study it can be concluded that the increase inflexibility of the disc due to annulotomy depends on the type of annulotomy and the annulotomy also produce asymmetrical deformations leading to increased facet loading.     

A methodological approach for the biomechanical cause analysis of golf-related lumbar spine injuries

A new methodological approach employing mechanical work (MW) determination and relative portion of its elemental analysis was applied to investigate the biomechanical causes of golf-related lumbar spine injuries. Kinematic and kinetic parameters at the lumbar and lower limb joints were measured during downswing in 18 golfers. The MW at the lumbar joint (LJ) was smaller than at the right hip but larger than the MWs at other joints. The contribution of joint angular velocity (JAV) to MW was much greater than that of net muscle moment (NMM) at the LJ, whereas the contribution of NMM to MW was greater rather than or similar to that of JAV at other joints. Thus, the contribution of JAV to MW is likely more critical in terms of the probability of golf-related injury than that of NMM. The MW-based golf-related injury index (MWGII), proposed as the ratio of the contribution of JAV to MW to that of NMM, at the LJ (1.55) was significantly greater than those at other joints ( < 1.05). This generally corresponds to the most frequent occurrence of golf-related injuries around the lumbar spine. Therefore, both MW and MWGII should be considered when investigating the biomechanical causes of lumbar spine injuries.     

Biomechanical assessment of a PEEK rod system for semi-rigid fixation of lumbar fusion constructs

The concept of semi-rigid fixation (SRF) has driven the development of spinal implants that utilize nonmetallic materials and novel rod geometries in an effort to promote fusion via a balance of stability, intra- and inter-level load sharing, and durability. The purpose of this study was to characterize the mechanical and biomechanical properties of a pedicle screw-based polyetheretherketone (PEEK) SRF system for the lumbar spine to compare its kinematic, structural, and durability performance profile against that of traditional lumbar fusion systems. Performance of the SRF system was characterized using a validated spectrum of experimental, computational, and in vitro testing. Finite element models were first used to optimize the size and shape of the polymeric rods and bound their performance parameters. Subsequently, benchtop tests determined the static and dynamic performance threshold of PEEK rods in relevant loading modes (flexion-extension (F/E), axial rotation (AR), and lateral bending (LB)). Numerical analyses evaluated the amount of anteroposterior column load sharing provided by both metallic and PEEK rods. Finally, a cadaveric spine simulator was used to determine the level of stability that PEEK rods provide. Under physiological loading conditions, a 6.35 mm nominal diameter oval PEEK rod construct unloads the bone-screw interface and increases anterior column load (approx. 75% anterior, 25% posterior) when compared to titanium (Ti) rod constructs. The PEEK construct's stiffness demonstrated a value lower than that of all the metallic rod systems, regardless of diameter or metallic composition (78%??80% reduction in F/E, p??70% reduction in LB, p??54% reduction in AR, p?相似文献   

Effects of external trunk loads on lumbar spine stability

Stability of the lumbar spine is an important factor in determining spinal response to sudden loading. Using two different methods, this study evaluated how various trunk load magnitudes and directions affect lumbar spine stability. The first method was a quick release procedure in which effective trunk stiffness and stability were calculated from trunk kinematic response to a resisted-force release. The second method combined trunk muscle EMG data with a biomechanical model to calculate lumbar spine stability. Twelve subjects were tested in trunk flexion, extension, and lateral bending under nine permutations of vertical and horizontal trunk loading. The vertical load values were set at 0, 20, and 40% of the subject's body weight (BW). The horizontal loads were 0, 10, and 20% of BW. Effective spine stability as obtained from quick release experimentation increased significantly (p<0.01) with increased vertical and horizontal loading. It ranged from 785 (S.D.=580) Nm/rad under no-load conditions to 2200 (S.D.=1015) Nm/rad when the maximum horizontal and vertical loads were applied to the trunk simultaneously. Stability of the lumbar spine achieved prior to force release and estimated from the biomechanical model explained approximately 50% of variance in the effective spine stability obtained from quick release trials in extension and lateral bending (0.53相似文献   

The LSP test for determining oscillation resistance of spinal implants]

QUESTION: New spinal implants need to be tested for primary stability in vitro under standardized laboratory conditions. To ensure the reliability of the test procedures, quality assurance standards in accordance with ISO 9000 were introduced to standardize testing including experimental set ups, loading and test frequency. These standards, however, require a relatively long time to implement. METHOD: The LSP test was used to compare various surface treatments by different shot peening processes applied to spinal rods for dorsal spine implant systems. 6 rods made of two different cp-Titanium materials (Ti-2 and Ti-4) were tested. Dynamic tests were performed with the MTS 810 mono-axial servo-hydraulic test equipment. Beginning with a load of 200 N the rods were subjected to tensile and compressive loads, which were increased in steps of 100 N after every 50.000 cycles until rod failure. RESULTS: Results were available after one to two weeks. The best results (LSP 167 million) were achieved with Ti-4 rods shot peened with steel balls and glass beads. In comparison, the lowest LSP value of 81 million was found with Ti-2 rods shot peened with glass beads only. CONCLUSION: This high speed testing method has reduced the development time from two years to 6 months.     

Effects of suppression of bone turnover on cortical and trabecular load sharing in the canine vertebral body

The relative biomechanical effects of antiresorptive treatment on cortical thickness vs. trabecular bone microarchitecture in the spine are not well understood. To address this, T-10 vertebral bodies were analyzed from skeletally mature female beagle dogs that had been treated with oral saline (n=8 control) or a high dose of oral risedronate (0.5 mg/kg/day, n=9 RIS-suppressed) for 1 year. Two linearly elastic finite element models (36-μm voxel size) were generated for each vertebral body—a whole-vertebra model and a trabecular-compartment model—and subjected to uniform compressive loading. Tissue-level material properties were kept constant to isolate the effects of changes in microstructure alone. Suppression of bone turnover resulted in increased stiffness of the whole vertebra (20.9%, p=0.02) and the trabecular compartment (26.0%, p=0.01), while the computed stiffness of the cortical shell (difference between whole-vertebra and trabecular-compartment stiffnesses, 11.7%, p=0.15) was statistically unaltered. Regression analyses indicated subtle but significant changes in the relative structural roles of the cortical shell and the trabecular compartment. Despite higher average cortical shell thickness in RIS-suppressed vertebrae (23.1%, p=0.002), the maximum load taken by the shell for a given value of shell mass fraction was lower (p=0.005) for the RIS-suppressed group. Taken together, our results suggest that—in this canine model—the overall changes in the compressive stiffness of the vertebral body due to suppression of bone turnover were attributable more to the changes in the trabecular compartment than in the cortical shell. Such biomechanical studies provide an unique insight into higher-scale effects such as the biomechanical responses of the whole vertebra.