Lumbar Functional Spinal Units Research Articles

Human and Porcine Lumbar Endplate Injury Risk in Repeated Flexion-Compression.

Low back pain (LBP) affects 50-80% of adults at some pointin their lifetime, yet the etiology of injury is not well understood. Those exposed to repeated flexion-compression are at a higher risk for LBP, such as helicopter pilots and motor vehicle operators. Animal injury models offer insight into in vivo injury mechanisms, but interspecies scaling is needed to relate animal results to human. Human (n = 16) and porcine (n = 20) lumbar functional spinal units (FSUs) were loaded in repeated flexion-compression (1Hz) to determine endplate fracture risk over long loading exposures. Flexion oscillated from 0 to 6° and peak applied compressivestress ranged from 0.65 to 2.38MPa for human and 0.64 to 4.68MPa for porcine specimens. Five human and twelve porcine injuries were observed. The confidence intervals for human and porcine 50% injury risk curves in terms of stress and cycles overlapped, indicating similar failure behavior for this loading configuration. However, porcine specimens were more tolerant to the applied loading compared to human, demonstrated by a longer time-to-failure for the same applied stress. Optimization revealed that time-to-failure in human specimens was approximately 25% that of porcine specimens at a given applied stress within 0.65-2.38MPa. This study determined human and porcine lumbar endplate fracture risks in long-duration repeated flexion-compression that can be directly used for future equipment and vehicle design, injury prediction models, and safety standards. The interspecies scale factor produced in this study can be used for previous and future porcine lumbar injury studies to scale results to relevant human injury.

Biomechanical analysis of functionally graded porous interbody cage for lumbar spinal fusion

Functionally graded porous (FGP) interbody cage might offer a trade-off between porosity-based reduction of stiffness and mechanical properties. Using finite element models of intact and implanted lumbar functional spinal unit (FSU), the study investigated the quantitative deviations in load transfer and adaptive changes in bone density distributions around FGP interbody cages. The cage had three graded porosities: FGP-A, -B, and –C corresponded to a maximum porosity levels of 48%, 65% and 78%, respectively. Efficacy of the FGP cages were evaluated by comparing the numerically predicted results of solid-Ti and uniformly porous 78% porosity (P78) cage. Variations in stiffness and interface condition affected the strain distribution and bone remodelling around the cages. Peak strains of 0.5–1% were observed in less number of peri-prosthetic bone elements for the FGP cages as compared to the solid-Ti cage. Strains and bone apposition were considerably higher for the bonded implant-bone interface condition than the debonded case. For the FGP-C with bonded interface condition, bone apposition of 11–20% was predicted in the L4 and L5 regions of interest (ROIs); whereas the debonded model exhibited 6–10% increase in bone density. The deviations in bone density change between FGP-C and P78 model were 3–8% for L4 and L5 ROIs. FGP resulted in a reduced average micromotion (∼70–106 μm) as compared to solid-Ti (116 μm), for all physiologic movements. Compared to solid-Ti and uniformly porous cages, the FGP cage seems to be a viable alternative considering the conflicting nature of strength and porosity.

Numerical analysis of the mechanical behaviour of intact and implanted lumbar functional spinal units: Effects of loading and boundary conditions.

The objective of this study was to develop an improved finite element (FE) model of a lumbar functional spinal unit (FSU) and to subsequently analyse the deviations in load transfer owing to implantation. The effects of loading and boundary conditions on load transfer in intact and implanted FSUs and its relationship with the potential risk of vertebral fracture were investigated. The FE models of L1-L5 and L3-L4 FSUs, intact and implanted, were developed using patient-specific CT-scan dataset and segmentation of cortical and cancellous bone regions. The effect of submodelling technique, as compared to artificial boundary conditions, on the elastic behaviour of lumbar spine was examined. Applied forces and moments, corresponding to physiologic movements, were used as loading conditions. Results indicated that the loading and boundary conditions considerably affect stress-strain distributions within a FSU. This study, based on an improved FE model of a vertebra, highlights the importance of using the submodelling technique to adequately evaluate the mechanical behaviour of a FSU. In the intact FSU, strains of 200-400 µε were observed in the cancellous bone of vertebral body and pedicles. High equivalent stresses of 10-25 MPa and 1-5 MPa were generated around the pars interarticularis for cortical and cancellous regions, respectively. Implantation caused reductions of 85%-92% in the range of motion for all movements. Insertion of the intervertebral cage resulted in major deviations in load transfer across a FSU for all movements. The cancellous bone around cage experienced pronounced increase in stresses of 10-15 MPa, which indicated potential risk of failure initiation in the vertebra.

Biomechanical evaluation of a novel decompression surgery: Transforaminal full-endoscopic lateral recess decompression (TE-LRD)

BackgroundTransforaminal full endoscopic lateral recess decompression (TE-LRD) can decompress lateral recess stenosis transforaminally under the endoscopy procedure. However, the biomechanical effects of the TE-LRD compared to the conventional decompression techniques are not reported. The purpose of this study is to compare the biomechanical effects of TE-LRD with conventional decompression techniques using finite element method. MethodsThree finite element models of lumbar functional spinal unit (FSU) of the L4-L5 levels were created: 1) normal disc 2) moderate grade disc degeneration 3) severe grade disc degeneration. For each of these three models, the following decompression techniques were simulated, 1) 50% TE-LRD, 2) 100% TE-LRD, 3) Unilateral laminectomy, 4) Bilateral laminectomy. The lower endplate of the fifth lumbar vertebra was fixed and 10Nm of moment in flexion/extension, left/right bending and axial rotation was applied to the upper endplate of the fourth lumbar vertebra, under a follower load of 400N. The range of motion, intervertebral disc stress, and facet joint stress were compared. Results50% TE-LRD was found to be the most stable decompression technique in all intervertebral disc models. Though the increase in the range of motion of 100% TE-LRD was higher than other decompression techniques in the normal disc model, it was not significantly different from 50% TE-LRD or unilateral laminectomy techniques in the degenerated disc models. The increase in the intervertebral disc stress was lowest for the 50% TE-LRD surgery in all intervertebral disc models. The increase in the facet stresses for 50% TE-LRD was lower than in the conventional decompression techniques for all intervertebral disc models. Conclusions50% TE-LRD was the decompression surgical technique with the least effect on spinal instability. 100% TE-LRD showed to be effective for cases with degenerative discs. 50% TE-LRD may decrease the risk of postoperative intervertebral disc and facet joint degeneration.

Development of a finite element biomechanical whole spine model for analyzing lumbar spine loads under caudocephalad acceleration

Background: Spine injury risk due to military conflict is an ongoing concern among defense organizations throughout the world. A better understanding of spine biomechanics could assist in developing protection devices to reduce injuries caused by caudocephalad acceleration (+Gz) in under-body blasts (UBB). Although some finite element (FE) human models have demonstrated reasonable lumbar spine biofidelity, they were either partial spine models or not validated for UBB-type loading modes at the lumbar functional spinal unit (FSU) level, thus limiting their ability to analyze UBB-associated occupant kinematics. Methods: An FE functional representation of the human spine with simplified geometry was developed to study the lumbar spine responses under +Gz loading. Fifty-seven load curves obtained from post mortem human subject experiments were used to optimize the model. Results: The model was cumulatively validated for compression, flexion, extension, and anterior-, posterior-, and lateral-shears of the lumbar spine and flexion and extension of the cervical spine. The thoracic spine was optimized for flexion and compression. The cumulative CORrelation and Analysis (CORA) rating for the lumbar spine was 0.766 and the cervical spine was 0.818; both surpassed the 0.7 objective goal. The model’s element size was confirmed as converged. Conclusions: An FE functional representation of the human spine was developed for +Gz lumbar load analysis. The lumbar and cervical spines were demonstrated to be quantitatively biofidelic to the FSU level for multi-directional loading and bending typically experienced in +Gz loading, filling the capability gap in current models.

Mechanisms of Failure Following Simulated Repetitive Lifting: A Clinically Relevant Biomechanical Cadaveric Study.

A biomechanical analysis correlating internal disc strains and tissue damage during simulated repetitive lifting. To understand the failure modes during simulated safe and unsafe repetitive lifting. Repetitive lifting has been shown to lead to lumbar disc herniation (LDH). In vitro studies have developed a qualitative understanding of the effect of repetitive loading on LDH. However, no studies have measured internal disc strains and subsequently correlated these with disc damage. Thirty human cadaver lumbar functional spinal units were subjected to an equivalent of 1 year of simulated repetitive lifting under safe and unsafe levels of compression, in combination with flexion (13-15°), and right axial rotation (2°) for 20,000 cycles or until failure. Safe or unsafe lifting were applied as a compressive load to mimic holding a 20 kg weight either close to, or at arm's length, from the body, respectively. Maximum shear strains (MSS) were measured, and disc damage scores were determined in nine regions from axial post-test magnetic resonance imaging (MRI) and macroscopic images. Twenty percent of specimens in the safe lifting group failed before 20,000 cycles due to endplate failure, compared with 67% in the unsafe group. Over half of the specimens in the safe lifting group failed via either disc protrusion or LDH, compared with only 20% via protrusion in the unsafe group. Significant positive correlations were found between MRI and macroscopic damage scores in all regions (rs > 0.385, P < 0.049). A significant positive correlation was observed in the left lateral region for MSS versus macroscopic damage score (rs = 0.486, P < 0.037) and MSS versus failure mode (rs = 0.724, P = 0.018, only specimens with disc failure). Pfirrmann Grade 3 discs were strongly associated with subsequent LDH (P = 0.003). Increased shear strains were observed in the contralateral side to the applied rotation as disc injury progressed from protrusion to LDH. Larger compressive loads applied to simulate unsafe lifting led to frequent early failure of the endplate, however, smaller compressive loads at similar flexion angles applied under safe lifting led to more loading cycles before failure, where the site of failure was more likely to be the disc. Our study demonstrated that unsafe lifting leads to greater risk of injury compared with safe lifting, and LDH and disc protrusion were more common in the posterior/posterolateral regions. N/A.

Improvement in determining the risk of damage to the human lumbar functional spinal unit considering age, height, weight and sex using a combination of FEM and RSM.

Human functional spinal unit (FSU) is comprised of two adjacent vertebrae, the intervertebral disc (IVD), the cartilage endplates and the connecting ligamentous tissues. As humans age, trabecular, cortical bone and IVD change in mechanical properties. These changes influence the movement of the FSU (displacements, rotations and disc bulges). This paper proposes a method to determine the influence of sex, age, weight and height on the movements of the FSU using a combination of the FEM and RSM. The work concentrates on the medium-sized human FSU (L3-L4 lumbar level with standard dimensions of IVD: width = 50mm, depth = 35mm and height = 11mm). Also, the proposed method could be an improvement to the determination of risk of damage based on body mass index (BMI), as well as an alternative method of searching for a healthy weight. The method was developed as follows: First, a three-dimensional parameterized human healthy FSU FE model was generated based on the standard test (for compression, flexion, extension, lateral bending, torsion and shear). The movements of the FSU FE model based on the aforementioned standardized test when the sex, age, weight and height varies in a range for each individual (30-80 years; 70-120 kg; 160-190 cm) were simulated. Quadratic regression models were generated for each standardized test using RSM. An ANOVA test showed that all of the tests have a significant influence on weight and age, whereas height influences only those tests that involve rotation of the FSU (flexion, extension and lateral bending). Normal weight, overweight and obesity BMI classifications were considered in determining the healthy weight with the following constraints: the limit angles of flexion and lateral bending, as well as the maximum posterior bulge limit of the IVD, could not exceed of one-third of the spinal canal. The proposed method concluded that: (1) normal BMI values do not cause damage to the FSU, (2) some overweight BMI values may cause damage to the FSU for a certain range of ages and heights and (3) for BMI obesity values, the risk of damage to the FSU may be imminent.

Influence of Deviated Centers of Rotation on Kinematics and Kinetics of a Lumbar Functional Spinal Unit: An In Vitro Study.

BackgroundCenter of rotation (COR) has been used for assessing spinal motion quality. However, the biomechanical influence of COR deviation towards different directions during flexion-extension (FE) remains largely unknown. This study aimed to investigate the alteration in the range of motion (ROM), compressive force, shear force, and neutral zone size (NZ) in a lumbar functional spinal unit (FSU), caused by the deviated COR in different directions during FE.Material/MethodsTwelve human cadaveric lumbar FSUs (6 for L2–L3, 6 for L4–L5) were tested in a 6-degree-of-freedom servo-hydraulic load frame. These FSUs were firstly applied a 7.5 Nm pure moment to perform FE to obtain their natural COR during FE. Subsequently, they were subjected to FE around 9 established deviated CORs with 6 Nm cyclical loading.ResultsIt was found that the ROM and NZ increased significantly when the COR moved from the superior plane to the inferior plane for the L2–L3 unit and when the COR located in the superior plane compared with the inferior plane for the L4–L5 unit. The compressive forces for both FSUs demonstrated significant changes caused by COR shift in the same horizontal plane, while the shear forces demonstrated significant changes caused by COR shift in the same vertical plane.ConclusionsThe ROM, NZ, and shear force of FSU are sensitive to the vertical COR shift, while the compressive force of FSU is highly sensitive to the horizontal COR shift. Additionally, the kinematics and kinetics of the L2–L3 unit are more sensitive to COR location than those of the L4–L5 unit.

Stand-alone lumbar cage subsidence: A biomechanical sensitivity study of cage design and placement.

Background and objectiveSpinal degeneration and instability are commonly treated with interbody fusion cages either alone or supplemented with posterior instrumentation with the aim to immobilise the segment and restore intervertebral height. The purpose of this work is to establish a tool which may help to understand the effects of intervertebral cage design and placement on the biomechanical response of a patient-specific model to help reducing post-surgical complications such as subsidence and segment instability. MethodsA 3D lumbar functional spinal unit (FSU) finite element model was created and a parametric model of an interbody cage was designed and introduced in the FSU. A Drucker–Prager Cap plasticity formulation was used to predict plastic strains and bone failure in the vertebrae. The effect of varying cage size, cross-sectional area, apparent stiffness and positioning was evaluated under 500 N preload followed by 7.5 Nm multidirectional rotation and the results were compared with the intact model. ResultsThe most influential cage parameters on the FSU were size, curvature congruence with the endplates and cage placement. Segmental stiffness was higher when increasing the cross-sectional cage area in all loading directions and when the cage was anteriorly placed in all directions but extension. In general, the facet joint forces were reduced by increasing segmental stiffness. However, these forces were higher than in the intact model in most of the cases due to the displacement of the instantaneous centre of rotation. The highest plastic deformations took place at the caudal vertebra under flexion and increased for cages with greater stiffness. Thus, wider cages and a more anteriorly placement would increase the volume of failed bone and, therefore, the risk of subsidence. ConclusionsCage geometry plays a crucial role in the success of lumbar surgery. General considerations such as larger cages may be applied as a guideline, but parameters such as curvature or cage placement should be determined for each specific patient. This model provides a proof-of-concept of a tool for the preoperative evaluation of lumbar surgical outcomes.

Biomechanics of Lumbar Motion-Segments in Dynamic Compression.

Recent epidemiology studies have reported increase in lumbar spine injuries in frontal crashes. Whole human body finite element models (FEHBM) are frequently used to delineate mechanisms of such injuries. However, the accuracy of these models in mimicking the response of human spine relies on the characterization data of the spine model. The current study set out to generate characterization data that can be input to FEHBM lumbar spine, to obtain biofidelic responses from the models. Twenty-five lumbar functional spinal units were tested under compressive loading. A hydraulic testing machine was used to load the superior ends of the specimens. A 75N load was placed on the superior PMMA to remove the laxity in the joint and mimic the physiological load. There were three loading sequences, namely, preconditioning, 0.5 m/s (non-injurious) and 1.0 m/s (failure). Forces and displacements were collected using six-axis load cell and VICON targets. In addition, acoustic signals were collected to identify the times of failures. Finally, response corridors were generated for the two speeds. To demonstrate the corridors, GHBMC FE model was simulated in frontal impact condition with the default and updated lumbar stiffness. Bi-linear trend was observed in the force versus displacement plots. In the 0.5 m/s tests, mean toe- and linear-region stiffnesses were 0.96±0.37 and 2.44±0.92 kN/mm. In 1.0 m/s tests, the toe and linear-region stiffnesses were 1.13±0.56 and 4.6±2.5 kN/mm. Lumbar joints demonstrated 2.5 times higher stiffness in the linear-region when the loading rate was increased by 0.5 m/s.

Effect of cage design, supplemental posterior instrumentation and approach on primary stability of a lumbar interbody fusion – A biomechanical in vitro study

BackgroundThere are various techniques and approaches for lumbar interbody fusion differing in access, cage type and type of supplemental posterior instrumentation. While a transforaminal access usually includes a hemifacetectomy, the facet joint can be preserved with a more lateral extraforaminal access. The supplemental posterior instrumentation required for both fusion techniques is still debated. The purpose of the present study was to compare primary stability of the two accesses for two different cage types with none, unilateral and bilateral supplemental posterior instrumentation. MethodsSix monosegmental lumbar functional spinal units (FSUs) were included in each of the two groups, and subjected to a flexibility test. As cages, a newly designed cage was compared to a standard cage in the following states: (a) native, (b) stand-alone cage, (c) bilateral internal fixator, (d) unilateral internal fixator, (e) unilateral facetectomy+bilateral internal fixator, (f) unilateral facetectomy+unilateral internal fixator and (g) unilateral facetectomy with stand-alone cage. For comparison the range of motion was normalized to the native state and the effects of the facetectomy, cage type, and supplemental instrumentation was compared. FindingsWithin the subject comparison showed a significantly higher flexibility for the unilateral facetectomy in all motion directions (p<0.001).In between subject comparison showed a significant effect of cage type on flexibility in flexion/extension (p=0.002) and lateral bending (p=0.028) but not in axial rotation (p=0.322). The type of supplemental posterior fixation had a significant effect on the flexibility in all motion directions (stand-alone>unilateral fixator>bilateral fixator). InterpretationCage design and approach type are affecting the primary stability of lumbar interbody fusion procedures while the type of posterior instrumentation is the most influencing factor.

The effect of repetitive flexion and extension fatigue loading on the young porcine lumbar spine, a feasibility study of MRI and histological analyses

BackgroundThe biomechanical mechanisms of failure of FSUs have been studied but the correlation of repetitive flexion and extension loadings to the initial phase of fatigue in young FSUs are still not known. The purpose of the study was to examine the fatigue results of low magnitude repetitive flexion and extension loading on porcine lumbar Functional Spinal Units (FSUs) with Magnetic Resonance Imaging (MRI) and histology.MethodsEight FSUs were subject to repetitive pivot flexion and eight to extension loading by a protocol of 20 000 cycles at 1 Hz with a load of 700 N. All loaded FSUs (N = 16) were examined with MRI and histology post loading. Three FSUs were examined with MRI as controls. Further three FSUs were non loaded histology controls.ResultsFifteen (94%) of the loaded FSUs have decreased MRI signal in the growth zone of the superior vertebra and 12 (75%) in the inferior vertebrae. Fourteen (88%) FSUs have increased signal in the superior vertebral body. Fourteen (88%) FSUs have a reduced signal in all or any endplate. The histology morphometry displayed that the unstained parts of the epiphyseal growth zone were larger among the loaded FSUs (mean 29% vs 4%) and that the chondrocytes in the endplate and growth zones had abnormal structure and deformed extracellular matrix.ConclusionRepetitive loading of young porcine FSUs in both extension and flexion causes concurrent MRI and histological changes in the growth zones and endplates, which could be a first sign of fatigue and an explanation for the disc, apophyseal and growth zone injuries seen among adolescent athletes.

Longitudinal changes in the structure and inflammatory response of the intervertebral disc due to stab injury in a murine organ culture model.

Despite the significant public health impact of intervertebral disc (IVD) degeneration and low back pain, it remains challenging to investigate the multifactorial molecular mechanisms that drive the degenerative cascade. Organ culture model systems offer the advantage of allowing cells to live and interact with their native extracellular matrix, while simultaneously reducing the amount of biological variation and complexity present at the organismal level. Murine organ cultures in particular also allow the use of widely available genetically modified animals with molecular level reporters that would reveal insights on the degenerative cascade. Here, we utilize an organ culture system of murine lumbar functional spinal units where we are able to maintain the cellular, metabolic, and structural, and mechanical stability of the whole organ over a 21-day period. Furthermore, we describe a novel approach in organ culture by using tissues from animals with an NF-κB-luc reporter in combination with a mechanical injury model, and are able to show that proinflammatory factors and cytokines such as NF-κB and IL-6 produced by IVD cells can be monitored longitudinally during culture in a stab injury model. Taken together, we utilize a murine organ culture system that maintains the cellular and tissue level behavior of the intervertebral disc and apply it to transgenic animals that allow the monitoring of the inflammatory profile of IVDs. This approach could provide important insights on the molecular and metabolic mediators that regulate the homeostasis of the IVD. © 2016 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 34:1431-1438, 2016.

A description of spinal fatigue strength

Understanding fatigue failure of the spine is important to establish dynamic loading limits for occupational health and safety. In this study experimental data were combined with published data to develop a description of the predictive parameters for spinal fatigue failure. 41 lumbar functional spinal units (FSUs) from cadaveric spines (age 49.0±11.9yr) where cyclically loaded. Three different levels of sinusoidal axial compression (0–3kN, 0–2kN or 1–3kN) were applied for 300,000 cycles. Further, published data consisted of 70 thoracic and lumbar FSUs loaded in axial compression for 5000 cycles. Cyclic forces ranged from lower peaks (Fmin) of 0.7–1kN to upper peaks (Fmax) of 1.2–7.1kN. Based on Wöhler analysis, a fatigue model was developed accounting for three parameters: I) specimen-specific scaling based on the endplate area, II) specimen-specific strength dependency on age or bone mineral density, III) load-specific correction factors based on Fmax and Fmin.The most predictive model was achieved for a combination of Fmax, endplate area and bone mineral density; this model explained 61% of variation (p<0.001). A model including Fmax, endplate area and age explained only 28% of variation (p<0.001). Inclusion of a load-specific correction factor did not significantly improve model prediction of fatigue failure.This analysis presents the basis for the prediction of specimen-specific fatigue failure of the lumbar spine, provided the endplate area and bone mineral density can be derived.

Biological responses to flexion/extension in spinal segments ex-vivo.

Mechanical loading is a salient factor in the progression of spinal disorders that contribute to back pain. Biological responses to loading modes like flexion/extension (F/E) in relevant spinal tissues remain unstudied. A novel, multi-axial experimental system was developed to subject viable functional spinal units (FSUs) to complex, in-situ loading. The objective was to determine biological effects of F/E in multiple spinal tissues-annulus fibrosus, nucleus pulposus, facet cartilage, and ligamentum flavum. Rabbit lumbar FSUs were mounted in a bioreactor within a robotic testing system. FSUs underwent small (0.17/0.05 Nm) and large (0.5/0.15 Nm) range-of-motion F/E for 1 or 2 h of cycling. Outcomes in each tissue, compared to unloaded FSUs, included (i) relative mRNA expression of catabolic (MMP-1, 3 and ADAMTS-5), pro-inflammatory (COX-2), and anabolic (ACAN) genes and (ii) immunoblotting of aggrecan degradation. Total energy applied to FSUs increased in groups subject to large range-of-motion and 2-h cycling, and moment relaxation was higher with large range-of-motion. F/E significantly modulated MMP1,-3 and COX-2 in facet cartilage and MMP-3 and ACAN in annulus fibrosus. Large range-of-motion loading increased MMP-mediated aggrecan fragmentation in annulus fibrosus. Biological responses to complex loading ex vivo showed variation among spinal tissues that reflect tissue structure and mechanical loading in F/E.

Needle puncture in rabbit functional spinal units alters rotational biomechanics.

An in vitro biomechanical study for rabbit lumbar functional spinal units (FSUs) using a robot-based spine testing system. To elucidate the effect of annular puncture with a 16 G needle on mechanical properties in flexion/extension, axial rotation, and lateral bending. Needle puncture of the intervertebral disk has been shown to alter mechanical properties of the disk in compression, torsion, and bending. The effect of needle puncture in FSUs, where intact spinal ligaments and facet joints may mitigate or amplify these changes in the disk, on spinal motion segment stability subject to physiological rotations remains unknown. Rabbit FSUs were tested using a robot testing system whose force/moment and position precision were assessed to demonstrate system capability. Flexibility testing methods were developed by load-to-failure testing in flexion/extension, axial rotation, and lateral bending. Subsequent testing methods were used to examine a 16 G needle disk puncture and No. 11 blade disk stab (positive control for mechanical disruption). Flexibility testing was used to assess segmental range-of-motion (degrees), neutral zone stiffness (N m/degrees) and width (degrees and N m), and elastic zone stiffness before and after annular injury. The robot-based system was capable of performing flexibility testing on FSUs-mean precision of force/moment measurements and robot system movements were <3% and 1%, respectively, of moment-rotation target values. Flexibility moment targets were 0.3 N m for flexion and axial rotation and 0.15 N m for extension and lateral bending. Needle puncture caused significant (P<0.05) changes only in flexion/extension range-of-motion and neutral zone stiffness and width (N m) compared with preintervention. No. 11 blade-stab significantly increased range-of-motion in all motions, decreased neutral zone stiffness and width (N m) in flexion/extension, and increased elastic zone stiffness in flexion and lateral bending. These findings suggest that disk puncture and stab can destabilize FSUs in primary rotations.

Effect of potting technique on the measurement of six degree-of-freedom viscoelastic properties of human lumbar spine segments.

Polymethyl methacrylate (PMMA) and Wood's Metal are fixation media for biomechanical testing; however, the effect of each potting medium on the measured six degree-of-freedom (DOF) mechanical properties of human lumbar intervertebral discs is unknown. The first aim of this study was to compare the measured 6DOF elastic and viscoelastic properties of the disc when embedded in PMMA compared to repotting in Wood's Metal. The second aim was to compare the surface temperature of the disc when potted with PMMA and Wood's Metal. Six human lumbar functional spinal units (FSUs) were first potted in PMMA, and subjected to overnight preload in a saline bath at 37 °C followed by five haversine loading cycles at 0.1 Hz in each of 6DOF loading directions (compression, left/right lateral bending, flexion, extension, left/right axial rotation, anterior/posterior, and lateral shear). Each specimen was then repotted in Wood's Metal and subjected to a 2-h re-equilibrating preload followed by repeating the same 6DOF tests. Outcome measures of stiffness and phase angle were calculated from the final loading cycle in each DOF and were expressed as normalized percentages relative to PMMA (100%). Disc surface temperatures (anterior, left/right lateral) were measured during potting. Paired t-tests (with alpha adjusted for multiple DOF) were conducted to compare the differences in each outcome parameter between PMMA and Wood's Metal. No significant differences in stiffness or phase angle were found between PMMA and Wood's Metal. On average, the largest trending differences were found in the shear DOFs for both stiffness (approximately 35% greater for Wood's Metal compared to PMMA) and phase angle (approximately 15% greater for Wood's Metal). A significant difference in disc temperature was found at the anterior surface after potting with Wood's Metal compared to PMMA, which did not exceed 26 °C. Wood's Metal is linear elastic, stiffer than PMMA and may reduce measurement artifact of potting medium, particularly in the shear directions. Furthermore, it is easier to remove than PMMA, reuseable, and cost effective.

Characterization of the behavior of a novel low-stiffness posterior spinal implant under anterior shear loading on a degenerative spinal model.

Dynamic implants have been developed to address potential adjacent level effects due to rigid instrumentation. Rates of revision surgeries may be reduced by using improved implants in the primary surgery. Prior to clinical use, implants should be rigorously tested ex vivo. The objective of our study was to characterize the load-sharing and kinematic behavior of a novel low-stiffness spinal implant. A human cadaveric model of degenerative spondylolisthesis was tested in shear. Lumbar functional spinal units (N = 15) were tested under a static 300 N axial compression force and a cyclic anterior shear force (5-250 N). Translation was tracked with a motion capture system. A novel implant was compared to three standard implants with shear stiffness ranging from low to high. All implants were instrumented with strain gauges to measure the supported shear force. Each implant was affixed to each specimen, and the specimens were tested intact and in two progressively destabilized states. Specimen condition and implant type affected implant load-sharing and specimen translation (p < 0.0001). Implant load-sharing increased across all degeneration-simulating specimen conditions and decreased across the three standard implants (high- to low-stiffness). Translation increased with the three standard implants (trend). The novel implant behaved similarly to the medium-stiffness implant (p > 0.2). The novel implant behaved similarly to the medium-stiffness implant in both load-sharing and translation despite having a different design and stiffness. Complex implant design and specimen-implant interaction necessitate pre-clinical testing of novel implants. Further in vitro testing in axial rotation and flexion-extension is recommended as they are highly relevant loading directions for non-rigid implants.

The analysis of mechanical changes of percutaneous vertebroplasty

In this article, a numerical analysis method of mechanical changes for percutaneous vertebroplasty is presented. Firstly, a 3D geometric model of lumbar functional spinal units has been built by segmenting computed tomography images and performing 3D reconstruction. On the basis of the geometric model, two kinds of 3D finite element models (FEM) for preoperative and postoperative vertebrae are created. Then, a numerical calculation method on FEM for the analysis of mechanical changes has been developed. The simulating calculation can reveal the stress and strain distribution and deformation of the preoperative and postoperative vertebrae. The approach could provide useful mechanical changes analysis and a fresh perspective into how the procedure can be implemented more effectively toward the goal of preventing osteoporosis-related fractures.

New Cage for Posterior Minimally Invasive Lumbar Interbody Fusion: a Study in Vitro and in Vivo

To design a new type of interbody fusion device made of nickel titanium NiTi shape memory alloy and to compare segmental stiffness after various posterior lumbar interbody fusion (PLIF) procedures in vitro and in vivo. Twelve sheep lumbar functional spinal units were randomly allocated to four groups. One acted as controls (N); the other three were treated with autogenous iliac crest bone dowel graft (L), a threaded cylindrical titanium (KC) interbody fusion device (TFC) or a new type of interbody fusion device made of NiTi shape memory alloy (NT) containing autogenous iliac crest graft. In addition, 15 sheep were allocated to three groups; one served as controls and the other two underwent TFC (KC) or NiTi-FC (NT). Nondestructive mechanical tests were performed in pure compression, extension, lateral bending and torsion. The operated spines were photographed regularly to assess changes in interbody height and degree of fusion. The animals were killed at 6 months for histologic testing. Biomechanical tests showed both the strength and axial stiffness of the NT and KC groups were significantly higher than those of the control group and L group (P < 0.05). When the mechanical performance in torque and torsion of each group were compared, the same results could be obtained. The maximal destructive load of the NiTi-TFC was 11 200 N and the safety coefficient was above 1.2. Radiological observations revealed that the bone callus around the interbody fusion device were gradually increased postoperatively (2 months, no obvious; 4 months, poorly define; 6 months, dense). The KC and NT group had lost 16% and 16.5% of their postoperative height but remained well above normal disc height (P < 0.05). Histologic examination showed new trabeculation connected with that of the host. The mechanical characteristics of the NiTi-TFC are excellent and it is safe and reliable.