Effect of axial compression on stiffness and deformation of human lumbar spine in flexion-extension

OBJECTIVE

The goal of this study was to evaluate the effect of axial compression, employed with a follower-load mechanism, on the response of the lumbar spine in flexion and extension bending. Additional goals include measurement of both the kinetic (stiffness) and kinematic (deformation distribution) responses, evaluating how the responses vary across specimens, and to develop response corridors that can be used to evaluate human body models (HBMs) and anthropomorphic test devices (ATDs).

METHODS

Seven mid-sized male adult lumbar spines (T12-S1) from postmortem human surrogates were tested in subinjurious flexion and extension bending with 0, 900, and 1800 N of superimposed axial compression. Tests were performed in load-control with a 6-DOF robotic test system that applied pure flexion and extension moments to the specimens, and axial compression was directed along the spine's curvature via a follower load mechanism powered by force-controlled linear actuators. Load-deformation response data were captured and used to characterize the kinetic response of the lumbar spine in flexion/extension, and how it varies with axial compression. Individual vertebral kinematics were captured using 3D motion capture and the data was used to illustrate the distribution of bending deformation across each intervertebral joint of the spine, as well has how that distribution changes with axial compression. These response data were used to develop elliptical path-length parameterized response corridors for surrogate biofidelity evaluation.

RESULTS

The lumbar spine was found to be generally stiffer in extension than in flexion, but this difference decreased with increasing axial compression. The lumbar spine exhibited a nonlinear kinetic (moment vs. angle) response in flexion that became more linear and stiffer with the addition of axial compression. In flexion without axial load, the majority of the bending deformation occurred at the L5-S1 joint, whereas in extension, deformation was more evenly distributed across the different intervertebral levels, but the locus of deformation was located in the mid-proximal lumbar at L2-L3.

CONCLUSIONS

The superposition of axial compression in the lumbar spine affects the kinetic and kinematic response of the lumbar spine in flexion and extension. The response data and approach detailed in this study permit better assessment of ATD and HBM biofidelity.

In Silico Meta-Analysis of Boundary Conditions for Experimental Tests on the Lumbar Spine

The study of the spine range of motion under given external load has been the object of many studies in literature, finalised to a better understanding of the spine biomechanics, its physiology, eventual pathologic conditions and possible rehabilitation strategies. However, the huge amount of experimental work performed so far cannot be straightforwardly analysed due to significant differences among loading set-ups. This work performs a meta-analysis of various boundary conditions in literature, focusing on the flexion/extension behaviour of the lumbar spine. The comparison among range of motions is performed virtually through a validated multibody model. Results clearly illustrated the effect of various boundary conditions which can be met in literature, so justifying differences of biomechanical behaviours reported by authors implementing different set-up: for example, a higher value of the follower load can indeed result in a stiffer behaviour; the application of force producing spurious moments results in an apparently more deformable behaviour, however the respective effects change at various segments along the spine due to its natural curvature. These outcomes are reported not only in qualitative, but also in quantitative terms. The numerical approach here followed to perform the meta-analysis is original and it proved to be effective thanks to the bypass of the natural variability among specimens which might completely or partially hinder the effect of some boundary conditions. In addition, it can provide very complete information since the behaviour of each functional spinal unit can be recorded. On the whole, the work provided an extensive review of lumbar spine loading in flexion/extension.

A finite element study on the effects of follower load on the continuous biomechanical responses of subaxial cervical spine

In spine biomechanics, follower loads are used to mimic the in vivo muscle forces acting on a human spine. However, the effects of the follower load on the continuous biomechanical responses of the subaxial cervical spines (C2-T1) have not been systematically clarified. This study aims at investigating the follower load effects on the continuous biomechanical responses of C2-T1. A nonlinear finite element model is reconstructed and validated for C2-T1. Six levels follower loads are considered along the follower load path that is optimized through a novel range of motion-based method. A moment up to 2 Nm is subsequently superimposed to produce motions in three anatomical planes. The continuous biomechanical responses, including the range of motion, facet joint force, intradiscal pressure and flexibility are evaluated for each motion segment. In the sagittal plane, the change of the overall range of motion arising from the follower loads is less than 6%. In the other two anatomical planes, both the magnitude and shape of the rotation-moment curves change with follower loads. At the neutral position, over 50% decrease in flexibility occurs as the follower load increases from zero to 250 N. In all three anatomical planes, over 50% and 30% decreases in flexibility occur in the first 0.5 Nm for small (≤100 N) and large (≥150 N) follower loads, respectively. Moreover, follower loads tend to increase both the facet joint forces and the intradiscal pressures. The shape of the intradiscal pressure-moment curves changes from nonlinear to roughly linear with increased follower load, especially in the coronal and transverse planes. The results obtained in this work provide a comprehensive understanding on the effects of follower load on the continuous biomechanical responses of the C2-T1.

Moment-rotation behavior of intervertebral joints in flexion-extension, lateral bending, and axial rotation at all levels of the human spine: A structured review and meta-regression analysis

Spinal intervertebral joints are complex structures allowing motion in multiple directions, and many experimental studies have reported moment-rotation response. However, experimental methods, reporting of results, and levels of the spine tested vary widely, and a comprehensive assessment of moment-rotation response across all levels of the spine is lacking. This review aims to characterize moment-rotation response in a consistent manner for all levels of the human spine. A literature search was conducted in PubMed for moment versus rotation data from mechanical testing of intact human cadaveric intervertebral joint specimens in flexion-extension, lateral bending, and axial rotation. A total of 45 studies were included, providing data from testing of an estimated 1,648 intervertebral joints from 518 human cadavers. We used mixed-effects regression analysis to create 75 regression models of moment-rotation response (25 intervertebral joints × 3 directions). We found that a cubic polynomial model provides a good representation of the moment-rotation behavior of most intervertebral joints, and that compressive loading increases rotational stiffness throughout the spine in all directions. The results allow for the direct evaluation of intervertebral ranges of motion across the whole of the spine for given loading conditions. The random-effects outcomes, representing standard deviations of the model coefficients across the dataset, can aid understanding of normal variations in moment-rotation responses. Overall these results fill a large gap, providing the first realistic and comprehensive representations of moment-rotation behavior at all levels of the spine, with broad implications for surgical planning, medical device design, computational modeling, and understanding of spine biomechanics.

Optimization of compressive loading parameters to mimic in vivo cervical spine kinematics in vitro

The human cervical spine supports substantial compressive load in vivo. However, the traditional in vitro testing methods rarely include compressive loads, especially in investigations of multi-segment cervical spine constructs. Previously, a systematic comparison was performed between the standard pure moment with no compressive loading and published compressive loading techniques (follower load - FL, axial load - AL, and combined load - CL). The systematic comparison was structured a priori using a statistical design of experiments and the desirability function approach, which was chosen based on the goal of determining the optimal compressive loading parameters necessary to mimic the segmental contribution patterns exhibited in vivo. The optimized set of compressive loading parameters resulted in in vitro segmental rotations that were within one standard deviation and 10% of average percent error of the in vivo mean throughout the entire motion path. As hypothesized, the values for the optimized independent variables of FL and AL varied dynamically throughout the motion path. FL was not necessary at the extremes of the flexion-extension (FE) motion path but peaked through the neutral position, whereas, a large negative value of AL was necessary in extension and increased linearly to a large positive value in flexion. Although further validation is required, the long-term goal is to develop a "physiologic" in vitro testing method, which will be valuable for evaluating adjacent segment effect following spinal fusion surgery, disc arthroplasty instrumentation testing and design, as well as mechanobiology experiments where correct kinematics and arthrokinematics are critical.

Influence of follower load application on moment-rotation parameters and intradiscal pressure in the cervical spine

The objective of this study was to implement a follower load (FL) device within a robotic (universal force-moment sensor) testing system and utilize the system to explore the effect of FL on multi-segment cervical spine moment-rotation parameters and intradiscal pressure (IDP) at C45 and C56. Twelve fresh-frozen human cervical specimens (C3-C7) were biomechanically tested in a robotic testing system to a pure moment target of 2.0 Nm for flexion and extension (FE) with no compression and with 100 N of FL. Application of FL was accomplished by loading the specimens with bilateral cables passing through cable guides inserted into the vertebral bodies and attached to load controlled linear actuators. FL significantly increased neutral zone (NZ) stiffness and NZ width but resulted in no change in the range of motion (ROM) or elastic zone stiffness. C45 and C56 IDP measured in the neutral position were significantly increased with application of FL. The change in IDP with increasing flexion rotation was not significantly affected by the application of FL, whereas the change in IDP with increasing extension rotation was significantly reduced by the application of FL. Application of FL did not appear to affect the specimen's quantity of motion (ROM) but did affect the quality (the shape of the curve). Regarding IDP, the effects of adding FL compression approximates the effect of the patient going from supine to a seated position (FL compression increased the IDP in the neutral position). The change in IDP with increasing flexion rotation was not affected by the application of FL, but the change in IDP with increasing extension rotation was, however, significantly reduced by the application of FL.

The rib cage reduces intervertebral disc pressures in cadaveric thoracic spines by sharing loading under applied dynamic moments

The effects of the rib cage on thoracic spine loading are not well studied, but the rib cage may provide stability or share loads with the spine. Intervertebral disc pressure provides insight into spinal loading, but such measurements are lacking in the thoracic spine. Thus, our objective was to examine thoracic intradiscal pressures under applied pure moments, and to determine the effect of the rib cage on these pressures. Human cadaveric thoracic spine specimens were positioned upright in a testing machine, and Dynamic pure moments (0 to ±5 N·m) with a compressive follower load of 400 N were applied in axial rotation, flexion - extension, and lateral bending. Disc pressures were measured at T4-T5 and T8-T9 using needle-mounted pressure transducers, first with the rib cage intact, and again after the rib cage was removed. Changes in pressure vs. moment slopes with rib cage removal were examined. Pressure generally increased with applied moments, and pressure-moment slope increased with rib cage removal at T4-T5 for axial rotation, extension, and lateral bending, and at T8-T9 for axial rotation. The results suggest the intact rib cage carried about 62% and 56% of axial rotation moments about T4-T5 and T8-T9, respectively, as well as 42% of extension moment and 36-43% of lateral bending moment about T4-T5 only. The rib cage likely plays a larger role in supporting moments than compressive loads, and may also play a larger role in the upper thorax than the lower thorax.

Biomechanical comparison between concentrated, follower, and muscular loads of the lumbar column

Experimental and numerical methods have been extensively used to simulate the lumbar kinematics and mechanics. One of the basic parameters is the lumbar loads. In the literature, both concentrated and distributed loads have been assumed to simulate the in vivo lumbar loads. However, the inconsistent loads between those studies exist and make the comparison of their results controversial. Using finite-element method, this study aimed to numerically compare the effects of the concentrated, follower, and muscular loads on the lumbar biomechanics during flexion. Two conditions of equivalent and simple constraints were simulated. The equivalent condition assumes the identical flexion at the L1 level and loads at the L5 level for the three types of loads. Another condition is to remove such kinematic and mechanical constraints on the lumbar. The comparison indices were flexed profile, distributed stress, and transferred loads of the discs and vertebrae at the different levels. The results showed that the three modes in the equivalent condition show the nearly same flexed profiles. In the simple condition, however, the L1 vertebra of the concentrated mode anteriorly translates about 3 and 5 times that of the follower and muscular mode, respectively. By contrast, the flexion profiles of the follower and muscular are comparable. In the equivalent condition, all modes consistently show the gradually increasing stress and loads toward the caudal levels. The results of both concentrated and muscular modes exhibit the quite comparable trends and even magnitudes. In the simple condition, however, the removal of flexion and load constraints makes the results of the concentrated mode significantly different from its counterparts. In both conditions, the predictedindices of the follower mode are more uniform along the lumbar. In conclusion, the kinematic and mechanical constraints significantly affect the profile, stress, and loads of the three modes. In the equivalent condition, the concentrated mode can simulate the similar results to the muscular mode and top-loading fashion seems to be more practicable for experimental setup. In the simple condition, the follower mode can serve as the alternative to avoid the unreasonably higher flexion at the L1 level and shear at the L5 level. In the future, the detailed studies about the load-related effects on both load-transferring mechanism and failure mode of the lumbar-implant construct should be conducted.

Effect of follower load on motion and stiffness of the human thoracic spine with intact rib cage

Researchers have reported on the importance of the rib cage in maintaining mechanical stability in the thoracic spine and on the validity of a compressive follower preload. However, dynamic mechanical testing using both the rib cage and follower load has never been studied. An in vitro biomechanical study of human cadaveric thoracic specimens with rib cage intact in lateral bending, flexion/extension, and axial rotation under varying compressive follower preloads was performed. The objective was to characterize the motion and stiffness of the thoracic spine with intact rib cage and follower preload. The hypotheses tested for all modes of bending were (i) range of motion, elastic zone, and neutral zone will be reduced with a follower load, and (ii) neutral and elastic zone stiffness will be increased with a follower load. Eight human cadaveric thoracic spine specimen (T1-T12) with intact rib cage were subjected to 5Nm pure moments in lateral bending, flexion/extension, and axial rotation under follower loads of 0-400N. Range of motion, elastic and neutral zones, and elastic and neutral zone stiffness values were calculated for functional spinal units and segments within the entire thoracic section. Combined segmental range of motion decreased by an average of 34% with follower load for every mode. Application of a follower load with intact rib cage impacts the motion and stiffness of the human cadaveric thoracic spine. Researchers should consider including both aspects to better represent the physiologic implications of human motion and improve clinically relevant biomechanical thoracic spine testing.

Biomechanical response of lumbar facet joints under follower preload: a finite element study

Background

Facet joints play a significant role in providing stability to the spine and they have been associated with low back pain symptoms and other spinal disorders. The influence of a follower load on biomechanics of facet joints is unknown. A comprehensive research on the biomechanical role of facets may provide insight into facet joint instability and degeneration.

Method

A nonlinear finite element (FE) model of lumbar spine (L1-S1) was developed and validated to study the biomechanical response of facets, with different values of follower preload (0 N,500 N,800 N,1200 N), under loadings in the three anatomic planes. In this model, special attention was paid to the modeling of facet joints, including cartilage layer. The asymmetry in the biomechanical response of facets was also discussed. A rate of change (ROC) and an average asymmetry factor (AAF) were introduced to explore and evaluate the preload effect on these facet contact parameters and on the asymmetry under different loading conditions.

Results

The biomechanical response of facets changed according to the loading condition. The preload amplified the facet force, contact area and contact pressure in flexion-extension; the same effect was observed on the ipsilateral facet while an opposite effect could be seen on the contralateral facet during lateral bending. For torsion loading, the preload increased contact area, decreased the mean contact pressure, but had almost no effect on facet force. However, all the effects of follower load on facet response became weaker with the increase of preload. The greatest asymmetry of facet response could be found on the ipsilateral side during lateral bending, followed by flexion, bending (contralateral side), extension and torsion. This asymmetry could be amplified by preload in the bending (ipsilateral), torsion loading group, while being reduced in the flexion group.

Conclusions

An analysis combining patterns of contact pressure distribution, facet load, contact area and contact pressure can provide more insight into the biomechanical role of facets under various moment loadings and follower loads. The effect of asymmetry on facet joint response should be fully considered in biomechanical studies of lumbar spine, especially in post structures subjected to physiological loadings.

Electronic supplementary material

The online version of this article (doi:10.1186/s12891-016-0980-4) contains supplementary material, which is available to authorized users.

Effects of follower load and rib cage on intervertebral disc pressure and sagittal plane curvature in static tests of cadaveric thoracic spines

The clinical relevance of mechanical testing studies of cadaveric human thoracic spines could be enhanced by using follower preload techniques, by including the intact rib cage, and by measuring thoracic intervertebral disc pressures, but studies to date have not incorporated all of these components simultaneously. Thus, this study aimed to implement a follower preload in the thoracic spine with intact rib cage, and examine the effects of follower load, rib cage stiffening and rib cage removal on intervertebral disc pressures and sagittal plane curvatures in unconstrained static conditions. Intervertebral disc pressures increased linearly with follower load magnitude. The effect of the rib cage on disc pressures in static conditions remains unclear because testing order likely confounded the results. Disc pressures compared well with previous reports in vitro, and comparison with in vivo values suggests the use of a follower load of about 400N to approximate loading in upright standing. Follower load had no effect on sagittal plane spine curvature overall, suggesting successful application of the technique, although increased flexion in the upper spine and reduced flexion in the lower spine suggest that the follower load path was not optimized. Rib cage stiffening and removal both increased overall spine flexion slightly, although with differing effects at specific spinal locations. Overall, the approaches demonstrated here will support the use of follower preloads, intact rib cage, and disc pressure measurements to enhance the clinical relevance of future studies of the thoracic spine.

Influence of varying compressive loading methods on physiologic motion patterns in the cervical spine

The human cervical spine supports substantial compressive load in-vivo arising from muscle forces and the weight of the head. However, the traditional in-vitro testing methods rarely include compressive loads, especially in investigations of multi-segment cervical spine constructs. Various methods of modeling physiologic loading have been reported in the literature including axial forces produced with inclined loading plates, eccentric axial force application, follower load, as well as attempts to individually apply/model muscle forces in-vitro. The importance of proper compressive loading to recreate the segmental motion patterns exhibited in-vivo has been highlighted in previous studies. However, appropriate methods of representing the weight of head and muscle loading are currently unknown. Therefore, a systematic comparison of standard pure moment with no compressive loading versus published and novel compressive loading techniques (follower load - FL, axial load - AL, and combined load - CL) was performed. The present study is unique in that a direct comparison to continuous cervical kinematics over the entire extension to flexion motion path was possible through an ongoing intra-institutional collaboration. The pure moment testing protocol without compression or with the application of follower load was not able to replicate the typical in-vivo segmental motion patterns throughout the entire motion path. Axial load or a combination of axial and follower load was necessary to mimic the in-vivo segmental contributions at the extremes of the extension-flexion motion path. It is hypothesized that dynamically altering the compressive loading throughout the motion path is necessary to mimic the segmental contribution patterns exhibited in-vivo.

Selecting boundary conditions in physiological strain analysis of the femur: Balanced loads, inertia relief method and follower load

Selection of boundary constraints may influence amount and distribution of loads. The purpose of this study is to analyze the potential of inertia relief and follower load to maintain the effects of musculoskeletal loads even under large deflections in patient specific finite element models of intact or fractured bone compared to empiric boundary constraints which have been shown to lead to physiological displacements and surface strains. The goal is to elucidate the use of boundary conditions in strain analyses of bones. Finite element models of the intact femur and a model of clinically relevant fracture stabilization by locking plate fixation were analyzed with normal walking loading conditions for different boundary conditions, specifically re-balanced loading, inertia relief and follower load. Peak principal cortex surface strains for different boundary conditions are consistent (maximum deviation 13.7%) except for inertia relief without force balancing (maximum deviation 108.4%). Influence of follower load on displacements increases with higher deflection in fracture model (from 3% to 7% for force balanced model). For load balanced models, follower load had only minor influence, though the effect increases strongly with higher deflection. Conventional constraints of fixed nodes in space should be carefully reconsidered because their type and position are challenging to justify and for their potential to introduce relevant non-physiological reaction forces. Inertia relief provides an alternative method which yields physiological strain results.

On the load-sharing along the ligamentous lumbosacral spine in flexed and extended postures: Finite element study

A harmonic synergy between the load-bearing and stabilizing components of the spine is necessary to maintain its normal function. This study aimed to investigate the load-sharing along the ligamentous lumbosacral spine under sagittal loading. A 3D nonlinear detailed Finite Element (FE) model of lumbosacral spine with realistic geometry was developed and validated using wide range of numerical and experimental (in-vivo and in-vitro) data. The model was subjected to 500 N compressive Follower Load (FL) combined with 7.5 Nm flexion (FLX) or extension (EXT) moments. Load-sharing was expressed as percentage of total internal force/moment developed along the spine that each spinal component carried. These internal forces and moments were determined at the discs centres and included the applied load and the resisting forces in the ligaments and facet joints. The contribution of the facet joints and ligaments in supporting bending moments produced additional forces and moments in the discs. The intervertebral discs carried up to 81% and 68% of the total internal force in case of FL combined with FLX and EXT, respectively. The ligaments withstood up to 67% and 81% of the total internal moment in cases of FL combined with EXT and FLX, respectively. Contribution of the facet joints in resisting internal force and moment was noticeable at levels L4-S1 only particularly in case of FL combined with EXT and reached up 29% and 52% of the internal moment and force, respectively. This study demonstrated that spinal load-sharing depended on applied load and varied along the spine.

In vitro evaluation of translating and rotating plates using a robot testing system under follower load

BACKGROUND CONTEXT

Various modifications to standard "rigid" anterior cervical plate designs (constrained plate) have been developed that allow for some degree of axial translation and/or rotation of the plate (semi-constrained plate)-theoretically promoting proper load sharing with the graft and improved fusion rates. However, previous studies about rigid and dynamic plates have not examined the influence of simulated muscle loading.

PURPOSE

The objective of this study was to compare rigid, translating, and rotating plates for single-level corpectomy procedures using a robot testing system with follower load.

STUDY DESIGN

In-vitro biomechanical test.

METHODS

N = 15 fresh-frozen human (C3-7) cervical specimens were biomechanically tested. The follower load was applied to the specimens at the neutral position from 0 to 100 N. Specimens were randomized into a rigid plate group, a translating plate group and a rotating plate group and then tested in flexion, extension, lateral bending and axial rotation to a pure moment target of 2.0 Nm under 100N of follower load. Range of motion, load sharing, and adjacent level effects were analyzed using a repeated measures analysis of variance (ANOVA).

RESULTS

No significant differences were observed between the translating plate and the rigid plate on load sharing at neutral position and C4-6 ROM, but the translating plate was able to maintain load through the graft at a desired level during flexion. The rotating plate shared less load than rigid and translating plates in the neutral position, but cannot maintain the graft load during flexion.

CONCLUSIONS

This study demonstrated that, in the presence of simulated muscle loading (follower load), the translating plate demonstrated superior performance for load sharing compared to the rigid and rotating plates.

Intervertebral disc degeneration alters lumbar spine segmental stiffness in all modes of loading under a compressive follower load

BACKGROUND CONTEXT

Previous studies have investigated the relationship between the degeneration grade of the intervertebral disc (IVD) and the flexibility of the functional spinal unit (FSU) but were completed at room temperature without the presence of a compressive follower load. This study builds on previous work by performing the testing under more physiological conditions of a compressive follower load at body temperature and at near 100% humidity.

PURPOSE

The present work evaluates the effects of IVD degeneration on segmental stiffness, range of motion (ROM), hysteresis area, and normalized hysteresis (hysteresis area/ROM). This study also briefly evaluates the effect of the segment level, temperature, and follower load on the same parameters.

STUDY DESIGN

In vitro human cadaveric biomechanical investigation.

METHODS

Twenty-one FSUs were tested in the three primary modes of loading at both body temperature and room temperature in a near 100% humidity environment. A compressive follower load of 440 N was applied to simulate the physiological conditions. Fifteen of the 21 segments were also tested without the follower load to determine the effects of the follower load on segmental biomechanics. The grade of degeneration for each segment was determined using the Thompson scale, and the torque-rotation curves were fit with the Dual-Inflection-Point Boltzmann sigmoid curve.

RESULTS

Intervertebral disc degeneration resulted in statistically significant changes in segmental stiffness, ROM, and hysteresis area in axial rotation (AR) and lateral bending (LB) and statistically significant changes in ROM and normalized hysteresis in flexion-extension (FE). The progression of these changes with increased degeneration is nonlinear, with changes in the FE and LB tending to respond in concert and opposite to the changes in AR. The lumbosacral joint was significantly stiffer and demonstrated a decreased ROM and hysteresis area as compared with other lumbar segments in AR and LB. Temperature had a significant effect on the stiffness and hysteresis area in AR and on the hysteresis area in LB. Application of a compressive follower load increased the stiffness in all three modes of loading but was significant only in AR and LB. It also reduced the ROM and increased normalized hysteresis in all three modes of loading.

CONCLUSIONS

The results from this testing quantify the effects of degeneration on spinal biomechanics. Because the testing was conducted under physiological conditions (including a compressive follower load and at body temperature), we expect the measured response to closely match the in vivo response. The testing results can be used to guide the selection of appropriate surgical treatments in the context of IVD degeneration and to validate the mathematical and engineering models of the lumbar spine, including finite element models.