A study of stiffness protocol as exemplified by testing of a burst fracture model in sagittal plane

A novel spinal cord surrogate for the study of compressive traumatic spinal cord injuries

Although in vitro studies are frequent for the study of traumatic spine and spinal cord injuries, few include the spinal cord due to its prompt post-mortem decay. Several materials have been proposed to mimic the spinal cord behaviour, but none matched its mechanical properties under transverse compression, which is vital for the study of burst fractures and other injury mechanisms leading to spinal cord compression. In this study, a new material named Soma Foama 15 (Reynolds Advanced Material, USA) was used to manufacture three spinal cord surrogates at 3 mixing ratios of elastomer to catalyst (1:1, 2:1 and 3:1) and tested at three different strain rates (0.5, 5 and 50 .s^-1). The mixing ratio 3:1 presents a mechanical behaviour comparable to that of the porcine spinal cord at each of these strain rates, making the surrogate a valid substitute up to 75 % of transverse compression.

The current testing protocols for biomechanical evaluation of lumbar spinal implants in laboratory setting: a review of the literature

In vitro biomechanical investigations have become a routinely employed technique to explore new lumbar instrumentation. One of the most important advantages of such investigations is the low risk present when compared to clinical trials. However, the best use of any experimental data can be made when standard testing protocols are adopted by investigators, thus allowing comparisons among studies. Experimental variables, such as the length of the specimen, operative level, type of loading (e.g., dynamic versus quasistatic), magnitude, and rate of load applied, are among the most common variables controlled during spinal biomechanical testing. Although important efforts have been made to standardize these protocols, high variability can be found in the current literature. The aim of this investigation was to conduct a systematic review of the literature to identify the current trends in the protocols reported for the evaluation of new lumbar spinal implants under laboratory setting.

Adaptive velocity-based six degree of freedom load control for real-time unconstrained biomechanical testing

Robotic biomechanics is a powerful tool for further developing our understanding of biological joints, tissues and their repair. Both velocity-based and hybrid force control methods have been applied to biomechanics but the complex and non-linear properties of joints have limited these to slow or stepwise loading, which may not capture the real-time behaviour of joints. This paper presents a novel force control scheme combining stiffness and velocity based methods aimed at achieving six degree of freedom unconstrained force control at physiological loading rates.

Load sharing in lumbar spinal segment as a function of location of center of rotation

Object

The center (axis) of rotation (COR) in the lumbar spine has been studied well. However, there is limited information on the kinetic and kinematic consequences of imposed shift in the location of the COR, although this type of shift can be seen after surgeries using motion preservation or dynamic stabilization devices. The objective of this study was to assess the kinetic and kinematic changes in the lumbar spinal segment due to various imposed CORs.

Methods

A 3D finite element model of the L4–5 segment was constructed and validated. The segment was loaded under a 7.5-Nm bending moment while constrained to rotate about various imposed CORs in the sagittal and axial motion planes. Range of motion, ligament forces, facet loads, and disc stresses were measured.

Results

The present model showed an agreement with previous in vitro and finite element studies under the same load and boundary conditions. Range of motion, facet forces, disc stresses, and ligament loads showed a strong association with the location of the COR.

Conclusions

Acute alterations in the location of the COR can significantly change the load sharing characteristics within the spine segment. The normal location of the COR is a result of the tendency of the vertebra to move in the path of least cumulative resistance.

Characterization and prediction of rate-dependent flexibility in lumbar spine biomechanics at room and body temperature

BACKGROUND CONTEXT

The soft tissues of the spine exhibit sensitivity to strain-rate and temperature, yet current knowledge of spine biomechanics is derived from cadaveric testing conducted at room temperature at very slow, quasi-static rates.

PURPOSE

The primary objective of this study was to characterize the change in segmental flexibility of cadaveric lumbar spine segments with respect to multiple loading rates within the range of physiologic motion by using specimens at body or room temperature. The secondary objective was to develop a predictive model of spine flexibility across the voluntary range of loading rates.

STUDY DESIGN

This in vitro study examines rate- and temperature-dependent viscoelasticity of the human lumbar cadaveric spine.

METHODS

Repeated flexibility tests were performed on 21 lumbar function spinal units (FSUs) in flexion-extension with the use of 11 distinct voluntary loading rates at body or room temperature. Furthermore, six lumbar FSUs were loaded in axial rotation, flexion-extension, and lateral bending at both body and room temperature via a stepwise, quasi-static loading protocol. All FSUs were also loaded using a control loading test with a continuous-speed loading-rate of 1-deg/sec. The viscoelastic torque-rotation response for each spinal segment was recorded. A predictive model was developed to accurately estimate spine segment flexibility at any voluntary loading rate based on measured flexibility at a single loading rate.

RESULTS

Stepwise loading exhibited the greatest segmental range of motion (ROM) in all loading directions. As loading rate increased, segmental ROM decreased, whereas segmental stiffness and hysteresis both increased; however, the neutral zone remained constant. Continuous-speed tests showed that segmental stiffness and hysteresis are dependent variables to ROM at voluntary loading rates in flexion-extension. To predict the torque-rotation response at different loading rates, the model requires knowledge of the segmental flexibility at a single rate and specified temperature, and a scaling parameter. A Bland-Altman analysis showed high coefficients of determination for the predictive model.

CONCLUSIONS

The present work demonstrates significant changes in spine segment flexibility as a result of loading rate and testing temperature. Loading rate effects can be accounted for using the predictive model, which accurately estimated ROM, neutral zone, stiffness, and hysteresis within the range of voluntary motion.

EXPERIMENTAL METHODS FOR THE BIOMECHANICAL INVESTIGATION OF THE HUMAN SPINE: A REVIEW

In vitro mechanical testing of spinal specimens is extremely important to better understand the biomechanics of the healthy and diseased spine, fracture, and to test/optimize surgical treatment. While spinal testing has extensively been carried out in the past four decades, testing methods are quite diverse. This paper aims to provide a critical overview of the in vitro methods for mechanical testing the human spine at different scales. Specimens of different type are used, according to the aim of the study: spine segments (two or more adjacent vertebrae) are used both to investigate the spine kinematics, and the mechanical properties of the spine components (vertebrae, ligaments, discs); single vertebrae (whole vertebra, isolated vertebral body, or vertebral body without endplates) are used to investigate the structural properties of the vertebra itself; core specimens are extracted to test the mechanical properties of the trabecular bone at the tissue-level; mechanical properties of spine soft tissue (discs, ligaments, spinal cord) are measured on isolated elements, or on tissue specimens. Identification of consistent reference frames is still a debated issue. Testing conditions feature different pre-conditioning and loading rates, depending on the simulated action. Tissue specimen preservation is a very critical issue, affecting test results. Animal models are often used as a surrogate. However, because of different structure and anatomy, extreme caution is required when extrapolating to the human spine. In vitro loading conditions should be based on reliable in vivo data. Because of the high complexity of the spine, such information (either through instrumented implants or through numerical modeling) is currently unsatisfactory. Because of the increasing ability of computational models in predicting biomechanical properties of musculoskeletal structures, a synergy is possible (and desirable) between in vitro experiments and numerical modeling. Future perspectives in spine testing include integration of mechanical and structural properties at different dimensional scales (from the whole-body-level down to the tissue-level) so that organ-level models (which are used to predict the most relevant phenomena such as fracture) include information from all dimensional scales.

Hybrid multidirectional test method to evaluate spinal adjacent-level effects

BACKGROUND

Several clinical studies have documented long-term adjacent-level effects of spinal fusion, due to stress concentration and motion loss at the fused segment. Non-fusion motion preservation devices are designed to eliminate or slow down such adverse effects. Therefore, appropriate biomechanical evaluation of the adjacent-level effects in spine is important and timely. Although many biomechanical studies are available and have provided some understanding of the adjacent-level effects, results have large variation and are conflicting, mostly due to the use of inappropriate and ill-defined methods. A new test method especially designed to study spinal adjacent-level effects is needed.

METHODS

The proposed Hybrid method uses unconstrained pure moment to provide rotation-input for multi-directional testing. The new method has four steps: (1) Intact spine specimen with entire mobile region is used. The specimen is prepared to measure various biomechanical parameters, e.g., disc pressures, ligament strains, and facet loads. (2) Appropriate unconstrained pure moment is applied to the intact specimen and total range of motion is determined. (3) Unconstrained pure moment is applied to the spinal construct (specimen with an implant) until the total range of motion of the construct equals that of the intact. (4) Statistical comparison of the biomechanical parameters between the construct and intact quantifies the adjacent-level effects.

FINDINGS

The uniqueness of the proposed method, to study the adjacent level effects due to fusion and non-fusion devices, is that it applies the needed rotation-input to the spine specimen, using available methodology with minimal modification.

INTERPRETATION

Previous studies have lacked appropriate and well-defined methodologies to evaluate spinal adjacent-level effects. The proposed method uses well-known methodology and yields high quality, and laboratory-independent results for the fusion and non-fusion devices.

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.

Compliance of the L5-S1 spinal unit: a comparative study between an unconstrained and a partially constrained system

A comparison between an unconstrained and a partially constrained system for in vitro biomechanical testing of the L5-S1 spinal unit was conducted. The objective was to compare the compliance and the coupling of the L5-S1 unit measured with an unconstrained and a partially constrained test for the three major physiological motions of the human spine. Very few studies have compared unconstrained and partially constrained testing systems using the same cadaveric functional spinal units (FSUs). Seven human L5-S1 units were therefore tested on both a pneumatic, unconstrained, and a servohydraulic, partially constrained system. Each FSU was tested along three motions: flexion-extension (FE), lateral bending (LB) and axial rotation (AR). The obtained kinematics on both systems is not equivalent, except for the FE case, where both motions are similar. The directions of coupled motions were similar for both tests, but their magnitudes were smaller in the partially constrained configuration. The use of a partially constrained system to characterize LB and AR of the lumbosacral FSU decreased significantly the measured stiffness of the segment. The unconstrained system is today's "gold standard" for the characterization of FSUs. The selected partially constrained method seems also to be an appropriate way to characterize FSUs for specific applications. Care should be taken using the latter method when the coupled motions are important.

Biomechanical in vitro comparison of different mono- and bisegmental anterior procedures with regard to the strategy for fracture stabilisation using minimally invasive techniques

Endoscopic minimally invasive techniques have become an established method of fracture stabilisation in the spine. In view of this fact, anterior stabilisation strategies must be reconsidered, as monosegmental A 3.1 compression fractures are increasingly being stabilised endoscopically from the anterior aspect using minimally invasive techniques. This study investigated the biomechanical necessity of anterior two-point or four-point stabilisation in the instrumentation of mono- and bisegmental fractures. In three biomechanical in vitro studies, burst fracture stabilisation was simulated, and anterior short fixation devices were tested under load with pure moments up to 3.75 Nm to evaluate the biomechanical stabilising characteristics of different kinds of instrumentations in flexion/extension, lateral bending, and axial rotation. Only anterior four-point stabilisation resulted in sufficient primary stability both in mono- and bisegmental instrumentation and therefore represents the standard procedure in open as well as in minimally invasive spinal surgery.

Kinematics of the Thoracic T10–T11 Motion Segment: Locus of Instantaneous Axes of Rotation in Flexion and Extension

The purpose of this study was to determine the locations and loci of instantaneous axes of rotation (IARs) of the T10-T11 motion segment in flexion and extension. An anatomically accurate three-dimensional model of thoracic T10-T11 functional spinal unit (FSU) was developed and validated against published experimental data under flexion, extension, lateral bending, and axial rotation loading configurations. The validated model was exercised under six load configurations that produced motions only in the sagittal plane to characterize the loci of IARs for flexion and extension. The IARs for both flexion and extension under these six load types were directly below the geometric center of the moving vertebra, and all the loci of IARs were tracked superoanteriorly for flexion and inferoposteriorly for extension with rotation. These findings may offer an insight to better understanding of the kinematics of the human thoracic spine and provide clinically relevant information for the evaluation of spinal stability and implant device functionality.

Distribution of anterior cortical shear strain after a thoracic wedge compression fracture

BACKGROUND CONTEXT

Vertebral compression fractures (VCFs) are a common clinical problem and may follow trauma or be pathological. Osteoporosis increases susceptibility to fracture by reducing bone mass and weakening bone architecture. Approximately 2.5 million osteoporotic fractures occur worldwide annually, usually involving the vertebrae, wrist and hip. In the United States 700,000 VCFs occur annually, causing significant morbidity, mortality and economic burden. An initial VCF often leads to subsequent VCFs. The strain distribution along the anterior cortex, the major load-bearing pathway in flexion, may be predictive of impending VCF. Regions of high strain distribution are likely to experience secondary fracture.

PURPOSE

To investigate the distribution of anterior cortical strain at, above and below an experimentally created index VCF to determine the vertebral body at risk of secondary fracture.

STUDY DESIGN

In vitro experimental study using cadaveric thoracic spinal segments.

METHODS

Seventeen thoracic spines underwent dual-energy X-ray absorptiometry (DEXA) to assess bone mineral density and were divided into T1-T3 (Subsegment 1), T4-T6 (Subsegment 2), T7-T9 (Subsegment 3) and T10-T12 (Subsegment 4). Rectangular rosette strain gauges were applied to the anterior cortices of the vertebrae of each subsegment (vertebrae in each specimen were denoted V1-superior, V2-intermediate and V3-inferior). V1 and V3 were partially embedded into polyester resin blocks, which were used to mount the specimens in a materials testing machine. Nondestructive predefect testing was performed in compression at 125 N and 250 N, followed by flexion at 1.25 Nm and 2.5 Nm. To ensure fracture reproducibility, V2 of each specimen had a trabecular defect created to a volume of 21.3+/-4.4% of the V2 centrum. Postdefect nondestructive compression and flexion were then performed in a manner similar to the predefect tests, followed by destructive testing in flexion. Anterior cortical shear strain on V1, V2 and V3, applied moments and applied flexion angle were all measured and analyzed.

RESULTS

A VCF occurred in 55 of the 59 subsegments. Fifty-one VCF (93%) were seen in V2 and 4 VCF (7%) were seen in V1. After the creation of the trabecular defect, the shear strain on V2 increased, but a comparison of the postdefect with the predefect nondestructive tests showed no significant differences. The pre- and postdefect shear strain distribution in compression and flexion was V1strain>V3strain>V2strain. Shear strain at failure was highest on V2, and in all subsegments there were significant differences between V2 and V3 (p<.05). In all subsegments there were no significant differences between V2 and V1 (p>.05) at failure with the exception of Subsegment 1 where V2 and V1 were significantly different (p<.05). The predominant strain pattern at failure was (V2strain>V1strain>V3strain V2strain>>V3strain). Using shear strain as the codeterminant of peak moment with bending stiffness and applied angle at failure, the strain on V1 was the greatest predictor (p=.0084; R2=0.78). These findings suggest that the events leading to a secondary fracture probably start before the index VCF occurs and continue with loading beyond the index VCF.

CONCLUSION

Anterior cortical strain is concentrated at the apex of a thoracic kyphotic curve. The vertebral body immediately above the index VCF has the next highest amount of strain and therefore the highest risk of secondary fracture.

Validation of T10-T11 finite element model and determination of instantaneous axes of rotations in three anatomical planes

STUDY DESIGN

A finite element (FE) model of thoracic spine (T10-T11) was constructed and used to determine instantaneous axes of rotation (IARs).

OBJECTIVES

To characterize the locations and loci of IARs in three anatomic planes.

SUMMARY OF BACKGROUND DATA

The center of rotation is a part of a precise method of documenting the kinematics of a spinal segment for spinal stability and deformity assessments and for implant devices study. There is little information about loci of IARs in thoracic spine.

METHODS

A FE model of thoracic spine (T10-T11) was developed and validated against published data. The validated model was then used to determine the locations and loci of IARs in three anatomic planes.

RESULTS

Within the validated range, The IARs locations and loci were found to vary with the applied pure moments. Under flexion and extension pure moments, the loci of IARs were tracked anterosuperiorly for flexion and posteroinferiorly for extension with rotation between the superior endplate and the geometrical center of the inferior vertebra T11. Under left and right lateral bending pure moments, the loci were detected to diverge latero-inferiorly from the mid-height of the intervertebral disc, then converge medio-inferiorly toward the geometrical center of the inferior vertebra T11. For axial rotation, the IARs were located between anterior nucleus and anulus and found to diverge in opposite direction latero-posteriorly with increasing left and right axial torque.

CONCLUSIONS

The results of IARs would provide further understanding to the kinematics and biomechanical responses of the human thoracic spine, which is important for the diagnosis of disc degeneration and implant study.