Pure Moment Testing Research Articles

Replicating spine loading during functional and daily activities: An in vivo, in silico, in vitro research pipeline

Lifestyle heavily influences intervertebral disc (IVD) loads, but measuring in vivo loads requires invasive methods, and the ability to apply these loads in vitro is limited. In vivo load data from instrumented vertebral body replacements is limited to patients that have had spinal fusion surgery, potentially resulting in different kinematics and loading patterns compared to a healthy population. Therefore, this study aimed to develop a pipeline for the non-invasive estimation of in vivo IVD loading, and the application of these loads in vitro. A full-body Opensim model was developed by adapting and combining two existing models. Kinetic data from healthy participants performing activities of daily living were used as inputs for simulations using static optimisation. After evaluating simulation results using in vivo data, the estimated six-axis physiological loads were applied to bovine tail specimens. The pipeline was then used to compare the kinematics resulting from the physiological load profiles (flexion, lateral bending, axial rotation) with a simplified pure moment protocol commonly used for in vitro studies. Comparing kinematics revealed that the in vitro physiological load protocol followed the same trends as the in silico and in vivo data. Furthermore, the physiological loads resulted in substantially different kinematics when compared to pure moment testing, particularly in flexion. Therefore, the use of the presented pipeline to estimate the complex loads of daily activities in different populations, and the application of those loads in vitro provides a novel capability to deepen our knowledge of spine biomechanics, IVD mechanobiology, and improve pre-clinical test methods.

Spinolaminar locking plates improve fixation strength compared to pedicle screws: a biomechanical analysis.

Pedicle screw loosening is a significant complication of posterior spinal fixation, particularly among osteoporotic patients and in deformity constructs. In orthopedic trauma surgery, locking plates and screws have revolutionized the fixation of osteoporotic fractures. We have combined the traumatology principle of fixed-angle locking plate fixation with the spine principles of segmental instrumentation. A novel spinolaminar locking plate was designed based on morphometric studies of human thoracolumbar vertebrae. The plates were fixed to cadaveric human lumbar spines and connected to form 1-level L1-L2 or L4-L5 constructs and compared to similar pedicle screw constructs. Pure moment testing was performed to assess range of motion before and after 30,000 cycles of cyclic fatigue. Post-fatigue fixture pullout strength was assessed by applying a continuous axial tensile force oriented to the principal axis of the pedicle until pullout was observed. Spinolaminar plate fixation resulted in superior pullout strength compared to pedicle screws (1,065 ± 400N vs. 714 ± 284N, p = 0.028). Spinolaminar plates performed equivalently to pedicle screws in range of motion reduction during flexion/extension and axial rotation. Pedicle screws outperformed the spinolaminar plates in lateral bending. Finally, no spinolaminar constructs failed during cyclic fatigue testing, whereas one pedicle screw construct did. The spinolaminar locking plate maintained adequate fixation post-fatigue, particularly in flexion/extension and axial rotation compared to pedicle screws. Moreover, spinolaminar plates were superior to pedicle screw fixation with respect to cyclic fatiguing and pullout strength. The spinolaminar plates offer a viable option for posterior lumbar instrumentation in the adult spine.

Explicit and interaction direct strength methods for combined web crippling and bending moment failure of first-generation trapezoidal steel sheeting

For trapezoidal steel sheeting, design codes so far calculate the web crippling strength via curve fitted rules; bending moment failure is predicted by the effective width method or the Direct Strength Method (DSM); and their combination is handled by a curve fitted interaction rule. As the DSM is easy to use, potentially more accurate, and closer to a mechanical description than curve fitted rules, this paper investigates whether it can be used for trapezoidal sheeting, limited here to the first-generation, i.e. without stiffeners. First, an existing set of experiments is presented, in which first-generation sheeting has been subjected to combined web crippling and bending moment via three-point bending tests. In these experiments, web-crippling deformation, beam deflection, and support rotations have been recorded, and failure behaviour has been studied well beyond the ultimate load. These experiments are simulated by the finite element method, and after verification, the simulations are used to record the first Eigenvalue, first yield, and the ultimate load of the experiments. Additionally, each experiment is also modelled as an Interior One Flange (IOF) web crippling test and a pure bending moment test. Finally, the DSM is developed based on the simulation data (not the experimental data), using two approaches. (i) For the interaction approach, only the web crippling strength and the bending moment capacity are predicted by the DSM, using the simulation data from the IOF web crippling tests and pure bending moment tests. Interaction is described by an interaction rule, calibrated to the simulations of the three-point bending tests. (ii) For the explicit approach, the DSM is used to predict the combined load capacity, based on the simulation data from the three-point bending tests. It is concluded that both approaches work, but the interaction approach performs better than the explicit approach. Using Interior Two Flange (ITF) Eigenvalues in combination with IOF yield loads and IOF ultimate loads increased the performance of the IOF web crippling DSM equations, however, it not leads to a further improvement of the interaction approach.

Increasing loads and diminishing returns: a biomechanical study of direct vertebral rotation.

Biomechanical simulation of DVR and pure-moment testing on thoracic spines. Characterize load-deformation response of thoracic spines under DVR maneuvers until failure, and compare to pure-moment testing of same spines. Despite reports of surgical complications, few studies exist on increase in ROM under DVR torque. Biomechanical models predicting increases from surgical releases have consistently used "pure-moments", a standard established for non-destructive measurement of ROM. Yet, DVR torque is not accurately modeled using pure moments and, moreover, magnitudes of torque applied during DVR maneuvers may be substantially higher than pure-moment testing. Cadaveric thoracic spines (N = 11) were imaged, then prepared. Polyaxial pedicle screws were implanted at T7-T10 after surgical releases. Bilateral facetectomies and Ponte osteotomies were completed at T10-T11. A custom apparatus, mounted into an 8-dof MTS load frame, was used to attach to pedicle screws, allowing simulation of surgical DVR maneuvers. Motions of vertebrae were measured using optical motion tracking. Torque was increased until rupture of the T10-T11 disc or fracture at the pedicle screw sites at any level. The torque-rotation behavior was compared to its behavior under pure-moment testing performed prior to the DVR maneuver. Under DVR maneuvers, failure of the T10-T11 discs accompanied in most cases by pedicle screw loosening, occurred at 13.7-54.7Nm torque, increasing axial rotation by 1.4°-8.9°. In contrast, pure-moment testing (4Nm) increased axial rotation by only 0.0°-0.9°. DVR resulted in substantially greater correction potential increases compared to pure-moment testing even at the same torque. These results suggest increased flexibility obtained by osteotomies and facetectomies is underestimated using pure-moment testing, misrepresenting clinical expectations. The present study is an important and necessary step toward the establishment of a more accurate and ultimately surgically applied model. III.

Wednesday, September 26, 2018 7:35 AM–9:00 AM ePosters: P13. Changes in spinous process tether looping method significantly alters segmental biomechanics for proximal junctional kyphosis prophylaxis

BACKGROUND CONTEXT Spinous process (SP) tethering is gaining interest as a prophylactic technique to alleviate load concentrations and reinforce the posterior ligament complex (PLC) at the uppermost instrumented vertebra (UIV) to reduce the risk of developing proximal junctional kyphosis (PJK) in adult spinal deformity (ASD) surgeries. Consensus has yet to be established on appropriate technique and there is a lack of biomechanical data to provide initial guidance. While multiple SP looping methods have recently been proposed, no study has evaluated the effect of tether looping method on segmental biomechanics. PURPOSE We sought to evaluate the effects of four tether looping methods on segmental biomechanics at UIV, UIV+1, and UIV+2. We hypothesized that the different looping methods would result in significant differences in flexion range-of-motion (ROM), intervertebral disc (IVD) pressures, and peak tether loop tensions. STUDY DESIGN/SETTING Biomechanical study. OUTCOME MEASURES Flexion ROM, IVD pressure, peak tether tension. METHODS Nine T1–T4 cadaveric motion segments were tested in flexion–extension bending to 5 Nm using a pure-moment testing machine. UIV was located at T3 using standard pedicle screws and rods. A rod crosslink (CL) was placed inferior to the pedicle screws. Motion of UIV, UIV+1, and UIV+2 was recorded using a motion capture system. IVD pressure was measured at UIV/UIV+1 and UIV+1/UIV+2 using custom pressure probes. A 5 mm braided polyester suture was used for tethering. The tether was looped under the CL at UIV, and through holes drilled at the base of UIV+1 and UIV+2 SPs. Tether tension was measured using custom in-line tension sensors for each independent loop. An untethered test was used for baseline values. Tethered tests included one single level (SL) and three double level (DL) tests: (1) SL, a single tether from CL at UIV to UIV+1 SP, (2) DL Common (CM), a tether from CL at UIV to UIV+1 SP and another tether from CL at UIV to UIV+2 SP, (3) DL Chained (CH), a tether from CL at UIV to UIV+1 SP and another tether from UIV+1 SP to UIV+2 SP, and (4) DL Figure-Eight (F8), a single tether from CL at UIV to UIV+1 SP and UIV+2 SP in a woven figure-eight loop. Loops were pretensioned to 22N before each test. RESULTS The SL method resulted in significant reductions in flexion ROM at UIV/UIV+1 (p=.001) but not at UIV+1/UIV+2 (p=.052). SL also yielded significant reductions in IVD pressure at both UIV/UIV+1 (p=.007) and UIV+1/UIV+2 (0.002). The choice of DL method had a significant univariate effect on flexion ROM reduction at UIV/UIV+1 (p=.004) but not at UIV+1/UIV+2 (0.14). The choice of DL method also had a significant univariate effect on IVD pressure reduction at UIV/UIV+1 (p CONCLUSIONS The use of different tether looping methods significantly alters the tether's effect on segmental biomechanics. double level techniques provide greater reductions in flexion ROM and IVD pressures; however, they yield greater tether tensions at the uppermost tethered level compared to SL. CM produced the best results for improved segmental stability. The transfer of peak SP loading to superjacent levels may provide benefits in protecting the UIV+1 SP and the PLC. New understanding in the effect of looping methods may provide improved ability to reduce PJK caused by ligamentous laxity.

Effects of cement augmentation on the mechanical stability of multilevel spine after vertebral compression fracture.

Studies on the effects of cement augmentation or vertebroplasty on multi-level spine after vertebral compression fractures are lacking. This paper seeks to establish a 3-vertebrae ovine model to determine the impact of compression fracture on spine biomechanics, and to discover if cement augmentation can restore mechanical stability to fractured spine. Five lumbar spine segments (L1-L3) were obtained from 5-year-old female Merino sheep. Standardized wedge-compression fractures were generated in each L2 vertebra, and then augmented with polymethyl methacrylate (PMMA) cement mixed with 30% barium sulphate powder. Biomechanical pure moment testing in axial rotation (AR), flexion/extension (FE) and lateral bending (LB) was carried out in the intact, fractured and repaired states. Range of motion (ROM) and neutral zone (NZ) parameters were compared, and plain radiographs taken at every stage. Except for a significant increase in ROM between the intact and fractured states in AR between L1 and L2 (P<0.05), there were no other significant differences in ROM or NZ between the other groups. There was a trend towards an increase in ROM and NZ in all directions after fracture, but this did not reach significance. Normal biomechanics was only minimally restored after augmentation. Results suggest that cement augmentation could not restore mechanical stability of fractured spine. Model-specific factors may have had a role in these findings. Caution should be exercised when applying these results to humans.

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.

Global and segmental kinematic changes following sequential resection of posterior osteoligamentous structures in the lumbar spine: An in vitro biomechanical investigation using pure moment testing protocols.

Lumbar spinal surgeries may compromise the integrity of posterior osteoligamentous structures implicating mechanical stability. Circumstances necessitating a concomitant surgery to achieve restabilisation are not well understood. The main objective of this in vitro study was to quantify global and segmental (index and adjacent levels) kinematic changes in the lumbar spine following sequential resection of the posterior osteoligamentous structures using pure moment testing protocols. Six fresh frozen cadaveric kangaroo lumbar spines (T12-S1) were tested under a bending moment in flexion-extension, bilateral bending, and axial torsion in a 6-degree-of-freedom Kinematic Spine Simulator. Specimens were tested in the following order: intact state (D0), after interspinous and supraspinous ligaments transection between L4 and L5 (D1), further after a total bilateral facetectomy between L4 and L5 (D2). Segmental motions at the cephalad, damaged, and caudal levels were recorded using an infrared-based motion tracking device. Following D1, no significant change in the global range of motion was observed in any of the bending planes. Following D2, a significant increase in the global range of motion from the baseline (D0) was observed in axial torsion (median normalised change +20%). At the damaged level, D2 resulted in a significant increase in the segmental range of motion in flexion-extension (+77%) and axial torsion (+492%). Additionally, a significant decrease in the segmental range of motion in axial torsion (-35%) was observed at the caudal level following D2. These results suggest that a multi-segment lumbar spine acts as a mechanism for transmitting motions, and that a compromised joint may significantly alter motion transfer to adjacent segments. We conclude that the interspinous and supraspinous ligaments play a modest role in restricting global spinal motions within physiologic limits. Following interspinous and supraspinous ligaments transection, a total bilateral facetectomy resulted in a significant increase in axial torsion motion, both at global and damaged levels, accompanied with a compensatory decrease in motion at the caudal level.

Biomechanical evaluation of a biomimetic spinal construct

BackgroundLaboratory spinal biomechanical tests using human cadaveric or animal spines have limitations in terms of disease transmission, high sample variability, decay and fatigue during extended testing protocols. Therefore, a synthetic biomimetic spine model may be an acceptable substitute. The goal of current study is to evaluate the properties of a synthetic biomimetic spine model; also to assess the mechanical performance of lateral plating following lateral interbody fusion.MethodsThree L3/4 synthetic spinal motion segments were examined using a validated pure moment testing system. Moments (±7.5 Nm) were applied in flexion-extension (FE), lateral bending (LB) and axial rotation (AR) at 1Hz for total 10000 cycles in MTS Bionix. An additional test was performed 12 hours after 10000 cycles. A ±10 Nm cycle was also performed to allow provide comparison to the literature. For implantation evaluation, each model was tested in the 4 following conditions: 1) intact, 2) lateral cage alone, 3) lateral cage and plate 4) anterior cage and plate. Results were analysed using ANOVA with post-hoc Tukey’s HSD test.ResultsRange of motion (ROM) exhibited logarithmic growth with cycle number (increases of 16%, 37.5% and 24.3% in AR, FE and LB respectively). No signification difference (p > 0.1) was detected between 4 cycles, 10000 cycles and 12 hour rest stages. All measured parameters were comparable to that of reported cadaveric values. The ROM for a lateral cage and plate construct was not significantly different to the anterior lumbar interbody construct for FE (p = 1.00), LB (p = 0.995) and AR (p = 0.837).ConclusionsBased on anatomical and biomechanical similarities, the synthetic spine tested here provides a reasonable model to represent the human lumbar spine. Repeated testing did not dramatically alter biomechanics which may allow non-destructive testing between many different procedures and devices without the worry of carry over effects. Small intra-specimen variability and lack of biohazard makes this an attractive alternative for in vitro spine biomechanical testing. It also proved an acceptable surrogate for biomechanical testing, confirming that a lateral lumbar interbody cage and plate construct reduces ROM to a similar degree as anterior lumbar interbody cage and plate constructs.

Examination of cervical spine kinematics in complex, multiplanar motions after anterior cervical discectomy and fusion and total disc replacement

BackgroundThe biomechanical behavior of total disc replacement (TDR) and anterior cervical discectomy and fusion (ACDF) in complex multiplanar motion is incompletely understood. The purpose of this study was to determine whether ACDF or TDR significantly affects in vitro kinematics through a range of complex, multiplanar motions. MethodsSeven human cervical spines from C4-7 were used for this study. Intact cervical motion segments with and without implanted TDR and ACDF were tested by use of unconstrained pure bending moment testing fixtures in 7 mechanical modes: axial rotation (AR); flexion/extension (FE); lateral bending (LB); combined FE and LB; combined FE and AR; combined LB and AR; and combined FE, LB, and AR. Statistical testing was performed to determine whether differences existed in range of motion (ROM) and stiffness among spinal segments and treatment groups for each mechanical test mode. ResultsACDF specimens showed increased stiffness compared with the intact and TDR specimens (P < .001); stiffness was not found to be different between TDR and intact specimens. ACDF specimens showed decreased ROM in all directions compared with TDR and intact specimens at the treated level. For the coupled motion test, including AR, LB, and FE, the cranial adjacent level (C4/C5) for the intact specimens (2.7°) showed significantly less motion compared with both the TDR (6.1°, P = .009) and ACDF (6.8°, P = .002) treatment groups about the LB axis. Testing of the C4/C5 and C6/C7 levels in all other test modes yielded no significant differences in ROM comparisons, although a trend toward increasing ROM in adjacent levels in ACDF specimens compared with intact and TDR specimens was observed. ConclusionsThis study compared multiplanar motion under load-displacement testing of subaxial cervical motion segments with and without implanted TDR and ACDF. We found a trend toward increased motion in adjacent levels in ACDF specimens compared with TDR specimens. Biomechanical multiplanar motion testing will be useful in the ongoing development and evaluation of spinal motion–preserving implants.

Verification of pure moment testing in a multi–degree of freedom spine testing apparatus

BackgroundPure moment testing is a common method used in cadaveric spine testing. The fundamental basis for the widespread acceptance of applying a pure moment is uniform loading along the column of the spine. To our knowledge, this protocol has not been experimentally verified on a multi–degree of freedom testing apparatus. Given its ubiquitous use in spine biomechanics laboratories, confirmation of this comparative cadaveric test protocol is paramount. MethodsGroup A specimens (n = 13) were used to test the pure moment protocol, by use of 3 constructs that changed the number of involved vertebrae, orientation, and rigidity of the spine construct. Group B specimens (n = 6) were used to determine whether potting orientation, testing order, or degradation affected the range of motion (ROM) by use of 8 constructs. Each group was subjected to 3 cycles of flexion-extension, lateral bending, and axial torsion. The data from the third cycle were used to calculate the ROM for each method. ResultsGroup A testing resulted in significant differences in ROM across the 3 constructs for lateral bending and axial torsion (P < .02) and trended toward a difference for flexion-extension (P = .055). Group B testing showed an increase in ROM across 8 constructs (P < .04) but no significant difference due to the orientation change. ConclusionThe increased ROM across constructs observed in both groups indicates that the cause is likely the testing order or degradation of the specimens, with orientation having no observed effect. The data do not invalidate pure moment testing, and its use should persist.

Pure moment testing for spinal biomechanics applications: Fixed versus 3D floating ring cable-driven test designs

Pure moment testing has become a standard protocol for in vitro assessment of the effect of surgical techniques or devices on the bending rigidity of the spine. Of the methods used for pure moment testing, cable-driven set-ups are popular due to their low requirements and simple design. Fixed loading rings are traditionally used in conjunction with these cable-driven systems. However, the accuracy and validity of the loading conditions applied with fixed ring designs have raised some concern, and discrepancies have been found between intended and prescribed loading conditions for flexion–extension. This study extends this prior work to include lateral bending and axial torsion, and compares this fixed ring design with a novel “3D floating ring” design. A complete battery of multi-axial bending tests was conducted with both rings in multiple different configurations using an artificial lumbar spine. Applied moments were monitored and recorded by a multi-axial load cell at the base of the specimen. Results indicate that the fixed ring design deviates as much as 77% from intended moments and induces non-trivial shear forces (up to 18N) when loaded to a non-destructive maximum of 4.5Nm. The novel 3D floating ring design largely corrects the inherent errors in the fixed ring design by allowing additional directions of unconstrained motion and producing uniform loading conditions along the length of the specimen. In light of the results, it is suggested that the 3D floating ring set-up be used for future pure moment spine biomechanics applications using a cable-driven apparatus.

Verification of Pure Moment Testing in a Multi-Degree of Freedom Spine Testing Apparatus

Background Pure moment testing is a common method used in cadaveric spine testing. The fundamental basis for the widespread acceptance of applying a pure moment is uniform loading along the column of the spine. To the author’s knowledge, this protocol has not been experimentally verified on a multi-degree-of-freedom testing apparatus. Due to its ubiquitous use in spine biomechanics laboratories, confirmation of this comparative cadaveric test protocol is paramount. Methods Group A specimens (n = 13) were used to test the pure moment protocol, using 3 constructs that changed the number of involved vertebrae, orientation and rigidity of the spine construct. Group B specimens (n = 6) were used to determine if potting orientation, testing order or degradation affected the range of motion (ROM) using 8 constructs. Each group was subjected to 3 cycles of flexion-extension, lateral bending and axial torsion. The data from the third cycle was used to calculate the ROM for each method. Results Group A testing resulted in significant differences in ROM across the 3 constructs for lateral bending and axial torsion ( P < 0.02), and trended towards a difference for flexion-extension ( P = 0.055). Group B testing showed an increase in ROM across 8 constructs ( P < 0.04), but no significant difference due to the orientation change. Conclusion The increased ROM across constructs observed in both groups indicates that the cause is likely due to the testing order or degradation of the specimens, with orientation having no observed effect. The data does not invalidate pure moment testing and its use should persist.

Biomechanical Cyclical Loading on Cadaveric Cervical Spines in a Corpectomy Model

Abstract Approved posterior cervical spinal fixation systems have been submitted to the U.S. Food and Drug Administration with results from standardized test protocols from the ASTM. The bench top mechanical studies are designed to minimize biologic and laboratory variability. However, for implant tests such as ASTM F1717 to be more clinically relevant, anatomical and physiologic considerations must be understood. The specific aim of this study was to determine if a human cadaveric cervical spine with a corpectomy and posterior fixation was effective in maintaining stability prior to and following cyclical loading. Six fresh frozen cadaveric human cervical specimens were harvested and prepared. A C5 corpectomy was performed. Posterior cervical instrumentation was implanted from C3 and C7 spanning across the C4–C6 defect. Each specimen followed an established pure moment test protocol to characterize the instrumented spine in flexion extension, lateral bending, and axial torsion at ±2.5 N m and axial compressive loading to 150 N. Subsequently, each specimen was subjected to 10 000 flexion extension cycles. Following the cyclical loading, each specimen was characterized a second time via the same test protocol. Statistical analyses were then performed on the third cycle data between the two pure moment tests. The mean FE bending range of motion (ROM) was 18.0° ± 10.7° prior to the 10 000 cyclical bending protocol. Following cyclical loading, the mean ROM measured 22.0° ± 19.9°. In axial compression, the mean ROM was 4.1 mm ± 1.9 mm prior to cycling and 4.2 mm ± 1.4 mm post-cycling. A statistically significant difference was detected only in the axial torsion mode of loading (p = 0.030). Although the ASTM standard provides consistent test methodologies, biomechanical cadaveric testing remains an important step in the validation of all spinal instrumentation. The pure moment biomechanical cadaveric test protocol for the same construct was capable of detecting significant changes pre- and post-flexion extension fatigue cycling only in the axial torsion mode of loading.

Pure moment testing for spinal biomechanics applications: Fixed versus sliding ring cable-driven test designs

In vitro multi-axial bending testing using pure moment loading conditions has become the standard in evaluating the effects of different types of surgical intervention on spinal kinematics. Simple, cable-driven experimental set-ups have been widely adopted because they require little infrastructure. Traditionally, “fixed ring” cable-driven experimental designs have been used; however, there have been concerns with the validity of this set-up in applying pure moment loading. This study involved directly comparing the loading state induced by a traditional “fixed ring” apparatus versus a novel “sliding ring” approach. Flexion-extension bending was performed on an artificial spine model and a single cadaveric test specimen, and the applied loading conditions to the specimen were measured with an in-line multiaxial load cell. The results showed that the fixed ring system applies flexion-extension moments that are 50–60% less than the intended values. This design also imposes non-trivial anterior–posterior shear forces, and non-uniform loading conditions were induced along the length of the specimen. The results of this study indicate that fixed ring systems have the potential to deviate from a pure moment loading state and that our novel sliding ring modification corrects this error in the original test design. This suggests that the proposed sliding ring design should be used for future in vitro spine biomechanics studies involving a cable-driven pure moment apparatus.

Modeling of springback, strain rate and Bauschinger effects for two-dimensional steady state cyclic flow of sheet metal subjected to bending under tension

An elastic–plastic mathematical model is presented for plane strain flow of sheet metal subjected to strain rate effects during cyclic bending under tension. The model calculates the stress, strain, strain rate, flow profile geometry, springback and residual stresses for steady state flow of sheet metal under plane strain along the width. Stress reversals were experimentally quantified using a pure bending moment test and were included in the model through Bauschinger factors. Modeling results for two materials, mild steel and aluminum alloy, were in good agreement with experimental results from bending under tension test devices. The iterative nature of the model, associated with a representative experimental framework proved a valuable approach to improving the modeling of sheet metal forming and springback control.

Flexion and extension structural properties and strengths for male cervical spine segments

New vehicle safety standards are designed to limit the amount of neck tension and extension seen by out-of-position motor vehicle occupants during airbag deployments. The criteria used to assess airbag injury risk are currently based on volunteer data and animal studies due to a lack of bending tolerance data for the adult cervical spine. This study provides quantitative data on the flexion–extension bending properties and strength on the male cervical spine, and tests the hypothesis that the male is stronger than the female in pure bending. An additional objective is to determine if there are significant differences in stiffness and strength between the male upper and lower cervical spine. Pure-moment flexibility and failure testing was conducted on 41 male spinal segments (O–C2, C4–C5, C6–C7) in a pure-moment test frame and the results were compared with a previous study of females. Failures were conducted at approximately 90 N-m/s. In extension, the male upper cervical spine (O–C2) fails at a moment of 49.5 (s.d. 17.6) N-m and at an angle of 42.4° (s.d. 8.0°). In flexion, the mean moment at failure is 39.0 (s.d. 6.3°) N-m and an angle of 58.7° (s.d. 5.1°). The difference in strength between flexion and extension is not statistically significant. The difference in the angles is statistically significant. The upper cervical spine was significantly stronger than the lower cervical spine in both flexion and extension. The male upper cervical spine was significantly stiffer than the female and significantly stronger than the female in flexion. Odontoid fractures were the most common injury produced in extension, suggesting a tensile mechanism due to tensile loads in the odontoid ligamentous complex.

An improved biomechanical testing protocol for evaluating spinal arthroplasty and motion preservation devices in a multilevel human cadaveric cervical model

An experimental study was performed to determine the biomechanical end-mounting configurations that replicate in vivo physiological motion of the cervical spine in a multiple-level human cadaveric model. The vertebral motion response for the modified testing protocol was compared to in vivo motion data and traditional pure-moment testing methods. Biomechanical tests were performed on fresh human cadaveric cervical spines (C2-T1) mounted in a programmable testing apparatus. Three different end-mounting conditions were studied: pinned-pinned, pinned-fixed, and translational/pinned-fixed. The motion response of the individual segmental vertebral rotations was statistically compared using one-way analysis of variance and Student-Newman-Keuls tests (p < 0.05 unless otherwise stated) to determine differences in the motion responses for different testing methods. A translational/pinned-fixed mounting configuration induced a bending-moment distribution across the cervical spine, resulting in a motion response that closely matched the in vivo case. In contrast, application of pure-moment loading did not reproduce the physiological response and is less suitable for studying disc arthroplasty and nonfusion devices.

Comparative strengths and structural properties of the upper and lower cervical spine in flexion and extension

The purpose of this study is to test the hypothesis that the upper cervical spine is weaker than the lower cervical spine in pure flexion and extension bending, which may explain the propensity for upper cervical spine injuries in airbag deployments. An additional objective is to evaluate the relative strength and flexibility of the upper and lower cervical spine in an effort to better understand injury mechanisms, and to provide quantitative data on bending responses and failure modes. Pure moment flexibility and failure testing was conducted on 52 female spinal segments in a pure-moment test frame. The average moment at failure for the O-C2 segments was 23.7±3.4 N m for flexion and 43.3±9.3 N m for extension. The ligamentous upper cervical spine was significantly stronger in extension than in flexion ( p=0.001). The upper cervical spine was significantly stronger than the lower cervical spine in extension. The relatively high strength of the upper cervical spine in tension and in extension is paradoxical given the large number of upper cervical spine injuries in out-of-position airbag deployments. This discrepancy is most likely due to load sharing by the active musculature.

The cervical facet capsule and its role in whiplash injury: a biomechanical investigation.

Cervical facet capsular strains were determined during bending and at failure in the human cadaver. To determine the effect of an axial pretorque on facet capsular strains and estimate the risk for subcatastrophic capsular injury during normal bending motions. Epidemiologic and clinical studies have identified the facet capsule as a potential site of injury and prerotation as a risk factor for whiplash injury. Unfortunately, biomechanical data on the cervical facet capsule and its role in whiplash injury are not available. Cervical spine motion segments were tested in a pure-moment test frame and the full-field strains determined throughout the facet capsule. Motion segments were tested with and without a pretorque in pure bending. The isolated facet was then elongated to failure. Maximum principal strains during bending were compared with failure strains, by paired t test. Statistically significant increases in principal capsular strains during flexion-extension loading were observed when a pretorque was applied. All measured strains during bending were significantly less than strains at catastrophic joint failure. The same was true for subcatastrophic ligament failure strains, except in the presence of a pretorque. Pretorque of the head and neck increases facet capsular strains, supporting its role in the whiplash mechanism. Although the facet capsule does not appear to be at risk for gross injury during normal bending motions, a small portion of the population may be at risk for subcatastrophic injury.