Cervical vertebral strain measurements under axial and eccentric loading

New anterior controllable antedisplacement and fusion surgery for cervical ossification of the posterior longitudinal ligament: a biomechanical study

OBJECTIVE

The traditional anterior approach for multilevel severe cervical ossification of the posterior longitudinal ligament (OPLL) is demanding and risky. Recently, a novel surgical procedure-anterior controllable antedisplacement and fusion (ACAF)-was introduced by the authors to deal with these problems and achieve better clinical outcomes. However, to the authors' knowledge, the immediate and long-term biomechanical stability obtained after this procedure has never been evaluated. Therefore, the authors compared the postoperative biomechanical stability of ACAF with those of more traditional approaches: anterior cervical discectomy and fusion (ACDF) and anterior cervical corpectomy and fusion (ACCF).

METHODS

To determine and assess pre- and postsurgical range of motion (ROM) (2 Nm torque) in flexion-extension, lateral bending, and axial rotation in the cervical spine, the authors collected cervical areas (C1-T1) from 18 cadaveric spines. The cyclic fatigue loading test was set up with a 3-Nm cycled load (2 Hz, 3000 cycles). All samples used in this study were randomly divided into three groups according to surgical procedures: ACDF, ACAF, and ACCF. The spines were tested under the following conditions: 1) intact state flexibility test; 2) postoperative model (ACDF, ACAF, ACCF) flexibility test; 3) cyclic loading (n = 3000); and 4) fatigue model flexibility test.

RESULTS

After operations were performed on the cadaveric spines, the segmental and total postoperative ROM values in all directions showed significant reductions for all groups. Then, the ROMs tended to increase during the fatigue test. No significant crossover effect was detected between evaluation time and operation method. Therefore, segmental and total ROM change trends were parallel among the three groups. However, the postoperative and fatigue ROMs in the ACCF group tended to be larger in all directions. No significant differences between these ROMs were detected in the ACDF and ACAF groups.

CONCLUSIONS

This in vitro biomechanical study demonstrated that the biomechanical stability levels for ACAF and ACDF were similar and were both significantly greater than that of ACCF. The clinical superiority of ACAF combined with our current results showed that this procedure is likely to be an acceptable alternative method for multilevel cervical OPLL treatment.

Load-Sharing and Kinematics of the Human Cervical Spine Under Multi-Axial Transverse Shear Loading: Combined Experimental and Computational Investigation

The cervical spine experiences shear forces during everyday activities and injurious events yet there is a paucity of biomechanical data characterizing the cervical spine under shear loading. This study aimed to (1) characterize load transmission paths and kinematics of the subaxial cervical spine under shear loading, and (2) assess a contemporary finite element cervical spine model using this data. Subaxial functional spinal units (FSUs) were subjected to anterior, posterior, and lateral shear forces (200 N) applied with and without superimposed axial compression preload (200 N) while monitoring spine kinematics. Load transmission paths were identified using strain gauges on the anterior vertebral body and lateral masses and a disc pressure sensor. Experimental conditions were simulated with cervical spine finite element model FSUs (GHBMC M50 version 5.0). The mean kinematics, vertebral strains, and disc pressures were compared to experimental results. The shear force-displacement response typically demonstrated a toe region followed by a linear response, with higher stiffness in anterior shear relative to lateral and posterior shear. Compressive axial preload decreased posterior and lateral shear stiffness and increased initial anterior shear stiffness. Load transmission patterns and kinematics suggest the facet joints play a key role in limiting anterior shear while the disc governs motion in posterior shear. The main cervical spine shear responses and trends are faithfully predicted by the GHBMC cervical spine model. These basic cervical spine biomechanics and the computational model can provide insight into mechanisms for facet dislocation in high severity impacts, and tissue distraction in low severity impacts.

Experimental mechanical strain measurement of tissues

Strain, an important biomechanical factor, occurs at different scales from molecules and cells to tissues and organs in physiological conditions. Under mechanical strain, the strength of tissues and their micro- and nanocomponents, the structure, proliferation, differentiation and apoptosis of cells and even the cytokines expressed by cells probably shift. Thus, the measurement of mechanical strain (i.e., relative displacement or deformation) is critical to understand functional changes in tissues, and to elucidate basic relationships between mechanical loading and tissue response. In the last decades, a great number of methods have been developed and applied to measure the deformations and mechanical strains in tissues comprising bone, tendon, ligament, muscle and brain as well as blood vessels. In this article, we have reviewed the mechanical strain measurement from six aspects: electro-based, light-based, ultrasound-based, magnetic resonance-based and computed tomography-based techniques, and the texture correlation-based image processing method. The review may help solving the problems of experimental and mechanical strain measurement of tissues under different measurement environments.

A laboratory study on effects of cycling helmet fit on biomechanical measures associated with head and neck injury and dynamic helmet retention

There is a scant biomechanical literature that tests, in a laboratory setting, whether or not determinants of helmet fit affect biomechanical parameters associated with injury. Using conventional cycling helmets and repeatable models of the human head and neck, integrated into a guided drop impact experiment at speeds up to 6m/s, this study tests the hypothesis that fit affects head kinematics, neck kinetics, and the extent to which the helmet moves relative to the underlying head (an indicator of helmet positional stability). While there were a small subset of cases where head kinematics were statistically significantly altered by fit, when viewed as a whole our measures of head kinematics suggest that fit does not systematically alter kinematics of the head secondary to impact. Similarly, when viewed as a whole our data suggests that fit does not systematically alter resultant neck compression and resultant moment and associated biomechanical measures. Our data suggests that backward fit helmets exhibit the worst dynamic stability, in particular when the torso is impacted before the helmeted head is impacted, suggesting that the typical certification method of dynamical loading of a helmet to quantify retention may not be representative of highly plausible cycling incident scenarios where impact forces are first applied to the torso leading to loading of the neck prior to the head. Further study is warranted so that factors of fit that affect injury outcome are uncovered in both laboratory and real world settings.

Comparison of Strain Rosettes and Digital Image Correlation for Measuring Vertebral Body Strain

Strain gages are commonly used to measure bone strain, but only provide strain at a single location. Digital image correlation (DIC) is an optical technique that provides the displacement, and therefore strain, over an entire region of interest on the bone surface. This study compares vertebral body strains measured using strain gages and DIC. The anterior surfaces of 15 cadaveric porcine vertebrae were prepared with a strain rosette and a speckled paint pattern for DIC. The vertebrae were loaded in compression with a materials testing machine, and two high-resolution cameras were used to image the anterior surface of the bones. The mean noise levels for the strain rosette and DIC were 1 με and 24 με, respectively. Bland–Altman analysis was used to compare strain from the DIC and rosette (excluding 44% of trials with some evidence of strain rosette failure or debonding); the mean difference ± 2 standard deviations (SDs) was −108 με ± 702 με for the minimum (compressive) principal strain and −53 με ± 332 με for the maximum (tensile) principal strain. Although the DIC has higher noise, it avoids the relatively high risk we observed of strain gage debonding. These results can be used to develop guidelines for selecting a method to measure strain on bone.

Biomechanical modelling of impact-related fracture characteristics and injury patterns of the cervical spine associated with riding accidents

BACKGROUND

Horse-related injuries are manifold and can involve the upper and lower limbs, the trunk, spine or head. Cervical spine injuries are not among the most common injuries. However, they can be fatal and often result in neurological symptoms. This study investigated the influence of the posture of the cervical spine on the ultimate strength and the pattern of vertebrae failure with the aim to provide some guidance for protective clothing design.

METHODS

Eighteen human cervical spines, each divided into two specimens (three vertebrae each), were subjected to a simulator test designed to mimic a spinal trauma in different postures of the specimen (neutral, flexion, extension). The stress-to-failure, the deformation at the time of fracture and the fracture patterns assessed based on CT scans were analysed.

FINDINGS

Stress-to-failure of the superior specimens was lower for the flexion group compared to the others (P=0.027). The superior specimens demonstrated higher stress-to-failure in comparison to the inferior specimens (P<0.001). Compression in a neutral or flexed position generated mild or moderate fracture patterns. On the contrary, the placement of the spine in extension resulted in severe fractures mostly associated with narrowing of the spinal canal.

INTERPRETATION

The results imply that a neutral cervical spine position during an impaction can be beneficial. In this position, the failure loads are high, and even if a vertebral fracture occurs, the generated injury patterns are expected to be mild or moderate.

Cervical spine injuries and flexibilities following axial impact with lateral eccentricity

PURPOSE

Determine the effects of dynamic injurious axial compression applied at various lateral eccentricities (lateral distance to the centre of the spine) on mechanical flexibilities and structural injury patterns of the cervical spine.

METHODS

13 three-vertebra human cadaver cervical spine specimens (6 C3-5, 3 C4-6, 2 C5-7, 2 C6-T1) were subjected to pure moment flexibility tests (±1.5 Nm) before and after impact trauma was applied in two groups: low and high lateral eccentricity (1 and 150 % of the lateral diameter of the vertebral body, respectively). Relative range of motion (ROM) and relative neutral zone (NZ) were calculated as the ratio of post and pre-trauma values. Injuries were diagnosed by a spine surgeon and scored. Classification functions were developed using discriminant analysis.

RESULTS

Low and high eccentric loading resulted in primarily bony fractures and soft tissue injuries, respectively. Axial impacts with high lateral eccentricities resulted in greater spinal motion in lateral bending [median relative ROM 3.5 (interquartile range, IQR 2.3) vs. 1.4 (IQR 0.5) and median relative NZ 4.7 (IQR 3.7) vs. 2.3 (IQR 1.1)] and in axial rotation [median relative ROM 5.3 (IQR 13.7) vs. 1.3 (IQR 0.5), p < 0.05 for all comparisons] than those that resulted from low eccentricity impacts. The developed classification functions had 92 % classification accuracy.

CONCLUSIONS

Dynamic axial compression loading of the cervical spine with high lateral eccentricities produced primarily soft tissue injuries resulting in more post-injury spinal flexibility in lateral bending and axial rotation than that associated with the bony fractures resulting from low eccentricity impacts.

The effect of lateral eccentricity on failure loads, kinematics, and canal occlusions of the cervical spine in axial loading

Current neck injury criteria do not include limits for lateral bending combined with axial compression and this has been observed as a clinically relevant mechanism, particularly for rollover motor vehicle crashes. The primary objectives of this study were to evaluate the effects of lateral eccentricity (the perpendicular distance from the axial force to the centre of the spine) on peak loads, kinematics, and spinal canal occlusions of subaxial cervical spine specimens tested in dynamic axial compression (0.5 m/s). Twelve 3-vertebra human cadaver cervical spine specimens were tested in two groups: low and high eccentricity with initial eccentricities of 1 and 150% of the lateral diameter of the vertebral body. Six-axis loads inferior to the specimen, kinematics of the superior-most vertebra, and spinal canal occlusions were measured. High speed video was collected and acoustic emission (AE) sensors were used to define the time of injury. The effects of eccentricity on peak loads, kinematics, and canal occlusions were evaluated using unpaired Student t-tests. The high eccentricity group had lower peak axial forces (1544 ± 629 vs. 4296 ± 1693 N), inferior displacements (0.2 ± 1.0 vs. 6.6 ± 2.0 mm), and canal occlusions (27 ± 5 vs. 53 ± 15%) and higher peak ipsilateral bending moments (53 ± 17 vs. 3 ± 18 Nm), ipsilateral bending rotations (22 ± 3 vs. 1 ± 2°), and ipsilateral displacements (4.5 ± 1.4 vs. -1.0 ± 1.3 mm, p<0.05 for all comparisons). These results provide new insights to develop prevention, recognition, and treatment strategies for compressive cervical spine injuries with lateral eccentricities.

Strain distribution in the lumbar vertebrae under different loading configurations

BACKGROUND CONTEXT

The stress/strain distribution in the human vertebrae has seldom been measured, and only for a limited number of loading scenarios, at few locations on the bone surface.

PURPOSE

This in vitro study aimed at measuring how strain varies on the surface of the lumbar vertebral body and how such strain pattern depends on the loading conditions.

METHODS

Eight cadaveric specimens were instrumented with eight triaxial strain gauges each to measure the magnitude and direction of principal strains in the vertebral body. Each vertebra was tested in a three adjacent vertebrae segment fashion. The loading configurations included a compressive force aligned with the vertebral body but also tilted (15°) in each direction in the frontal and sagittal planes, a traction force, and torsion (both directions). Each loading configuration was tested six times on each specimen.

RESULTS

The strain magnitude varied significantly between strain measurement locations. The strain distribution varied significantly when different loading conditions were applied (compression vs. torsion vs. traction). The strain distribution when the compressive force was tilted by 15° was also significantly different from the axial compression. Strains were minimal when the compressive force was applied coaxial with the vertebral body, compared with all other loading configurations. Also, strain was significantly more uniform for the axial compression, compared with all other loading configurations. Principal strains were aligned within 19° to the axis of the vertebral body for axial-compression and axial-traction. Conversely, when the applied force was tilted by 15°, the direction of principal strain varied by a much larger angle (15° to 28°).

CONCLUSIONS

This is the first time, to our knowledge, that the strain distribution in the vertebral body is measured for such a variety of loading configurations and a large number of strain sensors. The present findings suggest that the structure of the vertebral body is optimized to sustain compressive forces, whereas even a small tilt angle makes the vertebral structure work under suboptimal conditions.

Strain changes on the cortical shell of vertebral bodies due to spine ageing: A parametric study using a finite element model evaluated by strain measurements

The probability of fractures of the cortical shell of vertebral bodies increases as ageing progresses. Ageing involves all the spinal component changes. However, the effect of the spinal component ageing on the fracture risk of the cortical shell remains poorly understood. In this study, the influence of the ageing of the spinal components on cortical shell strain was investigated. A lumbar spinal specimen (L3–L5) was mechanically tested under a quasi-static axial compressive load. Clinical computed tomography images of the same specimen were used to create a corresponding finite element model. The material properties were determined by calibrating the finite element model using the L4 cortical shell strains of the anterior centre measurement site. The remaining experiment data (axial displacement, the intra-discal pressures, L4 cortical shell strain on the lateral measurement site) were used to evaluate the model. The individual ageing process of the six spinal components (cortical shell, cancellous bone, bony endplate, posterior elements, nucleus pulposus and annulus matrix) was simulated by changing their Young’s moduli and Poisson’s ratios, and the effect on cortical shell strain was investigated. Results show that the cortical shell strain is more sensitive to the ageing of the cortical shell and the cancellous bone than to the ageing of the nucleus pulposus, the annulus matrix, and the bony endplates and of the posterior elements. The results can help the clinicians focus on the aspects that mainly influence the vertebral cortex fracture risk factor.

An interactive multiblock approach to meshing the spine

Finite element (FE) analysis is a useful tool to study spine biomechanics as a complement to laboratory-driven experimental studies. Although individualized models have the potential to yield clinically relevant results, the demands associated with modeling the geometric complexity of the spine often limit its utility. Existing spine FE models share similar characteristics and are often based on similar assumptions, but vary in geometric fidelity due to the mesh generation techniques that were used. Using existing multiblock techniques, we propose mesh generation methods that ease the effort and reduce the time required to create subject-specific allhexahedral finite element models of the spine. We have demonstrated the meshing techniques by creating a C4-C5 functional spinal unit and validated it by comparing the resultant motions and vertebral strains with data reported in the literature.

Neural space and biomechanical integrity of the developing cervical spine in compression

STUDY DESIGN

A factorial study design was used to examine the biomechanical and neuroprotective integrity of the cervical spine throughout maturation using a postmortem baboon model.

OBJECTIVE

To investigate changes with spinal development that affect the neuroprotective ability of the cervical spine in compressive loading.

SUMMARY OF BACKGROUND DATA

Child spinal cord injuries claim and debilitate thousands of children in the United States each year. Many of these injuries are diagnostically and mechanistically difficult to classify, treat, and prevent. Biomechanical studies on maturing spinal tissues have identified decreased stiffness and tolerance characteristics for children compared with adults. Unfortunately, while neurologic deficit typically dictates functional outcome, no previous studies have examined the neuroprotective role of the pediatric cervical spine.

METHODS

Twenty-two postmortem baboon cervical spines across the developmental age spectrum were tested. Two functional spinal unit segments (Oc-C2, C3-C5, and C6-T1) were instrumented with transducers to measure dynamic changes in the spinal canal. These tissues were compressed to 70% strain dynamically, and the resultant mechanics and spinal canal occlusions were recorded.

RESULTS

Classic injury patterns were observed in all of the specimens tested. The compressive mechanics exhibited a significant age relationship (P < 0.0001). Furthermore, while the peak-percent spinal canal occlusion was not age dependent, the percent occlusion just before failure did demonstrate a significant decrease with advancing age (P = 0.0001).

CONCLUSIONS

The neuroprotective ability of the cervical spine preceding failure appears to be age dependent, where the young spine can produce greater spinal canal occlusions without failure than its adult counterpart. The overall percent of the spinal canal occluded during a compression injury was not age dependent; however, these data reveal the neuroprotective ability of the child spine to be more sensitive as an injury predictor than the biomechanical fracture data.

Development of numerical models for injury biomechanics research: a review of 50 years of publications in the Stapp Car Crash Conference

Numerical analyses frequently accompany experimental investigations that study injury biomechanics and improvements in automotive safety. Limited by computational speed, earlier mathematical models tended to simplify the system under study so that a set of differential equations could be written and solved. Advances in computing technology and analysis software have enabled the development of many sophisticated models that have the potential to provide a more comprehensive understanding of human impact response, injury mechanisms, and tolerance. In this article, 50 years of publications on numerical modeling published in the Stapp Car Crash Conference Proceedings and Journal were reviewed. These models were based on: (a) author-developed equations and software, (b) public and commercially available programs to solve rigid body dynamic models (such as MVMA2D, CAL3D or ATB, and MADYMO), and (c) finite element models. A clear trend that can be observed is the increasing use of the finite element method for model development. A review of these modeling papers clearly indicates the progression of the state-of-the-art in computational methods and technologies in injury biomechanics.

Validation of a clinical finite element model of the human lumbosacral spine

Very few finite element models on the lumbosacral spine have been reported because of its unique biomechanical characteristics. In addition, most of these lumbosacral spine models have been only validated with rotation at single moment values, ignoring the inherent nonlinear nature of the moment-rotation response of the spine. Because a majority of lumbar spine surgeries are performed between L4 and S1 levels, and the confidence in the stress analysis output depends on the model validation, the objective of the present study was to develop a unique finite element model of the lumbosacral junction. The clinically applicable model was validated throughout the entire nonlinear range. It was developed using computed tomography scans, subjected to flexion and extension, and left and right lateral bending loads, and quantitatively validated with cumulative variance analyses. Validation results for each loading mode and for each motion segment (L4-L5, L5-S1) and bisegment (L4-S1) are presented in the paper.

Effect of Loading Rate on the Compressive Mechanics of the Immature Baboon Cervical Spine

Thirty-four cervical spine segments were harvested from 12 juvenile male baboons and compressed to failure at displacement rates of 5, 50, 500, or 5000mm∕s. Compressive stiffness, failure load, and failure displacement were measured for comparison across loading rate groups. Stiffness showed a significant concomitant increase with loading rate, increasing by 62% between rates of 5 and 5000mm∕s. Failure load also demonstrated an increasing relationship with loading rate, while displacement at failure showed no rate dependence. These data may help in the development of improved pediatric automotive safety standards and more biofidelic physical and computational models.

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.

Compressive tolerance of the maturing cervical spine

While a number of experiments have reported injury thresholds for the adult cervical spine in compression, the compressive failure tolerance of the child spine has not been characterized. In order to develop useful safety measures for children, the biomechanical effects of maturation must be evaluated. Hence, this study examined the effects of spinal development on the compressive mechanics of the cervical spine. An animal model was used due to the lack of human tissues in the pediatric age range. Twenty-two fresh cadaveric baboon cervical spines (all male) were dissected into two functional spinal unit segments: Occiput-C2, C3-C5, C6-T1. The specimens ranged in age from 1 to 30-human equivalent years based upon radiographic assessment of their skeletal maturity. Dynamic (1.0-m/sec) haversine displacement inputs up to 70% strain were imparted on each specimen and the resulting loads were recorded. Significant increases in the compressive failure load were observed with increased maturation of the spinal tissues (ANOVA, p = 0.003). Differences were also observed between the spinal levels examined. The lower cervical spine, C6-T1, had the smallest failure load for specimens greater than 8-human-equivalent years, while the upper cervical spine was the most susceptible to injury at less than 8-human-equivalent years. The compressive failures generated are consistent with those observed clinically, consisting of primarily burst fractures and physis (growth plate) failures. These data clearly suggest that spinal compressive tolerance is directly related to maturation. Therefore, through scaling, these data may provide tolerance values applicable to anthropomorphic test dummies and computational models aimed at injury prevention for the pediatric occupant.

Biomechanical analyses of whiplash injuries using an experimental model

Neck pain and headaches are the two most common symptoms of whiplash. The working hypothesis is that pain originates from excessive motions in the upper and lower cervical segments. The research design used an intact human cadaver head-neck complex as an experimental model. The intact head-neck preparation was fixed at the thoracic end with the head unconstrained. Retroreflective targets were placed on the mastoid process, anterior regions of the vertebral bodies, and lateral masses at every spinal level. Whiplash loading was delivered using a mini-sled pendulum device. A six-axis load cell and an accelerometer were attached to the inferior fixation of the specimen. High-speed video cameras were used to obtain the kinematics. During the initial stages of loading, a transient decoupling of the head occurs with respect to the neck exhibiting a lag of the cranium. The upper cervical spine-head undergoes local flexion concomitant with a lag of the head while the lower column is in local extension. This establishes a reverse curvature to the head-neck complex. With continuing application of whiplash loading, the inertia of the head catches up with the neck. Later, the entire head-neck complex is under an extension mode with a single extension curvature. The lower cervical facet joint kinematics demonstrates varying local compression and sliding. While the anterior- and posterior-most regions of the facet joint slide, the posterior-most region of the joint compresses more than the anterior-most region. These varying kinematics at the two ends of the facet joint result in a pinching mechanism. Excessive flexion of the posterior upper cervical regions can be correlated to headaches. The pinching mechanism of the facet joints can be correlated to neck pain. The kinematics of the soft tissue-related structures explain the mechanism of these common whiplash associated disorders.

Evaluation of axial and flexural stresses in the vertebral body cortex and trabecular bone in lordosis and two sagittal cervical translation configurations with an elliptical shell model

BACKGROUND

Osteoarthritis and spinal degeneration are factors in neck and back pain. Calculations of stress in clinically occurring configurations of the sagittal cervical spine are rare.

OBJECTIVE

To calculate and compare combined axial and flexural stresses in lordosis versus cervical configurations in anterior and vertical sagittal head translated positions.

DESIGN

Digitized measurements from lateral cervical radiographs of 3 different shapes were used to calculate axial loads and bending moments on the vertebral bodies of C2-C7.

METHODS

An elliptical shell model was used to model horizontal cross-sections of the vertebral bodies of C2 through T1. Axial and flexural stresses were calculated with short compression block equations. Elliptical shell modeling permitted separation of stresses into cortical and inner medullary regions. Digitized radiographic points were used to create polynomials representing the shape of the sagittal cervical curvatures from C1 to T1. To calculate bending moments at each vertebral segment, moment arms from a vertical line through C1 were determined from digitizing.

RESULTS

Compared with the normal lordosis, stresses on the anterior vertebral body cortical margins of C5-T1 in the sagittal translated postures are compression rather than tension. At the posterior vertebral bodies in the anteriorly translated position and vertically translated postures, the stresses change from compression to tension at C5 through T1. In absolute value (ABS) compared with values at the same segments in a normal lordosis, the magnitude of the combined anterior stresses in the sagittal postures are higher at C5-C7 (eg, ABS[sigma(straight)/sigma(normal)] approximately 1.25 to 4.25).

CONCLUSIONS

Vertebral body stresses are reversed in direction at C5-T1 in sagittal translated postures compared to a normal lordosis. Stress analysis, with implications for bone remodeling, indicates that both sagittal head translation postures, anterior head carriage, and vertical head translation, are undesirable configurations in the cervical spine.

Contribution of disc degeneration to osteophyte formation in the cervical spine: a biomechanical investigation

Cervical spine disorders such as spondylotic radiculopathy and myelopathy are often related to osteophyte formation. Bone remodeling experimental-analytical studies have correlated biomechanical responses such as stress and strain energy density to the formation of bony outgrowth. Using these responses of the spinal components, the present study was conducted to investigate the basis for the occurrence of disc-related pathological conditions. An anatomically accurate and validated intact finite element model of the C4-C5-C6 cervical spine was used to simulate progressive disc degeneration at the C5-C6 level. Slight degeneration included an alteration of material properties of the nucleus pulposus representing the dehydration process. Moderate degeneration included an alteration of fiber content and material properties of the anulus fibrosus representing the disintegrated nature of the anulus in addition to dehydrated nucleus. Severe degeneration included decrease in the intervertebral disc height with dehydrated nucleus and disintegrated anulus. The intact and three degenerated models were exercised under compression, and the overall force-displacement response, local segmental stiffness, anulus fiber strain, disc bulge, anulus stress, load shared by the disc and facet joints, pressure in the disc, facet and uncovertebral joints, and strain energy density and stress in the vertebral cortex were determined. The overall stiffness (C4-C6) increased with the severity of degeneration. The segmental stiffness at the degenerated level (C5-C6) increased with the severity of degeneration. Intervertebral disc bulge and anulus stress and strain decreased at the degenerated level. The strain energy density and stress in vertebral cortex increased adjacent to the degenerated disc. Specifically, the anterior region of the cortex responded with a higher increase in these responses. The increased strain energy density and stress in the vertebral cortex over time may induce the remodeling process according to Wolff's law, leading to the formation of osteophytes.

Comparison of axial and flexural stresses in lordosis and three buckled configurations of the cervical spine

OBJECTIVE

To calculate and compare combined axial and flexural stresses in lordosis versus buckled configurations of the sagittal cervical curve.

DESIGN

Digitized measurements from lateral cervical radiographs of four different shapes were used to calculate axial loads and bending moments on the vertebral bodies of C2-C7.Background. Osteoarthritis and spinal degeneration are factors in neck and back pain. Calculations of stress in clinically occurring configurations of the sagittal cervical spine are rare.

METHODS

Center of gravity of the head (inferior-posterior sella turcica) and vertebral body margins were digitized on four different lateral cervical radiographs: lordosis, kyphosis, and two "S"-shapes. Polynomials (seventh degree) and stress concentrations on the concave and convex margins were derived for the shape of the sagittal cervical curvatures from C1 to T1. Moments of inertia were determined from digitizing and the use of an elliptical shell model of cross-section. Moment arms from a vertical line through the center of gravity of the head to the atlas and scaled neck extensor moment arms from the literature were used to compute the vertical component of extensor muscle effort. Segmental lever arms were calculated from a vertical line through C1 to each vertebra.

RESULTS

In lordosis, anterior and posterior stresses in the vertebral body are nearly uniform and minimal. In kyphotic areas, combined stresses changed from tension to compression at the anterior vertebral margins and were very large (6-10 times as large in magnitude) compared to lordosis. In kyphotic areas at the posterior vertebral body, the combined stresses changed from compression (in lordosis) to tension.

CONCLUSIONS

The stresses in kyphotic areas are very large and opposite in direction compared to a normal lordosis. This analysis provides the basis for the formation of osteophytes (Wolff's Law) on the anterior margins of vertebrae in kyphotic regions of the sagittal cervical curve. This indicates that any kyphosis is an undesirable configuration in the cervical spine. Relevance. Osteophytes and osteoarthritis are found at areas of altered stress and strain. Axial and flexural stresses at kyphotic areas in the sagittal cervical spine are abnormally high.

Biomechanics of the cervical spine Part 2. Cervical spine soft tissue responses and biomechanical modeling

OBJECTIVE

The responses and contributions of the soft tissue structures of the human neck are described with a focus on mathematical modeling. Spinal ligaments, intervertebral discs, zygapophysial joints, and uncovertebral joints of the cervical spine are included. Finite element modeling approaches have been emphasized. Representative data relevant to the development and execution of the model are discussed. A brief description is given on the functional mechanical role of the soft tissue components. Geometrical characteristics such as length and cross-sectional areas, and material properties such as force-displacement and stress-strain responses, are described for all components. Modeling approaches are discussed for each soft tissue structure. The final discussion emphasizes the normal and abnormal (e.g., degenerative joint disease, iatrogenic alteration, trauma) behaviors of the cervical spine with a focus on all these soft tissue responses. A brief description is provided on the modeling of the developmental biomechanics of the pediatric spine with a focus on soft tissues. Relevance. Experimentally validated models based on accurate geometry, material property, boundary, and loading conditions are useful to delineate the clinical biomechanics of the spine. Both external and internal responses of the various spinal components, a data set not obtainable directly from experiments, can be determined using computational models. Since soft tissues control the complex structural response, an accurate simulation of their anatomic, functional, and biomechanical characteristics is necessary to understand the behavior of the cervical spine under normal and abnormal conditions such as facetectomy, discectomy, laminectomy, and fusion.

A plate-rod device for treatment of cervicothoracic disorders: comparison of mechanical testing with established cervical spine in vitro load testing data

Posterior cervical internal fixation has long been accomplished using wires, hooks, and rods. More recently, the cervical lateral mass screw and plate or rod systems have been used effectively in unstable lower cervical spine disorders. Each form of fixation has its advantages and disadvantages. Interspinous wiring and lateral mass screw placement obviate canal penetration in the cervical region but are associated with a potential neurologic risk as a result of canal encroachment. Minor canal intrusion by laminar hooks in the thoracic spine pose a lesser neurologic risk than in the cervical region. To exploit the benefits and safety features of spinal instrumentation, a combination plate rod construct (PRC) has been developed that obviates canal penetration in the cervical region by way of lateral mass and cervical pedicle screw fixation and hooks or wires in the thoracic spine. A biomechanical analysis of the PRC device was performed and compared with the in vivo maximal load data of the cervical spine and established maximal load data of the Roy-Camille posterior cervical fixation system. The PRC has greater strength and resistance to failure than is necessary to sustain maximal in vivo cervical spine loads, and it has also compared favorably with the parameters of the Roy-Camille system. The PRC device, or variations on it, is an excellent option for spinal fixation across the cervicothoracic junction because of its superior biomechanical qualities and versatility in stabilizing a complex anatomic junction of the spine.

Finite element modeling of the cervical spine: role of intervertebral disc under axial and eccentric loads

An anatomically accurate, three-dimensional, nonlinear finite element model of the human cervical spine was developed using computed tomography images and cryomicrotome sections. The detailed model included the cortical bone, cancellous core, endplate, lamina, pedicle, transverse processes and spinous processes of the vertebrae; the annulus fibrosus and nucleus pulposus of the intervertebral discs; the uncovertebral joints; the articular cartilage, the synovial fluid and synovial membrane of the facet joints; and the anterior and posterior longitudinal ligaments, interspinous ligaments, capsular ligaments and ligamentum flavum. The finite element model was validated with experimental results: force-displacement and localized strain responses of the vertebral body and lateral masses under pure compression, and varying eccentric anterior-compression and posterior-compression loading modes. This experimentally validated finite element model was used to study the biomechanics of the cervical spine intervertebral disc by quantifying the internal axial and shear forces resisted by the ventral, middle, and dorsal regions of the disc under the above axial and eccentric loading modes. Results indicated that higher axial forces (compared to shear forces) were transmitted through different regions of the disc under all loading modes. While the ventral region of the disc resisted higher variations in axial force, the dorsal region transmitted higher shear forces under all loading modes. These findings may offer an insight to better understand the biomechanical role of the human cervical spine intervertebral disc.

Surface friction in near-vertex head and neck impact increases risk of injury

A computational head-neck model was developed to test the hypothesis that increases in friction between the head and impact surface will increase head and neck injury risk during near-axial impact. The model consisted of rigid vertebrae interconnected by assemblies of nonlinear springs and dashpots, and a finite element shell model of the skull. For frictionless impact surfaces, the model reproduced the kinematics and kinetics observed in near-axial impacts to cadaveric head-neck specimens. Increases in the coefficient of friction between the head and impact surface over a range from 0.0 to 1.0 resulted in increases of up to 40, 113, 9.8, and 43% in peak post-buckled resultant neck forces, peak moment at the occiput-C1 joint, peak resultant head accelerations, and HIC values, respectively. The most dramatic increases in injury-predicting quantities occurred for COF increases from 0.0 to 0.2, while further COF increases above 0.5 generally produced only nominal changes. These data suggest that safety equipment and impact environments which minimize the friction between the head and impact surface may reduce the risk of head and neck injury in near-vertex head impact.

Static and dynamic bending responses of the human cervical spine

The quasi-static and dynamic bending responses of the human mid-lower cervical spine were determined using cadaver intervertebral joints fixed at the base to a six-axis load cell. Flexion bending moment was applied to the superior end of the specimen using an electrohydraulic piston. Each specimen was tested under three cycles of quasi-static load-unload and one high-speed dynamic load. A total of five specimens were included in this study. The maximum intervertebral rotation ranged from 11.0 to 15.4 deg for quasi-static tests and from 22.9 to 34.4 deg for dynamic tests. The resulting peak moments at the center of the intervertebral joint ranged from 3.8 to 6.9 Nm for quasi-static tests and from 14.0 to 31.8 Nm for dynamic tests. The quasi-static stiffness ranged from 0.80 to 1.35 Nm/deg with a mean of 1.03 Nm/deg (+/- 0.11 Nm/deg). The dynamic stiffness ranged from 1.08 to 2.00 Nm/deg with a mean of 1.50 Nm/deg (+/- 0.17 Nm/deg). The differences between the two stiffnesses were statistically significant (p < 0.01). Exponential functions were derived to describe the quasi-static and dynamic moment-rotation responses. These results provide input data for lumped-parameter models and validation data for finite element models to better investigate the biomechanics of the human cervical spine.

Effect of age and loading rate on human cervical spine injury threshold

STUDY DESIGN

Statistical analysis of human cadaver cervical spine compression experiments.

OBJECTIVES

To quantify the cervical spine compressive injury threshold as a function of the person's age, gender, and external loading rate.

SUMMARY OF BACKGROUND DATA

Results of epidemiologic studies have indicated that most survivors of cervical spinal cord injury have spinal column fractures and dislocations that result from a compression or compression-flexion force vector. Cervical spinal column injury thresholds are dependent on many factors. Delineation of the injury thresholds according to age, gender, and loading rate is necessary to improve clinical assessments and prevention strategies.

METHODS

Twenty-five human cadaver head-neck compression tests were included in the analysis. Two statistical models were used to quantify the effects of age, gender, and loading rate on the force required to induce failure in the cervical spine. A multiple linear regression model provided a direct equation that quantified the effects of the variables, and a proportional hazards model was used to quantify probability of injury with each factor.

RESULTS

The regression model had a correlation coefficient of 0.87. There was an interactive effect between age and loading rate: Increasing age reduced the effect of loading rate and at approximately 82 years, loading rate had no effect. Men were consistently 600 N stronger than women. The 50% probability of failure for a 50-year-old man at a 4.5-m/sec loading rate was approximately 3.9 kN. Differences in probability curves followed the same trends as seen in the regression model.

CONCLUSIONS

The effects of age on cervical spine injury threshold are coupled with the rate of loading experienced through the external force vector that causes the trauma. Assessment of injury mechanisms and thresholds should be based on the person's age, gender, and loading rate to determine treatment and prevent injuries.

Cervical spine vertebral and facet joint kinematics under whiplash

Whiplash injuries sustained during a rear-end automobile collision have significant societal impact. The scientific literature on whiplash loading is both diverse and confusing. Definitive studies are lacking to describe the local mechanisms of injury that induce either acute or chronic pain symptoms. A methodology has been presented to quantify the kinematics of the cervical spine components by inducing controlled whiplash-type forces to intact human head-neck complexes. The localized facet joint kinematics and the overall segmental motions of the cervical spine are presented. It is anticipated that the use of this methodology will assist in a better delineation of the localized mechanisms of injury leading to whiplash pain.

Inertial loading of the human cervical spine

While the majority of experimental cervical spine biomechanics research has been conducted using slowly applied forces and/or moments, or dynamically applied forces with contact, little research has been performed to delineate the biomechanics of the human neck under inertial "noncontact" type forces. This study was designed to develop a comprehensive methodology to induce these loads. A minisled pendulum experimental setup was designed to test specimens (such as human cadaver neck) at subfailure or failure levels under different loading modalities including flexion, extension, and lateral bending. The system allows acceleration/deceleration input with varying wave form shapes. The test setup dynamically records the input and output strength information such as forces, accelerations, moments, and angular velocities; it also has the flexibility to obtain the temporal overall and local kinematic data of the cervical spine components at every vertebral level. These data will permit a complete biomechanical structural analysis. In this paper, the feasibility of the methodology is demonstrated by subjecting a human cadaver head-neck complex with intact musculature and skin under inertial flexion and extension whiplash loading at two velocities.