Biomechanics of coupled motion in the cervical spine during simulated whiplash in patients with pre-existing cervical or lumbar spinal fusion: A Finite Element Study.

OBJECTIVES: Loss of motion following spine segment fusion results in increased strain in the adjacent motion segments. However, to date, studies on the biomechanics of the cervical spine have not assessed the role of coupled motions in the lumbar spine. Accordingly, we investigated the biomechanics of the cervical spine following cervical fusion and lumbar fusion during simulated whiplash using a whole-human finite element (FE) model to simulate coupled motions of the spine. METHODS: A previously validated FE model of the human body in the driver-occupant position was used to investigate cervical hyperextension injury. The cervical spine was subjected to simulated whiplash exposure in accordance with Euro NCAP (the European New Car Assessment Programme) testing using the whole human FE model. The coupled motions between the cervical spine and lumbar spine were assessed by evaluating the biomechanical effects of simulated cervical fusion and lumbar fusion. RESULTS: Peak anterior longitudinal ligament (ALL) strain ranged from 0.106 to 0.382 in a normal spine, and from 0.116 to 0.399 in a fused cervical spine. Strain increased from cranial to caudal levels. The mean strain increase in the motion segment immediately adjacent to the site of fusion from C2-C3 through C5-C6 was 26.1% and 50.8% following single- and two-level cervical fusion, respectively (p = 0.03, unpaired two-way t-test). Peak cervical strains following various lumbar-fusion procedures were 1.0% less than those seen in a healthy spine (p = 0.61, two-way ANOVA). CONCLUSION: Cervical arthrodesis increases peak ALL strain in the adjacent motion segments. C3-4 experiences greater changes in strain than C6-7. Lumbar fusion did not have a significant effect on cervical spine strain.Cite this article: H. Huang, R. W. Nightingale, A. B. C. Dang. Biomechanics of coupled motion in the cervical spine during simulated whiplash in patients with pre-existing cervical or lumbar spinal fusion: A Finite Element Study. Bone Joint Res 2018;7:28-35. DOI: 10.1302/2046-3758.71.BJR-2017-0100.R1.

The influence of surface padding properties on head and neck injury risk.

A validated computational head-neck model was used to understand the mechanical relationships between surface padding characteristics and injury risk during impacts near the head vertex. The study demonstrated that injury risk can be decreased by maximizing the energy-dissipating ability of the pad, choosing a pad stiffness that maximizes pad deformation without bottoming out, maximizing pad thickness, and minimizing surface friction. That increasing pad thickness protected the head without increasing neck loads suggests that the increased cervical spine injury incidence previously observed in cadaveric impacts to padded surfaces relative to lubricated rigid surfaces was due to increased surface friction rather than pocketing of the head in the pad.

On the feasibility of remote palpation using acoustic radiation force.

A method of acoustic remote palpation, capable of imaging local variations in the mechanical properties of tissue, is under investigation. In this method, focused ultrasound is used to apply localized (on the order of 2 mm3) radiation force within tissue. and the resulting tissue displacements are mapped using ultrasonic correlation based methods. The tissue displacements are inversely proportional to the stiffness of the tissue, and thus a stiffer region of tissue exhibits smaller displacements than a more compliant region. In this paper, the feasibility of remote palpation is demonstrated experimentally using breast tissue phantoms with spherical lesion inclusions, and in vitro liver samples. A single diagnostic transducer and modified ultrasonic imaging system are used to perform remote palpation. The displacement images are directly correlated to local variations in tissue stiffness with higher contrast than the corresponding B-mode images. Relationships between acoustic beam parameters, lesion characteristics and radiation force induced tissue displacement patterns are investigated and discussed. The results show promise for the clinical implementation of remote palpation.

A finite element model of remote palpation of breast lesions using radiation force: factors affecting tissue displacement.

The early detection of breast cancer reduces patient mortality. The most common method of breast cancer detection is palpation. However, lesions that lie deep within the breast are difficult to palpate when they are small. Thus, a method of remote palpation, which may allow the detection of small lesions lying deep within the breast, is currently under investigation. In this method, acoustic radiation force is used to apply localized forces within tissue (to tissue volumes on the order of 2 mm3) and the resulting tissue displacements are mapped using ultrasonic correlation based methods. A volume of tissue that is stiffer than the surrounding medium (i.e., a lesion) distributes the force throughout the tissue beneath it, resulting in larger regions of displacement, and smaller maximum displacements. The resulting displacement maps may be used to image tissue stiffness. A finite-element-model (FEM) of acoustic remote palpation is presented in this paper. Using this model, a parametric analysis of the affect of varying tissue and acoustic beam characteristics on radiation force induced tissue displacements is performed. The results are used to evaluate the potential of acoustic remote palpation to provide useful diagnostic information in a clinical setting. The potential for using a single diagnostic transducer to both generate radiation force and track the resulting displacements is investigated.

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

STUDY DESIGN: Cervical facet capsular strains were determined during bending and at failure in the human cadaver. OBJECTIVE: To determine the effect of an axial pretorque on facet capsular strains and estimate the risk for subcatastrophic capsular injury during normal bending motions. SUMMARY OF BACKGROUND DATA: 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. METHODS: 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. RESULTS: 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. CONCLUSIONS: 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.

Comparison of soccer shin guards in preventing tibia fracture.

The goal of this study was to evaluate the effectiveness of a number of shin guards in protecting against tibia fracture in soccer players. A secondary purpose was to determine the relationship between the material and structural differences in shin guard design and the protection provided. Twenty-three commercially available shin guards were tested on a model leg containing a synthetic tibia that had been calibrated against human cadaver specimens. Each guard was categorized into one of four material types: plastic (N = 9), fiberglass (N = 6), compressed air (N = 4), and Kevlar (N = 4). The maximum combined force at the ends of the tibia, the principal strain on the posterior side of the tibia, and the contact time of the impact were measured using a drop track impact simulation. Shin guards provided significant protection from tibia fracture at all drop heights. The average guard reduced force by 11% to 17% and strain by 45% to 51% compared with the unguarded leg. At the higher drop heights, material composition and structural characteristics of the shin guards showed significant differences in protective abilities. These findings indicate that all shin guards provide some measure of protection against tibia fracture, although the level of protection may vary significantly among the different guards.

Inertial properties and loading rates affect buckling modes and injury mechanisms in the cervical spine.

Cervical spine injuries continue to be a costly societal problem. Future advancements in injury prevention depend on improved physical and computational models which, in turn, are predicated on a better understanding of the responses of the neck during dynamic loading. Previous studies have shown that the tolerance of the neck is dependent on its initial position and its buckling behavior. This study uses a computational model to examine the mechanical factors influencing buckling behavior during impact to the neck. It was hypothesized that the inertial properties of the cervical spine influence the dynamics during compressive axial loading. The hypothesis was tested by performing parametric analyses of vertebral mass, mass moments of inertia, motion segment stiffness, and loading rate. Increases in vertebral mass resulted in increasingly complex kinematics and larger peak loads and impulses. Similar results were observed for increases in stiffness. Faster loading rates were associated with higher peak loads and higher-order buckling modes. The results demonstrate that mass has a great deal of influence on the buckling behavior of the neck, particularly with respect to the expression of higher-order modes. Injury types and mechanisms may be substantially altered by loading rate because inertial effects may influence whether the cervical spine fails in a compressive mode, or a bending mode.

Tensile properties of the human muscular and ligamentous cervical spine.

Tensile neck injuries are amongst the most serious cervical injuries. However, because neither reliable human cervical tensile tolerance data nor tensile structural data are currently available, the quantification of tensile injury risk is limited. The purpose of this study is to provide previously unavailable kinetic and tolerance data for the ligamentous cervical spine and determine the effect of neck muscle on tensile load response and tolerance. Using six male human cadaver specimens, isolated ligamentous cervical spine tests (occiput - T1) were conducted to quantify the significant differences in kinetics due to head end condition and anteroposterior eccentricity of the tensile load. The spine was then separated into motion segments for tension failure testing. The upper cervical spine tolerance of 2400 +/- 270 N (occiput-C2) was found to be significantly greater (p < 0.01) than the lower cervical spine tolerance of 1780 +/- 230 N (C4-C5 and C6-C7 segments). Data from these experiments were used to develop and validate a computational model of the ligamentous spine. The model predicted the end condition and eccentricity responses for the tensile force-displacement relationship. Cervical muscular geometry data derived from cadaver dissection and MRI imaging were used to incorporate a muscular response into the model. The cervical musculature under maximal stimulation increased the tolerance of the cervical spine from 1800 N to 4160 N. In addition, the cervical musculature resulted in a shift in the site of injury from the lower cervical spine to the upper cervical spine and offers an explanation for the mechanism of upper cervical spine tension injuries observed clinically. The results from this study predict a range in tensile tolerance from 1.8 - 4.2 kN based on the varying role of the cervical musculature.

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.

The effects of padded surfaces on the risk for cervical spine injury.

STUDY DESIGN: This is an in vitro study comparing cervical spine injuries produced in rigid head impacts and in padded head impacts. OBJECTIVES: To test the hypothesis that deformable impact surfaces pose a greater risk for cervical spine injury than rigid surfaces using a cadaver-based model that includes the effects of the head and torso masses. SUMMARY OF BACKGROUND DATA: It is widely assumed that energy-absorbing devices that protect the head from injury also reduce the risk for neck injury. However, this has not been demonstrated in any experimental or epidemiologic study. On the contrary, some studies have shown that padded surfaces have no effect on neck injury risk, and others have suggested that they can increase risk. METHODS: Experiments were performed on 18 cadaveric cervical spines to test 6 combinations of impact angle and impact surface padding. The impact surface was oriented at -15 degrees (posterior impact), 0 degree (vertex impact), or +15 degrees (anterior impact). The impact surface was either a 3-mm sheet of lubricated Teflon or 5 cm of polyurethane foam. RESULTS: Impacts onto padded surfaces produced significantly larger neck impulses (P = 0.00023) and a significantly greater frequency of cervical spine injuries than rigid impacts (P = 0.0375). The impact angle was also correlated with injury risk (P < 0.00001). CONCLUSIONS: These experiments suggest that highly deformable, padded contact surfaces should be used carefully in environments where there is the risk for cervical spine injury. The results also suggest that the orientation of the head, neck, and torso relative to the impact surface is of equal if not greater importance in neck injury risk.

Strength and stability of posterior lumbar interbody fusion. Comparison of titanium fiber mesh implant and tricortical bone graft.

STUDY DESIGN: A paired comparison was done of the bending flexibility and compression strength of tricortical bone graft and titanium fiber mesh implants in a human cadaver model of posterior lumbar interbody fusion. OBJECTIVES: To test the hypothesis that a titanium fiber mesh implant and a tricortical bone graft provide adequate and equal mechanical strength and stability in posterior lumbar interbody fusion constructs. SUMMARY OF BACKGROUND DATA: Although studies of posterior lumbar interbody fusion constructs have been performed, the authors are unaware of any study in which the strength and stability of a titanium fiber mesh implant are compared with those of tricortical bone graft for posterior lumbar interbody fusion in the human cadaver lumbar spine. METHODS: Changes in neutral zone and range of motion were measured in a bending flexibility test before and after placement of posterior lumbar interbody fusion constructs. Tricortical bone graft and titanium fiber mesh implant construct stability than were compared in a paired analysis. The constructs than were loaded to failure to evaluate construct strength as a function of graft material and bone mineral density. RESULTS: The posterior lumbar interbody fusion procedure produced statistically significant decreases in neutral zone when compared with the intact spine. No statistically significant differences in neutral zone, range of motion, or strength were detected between the two implants. Construct strength correlated strongly with bone mineral density. CONCLUSIONS: Posterior lumbar interbody fusion procedures result in equal or improved acute stability for titanium fiber mesh implants and tricortical bone graft implants when used without additional posterior stabilization.

The role of imaging and in situ biomechanical testing in assessing pedicle screw pull-out strength.

STUDY DESIGN: This study determined the predictive ability of quantitative computed tomography, dual energy x-ray absorptiometry, pedicular geometry, and mechanical testing in assessing the strength of pedicle screw fixation in an in vitro mechanical test of intra-pedicular screw fixation in the human cadaveric lumbar spine. OBJECTIVE: To test several hypotheses regarding the relative predictive value of densitometry, pedicular geometry, and mechanical testing in describing pedicle screw pull-out. SUMMARY OF BACKGROUND DATA: Previous investigations have suggested that mechanical testing, geometry, and densitometry, determined by quantitative computed tomography or dual energy x-ray absorptiometry, predict the strength of the screw-bone system. However, no study has compared the relative predictive value of these techniques. METHODS: Forty-nine pedicle screw cyclic-combined flexion-extension moment-axial pull-out tests were performed on human cadaveric lumbar vertebrae. The predictive ability of quantitative computed tomography, dual energy x-ray absorptiometry, insertional torque, in situ stiffness, and pedicular geometry was assessed using multiple regression. RESULTS: Several variables correlated to force at failure. However, multiple regression analysis showed that bone mineral density of the pedicle determined by quantitative computed tomography, insertional torque, and in situ stiffness when used in combination resulted in the strongest prediction of pull-out force. No other measures provided additional predictive ability in the presence of these measures. CONCLUSIONS: Pedicle density determined by quantitative computed tomography when used with insertional torque and in situ stiffness provides the strongest predictive ability of screw pull-out. Geometric measures of the pedicle and density determined by dual energy x-ray absorptiometry do not provide additional predictive ability in the presence of these measures.

Experimental impact injury to the cervical spine: relating motion of the head and the mechanism of injury.

The purpose of this study was to analyze, with use of an impact model, the relationships among motion of the head, local deformations of the cervical spine, and the mechanisms of injury; the model consisted of the head and neck of a cadaver. Traditionally, the mechanisms of injury to the cervical spine have been associated with flexion and extension motions of the head and neck. However, the classification of the mechanisms is not always in agreement with the patient's account of the injury or with lacerations and contusions of the scalp, which indicate the site of the impact of the head. Eleven specimens were dropped in an inverted posture with the head and neck in an anatomically neutral position. Forces, moments, and accelerations were recorded, and the impacts were imaged at 1000 frames per second. The velocity at the time of impact was on the order of 3.2 meters per second. The angle and the padding of the impact surface varied. Observable motion of the head did not correspond to the mechanism of the injury to the cervical spine. Injury occurred 2.2 to 18.8 milliseconds after impact and before noticeable motion of the head. However, the classification of the mechanism of the injuries was descriptive of the local deformations of the cervical spine at the time of the injury. Accordingly, it is a useful tool in describing the local mechanism of injury. Buckling of the cervical spine, involving extension between the third and sixth cervical vertebrae and flexion between the seventh and eight cervical vertebrae, was observed. Other, more complex, buckling deformations were also seen, suggesting that the deformations that occur during impact are so complex that they can give rise to a number of different mechanisms of injury.

Dynamic responses of the head and cervical spine to axial impact loading.

This study explores the inertial effects of the head and torso on cervical spine dynamics with the specific goal of determining whether the head mass can provide a constraining cervical spine end condition. The hypothesis was tested using a low friction impact surface and a pocketing foam impact surface. Impact orientation was also varied. Tests were conducted on whole unembalmed heads and cervical spines using a drop track system to produce impact velocities on the order of 3.2 m s-1. Data for the head impact forces and the reactions at T1 were recorded and the tests were also imaged at 1000 frames s-1. Injuries occurred 2-19 ms following head impact and prior to significant head motion. Average compressive load a failure was 1727 +/- 387 N. Decoupling was observed between the head and T1. Cervical spine loading due to head rebound constituted up to 54 +/- 16% of the total axial neck load for padded impacts and up to 38 +/- 30% of the total axial neck load for rigid impacts. Dynamic buckling was also observed; including first-order modes and transient higher-order modes which shifted the structure from a primarily compressive mode of deformation to various bending modes. These experiments demonstrate that in the absence of head pocketing, the head mass can provide sufficient constraint to cause cervical spine injury. The results also show that cervical spinal injury dynamics are complex, and that a large sample size of experimentally produced injuries will be necessary to develop comprehensive neck injury models and criteria.

Mechanisms of basilar skull fracture.

Basilar skull fractures comprise a broad category of injuries that have been attributed to a variety of causal mechanisms. The objective of this work is to develop an understanding of the biomechanical mechanisms that result in basilar skull fractures, specifically focusing on mandibular impact and neck loading as potential mechanisms. In the characterization of the injury mechanisms, three experimental studies have been performed. The first study evaluated the response of the base of the skull to midsymphysis loading on the mental protuberance (chin) of the mandible. Five dynamic impacts using a vertical drop track and one quasi-static test in a servohydraulic test frame have been performed. In each test, clinically relevant mandibular fractures were produced but no basilar skull fractures were observed. The second study assessed the fracture tolerance of the base of the skull subject to direct loading on the temporomandibular joint in conjunction with tensile loading imposed locally around the foramen magnum to simulate the effect of the ligaments and musculature of the neck. Among four specimens that sustained either complete or incomplete basilar skull ring fractures remote from the sites of load application, the mean load at fracture was 4300 +/- 350 N. Energy to fracture was computed in three of those tests and averaged 13.0 +/- 1.7 J. Injuries produced were consistent with clinical observations that have attributed basilar skull ring fractures to mandibular impacts. In the third series of experimental tests, loading responses resulting from cranial vault impacts were investigated using unembalmed human cadaver heads and ligamentous cervical spines. Multiaxis load cells and accelerometers, coupled with high-speed digital video, were used to quantify impact dynamics. The results of these experiments suggest that while there is a greater probability of cervical spine injury, basilar skull ring fractures can result when the head is constrained on the impact surface and the inertia of the torso drives the vertebral column onto the occiput.