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Technical Paper

Comparative Structural Neck Responses of the THOR-NT, Hybrid III, and Human in Combined Tension-Bending and Pure Bending

This study evaluated the biofidelity of both the Hybrid III and the THOR-NT anthropomorphic test device (ATD) necks in quasistatic tension-bending and pure-bending by comparing the responses of both the ATDs with results from validated computational models of the living human neck. This model was developed using post-mortem human surrogate (PMHS) osteoligamentous response corridors with effective musculature added (Chancey, 2005). Each ATD was tested using a variety of end-conditions to create the tension-bending loads. The results were compared using absolute difference, RMS difference, and normalized difference metrics. The THOR-NT was tested both with and without muscle cables. The THOR-NT was also tested with and without the central safety cable to test the effect of the cable on the behavior of the ATD. The Hybrid III was stiffer than the model for all tension-bending end conditions.
Technical Paper

Mechanical Properties and Anthropometry of the Human Infant Head

The adult head has been studied extensively and computationally modeled for impact, however there have been few studies that attempt to quantify the mechanical properties of the pediatric skull. Likewise, little documentation of pediatric anthropometry exists. We hypothesize that the properties of the human pediatric skull differ from the human adult skull and exhibit viscoelastic structural properties. Quasi-static and dynamic compression tests were performed using the whole head of three human neonate specimens (ages 1 to 11 days old). Whole head compression tests were performed in a MTS servo-hydraulic actuator. Testing was conducted using nondestructive quasi-static, and constant velocity protocols in the anterior-posterior and right-left directions. In addition, the pediatric head specimens were dropped from 15cm and 30cm and impact force-time histories were measured for five different locations: vertex, occiput, forehead, right and left parietal region.
Technical Paper

Variation of Neck Muscle Strength Along the Human Cervical Spine

The aim of this study was to describe and explain the variation of neck muscle strength along the cervical spine. A three-dimensional model of the head-neck complex was developed to test the hypothesis that the moment-generating capacity of the neck musculature is lower in the upper cervical spine than in the lower cervical spine. The model calculations suggest that the neck muscles can protect the lower cervical spine from injury during extension and lateral bending. The maximum flexor moment developed in the lower cervical spine was 2 times higher than that developed in the upper spine. The model also predicted that the neck musculature is 30% stronger in the lower cervical spine during lateral bending. Peak compressive forces (up to 3 times body weight) were higher in the lower cervical spine. These results are consistent with the clinical finding that extension loading of the neck often leads to injuries in the upper cervical spine.
Technical Paper

Improved Estimation of Human Neck Tensile Tolerance: Reducing the Range of Reported Tolerance Using Anthropometrically Correct Muscles and Optimized Physiologic Initial Conditions

Unlike other modes of loading, the tolerance of the human neck in tension depends heavily on the load bearing capabilities of the muscles of the neck. Because of limitations in animal models, human cadaver, and volunteer studies, computational modeling of the cervical spine is the best way to understand the influence of muscle on whole neck tolerance to tension. Muscle forces are a function of the muscle's geometry, constitutive properties, and state of activation. To generate biofidelic responses for muscle, we obtained accurate three-dimensional muscle geometry for 23 pairs of cervical muscles from a combination of human cadaver dissection and 50th percentile male human volunteer magnetic resonance imaging and incorporated those muscles into a computational model of the ligamentous spine that has been previously validated against human cadaver studies.
Technical Paper

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).
Technical Paper

Cervical Facet Joint Mechanics: Its Application to Whiplash Injury

Epidemiological and clinical studies have identified the cervical facet capsule as a potential site of whiplash injury and prerotation of the head and neck as a risk factor for whiplash injury. However, biomechanical data related to the cervical facet capsule and its role in whiplash injury remain limited in the literature. In this study, cervical spine motion segments were tested in a pure moment test frame and the full field strains were determined throughout the facet capsule. Motion segments were tested with and without a pretorque in pure bending. Bending tests were followed by isolated facet elongation tests to failure. Maximum principal strains during bending were compared to failure strains. Statistically significant increases in principal capsular strains were observed in the facet which was contralateral to the pretorque. In contrast, no significant differences were present in the ipsilateral facet when large flexion-extension moments were applied.
Technical Paper

Experimental Flexibility Measurements for the Development of a Computational Head-Neck Model Validated for Near-Vertex Head Impact

A computational head-neck model was developed to more efficiently study dynamic responses of the head and neck to near-vertex head impact. The model consisted of rigid vertebrae interconnected by assemblies of nonlinear springs and dashpots, and a finite element shell model of the skull. Quasi-static flexion-extension characteristics of ten human cadaveric cervical spines were measured using a test frame capable of applying pure moments. The cadaveric motion segments demonstrated a nonlinear stiffening response without a no-load neutral zone. Computational model parameters were based upon these measurements and existing data reported in the literature. Geometric and inertial characteristics were derived from three-dimensional reconstructions of skull and vertebral CT images. The model reproduced the shape and timing of the cervical spine buckling deformations observed in high speed video of cadaveric studies of near-vertex head impact [1].
Technical Paper

The Dynamic Responses of the Cervical Spine: Buckling, End Conditions, and Tolerance in Compressive Impacts

This study explores the dynamics of head and cervical spine impact with the specific goals of determining the effects of head inertia and impact surface on injury risk. Head impact experiments were performed using unembalmed head and neck specimens from 22 cadavers. These included impacts onto compliant and a rigid surfaces with the surface oriented to produce both flexion and extension attitudes. Tests were conducted using a drop track system to produce impact velocities on the order of 3.2 m/s. Multiaxis transduction recorded the head impact forces, head accelerations, and the reactions at T1. The tests were also imaged at 1000 frames/sec. Injuries occurred 2 to 30 msec following head impact and prior to significant head motion. Head motions were not found to correlate with injury classification. Decoupling was observed between the head and T1, resulting in a lag in the force histories.
Technical Paper

The Influence of End Condition on Human Cervical Spine Injury Mechanisms

The passive combined flexion and axial loading responses of the unembalmed human cervical spine were measured in a dynamic test environment. The influence of end condition (the degree of constraint imposed on the head by the contact surface) was varied to determine its effect on observed column stiffness and on failure modes of the cervical spine. Multi-axis load cells were used to completely describe the forces and moments developed in the specimen. Twenty three specimens were studied. The Hybrid III neckform performance was assessed to determine its suitability as a mechanical simulator of the neck during head impact. Changes in end condition produced significant changes in axial stiffness in both the Hybrid III neckform and the cadaver neck. The mode of injury also varied as a function of end condition in a repeatable fashion. Separation of injuries based upon imposed end condition identified groups with significantly different axial load to failure.