Motion analysis of cervical vertebrae during whiplash loading

Metabotropic glutamate receptor-5 and protein kinase C-epsilon increase in dorsal root ganglion neurons and spinal glial activation in an adolescent rat model of painful neck injury

There is growing evidence that neck pain is common in adolescence and is a risk factor for the development of chronic neck pain in adulthood. The cervical facet joint and its capsular ligament is a common source of pain in the neck in adults, but its role in adolescent pain remains unknown. The aim of this study was to define the biomechanics, behavioral sensitivity, and indicators of neuronal and glial activation in an adolescent model of mechanical facet joint injury. A bilateral C6-C7 facet joint distraction was imposed in an adolescent rat and biomechanical metrics were measured during injury. Following injury, forepaw mechanical hyperalgesia was measured, and protein kinase C-epsilon (PKCɛ) and metabotropic glutamate receptor-5 (mGluR5) expression in the dorsal root ganglion and markers of spinal glial activation were assessed. Joint distraction induced significant mechanical hyperalgesia during the 7 days post-injury (p < 0.001). Painful injury significantly increased PKCɛ expression in small- and medium-diameter neurons compared to sham (p < 0.05) and naïve tissue (p < 0.001). Similarly, mGluR5 expression was significantly elevated in small-diameter neurons after injury (p < 0.05). Spinal astrocytic activation after injury was also elevated over sham (p < 0.035) and naïve (p < 0.0001) levels; microglial activation was only greater than naïve levels (p < 0.006). Mean strains in the facet capsule during injury were 32.8 ± 12.9%, which were consistent with the strains associated with comparable degrees of hypersensitivity in the adult rat. These results suggest that adolescents may have a lower tissue tolerance to induce pain and associated nociceptive response than do adults.

Neuronal hyperexcitability in the dorsal horn after painful facet joint injury

Excessive cervical facet capsular ligament stretch has been implicated as a cause of whiplash-associated disorders following rear-end impacts, but the pathophysiological mechanisms that produce chronic pain in these cases remain unclear. Using a rat model of C6-C7 cervical facet joint capsule stretch that produces sustained mechanical hyperalgesia, the presence of neuronal hyperexcitability was characterized 7 days after joint loading. Extracellular recordings of spinal dorsal horn neuronal activity between C6 and C8 (117 neurons) were obtained from anesthetized rats, with both painful and non-painful behavioral outcomes established by the magnitude of capsule stretch. The frequency of neuronal firing during noxious pinch (p<0.0182) and von Frey filaments applications (4-26g) to the forepaw was increased (p<0.0156) in the painful group compared to the non-painful and sham groups. In addition, the incidence and frequency of spontaneous and after discharge firing were greater in the painful group (p<0.0307) relative to sham. The proportion of cells in the deep laminae that responded as wide dynamic range neurons also was increased in the painful group relative to non-painful or sham groups (p<0.0348). These findings suggest that excessive facet capsule stretch, while not producing visible tearing, can produce functional plasticity of dorsal horn neuronal activity. The increase in neuronal firing across a range of stimulus magnitudes observed at day 7 post-injury provides the first direct evidence of neuronal modulation in the spinal cord following facet joint loading, and suggests that facet-mediated chronic pain following whiplash injury is driven, at least in part, by central sensitization.

Minimizing the source of nociception and its concurrent effect on sensory hypersensitivity: an exploratory study in chronic whiplash patients

Background

The cervical zygapophyseal joints may be a primary source of pain in up to 60% of individuals with chronic whiplash associated disorders (WAD) and may be a contributing factor for peripheral and centrally mediated pain (sensory hypersensitivity). Sensory hypersensitivity has been associated with a poor prognosis. The purpose of the study was to determine if there is a change in measures indicative of sensory hypersensitivity in patients with chronic WAD grade II following a medial branch block (MBB) procedure in the cervical spine.

Methods

Measures of sensory hypersensitivity were taken via quantitative sensory testing (QST) consisting of pressure pain thresholds (PPT's) and cold pain thresholds (CPT's). In patients with chronic WAD (n = 18), the measures were taken at three sites bilaterally, pre- and post- MBB. Reduced pain thresholds at remote sites have been considered an indicator of central hypersensitivity. A healthy age and gender matched comparison group (n = 18) was measured at baseline. An independent t-test was applied to determine if there were any significant differences between the WAD and normative comparison groups at baseline with respect to cold pain and pressure pain thresholds. A dependent t-test was used to determine whether there were any significant differences between the pre and post intervention cold pain and pressure pain thresholds in the patients with chronic WAD.

Results

At baseline, PPT's were decreased at all three sites in the WAD group (p < 0.001). Cold pain thresholds were increased in the cervical spine in the WAD group (p < 0.001). Post-MBB, the WAD group showed significant increases in PPT's at all sites (p < 0.05), and significant decreases in CPT's at the cervical spine (p < 0.001).

Conclusions

The patients with chronic WAD showed evidence of widespread sensory hypersensitivity to mechanical and thermal stimuli. The WAD group revealed decreased sensory hypersensitivity following a decrease in their primary source of pain stemming from the cervical zygapophyseal joints.

Synovial folds of the lateral atlantoaxial joints: in vivo quantitative assessment using magnetic resonance imaging in healthy volunteers

STUDY DESIGN

Analysis of magnetic resonance (MR) images of healthy volunteers.

OBJECTIVE

To develop and validate an imaging protocol and measurement technique to describe the morphology and quantify the dimensions of the synovial folds of the lateral atlantoaxial joints in vivo.

SUMMARY OF BACKGROUND DATA

The synovial folds of the lateral atlantoaxial joints are considered to be a potential source of neck pain and headache, especially following whiplash injury. Until recently, it has not been possible to image the synovial folds in vivo and consequently their normal morphology is not fully understood.

METHODS

MR images of the cervical spine of 17 volunteers (4 male and 13 female) were acquired using a 1.5-tesla scanner. The morphology of the synovial folds at the lateral atlantoaxial joints was described and their presence determined. The volume and cross-sectional area of the ventral and dorsal synovial folds of the right and left lateral atlantoaxial joints were measured and compared. The relationship between the dimensions of the synovial folds and subject age was examined. Twenty synovial folds were measured twice by one observer and once by a second observer for the determination of measurement reliability.

RESULTS

There was a significant difference in volume (chi [3] = 17.54, P = 0.000) and cross-sectional area (chi [3] = 18.95, P = 0.000) between the ventral and dorsal synovial folds of the left and right lateral atlantoaxial joints. There was no correlation between synovial fold dimensions and age. The reliability of the measurements ranged from intraclass correlation coefficient 0.95 to 0.99 (intraobserver reliability) and intraclass correlation coefficients 0.75 to 0.82 (interobserver reliability).

CONCLUSION

MR imaging was successfully implemented as a noninvasive method for visualizing the synovial folds of the lateral atlantoaxial joints and quantifying their dimensions in healthy volunteers. The results of this study provide a basis for future studies investigating synovial fold pathology in patients with neck pain and headache.

Is there a relationship between whiplash-associated disorders and concussion in hockey? A preliminary study

PRIMARY OBJECTIVE

To examine the relationship between the occurrence of whiplash-associated disorders and concussion symptoms in hockey players.

RESEARCH DESIGN

Prospective cohort observational study to examine the relationship between whiplash associated disorders and concussion in actual hockey play.

METHODS AND PROCEDURES

Twenty hockey teams were followed prospectively for one season. Team therapists completed acute and 7-10 day follow-up evaluation questionnaires for all of the players who received either a whiplash mechanism or a concussion.

MAIN OUTCOMES AND RESULTS

183 players were registered for this study; 13 received either a whiplash mechanistic injury or a concussion injury. Initial injuries ranged from WAD I to WAD III and all subjects reported concussion symptoms. Only three subjects reported full resolution of both WAD and concussion symptoms at the 7-10 day follow-up evaluation.

CONCLUSIONS

There is a strong association between whiplash induced neck injuries and the symptoms of concussion in hockey injuries. Both should be evaluated when dealing with athletes/patients suffering from either injury.

Joint distraction magnitude is associated with different behavioral outcomes and substance P levels for cervical facet joint loading in the rat

The facet joint is a common source of pain in both the neck and low back, and can be injured by abnormal loading of the spinal joints. Whereas a host of nociceptive changes including neuronal activation, neuropeptide expression, and inflammatory mediator responses has been reported for rat models of joint pain, no such responses have been explicitly investigated or quantified for painful mechanical injury to the facet joint. Two magnitudes of joint loading were separately imposed in a rat model of cervical facet joint distraction: Painful and nonpainful distractions. Behavioral outcomes were defined by assessing mechanical hyperalgesia in the shoulders and forepaws. Substance P (SP) mRNA and protein levels were quantified in the dorsal root ganglion (DRG) and spinal cord at days 1 and 7 following distraction. Painful distraction produced mechanical hyperalgesia that was significantly greater (P < .010) than that for a nonpainful distraction. Painful distraction significantly increased spinal SP mRNA (P = .048) and SP protein expression in the DRG (P = .013) at day 7 compared to nonpainful distraction. However, spinal SP protein for painful distraction was significantly less (P = .024) than that for nonpainful distraction at day 1. Joint distractions producing different behavioral outcomes modulate SP mRNA and protein in the DRG and spinal cord, suggesting that SP responses may be involved with different temporal responses in painful joint loading.

PERSPECTIVE

SP mRNA and protein in the DRG and spinal cord are quantified at 2 time points after cervical facet joint distractions that separately do or do not produce mechanical hyperalgesia. Studies describe a role for SP to contribute to pain produced by mechanical joint loading.

Characterization of acute and chronic whiplash-associated disorders

SYNOPSIS

The development of chronic pain and disability following whiplash injury is common and contributes substantially to personal and economic costs related with this condition. Emerging evidence demonstrates the clinical presence of alterations in the sensory and motor systems, including psychological distress in all individuals with a whiplash injury, regardless of recovery. However, individuals who transition to the chronic state present with a more complex clinical picture characterized by the presence of widespread sensory hypersensitivity, as well as significant posttraumatic stress reactions. Based on the diversity of the signs and symptoms experienced by individuals with a whiplash condition, clinicians must take into account the more readily observable/measurable differences in motor, sensory, and psychological dysfunction. The implications for the assessment and management of this condition are discussed. Further review into the pathomechanical, pathoanatomical, and pathophysiological features of the condition also will be discussed.

LEVEL OF EVIDENCE

Level 5.J

The anatomy and biomechanics of acute and chronic whiplash injury

Whiplash injury is the most common motor vehicle injury, yet it is also one of the most poorly understood. Here we examine the evidence supporting an organic basis for acute and chronic whiplash injuries and review the anatomical sites within the neck that are potentially injured during these collisions. For each proposed anatomical site--facet joints, spinal ligaments, intervertebral discs, vertebral arteries, dorsal root ganglia, and neck muscles--we present the clinical evidence supporting that injury site, its relevant anatomy, the mechanism of and tolerance to injury, and the future research needed to determine whether that site is responsible for some whiplash injuries. This article serves as a snapshot of the current state of whiplash biomechanics research and provides a roadmap for future research to better understand and ultimately prevent whiplash injuries.

Use of the finite element method to study the mechanism of spinal cord injury without radiological abnormality in the cervical spine

STUDY DESIGN

Three-dimensional C3-C5 and C3-C4 finite element (FE) models were used to analyze biomechanical responses under compression and extension moments.

OBJECTIVE

To validate our models against other published FE models and experimental studies and improve our understanding of the mechanism of spinal cord injury without radiologic abnormality (SCIWORA) in cervical spine.

SUMMARY OF BACKGROUND DATA

The underlying mechanism for SCIWORA remains unclear. We hypothesized that the incidence of SCIWORA was associated with facet joint morphology and bony pincers mechanism.

METHODS

FE models were constructed using data from computed tomography scans of the cervical spine of a healthy young man. The C3-C5 FE models consisted of bony vertebra, articulating facets, and intervertebral disc. Facet surfaces were oriented at 30 degrees , 45 degrees , and 60 degrees from the transverse plane. These models were constrained in all degrees of freedom at the C5 inferior vertebral body and a uniform axial displacement of 1 mm was applied to the superior nodes of C3. Three model versions changed to C3-C4 models with ligaments. The C4 inferior-most bony nodes were constrained, whereas the top of the C3 superior-most bony nodes were left unconstrained. These models were subjected to an axial compression load of 73.6 N with extension moments (1.8 Nm) applied to the upper bony section C3 vertebra. The predicted responses were compared with published results.

RESULTS

The response under axial compression was validated and corresponded closely with published results. Under sagittal moment, the C3-C4 FE model with 60 degrees facet was the most flexible in extension (4.22 degrees ). Total translation was highest for the model with 60 degrees facet.

CONCLUSION

The load displacement response of C3-C5 FE models was in agreement with published data. We confirmed that the C3-C4 FE model with 60 degrees facet was the most susceptible to SCIWORA and that the bony pincers mechanism was dependent on facet joint inclination.

MRI analysis of the size and shape of the oropharynx in chronic whiplash

OBJECTIVES

To quantify differences in the size/shape of the oropharynx between female subjects with whiplash and controls.

DESIGN

Retrospective cohort.

METHODS

A total of 113 subjects (79 whiplash, 34 controls) were included. T1-weighted MRI was used to measure 1) cross-sectional area (CSA [mm(2)]) and 2) shape ratios for the oropharynx. Reliability data were established.

RESULTS

Whiplash subjects had significantly smaller oropharynx CSAs (P < 0.001) and shape ratios (P < 0.001) compared with healthy controls. Self-reported levels of pain and disability and duration of symptoms were not associated with size and shape of the oropharynx in whiplash subjects (P = 0.75 and P = 0.99, respectively). Age and BMI did influence the size (P = 0.01) and shape of the oropharynx (P < 0.001) in the whiplash subjects, but only 20 to 30 percent of the variance could be explained by these factors.

CONCLUSION

Significant difference in the size and shape of the oropharynx was noted in subjects with chronic whiplash compared with controls. Future studies are required to investigate the relationships between oropharynx morphometry and symptoms in patients with chronic whiplash.

Influence of anthropometry on the kinematics of the cervical spine and the risk of injury in sled tests in female volunteers

The objective of this study was to investigate the influence of anthropometric data on the kinematics of the cervical spine and the risk factors for sustaining a neck injury during rear-end collisions occurring in a sled test. A rear-end collision with a velocity change (DeltaV) of 6.3 km/h was simulated in a sled test with eight healthy female subjects. The study analysed the association of anthropometric data with the initial distance between the head and the head restraint, defined kinematic characteristics, the neck injury criterion (NIC) and the neck injury criterion minor (NICmin). The head circumference is negatively associated (r=-0.598) with the initial distance between the head and the head restraint, the maximal head extension (r=-0.687) and the maximal dorsal angular head acceleration (r=-0.633). The body weight (r=0.800), body height (r=0.949) and thorax circumference (r=0.632) are positively associated with the maximal ventral head translation. The neck length correlates positively with the NIC (r=0.826) and negatively with the NICmin (r=-0.797). Anthropometric factors influence the kinematics of the cervical spine and the risk of injury. A high risk of injury may be assumed for individuals with a small head circumference, long neck, tall body height and high body weight.

Head-turned postures increase the risk of cervical facet capsule injury during whiplash

STUDY DESIGN

In vitro experiments using cadaveric cervical spine motion segments to quantify facet capsular ligament strain during whiplash-like loading.

OBJECTIVE

To quantify facet capsule strains during whiplash-like loading with an axial intervertebral prerotation simulating an initial head-turned posture and to then compare these strains to previously-published strains for partial failure and gross failure of the facet capsule for these specimens.

SUMMARY OF BACKGROUND DATA

Clinical data have shown that a head-turned posture at impact increases the severity and duration of whiplash-related symptoms.

METHODS

Thirteen motion segments were used from 7 women donors (50 +/- 10 years). Axial pretorques (+/-1.5 Nm), axial compressive preloads (45, 197, and 325 N), and quasi-static shear loads (posteriorly-directed horizontal forces from 0 to 135 N) were applied to the superior vertebral body to simulate whiplash kinematics with the head turned. Three-dimensional displacements of markers placed on the right facet capsular ligament were used to estimate the strain field in the ligament during loading. The effects of pretorque direction, compression, and posterior shear on motion segment motion and maximum principal strain in the capsule were examined using repeated-measures analyses of variance.

RESULTS

Axial pretorque affected peak capsule strains more than axial compression or posterior shear. Peak strains reached 34% +/- 18% and were higher for pretorques toward rather than away from the facet capsule (i.e.-, head rotation to the right caused higher strain in the right facet capsule).

CONCLUSION

Compared to previously-reported data for these specimens, peak capsule strains with a pretorque were double those without a pretorque (17% +/- 6%) and not significantly different from those at partial failure of the ligament (35% +/- 21%). Thus a head-turned posture increases facet capsular ligament strain compared to a neutral head posture-a finding consistent with the greater symptom severity and duration observed in whiplash patients who have their head turned at impact.

Are cervical multifidus muscles active during whiplash and startle? An initial experimental study

BACKGROUND

The cervical multifidus muscles insert onto the lower cervical facet capsular ligaments and the cervical facet joints are the source of pain in some chronic whiplash patients. Reflex activation of the multifidus muscle during a whiplash exposure could potentially contribute to injuring the facet capsular ligament. Our goal was to determine the onset latency and activation amplitude of the cervical multifidus muscles to a simulated rear-end collision and a loud acoustic stimuli.

METHODS

Wire electromyographic (EMG) electrodes were inserted unilaterally into the cervical multifidus muscles of 9 subjects (6M, 3F) at the C4 and C6 levels. Seated subjects were then exposed to a forward acceleration (peak acceleration 1.55 g, speed change 1.8 km/h) and a loud acoustic tone (124 dB, 40 ms, 1 kHz).

RESULTS

Aside from one female, all subjects exhibited multifidus activity after both stimuli (8 subjects at C4, 6 subjects at C6). Neither onset latencies nor EMG amplitude varied with stimulus type or spine level (p > 0.13). Onset latencies and amplitudes varied widely, with EMG activity appearing within 160 ms of stimulus onset (for at least one of the two stimuli) in 7 subjects.

CONCLUSION

These data indicate that the multifidus muscles of some individuals are active early enough to potentially increase the collision-induced loading of the facet capsular ligaments.

Cervical facet joint dysfunction: a review

OBJECTIVE

To review the relevant literature on cervical facet joint dysfunction and determine findings regarding its anatomy, etiology, prevalence, clinical features, diagnosis, and treatment.

DATA SOURCES

A computer-aided search of several databases was performed, including Medline (1966 to present), Ovid (1966 to present), and the Cochrane database (1993 to present).

STUDY SELECTION

Selected articles had the following criteria: (1) all articles analyzed cervical facet joint pain-anatomy, prevalence, etiology, diagnosis, treatment; (2) only full, published articles were studied, not abstracts; and (3) all articles were published in English.

DATA EXTRACTION

All articles were critically evaluated and included the following categories: randomized controlled trials, meta-analyses, uncontrolled clinical trials, uncontrolled comparison studies, nonquantitative systematic reviews, and literature-based reviews.

DATA SYNTHESIS

We examined 45 references that consisted of 44 journal articles and relevant sections from 1 textbook. Cervical facet joints have been well established in the literature as a common nociceptive pain generator, with an estimated prevalence that ranges from 25% to 66% of chronic axial neck pain. No studies have reported clinical examination findings that are diagnostic for cervical facet mediated pain.

CONCLUSIONS

Overall the literature provides very limited information regarding the treatment of this condition, with only radiofrequency neurotomy showing evidence of effectively reducing pain from cervical facet joint dysfunction.

Side impact causes multiplanar cervical spine injuries

BACKGROUND

Side impact may cause neck and upper extremity pain, paresthesias, and impaired neck motion. No studies have quantified the cervical spine mechanical instability and injury threshold acceleration due to side impact. The goals of the present study were to identify and quantify cervical spine soft tissue injury and the injury threshold acceleration for side impact, and to compare these results with previous findings.

METHODS

Six human cervical spine specimens (C0-T1) underwent 3.5, 5, 6.5, and 8 g impacts. Pre- and postimpact flexibility tests were performed. Soft tissue injury was defined as a significant increase (p < 0.05) in the average intervertebral flexibility above the baseline 2 g impact. The injury threshold was the lowest T1 horizontal peak acceleration that caused the injury.

RESULTS

The injury threshold acceleration was 6.5 g, with injuries occurring at C4-C5 through C7-T1 in flexion, axial rotation, or left lateral bending. After 8 g, three-plane injury was observed at C4-C5 and C6-C7, whereas two-plane injury occurred at C3-C4 in flexion and left lateral bending and at C5-C6 and C7-T1 in axial rotation and left lateral bending.

CONCLUSIONS

Side impact caused multiplanar injuries at C3-C4 through C7-T1 and significantly greater injury at C6-C7, as compared with head-forward rear impact.

Chronic whiplash and whiplash-associated disorders: an evidence-based approach

Whiplash is neck pain experienced as a result of a motor vehicle collision or similar trauma. Following a motor vehicle collision, 15% to 40% of patients with acute neck pain develop chronic neck pain. The cervical facet joint is the most common source of chronic neck pain after whiplash injury, followed by disk pain. Some patients experience pain from both structures. Initial management recommendations need not be directed toward an exact structural cause, but treatment includes advising the patient to remain active, prescribing medications when necessary, and providing advice regarding the generally favorable outcome. When neck pain persists, the physician should recommend medial branch blocks of the dorsal rami of the spinal nerves that supply the putative painful facet joint or joints; this is done to determine whether the facet joints are the cause of pain. When significant relief occurs on two occasions, radiofrequency neurotomy typically provides substantial relief for approximately 8 to 12 months and can be repeated indefinitely as needed. Occasionally, long-term treatment with medication may be indicated. Anterior cervical diskectomy and fusion is necessary on rare occasions.

Risk of injury of the cervical spine in sled tests in female volunteers

BACKGROUND

The description of cervical spine motion and the risk to sustain a cervical spine injury is mainly based on cadaver studies. As the active influence of soft tissue is neglected in cadaver studies, our understanding of the kinematic model for whiplash is limited. Therefore the objective of this study was, to describe the in vivo cervical spine motion and acceleration during rear-end sled collisions to identify sequences of motion in which the risk of injury to the cervical spine is increased.

METHODS

A rear-end collision with a speed change (DeltaV) of 6.3 km/h was simulated in a sled test with eight female subjects with no history of prior injury or pain of the cervical spine. A high-speed camera was used to document motion data. Acceleration data were recorded using accelerometers. Acceleration input to the cervical spine was assessed by the simultaneous analysis of the head angle and angular head acceleration as well as the simultaneous analysis of the relative motion and the relative acceleration between the head and T1 to define intervals of increased risk of cervical spine injuries during rear-end collision.

FINDINGS

The motion sequence is characterized by the same phases that have already been described for male volunteers. Increasing angular head acceleration can explain facet joint injuries during the extension movement (100-120 ms) and hence occur about 50 ms later than shown in cadaver models. In the late rebound the combination of maximal ventral head acceleration and head movement is underestimated and can be responsible for soft tissue injuries.

INTERPRETATION

The study shows that during the extension phase and the late rebound phase, acceleration and movement pattern occur that could lead to cervical spine injuries.

Incidence and outcome of whiplash injury after multiple trauma

STUDY DESIGN

A retrospective study of 101 consecutive polytrauma patients with regard to whiplash injury.

OBJECTIVES

To investigate the incidence and evaluate long-term outcome of whiplash injury following high-energy trauma.

SUMMARY OF BACKGROUND DATA

Chronic whiplash injury has been widely reported in the literature, following low-energy trauma. Very few studies exist on whiplash injury following high-energy trauma.

METHODS

A total of 101 consecutive polytrauma patients admitted to our Level I Trauma Center over a 2-year period, fulfilling the inclusion criteria (age >18 years, high-energy trauma [a fall from a height >2 m, road traffic accidents with speed >30 km/h], and Injury Severity Score >16), were assessed. Whiplash injury was defined according to Quebec Task Force guidelines. The study group (n = 13) included patients who developed whiplash injury symptoms and the control group (n = 88) those who did not. The Neck Disability Index was calculated as an outcome measure for patients complaining of whiplash injury symptoms. The mean follow-up was 17 months. The chi2 and Student t tests were used for the statistical analysis (SPSS 12.1; SPSS, Inc., Chicago, IL).

RESULTS

Only 13 out of 101 patients (1 female/12 male) (13%) complained of whiplash injury. There was a significantly higher rate of neck pain at triage (P < 0.001) and higher combined mean of Abbreviated Injury Score of upper torso (P < 0.0001) in the study group, elucidating the cause of whiplash injury. The Neck Disability Index was <24 points, indicating only mild-to-moderate disability in these patients. Whiplash injury incidence in this study (13%) was similar to the incidence of neck pain in the general population.

CONCLUSIONS

The incidence of whiplash injury following polytrauma was found to be low in our study. There is no dose-response relation between magnitude of trauma severity and incidence of whiplash injury.

Intracranial Hypotension Following Motor Vehicle Accident: An Overlooked Cause of Post-Traumatic Head and Neck Pain?

Motor vehicle accidents result in many patients with chronic head and neck pain, some of which meet the criteria for a "whiplash syndrome." The cervical zygapophysial joint synovium, muscular, and ligamentous strains and other anatomical sites are often implicated in the pathophysiology of these cases. Some patients have a characteristic constellation of vague neurological symptoms, often including headache, posterior neck discomfort, dizziness, nausea, and sometimes visual changes. Recently presented research has noted that some patients who have a whiplash-associated disorder have imaging findings consistent with a low-pressure cerebrospinal fluid leak. Some of these patients respond favorably to high-volume epidural blood patch. The following case presentation focuses on the differential diagnosis of a post-traumatic headache syndrome, specific imaging findings, and treatment strategies.

Cervical facet capsular ligament yield defines the threshold for injury and persistent joint-mediated neck pain

The cervical facet joint has been identified as a source of neck pain, and its capsular ligament is a likely candidate for injury during whiplash. Many studies have shown that the mechanical properties of ligaments can be altered by subfailure injury. However, the subfailure mechanical response of the facet capsular ligament has not been well defined, particularly in the context of physiology and pain. Therefore, the goal of this study was to quantify the structural mechanics of the cervical facet capsule and define the threshold for altered structural responses in this ligament during distraction. Tensile failure tests were preformed using isolated C6/C7 rat facet capsular ligaments (n=8); gross ligament failure, the occurrence of minor ruptures and ligament yield were measured. Gross failure occurred at 2.45+/-0.60 N and 0.92+/-0.17 mm. However, the yield point occurred at 1.68+/-0.56 N and 0.57+/-0.08 mm, which was significantly less than gross failure (p<0.001 for both measurements). Maximum principal strain in the capsule at yield was 80+/-24%. Energy to yield was 14.3+/-3.4% of the total energy for a complete tear of the ligament. Ligament yield point occurred at a distraction magnitude in which pain symptoms begin to appear in vivo in the rat. These mechanical findings provide insight into the relationship between gross structural failure and painful loading for the facet capsular ligament, which has not been previously defined for such neck injuries. Findings also present a framework for more in-depth methods to define the threshold for persistent pain and could enable extrapolation to the human response.

Tensile cervical facet capsule ligament mechanics: failure and subfailure responses in the rat

Clinical, epidemiological, and biomechanical studies suggest the involvement of the cervical facet joint in neck pain. Mechanical studies have suggested the facet capsular ligament to be at risk for subfailure tensile injury during whiplash kinematics of the neck. Ligament mechanical properties can be altered by subfailure injury and such loading can induce cellular damage. However, at present, there is no clear understanding of the physiologic context of subfailure facet capsular ligament injury and mechanical implications for whiplash-related pain. Therefore, this study aimed to define a relationship between mechanical properties at failure and a subfailure condition associated with pain for tension in the rat cervical facet capsular ligament. Tensile failure studies of the C6/C7 rat cervical facet capsular ligament were performed using a customized vertebral distraction device. Force and displacement at failure were measured and stiffness and energy to failure were calculated. Vertebral motions and ligament deformations were tracked and maximum principal strains and their directions were calculated. Mean tensile force at failure (2.96 +/- 0.69 N) was significantly greater (p < 0.005) than force at subfailure (1.17 +/- 0.48 N). Mean ligament stiffness to failure was 0.75 +/- 0.27 N/mm. Maximum principal strain at failure (41.3 +/- 20.0%) was significantly higher (p = 0.003) than the corresponding subfailure value (23.1 +/- 9.3%). This study determined that failure and a subfailure painful condition were significantly different in ligament mechanics and findings provide preliminary insight into the relationship between mechanics and pain physiology for this ligament. Together with existing studies, these findings offer additional considerations for defining mechanical thresholds for painful injuries.

Distribution of A-delta and C-fiber receptors in the cervical facet joint capsule and their response to stretch

BACKGROUND

It has been proposed that cervical facet joint capsules are a major source of whiplash pain. However, there is a paucity of neurophysiologic data to support this hypothesis. The purposes of this study were to determine the distribution of A-delta and C-fiber sensory receptors in the facet joint capsule and to test their patterns of response to stretch and related sensory function.

METHODS

Laminectomy from C4 to C7 was performed in seventeen goats, while they were under general anesthesia, to expose the C6 nerve roots. Customized dual bipolar electrodes were used to record neural activity from one of the C6 branches. An 8 or 15-V electrical stimulus was used to provoke receptor activity in nine designated areas on the dorsal part of the C5-C6 facet joint capsule. Receptors were classified on the basis of conduction velocities. The waveform of an identified receptor was set up as a template to determine its neural activity in response to capsular stretch. The characteristics of each single receptor's response to capsular stretch were analyzed to determine its sensory function as a mechanoreceptor or nociceptor.

RESULTS

Two hundred and forty-eight receptors on the dorsal part of the C5-C6 facet joint capsule were evoked by electrical stimulation in the seventeen goats. More C-fiber receptors were found on the dorsolateral aspect of the facet joint capsule, where tendons and muscles were attached. The response to stretch of 120 receptors, from twelve goats, were analyzed to classify them into one of four categories (high-threshold mechanoreceptors, non-saturated low-threshold mechanoreceptors, saturated low-threshold mechanoreceptors, and silent receptors) or as unclassified receptors.

CONCLUSIONS

The existence of receptors in the facet joint capsule indicates that the capsule has pain and proprioceptive sensory functions.

Mandibular kinematics associated with simulated low-velocity rear-end impacts

Rear-end-impact motor vehicle accidents may result in cervical and temporomandibular-related pain complaints. Head kimematics in simulated low-impact rear-end impacts have been investigated but mandibular kinematics have not been described. Thirty healthy adult subjects underwent three impacts (4.5 m s(-2) expected, 10.0 m s(-2) unexpected, and 10.0 m s(-2) expected). Onset time and peak magnitude of angular head acceleration, angular mandibular acceleration and angular mandibular displacement were measured. Significant mandibular opening acceleration was not identified with rearward head rotation. The peak magnitude of mandibular closing angular acceleration approximately doubled with increased impact magnitude. No differences in peak angular mandibular acceleration regarding expectation were identified. Gender differences were detected in the fast unexpected impact. The peak time for the angular mandibular acceleration (mandibular closure) was approximately 84-120 ms later than peak rearward angular head acceleration for all impacts. Onset and peak times for angular mandibular acceleration (mandibular closure) were similar to the onset and peak times for forward head acceleration. There was also a positive correlation between the magnitude of the forward angular acceleration of the head and angular acceleration of the mandible for the slow (0.65, P = 0.015) and fast expected (0.844, P = 0.001) impacts. The average angular mandibular angular displacement (mandibular closure) was approximately 6 degrees . The hyperextension hypothesis regarding mechanism of temporomandibular joint injury in low-impact rear-end collisions cannot be supported.

Active involvement and intervention in patients exposed to whiplash trauma in automobile crashes reduces costs: a randomized, controlled clinical trial and health economic evaluation

STUDY DESIGN

To examine and compare the costs and consequences in a partial economic evaluation of two competing interventions in patients exposed to whiplash trauma in automobile crashes. The interventions were an active involvement and intervention using early mobilization and a standard intervention of rest, recommended short-term immobilization in a cervical collar and a cautious, gradual self-exercise program according to a leaflet. The study was randomized and controlled.

OBJECTIVES

The aim of the study was to compare the costs of an active involvement and intervention versus a standard intervention and to relate them to the clinical benefits in patients exposed to whiplash trauma in automobile crashes to facilitate decision-making regarding intervention and resource allocation.

SUMMARY OF BACKGROUND DATA

There is very little known about the health economic aspects of various interventions in the target treatment group of patients.

METHODS

Based on a prospective, randomized, clinical trial, data on clinical effectiveness and resources used for the active involvement and intervention and standard intervention were collected for a comparative analysis of the costs related to physical therapy treatment and sick leave. A cost-consequence analysis consisting of a modified cost-effectiveness analysis was used.

RESULTS

The costs were significantly lower after 6 and 36 months with an active involvement and intervention as compared with the standard intervention. The active involvement and intervention were significantly superior in reducing experienced pain and reducing sick leave.

CONCLUSIONS

For patients exposed to whiplash trauma in a motor vehicle collision, an active involvement and intervention were both less costly and more effective than a standard intervention.

Neck injury mechanisms during direct face impact

STUDY DESIGN

Digitized measurements of the intervertebral motions using cervical cineradiographs of 10 volunteers during direct impacts applied to their faces.

OBJECTIVES

To clarify the cervical spine motion during direct face impact and postulate some mechanisms of neck injuries.

SUMMARY OF BACKGROUND DATA

Neck injury occurs mostly in traffic or falling accidents. Hyperextension of the neck is considered the most common mechanism of the injury because most victims have lacerations or contusions on their faces.

METHODS

A low-level backward impact load was applied to 10 healthy male volunteers' faces at the forehead and maxilla via a strap using a free-falling small mass. Cervical vertebral motion was recorded by radiograph cineradiography during the impact.

RESULTS

The upper cervical spine showed a flexion motion for both conditions. Consequently, the cervical spine had an S-shaped curvature similar to that in cervical retraction. Intervertebral motions of the cervical spine were evaluated using an radiograph frame taken at the maximum cervical retraction. For the forehead load, intervertebral motion at C1-C2 was flexion, and motions of the lower cervical spine were extension. For the maxilla load, intervertebral motions from occiput-C1 through C4-C5 were flexion. The inflection point of the curvature was influenced by the impact location.

CONCLUSION

We detected a flexion motion of the upper or middle cervical spine during direct face impact. In an actual accident, if the cervical spine is forced into similar motion, we speculate that neck injury would occur in this retraction-like curvature of the cervical spine.

Effect of head restraint backset on head-neck kinematics in whiplash

Although head restraints were introduced in the 1960s as a countermeasure for whiplash, their limited effectiveness has been attributed to incorrect positioning. The effect of backset on cervical segmental angulations, which were previously correlated with spinal injury, has not been delineated. Therefore, the practical restraint position to minimize injury remains unclear. A parametric study of increasing head restraint backset between 0 and 140mm was conducted using a comprehensively validated computational model. Head retraction values increased with increasing backset, reaching a maximum value of 53.5mm for backsets greater than 60mm. Segmental angulation magnitudes, greatest at levels C5-C6 and C6-C7, reached maximum values during the retraction phase and increased with increasing backset. Results were compared to a previously published head restraint rating system, wherein lower cervical extension magnitudes from this study exceeded mean physiologic limits for restraint positions rated good, acceptable, marginal, and poor. As head restraint contact was the limiting factor in head retraction and segmental angulations, the present study indicates that minimizing whiplash injury may be accomplished by limiting head restraint backset to less than 60mm either passively or actively after impact.

Localized cervical facet joint kinematics under physiological and whiplash loading

OBJECT

Although facet joints have been implicated in the whiplash injury mechanism, no investigators have determined the degree to which joint motions in whiplash are nonphysiological. The purpose of this investigation was to quantify the correlation between facet joint and segmental motions under physiological and whiplash loading.

METHODS

Human cadaveric cervical spine specimens were exercise tested under physiological extension loading, and intact human head-neck complexes were exercise tested under whiplash loading to correlate the localized component motions of the C4-5 facet joint with segmental extension. Facet joint shear and distraction kinematics demonstrated a linear correlation with segmental extension under both loading modes. Facet joints responded differently to whiplash and physiological loading, with significantly increased kinematics for the same-segmental angulation. The limitations of this study include removal of superficial musculature and the limited sample size for physiological testing.

CONCLUSIONS

The presence of increased facet joint motions indicated that synovial joint soft-tissue components (that is, synovial membrane and capsular ligament) sustain increased distortion that may subject these tissues to a greater likelihood of injury. This finding is supported by clinical investigations in which lower cervical facet joint injury resulted in similar pain patterns due to the most commonly reported whiplash symptoms.

Reflex muscle contraction in the unaware occupant in whiplash injury

STUDY DESIGN

Computer modeling and parametric analysis were used to determine the effect of reflex contraction of the neck muscles in the unaware occupant in whiplash.

OBJECTIVE

To delineate effects of reflex contraction on spinal segmental kinematics during the retraction phase.

SUMMARY OF BACKGROUND DATA

The ability of reflex neck muscle contraction to mitigate whiplash injury in the unaware occupant remains unclear. Analyzing relative timing between electromyographic and head-neck kinematics, previous investigators theorized that muscle contraction alters spinal kinematics, decreasing injury likelihood. Other investigators suggested that injury occurs during the initial (retraction) phase of head-neck kinematics, before significant muscle force generation.

METHODS

Computer modeling was used to determine reflex contraction effects on segmental angulations, implementing parametric analysis techniques to vary reflex delay and impact severity.

RESULTS

Shorter reflex delays had a greater effect on segmental angulations later in the event and at lower impact severities. However, the magnitude of this effect, particularly at higher impact severities and during maximum cervical S-curvature (factors implicated in the whiplash injury mechanism) was minimal, altering segmental angulations by a maximum of 19%.

CONCLUSIONS

Because reflex contraction did not substantially alter spinal kinematics, muscle contraction likely does not initiate in sufficient time to mitigate whiplash injuries that may occur during the retraction phase.

Intervertebral neck injury criterion for prediction of multiplanar cervical spine injury due to side impacts

OBJECTIVE

Intervertebral Neck Injury Criterion (IV-NIC) is based on the hypothesis that dynamic three-dimensional intervertebral motion beyond physiological limits may cause multiplanar injury of cervical spine soft tissues. Goals of this study, using a biofidelic whole human cervical spine model with muscle force replication and surrogate head in simulated side impacts, were to correlate IV-NIC with multiplanar injury and determine IV-NIC injury threshold for each intervertebral level.

METHODS

Using a bench-top apparatus, side impacts were simulated at 3.5, 5, 6.5, and 8 g horizontal accelerations of the T1 vertebra. Pre- and post-impact flexibility testing in three-motion planes measured the soft tissue injury, i.e., significant increase (p < 0.05) in neutral zone (NZ) or range of motion (RoM) at any intervertebral level, above corresponding physiological limit.

RESULTS

IV-NIC in left lateral bending correlated well with total lateral bending RoM (R = 0.61, P < 0.001) and NZ (R = 0.55, P < 0.001). Additionally, the same IV-NIC correlated well with left axial rotation RoM (R = 0.50, P < 0.001). IV-NIC injury thresholds (95% confidence limits) varied among intervertebral levels and ranged between 1.5 (0.6-2.4) at C3-C4 and 4.0 (2.4-5.7) at C7-T1. IV-NIC injury threshold times were attained beginning at 84.5 ms following impact.

CONCLUSIONS

Present results suggest that IV-NIC is an effective tool for determining multiplanar soft tissue neck injuries by identifying the intervertebral level, mode, time, and severity of injury.

Evaluation of head-to-trunk control in whiplash patients using digital CranioCorpoGraphy during a stepping test

We used digital-CranioCorpoGraphy (dCCG) to document whiplash effects on head-on-trunk stabilization while subjects walked on the spot (Fukuda test). Twenty-five healthy subjects were compared with 33 whiplash patients. Whiplash patients were classified as chronic (more than 6 months after injury) and recent (less than 6 months after injury). Clear differences between healthy subjects and patients were seen and three different strategies were detected. Pattern A appeared similar to normals but quantitative analysis showed significant differences with paradoxical head over-stabilization (collar-effect); pattern B decreased head stability; pattern C decreased head stabilization with reduced displacements of the body. Due to the complexity of the data analyzed for each marker, a coefficient of performance (CP) was calculated to simplify the cut-off between normal and pathological tests. CP clearly identified an abnormal stepping pattern. Our findings indicate that firstly dCCG identified a specific vestibular task in whiplash patients, secondly whiplash did not always provoke vestibular involvement and thirdly dCCG could be a tool to discriminate patients with and without vestibular involvement after whiplash.

Neural response of cervical facet joint capsule to stretch: a study of whiplash pain mechanism

Cervical facet joints are implicated as a major source of pain after whiplash injury. The purpose of this study was to investigate the proposed capsule strain injury mechanism of whiplash pain using neurophysiologic methods. Strain thresholds, threshold distribution, saturation strains and afterdischarge responses of capsule neural receptors were characterized in vivo. Goat C5-C6 facet joint capsules were used to identify and characterize capsule receptors in response to controlled uniaxial stretch by recording C6 dorsal rootlet nerve discharge. The joints were stretched at 0.5 mm/sec in a series of tests with 2 mm increments until the capsule ruptured. Ninety-two identified units were responsive to physiologic or noxious stretch while 28 were silent receptors. Among the 50 characterized responsive units, 42 showed low strain thresholds at 10.2+/-4.6% while 8 had high strain thresholds at 47.2+/-9.6%. Further, 35 of the 42 low-threshold units displayed discharge saturation at various strains (44.2+/-16.7%). A significant finding was that twelve low-threshold units exhibited afterdischarge for greater than 30 sec after stretch release at 36.6+/-12.5% strains, and displayed longer-lasting afterdischarge (greater than 4 min) at higher strains (39.0+/-14.4%) with significant difference (p = 0.019) in strains. Two high-threshold units had afterdischarges for greater than 30 sec or 4 min at 50.3+/-5.9% and 57.7+/-10.6% strains, respectively. In addition, the spatial distribution of the 42 low-threshold receptors demonstrated that the receptors on the joint gap were more strain-sensitive, with significantly lower strain thresholds compared to the rostral and caudal regions. No significant difference in strain threshold was observed in the medial-lateral direction. When compared to the reported strains that facet joint capsules experienced in whiplash (35-60%) and the reported capsule subfailure strains (35-67%), the low strain thresholds are substantially lower whereas the high thresholds and afterdischarge strains are within that range. Thus, low threshold units appear to signal proprioception within the physiologic range. High threshold units likely signal nociception (pain sensation) while afterdischarge may signal capsule strain injury and contribute to persistent pain.

Significant Spinal Injury Resulting From Low-Level Accelerations: A Case Series of Roller Coaster Injuries

OBJECTIVES

To describe a cohort of significantly injured roller coaster riders and the likely levels of acceleration at which the injuries occurred, and to compare these data with contemporary efforts to define a lower limit of acceleration below which no significant spinal injury is likely to occur.

DESIGN

A retrospective case series of roller coaster ride-induced significant spinal injuries.

SETTING

Injury incident records and emergency medical service records for the Rattler roller coaster in San Antonio, TX, were evaluated for a 19-month period in 1992 and 1993. Medical records for the more significant injuries were also reviewed and the specific injuries were tabulated, along with the demographics of the cohort.

PARTICIPANTS

There were 932,000 riders of the Rattler roller coaster, estimated to represent between 300,000 and 600,000 individual riders.

INTERVENTIONS

Not applicable.

MAIN OUTCOME MEASURES

Injury incident reports and medical record review.

RESULTS

It is estimated that there were a total of 656 neck and back injuries during the study period, and 39 were considered significant by the study inclusion criteria. Seventy-two percent (28/39) of the injured subjects sustained a cervical disk injury; 71% of these injuries were at C5-6 (15 disk herniations, 5 symptomatic disk bulges) and 54% were at C6-7 (11 disk herniations, 4 symptomatic disk bulges). In the lumbar spine, the most frequent injury was a symptomatic disk bulge (20% of the cohort), followed by vertebral body compression fracture (18%), and L4-5 or L5-S1 disk herniation (13%). Accelerometry testing of passengers and train cars indicated a peak of 4.5 to 5g of vertical or axial acceleration and 1.5g of lateral acceleration over approximately 100ms (0.1s) on both.

CONCLUSIONS

The results of this study suggest that there is no established minimum threshold of significant spine injury. The greatest explanation for injury from traumatic loading of the spine is individual susceptibility to injury, an unpredictable variable.

The role of sternocleidomastoid muscle in simulated low velocity rear-end impacts

The objectives of this study were to evaluate behavior of sternocleidomastoid muscle (SCM) electromyogram (EMG) related to impact velocity, gender, awareness and kinematics of head movement in simulated low velocity rear-end impacts. Twenty-nine healthy adults (17 male) were subjected in random order to three rear-end impacts: Two unexpected impacts causing chair accelerations of 4.5 m/s(2) (slow) and 10.1 m/s(2) (fast) and one 10.1 m/s(2) expected impact. Normalized left and right SCM EMG, linear head acceleration, angular head acceleration and maximum angular head displacement were recorded. The magnitude of normalized SCM EMG peak response ranged 2-3 times higher (P< 0.001) in female subjects than their counterpart male subjects. SCM EMG magnitude was 3-4 times higher (P< 0.001) for the fast unexpected than slow unexpected impacts, but there was no significant difference (P> 0.05) for the fast expected compared to the fast unexpected impacts. The onset time of SCM peak EMG ranged from 78 ms to 114 ms later than peak of linear head acceleration for all groups. Onset time of peak SCM EMG was not significantly different (P> 0.05) than onset of angular acceleration for the slow and fast-unexpected impacts, but onset peak SCM EMG was significantly earlier than peak angular head acceleration (30 ms) (P </= 0.05) for the fast expected impact. SCM EMG magnitude increased with increased impact velocity. Gender differences exist for SCM EMG magnitude. Temporal and amplitude awareness of a simulated impact do not produce different magnitude of SCM EMG response. The temporal relationship between the SCM and angular head acceleration is different from the temporal relationship between the SCM and linear head acceleration.

Effects of abnormal posture on capsular ligament elongations in a computational model subjected to whiplash loading

Although considerable biomechanical investigations have been conducted to understand the response of the cervical spine under whiplash (rear impact-induced postero-anterior loading to the thorax), studies delineating the effects of initial spinal curvature are limited. This study advanced the hypothesis that abnormal curvatures (straight or kyphotic) of the cervical column affect spinal kinematics during whiplash loading. Specifically, compared to the normal lordotic curvature, abnormal curvatures altered facet joint ligament elongations. The quantifications of these elongations were accomplished using a validated mathematical model of the human head-neck complex that simulated three curvatures. The model was validated using companion experiments conducted in our laboratory that provided facet joint kinematics as a function of cervical spinal level. Regional facet joint ligament elongations were investigated as a function of whiplash loading in the four local anatomic regions of each joint. Under the normal posture, greatest elongations occurred in the dorsal anatomic region at the C2-C3 level and in the lateral anatomic region from C3-C4 to C6-C7 levels. Abnormal postures increased elongation magnitudes in these regions by up to 70%. Excessive ligament elongations induce laxity to the facet joint, particularly at the local regions of the anatomy in the abnormal kyphotic posture. Increased laxity may predispose the cervical spine to accelerated degenerative changes over time and lead to instability. Results from the present study, while providing quantified level- and region-specific kinematic data, concur with clinical findings that abnormal spinal curvatures enhance the likelihood of whiplash injury and may have long-term clinical and biomechanical implications.

The effect of trunk flexion in healthy volunteers in rear whiplash-type impacts

STUDY DESIGN

Twenty young, healthy volunteers in a laboratory were subjected to rear-end impacts 4.4, 7.9, 10.9, and 13.1 m/s acceleration with head rotation to right and left.

OBJECTIVES

The purpose of this study was to determine the response of the cervical muscles to increasing low-velocity rear impacts when the head is rotated at the time of impact.

SUMMARY OF BACKGROUND DATA

A previous study of rear impacts with head in neutral posture suggests that the burden of impact is borne primarily by the sternocleidomastoid muscles bilaterally. To improve automobile designs to prevent whiplash injury, we need to understand the response of the cervical muscles to whiplash-type perturbations in other conditions that mimic road collisions, such as when the head is rotated to the right and left at the time of rear-end impact.

METHODS

Triaxial accelerometers recorded the acceleration of the sled, torso at the shoulder level, and head of the participant, while bilateral electromyograms (EMGs) of the sternocleidomastoids, trapezii, and splenii capitis were also recorded on 20 subjects (10 males and 10 females, mean age of 23.6 +/- 3 years)

RESULTS

For participants experiencing a rear-end impact, whether having the head rotated to the left or right at the time of impact, the muscle responses increased with increasing levels of acceleration (P < 0.01). The time to onset and time to peak EMG for all muscles progressively decreased with increasing levels of acceleration (P < 0.01). Which muscle responded most to a whiplash-type neck perturbation was determined by the direction of head rotation. With the head rotated to the left, the right sternocleidomastoid generated 88% of its maximal voluntary contraction EMG (at least triple the response of other muscles). In comparison, the left sternocleidomastoid, both trapezii, and the splenii capitis generated on average only 10 to 30% of their maximal voluntary contraction EMG with head rotated to the left. On the other hand, with the head rotated to the right, the left sternocleidomastoid generated 94% of its maximal voluntary contraction EMG (again, at least triple the response of other muscles).

CONCLUSIONS

If the head is rotated out of neutral posture at the time of rear impact, the injury risk tends to be greater for the sternocleidomastoid muscle contralateral to the side of rotation. Measures to prevent whiplash injury may have to account for the asymmetric response because many whiplash victims are expected to be looking to the left or right at the time of collision.

Cervical muscle response to trunk flexion in whiplash-type lateral impacts

The purpose of this study was to determine the response of the cervical muscles to increasing low-velocity, whiplash-type lateral impacts when the occupant is seated out of the recommended driving position (neutral posture). Twenty healthy volunteers were subjected to left lateral impacts of 4.1, 7.7, 10.5, and 13.7 m/s(2) acceleration, with their trunk flexed by 45 degrees and laterally flexed to the right and left also by 45 degrees at the time of impact. Bilateral electromyograms of the sternocleidomastoids, trapezii, and splenii capitis were recorded. Under these conditions of trunk-flexed postures, in a left lateral impact, muscle responses were of generally low magnitude with the trunk flexed to either the left or right. Even at the highest acceleration of 13.7 m/s(2), all muscles generated less than 37% of their known maximal voluntary contraction electromyogram. Also, in these left lateral impacts, the right splenius capitis showed a greater EMG response than the left splenius capitis regardless of whether the subject was flexed to the right or left at the time of impact. The right splenius capitis (the one contralateral to the left lateral impact direction) was more active than its counterpart. Compared to what is known for EMG responses with an occupant in the neutral posture, the right sternocleidomastoid (usually the most active muscle in a left lateral collision) was significantly less-active with trunk flexion than with neutral posture conditions (P<0.01). In the absence of bodily impact, the flexed trunk posture does not produce a biomechanical response that would increase the likelihood of cervical muscle injury in low velocity lateral impacts, and may lessen the risk of injury for some muscles.

Kinematic and electromyographic response to whiplash-type impacts. Effects of head rotation and trunk flexion: summary of research

Despite the fact that whiplash patients often report they had their head rotated or were in a twisted posture at the time of impact, the effect of these postures on the cervical muscle response to impact remains uninvestigated in impact studies. Prior impact studies have positioned the volunteers in the recommended driving position, for example, with head and trunk in a neutral posture. Using an approach of sled impacts with volunteers in very-low velocity impacts to describe the head kinematics and cervical muscle electromyography in response has provided a wealth of data. From this approach, the effect of varying impact direction and level of impact awareness can be discerned without subjecting the volunteers to injury. In part 1 of this review, a further series of results of impacts from eight directions is presented, revealing that the cervical electromyography response to whiplash-type impacts varies according to the presence and direction of head rotation. In part 2, additional data is summarised concerning whiplash-type impacts from 8 directions in the presence of trunk flexion. Contrary to a popular notion, head rotation or trunk flexion at the time of impact are factors that probably reduce injury risk. This data adds to attempts to approach an understanding of the human response to more complex scenarios of low-velocity road collisions.

Biofidelic whole cervical spine model with muscle force replication for whiplash simulation

Whiplash has been simulated using volunteers, whole cadavers, mathematical models, anthropometric test dummies, and whole cervical spines. Many previous in vitro whiplash models lack dynamic biofidelity. The goals of this study were to (1) develop a new dynamic whole cervical spine whiplash model that will incorporate anterior, lateral and posterior muscle force replication, (2) evaluate its performance experimentally and (3) compare the results with in vivo data. To evaluate the new model, rear-impact whiplash simulations were performed using the incremental trauma approach at maximum measured T1 horizontal accelerations of 3.6 g, 4.7 g, 6.6 g, and 7.9 g. The kinematic response of the new model, e.g., peak head-T1 extension and peak intervertebral rotations, were compared with the corresponding in vivo data. The average peak head-T1 extension was within the in vivo corridor during the 3.6 g whiplash simulation (9.1 kph delta V). The peak in vivo intervertebral rotations obtained during a 4.6 g whiplash simulation of a young volunteer were within, or only marginally in excess of, the 95% confidence limits of the average peak intervertebral rotations measured during the 4.7 g whiplash simulation of the present study. Thus, the new whole cervical spine model with muscle force replication produced biofidelic dynamic responses to simulated whiplash. The new model is capable of generating important biomechanical data that may help improve our understanding of whiplash injuries and injury mechanisms.

Cervical intervertebral disc injury during simulated frontal impact

Cervical disc injury due to frontal impact has been observed in both clinical and biomechanical investigations; however, there is a lack of data that elucidate the mechanisms of disc injury during these collisions. The goals of the current study were to determine the peak dynamic disc annular tissue strain and disc shear strain during simulated frontal impact of the whole human cervical spine model with muscle force replication at 4 g, 6 g, 8 g and 10 g horizontal accelerations of the T1 vertebra. These data were compared with those obtained during physiological loading, and with previously reported rear impact data. Peak disc shear strain and peak annular tissue strain during frontal impact exceeded (p<0.05) corresponding physiological limits at the C2-C3 intervertebral level, beginning at 4 g and 6 g, respectively. These subsequently spread throughout the entire cervical spine at 10 g, with the exception of C4-C5. The C5-C6 intervertebral level was at high risk for injury during both frontal and rear impacts, while during frontal impact, in addition to C5-C6, subfailure injuries were likely at superior intervertebral levels, including C2-C3. The disc injuries occurred at lower impact accelerations during rear impact as compared with frontal impact. The subfailure injuries of the cervical intervertebral disc that occur during frontal impact may lead to the chronic symptoms reported by patients, such as head and neck pain.