The anatomy and biomechanics of acute and chronic whiplash injury

The tolerance of the human body to automobile collision impact - a systematic review of injury biomechanics research, 1990-2009

Road traffic injuries account for 1.3 million deaths per year world-wide. Mitigating both fatalities and injuries requires a detailed understanding of the tolerance of the human body to external load. To identify research priorities, it is necessary to periodically compare trends in injury tolerance research to the characteristics of injuries occurring in the field. This study sought to perform a systematic review on the last twenty years of experimental injury tolerance research, and to evaluate those results relative to available epidemiologic data. Four hundred and eight experimental injury tolerance studies from 1990-2009 were identified from a reference index of over 68,000 papers. Examined variables included the body regions, ages, and genders studied; and the experimental models used. Most (20%) of the publications studied injury to the spine. There has also been a substantial volume of biomechanical research focused on upper and lower extremity injury, thoracic injury, and injury to the elderly - although these injury types still occur with regularity in the field. In contrast, information on pediatric injury and physiological injury (especially in the central nervous system) remains lacking. Given their frequency of injury in the field, future efforts should also include improving our understanding of tolerances and protection of vulnerable road users (e.g., motorcyclists, pedestrians).

[Expert evidence in whiplash injury: interdisciplinary orthopaedic and biomechanical approach]

Considering the controversially discussed issue of whiplash injury a pragmatic approach based on our own experience in the area of forensic expert opinion is presented. Findings of accident analysis and biomechanics are correlated with the individual situation after the accident (initial clinical appearance), the course of the ailment and the indispensable physical examination. The latter leads to determination of the individual vulnerability (not increased/increased) which is important for the evaluation of the physical condition and estimation of the physical stress limit. These limits vary widely between individuals and must be considered carefully when relating dose and effect of accident severity to a possible physical injury. Determination of the accident severity is especially important when there are no objective signs of injury and the existence of a minor whiplash injury (Quebec Task Force degree 1 or 2) is in question.

Chronic whiplash pain

Although most patients recover from acute whiplash injuries, those with chronic whiplash syndrome develop signs of central nervous system (CNS) amplification of pain and have a poor prognosis. In this context, specific pain generators from acute whiplash have been identified through clinical, biomechanical, and animal studies. This article gives a clinical perspective on current understanding of these pain generators, including the phenomenon of CNS sensitization.

Out-of-Position Rear Impact Tissue-Level Investigation Using Detailed Finite Element Neck Model

OBJECTIVE

Whiplash injuries can occur in automotive crashes and may cause long-term health issues such as neck pain, headache, and visual and auditory disturbance. Evidence suggests that nonneutral head posture can significantly increase the potential for injury in a given impact scenario, but epidemiological and experimental data are limited and do not provide a quantitative assessment of the increased potential for injury. Although there have been some attempts to evaluate this important issue using finite element models, none to date have successfully addressed this complex problem.

METHODS

An existing detailed finite element neck model was evaluated in nonneutral positions and limitations were identified, including musculature implementation and attachment, upper cervical spine kinematics in axial rotation, prediction of ligament failure, and the need for repositioning the model while incorporating initial tissue strains. The model was enhanced to address these issues and an iterative procedure was used to determine the upper cervical spine ligament laxities. The neck model was revalidated using neutral position impacts and compared to an out-of-position cadaver experiment in the literature. The effects of nonneutral position (axial head rotation) coupled with muscle activation were studied at varying impact levels.

RESULTS

The laxities for the ligaments of the upper cervical spine were determined using 4 load cases and resulted in improved response and predicted failure loads relative to experimental data. The predicted head response from the model was similar to an experimental head-turned bench-top rear impact experiment. The parametric study identified specific ligaments with increased distractions due to an initial head-turned posture and the effect of active musculature leading to reduced ligament distractions.

CONCLUSIONS

The incorporation of ligament laxity in the upper cervical spine was essential to predict range of motion and traumatic response, particularly for repositioning of the neck model prior to impact. The results of this study identify a higher potential for injury in out-of-position rear collisions and identified at-risk locations based on ligament distractions. The model predicted higher potential for injury by as much as 50% based on ligament distraction for the out-of-position posture and reduced potential for injury with muscle activation. Importantly, this study demonstrated that the location of injury or pain depends on the initial occupant posture, so that both the location of injury and kinematic threshold may vary when considering common head positions while driving.

Thrombospondin-4 and excitatory synaptogenesis promote spinal sensitization after painful mechanical joint injury

Facet joint injury induces persistent pain that may be maintained by structural plasticity in the spinal cord. Astrocyte-derived thrombospondins, especially thrombospondin-4 (TSP4), have been implicated in synaptogenesis and spinal sensitization in neuropathic pain, but the TSP4 response and its relationship to synaptic changes in the spinal cord have not been investigated for painful joint injury. This study investigates the role of TSP4 in the development and maintenance of persistent pain following injurious facet joint distraction in rats and tests the hypothesis that excitatory synaptogenesis contributes to such pain. Painful facet joint loading induces dorsal horn excitatory synaptogenesis along with decreased TSP4 in the DRG and increased astrocytic release of TSP4 in the spinal cord, all of which parallel the time course of sustained tactile allodynia. Blocking injury-induced spinal TSP4 expression with antisense oligonucleotides or reducing TSP4 activity at its neuronal receptor in the spinal cord with gabapentin treatment both attenuate the allodynia and dorsal horn synaptogenesis that develop after painful facet joint loading. Increased spinal TSP4 also facilitates the development of allodynia and spinal hyperexcitability, even after non-painful physiological loading of the facet joint. These results suggest that spinal TSP4 plays an important role in the development and maintenance of persistent joint-mediated pain by inducing excitatory synaptogenesis and facilitating the transduction of mechanical loading of the facet joint that leads to spinal hyperexcitability.

Symptoms, disabilities, and life satisfaction five years after whiplash injuries

Background Chronic whiplash-associated disorders (WADs) are often associated with social functioning problems and decreased ability to perform previous activities. This may lead to decreased life satisfaction, which is insufficiently studied in the context of whiplash injuries. Symptoms included in chronic WAD are similar to symptoms frequently reported by persons who have sustained mild traumatic brain injury (MTBI)/concussion. In cases of MTBI, the severity and number of symptoms have been suggested to have a diagnostic value. The corresponding importance of symptoms in chronic WAD has not been documented. Most studies of whiplash injuries have focused on neck pain because this is the dominant complaint, while other symptoms are less studied. The frequency of long-term symptoms after whiplash injuries seems to vary. It is difficult to compare the long-term outcome since the follow-up after whiplash injury in most studies has been rather short. Therefore, the primary aim of this investigation was to study neck pain and other symptoms, disability, and life satisfaction five years after whiplash injury in a defined population and geographical area. Methods The study was carried out at a public hospital in northern Sweden and was a cross-sectional survey of patients five years after the injury event in a cohort of whiplash-injured patients. Five years after the emergency department visit, 186 persons aged 18-64 answered questionnaires on symptoms (Rivermead Post-Concussion Symptoms Questionnaire, RPQ), disabilities (Rivermead Head Injury Follow Up Questionnaire, RHFUQ), and life satisfaction (LiSat-11). The answers were compared to those of a comparison cohort. Results The most common symptoms five years after whiplash injury were fatigue (41%), poor memory (39%), and headache (37%). Inability to sustain previous workload (44%) and fatigue at work (43%) were frequently reported disabilities. Only 39% were satisfied with their somatic health and 60% with their psychological health. Compared with healthy controls, the whiplash injured exhibited more symptoms and had lower life satisfaction. Women reported significantly higher pain intensity than men. Few significant differences between women and men regarding the other parameters were found. Conclusions This study shows that five years after a whiplash injury, patients reported symptoms that are typical of mild traumatic brain injury. Further, this study emphasizes the possibility of screening patients with chronic WAD for these symptoms as a complement to the assessment. Implications Untreated symptoms may negatively affect the outcome of pain rehabilitation. This implies that it might be clinically meaningful to quantify symptoms earlier in the rehabilitation process.

Failure Properties and Damage of Cervical Spine Ligaments, Experiments and Modeling

Cervical spine ligaments have an important role in providing spinal cord stability and restricting excessive movements. Therefore, it is of great importance to study the mechanical properties and model the response of these ligaments. The aim of this study is to characterize the aging effects on the failure properties and model the damage of three cervical spine ligaments: the anterior and the posterior longitudinal ligament and the ligamentum flavum. A total of 46 samples of human cadaveric ligaments removed within 24–48 h after death have been tested. Uniaxial tension tests along the fiber direction were performed in physiological conditions. The results showed that aging decreased the failure properties of all three ligaments (failure load, failure elongation). Furthermore, the reported nonlinear response of cervical ligaments has been modeled with a combination of the previously reported hyperelastic and damage model. The model predicted a nonlinear response and damage region. The model fittings are in agreement with the experimental data and the quality of agreement is represented with the values of the coefficient of determination close to 1.

Finite element modeling of potential cervical spine pain sources in neutral position low speed rear impact

The rate of soft tissue sprain/strain injuries to the cervical spine and associated cost continue to be significant; however, the physiological nature of this injury makes experimental tests challenging while aspects such as occupant position and musculature may contribute to significant variability in the current epidemiological data. Several theories have been proposed to identify the source of pain associated with whiplash. The goal of this study was to investigate three proposed sources of pain generation using a detailed numerical model in rear impact scenarios: distraction of the capsular ligaments; transverse nerve root compression through decrease of the intervertebral foramen space; and potential for damage to the disc based on the extent of rotation and annulus fibre strain. There was significant variability associated with experimental measures, where the range of motion data overlapped ultimate failure data. Average data values were used to evaluate the model, which was justified by the use of average mechanical properties within the model and previous studies demonstrating predicted response and failure of the tissues was comparable to average response values. The model predicted changes in dimension of the intervertebral foramen were independent of loading conditions, and were within measured physiological ranges for the impact severities considered. Disc response, measured using relative rotation between intervertebral bodies, was below values associated with catastrophic failure or avulsion but exceeded the average range of motion values. Annulus fibre strains exceeded a proposed threshold value at three levels for 10g impacts. Capsular ligament strain increased with increasing impact severity and the model predicted the potential for injury at impact severities from 4g to 15.4g, when the range of proposed distraction corresponding to sub-catastrophic failure was exceeded, in agreement with the typically reported values of 9-15g. This study used an enhanced neck finite element model with active musculature to investigate three potential sources of neck pain resulting from rear impact scenarios and identified capsular ligament strain and deformation of the disc as potential sources of neck pain in rear impact scenarios.

Muscle pathologies after cervical spine distortion-like exposure--a porcine model

OBJECTIVE

Histological evaluation of porcine posterior cervical muscles after a forceful translational and extensional head retraction simulating high-speed rear end impact.

METHODS

Four anesthetized pigs were exposed to a cervical spine distortion (CSD)-like motion in a lying position. After 2 different survival times of 4 and 6 h (posttrauma), the pigs were euthanized and tissue sampling of posterior cervical muscles was performed. A standard histological staining method involving paraffin-embedded sections was used to analyze the muscles, focusing on injury signs like hemorrhage and inflammatory cell reaction. A pig that was not subjected to impact was used as a control pig and was subjected to the same procedure to exclude any potential artifacts from the autopsy.

RESULTS

The differentiation of 8 different posterior neck muscles in the dissection process was successful in more than 50 percent for each muscle of interest. Staining and valid analysis was possible from all extracted samples. Muscle injuries to the deepest posterior neck muscles could be found, especially in the musculus obliquus samples, which showed laminar bleedings in 4 out of 4 samples. In addition, in 4 out of 4 samples we were able to see increased cellular reactions. The splenius muscle also showed bleeding in all 4 samples. All animals showed muscle injury signs in more than three quarters of analyzed neck muscles. Differences between survival times of 4 and 6 h in terms of muscular injury were not of primary interest and could not be found.

CONCLUSIONS

By simulating a CSD-like motion we were able to confirm injuries in the posterior cervical muscles under severe loading conditions. Further studies need to be conducted to determine whether these muscle injuries also occur under lower exposure forces.

The startle response during whiplash: a protective or harmful response?

Whiplash injuries are common following rear-end collisions. During such collisions, initially relaxed occupants exhibit brisk, stereotypical muscle responses consisting of postural and startle responses that may contribute to the injury. Using prestimulus inhibition, we sought to determine if the startle response elicited during a rear-end collision contributes to head stabilization or represents a potentially harmful overreaction of the body. Three experiments were performed. In the first two experiments, two groups of 14 subjects were exposed to loud tones (124 dB) preceded by prestimulus tones at either four interstimulus intervals (100-1,000 ms) or five prestimulus intensities (80-124 dB). On the basis of the results of the first two experiments, 20 subjects were exposed to a simulated rear-end collision (peak sled acceleration = 2 g; speed change = 0.75 m/s) preceded by one of the following: no prestimulus tone, a weak tone (85 dB), or a loud tone (105 dB). The prestimulus tones were presented 250 ms before sled acceleration onset. The loud prestimulus tone decreased the amplitude of the sternocleidomastoid (16%) and cervical paraspinal (29%) muscles, and key peak kinematics: head retraction (17%), horizontal head acceleration (23%), and head angular acceleration in extension (23%). No changes in muscle amplitude or kinematics occurred for the weak prestimulus. The reduced muscle and kinematic responses observed with loud tones suggest that the startle response represents an overreaction that increases the kinematics in a way that potentially increases the forces and strains in the neck tissues. We propose that minimizing this overreaction during a car collision may decrease the risk of whiplash injuries.

How can animal models inform on the transition to chronic symptoms in whiplash?

STUDY DESIGN

A nonsystematic review of the literature.

OBJECTIVE

The objective was to present general schema for mechanisms of whiplash pain and review the role of animal models in understanding the development of chronic pain from whiplash injury.

SUMMARY OF BACKGROUND DATA

Extensive biomechanical and clinical studies of whiplash have been performed to understand the injury mechanisms and symptoms of whiplash injury. However, only recently have animal models of this painful disorder been developed based on other pain models in the literature.

METHODS

A nonsystematic review was performed and findings were integrated to formulate a generalized picture of mechanisms by which chronic whiplash pain develops from mechanical tissue injuries.

RESULTS

The development of chronic pain from tissue injuries in the neck due to whiplash involves complex interactions between the injured tissue and spinal neuroimmune circuits. A variety of animal models are beginning to define these mechanisms.

CONCLUSION

Continued work is needed in developing appropriate animal models to investigate chronic pain from whiplash injuries and care must be taken to determine whether such models aim to model the injury event or the pain symptoms.

What occupant kinematics and neuromuscular responses tell us about whiplash injury

STUDY DESIGN

Literature-based review.

OBJECTIVE

To review the published data on occupant kinematic and neuromuscular responses during low-speed impacts and analyze how these data inform our understanding of whiplash injury.

SUMMARY OF BACKGROUND DATA

A stereotypical kinematic and neuromuscular response has been observed in human subjects exposed to rear-end impacts. Combined with various models of injury, these response data have been used to develop anti-whiplash seats that prevent whiplash injury in many, but not all, individuals exposed to a rear-end crash.

METHODS

Synthesis of the literature.

RESULTS

Understanding of the occupant kinematics and neuromuscular responses, combined with data from various seat-related interventions, have shown that differential motion between the superior and inferior ends of the cervical spine is responsible for many whiplash injuries. The number of whiplash injuries not prevented by current anti-whiplash seats suggests than further work remains, possibly related to designing seats that respond dynamically to the occupant and collision properties. Neck muscles alter the head and neck kinematics during the interval in which injury likely occurs, even in initially relaxed occupants. It remains unclear whether muscle activation mitigates or exacerbates whiplash injury. If muscle activation mitigates injury, then advance warning could be used to help occupant tense their muscles before impact. Alternatively, if muscle activation exacerbates whiplash injury, then a loud preimpact sound that uncouples the startle and postural components of the muscle response could reduce peak muscle activation during a whiplash exposure.

CONCLUSION

Our improved understanding of whiplash injury has led to anti-whiplash seats that have prevented many whiplash injuries. Further work remains to optimize these and possibly other systems to further reduce the number of whiplash injuries.

[Assessment of whiplash and cervical spine injury]

Formulating an expert opinion on whiplash injuries requires that consideration be given to biomechanical, orthopedic, neurological, psychiatric and medicolegal aspects. The greatest difficulties are encountered in cases of mild whiplash where patients complain of constant pain without any physical correlative. Diverse assessments and principles for approving a claim are reflected in the fact that the prevalence of chronic spine pain after whiplash injuries (late whiplash syndrome) varies between 16% and 71% in different countries, and the proportion of whiplash injuries involved in petitions for compensation differs greatly across Europe (UK 75%, Germany 47%, Finland 8.5% and France 3% of all personal injuries).Important biomechanical, orthopedic, neurological, psychiatric and medicolegal aspects of expert testimony on whiplash associated disorders (WAD) are delineated.

Whiplash-associated disorder: musculoskeletal pain and related clinical findings

The aim of this paper was to review the physical and psychological processes associated with whiplash-associated disorders. There is now much scientific data available to indicate the presence of disturbed nociceptive processing, stress system responses, muscle and motor changes as well as psychological factors in both acute and chronic whiplash-associated disorders. Some of these factors seem to be associated with the transition from acute to chronic pain and have demonstrated prognostic capacity. Further investigation is required to determine if these processes can be modified and if modification will lead to improved outcomes for this condition. The burden of whiplash injuries, the high rate of transition to chronicity, and evidence of limited effects of current management on transition rates demand new directions in evaluation and management. The understanding of processes underlying this condition is improving and this lays the foundation for the development of more effective management approaches.

Facet joint and disc kinematics during simulated rear crashes with active injury prevention systems

STUDY DESIGN

Experimental and computational biomechanical analyses of simulated rear crashes.

OBJECTIVE

The objectives were to determine cervical facet joint and disc kinematics and ligament strains during simulated rear crashes with the Whiplash Protection System (WHIPS) and active head restraint (AHR) and to compare these data with those obtained with no head restraint (NHR).

SUMMARY OF BACKGROUND DATA

Previous biomechanical studies document abnormal cervical facet kinematics and potentially injurious ligament strains during simulated rear crashes with no injury prevention system.

METHODS

A human model of the neck, consisting of a neck specimen mounted to the torso of BioRID II and carrying a surrogate head and stabilized with muscle force replication, was subjected to simulated rear crashes in a WHIPS seat (n = 6, 12.0 g, ΔV 11.4 km/h) or AHR seat and subsequently with NHR (n = 6: 11.0 g, ΔV 10.2 km/h with AHR; 11.5 g, ΔV 10.7 km/h with NHR). Lower cervical spine facet and disc motions and ligament strains during the crashes were computed and average peak values statistically compared (P < 0.05) between WHIPS, AHR, and NHR.

RESULTS

Average peak facet and disc translations and ligament strains could not be statistically differentiated between WHIPS and AHR or between AHR and NHR. WHIPS significantly reduced peak capsular ligament strain and peak disc separation at C6/C7 as compared with NHR. Facet compression at C6/C7 reached 2.9 mm with WHIPS, 1.9 mm with AHR, and 3.2 mm with NHR.

CONCLUSION

WHIPS and AHR generally reduced peak disc separation and anterior longitudinal ligament strain as compared with NHR. WHIPS and AHR limited capsular strain below the subfailure threshold but did not protect against potential facet joint compression injuries, which may occur during or after contact of the head with the head restraint.

Head restraint backset during routine automobile driving: drivers usually exceed the recommended guidelines

OBJECTIVE

Although several previous studies have evaluated horizontal head restraint backset distances, few studies have evaluated them during driving. The purpose of this study was to measure this backset during routine automobile driving and to specifically evaluate the backset during individual driving tasks such as turning, stopping, starting, and lane changes.

METHODS

Fourteen subjects drove around a specified route through the city of Guelph, Ontario, Canada, that included residential, thruway, and highway driving; additional minor driving tasks, such as lane changes, were evaluated. The distance of head restraint to posterior aspect of the head was measured continuously throughout the drive using an ultrasonic measurement system. The timing of specific tasks was documented using a video camera.

RESULTS

The average head-to-restraint distance throughout the driving route was 78.1 mm (standard deviation [SD] 24.8 mm); this distance did not vary significantly between the global measures during various driving areas (residential, thruway, and highway). We observed that the head restraint backsets during right turns (93.6 mm; SD 34.8 mm) were significantly larger compared to the other driving tasks (p < .001). The 7 males and 7 females showed similar backset distances: 84.52 mm (SD 12.08) and 71.68 mm (SD 5.53), respectively (p = .0785).

CONCLUSIONS

We observed that most subjects maintain a relatively consistent head-to-restraint distance throughout their driving route; 2 subjects adopted very large head restraint backset distances throughout their drive and 2 others adopted very small head restraint backset distances-this appears to reflect driver posture. Twelve of 14 subjects had average backset distances that exceed the National Highway Traffic Safety Administration and the Federal Motor Vehicle Safety Standard guidelines, indicating that most drivers may be at risk for whiplash-like disorders if exposed to a rear impact while driving. Of the monitored driving tasks, turning, especially right turns, caused drivers to increase their head-to-restraint backset distance.

A case-control study of cerebellar tonsillar ectopia (Chiari) and head/neck trauma (whiplash)

PRIMARY OBJECTIVE

Chiari malformation is defined as herniation of the cerebellar tonsils through the foramen magnum, also known as cerebellar tonsillar ectopia (CTE). CTE may become symptomatic following whiplash trauma. The purpose of the present study was to assess the frequency of CTE in traumatic vs non-traumatic populations.

STUDY DESIGN

Case-control.

METHODS AND PROCEDURES

Cervical MRI scans for 1200 neck pain patients were reviewed; 600 trauma (cases) and 600 non-trauma (controls). Half of the groups were scanned in a recumbent position and half were scanned in an upright position. Two radiologists interpreted the scans for the level of the cerebellar tonsils.

MAIN OUTCOMES AND RESULTS

A total of 1195 of 1200 scans were read. CTE was found in 5.7% and 5.3% in the recumbent and upright non-trauma groups vs 9.8% and 23.3% in the recumbent and upright trauma groups (p = 0.0001).

CONCLUSIONS

The results described in the present investigation are first to demonstrate a neuroradiographic difference between neck pain patients with and without a recent history of whiplash trauma. The results of prior research on psychosocial causes of chronic pain following whiplash are likely confounded because of a failure to account for a possible neuropathologic basis for the symptoms.

WHIPS seat and occupant motions during simulated rear crashes

OBJECTIVE

Objectives of this study were to investigate the motions of Volvo's Whiplash Protection System (WHIPS) seat and occupant during simulated rear crashes of a human model of the neck (HUMON).

METHODS

HUMON consisted of a human neck specimen (n = 6) mounted to the torso of BioRID II and carrying an anthropometric head stabilized with muscle force replication. HUMON was seated and secured in a 2005 Volvo XC90 minivan seat that included WHIPS and a fixed head restraint. Rear crashes of 9.9 g (ΔV 9.2 kph), 12.0 g (ΔV 11.4 kph), and 13.3 g (ΔV 13.4 kph) were simulated and WHIPS and occupant motions were monitored. Linear regression analyses (P < .05) were used to determine relationships between WHIPS and occupant motion peaks using data from all crashes combined.

RESULTS

WHIPS motions consisted of simultaneous rearward and downward translations and extension of the seatback and plastic deformation of the bilateral WHIPS energy-absorbing components. Peak WHIPS motions were linearly correlated only with peak rearward occupant translations. Less rearward pelvis translation was required to cause WHIPS activation as compared to T1 translation.

CONCLUSIONS

WHIPS reduced peak T1 horizontal acceleration by 39 percent compared to sled acceleration. This was within the range previously reported for WHIPS, between 30 and 60 percent, but higher than the 16 percent reduction previously reported due to active head restraint. Absorption of crash energy occurred during the initial 75 ms and the onset of head support occurred at 114 ms. Differential head-torso motions occurred prior to and during head support, indicating the potential for neck injury even with WHIPS.

Differential development of sensory hypersensitivity and a measure of spinal cord hyperexcitability following whiplash injury

Widespread sensory hypersensitivity is present in acute whiplash and is associated with poor recovery. Decreased nociceptive flexion reflex (NFR) thresholds (spinal cord hyperexcitability) are a feature of chronic whiplash but have not been investigated in the acute to chronic injury stage. This study compared the temporal development of sensory hypersensitivity and NFR responses from soon after injury to either recovery or to transition to chronicity. It also aimed to identify predictors of persistent spinal cord hyperexcitability. Pressure and cold pain thresholds, NFR responses (threshold and pain VAS) were prospectively measured in 62 participants at <3 weeks, 3 and 6 months post whiplash injury and in 22 healthy controls on two occasions a month apart. Pain levels and psychological distress (GHQ-28; IES) were measured at baseline. Whiplash participants were classified at 6 months post-injury using the Neck Disability Index: recovered (8%), mild pain and disability (10-28%) or moderate/severe pain and disability (30%). All whiplash groups demonstrated spinal cord hyperexcitability (lowered NFR thresholds) at 3 weeks post-injury. This hyperexcitability persisted in those with moderate/severe symptoms at 6 months but resolved in those who recovered or reported lesser symptoms at 6 months. In contrast generalized sensory hypersensitivity (pressure and cold) was only ever present in those with persistent moderate/severe symptoms and remained unchanged throughout the study period. This suggests different mechanisms underlie sensory hypersensitivity and NFR responses. In multivariate analyses only initial NDI scores (p=0.003) were a unique predictor of persistent spinal cord hyperexcitability indicating possible ongoing peripheral nociception following whiplash injury.

Comparison of the whiplash injury criteria

Whiplash injury criteria are based upon the hypothesis that neck injuries are caused by excessive loads, displacements, or head/T1 relative acceleration and velocity. The objectives of this study were to evaluate and compare the whiplash injury criteria (IV-NIC, NIC, Nkm, Nij, and NDC) during simulated rear impacts of a new Human Model of the Neck (HUMON) with and without an active head restraint (AHR). HUMON consisted of a neck specimen mounted to the torso of BioRID II and carrying an anthropometric head stabilized with muscle force replication. HUMON was seated and secured in a Kia Sedona seat with AHR on a sled. Rear impacts (7.1 and 11.1g) were simulated with the AHR in five different positions followed by an impact with no HR. Statistical differences (P < 0.05) were determined in the peak NIC and NDC due to the AHR, as compared to no HR, and in the peak IV-NIC relative to physiologic limits. Linear regression analyses identified correlation between IV-NIC and NIC, Nkm, Nij, and NDC (R(2) > or = 0.35 and P < 0.001). The AHR caused significant decreases in peak NIC and NDC as compared to no HR. The IV-NIC identified significantly increased motion above the physiologic limit at the middle and lower cervical spine with and without the AHR. Correlation was observed between IV-NIC and NIC, Nkm, Nij, and NDC. Extrapolation using the present correlations and the IV-NIC injury thresholds suggests neck injuries may occur at peak NIC of 14.4m(2)/s(2), Nkm of 0.33, or Nij of 0.09. Nonphysiologic spinal rotation at one or more spinal levels may occur even if head/T1 motions are small.