Effect of rotated head posture on dynamic vertebral artery elongation during simulated rear impact

Vertebral artery compression of the medulla oblongata: A benign radiological finding?

Background:

To the best of our knowledge, no study has documented the natural history of rostral medullary compression of the vertebral artery (RMCVA) as radiological finding. The aim of this study was to explore it.

Methods:

A total of 57 patients with RMCVA and not presenting symptoms of medullary compression syndrome were enrolled. These participants underwent cerebral magnetic resonance imaging with contrast, and 19 of them who were followed for 5.7 ± 1.9 years (range: 3.0–10.3 years) were analyzed in detail. For comparison, clinical courses of two other patients with vertebrobasilar dolichoectasia (VBDE) were presented.

Results:

RMCVA was well delineated in all 57 patients. In the 19 patients analyzed, RMCVA was found in 17 sides on the right and 15 on the left. Moreover, the ventrolateral medulla was the most frequent compression site, and it was found in 69% of cases, with 84.2% presenting as mild compression and 15.8% as considerable compression. During the follow-up period, no patients showed neurological deterioration or radiological progression. In contrast, the two VBDE patients demonstrated both neurological and radiological progressions during the follow-up period.

Conclusion:

Unlike VBDE, RMCVA seems to be a benign condition without progression, even when with a considerable compression. Degree of the compression in RMCVA may not be relevant to the patient’s neurological status.

Muscle fat infiltration following whiplash: A computed tomography and magnetic resonance imaging comparison

Here we present a secondary analysis from a parent database of 97 acutely injured participants enrolled in a prospective inception cohort study of whiplash recovery after motor vehicle collision (MVC). The purpose was to investigate the deep and superficial neck extensor muscles with peri-traumatic computed tomography (CT) and longitudinal measures of magnetic resonance imaging (MRI) in participants with varying levels of whiplash-related disability. Thirty-six underwent standard care imaging of the cervical spine with CT at a level-1 trauma designated emergency department. All 36 participants were assessed with MRI of the cervical spine at <1-week, 2-weeks, 3-, and 12-months post-injury and classified into three groups using initial pain severity and percentage scores on the Neck Disability Index (recovered (NDI of 0–8%), mild (NDI of 10–28%), or severe (NDI ≥ 30%)) at 3-months post MVC. CT muscle attenuation values were significantly correlated to muscle fat infiltration (MFI) on MRI at one-week post MVC. There was no significant difference in muscle attenuation across groups at the time of enrollment. A trend of lower muscle attenuation in the deep compared to the superficial extensors was observed in the severe group. MFI values in the deep muscles on MRI were significantly higher in the severe group when compared to the mild group at 1-year post MVC. This study provides further evidence that the magnitude of 1) deep MFI appears unique to those at risk of and eventually transitioning to chronic WAD and that 2) pre- or peri-traumatic muscular health, determined by CT muscle attenuation, may be contribute to our understanding of long-term recovery.

Head postures during naturalistic driving

OBJECTIVE

A rotated head posture at the time of a rear-end impact is associated with a higher risk of acute and chronic whiplash injury. The objective of this study was to quantify the amplitude and duration of rotated head postures observed in drivers during naturalistic driving.

METHODS

Twenty volunteers (14 males: 36 ± 12 years, 6 females: 27 ± 5 years) drove a 2010 Subaru Impreza on public roads while their 3D head angular position relative to the car was recorded using inertial measurement units. An experimenter rode in the passenger seat (right side) and logged when subjects performed one of 6 head movements: Bilateral shoulder and side mirror checks, looking at the rearview mirror, and looking at the front seat passenger. Video of the subjects was used to confirm the logged head movements and identify movements that the experimenter missed. The duration and amplitude of all 6 head movements were tabulated and then compared between periods when the car was moving and when the car was stationary.

RESULTS

During a 68 ± 5-min drive, subjects performed a median (range) of 15 (5-39) left shoulder checks, 82.5 (29-167) left mirror checks, 40.5 (10-168) rearview mirror checks, 27.5 (3-113) right mirror checks, 60 (0-185) passenger looks, and 12.5 (1-28) right shoulder checks. Peak yaw angles of the head relative to the vehicle for these 6 movements averaged -81.5°, -34.3°, 16.2°, 42.1°, 58.2°, and 84.3°, respectively. Drivers spent a larger proportion of time in nonneutral postures when the vehicle was stopped (17.5%) compared to moving (8.2%) (Z = 3.92, P < .0001). Drivers also moved their head further from neutral during the movements when the car was stationary compared to moving (t₁₉ = 5.90, P < .0001).

CONCLUSIONS

Drivers use larger and longer duration head movements when stationary than when driving. Given an increased risk of whiplash injury for initially rotated head postures, these findings provide a possible explanation for why drivers are more likely to be injured when hit from behind while their vehicle is stationary. Further, the head postures characterized in this study can be used as initial conditions in volunteer and computational studies to improve our understanding of why nonneutral head postures are associated with increased whiplash injury risk.

On the importance of retaining stresses and strains in repositioning computational biomechanical models of the cervical spine

Human body models are created in a specific posture and often repositioned and analyzed without retaining stresses that result from repositioning. For example, repositioning a human neck model within the physiological range of motion to a head-turned posture prior to an impact results in initial stresses within the tissues distracted from their neutral position. The aim of this study was to investigate the effect of repositioning on the subsequent kinetics, kinematics, and failure modes, of a lower cervical spine motion segment, to support future research at the full neck level. Repositioning was investigated for 3 modes (tension, flexion, and extension) and 3 load cases. The model was repositioned and loaded to failure in one continuous load history (case 1), or repositioned then restarted with retained stresses and loaded to failure (case 2). In case 3, the model was repositioned and then restarted in a stress-free state, representing current repositioning methods. Not retaining the repositioning stresses and strains resulted in different kinetics, kinematics, or failure modes, depending on the mode of loading. For the motion segment model, the differences were associated with the intervertebral disc fiber reorientation and load distribution, because the disc underwent the largest deformation during repositioning. This study demonstrated that repositioning led to altered response and tissue failure, which is critical for computational models intended to predict injury at the tissue level. It is recommended that stresses and strains be included and retained for subsequent analysis when repositioning a human computational neck model.

Advancements in Imaging Technology: Do They (or Will They) Equate to Advancements in Our Knowledge of Recovery in Whiplash?

Synopsis It is generally accepted that up to 50% of those with a whiplash injury following a motor vehicle collision will fail to fully recover. Twenty-five percent of these patients will demonstrate a markedly complex clinical picture that includes severe pain-related disability, sensory and motor disturbances, and psychological distress. A number of psychosocial factors have shown prognostic value for recovery following whiplash from a motor vehicle collision. To date, no management approach (eg, physical therapies, education, psychological interventions, or interdisciplinary strategies) for acute whiplash has positively influenced recovery rates. For many of the probable pathoanatomical lesions (eg, fracture, ligamentous rupture, disc injury), there remains a lack of available clinical tests for identifying their presence. Fractures, particularly at the craniovertebral and cervicothoracic junctions, may be radiographically occult. While high-resolution computed tomography scans can detect fractures, there remains a lack of prevalence data for fractures in this population. Conventional magnetic resonance imaging has not consistently revealed lesions in patients with acute or chronic whiplash, a "failure" that may be due to limitations in the resolution of available devices and the use of standard sequences. The technological evolution of imaging techniques and sequences eventually might provide greater resolution to reveal currently elusive anatomical lesions (or, perhaps more importantly, temporal changes in physiological responses to assumed lesions) in those patients at risk of poor recovery. Preliminary findings from 2 prospective cohort studies in 2 different countries suggest that this is so, as evidenced by changes to the structure of skeletal muscles in those who do not fully recover. In this clinical commentary, we will briefly introduce the available imaging decision rules and the current knowledge underlying the pathomechanics and pathophysiology of whiplash. We will then acknowledge known prognostic factors underlying functional recovery. Last, we will highlight emerging evidence regarding the pathobiology of muscle degeneration/regeneration, as well as advancements in neuroimaging and musculoskeletal imaging techniques (eg, functional magnetic resonance imaging, magnetization transfer imaging, spectroscopy, diffusion-weighted imaging) that may be used as noninvasive and objective complements to known prognostic factors associated with whiplash recovery, in particular, poor functional recovery. J Orthop Sports Phys Ther 2016;46(10):861-872. doi:10.2519/jospt.2016.6735.

Fatal subarachnoid hemorrhage associated with internal carotid artery dissection resulting from whiplash trauma

Spinal injury following inertial loading of the head and neck (whiplash) is a common sequel of low speed traffic crashes. A variety of non-musculoskeletal injuries have been described in association with injury to the spine following whiplash trauma, including traumatic brain injury, vestibular derangement, and cranial nerve injury, among others. Vascular injuries in the head and neck have, however, only rarely been described. We present the case of a middle-aged male who sustained an ultimately fatal injury that resulted from injury to the internal carotid artery (ICA) and intracerebral vascular structures following a hard braking maneuver, with no direct head- or neck contact with the vehicular interior. Based on this unusual mechanism of injury we reviewed hospital data from the United States nationwide inpatient database (NIS) to assess the frequency of similar injuries reportedly resulting from traffic crashes. The post-mortem examination revealed a left internal carotid artery dissection associated with subarachnoid hemorrhage (SAH). Based on the close temporal association, the absent prior history, and the plausibility of the injury mechanism, the injury was attributed to the braking maneuver. An analysis of NIS data demonstrated that the prevalence of subarachnoid hemorrhage is significantly higher when there is a traumatic etiology, and higher yet when the trauma is a traffic crash (odds ratio 3.3 and 4.3, respectively). The presented case, together with the hospital inpatient data analysis, indicate that although SAH in combination with ICA dissection is relatively rare, it is substantially more probable following a traffic crash. In a clinical or forensic setting the inference that magnitude of a trauma was low should not serve as a basis for either excluding a cervical artery dissection from a differential diagnosis, or for excluding the trauma as a cause of a diagnosed dissection. This case report illustrates a rare fatal outcome of inertial load to the head and neck induced by a sudden braking event in a commonly experienced non-collision traffic incident. The likely mechanism of injury resulted from interaction between the occupant and the 3-point seat belt. These findings indicate that ICA dissections are substantially more likely to be associated with SAH following head and neck trauma, regardless of the magnitude of the traumatic event or whether an impact was involved.

Injury mechanisms of the ligamentous cervical C2-C3 Functional Spinal Unit to complex loading modes: Finite Element study

The cervical spine sustains high rate complex loading modes during Motor Vehicle Crashes (MVCs) which may produce severe injuries accompanied with soft and/or hard tissue failure. Although previous numerical and experimental studies have provided insights on the cervical spine behavior under various loading scenarios, its response to complex impact loads and the resulting injury mechanisms are not fully understood. A validated Finite Element (FE) model of the ligamentous cervical C2-C3 Functional Spinal Unit (FSU) was utilized to assess the spinal response to six combined impact loading modes; flexion-extension combined with compression and distraction, and lateral bending and axial rotation combined with distraction. The FE model used time and rate-dependent material laws which permit assessing bone fracture and ligament failure. Spinal load-sharing, stresses in the spinal components, intradiscal pressure (IDP) change in the nucleus as well as contact pressure in the facet joints were predicted. Bone and ligaments failure occurrence and initiation instants were investigated. Results showed that spinal load-sharing varied with loading modes. Lateral bending combined with distraction was the most critical loading mode as it increased stresses and strains significantly and produced failure in most of the spinal components compared to other modes. The facet joints and surrounding cancellous bone as well as ligaments particularly the capsular (CL) and flavum (FL) ligaments were the most vulnerable structures to rapid flexion-extension, axial rotation and lateral bending combined with distraction or compression. The excessive stress and strain resulted from these loading modes produced rupture of the CL and FL ligaments and failure in the cancellous bone. The detection of failure initiation as well as fracture assessment demonstrated the vulnerability of ligaments to tensile combined loads and the major contribution of the bony structures in resisting compressive combined loads. Findings of this study may potentially assist in the development of injury prevention and treatment strategies.

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.

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.

Investigating cervical muscle response and head kinematics during right, left, frontal and rear-seated perturbations

OBJECTIVE

Whiplash research has largely focused on rear collisions because they account for the majority of whiplash injuries. The purpose of this study was to evaluate the effects of 4 perturbation directions (anterior, posterior, right, and left) on muscle activity and head kinematics to provide insight into the whiplash mechanism of injury.

METHODS

The effects of 4 perturbation directions induced by a parallel robotic platform, with peak acceleration of 8.50 m/s2, were analyzed on 10 subjects. Surface electromyography (EMG) measures were collected from the sternocleidomastoid (SCM), trapezius, and splenius capitus muscles. Kinematics of the head, thorax, and head relative to thorax were also measured.

RESULTS

We observed stereotypic responses for kinematics and SCM EMG for the various perturbation directions; the trapezius and splenius capitus muscles showed amplitudes that were less than 5 percent maximum voluntary contraction (MVC). Rear perturbations elicited the smallest onset latencies for the SCM (30 ms) and kinematic variables and greatest linear head center of mass (COM) accelerations. Frontal perturbations resulted in an average SCM onset latency of 143 ms and demonstrated the greatest magnitude of head translations and rotations relative to the thorax. Left and right perturbations demonstrated similar kinematics and SCM onset latencies (55 and 65 ms, respectively).

CONCLUSIONS

Compared to frontal, left, and right directions, rear perturbations showed smaller SCM onset latencies, greater SCM amplitudes, and larger head accelerations, relating to a greater potential for injury. We suggest that the greater contact area and stiffness of the seatback, in the posterior direction, compared to restrictions in other directions, led to increased peak head accelerations and shorter SCM onset latencies.

The role of tissue damage in whiplash-associated disorders: discussion paper 1

STUDY DESIGN

Nonsystematic review of cervical spine lesions in whiplash-associated disorders (WAD).

OBJECTIVE

To describe whiplash injury models in terms of basic and clinical science, to summarize what can and cannot be explained by injury models, and to highlight future research areas to better understand the role of tissue damage in WAD.

SUMMARY OF BACKGROUND DATA

The frequent lack of detectable tissue damage has raised questions about whether tissue damage is necessary for WAD and what role it plays in the clinical context of WAD.

METHODS

Nonsystematic review.

RESULTS

Lesions of various tissues have been documented by numerous investigations conducted in animals, cadavers, healthy volunteers, and patients. Most lesions are undetected by imaging techniques. For zygapophysial (facet) joints, lesions have been predicted by bioengineering studies and validated through animal studies; for zygapophysial joint pain, a valid diagnostic test and a proven treatment are available. Lesions of dorsal root ganglia, discs, ligaments, muscles, and vertebral artery have been documented in biomechanical and autopsy studies, but no valid diagnostic test is available to assess their clinical relevance. The proportion of WAD patients in whom a persistent lesion is the major determinant of ongoing symptoms is unknown. Psychosocial factors, stress reactions, and generalized hyperalgesia have also been shown to predict WAD outcomes.

CONCLUSION

There is evidence supporting a lesion-based model in WAD. Lack of macroscopically identifiable tissue damage does not rule out the presence of painful lesions. The best available evidence concerns zygapophysial joint pain. The clinical relevance of other lesions needs to be addressed by future research.

The influence of morphology on cervical injury characteristics

STUDY DESIGN

Review of peer-reviewed literature.

OBJECTIVE

Outline the effects of neck and cervical spine morphology on soft tissue injury Potential during low velocity automotive rear impacts.

SUMMARY OF BACKGROUND DATA

Automotive rear impacts are mechanical events and the response of the human head-neck complex can be thought of in biomechanical terms. This manuscript reviews evidence from peer-reviewed studies implicating occupant-related factors in the onset and severity of cervical spine soft-tissue injury.

METHODS

Effects of anatomical characteristics, head-neck and spine orientation, facet joints, and neck muscles were reviewed.

RESULTS

On the basis of existing biomechanically based research, the following occupant-related characteristics can influence the response of the cervical spine during automotive rear impacts: anatomical dimensions of the cervical spine, head-neck and cervical spine orientation at the time of impact, facet joint orientation, and neck muscle size and orientation.

CONCLUSION

The response of the cervical spine to rear impacts can be described using biomechanical concepts. This review has identified occupant-related factors that can influence injury susceptibility and cited biomechanically related research to outline the method by which those factors affect the overall head-neck and cervical spine response in such a way as to increase the susceptibility or severity of injury for a given rear impact event.

Axial head rotation increases facet joint capsular ligament strains in automotive rear impact

Axial head rotation prior to low speed automotive rear impacts has been clinically identified to increase morbidity and symptom duration. The present study was conducted to determine the effect of axial head rotation on facet joint capsule strains during simulated rear impacts. The study was conducted using a validated intact head to first thoracic vertebra (T1) computational model. Parametric analysis was used to assess effects of increasing axial head rotation between 0 and 60° and increasing impact severity between 8 and 24 km/h on facet joint capsule strains. Rear impacts were simulated by horizontally accelerating the T1 vertebra. Characteristics of the acceleration pulse were based on the horizontal T1 acceleration pulse from a series of simulated rear impact experiments using full-body post mortem human subjects. Joint capsule strain magnitudes were greatest in ipsilateral facet joints for all simulations incorporating axial head rotation (i.e., head rotation to the left caused higher ligament strain at the left facet joint capsule). Strain magnitudes increased by 47-196% in simulations with 60° head rotation compared to forward facing simulations. These findings indicate that axial head rotation prior to rear impact increases the risk of facet joint injury.

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.

Dynamic vertebral artery elongation during frontal and side impacts

BACKGROUND CONTEXT

Elongation-induced vertebral artery (VA) injury has been hypothesized to occur during nonphysiological coupled head motions during automobile impacts. Although previous work has investigated VA elongation during head-turned and head-forward rear impacts, no studies have performed similar investigations for frontal or side impacts.

PURPOSE

The present study quantified dynamic VA elongations during simulated frontal and side automotive collisions, and compared these data with corresponding physiological limits.

STUDY DESIGN/SETTING

In vitro biomechanical study of dynamic VA elongation during simulated impacts.

METHODS

A biofidelic whole cervical spine model with muscle force replication and surrogate head underwent simulated frontal impacts (n=6) of 4, 6, 8, and 10 g or left side impacts (n=6) of 3.5, 5, 6.5, and 8 g.

RESULTS

Average (SD) maximum physiological VA elongation was 7.1 (3.2) mm, measured during intact flexibility testing. Average peak dynamic elongation of right VA during left side impact, up to 17.4 (2.6) mm, was significantly greater (p<.05) than physiological beginning at 6.5 g, whereas the highest average peak VA elongation during frontal impact was 2.5 (2.4) mm, which did not exceed the physiological limit. Side impact, as compared with frontal impact, caused earlier occurrence of average peak VA elongation, 113.8 (13.5) ms versus 155.0 (46.2) ms, and higher average peak VA elongation rate, 608.8 (99.0) mm/s versus 130.0 (62.9) mm/s.

CONCLUSIONS

Elongation-induced VA injury is more likely to occur during side impact as compared with frontal impact.