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Study Design: An in vitro biomechanical study. Objectives: The objectives were to: quantify dynamic canal pinch diameter (CPD) narrowing during simulated bilateral facet dislocation of a cervical functional spinal unit model with muscle force replication, determine if peak dynamic CPD narrowing exceeded that observed post-trauma, and evaluate dynamic cord compression. Summary Of Background Data: Previous biomechanical models are limited to quasi-static loading or manual ligament transection. No studies have comprehensively analyzed dynamic CPD narrowing during simulated dislocation. Methods: Bilateral facet dislocation was simulated using 10 cervical functional spinal units (C3-C4: n = 4; C5-C6: n = 3; C7-T1: n = 3) with muscle force replication by frontal impact of the lower vertebra. Rigid body transformation of kinematic data recorded optically was used to compute the CPD in neutral posture (before dislocation), during dynamic impact (peak during dislocation), and post-impact (flexion rotation = 0(0) degrees ). Peak dynamic impact and post-impact CPD narrowing were statistically compared. Results: Average peak dynamic impact CPD narrowing significantly exceeded (P < 0.05) post-impact narrowing and occurred as early as 71.0 ms following impact. The greatest dynamic impact narrowing of 7.2 mm was observed at C3-C4, followed by 6.4 mm at C5-C6, and 5.1 mm at C7-T1, with average occurrence times ranging between 71.0 ms at C7-T1 and 97.0 ms at C5-C6. Conclusion: Extrapolation of the present results indicated dynamic spinal cord compression of up to 88% in those with stenotic canals and 35% in those with normal canal diameters. These results are consistent with the wide range of neurologic injury severity observed clinically due to bilateral facet dislocation. [ABSTRACT FROM AUTHOR]

STUDY DESIGN.: Review of whiplash injury mechanisms and effects of anti-whiplash systems including active head restraint (AHR) and Whiplash Protection System (WHIPS). OBJECTIVE.: This article provides an overview of previous biomechanical and epidemiological studies of AHR and WHIPS and investigates whether seat design and biomechanical knowledge of proposed whiplash injury mechanisms translates to understanding outcomes of rear crash occupants. SUMMARY OF BACKGROUND DATA.: In attempt to reduce whiplash injuries, some newer automobiles incorporate anti-whiplash systems such as AHR or WHIPS. During a rear crash, mechanically based systems activate by occupant momentum pressing into the seatback whereas electronically based systems activate using crash sensors and an electronic control unit linked to the head restraint. METHODS.: To investigate the effects of AHR and WHIPS on occupant responses including head and neck loads and motions, biomechanical studies of simulated rear crashes have been performed using human volunteers, mathematical models, crash dummies, whole cadavers, and hybrid cadaveric/surrogate models. Epidemiological studies have evaluated the effects of AHR and WHIPS on reducing whiplash injury claims and lessening subjective complaints of neck pain after rear crashes. RESULTS.: Biomechanical studies indicate that AHR and WHIPS reduced the potential for some whiplash injuries but did not completely eliminate the injury risk. Epidemiological outcomes indicate reduced whiplash injury claims or subjective complaints of crash-related neck pain between 43 and 75% due to AHR and between 21% and 49% due to WHIPS as compared to conventional seats and head restraints. CONCLUSION.: Yielding energy-absorbing seats aim to reduce occupant loads and accelerations whereas AHRs aim to provide early head support to minimize head and neck motions. Continued objective biomechanical and epidemiological studies of anti-whiplash systems together with industry, governmental, and clinical initiatives will ultimately lead to reduced whiplash injuries through improved prevention strategies. [ABSTRACT FROM AUTHOR]

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, [Delta]V 11.4 km/h) or AHR seat and subsequently with NHR (n = 6: 11.0 g, [Delta]V 10.2 km/h with AHR; 11.5 g, [Delta]V 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. [ABSTRACT FROM AUTHOR]

STUDY DESIGN: An in vitro biomechanical study of the effectiveness of halo-vest fixation. OBJECTIVE: The objective was to evaluate motion of the injured cervical spine with normal halo-vest application and vest loose in the prone and supine positions. SUMMARY OF BACKGROUND DATA: Snaking motion of the neck is defined as rotation in opposing directions throughout the cervical spine. Previous clinical studies have suggested snaking neck motion due to the halo-vest may lead to inadequate healing or nonunion. METHODS: The halo-vest was applied to a Human Model of the Neck, which consisted of a cervical spine specimen mounted to the torso of an anthropometric test dummy and carrying a surrogate head. The model was transitioned from prone, to upright, to supine with the halo-vest applied normally and with the vest loose. Average peak spinal motions were computed in the prone and supine positions and contrasted with the physiologic rotation range, obtained from the intact flexibility test, and statistically compared (P < 0.05) between normal halo-vest application and vest loose. RESULTS: Snaking motion of the neck was observed in the prone and supine positions, consisting of extension at head/C1 and C1/2 and flexion at the inferior spinal levels. The intervertebral rotation peaks generally exceeded the physiologic range throughout the cervical spine due to the loose vest in the prone position. Significant increases in the extension peaks at head/C1 (16.9 degrees vs. 5.7 degrees) and flexion peaks at C4/5 (6.9 degrees vs. 3.6 degrees) and C7-T1 (5.2 degrees vs. 0.7 degrees) were observed in the prone position due to the loose vest, as compared to normal halo-vest application. Axial neck separation was consistently observed in the prone and supine positions. CONCLUSION: The present results, which document snaking motion of the cervical spine due to the halo-vest, indicate that an inadequately fitting or loose vest may significantly diminish its immobilization capacity leading to delayed healing or nonunion. [ABSTRACT FROM AUTHOR]

Synopsis Head and spinal injuries commonly occur during motor vehicle crashes (MVCs). The goal of this clinical commentary is to discuss real-life versus simulated MVCs and to present clinical, biomechanical, and epidemiological evidence of MVC-related injury mechanisms. It will also address how this knowledge may guide and inform the design of injury mitigation devices and assist in clinical decision making. Evidence indicates that there exists no universal injury tolerance applicable to the entire population of the occupants of MVCs. Injuries sustained by occupants depend on a number of factors, including occupant characteristics (age, height, weight, sex, bone mineral density, and pre-existing medical and musculoskeletal conditions), pre-MVC factors (awareness of the impending crash, occupant position, usage of and position of the seatbelt and head restraint, and vehicle specifications), and MVC-related factors (crash orientation, vehicle dynamics, type of active or passive safety systems, and occupant kinematic response). Injuries resulting from an MVC occur due to blunt impact and/or inertial loading. An S-shaped curvature of the cervical spine and associated injurious strains have been documented during rear-, frontal-, and side-impact MVCs. Data on the injury mechanism and the quantification of spinal instability guide and inform the emergent and subsequent conservative or surgical care. Such care may require determining optimal patient positioning during transport, which injuries may be treated conservatively, whether reduction should be performed, optimal patient positioning intraoperatively, and whether bracing should be worn prior to and/or following surgery. The continued improvement of traditional injury mitigation systems, such as seats, seatbelts, airbags, and head restraints, together with research of newer collision-avoidance technologies, will lead to safer motor vehicles and ultimately more effective injury management strategies. J Orthop Sports Phys Ther 2016;46(10):826-833. Epub 3 Sep 2016. doi:10.2519/jospt.2016.6716.

Background: We report a case of a young male athlete who sustained a three column displaced teardrop fracture of the C5 vertebra due to a head-first impact in hockey, suffered neurapraxia, yet made full neurological recovery. This full recovery was in sharp contrast to multiple case series which reported permanent quadriplegia in the vast majority of teardrop fracture patients. We investigate the etiology and biomechanical mechanisms of injury., Methods: Admission imaging revealed the teardrop fracture which consisted of: a frontal plane fracture which separated an anterior quadrilateral-shaped fragment from the posterior vertebral body; a vertical fracture of the posterior vertebral body in the sagittal plane; and incomplete fractures of the neural arch that initiated superiorly at the anterior aspect of the spinous process and left lamina adjacent to the superior facet. Epidural hematoma in the region of the C5 vertebra was observed in addition to disc and ligamentous disruptions at C4-5 and C5-6. Our patient was ultimately treated surgically with anterior fusion from C4 through C6 and subsequently with bilateral posterior fusion at C5-6., Results: The injuries were caused by high-energy axial compression with the neck in a pre-flexed posture. The first fracture event consisted of the anterior vertebral body fragment being sheared off of the posterior fragment under the compression load due in part to the sagittal plane concavity of the C5 inferior endplate. The etiology of the vertical fracture of the posterior vertebral body fragment in the sagittal plane was consistent with a previously described hypothesis of the mechanistic injury events. First, the C4-5 disc height decreased under load which increased its hoop stress. Next, this increased hoop stress transferred lateral forces to the C5 uncinate processes which caused their outward expansion. Finally, the outward expansion of the uncinate processes caused the left and right sides of the vertebral body to split and spread. Evidence in support of this mechanistic event sequence was provided by the neural arch fractures which initiated superiorly, average angulation of the C5 uncinate processes, and similar well-established mechanisms causing vertical fractures at other spinal regions., Conclusions: Our case study and analyses provide insight into the etiology of the specific teardrop fracture patterns observed clinically.

Study Design: In vitro biomechanical study., Objectives: To investigate mechanisms of odontoid fracture., Summary of Background Data: Odontoid fractures in younger adults occur most often due to high-energy trauma including motor vehicle crashes and in older adults due to fall from standing height., Methods: Horizontally aligned head impacts into a padded barrier were simulated using a human upper cervical spine specimen (occiput through C3) mounted to a surrogate torso mass on a sled and carrying a surrogate head. We divided 13 specimens into 3 groups on the basis of head impact location: upper forehead in the midline, upper lateral side of the forehead, and upper lateral side of the head. Post-impact fluoroscopy and anatomical dissection documented the injuries. Time-history biomechanical responses were determined., Results: Four of the 5 specimens subjected to impact to the upper forehead in the midline sustained type II or high type III odontoid fractures due to abrupt deceleration of the head and continued forward torso momentum. Average peak force reached 1787.1 N at the neck at 50.3 milliseconds. Subsequently, the motion peaks occurred for the head relative to C3 reaching 15.2° for extension, 2.1 cm for upward translation, and 5.3 cm for horizontal compression, between 62 and 68 milliseconds., Conclusion: We identified impact to the upper forehead in the midline as a mechanism that produced odontoid fracture and associated atlas and ligamentous injuries similar to those observed in real-life trauma. We were not able to create odontoid fractures during impacts to the upper lateral side of the forehead or upper lateral side of the head. Dynamic odontoid fracture was caused by rapid deceleration of the head, which transferred load inferiorly combined with continued torso momentum, which caused spinal compression and anterior shear force and forward displacement of the axis relative to the atlas.

Purpose: "Plough" fracture, in which the odontoid ploughs through and causes a high-energy shear fracture of the anterior arch of the atlas, has been documented in clinical case studies and classified as clinically unstable. Our objectives were to develop a biomechanical model to simulate atlantal plough fracture and investigate injury mechanisms., Methods: Horizontally aligned head impacts into a padded barrier were simulated using a human upper cervical spine specimen (occiput through C3) mounted to a surrogate torso mass on a sled and carrying a surrogate head. We divided 13 specimens into 3 groups based upon head-impact location: upper forehead in the midline, upper lateral side of the forehead, and upper lateral side of the head. Post-impact fluoroscopy and anatomical dissection documented the injuries. Time-history biomechanical responses were determined for neck loads, accelerations, and motions., Results: A single specimen sustained a plough fracture variant to the atlantal anterior arch due to impact to the upper forehead and continued forward torso momentum. Horizontal velocity of C3 at the time of forehead impact was 2.7 m/s. This specimen had an anteriorly displaced fracture fragment consisting of the inferior portion of the atlantal anterior arch together with multiple complete fractures of the axis. Peak force occurred first at the impact barrier (1,903.0 N; 47 ms) followed by the neck (1,715.9 N; 58 ms). Forward translation ended at 48 ms for the head and 72 ms for the C3 vertebra., Conclusions: Our present results, though preliminary, indicate that plough fracture of the anterior arch of the atlas likely occurred immediately following or simultaneously with associated axis fractures at approximately 58 ms following impact to the upper forehead. The present injury response data highlighted the role of load transfer from torso momentum to the upper cervical spine to produce anterior shear force and forward displacement of the dens and bony fragment of the anterior arch of the atlas relative to the C1 ring.

Background: We investigated mechanisms of axis ring fractures due to simulated head impacts., Methods: Our model consisted of a human upper cervical spine specimen (occiput through C3) mounted to a surrogate torso mass on a sled and carrying a surrogate head. We divided 13 specimens into 3 groups based upon head impact location: upper forehead in the midline, upper lateral side of the forehead, and upper lateral side of the head. Post-impact fluoroscopy and anatomical dissection documented the injuries. Average occurrence times of the peak loads and accelerations were statistically compared (P<0.05) using ANOVA and Bonferroni pair-wise post-hoc tests., Findings: Of the 13 upper cervical spines tested, 5 specimens sustained axis ring fractures with the most common mechanism being impact to the upper left lateral side of the forehead. The first local force peaks at the impact barrier and neck and all peak head accelerations occurred between 18.0 and 22.8 ms, significantly earlier than the absolute force peaks. The average peak neck loads reached 1761.2N and the axis ring fractures occurred within 50 ms., Interpretation: We observed asymmetrical fractures of the axis ring including fractures of the superior and inferior facets, laminae, posterior wall of the vertebral body, pars interarticularis, and pedicles. The fracture patterns were related to the morphology of the axis as a transitional vertebra of the upper cervical spine. Understanding the mechanisms of axis ring fractures may help in choosing the optimal reduction technique and stabilization method based upon the specific fracture pattern., (Copyright © 2014 Elsevier Ltd. All rights reserved.)

Study Design In vitro biomechanical study. Objective To investigate the biomechanics of thoracolumbar burst and Chance-type fractures during fall from height. Methods Our model consisted of a three-vertebra human thoracolumbar specimen (n = 4) stabilized with muscle force replication and mounted within an impact dummy. Each specimen was subjected to a single fall from an average height of 2.1 m with average velocity at impact of 6.4 m/s. Biomechanical responses were determined using impact load data combined with high-speed movie analyses. Injuries to the middle vertebra of each spinal segment were evaluated using imaging and dissection. Results Average peak compressive forces occurred within 10 milliseconds of impact and reached 40.3 kN at the ground, 7.1 kN at the lower vertebra, and 3.6 kN at the upper vertebra. Subsequently, average peak flexion (55.0 degrees) and tensile forces (0.7 kN upper vertebra, 0.3 kN lower vertebra) occurred between 43.0 and 60.0 milliseconds. The middle vertebra of all specimens sustained pedicle and endplate fractures with comminution, bursting, and reduced height of its vertebral body. Chance-type fractures were observed consisting of a horizontal split fracture through the laminae and pedicles extending anteriorly through the vertebral body. Conclusions We hypothesize that the compression fractures of the pedicles and vertebral body together with burst fracture occurred at the time of peak spinal compression, 10 milliseconds. Subsequently, the onset of Chance-type fracture occurred at 20 milliseconds through the already fractured and weakened pedicles and vertebral body due to flexion-distraction and a forward shifting spinal axis of rotation.

Background: Axial compression injuries of the cervical spine occur during contact sports, automobile collisions, and falls. The objective of this study was to use flexibility tests to determine biomechanical instability of the cervical spine due to simulated axial compression injuries., Hypothesis: We hypothesized that the axial compression injuries cause severe biomechanical instability throughout the cervical spine., Materials and Methods: The injuries were simulated using 2.4m/s head-first impacts of a cadaveric cervical spine model (n=10) mounted horizontally to a torso-equivalent mass on a sled and carrying a surrogate head in protruded posture. Intact and post-impact flexibility tests were performed up to 1.5, 3, and 1.5 Nm in flexion-extension, axial torque, and lateral bending, respectively. Instability parameters of range of motion (RoM) and neutral zone (NZ) were determined for injured spinal levels and statistically compared (P<0.05) between intact and post-impact., Results: The sagittal instability parameters indicated extension-compression injuries at the upper and middle cervical spine and flexion-compression injuries at the lower cervical spine. Increases in extension RoM were 14.9° at the upper cervical spine and 24.9° (P<0.05) at the middle cervical spine and in flexion RoM at C7/T1 were 25.6°. RoM and NZ increases in axial rotation and lateral bending were nearly symmetric among left and right., Discussion: Multidirectional instability of the upper cervical spine caused by atlas and dens fractures was evidenced by increases between 36% and 53% in RoM and NZ due to the impacts. The sagittal RoM of injured spinal levels of the middle and lower cervical spine exceeded a proposed threshold for clinical instability by between 67% and 114%. The instability documented throughout the cervical spine was consistent with clinical observations of cord injuries and paralysis in patients., Level of Evidence: Level IV, controlled laboratory investigation., (Copyright © 2013 Elsevier Masson SAS. All rights reserved.)

Background: Our goal was to determine 3-dimensional interfragmentary motions due to simulated transverse fracture and volar wedge osteotomy of the scaphoid during physiologic flexion-extension of a cadaveric wrist model., Methods: The model consisted of a cadaveric wrist (n = 8) from the metacarpals through the distal radius and ulna with load applied through the major flexor-extensor tendons. Flexibility tests in flexion-extension were performed in the following 3 test conditions: intact and following transverse fracture and wedge osteotomy of the scaphoid. Scaphoid interfragmentary motions were measured using optoelectronic motion tracking markers. Average peak scaphoid interfragmentary motions due to transverse fracture and wedge osteotomy were statistically compared (P<0.05) to intact., Findings: The accuracy of our computed interfragmentary motions was ± 0.24 mm for translation and ± 0.54° for rotation. Average peak interfragmentary motions due to fracture ranged between 0.9 mm to 1.9 mm for translation and 5.3° to 10.8° for rotation. Significant increases in interfragmentary motions were observed in volar/dorsal translations and flexion/extension due to transverse fracture and in separation and rotations in all 3 motion planes due to wedge osteotomy., Interpretation: Comparison of our results with data from previous in vitro and in vivo biomechanical studies indicates a wide range of peak interfragmentary rotations due to scaphoid fracture, from 4.6° up to 30°, with peak interfragmentary translations on the order of several millimeters. Significant interfragmentary motions, indicating clinical instability, likely occur due to physiologic flexion-extension of the wrist in those with transverse scaphoid fracture with or without volar bone loss., (Copyright © 2013 Elsevier Ltd. All rights reserved.)

A fall from high height can cause thoracolumbar spine fracture with retropulsion of endplate fragments into the canal leading to neurological deficit. Our objectives were to develop a hybrid cadaveric/surrogate model for producing thoracolumbar spine injury during simulated fall from height, evaluate the feasibility and performance of the model, and compare injuries with those observed clinically. Our model consisted of a 3-vertebra human lumbar specimen (L3-L4-L5) stabilized with muscle force replication and mounted within an impact dummy. The model was subjected to a fall from height of 2.2 m with impact velocity of 6.6 m/s. Kinetic and kinematic time-history responses were determined using spinal and pelvis load cell data and analyses of high-speed video. Injuries to the L4 vertebra were evaluated by fluoroscopy, radiography, and detailed anatomical dissection. Peak compression forces during the fall from height occurred at 7 ms and reached 44.7 kN at the ground, 9.1 kN at the pelvis, and 4.5 kN at the spine. Pelvis acceleration peaks reached 209.9 g at 8 ms for vertical and 62.8 g at 12 ms for rearward. Tensile load peaks were then observed (spine: 657.0 N at 47 ms; pelvis: 569.4 N at 61 ms). T1/pelvis peak flexion of 68.3° occurred at 38 ms as the upper torso translated forward while the pelvis translated rearward. Complete axial burst fracture of the L4 vertebra was observed including endplate comminution, retropulsion of bony fragments into the canal, loss of vertebral body height, and increased interpedicular distance due to fractures anterior to the pedicles and a vertical split fracture of the left lamina. Our dynamic injury model closely replicated the biomechanics of real-life fall from height and produced realistic, clinically relevant burst fracture of the lumbar spine. Our model may be used for further study of thoracolumbar spine injury mechanisms and injury prevention strategies., (Copyright © 2013 Elsevier Ltd. All rights reserved.)

Study Design: In vitro biomechanical study., Objective: Our objective was to determine the effectiveness of cervical collars and cervicothoracic orthoses for stabilizing clinically relevant, experimentally produced cervical spine injuries., Summary of Background Data: Most previous in vitro studies of cervical orthoses used a simplified injury model with all ligaments transected at a single spinal level, which differs from real-life neck injuries. Human volunteer studies are limited to measuring only sagittal motions or 3-dimensional motions only of the head or 1 or 2 spinal levels., Methods: Three-plane flexibility tests were performed to evaluate 2 cervical collars (Vista Collar and Vista Multipost Collar) and 2 cervicothoracic orthoses (Vista TS and Vista TS4) using a skull-neck-thorax model with 8 injured cervical spine specimens (manufacturer of orthoses: Aspen Medical Products Inc, Irvine, CA). The injuries consisted of flexion-compression at the lower cervical spine and extension-compression at superior spinal levels. Pair-wise repeated measures analysis of variance (P < 0.05) and Bonferroni post hoc tests determined significant differences in average range of motions of the head relative to the base, C7 or T1, among experimental conditions. RESULTS.: All orthoses significantly reduced unrestricted head/base flexion and extension. The orthoses allowed between 8.4% and 25.8% of unrestricted head/base motion in flexion/extension, 57.8% to 75.5% in axial rotation, and 53.8% to 73.7% in lateral bending. The average percentages of unrestricted motion allowed by the Vista Collar, Vista Multipost Collar, Vista TS, and Vista TS4 were: 14.0, 9.7, 6.1, and 4.7, respectively, for middle cervical spine extension and 13.2, 11.8, 3.3, and 0.4, respectively, for lower cervical spine flexion., Conclusion: Successive increases in immobilization were observed from Vista Collar to Vista Multipost Collar, Vista TS, and Vista TS4 in extension at the injured middle cervical spine and in flexion at the injured lower cervical spine. Our results may assist clinicians in selecting the most appropriate orthosis based upon patient-specific cervical spine injuries.

Background: Our objectives were to use a hybrid cadaveric/surrogate model to evaluate the effects of the cervicothoracic orthosis and collar on head and neck biomechanical responses during transitioning from supine to upright., Methods: The model consisted of an adult-male surrogate dummy with its artificial neck replaced by a human neck specimen (n=10). The model was transitioned from supine to upright using a rotation apparatus. A high-speed digital camera tracked motions of the head, vertebrae, cervicothoracic orthosis, pelvis, and rotation apparatus. Head load cell data were used to compute occipital condyle loads. Average peak spinal loads and motions were statistically compared (P<0.05) among experimental conditions (cervicothoracic orthosis: anterior strut locked and unlocked; collar; and unrestricted)., Findings: Loads at the occipital condyles consisted of anterior shear, compression, and extension moment. The most rigid device tested, cervicothoracic orthosis with anterior strut locked, significantly reduced axial compression neck force and increased anterior shear neck force and provided the greatest immobilization by significantly reducing spinal rotations as compared to other experimental conditions. Similar neck biomechanical responses were observed between the cervicothoracic orthosis, anterior strut unlocked, and collar., Interpretation: The simple maneuver of supine-to-upright transitioning, commonly performed clinically, produced complex neck loads and motions including head protrusion which caused cervical spine snaking. Neck motions consisted of extension at the upper cervical spine and flexion at the subaxial cervical spinal levels. Of the devices tested, the cervicothoracic orthosis, with anterior strut locked, provided the greatest cervical spine immobilization thereby reducing the risk of potential secondary neck injuries., (Copyright © 2013 Elsevier Ltd. All rights reserved.)

Previous in vivo studies have observed flexion of the upper or upper/middle cervical spine and extension at inferior spinal levels due to direct head impacts. These studies hypothesized that hyperflexion may contribute to injury of the upper or middle cervical spine during real-life head impact. Our objectives were to determine the cervical spine injury response to direct head impact, document injuries, and compare our results with previously reported in vivo data. Our model consisted of a human cadaver neck (n=6) mounted to the torso of a rear impact dummy and carrying a surrogate head. Rearward force was applied to the model's forehead using a cable and pulley system and free-falling mass of 3.6kg followed by 16.7kg. High-speed digital cameras tracked head, vertebral, and pelvic motions. Average peak spinal rotations observed during impact were statistically compared (P<0.05) to physiological ranges obtained from intact flexibility tests. Peak head impact force was 249 and 504N for the 3.6 and 16.7kg free-falling masses, respectively. Occipital condyle loads reached 205.3N posterior shear, 331.4N compression, and 7.4Nm extension moment. We observed significant increases in intervertebral extension peaks above physiologic at C6/7 (26.3° vs. 5.7°) and C7/T1 (29.7° vs. 4.6°) and macroscopic ligamentous and osseous injuries at C6 through T1 due to the 504N impacts. Our results indicate that a rearward head shear force causes complex neck loads of posterior shear, compression, and extension moment sufficient to injure the lower cervical spine. Real-life neck injuries due to motor vehicle crashes, sports impacts, or falls are likely due to combined loads transferred to the neck by direct head impact and torso inertial loads., (Copyright © 2012 Elsevier Ltd. All rights reserved.)

Purpose: Our objectives were to develop a skull-neck-thorax model capable of quantifying spinal motions in an intact human cadaver neck with and without cervical orthoses, determine the effect of orthoses on three-dimensional load-displacement properties of all cervical spinal levels, and compare and contrast our results with previously reported in vivo data., Methods: Load input flexibility tests were performed to evaluate two cervical collars (Vista(®) collar and Vista(®) Multipost collar) and two cervicothoracic orthoses (CTOs: Vista(®) TS and Vista(®) TS4) using the skull-neck-thorax model with 10 intact whole cervical spine specimens. The physiologic range of motion (RoM) limit was the peak obtained from flexibility tests with no orthosis. Pair-wise repeated measures, analysis of variance (p < 0.05), and Bonferroni post hoc tests determined significant differences in average peak RoM at each spinal level among the experimental conditions., Results: Significant reductions below physiologic limits were observed due to all orthoses in: three-dimensional head/T1 RoMs, all sagittal intervertebral RoMs, and lateral bending at C4/5 through C7/T1. Both CTOs significantly reduced C6/7 sagittal RoM as compared to both collars. Intervertebral RoMs with the orthoses could not be differentiated from physiologic limits at the upper cervical spine in lateral bending and throughout the entire cervical spine in axial rotation, with the exception of C1/2., Conclusions: Our results indicate that cervical orthoses effectively immobilized the entire cervical spine in flexion/extension and the lower cervical spine in lateral bending. The CTOs improved immobilization of the lower cervical spine in flexion/extension as compared to the collars. The orthoses were least effective at restricting lateral bending of the upper spinal levels and axial rotation of all spinal levels, except C1/2. Understanding immobilization provided by orthoses will assist clinicians in selecting the most appropriate brace based upon patient-specific immobilization requirements.

Objective: To simulate horizontally aligned head-first impacts with initial head protrusion using a human cadaveric neck model and to determine biomechanical responses, injuries, and injury severity., Design: Head-first impacts with initial head protrusion were simulated at 2.4 m/s using a human cadaver neck model (n = 10) mounted horizontally to a torso-equivalent mass on a sled and carrying a surrogate head. Macroscopic neck injuries were determined, and ligamentous injuries were quantified using fluoroscopy and visual inspection after the impacts. Representative time-history responses for injured specimens were determined during impact using load cell data and analyses of high-speed video., Setting: Biomechanics research laboratory., Participants: Cervical spines of 10 human cadavers., Main Outcome Measures: Injury severity at the middle and lower cervical spine was statistically compared using a 2-sample t test (P < 0.05)., Results: Neck buckling consisted of hyperflexion at C6/7 and C7/T1 and hyperextension at superior spinal levels. Noncontiguous neck injuries included forward dislocation at C7/T1, spinous process fracture and compression-extension injuries at the middle cervical spine, and atlas and odontoid fractures. Ligamentous injury severity at C7/T1 was significantly greater than at the middle cervical spine., Conclusions: Distinct injury mechanisms were observed throughout the neck, consisting of extension-compression and posterior shear at the upper and middle cervical spine and flexion-compression and anterior shear at C6/7 and C7/T1. Our experimental results highlight the importance of clinical awareness of potential noncontiguous cervical spine injuries due to head-first sports impacts.

Context: Head-first sports-induced impacts cause cervical fractures and dislocations and spinal cord lesions. In previous biomechanical studies, researchers have vertically dropped human cadavers, head-neck specimens, or surrogate models in inverted postures., Objective: To develop a cadaveric neck model to simulate horizontally aligned, head-first impacts with a straightened neck and to use the model to investigate biomechanical responses and failure mechanisms., Design: Descriptive laboratory study., Setting: Biomechanics research laboratory., Patients or Other Participants: Five human cadaveric cervical spine specimens., Intervention(s): The model consisted of the neck specimen mounted horizontally to a torso-equivalent mass on a sled and carrying a surrogate head. Head-first impacts were simulated at 4.1 m/s into a padded, deformable barrier., Main Outcome Measure(s): Time-history responses were determined for head and neck loads, accelerations, and motions. Average occurrence times of the compression force peaks at the impact barrier, occipital condyles, and neck were compared., Results: The first local compression force peaks at the impact barrier (3070.0 ± 168.0 N at 18.8 milliseconds), occipital condyles (2868.1 ± 732.4 N at 19.6 milliseconds), and neck (2884.6 ± 910.7 N at 25.0 milliseconds) occurred earlier than all global compression peaks, which reached 7531.6 N in the neck at 46.6 milliseconds (P < .001). Average peak head motions relative to the torso were 6.0 cm in compression, 2.4 cm in posterior shear, and 6.4° in flexion. Neck compression fractures included occipital condyle, atlas, odontoid, and subaxial comminuted burst and facet fractures., Conclusions: Neck injuries due to excessive axial compression occurred within 20 milliseconds of impact and were caused by abrupt deceleration of the head and continued forward torso momentum before simultaneous rebound of the head and torso. Improved understanding of neck injury mechanisms during sports-induced impacts will increase clinical awareness and immediate care and ultimately lead to improved protective equipment, reducing the frequency and severity of neck injuries and their associated societal costs.

Study Design: An in vitro biomechanical study., Objective: To investigate atlas injury mechanisms due to horizontally aligned head-first impacts of a cadaveric neck model and to document atlas fracture patterns and associated injuries., Summary of Background Data: Experimental atlas injuries have been created by applying compression or radial forces to isolated C1 vertebrae, dropping weight or applying sagittal moments to the upper cervical spine segments, or vertical drop testing of head-neck specimens or whole cadavers. Atlas injuries that commonly occur due to horizontally aligned head-first impacts have not been previously investigated., Methods: Horizontally aligned head-first impacts into a padded barrier were simulated at 4.1 m/s, using a human cadaver neck model mounted horizontally to a torso-equivalent mass on a sled and carrying a surrogate head. Atlantal radial force was computed using head and neck load cell data. Postimpact dissection documented atlas and associated injuries. Average atlantal radial force peaks and their occurrence times were statistically compared (P < 0.05) among the first local and global peaks using paired t tests., Results: The first average local peak in radial atlantal force was significantly smaller (1240 vs. 2747 N) and occurred significantly earlier (24 ms vs. 46 ms) than the global force peak. Atlas injuries consisted of either 3- or 4-part burst fractures or incomplete lateral mass fracture unilaterally. Associated injuries included bony avulsion of the transverse ligament unilaterally and fractures of the occipital condyles, superior facets of the axis, or odontoid., Conclusion: The results indicated that the varied atlas fracture patterns were due primarily to radial forces causing outward lateral expansion of its lateral masses. Anterior and posterior arch fracture locations are dependent, in part, upon the cross-sectional arch dimensions. Transverse ligament rupture or bony avulsion is likely associated with real-life atlantal burst fractures.

Purpose: Chronic radicular symptoms have been documented in whiplash patients, potentially caused by cervical neural tissue compression during an automobile rear crash. Our goals were to determine neural space narrowing of the lower cervical spine during simulated rear crashes with whiplash protection system (WHIPS) and active head restraint (AHR) and to compare these data to those obtained with no head restraint (NHR). We extrapolated our results to determine the potential for cord, ganglion, and nerve root compression., Methods: Our model, consisting of a human neck specimen within a BioRID II crash dummy, was subjected to simulated rear crashes in a WHIPS seat (n = 6, peak 12.0 g and ΔV 11.4 kph) or AHR seat and subsequently with NHR (n = 6, peak 11.0 g and ΔV 10.2 kph with AHR; peak 11.5 g and ΔV 10.7 kph with NHR). Cervical canal and foraminal narrowing were computed and average peak values statistically compared (P < 0.05) between WHIPS, AHR, and NHR., Results: Average peak canal and foramen narrowing could not be statistically differentiated between WHIPS, AHR, or NHR. Peak narrowing with WHIPS or AHR was 2.7 mm for canal diameter and 1.6 mm, 2.7 mm, and 5.9 mm(2) for foraminal width, height and area, respectively., Conclusions: While lower cervical spine cord compression during a rear crash is unlikely in those with normal canal diameters, our results demonstrated foraminal kinematics sufficient to compress spinal ganglia and nerve roots. Future anti-whiplash systems designed to reduce cervical neural space narrowing may lead to reduced radicular symptoms in whiplash patients.

Objectives: Various models for rear crash simulation exist and each has unique advantages and limitations. Our goals were to: determine the neck load and motion responses of a human model of the neck (HUMON) during simulated rear crashes; evaluate HUMON's biofidelity via comparisons with in vivo data; and investigate mechanisms of whiplash injury and prevention., Methods: HUMON, consisting of a neck specimen (n=6) 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 an energy-absorbing seat with fixed head restraint (HR) at peak sled accelerations of 9.9g (ΔV 9.2kph), 12.0g (ΔV 11.4kph), and 13.3g (ΔV 13.4kph). Physiologic spinal rotation ranges were determined from intact flexibility tests. Average time-history response corridors (±1 standard deviation) were computed for spinal motions, loads, and injury criteria., Results: Neck loads generally increased caudally and consisted of shear, compression, and flexion moment caused by straightening of the kyphotic thoracic and lordotic lumbar curvatures, upward torso ramping, and head inertial and head/HR contact loads. Nonphysiologic rotation occurred in flexion at C7/T1 prior to head/HR contact and in extension at C6/7 and C7/T1 during head/HR contact., Conclusions: HUMON's neck load and motion responses compared favorably with in vivo data. Lower cervical spine flexion-compression injuries prior to head/HR contact and extension-compression injuries during head/HR contact may be reduced by refinement of existing seatback, lapbelt, and HR designs and/or development of new injury prevention systems., (Copyright © 2011 Elsevier Ltd. All rights reserved.)

Background Context: Epidemiological studies indicate potential benefits of the Whiplash Protection System (WHIPS) for reducing neck injury risk., Purpose: Our goal was to evaluate cervical spine curvature during simulated rear crashes of a Human Model of the Neck (HUMON) within a WHIPS seat with fixed head restraint (HR)., Study Design: In vitro biomechanical study., Methods: The HUMON consisted of a human neck specimen mounted to the torso of BioRID II (Denton ATD, Inc., Milan, OH, USA) and carrying a surrogate head stabilized with muscle force replication. The HUMON was subjected to simulated rear crashes in a WHIPS seat (n=6) at 9.9, 12.0, and 13.3 g and in a seat with no WHIPS or HR (n=6) at 11.5 g. Statistical tests (p<.05) determined significant increases in spinal motion peaks during the crashes with WHIPS relative to physiologic and significant differences in spinal curvature peaks between WHIPS (12.0 g) and no WHIPS or HR (11.5 g)., Results: The WHIPS absorbed crash energy during the initial 75 milliseconds, while peak lower cervical spine (LCS) extension occurred as late as 179 milliseconds. The average C7/T1 rotation peaks during the 13.3-g rear crashes with WHIPS significantly exceeded physiologic by 95% in flexion (4.3° vs. 2.2°) and more than 225% in extension (9.8° vs. 3.0°). The WHIPS caused a significant reduction in average peak normalized LCS extension as compared with no WHIPS or HR (1.2 vs. 3.7)., Conclusions: Although the peak LCS extension was significantly reduced due to WHIPS as compared with no WHIPS or HR, it exceeded physiologic as the cervical spine maintained a prolonged S-shaped curvature. Nonphysiologic LCS motion may occur even if head/HR contact occurs early, and injury is possible before head/HR contact even with a modern energy-absorbing seat. Future whiplash-reduction systems will most likely integrate active injury prevention systems with advanced features such as accident avoidance technology., (Copyright © 2011 Elsevier Inc. All rights reserved.)

Study Design: An in vitro study of simulated whiplash using a hybrid cadaveric/surrogate model., Objective: The goal of the present study was to determine the effect of the active head restraint (AHR) on residual neck instability due to simulated rear impacts of a human model of the neck., Summary of Background Data: Previous studies have indicated potential benefits of active injury prevention systems in reducing neck injuries during rear impacts., Methods: Six osteoligamentous whole cervical spine specimens (occiput-T1) were prepared with vertebral motion tracking flags. The model, consisting of the neck specimen mounted to the torso of BioRID II and carrying an anthropometric surrogate head, was rear impacted (7.1 and 11.1 g) with and without the AHR. Pre- and post-impact flexibility tests identified significant residual instability (P < 0.05) above physiologic values and among experimental conditions. Linear regression analyses were used to identify correlation between spinal rotation peaks measured during impact and the resulting flexibility parameter increases (R² > 0.35 and P < 0.001)., Results: Our results indicated significant increases in the average flexibility parameters, up to 3.1°, at C2-C3, C3-C4, and C5-C6 due to 7.1 g rear impacts even in the presence of the AHR. Subsequently, increases in the flexibility parameters progressed and spread to head/C1 and to the inferior spinal levels following the 11.1 g impacts. Correlation was observed between the C7-T1 extension peaks measured during impact and the flexibility parameter increases measured following impact. The flexibility parameter increases were generally larger due to the impacts with no head restraint, as compared with the AHR., Conclusion: Extrapolation of our results indicated that every 1° of extension beyond the physiologic limit during whiplash contributed approximately 0.5° of residual neck rotation following whiplash. The present data underscore the protective effect of the AHR in reducing residual neck instability due to whiplash.

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.

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.

Background: Previous epidemiological studies have observed that an initial head restraint backset greater than 10 cm is associated with a higher risk of neck injury and persistent symptoms. The objective of this study was to investigate the relation between the active head restraint position and peak neck motion using a new human model of the neck., Methods: The model consisted of an osteoligamentous neck specimen mounted to the torso of a rear impact dummy and carrying an anthropometric head stabilized with muscle force replication. Rear impacts (7.1 and 11.1g) were simulated with and without the active head restraint. Physiologic rotation was determined from intact flexibility tests. Significant reductions (P<0.05) in the spinal motion peaks with the active head restraint, as compared to without, were identified. Linear regression analyses identified correlation between head restraint backset and peak spinal rotations (R(2)>0.3 and P<0.001)., Findings: The active head restraint significantly reduced the average peak spinal rotations, however, these peaks exceeded the physiologic range in flexion at head/C1 and in extension at C4/5 through C7/T1. Correlation was observed between the head restraint backset and the extension peaks at C4/5 and C5/6., Interpretation: Correlation between head restraint backset and spinal rotation peaks indicated that a head restraint backset in excess of 8.0 cm may cause hyperextension injuries at the middle and lower cervical spine. The active head restraint may not be fully activated at the time of peak spinal motions, thus reducing its potential protective effects.

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.

Study Design: An in vitro biomechanical study of halo-vest and odontoid screw fixation of Type II dens fracture., Objective: The objective were to determine upper cervical spine instability due to simulated dens fracture and investigate stability provided by the halo-vest and odontoid screw, applied individually and combined., Summary of Background Data: Previous studies have evaluated posterior fixation techniques for stabilizing dens fracture. No previous biomechanical study has investigated the halo-vest and odontoid screw for stabilizing dens fracture., Methods: A biofidelic skull-neck-thorax model was used with 5 osteoligamentous whole cervical spine specimens. Three-dimensional flexibility tests were performed on the specimens while intact, following simulated dens fracture, and following application of the halo-vest alone, odontoid screw alone, and halo-vest and screw combined. Average total neutral zone and total ranges of motion at C0/1 and C1/2 were computed for each experimental condition and statistically compared with physiologic motion limits, obtained from the intact flexibility test. Significance was set at P < 0.05 with a trend toward significance at P < 0.1., Results: Type II dens fracture caused trends toward increased sagittal neutral zone and lateral bending range of motion at C1/2. Spinal motions with the dens screw alone could not be differentiated from physiologic limits. Significant reductions in motion were observed at C0/1 and C1/2 in flexion-extension and axial rotation due to the halo-vest, applied individually or combined with the dens screw. At C1/2, the halo-vest combined with the dens screw generally allowed the smallest average percentages of intact motion: 3% in axial rotation, 17% in flexion-extension, and 18% in lateral bending., Conclusion: The present reduction in C1/2 motion observed, due to the halo-vest and dens screw combined is similar to previously reported immobilization provided by the polyaxial screw/rod system and transarticular screw fixation combined with wiring. The present biomechanical data may be useful to clinicians when choosing an appropriate treatment for those with Type II dens fracture.

Study Design: An in vitro biomechanical study., Objectives: The objectives were to develop a new biofidelic skull-neck-thorax model capable of quantifying motion patterns of the cervical spine in the presence of a halo-vest; to investigate the effects of vest loosening, superstructure loosening, and removal of the posterior uprights; and to evaluate the ability of the halo-vest to stabilize the neck within physiological motion limits., Summary of Background Data: Previous clinical and biomechanical studies have investigated neck motion with the halo-vest only in the sagittal plane or only at the injured spinal level. No previous studies have quantified three-dimensional intervertebral motion patterns throughout the injured cervical spine stabilized with the halo-vest or studied the effect of halo-vest components on these motions., Methods: The halo-vest was applied to the skull-neck-thorax model. Six osteoligamentous whole cervical spine specimens (occiput through T1 vertebra) were used that had sustained multiplanar ligamentous injuries at C3/4 through C7-T1 during a previous protocol. Flexibility tests were performed with normal halo-vest application, loose vest, loose superstructure, and following removal of the posterior uprights. Average total range of motion for each experimental condition was statistically compared (P < 0.05) with the physiologic rotation limit for each spinal level., Results: Cervical spine snaking was observed in both the sagittal and frontal planes. The halo-vest, applied normally, generally limited average spinal motions to within average physiological limits. No significant increases in average spinal motions above physiologic were observed due to loose vest, loose superstructure, or removal of the posterior uprights. However, a trend toward increased motion at C6/7 in lateral bending was observed due to loose superstructure., Conclusion: The halo-vest, applied normally, effectively immobilized the cervical spine. Sagittal or frontal plane snaking of the cervical spine due to the halo-vest may reduce its immobilization capability at the upper cervical spine and cervicothoracic junction.

Objectives: The goal of this study was to compute the dynamic neck loads during simulated high-speed bilateral facet dislocation and investigate the injury mechanism., Methods: Ten osteoligamentous functional spinal units (C3/4, n = 4; C5/6, n = 3; C7/T1, n = 3) were prepared with muscle force replication, motion tracking flags, and a 3.3-kg mass rigidly attached to the upper vertebra. Frontal impacts of increasing severity were applied to the lower vertebra until dislocation was achieved. Inverse dynamics was used to calculate the dynamic neck loads during dislocation. Average peak impact acceleration required to cause dislocation ranged between 7.6 and 11.6 g. This resulted in dynamic neck loads applied at average peak rates of 906 Nm/s for flexion moment, 8017 N/ for anterior shear, and 8100 N/s for axial compression. To determine the temporal event patterns, the average occurrence times of the load and motion peaks were statistically compared (P <0.05)., Results: Among average peak loads, axial compression of 233.6 N was first to occur followed by anterior shear force of 73.1 N and flexion moment of 30.7 Nm. Among average peak motions, axial separation of 5.3 mm was first to occur followed by flexion rotation of 63.1 degrees and anterior shear of 21.5 mm. Subsequently, average peak posterior shear force of 110.3 N was observed as the upper facet became locked in the intervertebral foramina. Average peak axial compression of 6.6 mm occurred significantly later than all preceding events., Conclusions: During bilateral facet dislocation, the main loads included flexion moment and forces of axial compression and anterior shear. These loads caused flexion rotation, facet separation, and anterior translation of the upper facet relative to the lower. The present data help elucidate the injury mechanism of cervical facet dislocation.

Background: Previous clinical studies have identified the cervical facet joint, including the capsular ligaments, as sources of pain in whiplash patients. The goal of this study was to determine whether whiplash caused increased capsular ligament laxity by applying quasi-static loading to whiplash-exposed and control capsular ligaments., Methods: A total of 66 capsular ligament specimens (C2/3 to C7/T1) were prepared from 12 cervical spines (6 whiplash-exposed and 6 control). The whiplash-exposed spines had been previously rear impacted at a maximum peak T1 horizontal acceleration of 8 g. Capsular ligaments were elongated at 1mm/s in increments of 0.05 mm until a tensile force of 5 N was achieved and subsequently returned to neutral position. Four pre-conditioning cycles were performed and data from the load phase of the fifth cycle were used for subsequent analyses. Ligament elongation was computed at tensile forces of 0, 0.25, 0.5, 0.75, 1.0, 2.5, and 5.0 N. Two factor, non-repeated measures ANOVA (P<0.05) was performed to determine significant differences in the average ligament elongation at tensile forces of 0 and 5 N between the whiplash-exposed and control groups and between spinal levels., Findings: Average elongation of the whiplash-exposed capsular ligaments was significantly greater than that of the control ligaments at tensile forces of 0 and 5 N. No significant differences between spinal levels were observed., Interpretation: Capsular ligament injuries, in the form of increased laxity, may be one component perpetuating chronic pain and clinical instability in whiplash patients.

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.

Background Context: Most previous studies have investigated ligament mechanical properties at slow elongation rates of less than 25 mm/s., Purpose: To determine the tensile mechanical properties, at a fast elongation rate, of intact human cervical anterior and posterior longitudinal, capsular, and interspinous and supraspinous ligaments, middle-third disc, and ligamentum flavum., Study Design/setting: In vitro biomechanical study., Methods: A total of 97 intact bone-ligament-bone specimens (C2-C3 to C7-T1) were prepared from six cervical spines (average age: 80.6 years, range, 71 to 92 years) and were elongated to complete rupture at an average (SD) peak rate of 723 (106) mm/s using a custom-built apparatus. Nonlinear force versus elongation curves were plotted and peak force, peak elongation, peak energy, and stiffness were statistically compared (p<.05) among ligaments. A mathematical model was developed to determine the quasi-static physiological ligament elongation., Results: Highest average peak force, up to 244.4 and 220.0 N in the ligamentum flavum and capsular ligament, respectively, were significantly greater than in the anterior longitudinal ligament and middle-third disc. Highest peak elongation reached 5.9 mm in the intraspinous and supraspinous ligaments, significantly greater than in the middle-third disc. Highest peak energy of 0.57 J was attained in the capsular ligament, significantly greater than in the anterior longitudinal ligament and middle-third disc. Average stiffness was generally greatest in the ligamentum flavum and least in the intraspinous and supraspinous ligaments. For all ligaments, peak elongation was greater than average physiological elongation computed using the mathematical model., Conclusions: Comparison of the present results with previously reported data indicated that high-speed elongation may cause cervical ligaments to fail at a higher peak force and smaller peak elongation and they may be stiffer and absorb less energy, as compared with a slow elongation rate. These comparisons may be useful to clinicians for diagnosing cervical ligament injuries based upon the specific trauma.

Previous biomechanical models of cervical bilateral facet dislocation (BFD) are limited to quasi-static loading or manual ligament transection. The goal of the present study was to determine the facet joint kinematics during high-speed BFD. Dislocation was simulated using ten cervical functional spinal units with muscle force replication by frontal impact of the lower vertebra, tilted posteriorly by 42.5 degrees. Average peak rotations and anterior sliding (displacement of upper articulating facet surface along the lower), separation and compression (displacement of upper facet away from and towards the lower), and lateral shear were determined at the anterior and posterior edges of the right and left facets and statistically compared (P < 0.05). First, peak facet separation occurred, and was significantly greater at the left posterior facet edge, as compared to the anterior edges. Next, peak flexion rotation and anterior facet sliding occurred, followed by peak facet compression. The highest average facet translation peaks were 22.0 mm for anterior sliding, 7.9 mm for separation, 9.9 mm for compression and 3.6 mm for lateral shear. The highest average rotation of 63 degrees occurred in flexion, significantly greater than all other directions. These events occurred, on average, within 0.29 s following impact. During BFD, the main sagittal motions included facet separation, flexion rotation, anterior sliding, followed by compression, however, non-sagittal motions also existed. These motions indicated that unilateral dislocation may precede bilateral dislocation.

The goal of the present study was to determine the dynamic sagittal flexibility coefficients, including coupling coefficients, throughout the human cervical spine using rear impacts. A biofidelic whole cervical spine model (n=6) with muscle force replication and surrogate head was rear impacted at 5 g peak horizontal accelerations of the T1 vertebra within a bench-top mini-sled. The dynamic main and coupling sagittal flexibility coefficients were calculated at each spinal level, head/C1 to C7/T1. The average flexibility coefficients were statistically compared (p<0.05) throughout the cervical spine. To validate the coefficients, the average computed displacement peaks, obtained using the average flexibility matrices and the measured load vectors, were statistically compared to the measured displacement peaks. The computed and measured displacement peaks showed good overall agreement, thus validating the computed flexibility coefficients. These peaks could not be statistically differentiated, with the exception of extension rotation at head/C1 and posterior shear translation at C7/T1. Head/C1 was significantly more flexible than all other spinal levels. The cervical spine was generally more flexible in posterior shear, as compared to axial compression. The coupling coefficients indicated that extension moment caused coupled posterior shear translation while posterior shear force caused coupled extension rotation. The present results may be used towards the designs of anthropometric test dummies and mathematical models that better simulate the cervical spine response during dynamic loading.

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.

Background: Previous clinical studies have documented successful neck pain relief in whiplash patients using nerve block and radiofrequency ablation of facet joint afferents, including capsular ligament nerves. No previous study has documented injuries to the neck ligaments as determined by altered dynamic mechanical properties due to whiplash. The goal of the present study was to determine the dynamic mechanical properties of whiplash-exposed human cervical spine ligaments. Additionally, the present data were compared to previously reported control data. The ligaments included the anterior and posterior longitudinal, capsular, and interspinous and supraspinous ligaments, middle-third disc, and ligamentum flavum., Methods: A total of 98 bone-ligament-bone specimens (C2-C3 to C7-T1) were prepared from six cervical spines following 3.5, 5, 6.5, and 8 g rear impacts and pre- and post-impact flexibility testing. The specimens were elongated to failure at a peak rate of 725 (SD 95) mm/s. Failure force, elongation, and energy absorbed, as well as stiffness were determined. The mechanical properties were statistically compared among ligaments, and to the control data (significance level: P < 0.05; trend: P < 0.1). The average physiological ligament elongation was determined using a mathematical model., Results: For all whiplash-exposed ligaments, the average failure elongation exceeded the average physiological elongation. The highest average failure force of 204.6 N was observed in the ligamentum flavum, significantly greater than in middle-third disc and interspinous and supraspinous ligaments. The highest average failure elongation of 4.9 mm was observed in the interspinous and supraspinous ligaments, significantly greater than in the anterior longitudinal ligament, middle-third disc, and ligamentum flavum. The average energy absorbed ranged from 0.04 J by the middle-third disc to 0.44 J by the capsular ligament. The ligamentum flavum was the stiffest ligament, while the interspinous and supraspinous ligaments were most flexible. The whiplash-exposed ligaments had significantly lower (P = 0.036) failure force, 149.4 vs. 186.0 N, and a trend (P = 0.078) towards less energy absorption capacity, 308.6 vs. 397.0 J, as compared to the control data., Conclusion: The present decreases in neck ligament strength due to whiplash provide support for the ligament-injury hypothesis of whiplash syndrome.

Objective: To quantify the dynamic loads and intervertebral motions throughout the cervical spine during simulated rear impacts., Methods: Using a biofidelic whole cervical spine model with muscle force replication and surrogate head and bench-top mini-sled, impacts were simulated at 3.5, 5, 6.5, and 8 g horizontal accelerations of the T1 vertebra. Inverse dynamics was used to calculate the dynamic cervical spine loads at the centers of mass of the head and vertebrae (C1-T1). The average peak loads and intervertebral motions were statistically compared (P < 0.05) throughout the cervical spine., Results: Load and motion peaks generally increased with increasing impact acceleration. The average extension moment peaks at the lower cervical spine, reaching 40.7 Nm at C7-T1, significantly exceeded the moment peaks at the upper and middle cervical spine. The highest average axial tension peak of 276.9 N was observed at the head, significantly greater than at C4 through T1. The average axial compression peaks, reaching 223.2 N at C5, were significantly greater at C4 through T1, as compared to head-C1. The highest average posterior shear force peak of 269.5 N was observed at T1., Conclusion: During whiplash, the cervical spine is subjected to not only bending moments, but also axial and shear forces. These combined loads caused both intervertebral rotations and translations.

Objective: Intervertebral Neck Injury Criterion (IV-NIC) hypothesizes that dynamic three-dimensional intervertebral motion beyond physiological limit may cause multiplanar soft-tissue injury. Present goals, using biofidelic whole human cervical spine model with muscle force replication and surrogate head in head-turned rear impacts, were to: (1) correlate IV-NIC with multiplanar injury, (2) determine IV-NIC injury threshold at each intervertebral level, and (3) determine time and mode of dynamic intervertebral motion that caused injury., Methods: Impacts were simulated at 3.5, 5, 6.5, and 8 g horizontal accelerations of T1 vertebra (n = 6; average age: 80.2 years; four male, two female donors). IV-NIC was defined at each intervertebral level and in each motion plane as dynamic intervertebral rotation divided by physiological limit. Three-plane pre- and post-impact flexibility testing measured soft-tissue injury; that is significant increase in neutral zone (NZ) or range of motion (RoM) at any intervertebral level, above baseline. IV-NIC injury threshold was average IV-NIC peak at injury onset., Results: IV-NIC extension peaks correlated best with multiplanar injuries (P < 0.001): extension RoM (R = 0.55) and NZ (R = 0.42), total axial rotation RoM (R = 0.42) and NZ (R = 0.41), and total lateral bending NZ (R = 0.39). IV-NIC injury thresholds ranged between 1.1 at C0-C1 and C3-C4 to 2.9 at C7-T1. IV-NIC injury threshold times were attained between 83.4 and 150.1 ms following impact., Conclusions: Correlation between IV-NIC and multiplanar injuries demonstrated that three-plane intervertebral instability was primarily caused by dynamic extension beyond the physiological limit during head-turned rear impacts.

Between 23 and 70% of occupants involved in frontal impacts sustain cervical spine injuries, many with neurological involvement. It has been hypothesized that cervical spinal cord compression and injury may explain the variable neurological profile described by frontal impact victims. The goals of the present study, using a biofidelic whole cervical spine model with muscle force replication, were to quantify canal pinch diameter (CPD) narrowing during frontal impact and to evaluate the potential for cord compression. The biofidelic model and a sled apparatus were used to simulate frontal impacts at 4, 6, 8, and 10 g horizontal accelerations of the T1 vertebra. The CPD was measured in the intact specimen in the neutral posture (neutral posture CPD), under static sagittal pure moments of 1.5 Nm (pre-impact CPD), during dynamic frontal impact (dynamic impact CPD), and again under static pure moments following each impact (post-impact CPD). Frontal impact caused significant (P<0.05) dynamic CPD narrowing at C0-dens, C2-C3, and C6-C7. The narrowest dynamic CPD was observed at C0-dens during the 10 g impact and was 25.9% narrower than the corresponding neutral posture CPD. Interpretation of the present results indicate that the neurological symptomatology reported by frontal impact victims is most likely not due to cervical spinal cord compression. Cord compression due to residual spinal instability is also not likely.

Object: A rotated head posture at the time of vehicular rear impact has been correlated with a higher incidence and greater severity of chronic radicular symptoms than accidents occurring with the occupant facing forward. No studies have been conducted to quantify the dynamic changes in foramen dimensions during head-turned rear-impact collisions. The objectives of this study were to quantify the changes in foraminal width, height, and area during head-turned rear-impact collisions and to determine if dynamic narrowing causes potential cervical nerve root or ganglion impingement., Methods: The authors subjected a whole cervical spine model with muscle force replication and a surrogate head to simulated head-turned rear impacts of 3.5, 5, 6.5, and 8 G following a noninjurious 2-G baseline acceleration. Continuous dynamic foraminal width, height, and area narrowing were recorded, and peaks were determined during each impact; these data were then statistically compared with those obtained at baseline. The authors observed significant increases (p < 0.05) in mean peak foraminal width narrowing values greater than baseline values, of up to 1.8 mm in the left C5-6 foramen at 8 G. At the right C2-3 foramen, the mean peak dynamic foraminal height was significantly narrower than baseline when subjected to rear-impacts of 5 and 6.5 G, but no significant increases in foraminal area were observed. Analysis of the results indicated that the greatest potential for cervical ganglion compression injury existed at C5-6 and C6-7. Greater potential for ganglion compression injury existed at C3-4 and C4-5 during head-turned rear impact than during head-forward rear impact., Conclusions: Extrapolation of present results indicated potential ganglion compression in patients with a non-stenotic foramen at C5-6 and C6-7; in patients with a stenotic foramen the injury risk greatly increases and spreads to include the C3-4 through C6-7 as well as C4-5 through C6-7 nerve roots.

Study Design: Determination of alar, transverse, and apical ligament strains during simulated head-turned rear impact., Objectives: To quantify the alar, transverse, and apical ligament strains during head-turned rear impacts of increasing severity, to compare peak strains with baseline values, and to investigate injury mechanisms., Summary of Background Data: Clinical and epidemiologic studies have documented upper cervical spine ligament injury due to severe whiplash trauma. There are no previous biomechanical studies investigating injury mechanisms during head-turned rear impacts., Methods: Whole cervical spine specimens (C0-T1) with surrogate head and muscle force replication were used to simulate head-turned rear impacts of 3.5, 5, 6.5, and 8 g horizontal accelerations of the T1 vertebra. The peak ligament strains during impact were compared (P < 0.05) to baseline values, obtained during a noninjurious 2 g acceleration., Results: The highest right and left alar ligament average peak strains were 41.1% and 40.8%, respectively. The highest transverse and apical ligament average strain peaks were 17% and 21.3%, respectively. There were no significant increases in the average peak ligament strains at any impact acceleration compared with baseline., Conclusions: The alar, transverse, and apical ligaments are not at risk for injury due to head-turned rear impacts up to 8 g. The upper cervical spine symptomatology reported by whiplash patients may, therefore, be explained by other factors, including severe whiplash trauma in excess of 8 g peak acceleration and/or other impact types, e.g., offset, rollover, and multiple collisions.

Objective: Previous methods to determine spinal ligament deformation have included either custom-designed transducers or computational methods using rigid body transformation of kinematic data. Goals of the present study were to describe a computational methodology to determine dynamic deformations of an arbitrarily oriented ligament in a spine specimen and its associated errors., Methods: Calculation of ligament deformation in a spinal segment with vertebral motion tracking flags utilized digital stereophotography, lateral neutral posture radiograph, and detailed quantitative anatomy to develop geometrical relationships between flag markers and ligament attachment points. A custom jig, consisting of two flags each with four markers, was constructed to quantify errors associated with computed ligament deformation, flag marker translation, and flag rotation., Results: Average error in ligament deformation was dependent upon motion direction and ranged between 0.03 mm (SD 0.45 mm) and 0.28 mm (SD 0.18 mm). Average error for flag marker translation ranged between 0.02 mm (SD 0.14 mm) and 0.11 mm (SD 0.39 mm), and for flag rotation ranged between -0.06 degrees (SD 0.17 degrees ) and 0.07 degrees (SD 0.12 degrees )., Conclusions: Accuracy of the present technique was equivalent to or greater than that of previous methods. The present technique utilized relatively cost-effective digital stereophotography, and may be used to calculate strain in ligaments not readily accessible for transducer application. The methodology has wide-spread applicability for analyses of dynamic or static spinal or other ligament strains, and may be used to determine spinal canal and intervertebral foramen narrowing and area reduction.

Study Design: A biomechanical study of intervertebral foraminal narrowing during simulated automotive rear impacts., Objectives: To quantify foraminal width, height, and area narrowing during simulated rear impact, and evaluate the potential for nerve root and ganglion impingement in individuals with and without foraminal spondylosis., Summary of Background Data: Muscle weakness and paresthesias, documented in whiplash patients, have been associated with neural compression within the cervical intervertebral foramen. To our knowledge, no studies have comprehensively examined dynamic changes in foramen dimensions., Methods: There were 6 whole cervical spine specimens (average age 70.8 years) with muscle force replication and surrogate head that underwent simulated rear impact at 3.5, 5, 6.5, and 8 g, following noninjurious baseline 2 g acceleration. Peak dynamic narrowing of foraminal width, height, and area were determined during each impact and statistically compared to baseline narrowing., Results: Significant increases (P < 0.05) in average peak foraminal width narrowing above baseline were observed at C5-C6 beginning with 3.5 g impact. No significant increases in average peak foraminal height narrowing were observed, while average peak foraminal areas were significantly narrower than baseline at C4-C5 at 3.5, 5, and 6.5 g., Conclusions: Extrapolation of the present results indicated that the highest potential for ganglia compression injury was at the lower cervical spine, C5-C6 and C6-C7. Acute ganglia compression may produce a sensitized neural response to repeat compression, leading to chronic radiculopathy following rear impact.

Background: Elongation-induced vertebral artery injury has been hypothesized to occur during non-physiological coupled axial rotation and extension of head. No studies have quantified dynamic vertebral artery elongation during head-turned rear impacts. Therefore, we evaluated effect of rotated head posture vs. forward head posture at the time of impact on dynamic vertebral artery elongation during simulated rear impacts., Methods: A whole cervical spine model with surrogate head and muscle force replication underwent either simulated head-turned (n = 6) or head-forward (n = 6) rear impacts of 3.5, 5, 6.5 and 8 g. Continuous dynamic vertebral artery elongation was recorded using custom transducer and compared to physiological values obtained during intact flexibility testing., Findings: Average (SD) peak dynamic vertebral artery elongation of up to 30.5 (2.6) mm during head-turned rear-impact significantly exceeded (P < 0.05) the physiological beginning at 5 g. Highest peak elongation of 5.8 (2.1) mm during head-forward rear impact did not exceed physiological limit. Head-turned rear impact caused earlier occurrence of average peak vertebral artery elongation, 84.5 (4.2) ms vs. 161.0 (43.8) ms, and higher average peak vertebral artery elongation rate, 1336.7 (74.5) mm/s vs. 211.5 (97.4) mm/s, as compared to head-forward rear impact., Interpretation: Elongation-induced vertebral artery injury is more likely to occur in those with rotated head posture at the time of rear impact, as compared to head-forward.

Study Design: Head-turned whole cervical spine model was stabilized with muscle force replication and subjected to simulated rear impacts of increasing severity. Multiplanar flexibility testing evaluated any resulting injury., Objectives: To identify and quantify cervical spine soft tissue injury and injury threshold acceleration for head-turned rear impact, and to compare these data with previously published head-forward rear and frontal impact results., Summary of Background Data: Epidemiologically and clinically, head-turned rear impact is associated with increased injury severity and symptom duration, as compared to forward facing. To our knowledge, no biomechanical data exist to explain this finding., Methods: Six human cervical spine specimens (C0-T1) with head-turned and muscle force replication were rear impacted at 3.5, 5, 6.5, and 8 g, and flexibility tests were performed before and after each impact. Soft tissue injury was defined as a significant increase (P < 0.05) in intervertebral flexibility above baseline. Injury threshold was the lowest T1 horizontal peak acceleration that caused the injury., Results: The injury threshold acceleration was 5 g with injury occurring in extension or axial rotation at C3-C4 through C7-T1, excluding C6-C7. Following 8 g, 3-plane injury occurred in extension and axial rotation at C5-C6, while 2-plane injury occurred at C7-T1., Conclusions: Head-turned rear impact caused significantly greater injury at C0-C1 and C5-C6, as compared to head-forward rear and frontal impacts, and resulted in multiplanar injuries at C5-C6 and C7-T1.