Cervical facet joint kinematics during bilateral facet dislocation

Facet deflection and strain are dependent on axial compression and distraction in C5-C7 spinal segments under constrained flexion

Background

Facet fractures are frequently associated with clinically observed cervical facet dislocations (CFDs); however, to date there has only been one experimental study, using functional spinal units (FSUs), which has systematically produced CFD with concomitant facet fracture. The role of axial compression and distraction on the mechanical response of the cervical facets under intervertebral motions associated with CFD in FSUs has previously been shown. The same has not been demonstrated in multi-segment lower cervical spine specimens under flexion loading (postulated to be the local injury vector associated with CFD).

Methods

This study investigated the mechanical response of the bilateral inferior C6 facets of thirteen C5-C7 specimens (67±13 yr, 6 male) during non-destructive constrained flexion, superimposed with each of five axial conditions: (1) 50 N compression (simulating weight of the head); (2-4) 300, 500, and 1000 N compression (simulating the spectrum of intervertebral compression resulting from neck muscle bracing prior to head-first impact and/or externally applied compressive forces); and, (5) 2 mm of C6/C7 distraction (simulating the intervertebral distraction present during inertial loading of the cervical spine by the weight of the head). Linear mixed-effects models (α = 0.05) assessed the effect of axial condition.

Results

Increasing amounts of intervertebral compression superimposed on flexion rotations, resulted in increased facet surface strains (range of estimated mean difference relative to Neutral: maximum principal = 77 to 110 με, minimum principal = 126 to 293 με, maximum shear = 203 to 375 με) and angular deflection of the bilateral inferior C6 facets relative to the C6 vertebral body (range of estimated mean difference relative to Neutral = 0.59° to 1.47°).

Conclusions

These findings suggest increased facet engagement and higher load transfer through the facet joint, and potentially a higher likelihood of facet fracture under the compressed axial conditions.

The Effect of Axial Compression and Distraction on Cervical Facet Cartilage Apposition During Shear and Bending Motions

During cervical spine trauma, complex intervertebral motions can cause a reduction in facet joint cartilage apposition area (CAA), leading to cervical facet dislocation (CFD). Intervertebral compression and distraction likely alter the magnitude and location of CAA, and may influence the risk of facet fracture. The aim of this study was to investigate facet joint CAA resulting from intervertebral distraction (2.5 mm) or compression (50, 300 N) superimposed on shear and bending motions. Intervertebral and facet joint kinematics were applied to multi rigid-body kinematic models of twelve C6/C7 motion segments (70 ± 13 year, nine male) with specimen-specific cartilage profiles. CAA was qualitatively and quantitatively compared between distraction and compression conditions for each motion; linear mixed-effects models (α = 0.05) were applied. Distraction significantly decreased CAA throughout all motions, compared to the compressed conditions (p < 0.001), and shifted the apposition region towards the facet tip. These observations were consistent bilaterally for both asymmetric and symmetric motions. The results indicate that axial neck loads, which are altered by muscle activation and head loading, influences facet apposition. Investigating CAA in longer cervical spine segments subjected to quasistatic or dynamic loading may provide insight into dislocation and fracture mechanisms.

Estimating Facet Joint Apposition with Specimen-Specific Computer Models of Subaxial Cervical Spine Kinematics

Computational models of experimental data can provide a noninvasive method to estimate spinal facet joint biomechanics. Existing models typically consider each vertebra as one rigid-body and assume uniform facet cartilage thickness. However, facet deflection occurs during motion, and cervical facet cartilage is nonuniform. Multi rigid-body computational models were used to investigate the effect of specimen-specific cartilage profiles on facet contact area estimates. Twelve C6/C7 segments underwent non-destructive intervertebral motions. Kinematics and facet deflections were measured. Three-dimensional models of the vertebra and cartilage thickness estimates were obtained from pre-test CT data. Motion-capture data was applied to two model types (2RB: C6, C7 vertebrae each one rigid body; 3RB: left and right C6 posterior elements, and C7 vertebrae, each one rigid body) and maximum facet mesh penetration was compared. Constant thickness cartilage (CTC) and spatially-varying thickness cartilage (SVTC) profiles were applied to the facet surfaces of the 3RB model. Cartilage apposition area (CAA) was compared. Linear mixed-effects models were used for all quantitative comparisons. The 3RB model significantly reduced penetrating mesh elements by accounting for facet deflections (p = 0.001). The CTC profile resulted in incongruent facet articulation, whereas realistic congruence was observed for the SVTC profile. The SVTC profile demonstrated significantly larger CAA than the CTC model (p < 0.001).

Investigating the Effect of Axial Compression and Distraction on Cervical Facet Mechanics During Supraphysiologic Anterior Shear

Bilateral cervical facet dislocation (BFD) with facet fracture (Fx) often causes tetraplegia but is rarely recreated experimentally, possibly due to a lack of muscle replication. Intervertebral axial compression (due to muscle activation) or distraction (due to inertial loading), when combined with excessive anterior translation, may influence interfacet contact or separation and the subsequent production of BFD with or without Fx. This paper presents a methodology to produce C6/C7 BFD+Fx using anterior shear motion superimposed with 300 N compression or 2.5 mm distraction. The effect of these superimposed axial conditions on six-axis loads, and C6 inferior facet deflections and surface strains, was assessed. Twelve motion segments (70 ± 13 yr) achieved 2.19 mm of supraphysiologic anterior shear without embedding failure (supraphysiologic shear analysis point; SSP), and BFD+Fx was produced in all five specimens that reached 20 mm of shear. Linear mixed-effects models (α = 0.05) assessed the effect of axial condition. At the SSP, the compressed specimens experienced higher axial forces, facet shear strains, and sagittal facet deflections, compared to the distracted group. Facet fractures had similar radiographic appearance to those that are observed clinically, suggesting that intervertebral anterior shear motion contributes to BFD+Fx.

Radiological Signs in Traumatic Cervical Facet Joint Dislocations

Unilateral cervical facet joint dislocation (UCFJD) is the most frequently missed cervical spine injury on plain radiographs. If left untreated, UCFJD can progress to bilateral cervical facet joint dislocation. Given the complexity of cervical facet joint dislocations, radiologists rely on metaphorical signs to identify them on radiographs. The “Bow-tie” and “laminar space” signs represent UCFJD on plain radiographs. The “reversed hamburger”, “naked facet” and “headphones” signs represent cervical facet joint dislocations on axial cross-sectional imaging. Illustrating these signs in an engaging manner facilitates pattern-based recognition, which can benefit trainees and radiologists. Moreover, pattern-based recognition can be applied to machine learning.

An Experimental Study on the Safety and Mechanism of Reduction of Subaxial Cervical Facet Dislocation Using Z-Shape Elevating-Pulling Reduction Technique

OBJECTIVE

We sought to clarify the safety and unlocking mechanism of the Z-shape elevating-pulling closed reduction (ZR) technique and to analyze the differences in facet contact force and intraspinal pressure during subaxial facet dislocation reduction using the ZR technique and traditional skull traction closed reduction (SR).

METHODS

In 15 human cadaveric skull-neck-thorax specimens, reproducible unilateral and bilateral facet dislocations (UFDs/BFDs) were created at the C5-C6 level and then reduced by applying the ZR and SR techniques, respectively. Tekscan FlexiForce A-201 pressure sensors were used to measure the anterior and posterior intraspinal pressure and injured facet contact force under physiological conditions and before and after reduction. The maximum pressures during the reduction process were recorded.

RESULTS

After creation of the facet dislocation, the anterior and posterior intraspinal pressure and facet contact force were significantly increased relative to normal (P < 0.001). The UFDs and BFDs of all specimens were successfully reduced by both ZR and SR, and the intraspinal pressure and facet contact force were significantly reduced compared with before reduction (P < 0.001). Compared with SR, the maximum posterior intraspinal pressure during BFD reduction (P = 0.027) and the maximum facet contact force during UFD reduction (P < 0.001) were lower when ZR was used for closed reduction.

CONCLUSIONS

Our findings suggest that ZR and SR can both be used to reduce subaxial facet dislocation and decompress the spinal cord. However, the ZR technique appears to safer and more effective than the SR technique for closed reduction of subaxial facet dislocations.

A Novel Anterior-Only Surgical Approach for Reduction and Fixation of Cervical Facet Dislocation

BACKGROUND

The anterior-only surgical procedure, including discectomy, open reduction, fusion, and fixation, is a recommended approach in the treatment of cervical facet dislocations. This approach has a reduction failure rate of up to 40%. When it fails, a posterior approach is usually required.

OBJECTIVE

To report a novel anterior-only surgical approach for reduction and fixation in patients with cervical facet dislocation, even for severe vertebral fracture, articular process fracture, and delayed surgical management.

METHODS

Sixty-three consecutive patients with unilateral/bilateral facet dislocation of the subaxial cervical spine were treated with this anterior-only procedure. After discectomy, kyphotic paramedian distraction reduction with Caspar pins plus vertebral screw plate fixation was performed. If the reduction failed, anterior facetectomy reduction plus anterior cervical pedicle screw plate fixation was introduced.

RESULTS

Among the 63 patients treated, 52 patients achieved successful reduction with the technique of kyphotic paramedian distraction with Caspar pins; the remaining 11 patients with failure of the former technique experienced successful reduction with the anterior facetectomy technique. No supplemental posterior approach surgery was performed. The American Spinal Injury Association grade was improved by at least 1 grade in 23 patients after this procedure, and no neurologic deterioration occurred in any of the patients. After at least 12 months of follow-up, all patients achieved satisfactory fusion, and there was no implant failure.

CONCLUSIONS

An anterior-only surgical procedure including kyphotic paramedian distraction with Caspar pins, anterior facetectomy, and anterior pedicle screw plate fixation is safe and effective for subaxial cervical facet dislocations.

On the relative importance of bending and compression in cervical spine bilateral facet dislocation

BACKGROUND

Cervical bilateral facet dislocations are among the most devastating spine injuries in terms of likelihood of severe neurological sequelae. More than half of patients with tetraparesis had sustained some form of bilateral facet fracture dislocation. They can occur at any level of the sub-axial cervical spine, but predominate between C5 and C7. The mechanism of these injuries has long been thought to be forceful flexion of the chin towards the chest. This "hyperflexion" hypothesis comports well with intuition and it has become dogma in the clinical literature. However, biomechanical studies of the human cervical spine have had little success in producing this clinically common and devastating injury in a flexion mode of loading.

METHODS

The purpose of this manuscript is to review the clinical and engineering literature on the biomechanics of bilateral facet dislocations and to describe the mechanical reasons for the causal role of compression, and the limited role of head flexion, in producing bilateral facet dislocations.

FINDINGS

Bilateral facet dislocations have only been produced in experiments where compression is the primary loading mode. To date, no biomechanical study has produced bilateral facet dislocations in a whole spine by bending. Yet the notion that it is primarily a hyper-flexion injury persists in the clinical literature.

INTERPRETATION

Compression and compressive buckling are the primary causes of bilateral facet dislocations. It is important to stop using the hyper-flexion nomenclature to describe this class of cervical spines injuries because it may have a detrimental effect on designs for injury prevention.

The effect of axial compression and distraction on cervical facet mechanics during anterior shear, flexion, axial rotation, and lateral bending motions

The subaxial cervical facets are important load-bearing structures, yet little is known about their mechanical response during physiological or traumatic intervertebral motion. Facet loading likely increases when intervertebral motions are superimposed with axial compression forces, increasing the risk of facet fracture. The aim of this study was to measure the mechanical response of the facets when intervertebral axial compression or distraction is superimposed on constrained, non-destructive shear, bending and rotation motions. Twelve C6/C7 motion segments (70 ± 13 yr, nine male) were subjected to constrained quasi-static anterior shear (1 mm), axial rotation (4°), flexion (10°), and lateral bending (5°) motions. Each motion was superimposed with three axial conditions: (1) 50 N compression; (2) 300 N compression (simulating neck muscle contraction); and, (3) 2.5 mm distraction. Angular deflections, and principal and shear surface strains, of the bilateral C6 inferior facets were calculated from motion-capture data and rosette strain gauges, respectively. Linear mixed-effects models (α = 0.05) assessed the effect of axial condition. Minimum principal and maximum shear strains were largest in the compressed condition for all motions except for maximum principal strains during axial rotation. For right axial rotation, maximum principal strains were larger for the contralateral facets, and minimum principal strains were larger for the left facets, regardless of axial condition. Sagittal deflections were largest in the compressed conditions during anterior shear and lateral bending motions, when adjusted for facet side.

Traumatic high-grade spondylolisthesis at C7-T1 with no neurological deficits: Case series, literature review, and biomechanical implications

Traumatic high-grade spondylolisthesis in subaxial cervical spine is frequently associated with acute spinal cord injury and quadriparesis. There have been rare cases where such pathology demonstrates minimal to no neurological deficits. Assessment of the underlying biomechanics may provide insight into the mechanism of injury and associated neurological preservation. Patient 1 is a 63-year-old female presenting after a motor vehicle collision with significant right arm pain without neurological deficits. Imaging demonstrated C7/T1 spondyloptosis, associated with a locked facet on the left at C6/7 and a locked facet on the right at C7/T1, with a fracture of the left C7 pedicle and right C7 lamina. Patient 2 is a 60-year-old male presenting after a bicycle collision with transient bilateral upper extremity paresthesias without neurological deficits. Imaging demonstrated C7/T1 spondyloptosis, with fractures of bilateral C7 pedicles, C7/T1 facets, and C7 lamina. Patient 3 is a 36-year-old male presenting after a motor vehicle collision with diffuse tingling sensation throughout all extremities. His neurological examination was nonfocal. Imaging demonstrated a grade 4 spondylolithesis at C7/T1, associated with bilateral C7/T1 locked facets. From literature, most cases were noted to be dislocations resulting from fractures of the posterior elements. A minority of cases has been found to involve facet dislocations without fractures. Further biomechanical studies are needed to understand the underlying mechanisms.

Influence of graft size on spinal instability with anterior cervical plate fixation following in vitro flexion-distraction injuries

BACKGROUND CONTEXT

Anterior cervical discectomy and fusion with plating (ACDFP) is commonly used for the treatment of distractive-flexion cervical spine injuries. Despite the prevalence of ACDFP, there is little biomechanical evidence for graft height selection in the unstable trauma scenario.

PURPOSE

This study aimed to investigate whether changes in graft height affect the kinematics of instrumented ACDFP C5-C6 motion segments in the context of varying degrees of simulated facet injuries.

STUDY DESIGN

In vitro cadaveric biomechanical study was used as study design.

METHODS

Seven C5-C6 motion segments were mounted in a custom spine simulator and taken through flexibility testing in axial rotation, lateral flexion, and flexion-extension. Specimens were first tested intact, followed by a standardized injury model (SIM) for a unilateral facet perch at C5-C6. The stability of the ACDFP approach was then examined with three graft heights (computed tomography-measured disc space height, disc space height undersized by 2.5 mm, and disc space height oversized by 2.5 mm) within three increasing unstable injuries (SIM, an added unilateral facet fracture, and a simulated bilateral facet dislocation injury).

RESULTS

In all motions, regardless of graft size, ACDFP reduced range of motion (ROM) from the SIM state. For flexion-extension, the oversized graft had a larger decrease in ROM compared with the other graft sizes (p<.05). Between graft sizes and injury states, there were a number of interactions in axial rotation and lateral flexion, where specifically in the most severe injury, the undersized graft had a larger decrease in ROM than the other two sizes (p<.05).

CONCLUSIONS

This study found that graft size did affect the kinematic stability of ACDFP in a series of distractive-flexion injuries; the undersized graft resulted in both facet overlap and locking of the uncovertebral joints leading to decreased ROM in lateral bending and axial rotation, whereas an oversized graft provided larger ROM decreases in flexion-extension. As such, a graft that engages the uncovertebral joint may be more advantageous in providing a rigid environment for fusion with ACDFP.

Treatment of traumatic spondylolisthesis of the lower cervical spine with concomitant bilateral facet dislocations: risk of respiratory deterioration

BACKGROUND

This study aimed to retrospectively examine 36 cases of bilateral cervical facet dislocations (BCFD) of the lower cervical spine who were at risk for respiratory deterioration.

METHODS

The cases of 36 subjects with BCFD of the lower cervical spine who failed to achieve closed reduction were retrospectively studied. The extents of neurological injuries included posterior neck pain without neurological deficit (n=2), incomplete spinal cord injury (ISCI) (n=21), and complete spinal cord injury (CSCI) (n=13).

RESULTS

Among the subjects, 26 (72.22%) had dyspnea, 6 required mechanical ventilation due to respiratory muscle paralysis, 11 required tracheostomy, and 9 required intubation. All patients received posterior approach reduction, stabilization, and fusion treatment for BCFD in one operative session. For the 26 quadriparetic patients with dyspnea, priority was given to treating their respiratory problems. For the other 10 patients without dyspnea, surgical treatment for irreducible lower cervical spine dislocation was given priority. After an average follow-up period of 63 months, 21 complications were found, but all patients exhibited fusion. Twenty-one patients with ISCI exhibited improvements in their conditions of 1 or 2 grades on the American Spinal Injury Association scale, whereas those with CSCI did not improve. All 26 apnea cases improved. The majority (26) of the 36 cases with BCFD of the lower cervical spine suffered dyspnea.

CONCLUSIONS

Although further study is required, our study suggests that the posterior surgical approach to the cervical spine is safe and effective for patients with traumatic spondylolisthesis of the lower cervical spine concomitant with BCFD who are at risk of respiratory deterioration.

A delayed diagnosis of bilateral facet dislocation of the cervical spine: a case report

OBJECTIVE

To review the case of a patient suffering from bilateral facet dislocation of the cervical spine.

CLINICAL FEATURES

A 53-year-old male was involved in a car accident and was transported to the hospital. Cervical radiographs were taken at the emergency department and interpreted as normal. Four days later, he consulted a chiropractor where radiographs of the cervical spine were repeated. The examination revealed bilateral cervical facet joint dislocation at C5-C6 as well as a fracture involving the spinous process and laminae of C6.

INTERVENTION AND OUTCOME

The patient was referred to the hospital and underwent surgery.

CONCLUSION

Patients involved in motor vehicle accidents often consult chiropractors for neck pain treatment. A high index of suspicion due to significant history and physical examination findings should guide the clinician in determining the need for reviewing the initial radiographs (if taken and available) or request repeat studies, regardless of the initial imaging status.

Three-dimensional motion analysis of the cervical spine for comparison of anterior cervical decompression and fusion versus artificial disc replacement in 17 patients

Object

Cervical arthroplasty with an artificial disc (AD) has emerged as an alternative to anterior cervical discectomy and fusion (ACDF) for the management of cervical spondylosis. This study aims to provide 3D motion analysis data comparing patients after ACDF and AD replacement.

Methods

Ten patients who underwent C5–6 ACDF and 7 who underwent C5–6 AD replacement were enrolled. Using biplanar fluoroscopy and a model-based track technique (accurate up to 0.6 mm and 0.6°), motion analysis of axial rotation and flexion-extension of the neck was performed. Three nonoperative segments (C3–4, C4–5, and C6–7) were assessed for both intervertebral rotation (coronal, sagittal, and axial planes) and facet shear (anteroposterior and mediolateral).

Results

There was no difference in total neck motion comparing ACDF and AD replacement for neck extension (43.3° ± 10.2° vs 44.3° ± 12.6°, p = 0.866) and rotation (36.0° ± 6.5° vs 38.2° ± 9.3°, p = 0.576). For extension, when measured as a percentage of total neck motion, there was a greater amount of rotation at the nonoperated segments in the ACDF group than in the AD group (p = 0.003). When comparing specific motion segments, greater normalized rotation was seen in the ACDF group at C3–4 (33.2% ± 4.9% vs 26.8% ± 6.6%, p = 0.036) and C6–7 (28.5% ± 6.7% vs 20.5% ± 5.5%, p = 0.009) but not at C4–5 (33.5% ± 6.4% vs 31.8% ± 4.0%, p = 0.562). For neck rotation, greater rotation was observed at the nonoperative segments in the ACDF group than in the AD group (p = 0.024), but the differences between individual segments did not reach significance (p ≥ 0.146). Increased mediolateral facet shear was seen on neck extension with ACDF versus AD replacement (p = 0.008). Comparing each segment, C3–4 (0.9 ± 0.5 mm vs 0.4 ± 0.1 mm, p = 0.039) and C4–5 (1.0 ± 0.4 mm vs 0.5 ± 0.2 mm, p = 0.022) showed increased shear while C6–7 (1.0 ± 0.4 mm vs 1.0 ± 0.5 mm, p = 0.767) did not.

Conclusions

This study illustrates increased motion at nonoperative segments in patients who have undergone ACDF compared with those who have undergone AD replacement. Further studies will be required to examine whether these changes contribute to adjacent-segment disease.

Contact Pressure in the Facet Joint During Sagittal Bending of the Cadaveric Cervical Spine

The facet joint contributes to the normal biomechanical function of the spine by transmitting loads and limiting motions via articular contact. However, little is known about the contact pressure response for this joint. Such information can provide a quantitative measure of the facet joint’s local environment. The objective of this study was to measure facet pressure during physiologic bending in the cervical spine, using a joint capsule-sparing technique. Flexion and extension bending moments were applied to six human cadaveric cervical spines. Global motions (C2-T1) were defined using infra-red cameras to track markers on each vertebra. Contact pressure in the C5-C6 facet was also measured using a tip-mounted pressure transducer inserted into the joint space through a hole in the postero-inferior region of the C5 lateral mass. Facet contact pressure increased by 67.6 ± 26.9 kPa under a 2.4 Nm extension moment and decreased by 10.3 ± 9.7 kPa under a 2.7 Nm flexion moment. The mean rotation of the overall cervical specimen motion segments was 9.6 ± 0.8° and was 1.6 ± 0.7° for the C5-C6 joint, respectively, for extension. The change in pressure during extension was linearly related to both the change in moment (51.4 ± 42.6 kPa/Nm) and the change in C5-C6 angle (18.0 ± 108.9 kPa/deg). Contact pressure in the inferior region of the cervical facet joint increases during extension as the articular surfaces come in contact, and decreases in flexion as the joint opens, similar to reports in the lumbar spine despite the difference in facet orientation in those spinal regions. Joint contact pressure is linearly related to both sagittal moment and spinal rotation. Cartilage degeneration and the presence of meniscoids may account for the variation in the pressure profiles measured during physiologic sagittal bending. This study shows that cervical facet contact pressure can be directly measured with minimal disruption to the joint and is the first to provide local pressure values for the cervical joint in a cadaveric model.

Pressure measurement in the cervical spinal facet joint: considerations for maintaining joint anatomy and an intact capsule

STUDY DESIGN

A novel noninvasive approach to measure facet joint pressure in the cervical spine was investigated using a tip-mounted transducer that can be inserted through a hole in the bony lateral mass. This technique is advantageous because it does not require resection of the joint capsule, but there are potential issues regarding its applicability that are addressed.

OBJECTIVE

The objective was to evaluate the effect of a tip-mounted pressure probe's position and orientation on contact pressure measurements in biomechanical experiments.

SUMMARY OF BACKGROUND DATA

Measurements of direct contact pressure in the facet joint of cadaveric spines have been obtained via pressure-sensitive films. However, that method requires the resection of the facet capsule, which can alter the overall joint's mechanical behavior and can affect the measured contact pressures.

METHODS

Influence of position and orientation on probe measurements was evaluated in companion surrogate and cadaveric investigations. The probe was placed in the facet of an anatomic vertebral C4/5 surrogate undergoing sagittal bending moments. Pressure-sensitive paper was used to map contact regions in the joint of the surrogate and cadaveric cervical segments (n = 3) during extension. The probe also underwent uniaxial compression in cadaveric facets to evaluate the effect of orientation relative to the contact surface on the probe signal.

RESULTS

Although experimental and theoretical pressure profiles followed the same trends, measured maximum pressures were half of the theoretical ones. In the orientation study, maximum pressures were not different for probe orientations of 0° and 5°, but no signal was recorded at orientations greater than 15°.

CONCLUSION

This approach to measure pressure was selected to provide a minimally-invasive method to quantify facet joint pressures during clinically relevant applications. Both the position and orientation of the probe are critical factors in monitoring local pressure profiles in this mobile synovial joint.

Biomechanics of cervical facet dislocation

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.