The Journal of Bone & Joint Surgery

The Journal of Bone & Joint Surgery, Volume 94, Issue 21

Scientific Articles | November 07, 2012

A Biomechanical Assessment of Soft-Tissue Damage in the Cervical Spine Following a Unilateral Facet Injury

Mélissa Nadeau, MD¹; Stewart D. McLachlin, MESc¹; Stewart I. Bailey, MD¹; Kevin R. Gurr, MD¹; Cynthia E. Dunning, PhD¹; Christopher S. Bailey, MD¹

¹ Division of Orthopaedics, Departments of Surgery (M.N., S.I.B., K.R.G., and C.S.B.) and Mechanical & Materials Engineering (S.D.M. and C.E.D.), Western University, 1151 Richmond Street, London, ON N6A 5B9, Canada. E-mail address for C.E. Dunning: cdunning@uwo.ca

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Investigation performed at the Jack McBain Biomechanical Testing Laboratory, Western University, London, Ontario, Canada

Disclosure: One or more of the authors received payments or services, either directly or indirectly (i.e., via his or her institution), from a third party in support of an aspect of this work. In addition, one or more of the authors, or his or her institution, has had a financial relationship, in the thirty-six months prior to submission of this work, with an entity in the biomedical arena that could be perceived to influence or have the potential to influence what is written in this work. No author has had any other relationships, or has engaged in any other activities, that could be perceived to influence or have the potential to influence what is written in this work. The complete Disclosures of Potential Conflicts of Interest submitted by authors are always provided with the online version of the article.

J Bone Joint Surg Am, 2012 Nov 07;94(21):e156 1-6. doi: 10.2106/JBJS.K.00694

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Abstract

Background:

Unilateral cervical spine facet injuries encompass a wide spectrum, including subluxations, dislocations, and fractures, and the instability produced varies greatly. The extent of anatomical disruption secondary to a unilateral facet injury is poorly understood, and few biomechanical studies have quantified the associated kinematics. The purpose of this study was to develop an experimental method that reliably produces an impending unilateral facet dislocation (perched facet) in cadaveric cervical spines and to identify the soft-tissue damage and resulting changes in cervical spine range of motion and neutral zone associated with this injury.

Methods:

Nine fresh-frozen cadaveric human spinal motion segments (C4-C5 or C6-C7) were mounted in a spinal loading simulator to induce a perched unilateral facet injury based on a previously described mechanism of flexion and bending with increasing rotation. Loads were applied to simulate and measure flexion-extension, lateral bending, and axial rotation motions before and after achieving a perched facet. Preinjury and postinjury range of motion and neutral zone were analyzed with use of paired t tests for each movement. Systematic qualitative inspection and gross dissection were then performed to define the soft-tissue injury pattern.

Results:

Range of motion and neutral zone increased following the reduction of this injury; the largest increase (294%) occurred in contralateral axial rotation (i.e., right axial rotation after a perched left facet). Postinjury dissections revealed bilateral capsular tears, 50% disc disruption, and 50% tearing of the ligamentum flavum in most specimens. The interspinous and supraspinous ligaments were stretched in less than half of the specimens and were never completely disrupted. The longitudinal ligaments were occasionally torn as extensions of anulus fibrosus disruptions.

Conclusions:

This study indicates that the anulus fibrosus, nucleus pulposus, and ligamentum flavum are important cervical spine stabilizers. Facet capsules were often torn bilaterally, implying a more advanced injury than a unilateral facet injury. These discoligamentous injuries result in increases in range of motion and neutral zone.

Clinical Relevance:

The results from this work provide further insight into the expected injury and associated instability present in a traumatic unilateral facet injury in the cervical spine.

Figures in this Article

Introduction

Introduction | Materials and Methods | Results | Discussion | Appendix

Trauma to the subaxial cervical spine encompasses a wide spectrum of injury, ranging from nondisplaced fractures of the articular process or capsular sprains to subluxations or dislocations. The severity of injury is dependent on the extent of damage to the stabilizing osseous and soft-tissue structures of the subaxial spine, which is influenced by patient factors, including age and bone health¹. Specific to a unilateral facet injury, the posterior osteoligamentous complex is likely injured initially following a distraction-type injury mechanism; this injury then spreads to the anterior discoligamentous complex with the addition of a further rotation or translation injury mechanism^2-5.

Few studies have attempted to identify the extent of anatomical disruption secondary to unilateral facet dislocation in the cervical spine and also quantify the associated kinematics^4,6,7. Sim et al.⁶ identified the ipsilateral facet capsule, anulus fibrosus, and ligamentum flavum as passive stabilizers resisting unilateral cervical spine facet dislocations. However, their methodology used an unlikely mechanism of injury (lateral distraction) in a static model and did not measure the posttraumatic stability. Vaccaro et al.⁷ reviewed cervical spine magnetic resonance imaging (MRI) of patients who had sustained a unilateral facet dislocation and noted that the ligamentum flavum, interspinous and supraspinous ligaments, nucleus pulposus, and capsules were commonly damaged. In cadavers, Crawford et al.⁴ created a unilateral facet dislocation in nine specimens and demonstrated increased cervical spine motion following the reduction of this injury, which was greatest in axial rotation. They did not comment on the extent of soft-tissue disruption responsible for the change in their reported kinematics measures. Other cadaveric and imaging studies have focused on the important role of the posterior longitudinal ligament and the interspinous ligament^8-10. Biomechanical studies evaluating the use of spinal fixation devices have also examined the effects of soft-tissue injury in relation to the stability provided by the device, but have not considered the effect of injury alone^11-13.

The aims of this study were (1) to develop an experimental method that reliably produces an impending unilateral facet dislocation (facet perch) in cadaveric subaxial cervical spinal segments, (2) to identify the associated soft-tissue injuries, and (3) to quantify the resulting change in cervical spine motion and neutral zone due to soft-tissue injuries.

Materials and Methods

Introduction | Materials and Methods | Results | Discussion | Appendix

Nine fresh-frozen cadaveric cervical spine motion segments (C4-C5 or C6-C7) (mean age and standard deviation, 63 ± 9 years) were used in this study. Each complete (C1 to C7) cervical spine specimen was imaged with computed tomography (CT) scanning to rule out the presence of any abnormal degeneration or pathology. The specimens were then thawed and cleaned of musculature, without disruption of ligaments, capsules, bones, or discs. The C4-C5 and/or C6-C7 functional spinal units were isolated from the cervical spine specimens for single motion segment testing.

The specimens were potted from their rostral and caudal ends with use of a previously described technique^4,14,15. Potting fixation required four screws with bicortical purchase placed in each vertebral body: two entering the end plate and exiting at the posterior vertebral body cortex, and two penetrating each lamina and going into the posterior vertebral body cortex. The protruding screw heads were potted in 10-cm-diameter polyvinyl chloride piping, held rigidly within stainless steel potting fixtures, with use of a 2.5-cm-thick layer of cement (Denstone; Heraeus, South Bend, Indiana).

Flexibility testing was performed on each specimen with use of a spinal loading simulator, developed as a modification to an existing materials testing machine (Instron 8874; Instron, Canton, Massachusetts). Simulated motions included independent flexion-extension, lateral bending, and axial rotation. Loading was applied to the rostral vertebra from the actuator via a loading arm with universal joints at each end, which allowed five degrees of freedom in addition to the loading direction (Fig. 1). This loading technique was similar to the methods described previously by Goertzen et al.¹⁶. The caudal vertebra was fixed to the testing platform. To standardize the starting location, the specimen was initially aligned to approximate the axis of movement of the motion segment, which is central in the coronal plane and in the posterior third of the vertebral body in the sagittal plane^17,18.

Fig. 1

Illustration showing the spine simulator that was built as a modification to a customized Instron 8874, which has a main axial and torque actuator along with an additional off-axis torque actuator. The two loading arms, ball splines with universal joints at each end, connect the actuators to the upper fixture to apply bending moments to the specimen. Inset photograph showing cadaveric cervical spine segments that were mounted in the upper (A) and lower (B) potting fixtures of the spinal loading simulator, with Optotrak Smart Markers (C) attached to each fixture for motion tracking.

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An Optotrak Certus motion capture system (NDI, Waterloo, Ontario, Canada) recorded the motion of the each vertebra in six degrees of freedom. The camera captured infrared signals sent by Optotrak Smart Markers attached to the upper and lower potting fixtures to represent the rostral and caudal vertebrae, respectively (Fig. 1). A local bone coordinate system for each vertebra was defined from four osseous landmarks: the left and right transverse processes and the centers of the superior and inferior end plates anteriorly. This bone coordinate system was used to transform marker motion to vertebral motion.

The flexibility testing protocol began with sequential preinjury kinematics data collection in flexion-extension, lateral bend, and axial rotation. For each of these motions, specimens were loaded at 3°/s, up to a target load of ±1.5 Nm. This loading method has been previously reported as producing nondestructive physiologic-type motion in cadaveric cervical spines^4,15,19. An axial preload of 40 N was applied during the simulated motions to approximate the compressive force that would be seen in vivo. As such, the extremes of physiologic motion and, in turn, the possible creation of soft-tissue damage were avoided. In order to minimize the viscoelastic effects, two preconditioning cycles in each of the three motions were performed before data from the third cycle were collected for analysis. Kinematic data collected for left lateral bend and left axial rotation motions were considered to be ipsilateral (the side of the injury) rotations; in contrast, rotations to the right side were considered to be contralateral (the opposite side of the injury). Throughout testing, the specimens were kept moist with use of normal saline solution¹⁹.

A unilateral facet perch was then induced in the left facet joint of each specimen. The simulated mechanism of injury of a unilateral facet dislocation has been described in the literature as being a combination of flexion, contralateral bending, and axial rotation^3,4,9,20-25. Initially, the potting fixture of the rostral vertebra was loaded with deadweights to position the specimen in flexion and contralateral bend, necessary to moderately distract the facet joint of interest. A contralateral axial rotational torque was then applied via the spinal loading simulator at a rate of 0.5°/s until impending dislocation (i.e., facet perch) was achieved in the left facet joint. Direct visualization of the facet perch was possible by virtue of a small lateral capsular slit performed during specimen preparation (Fig. 2-A). Following the facet perch (Fig. 2-B), the rostral vertebra was rotated back into a reduced position, and postinjury flexibility testing was performed with use of the same protocol as in the intact state.

Fig. 2

Fig. 2-A Photograph showing the direct visualization of the facet to be injured prior to testing, by virtue of a lateral capsular surgical slit. The solid white lines indicate the initial positions of the inferior articular process of the cranial vertebra and the superior articular process of the caudal vertebra (i.e., the facet joint). Small marks were defined on the anterior and posterior aspects of the articular processes to assist with identifying the instance of facet perch. Fig. 2-B Photograph showing that when this position was achieved (as identified by the solid white lines), the mechanism of injury was halted (axial torque component) and rotated back into a reduced position.

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Each specimen was then removed from the simulator and systematically inspected by a single observer with use of gross dissection techniques. Specifically, the integrity of the facet capsules, supraspinous and interspinous ligaments, ligamentum flavum, anterior and posterior longitudinal ligaments, anulus fibrosus, and nucleus pulposus was assessed. All structures were evaluated ipsilaterally (the side of the facet injury) and contralaterally (the side opposite to the facet injury). Evaluation of the disc included an independent assessment of the anulus and the nucleus pulposus. A standardized data sheet was used to record whether the structures were intact, were stretched (but were in continuity), or were disrupted. Structures that were stretched or disrupted were grouped as injured structures for purposes of data analysis. Data from all specimens were tabulated and the most consistently injured structures were identified to extrapolate a common pattern of injury.

Kinematic data analysis of preinjury and postinjury testing was performed post hoc with use of custom-written software (LabVIEW; National Instruments, Austin, Texas). Kinematic parameters that were examined included the magnitude of segmental (i.e., C4-C5 and C6-C7) ranges of motion for each of the three motions simulated, along with the neutral zone calculation (based on the width of the hysteresis curve at 0 Nm) as a measure of specimen laxity^19,26. Range of motion and neutral zone for each simulated motion, both preinjury and postinjury, were analyzed with use of paired t tests. The effect of segment level was compared with use of an analysis of variance (ANOVA) looking at the maximum rotation and torque achieved during the facet perch.

Source of Funding

Supplies for this research were directly funded by a grant from the Physicians’ Services Incorporated Foundation. Indirectly, equipment and salaries were provided by the Natural Sciences and Engineering Research Council of Canada, Canadian Foundation for Innovation, Ontario Innovation Trust, and The University of Western Ontario (later renamed as Western University).

Results

Introduction | Materials and Methods | Results | Discussion | Appendix

A unilateral facet perch was achieved in all specimens, without creating any dislocations. Three specimens also sustained a facet fracture, with two of these fractures occurring at 90% of subluxation of the facets. The third fracture occurred at 50% of subluxation, but involved the posterior 10% of the superior articular process only and was nondisplaced. Between the non-fractured and fractured specimens, there were similar injuries to the soft-tissue structures. Furthermore, subgroup analysis showed no difference in the range of motion or neutral zone. Therefore, these specimens were included in data analysis.

Specimen data for all nine specimens, including age, level, presence of fracture, and the maximum torque and rotation achieved during the facet perch injury mechanism, were tabulated (see Appendix). No significant differences in specimen rotation or maximum torque applied were identified for the specimen level (p > 0.05).

Postinjury dissections revealed that the capsules and anulus fibrosus were the most commonly damaged structures (see Appendix). All specimens demonstrated capsular injury, with eight of the injuries being bilateral. All specimens sustained disruptions of the anulus fibrosis, which extended into the nucleus pulposus to involve >50% of the disc substance in seven specimens. Most commonly, the injured portion of the disc was contralateral to the facet perch, spanning this entire portion from anterior to posterior. Eight of the nine specimens had at least 50% of the ligamentum flavum injured, with the ipsilateral side more often affected (67%). The interspinous ligament in three specimens and the supraspinous ligament in four specimens were stretched but never completely torn. A partial intrasubstance tear was only sustained in the anterior longitudinal ligament for two specimens and in the posterior longitudinal ligament for one specimen. Longitudinal ligament tears were associated with injuries to the outer disc where the fibers of the longitudinal ligaments blend with those of the anulus fibrosus.

Both the range of motion and the neutral zone for all three simulated motions increased following reduction of the unilateral facet injury (Table I). The largest change (p < 0.001) was seen in axial rotation, with a near-threefold increase in the neutral zone (256%) and contralateral axial range of motion (right axial rotation following a left facet injury) (294%). In contrast, the increase in ipsilateral axial range of motion (rotation toward the injured facet) was small (32%) and not significant. In lateral bending, a 100% increase was seen in the neutral zone (p < 0.001), with an 87% increase in ipsilateral range of motion (p = 0.04). In the opposing direction, contralateral bending showed an increase of 63%, but was not significant (p = 0.12). Flexion-extension data revealed a significant change only in flexion (52% increase) (p < 0.02).

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TABLE I Average Range of Motion and Neutral Zone Values Before and After a Unilateral Facet Injury

Motion	Preinjury*(°)	Postinjury*(°)	Change (%)	P Value
Range of motion
Flexion	5.6 ± 2.1	8.5 ± 3.1	52	0.02
Extension	3.3 ± 0.6	3.9 ± 1.4	18	0.13
Ipsilateral axial rotation	3.1 ± 0.9	4.1 ± 3.4	32	0.37
Contralateral axial rotation	3.5 ± 1.2	13.8 ± 4.2	294	<0.001
Ipsilateral lateral bend	3.9 ± 2.3	7.3 ± 4.9	87	0.04
Contralateral lateral bend	4.1 ± 1.1	6.7 ± 4.0	63	0.12
Neutral zone†
Flexion-extension	2.0 ± 1.4	3.8 ± 3.6	90	0.18
Ipsilateral-contralateral axial rotation	2.5 ± 1.2	8.9 ± 3.3	256	<0.001
Ipsilateral-contralateral lateral bend	1.7 ± 0.7	3.4 ± 1.1	100	<0.001

*Values are given as the mean and the standard deviation.
†Neutral zone includes both directions in one plane of motion (i.e., flexion-extension combined).

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TABLE I Average Range of Motion and Neutral Zone Values Before and After a Unilateral Facet Injury

Motion	Preinjury*(°)	Postinjury*(°)	Change (%)	P Value
Range of motion
Flexion	5.6 ± 2.1	8.5 ± 3.1	52	0.02
Extension	3.3 ± 0.6	3.9 ± 1.4	18	0.13
Ipsilateral axial rotation	3.1 ± 0.9	4.1 ± 3.4	32	0.37
Contralateral axial rotation	3.5 ± 1.2	13.8 ± 4.2	294	<0.001
Ipsilateral lateral bend	3.9 ± 2.3	7.3 ± 4.9	87	0.04
Contralateral lateral bend	4.1 ± 1.1	6.7 ± 4.0	63	0.12
Neutral zone†
Flexion-extension	2.0 ± 1.4	3.8 ± 3.6	90	0.18
Ipsilateral-contralateral axial rotation	2.5 ± 1.2	8.9 ± 3.3	256	<0.001
Ipsilateral-contralateral lateral bend	1.7 ± 0.7	3.4 ± 1.1	100	<0.001

*Values are given as the mean and the standard deviation.
†Neutral zone includes both directions in one plane of motion (i.e., flexion-extension combined).

Discussion

Introduction | Materials and Methods | Results | Discussion | Appendix

To date, only a few studies exist that examine the extent of injury inflicted to the stabilizing structures of the subaxial cervical spine, following unilateral facet injuries or the resulting spinal instability^4,6,7. In previous literature, some authors have suggested that unilateral injuries account for 12% to 16% of all cervical spine injuries²⁷. One of these prior studies identified soft-tissue injury patterns through MRI⁷, but such techniques may not yield the desired specificity. Previous research in this area has shown that MRI may “overread” the extent of injury to the posterior ligamentous complex²⁸ and is not always reliable for evaluating injuries to the cervical disc, anterior longitudinal ligament, and posterior longitudinal ligament⁹. In contrast, in vitro biomechanical studies can induce cervical spine injuries similar to the clinical scenario⁴ and have the benefit of allowing dissection to reveal injured structures.

This investigation was largely successful in utilizing an in vitro model to produce a unilateral perched facet. The most common soft-tissue injury pattern observed consisted of disruptions of the facet capsules bilaterally, the ipsilateral ligamentum flavum, and >50% of the contralateral anulus fibrosus and nucleus pulposus. These findings are similar to those of Vaccaro et al.⁷, who identified the ligamentum flavum, nucleus pulposus, and facet capsules as the most commonly disrupted structures seen on MRI. However, this same study found 60% disruption of the interspinous ligament and 40% disruption of the supraspinous ligament in the same patient population. In the current investigation, the authors observed that the interspinous and supraspinous ligaments were stretched in only a small portion of the specimens tested. This difference may be attributed to the poor specificity associated with the ability of MRI to diagnose cervical spine soft-tissue injury²⁹. A cadaveric study by Sim et al.⁶ and a subsequent study by Ebraheim et al.³⁰ identified the ipsilateral facet capsule, ipsilateral ligamentum flavum, and >50% of the ipsilateral anulus fibrosus as structures requiring resection to produce a unilateral facet dislocation. These injury data further demonstrate the importance of the anterior discoligamentous complex as a passive restraint to unilateral facet subluxation or dislocation. The disruption of the nucleus pulposus is likely predisposed to the recognized “disc-at-risk” phenomenon, which can occur with further vertebral body dislocation beyond the perched facet position, followed by spontaneous or induced reduction.

The discrepancy as to which side of the anulus was injured, ipsilateral (Sim et al.⁶) compared with contralateral (the current study), is best explained by the difference between the models and injuring force vectors. Although both models applied ipsilateral facet distraction, they did so differently: the current model used static lateral distraction and flexion loads coupled with a rotational moment, whereas Sim et al.⁶ utilized lateral distraction alone, thereby predictably producing an ipsilateral anular disruption. In addition, the current model reproduced one of the proposed mechanisms of injury commonly attributed to unilateral facet injuries^3,4,9,20-25 and demonstrated that the contralateral anulus fibrosus resists the rotational moment, producing the unilateral facet subluxation.

The results of the current study also identified bilateral facet capsular disruption commonly attributed to unilateral facet injuries, whereas Sim et al.⁶ demonstrated only a unilateral facet capsular injury. Again, the explanation for this disparity could be based on the difference in injury mechanisms; their model of lateral distraction, followed by sequential ablation of taut soft tissues, implied that the contralateral side would never experience distraction forces. In agreement with the present findings, Vaccaro et al.⁷ identified bilateral facet capsule injury in their MRI observations. Therefore, the results of the current study reinforce and further demonstrate the important concept that although the injuries are referred to as unilateral facet injuries, there is bilateral injury to the facets.

With regard to kinematics data, the current model revealed the most dramatic instability in contralateral axial rotation, as demonstrated by a near-threefold increase in the neutral zone and range of motion. Crawford et al.⁴ demonstrated similar findings, in which the largest range of motion increase (and standard deviation) occurring in contralateral rotation was 8.0° ± 2.8°, compared with the increase of 13.8° ± 4.2° that was found in the present study. However, Crawford et al.⁴ also demonstrated a large increase in ipsilateral axial rotation, which the current study did not, possibly because the injury induced by Crawford et al.⁴ was a unilateral facet dislocation, whereas, in the present study, it was limited to a unilateral facet perch. The age of the specimens in the present study was also older than those used in the study by Crawford et al.⁴. Other biomechanical studies, such as those by Pitzen et al.¹³ and Zdeblick et al.³¹, revealed similar kinematics findings for the effect of bilateral capsular injuries.

A number of limitations of this work have been identified. Similar to most in vitro biomechanical studies^4,6,11,13, the number of cadaver specimens available for testing in the current study was small (i.e., n = 9). We could not speculate whether the results obtained would have changed with a larger number of specimens; however, the relative consistency in the soft-tissue disruptions observed may suggest a potential common injury pattern. Additionally, the smaller sample size and destructive nature of the experimental protocol allowed for only a single mechanism of injury to be studied. Although this injury mechanism was chosen on the basis of the previous literature^3,4,9,20-25, it is almost certain that other injury mechanisms occur in vivo, to which the current results may not be applicable. Furthermore, this mechanism was induced at a much slower rate than could generally be expected to occur with traumatic forces, which may alter the nature and/or the extent of soft-tissue damage. However, the identified soft-tissue injury pattern coincided with that shown in previous literature⁷. Although a high-speed mechanism of injury would have been more clinically relevant, it was necessary to apply rotation at a slower rate to consistently stop the rotation at the moment of facet perch. The age of the specimens used in this study was older than that of the population that typically sustains such traumatic ligamentous injuries³². One may consider the surgical capsular slit performed prior to all testing to be a source of error; however, previous studies have demonstrated that the ipsilateral capsule is injured in unilateral facet dislocations^5,7,9,33. Furthermore, initial pilot testing of the spinal kinematics prior to and after the creation of this slit confirmed that the range of motion and the neutral zone of the motion segment were unaffected. Unfortunately, the laboratory where this study was completed does not have fluoroscopy to determine the instance of facet perch.

This study succeeds in determining both the changes in the range of motion and the neutral zone and the extent of anatomic disruption produced in a unilateral cervical spine facet injury. The dissection results from this work demonstrate that a substantial injury occurs to the anterior discoligamentous complex following a unilateral rotary subluxation to a perched position and that the capsular injury occurs bilaterally. Furthermore, the anterior longitudinal ligament and posterior longitudinal ligament are not important passive restraints to a perched unilateral facet injury. The collected kinematic data showed a large increase in both the range of motion and the neutral zone following the unilateral facet injury, especially in axial rotation.

Appendix

Introduction | Materials and Methods | Results | Discussion | Appendix

Tables showing specimen demographic characteristics and the number of specimens that had specific soft-tissue structures with injuries are available with the online version of this article as a data supplement at jbjs.org.

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Topics

range of motion ; cervical spine ; unilateral facet dislocation ; neutral zone

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Mélissa Nadeau, Stewart D. McLachlin, Stewart I. Bailey, Kevin R. Gurr, Cynthia E. Dunning, Christopher S. Bailey; A Biomechanical Assessment of Soft-Tissue Damage in the Cervical Spine Following a Unilateral Facet Injury. The Journal of Bone & Joint Surgery. 2012 Nov;94(21):e156 1-6.

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