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Morphological changes in the cervical intervertebral foramen dimensions with unilateral facet joint dislocation
Injury, Int. J. Care Injured 40 (2009) 1157–1160
Contents lists available at ScienceDirect
Injury journal homepage: www.elsevier.com/locate/injury
Morphological changes in the cervical intervertebral foramen dimensions with unilateral facet joint dislocation Nabil A. Ebraheim, Jiayong Liu *, Satheesh K. Ramineni, Xiaochen Liu, Joe Xie, Ryan G. Hartman, Vijay K. Goel Department of Orthopaedic Surgery, University of Toledo Medical Center, 3065 Arlington Avenue, Toledo, OH 43614, United States
A R T I C L E I N F O
A B S T R A C T
Article history: Accepted 14 January 2009
Background: Many investigators have conducted studies to determine the biomechanics, causes, complications and treatment of unilateral facet joint dislocation in the cervical spine. However, there is no quantitative data available on morphological changes in the intervertebral foramen of the cervical spine following unilateral facet joint dislocation. These data are important to understand the cause of neurological compromise following unilateral facet joint dislocation. Methods: Eight embalmed human cadaver cervical spine specimens ranging from level C1-T1 were used. The nerve roots of these specimens at C5-C6 level were marked by wrapping a 0.12 mm diameter wire around them. Unilateral facet dislocation at C5-C6 level was simulated by serially sectioning the corresponding ligamentous structures. A CT scan of the specimens was obtained before and after the dislocation was simulated. A sagittal plane through the centre of the pedicle and facet joint was constructed and used for measurement. The height and area of the intervertebral foramen, the facet joint space, nerve root diameter and area, and vertebral alignment both before and after dislocation were evaluated. Results: The intervertebral foramen area changed from 50.72 0.88 mm2 to 67.82 4.77 mm2 on the non-dislocated side and from 41.39 1.11 mm2 to 113.77 5.65 mm2 on the dislocated side. The foraminal heights changed from 9.02 0.30 mm to 10.52 0.50 mm on the non-dislocated side and 10.43 0.50 mm to 17.04 0.96 mm on the dislocated side. The facet space area in the sagittal plane changed from 6.80 0.80 mm2 to 40.02 1.40 mm2 on the non-dislocated side. The C-5 anterior displacement showed a great change from 0 mm to 5.40 0.24 mm on the non-dislocated side and from 0 mm to 3.42 0.20 mm on the dislocated side. Neither of the nerve roots on either side showed a significant change in size. Conclusions: The lack of change in nerve root area indicates that the associated nerve injury with unilateral facet joint dislocation is probably due to distraction rather than due to direct nerve root compression. Published by Elsevier Ltd.
Keywords: Unilateral facet dislocation Intervertebral foramen Facet joint Nerve injury
Introduction Unilateral facet dislocations occur most often in motor vehicle accidents or with sports injuries.1,12,18 They usually occur in the lower cervical spine. It is well accepted that the causative mechanism of the injury is a combination of flexion, traction and rotation. When this occurs, the inferior articular facet of the superior vertebra will move anterior to the superior articular facet of the inferior vertebra resulting in disarticulation and the disarticulation will be locked by the contracted supporting musculature.5,9,16 Some studies have also claimed that the mobility and the shape of the facet joints in the lower cervical
* Corresponding author. Tel.: +1 419 383 6558; fax: +1 419 383 3526. E-mail address: [email protected] (J. Liu). 0020–1383/$ – see front matter . Published by Elsevier Ltd. doi:10.1016/j.injury.2009.01.112
spine perpetuate this injury due to the above mechanism.13 The associated complications and injuries caused by unilateral facet dislocation are also well understood. The interspinous ligament can be weakened and the ligamentum flavum, facet capsule, and intervertebral disc will be ruptured either partially or completely. Additionally, it is known that the nerve roots and the spinal cord at the level of dislocation are often affected.7,15 Levine9 states that 24% of patients with unilateral facet dislocation had spinal cord injuries and 68% had some nerve root or radicular injury. It has also been found that while a facet is locked, there is a severe limitation to motion in the cervical spine. However, after reduction the spine has a greater range of motion than before on both sides, which could lead to post-treatment instability and dislocation.5 To date, the mechanism, complications, and treatment modalities for unilateral facet dislocation have been well documented2,3; however, no data is currently available on the morphological
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changes of the intervertebral foramen dimensions subsequent to unilateral facet joint dislocation. The purpose of this study was to arrive at accurate quantitative measurements of the intervertebral foramen before and after unilateral facet joint dislocation. This study may provide more detailed information to further understand the mechanism of neurological injury following unilateral facet joint dislocation which ultimately may help guide treatment. Materials and methods Eight embalmed human cadaver cervical spine specimens from adults with a mean age of 52 years (range, 44–65 years) were used for the study. Five were male and three female. Soft tissue dissection was performed at the level of C5-C6 to expose the corresponding nerve root, intervertebral disc and ligamentous structures including the facet joint. The nerve root was marked at the level of intervertebral foramen by encircling it with a 0.12 mm diameter wire. This enabled us to measure the dimensions of the nerve root by CT scan. After CT scans were obtained, 3.5 mm Schanz screws were placed in the C5 and C6 vertebral bodies parallel to the vertebral body endplates in the anteroposterior plane and in the centre of the vertebral bodies in the lateral plane. After sequentially sectioning the articular capsule of the facet joint, ipsilateral intertransverse ligament, ipsilateral ligamentum flavum, and slightly more than 50 percent of the annulus fibrosus,16 unilateral facet dislocation was then produced by applying flexion and traction forces to the Schanz screws. After the dislocation was simulated, the Schanz screws were removed and CT scans of the specimens were obtained. Computerised tomography (CT) was performed on each cervical spine specimen both before and after simulating unilateral facet joint dislocation using a Toshiba AquilionTM 16 slice scanner (Toshiba Medical Systems Europe, Zoetermeer, Netherlands), at a 1-mm slice thickness so that the metal around the roots could be used as a guide for measurements. Reconstructed CT scans in the plane passing through the centre of the C5-6 pedicles on the right and left sides were obtained from the specimens (Fig. 1, Table 1). Measurements were then taken of the height and area of the intervertebral foramen, the facet joint space on the non-dislocated side, the nerve root dimensions, and the C5 vertebral body displacement in both injured and normal scenarios (Figs. 2 and 3) as well as on both the dislocated and the non-dislocated sides. This was done using CT and Aquarius Image software (Microsoft, Corp., Redmond, WA) in the Tera Recon Aquarius workstation of the CT (Tables 2 and 3).
Fig. 1. The intervertebral foramen, the facet joint and the marked nerve root in a reconstructed CT scan in the plane passing through the center of the C5-6 pedicle.
Table 1 Nerve area and diameter of normal and injured non-dislocated and dislocated sides in the C5-6 level. Sagittal alignment
Nerve area
Normal
Injured
Normal
Injured
Normal
Nerve diameter Injured
Non-dislocated side Mean 0 S.D. 0
5.40 0.24
15.25 0.55
15.18 0.95
4.19 0.19
4.20 0.10
Dislocated side Mean 0 S.D. 0
3.42 0.20
18.03 0.94
17.95 0.96
4.34 0.19
4.35 0.13
The Student’s statistical t-test was performed using Microsoft Excel software. Statistical significance was defined by a p value <0.05. Results In a comparison of the pre and post-dislocation scenarios, the following results were obtained: the intervertebral foramen area significantly changed from 50.72 0.88 mm2 to 67.82 4.77 mm2 on the non-dislocated side, and from 41.39 1.11 mm2 to 113.77 5.65 mm2 on the dislocated side. The foraminal heights significantly changed from 9.02 0.30 mm to 10.52 0.50 mm on the nondislocated side, and from 10.43 0.50 mm to 17.04 0.96 mm on the dislocated side. The facet space area in the sagittal plane significantly increased from 6.80 0.80 mm2 on the dislocated side to 40.02 1.40 mm2 on the non-dislocated side. The C-5 anterior
Fig. 2. The changes in the intervertebral foramen area, the facet joint space, the vertebral body displacement, and the nerve root area following unilateral dislocation on the non-dislocated side.
Fig. 3. The changes in the intervertebral foramen area, the facet joint space, the vertebral body displacement, and the nerve root area following unilateral dislocation on the dislocated side.
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Table 2 Intervertebral foramen area, height, and widths of normal and injured non-dislocated and dislocated sides in the C5-6 level. IVF area Normal
IVF height Injured
Normal
Non-dislocated side Mean 50.72 S.D. 0.88
67.82 4.77
Dislocated side Mean 41.39 S.D. 1.11
113.77 5.65
Superior width
Middle width
Inferior width
Injured
Normal
Injured
Normal
Injured
Normal
Injured
9.02 0.30
10.52 0.50
5.97 0.13
7.60 0.08
6.45 0.20
7.14 0.53
5.87 0.24
6.63 0.18
10.43 0.50
17.04 0.96
7.38 0.47
7.40 0.48
7.93 0.40
7.94 0.39
7.45 0.11
7.48 0.15
Table 3 Facet area and widths of normal and injured non-dislocated side in the C5-6 level. Facet area Normal Non-dislocated side Mean 6.80 S.D. 0.80
Superior width
Middle width
Inferior width
Injured
Normal
Injured
Normal
Injured
Normal
Injured
40.02 1.40
0.74 0.09
2.41 0.38
0.86 0.05
3.13 0.19
0.62 0.10
5.60 0.21
displacement on both sides showed a significant change from 0 mm to 5.40 0.24 mm on the non-dislocated side, and from 0 mm to 3.42 0.20 mm on the dislocated side. The nerve root area was 15.25 0.55 mm2 and 18.03 0.94 mm2 on the non-dislocated and dislocated side, respectively. Following injury, the non-dislocated side nerve root area measured 15.18 0.95 mm2 while the dislocated side nerve root area measured 17.95 0.96 mm2. Neither of the nerve root areas on either side showed a significant change in size. Discussion The vertebral facet joints in the cervical spine differentiate on going caudally from the atlas. Despite the fact that all of the segments of the neck are highly mobile,6 there are some segments, particularly in the lower cervical spine, that are more mobile than others.9,13,6 In the lower cervical spine, the articular facets are oriented more posterolaterally which contribute to the motions required for unilateral dislocation, thus explaining why this injury is most common around C5-C6 and C6-C7.17 It has also been reported that females have less facet cartilage than males which may lead to a greater risk of injury in females.19 Beyer et al.1 reported that 12–16% of injuries to the cervical spine are unilateral facet joint dislocations or fracture-dislocations. The injury itself is a traction injury and it has been noted that the rotational component is the most significant one in creating the unilateral facet joint dislocation.5,9,3 The negative biomechanical consequences due to this mechanism include greater spinal instability even after reduction as well as a greater chance that dislocation will occur on the contralateral side.5 Unilateral facet dislocations are most often associated with facet fractures.1,5 There is also some associated soft tissue damage. In order for the injury to occur, the ipsilateral facet capsule, the ligamentum flavum and the intertransverse ligament along with more than half of the disc must be ruptured.5,9,16 Damage to both anterior and posterior longitudinal ligaments and the vertebral artery are common as well.5,16 There is not always a neurological problem associated with unilateral dislocation.9 However, the more severe is the injury, the more likely there is to be some root damage or even possibly a cord injury.9 Rorabeck et al.15 reported that 11 of the 26 subjects in a unilateral facet joint dislocation analysis had nerve root injuries. Hadley et al.7 reported that out of 30 subjects with unilateral dislocations, only 6 were without any neurological
injury while the other 24 had nerve root injury, incomplete or complete (90% claimed neurological morbidity).7 The current study matched up considerably well with the existing data. The average normal foraminal area measurements were consistent with those in the past studies by Lentell et al.10, Panjabi et al.14, Kitagawa et al.8, and Yoo et al.20. It was found that the intervertebral foramen on both the dislocated and nondislocated sides has increased in size. This increase in size was more on the dislocated side. Also, the normal cross-sectional kidney shape of the foramen took on a more deformed appearance on the dislocated side.11 With regard to the nerve roots, the current diameter and area results had good concordance with those previously recorded; however, there were no significant postinjury changes. With the intervertebral foramen actually obtaining a larger area during dislocation, it was unexpectedly found that there was no nerve root compression. Rotational abnormality is always present with unilateral facet dislocations.9 In fact, earlier findings show that the superior vertebral body will not protrude more than 50% of its length in a unilateral facet dislocation injury.3,17 The same results were found in the present study as well. Unilateral facet joint dislocation has been simulated before on fresh frozen cadaveric specimens, most notably in studies by Sim et al.16 and Crawford et al.5 In the Crawford et al. study,5 the dislocation was created using an MTS machine to apply an axial rotational force combined with lateral bending weights after the right facet capsule had been cut. Sim et al.16 used fresh frozen cadaver specimens to simulate unilateral facet dislocation after sequential sectioning of disco-ligamentous structures. Embalmed specimens were used in the current study and it did not influence the results of the study as the aim of the study was to measure the changes in the bony dimensions of the intervertebral foramen and their influence on the exiting nerve root. With this method, unilateral facet dislocation was created without causing facet fractures or other peripheral damage to the specimens. From the results of the study, it appears that the injury to the exiting nerve root at the level of dislocation appears to result from causes other than compression from a decrease in the dimensions of the intervertebral foramen. The neurological injury may be due traction force resulting from the rotational displacement of the superior vertebral body over the inferior vertebra. This speculation can be proven clinically. It is well known that not all patients with unilateral facet joint dislocation have nerve injuries.7,9,15 Dislocation
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happens regardless of the stretched length once the minimum threshold is reached. Therefore, some patients may only just reach the threshold for the facet joint ‘‘jump’’ to occur; the traction in this situation would not have damaged the nerve because the nerve can still tolerate the stretch. In other cases, the nerve would be damaged if it was subjected to excess stretching. As the displacement present in the patient at the hospital is the result of contracted supporting musculature, it may be less than the excursion at the time of impact.5,16 Bohlman4 reported four cases of unilateral facet joint dislocation with impairment of major radicular feeder vessels but no direct nerve compression injuries resulted in delayed nerve necrosis. This type of vessel injuries could also be caused by traction. In addition, the injury may result from other causes such as compression from displaced fracture fragments and extruded intervertebral disc. There is, however, a limitation to the study: the conclusion is based on speculation. Neither functional tests nor pathological evidence could be conducted or gathered to confirm the speculation since the subjects in question are cadavers. Conclusion This study found that the intervertebral foramen on both sides became larger following unilateral facet joint dislocation. The facet joint space on the non-dislocated side showed a significant increase following injury. This finding implies spinal instability at this level. The alignment change showed that both sides sustained displacement. The non-dislocated side showed a more severe displacement than the dislocated side. This inequality showed that the superior vertebral body undergoes axial rotation during unilateral dislocation. The lack of change in nerve root area reveals that patients with unilateral facet joint dislocation with associated nerve injuries may have traction of the nerve root rather than direct nerve root compression. Conflict of interest statement The authors did not receive grants or outside funding in support of this research or preparation of this manuscript. They did not receive payments or other benefits or a commitment or agreement to provide such benefits or a commitment or agreement to provide such benefits from a commercial entity. No commercial entity paid or directed, or agreed to pay or direct, any benefits to any research
fund, foundation, educational institution, or other charitable or nonprofit organization with which the authors are affiliated or associated. References 1. Beyer CA, Cabanela ME, Berquist TH. Unilateral facet dislocations and fracturedislocations of the cervical spine. J Bone Joint Surg Br 1991;73:977–81. 2. Bauze RJ, Ardran GM. Experimental production of forward dislocation in the human cervical spine. J Bone Joint Surg 1978;60:239–45. 3. Beatson TR. Fractures and dislocations of the cervical spine. J Bone Joint Surg 1963;45:21–35. 4. Bohlman HH. Acute fractures and dislocations of the cervical spine. An analysis of 300 hospitalized patients and review of the literature. J Bone Joint Surg Am 1979;61(8):1119–42. 5. Crawford NR, Duggal N, Chamberlain RH. Unilateral cervical facet dislocation: injury mechanism and biomechanical consequences. Spine 2002;27:1858–64. 6. Daffner SD, Daffner RH. Computed tomography diagnosis of facet dislocations: The ‘‘Hamburger Bun’’ and ‘‘Reverse Hamburger Bun’’ signs. J Emerg Med 2002;23:387–94. 7. Hadley MN, Fitzpatrick BC, Sonntag VK. Facet fracture-dislocation injuries of the cervical spine. Neurosurgery 1992;30:661–6. 8. Kitagawa T, Fujiwara A, Kobayashi N. Morphologic changes in the cervical neural foramen due to flexion and extension: in vivo imaging study. Spine 2004;29:2821–5. 9. Levine AM. Facet fractures and dislocations. In: Levine T, Eismont T, Garfin T, Zigler T, editors. Spine trauma. United States of America: W.B. Saunders; 1998. p. 331–66. 10. Lentell G, Kruse M, Chock B. Dimensions of the cervical neural foramina in resting and retracted positions using magnetic resonance imaging. J Orthop Sports Phys Ther 2002;32:380–90. 11. Nuckley DJ, Konodi MA, Raynak GC. Neural space integrity of the lower cervical spine. Spine 2002;27:587–95. 12. O’Connor PA, McCormack O, Noe¨l J. Anterior displacement correlates with neurological impairment in cervical facet dislocations. Int Orthop 2003;27: 190–3. 13. Pal GP, Routal RV, Saggu SK. The orientation of the articular facets of the zygapophyseal joints at the cervical and upper thoracic region. J Anat 2001;198: 431–41. 14. Panjabi MM, Maak TG, Ivancic PC. Dynamic intervertebral foramen narrowing during simulated rear impact. Spine 2006;31:128–34. 15. Rorabeck CH, Rock MG, Hawkins RJ. Unilateral facet dislocation of the cervical spine—an analysis of the results of treatment in 26 patients. Spine 1987;12: 23–7. 16. Sim E, Vaccaro AR, Berzlanovich A. In vitro genesis of sub axial cervical unilateral facet dislocations through sequential soft tissue ablation. Spine 2001;26:1317–23. 17. Shapiro SA. Management of unilateral locked facet of the cervical spine. Neurosurgery 1993;33:832–7. 18. Vives MJ, Garfin SR. Flexion Injuries. In: Clark CR, editor. The Cervical Spine. Philadelphia, PA: Lippincott Williams & Wilkins; 2005. p. 660–70. 19. Yoganandan N, Knowles SA, Maiman DJ. Anatomic study of the morphology of human cervical facet joint. Spine 2003;28:2317–23. 20. Yoo JU, Zou D, Edwards T. Effect of cervical spine motion on the neuroforaminal dimensions of human cervical spine. Spine 1992;17:1131–6.
Contents lists available at ScienceDirect
Injury journal homepage: www.elsevier.com/locate/injury
Morphological changes in the cervical intervertebral foramen dimensions with unilateral facet joint dislocation Nabil A. Ebraheim, Jiayong Liu *, Satheesh K. Ramineni, Xiaochen Liu, Joe Xie, Ryan G. Hartman, Vijay K. Goel Department of Orthopaedic Surgery, University of Toledo Medical Center, 3065 Arlington Avenue, Toledo, OH 43614, United States
A R T I C L E I N F O
A B S T R A C T
Article history: Accepted 14 January 2009
Background: Many investigators have conducted studies to determine the biomechanics, causes, complications and treatment of unilateral facet joint dislocation in the cervical spine. However, there is no quantitative data available on morphological changes in the intervertebral foramen of the cervical spine following unilateral facet joint dislocation. These data are important to understand the cause of neurological compromise following unilateral facet joint dislocation. Methods: Eight embalmed human cadaver cervical spine specimens ranging from level C1-T1 were used. The nerve roots of these specimens at C5-C6 level were marked by wrapping a 0.12 mm diameter wire around them. Unilateral facet dislocation at C5-C6 level was simulated by serially sectioning the corresponding ligamentous structures. A CT scan of the specimens was obtained before and after the dislocation was simulated. A sagittal plane through the centre of the pedicle and facet joint was constructed and used for measurement. The height and area of the intervertebral foramen, the facet joint space, nerve root diameter and area, and vertebral alignment both before and after dislocation were evaluated. Results: The intervertebral foramen area changed from 50.72 0.88 mm2 to 67.82 4.77 mm2 on the non-dislocated side and from 41.39 1.11 mm2 to 113.77 5.65 mm2 on the dislocated side. The foraminal heights changed from 9.02 0.30 mm to 10.52 0.50 mm on the non-dislocated side and 10.43 0.50 mm to 17.04 0.96 mm on the dislocated side. The facet space area in the sagittal plane changed from 6.80 0.80 mm2 to 40.02 1.40 mm2 on the non-dislocated side. The C-5 anterior displacement showed a great change from 0 mm to 5.40 0.24 mm on the non-dislocated side and from 0 mm to 3.42 0.20 mm on the dislocated side. Neither of the nerve roots on either side showed a significant change in size. Conclusions: The lack of change in nerve root area indicates that the associated nerve injury with unilateral facet joint dislocation is probably due to distraction rather than due to direct nerve root compression. Published by Elsevier Ltd.
Keywords: Unilateral facet dislocation Intervertebral foramen Facet joint Nerve injury
Introduction Unilateral facet dislocations occur most often in motor vehicle accidents or with sports injuries.1,12,18 They usually occur in the lower cervical spine. It is well accepted that the causative mechanism of the injury is a combination of flexion, traction and rotation. When this occurs, the inferior articular facet of the superior vertebra will move anterior to the superior articular facet of the inferior vertebra resulting in disarticulation and the disarticulation will be locked by the contracted supporting musculature.5,9,16 Some studies have also claimed that the mobility and the shape of the facet joints in the lower cervical
* Corresponding author. Tel.: +1 419 383 6558; fax: +1 419 383 3526. E-mail address: [email protected] (J. Liu). 0020–1383/$ – see front matter . Published by Elsevier Ltd. doi:10.1016/j.injury.2009.01.112
spine perpetuate this injury due to the above mechanism.13 The associated complications and injuries caused by unilateral facet dislocation are also well understood. The interspinous ligament can be weakened and the ligamentum flavum, facet capsule, and intervertebral disc will be ruptured either partially or completely. Additionally, it is known that the nerve roots and the spinal cord at the level of dislocation are often affected.7,15 Levine9 states that 24% of patients with unilateral facet dislocation had spinal cord injuries and 68% had some nerve root or radicular injury. It has also been found that while a facet is locked, there is a severe limitation to motion in the cervical spine. However, after reduction the spine has a greater range of motion than before on both sides, which could lead to post-treatment instability and dislocation.5 To date, the mechanism, complications, and treatment modalities for unilateral facet dislocation have been well documented2,3; however, no data is currently available on the morphological
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changes of the intervertebral foramen dimensions subsequent to unilateral facet joint dislocation. The purpose of this study was to arrive at accurate quantitative measurements of the intervertebral foramen before and after unilateral facet joint dislocation. This study may provide more detailed information to further understand the mechanism of neurological injury following unilateral facet joint dislocation which ultimately may help guide treatment. Materials and methods Eight embalmed human cadaver cervical spine specimens from adults with a mean age of 52 years (range, 44–65 years) were used for the study. Five were male and three female. Soft tissue dissection was performed at the level of C5-C6 to expose the corresponding nerve root, intervertebral disc and ligamentous structures including the facet joint. The nerve root was marked at the level of intervertebral foramen by encircling it with a 0.12 mm diameter wire. This enabled us to measure the dimensions of the nerve root by CT scan. After CT scans were obtained, 3.5 mm Schanz screws were placed in the C5 and C6 vertebral bodies parallel to the vertebral body endplates in the anteroposterior plane and in the centre of the vertebral bodies in the lateral plane. After sequentially sectioning the articular capsule of the facet joint, ipsilateral intertransverse ligament, ipsilateral ligamentum flavum, and slightly more than 50 percent of the annulus fibrosus,16 unilateral facet dislocation was then produced by applying flexion and traction forces to the Schanz screws. After the dislocation was simulated, the Schanz screws were removed and CT scans of the specimens were obtained. Computerised tomography (CT) was performed on each cervical spine specimen both before and after simulating unilateral facet joint dislocation using a Toshiba AquilionTM 16 slice scanner (Toshiba Medical Systems Europe, Zoetermeer, Netherlands), at a 1-mm slice thickness so that the metal around the roots could be used as a guide for measurements. Reconstructed CT scans in the plane passing through the centre of the C5-6 pedicles on the right and left sides were obtained from the specimens (Fig. 1, Table 1). Measurements were then taken of the height and area of the intervertebral foramen, the facet joint space on the non-dislocated side, the nerve root dimensions, and the C5 vertebral body displacement in both injured and normal scenarios (Figs. 2 and 3) as well as on both the dislocated and the non-dislocated sides. This was done using CT and Aquarius Image software (Microsoft, Corp., Redmond, WA) in the Tera Recon Aquarius workstation of the CT (Tables 2 and 3).
Fig. 1. The intervertebral foramen, the facet joint and the marked nerve root in a reconstructed CT scan in the plane passing through the center of the C5-6 pedicle.
Table 1 Nerve area and diameter of normal and injured non-dislocated and dislocated sides in the C5-6 level. Sagittal alignment
Nerve area
Normal
Injured
Normal
Injured
Normal
Nerve diameter Injured
Non-dislocated side Mean 0 S.D. 0
5.40 0.24
15.25 0.55
15.18 0.95
4.19 0.19
4.20 0.10
Dislocated side Mean 0 S.D. 0
3.42 0.20
18.03 0.94
17.95 0.96
4.34 0.19
4.35 0.13
The Student’s statistical t-test was performed using Microsoft Excel software. Statistical significance was defined by a p value <0.05. Results In a comparison of the pre and post-dislocation scenarios, the following results were obtained: the intervertebral foramen area significantly changed from 50.72 0.88 mm2 to 67.82 4.77 mm2 on the non-dislocated side, and from 41.39 1.11 mm2 to 113.77 5.65 mm2 on the dislocated side. The foraminal heights significantly changed from 9.02 0.30 mm to 10.52 0.50 mm on the nondislocated side, and from 10.43 0.50 mm to 17.04 0.96 mm on the dislocated side. The facet space area in the sagittal plane significantly increased from 6.80 0.80 mm2 on the dislocated side to 40.02 1.40 mm2 on the non-dislocated side. The C-5 anterior
Fig. 2. The changes in the intervertebral foramen area, the facet joint space, the vertebral body displacement, and the nerve root area following unilateral dislocation on the non-dislocated side.
Fig. 3. The changes in the intervertebral foramen area, the facet joint space, the vertebral body displacement, and the nerve root area following unilateral dislocation on the dislocated side.
N.A. Ebraheim et al. / Injury, Int. J. Care Injured 40 (2009) 1157–1160
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Table 2 Intervertebral foramen area, height, and widths of normal and injured non-dislocated and dislocated sides in the C5-6 level. IVF area Normal
IVF height Injured
Normal
Non-dislocated side Mean 50.72 S.D. 0.88
67.82 4.77
Dislocated side Mean 41.39 S.D. 1.11
113.77 5.65
Superior width
Middle width
Inferior width
Injured
Normal
Injured
Normal
Injured
Normal
Injured
9.02 0.30
10.52 0.50
5.97 0.13
7.60 0.08
6.45 0.20
7.14 0.53
5.87 0.24
6.63 0.18
10.43 0.50
17.04 0.96
7.38 0.47
7.40 0.48
7.93 0.40
7.94 0.39
7.45 0.11
7.48 0.15
Table 3 Facet area and widths of normal and injured non-dislocated side in the C5-6 level. Facet area Normal Non-dislocated side Mean 6.80 S.D. 0.80
Superior width
Middle width
Inferior width
Injured
Normal
Injured
Normal
Injured
Normal
Injured
40.02 1.40
0.74 0.09
2.41 0.38
0.86 0.05
3.13 0.19
0.62 0.10
5.60 0.21
displacement on both sides showed a significant change from 0 mm to 5.40 0.24 mm on the non-dislocated side, and from 0 mm to 3.42 0.20 mm on the dislocated side. The nerve root area was 15.25 0.55 mm2 and 18.03 0.94 mm2 on the non-dislocated and dislocated side, respectively. Following injury, the non-dislocated side nerve root area measured 15.18 0.95 mm2 while the dislocated side nerve root area measured 17.95 0.96 mm2. Neither of the nerve root areas on either side showed a significant change in size. Discussion The vertebral facet joints in the cervical spine differentiate on going caudally from the atlas. Despite the fact that all of the segments of the neck are highly mobile,6 there are some segments, particularly in the lower cervical spine, that are more mobile than others.9,13,6 In the lower cervical spine, the articular facets are oriented more posterolaterally which contribute to the motions required for unilateral dislocation, thus explaining why this injury is most common around C5-C6 and C6-C7.17 It has also been reported that females have less facet cartilage than males which may lead to a greater risk of injury in females.19 Beyer et al.1 reported that 12–16% of injuries to the cervical spine are unilateral facet joint dislocations or fracture-dislocations. The injury itself is a traction injury and it has been noted that the rotational component is the most significant one in creating the unilateral facet joint dislocation.5,9,3 The negative biomechanical consequences due to this mechanism include greater spinal instability even after reduction as well as a greater chance that dislocation will occur on the contralateral side.5 Unilateral facet dislocations are most often associated with facet fractures.1,5 There is also some associated soft tissue damage. In order for the injury to occur, the ipsilateral facet capsule, the ligamentum flavum and the intertransverse ligament along with more than half of the disc must be ruptured.5,9,16 Damage to both anterior and posterior longitudinal ligaments and the vertebral artery are common as well.5,16 There is not always a neurological problem associated with unilateral dislocation.9 However, the more severe is the injury, the more likely there is to be some root damage or even possibly a cord injury.9 Rorabeck et al.15 reported that 11 of the 26 subjects in a unilateral facet joint dislocation analysis had nerve root injuries. Hadley et al.7 reported that out of 30 subjects with unilateral dislocations, only 6 were without any neurological
injury while the other 24 had nerve root injury, incomplete or complete (90% claimed neurological morbidity).7 The current study matched up considerably well with the existing data. The average normal foraminal area measurements were consistent with those in the past studies by Lentell et al.10, Panjabi et al.14, Kitagawa et al.8, and Yoo et al.20. It was found that the intervertebral foramen on both the dislocated and nondislocated sides has increased in size. This increase in size was more on the dislocated side. Also, the normal cross-sectional kidney shape of the foramen took on a more deformed appearance on the dislocated side.11 With regard to the nerve roots, the current diameter and area results had good concordance with those previously recorded; however, there were no significant postinjury changes. With the intervertebral foramen actually obtaining a larger area during dislocation, it was unexpectedly found that there was no nerve root compression. Rotational abnormality is always present with unilateral facet dislocations.9 In fact, earlier findings show that the superior vertebral body will not protrude more than 50% of its length in a unilateral facet dislocation injury.3,17 The same results were found in the present study as well. Unilateral facet joint dislocation has been simulated before on fresh frozen cadaveric specimens, most notably in studies by Sim et al.16 and Crawford et al.5 In the Crawford et al. study,5 the dislocation was created using an MTS machine to apply an axial rotational force combined with lateral bending weights after the right facet capsule had been cut. Sim et al.16 used fresh frozen cadaver specimens to simulate unilateral facet dislocation after sequential sectioning of disco-ligamentous structures. Embalmed specimens were used in the current study and it did not influence the results of the study as the aim of the study was to measure the changes in the bony dimensions of the intervertebral foramen and their influence on the exiting nerve root. With this method, unilateral facet dislocation was created without causing facet fractures or other peripheral damage to the specimens. From the results of the study, it appears that the injury to the exiting nerve root at the level of dislocation appears to result from causes other than compression from a decrease in the dimensions of the intervertebral foramen. The neurological injury may be due traction force resulting from the rotational displacement of the superior vertebral body over the inferior vertebra. This speculation can be proven clinically. It is well known that not all patients with unilateral facet joint dislocation have nerve injuries.7,9,15 Dislocation
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happens regardless of the stretched length once the minimum threshold is reached. Therefore, some patients may only just reach the threshold for the facet joint ‘‘jump’’ to occur; the traction in this situation would not have damaged the nerve because the nerve can still tolerate the stretch. In other cases, the nerve would be damaged if it was subjected to excess stretching. As the displacement present in the patient at the hospital is the result of contracted supporting musculature, it may be less than the excursion at the time of impact.5,16 Bohlman4 reported four cases of unilateral facet joint dislocation with impairment of major radicular feeder vessels but no direct nerve compression injuries resulted in delayed nerve necrosis. This type of vessel injuries could also be caused by traction. In addition, the injury may result from other causes such as compression from displaced fracture fragments and extruded intervertebral disc. There is, however, a limitation to the study: the conclusion is based on speculation. Neither functional tests nor pathological evidence could be conducted or gathered to confirm the speculation since the subjects in question are cadavers. Conclusion This study found that the intervertebral foramen on both sides became larger following unilateral facet joint dislocation. The facet joint space on the non-dislocated side showed a significant increase following injury. This finding implies spinal instability at this level. The alignment change showed that both sides sustained displacement. The non-dislocated side showed a more severe displacement than the dislocated side. This inequality showed that the superior vertebral body undergoes axial rotation during unilateral dislocation. The lack of change in nerve root area reveals that patients with unilateral facet joint dislocation with associated nerve injuries may have traction of the nerve root rather than direct nerve root compression. Conflict of interest statement The authors did not receive grants or outside funding in support of this research or preparation of this manuscript. They did not receive payments or other benefits or a commitment or agreement to provide such benefits or a commitment or agreement to provide such benefits from a commercial entity. No commercial entity paid or directed, or agreed to pay or direct, any benefits to any research
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