Anterior Cervical Subluxation: An Unstable Position

275 Anterior Cervical Subluxation: An Unstable Position, 1 A. T. Scher1 The radioiogic signs of cervical anterior subluxation are subtle. Even when recognized, the injury may not be considered significant. However, anterior subluxation is the most unstable cervical spinal injury. The two case histories illustrate instability of the cervical spine in this type of injury. The radiologically diagnostic features on the lateral radiograph are described and the recognition of instability on mobility studies is discussed. Anterior cervical subluxation is sustained as result of flexion-rotation injury in which the traumatic force is not sufficient to cause disruption of the apophyseal joints and interlocking of articular facets [1 ]. Anterior subluxation is considered to be due to injury of the posterior ligamentous complex. Surgical and pathologic observations have confirmed this conception [2]. Because of the angle of the articular facets and the elasticity of the intervertebral discs and ligaments, the vertebral bodies can slide anteriorly on flexion in the normal person. In a stable spine the posterior ligament complex, the intervertebral discs, and the apophyseal joints limit anteropostenior motion to less than 3 mm [3]. Instability is defined as abnormal mobility between any pair of vertebrae, with or without pain or other clinical manifestation. A displacement of more than 3 mm on flexion-extension spine radiographs is considered abnormal and indicates instability [3]. There is considerable confusion in the literature, as well as imprecise use of the terms subluxation and dislocation. I define subluxation as anterior displacement of a vertebra in relation to the vertebra below, when the two articular surfaces of the apophyseal joints remain in contact [4]. In this situation the inferior articular facets of the upper vertebra have been displaced anteriorly in relation to the superior facets of the lower vertebra without interlocking... Dislocation is used when the articular facets of the apophyseal joints are no longer in contact and both inferior articular facets of the upper vertebra are Received September i9, 1978; accepted after locked in front of the superior articular facets of the lower vertebra. revision April 12, 1979. Flexion-rotation injuries of the cervical spine, resulting in either unilateral or Department of Radiology and Spinal Cord In- bilateral interlocking of facets, are easily discernible radiologically and should juries Centre. Conradie Hospital, Cape Town,.... South Africa. ensure careful treatment and ImmobIlizatIon of the neck for those patients who have sustained such injury. In contrast, anterior subluxations are sometimes AJR 133:275-280, August 1979.. 036i-803X/79/i332-0275 $00 00 overlooked due to the less dramatic radiographic features. As a result less American Roentgen Ray Society attention may be given to initial treatment and continued observation of these

276 SCHER AJR:133, August 1979 patients. It is not always appreciated that anterior subluxation is associated with damage to the posterior ligamentous structures, upon which the stability of the cervical spine depends. Two patients were recently admitted who had sustained traumatic anterior cervical subluxation, and in whom progressive intervertebral displacement rapidly developed. The case histories and radiologic features of these two patients provide interesting examples of subtle initial radiologic signs and evidence that this apparently unimportant injury may be accompanied by severe ligamentous damage and vertebral instability. Case Case 1 Reports A 3i -year-old man sustained injury to his neck during a rugby match. There was no evidence of neurologic deficit at admission. Radiography of the cervical spine revealed slight anterior subluxation of C5 on C6 with narrowing of the C5, C6 disc space anteriorly. Widening of the interspinous distance and overriding of the apophyseal joints at the C5, C6 level were also present (fig. 1 A). The patient was treated by halter traction weighted to 3 kg for 2 weeks. He was then provided with a soft collar and discharged. He was referred to Conradie Hospital 2 weeks later for follow-up examination. He complained of paresthesia in both arms in the distribution of the ulnar nerves. Radiography of the cervical spine showed extreme anterior subluxation of CS on C6 (fig. 1 B). After admission, the subluxation was reduced without difticulty by traction with Crutchfield tongs and 4 /2 kg weight. A posterior fusion was subsequently performed and neurologic deficit disappeared. Case 2 A 27-year-old man fell off a roof while intoxicated. On admission there was no evidence of neunologic deficit, but radiography of the cervical spine (fig. 2A) showed divergence of CS and C6 spinous processes, minimal anterior subluxation of C5 on C6, narrowing of the anterior CS/C6 disc space, and anterior displacement of the inferior articular facets of C5 on the superior articulan facets of C6. The patient was placed on skull traction with 4 /2 kg weight; further radiography showed that the subluxation had been reduced. The skull tongs became dislodged 6 weeks after admission and the patient, who had been extremely uncooperative, left the hospital. He was readmitted 2 weeks later complaining of severe cervical pain. Radiography (fig. 2B) showed a marked deterioration in appearance, the subluxation having notably increased. There was no evidence of neunologic deficit on clinical examination. A posterior cervical spinal fusion was performed and the patient was subsequently discharged in satisfactory condition. This case demonstrates that even with a prolonged period of immobilization on skull traction, deterioration of position at the injury site may occur. This patient was fortunate to have escaped delayed spinal cord injury, but he did have severe pain, another symptom of advanced subluxation. Discussion Anterior Subluxation on the Neutral Lateral Radiograph The range of motion of the normal cervical spine depends on the integrity and elasticity of the posterior ligamentous complex, consisting of the supraspinous and interspinous ligaments, the ligamentum flavae, and the capsules of the apophyseal joints [5]. Traumatic disruption of these structures allows forward vertebral slipping. Because injury is confined to cervical soft tissues and ligaments not radiologically demonstrable, recognition of anterior subluxation can only be made by noting changes in the relations of the vertebral components (vertebral bodies, articular facets, and spinous processes) at the level of injury. The following radiologic features were found to be valuable in the diagnosis of anterior subluxation (fig. 3). Divergence of the spinous processes. This is an important sign of damage to the posterior ligamentous complex [6]. The distance between the spinous processes at the affected level is greater than the interspinous distance above and below the injury. There is also a change of alignment of the long axes of the spinous processes that tend to diverge instead of converge toward a posteriorly placed center. Narrowing of the anterior disc space. In addition to the damaged posterior ligamentous complex, a tear in the postenor part of the intervertebral disc may be present, causing widening of the disc space posteriorly and compression anteriorly. Displacement at the apophyseal joints. Damage to the capsules of the apophyseal joints allows forward displacement of the cranial facet surfaces, loss of parallelism of the articular surfaces, and widening of the joint space posteriorly. Because of these changes, the function of the apophyseal joints in maintaining stability is impaired, allowing forward and angular vertebral displacement. Vertebral body displacement. Anterior vertebral displacement is not often marked, usually only 1-2 mm. In figure 4, minimal displacement with interspinous widening and antenor disc space narrowing is demonstrated. This patient sustained complete tetraplegia as result of a flexion injury during a rugby scrummage. Paravertebral hematoma. Widening of the prevertebral soft tissue space due to hematoma is rarely encountered, as these changes are usually from hyperextension or vertical compression injuries, whereas anterior cervical subluxations are flexion-rotation injuries. In those few cases in which a small vertebral body compression fracture is associated with subluxation, some prevertebral widening may be noted. Webb et al. [2] described two other radiologic signs they consider are of diagnostic value: (1 ) vertebral compression fractures and (2) loss of normal cervical lordosis. I have seldom seen vertebral compression fractures in cases of anterior subluxation. Loss of normal cervical lordosis which has been attributed to muscular spasm may be a normal finding in patients without history of trauma [3]. Since many patients tend to tuck their chins in during radiography in the lateral position (which straightens the cervical spine), we do not consider the absence of normal cervical lordosis to be a useful diagnostic feature. Since the radiologic signs of subluxation are subtle and may even border on the limits of normality, a control series of radiographs was obtained as a basis for assessment. Neutral, flexion, and extension lateral views of the cervical spine were taken on 25 asymptomatic subjects between the ages of 1 8 and 30 years. The subjects had not suffered any

AJR:133, August 1979 ANTERIOR CERVICAL SUBLUXATION 277 Fig. 1 -Case 1. A, Minimal anterior displacement of C5 on C6 with anterior narrowing of C5/C6 disc space. Divergence of spinous processes of C5 and C6 and overriding of apophyseal joints. B, Extreme anterior subluxation of C5 and C6. Fig. 2.-Case 2. A, Minimal anterior displacement of C5 on C6 with anterior narrowing of C5/C6 disc space. Divergence of spinous processes of C5 and C6 and overriding of apophyseal joints. B, Increased anterior subluxation of C5 on C6.

278 SCHER AJR:133, August 1979 Fig. 3.-Spinal injury with incomplete tetraplegia. Divergence of spinous processes, narrowing of anterior disc space, displacement at apophyseal joints, and vertebral body displacement. previous spinal injury. The presence and degree of spinous divergence, vertebral displacement, and the relations of the articular facets of the apophyseal joints with regard to parallelism of the surfaces and anterior slipping were analyzed. In this series of neutral lateral views of nontraumatized cervical spines, there was no divergence of the spinous processes. In five subjects there was no cervical lordosis, which is normal in some individuals. There was no lack of parallelism in the normal apophyseal joints, nor did the cranial joint surfaces project anteriorly over the caudal surfaces. There was no anterior vertebral displacement at any level. Flexion and extension views showed varying ranges of motion between individual vertebral elements. Diagnosis of Instability Anterior subluxation is the most unstable of all cervical injuries [7]. Failure to diagnose or be aware of the clinical significance of this injury may lead to serious consequences for the patient. The soft tissue and ligamentous damage is similar to that found in anterior dislocation. Both lesions are unstable, but in subluxation the original displacement is not as gross; therefore, there is less risk of immediate damage to the spinal cord or cervical nerve roots. However, an increase in instability could lead to progressive neurologic deficit. Webb et al. [2] reported three late complications of Fig. 4.-Spinal cord injury with complete tetraplegia. Only minimal vertebral displacement, slight divergence of spinous processes, and anterior disc space narrowing at C4/C5 level. anterior subluxation: (1 ) instability associated with pain, (2) delayed vertebral dislocation, and (3) progressive vertebral body collapse. In that report, instability was only detected on flexion-extension studies of patients complaining of persistent neck pain several months after injury. The initial radiographs of these patients were regarded as normal. In an analysis of 257 conservatively treated acute cervical injury patients, Cheshire [7] found a 21% incidence of instability in those with anterior subluxation when mobility studies were performed at the completion of treatment. This was considerably higher than the incidence found in patients with unilateral or bilateral apophyseal joint dislocation. He suggested that this discrepancy could be due to inadequate immobilization and treatment of subluxation. Holdsworth [8] stated, When the posterior ligamentous complex has been ruptured, healing such as to restore the original strength does not occur. My two cases clearly show that anterior subluxation is an unstable injury, in which increased vertebral malposition developed despite initial immobilization. Indications of instability on the neutral lateral radiograph. White et al. [9], in an experimental investigation on cadavers, analyzed the role of the cervical spinal ligaments and articular facets of the apophyseal joints in maintaining stability of the spine. The anatomic elements in the specimens were systematically and progressively destroyed to the point of instability. A quantitative biomechanical analysis of the importance of each structure was then made. Two measure-

AJR:133, August 1979 ANTERIOR CERVICAL SUBLUXATION 279 Fig. 5.-A, Extension view. Divergence of spinous processes of C5 and C6. No evidence of displacement of vertebral bodies or derangement of apophyseal joints. B, Flexion view. Anterior displacement of CS on C6 and posterior widening of C5/C6 apophyseal joint spaces. ments were identified on the lateral radiograph that indicate the spine is unstable, or on the brink of instability, due to Iigamentous damage. These are (measured on resting or flexion-extension radiographs): (1 ) more than 3.5 mm of horizontal displacement of one vertebra relative to the adjacent vertebra and (2) more than 1 1 #{176}of kyphotic angulation at the involved level. In practice, I have found that displacement of a magnitude sufficient to comply with the measurements of White et al. [9] is seldom present. Moreover, patients with a lesser degree of displacement have been found to show definite evidence of instability. In some cases it is apparent from the degree of spinal cord damage that the displacement that occurred at the moment of injury was greater than reflected on the initial radiographs (fig. 4). In these cases spontaneous reduction, or reduction while being transported to the hospital, must have taken place. The radiologic feature I have found most valuable in the diagnosis of potential instability is divergence of the spinous processes. This is a definite sign of ligamentous damage and should alert the clinician of the possibility that the patient has sustained a potentially unstable cervical spine injury. Fieding and Hawkins [1 0] referred to the divergence as fanning of the spinous processes. Mobility studies for demonstrating instability. The abnormal vertebral mobility of subluxation can only be confirmed by flexion-extension studies. On flexion of the normal spine there is a gliding motion of each vertebral body on the one below, resulting in slight forward displacement. In our control series this displacement of the vertebral bodies did not exceed 1 mm. The capsule of the apophyseal joint is loose and permits a relatively large oscillation, enabling the cranial joint surface to ascend in flexion and descend in extension. On flexion the joint surfaces may remain in apposition through 5 mm displacement in the normal spine [1 1 ]. Therefore, forward displacement of the cranial joint of more than 5 mm is abnormal. A more important sign of apophyseal joint instability is loss of parallelism at the surfaces induced by flexion. In the flexed spine of normal subjects the spinous processes are separated by nearly equal distances; localized divergence of the long axis processes suggests ligamentous damage and instability. Analysis of the flexion radiographs in our control series showed no isolated spinous process divergence in any individuals. Flexion-extension views of the cervical spine would seem to be the ideal technique for demonstrating instability, but in practice they cannot always be obtained. Muscular spasm, often present after cervical spine injury, and associated pain prevent adequate movement; therefore, radiographs obtamed under these circumstances are of little value. The possibility of inducing spinal cord or nerve root damage by injudicious movement of the neck is another contraindication to performing mobility studies in the acute stage. There is considerable controversy about the timing and method of mobility studies after cervical spinal injury, but there is general agreement that this potentially hazardous procedure should be performed under medical supervision. Mobility studies are of particular value in confirming the site of injury in patients whose initial static radiographs show little or no abnormality. Figure 5 illustrates the findings in a patient involved in a motor accident. The original static radiographs of his cervical spine were passed as normal. The significance of divergence of the CS and C6 spinous

280 SCHER AJR:133, August 1979 processes was not appreciated and the patient was discharged without treatment. He returned 6 weeks later complaining of severe neck pain. The flexion view (fig. SB) then confirmed that he had in fact sustained cervical injury. During mobility studies, flexion and extension of the neck by the patient is preferred. If movement is markedly limited the examination is abandoned until pain and spasm have disappeared. We consider muscular spasm an important protective mechanism in cervical injuries generally, and in unstable injuries particularly. This was confirmed by Hubbard [1 2] in an analysis of spinal injuries in children and adolescents. He noted that muscular spasm was present in 42.8% of patients with unstable lesions, but in only 3.5% of those with stable lesions. Not all authors are as cautious in their approach to mobility studies. Dolan [1 3] states that they should be obtained by passive movement of the neck, even on unconscious patients. Other authors [1, 2], recognizing the problems of limitation of movement due to muscular spasm, suggest that muscular relaxation be achieved by either sedation or general anesthesia and the neck then flexed while being visualized on an image intensifier. REFERENCES 1. Evans KD: Anterior cervical subluxation. J Bone Joint Surg [BrJ 58:3i8-321, 1976 2. Webb JK, Broughton RBK, McSweeney T, Park WM: Hidden flexion injury of the cervical spine. J Bone Joint Surg [Br] 58: 322-327, i976 3. Wein CD: Roentgenographic signs of cervical injury. Clin Orthop io9:9-i7, i975 4. Kewalnamani LS, Taylor AG: Injuries to the cervical spine from diving accidents. J Trauma i5:i30-142, i975 S. Holdsworth F: Fractures, dislocations and fracture-dislocations of the spine. J Bone Joint Surg [Am] 52: 1 534-1 55i, 1970 6. Scher AT: Ligamentous injury to the cervical spine-two radiological signs. S Aft Med J 53:802-804, 1978 7. Cheshire DJE: The stability of the cervical spine following conservative treatment of fractures and dislocations. Int J Paraplegia 7:193-203, i970 8. Holdsworth F: Fractures, dislocations and fracture-dislocations of the spine. J Bone Joint Surg [Br] 45:6-20, i 963 9. White AA, Johnson RN, Panjabi MM, Southwick WO: Biomechanical analysis of clinical stability in the cervical spine. C!in Orthop 109:85-95, 1975 10. Fielding JW, Hawkins JR: Roentgenognaphic diagnosis of the injured neck, in Instructional Course Lectures, vol 25, Chicago, American Academy of Orthopaedic Surgeons, i 976 pp i 49-170 1 1. Penning L: Diagnostic clues by x-ray of injuries of the lower cervical spine. Acta Neurochir(Wien) 22:234-244, 1970 1 2. Hubbard DD: Injuries of the spine in children and adolescents. C!in Orthop 100:56-65, 1974 1 3. Dolan KD: Cervical spine injuries below the axis. Radio! C!in NorthAm 15:247-259, 1977