Upper Cervical Spine Trauma

Jens R. Chapman, MD, and Richard J. Bransford, MD

1. What are the major types of injuries involving the upper cervical (occiput-C2) region?

The major types of injuries can be classified according to location:

1. Occipitocervical articulation 3. Axis (C2)

• Occipital condyle fractures • Odontoid fractures

• Atlanto-occipital dislocation • Hangman's fractures

• Atlas fractures

• Transverse ligament injuries

2. How are upper cervical spine injuries diagnosed?

Any patient with a suspected cervical spine injury requires a thorough evaluation. Frequently, no specific symptoms or findings on physical examination strongly point to the presence of a significant osseous or ligamentous injury involving the upper cervical region. Symptoms are notoriously vague and may include headaches or suboccipital pain. Not infrequently, the patient may be unconscious following trauma. A neurologic evaluation is performed, according to the American Spinal Injury Association (ASIA) guidelines. Assessment of the upper cervical spine should include evaluation of lower cranial nerve function. Most upper cervical spine injuries can be diagnosed on the lateral cervical spine radiograph. An open-mouth anteroposterior (AP) odontoid view and lateral skull radiograph should be obtained if radiographs are used for cervical spine clearance. Computed tomography (CT) with sagittal and coronal plane reformatted views is required to assess the full magnitude of injury and is considered the imaging modality of choice for initial workup in the trauma setting. Magnetic resonance imaging (MRI) is indicated for patients with cervical spinal cord injury and for evaluation of suspected ligament injuries that are not evident with other imaging modalities. Immobilization with a cervical collar (for stable injuries) or Gardner-Wells tong traction (for most unstable injuries) should be maintained in the emergency setting.

3. What is the role of flexion-extension radiographs in the assessment of acute upper cervical spine injuries?

Although flexion-extension radiographs can identify instability of the atlantoaxial motion segment, they are of limited value and even potentially dangerous in the acute trauma setting. Physician-supervised traction films are preferable to assess stability of the upper cervical spine.

4. How is a cervical traction test performed?

With the patient in the supine position, an image intensifier is used to obtain a baseline lateral radiographic image of the cervical spine. Traction weights are added in 5-lb increments (20-lb limit) using a head halter or skeletal traction device (halo, Gardner-Wells tongs) as the cervical region is monitored radiographically. If distraction of more than 3 mm between the occipital condyles and atlas or between the atlas and axis occurs, the test is considered positive and is discontinued.

5. Are upper cervical spine injuries common?

Because of the fragile nature of the bony and ligamentous components of the upper cervical spine, injuries are relatively common, especially in the setting of closed head trauma. Typical injury mechanisms include flexion, extension, or compressive forces applied to the head during motor vehicle accidents, falls from a height, or sporting injuries. Approximately 50% of fractures involving the atlas are accompanied by a second spine fracture. Fractures of the axis account for 27% of associated injuries; odontoid fractures account for 41%. The exact incidence of upper cervical ligamentous injuries is undetermined. Disruption of the craniocervical ligaments is reported to be the leading cause of fatal motor vehicle occupant trauma (Fig. 55-1).

Figure 55-1. Craniocervical ligaments. The tectorial membrane is the uppermost extension of the posterior longitudinal ligament (PLL) and attaches to the occipital condyles providing for stability against cranial traction and flexion forces. The alar ligaments extend from the tip of the odontoid process and attach to the anterior aspect of the foramen magnum serving as checkreins against rotation and distraction. These ligaments run from the occiput to C2 without attaching directly to C-1, which serves as a bushing. The transverse atlantal ligament (TAL) restricts translation of C1 on C2. (© Jens R. Chapman.)

6. Are upper cervical spine injuries commonly associated with neurologic deficits?

Because of the relatively large size of the upper cervical spinal canal, neurologic deficits are relatively rare in association with upper cervical injuries. However, when upper cervical spinal cord injuries occur, they are often fatal because of injury to the respiratory and cardiac centers in the medulla and upper cervical cord. Incomplete spinal cord injuries in the upper cervical region may present as a cervicomedullary syndrome or cranial nerve injury.

OCCIPITOCERVICAL ARTICULATION

7. What is the mechanism for occipital condyle fractures and how are these injuries classified?

Occipital condyle fractures typically result from a direct blow to the head or from a rapid deceleration injury. These injuries are frequently associated with C1 fractures and cranial nerve injuries. CT is used to classify these injuries into three subtypes according to the classification developed by Anderson and Montessano:

• Type 1: a stable comminuted fracture resulting from an axial loading injury

• Type 2: a stable skull base fracture that extends into the occipital condyle

• Type 3: an avulsion fracture of the condyle at the attachment of the alar ligament. This fracture type is potentially unstable and may be associated with an atlanto-occipital dislocation.

8. How are occipital condyle fractures treated?

Unilateral Type 1 and 2 injuries are usually treated with a rigid cervical orthosis. Isolated Type 3 avulsion injuries are managed with a halo orthosis. Type 3 injuries associated with atlanto-occipital dislocation require posterior occipitocervical fusion.

9. What is an atlanto-occipital dislocation (AOD)?

High-speed deceleration injuries may result in disruption of important craniocervical ligaments (tectorial membrane, anterior occipito-atlantal membrane, alar ligaments) resulting in craniocervical junction instability. These injuries are frequently fatal. Survivors frequently present with a spinal cord injury above the C4 segment. Incomplete spinal cord injuries associated with AOD include respiratory impairment and cranial nerve injuries (cervicomedullary syndrome). Young children are the most commonly injured age group due to their relatively large head size, shallow atlanto-occipital joints, and ligamentous laxity compared with adults. In the pediatric age group, AOD may be the result of shaken baby syndrome, pedestrian versus car injuries, or deceleration injuries in a car crash with the child immobilized in a car seat.

10. How is atlanto-occipital dislocation identified?

The most effective initial screening test remains the lateral cervical spine radiograph. The most important radiographic parameters to assess include:

• Soft tissue swelling adjacent to the upper cervical vertebral bodies (> 6mm)

• Diastasis or subluxation of atlanto-occipital articulation (Fig. 55-2A)

• Disruption of Harris' lines—this is a combination of two measurements that have been termed the rule of twelve (Fig. 55-2C):

° Dens-basion interval (DBI): The distance from the dens to the basion should be less than 12 mm ° Basion-atlantal interval (BAI): A measurement from a perpendicular line extending along the posterior margin of the C2 vertebral body (posterior axis line [PAL-B]) should not be more than 4 mm anterior and should be less than 12 mm posterior to the basion Additional measurements have been described but are less reliable than Harris's lines. These include Wackenheim's line and Power's ratio (Fig. 55-2B). A Power's ratio greater than 1 suggests an anterior dislocation of the atlanto-occipital joint. This ratio between the distance from the basion to the posterior arch of C1 and the distance from the opisthion to the anterior arch of C1 is usually less than 1.

LADI within

± 2 mm

Joint "spaces'

N /T

V\ l-""1^ mm

\y j

\ yjK/2-3 mm

^^^No overhang

A

Figure 55-2. A, The lateral masses of the atlas should closely articulate with the superior articular processes of the atlas. The odontoid should be centered symmetrically between the lateral masses of the atlas (lateral atlantodens interval [LADI]). B, Additional screening lines include Wackenheim's line and the C1 to C3 spinolami-nar line. C, The tip of the odontoid should remain in close proximity to the basion, as shown with the reference lines described by Harris. ADI, atlantodens interval; PAL-B, posterior axis line. (© Jens R. Chapman.)

<15% Normals

The diagnosis of AOD is confirmed with a fine-cut CT scan and/or MRI. Occasionally, a cervical traction test is necessary to confirm the presence of an occult AOD.

11. How is an atlanto-occipital dislocation classified?

There are two major methods used to classify AODs. The initial classification of Traynelis assessed the direction of displacement of the head relative to the cervical spine and described anterior, vertical, posterior, and oblique dislocations. However, classification according to displacement in the presence of global ligamentous failure is somewhat arbitrary as displacement can be altered by patient positioning. In addition, such a classification does not grade injury severity or the potential for spontaneously reduced dislocations, which would be overlooked if the sole criterion for injury is displacement. It is important to distinguish incomplete injuries that retain partial meaningful craniocervical ligementous integrity from occult injuries in which a rebound phenomenon led to partial or complete deformity reduction. Spontaneously reduced injuries are easily overlooked yet may have catastrophic consequences if left untreated.

An alternative classification, the Harborview classification, attempts to stratify injuries according to severity:

• Type I: These injuries are relatively stable and can be treated nonoperatively. MRI shows edema or hemorrhage at the craniocervical junction, but Harris' lines show normal cervical alignment. A traction test performed with 25 pounds of traction is normal and rules out a spontaneously reduced injury

• Type II: These injuries feature complete disruption of key ligaments of the cranio-cervical junction and are innately unstable, requiring surgical treatment. MRI shows edema or hemorrhage at the craniocervical junction, but Harris's lines show borderline screening measurement values. A spontaneous partial reduction of the cranium to its cervical location, through remaining residual ligamentous attachments, has occurred and is potentially misleading. Traction at weights less than 25 pounds shows sufficient distraction to meet craniocervical dissociation criteria, according to Harris's lines

• Type III: These injuries demonstrate obvious major cranio-cervical displacement on static plain radiographs

12. What is the treatment for an atlanto-occipital dislocation?

Stage I lesions are usually treated with 8 to 12 weeks of halo vest immobilization. Stage II and III AOD are potentially life-threatening injuries. Emergent reduction and external immobilization attempts can be made with a halo vest or head immobilization using a neck collar and sand bags. Definitive treatment of stage II and III lesions consists of posterior occipitocervical arthrodesis with rigid segmental spinal instrumentation. Attempts at occiput to C1 fusion are unwarranted because this treatment does not address the disrupted alar ligaments and tectorial membrane, which extend between C2 and the occiput. Significant ethical challenges, in terms of sustaining life-preserving support measures, may arise in cases of patients with associated anoxic or traumatic brain injury.

ATLAS FRACTURES AND TRANSVERSE ATLANTAL LIGAMENT (TAL)

INJURIES

13. How are C1 fractures and TAL injuries diagnosed?

Radiographs should include an open-mouth odontoid view and a lateral C1-C2 view. On the lateral view, the atlantodens interval (ADI) should be less than 3 mm in adults and less than 5 mm in children. On the open-mouth view, the symmetry of the dens in relation to the adjacent lateral masses should be assessed. Any outward displacement of the lateral masses of C1 in relation to C2 should be noted. Atlantoaxial offset greater than 7 mm indicates C1-C2 instability and disruption of the TAL (Fig. 55-3), although TAL disruption may also be present if atlantoaxial offset is less than 7 mm. Definitive assessment is achieved with a fine-cut CT scan with reformatted images. Efforts at visualizing the TAL on MRI have remained unreliable. Isolated TAL injuries may occasionally require flexion-extension radiographs to assess for atlantoaxial instability.

Figure 55-3. A, Lateral displacement of the C1 lateral masses of more than 7 mm indicates disruption of the transverse ligament. B, The sum of the displacements of the left and right sides (a + b) is used to determine the total displacement. (From Browner BD, Jupiter JB, Levine AM, editors. Skeletal Trauma. Philadelphia: Saunders; 1998.)

Figure 55-3. A, Lateral displacement of the C1 lateral masses of more than 7 mm indicates disruption of the transverse ligament. B, The sum of the displacements of the left and right sides (a + b) is used to determine the total displacement. (From Browner BD, Jupiter JB, Levine AM, editors. Skeletal Trauma. Philadelphia: Saunders; 1998.)

14. How are C1 fractures classified?

Five primary types of atlas fractures have been defined (Fig. 55-4):

• Type 1: Transverse process fracture

• Type 2: Posterior arch fracture

• Type 3: Lateral mass fracture

• Type 4: Anterior arch fracture

• Type 5: Burst fracture (Jefferson's fracture). Burst fractures may consist of three or four parts. This injury may occur as a ligamentous combination injury with associated TAL disruption.

Types 3, 4, and 5 fractures can be inherently stable or unstable, depending on fracture comminution, displacement, and concurrent ligamentous disruption. In general, Type 3 injuries with a sagittal fracture line and segmental anterior arch fractures are unstable.

Figure 55-4. Classification of atlas fractures. (From Browner BD, Jupiter JB, Levine AM, editors. Skeletal Trauma. Philadelphia: Saunders; 1998.)

15. How are atlas fractures treated?

Type 1 injuries are treated with a cervical collar.

Type 2 injuries are treated with a cervical collar if they occur as an isolated injury. However, there is a greater than 50% chance of an association injury (e.g. odontoid fracture), and the presence of additional injury alters the treatment plan.

Type 3 injuries require close follow-up for potential loss of reduction and secondary collapse. If subsidence of the occipital condyle through the lateral mass of the atlas occurs, treatment options consist of closed reduction with skeletal cranial traction over a period of several weeks, followed by halo immobilization or primary posterior atlantoaxial arthrodesis.

Type 4 fractures, in which the odontoid has displaced through the anterior ring of the atlas, are highly unstable. If atlantoaxial alignment is maintained, nonoperative treatment with a halo vest is considered. Closed reduction and atlantoaxial arthrodesis are usually required.

Type 5 (Jefferson-type) fractures are treated based on integrity of the TAL. Disruption of the TAL is suspected if the lateral masses of C1 overhang those of C2 by the sum of 7 mm or more on the AP open-mouth radiograph (Spence's rule). TAL disruption is also present if there is translational atlantoaxial displacement of 3 mm or more in any direction. Nonoperative treatment of type 5 atlas fractures consists of fracture reduction with traction and conversion to a halo vest after a period of days to weeks. Upon mobilization, maintenance of satisfactory alignment is checked with upright lateral and open mouth odontoid views. Surgical stabilization has been advocated based upon an unstable fracture configuration, for patients with purely ligamentous injury of the TAL, or if recumbent traction or halo treatment is unsuccessful or contraindicated.

Certain variants of C1 injuries can be treated with primary open reduction and internal fixation.

16. What are the different types of TAL injuries?

TAL injuries have been differentiated into:

• Type 1 injuries (bony avulsion)

• Type 2 injuries (purely ligamentous injuries)

17. How are TAL injuries treated?

Treatment options depend on the type of TAL injury.

• Type 1 injuries (bony avulsion) can be successfully treated with rigid immobilization in a significant number of patients

• Type 2 injuries (purely ligamentous injuries) are unlikely to heal with nonsurgical management and require reduction of the deformity and atlantoaxial arthrodesis

18. What surgical techniques are used for posterior stabilization of atlas fractures and TAL injuries?

Historically, fusion of the C1-C2 motion segment was performed utilizing wire or cable fixation. The Gallie technique places wires under the posterior arch of C1 and around the C2 spinous process. The Brooks technique places wires under the lamina of C1 and C2. However, in the presence of a fracture through the ring of C1, neither technique is applicable. Today, wiring techniques are rarely used as a stand-alone option due to advances in spinal instrumentation techniques.

Placement of transarticular screws has been described for types 3, 4, and 5 atlas fractures. This technique requires anatomic reduction of the atlas on the axis to ensure safe placement and optimal fixation of each transarticular screw. It is also limited by anatomic factors such as patient size and body habitus, as well as the location of the vertebral artery within the C2 segment. Alternatively, placement of lateral mass screws into the atlas and pedicle screws or laminar screws in the axis, linked to a cervical rod on each side, has gained popularity. Advantages of C1-C2 screw-rod fixation (Harm's technique) include flexibility to adapt fixation according to individual patient anatomy and provision for intraoperative fracture reduction by manipulation of independent screws in C1 and C2. Occipitocervical instrumentation and fusion have been utilized to stabilize severe C1 injuries by spanning the injured level. Some experts have advocated treatment of select C1 fractures with osteosynthesis techniques utilizing lateral mass screws placed from either a posterior or transoral approach to achieve direct fracture repair and avoid fusion across motion segments.

AXIS FRACTURES: ODONTOID FRACTURES AND HANGMAN'S FRACTURES

19. What is the usual mechanism of injury for an odontoid fracture?

Odontoid fractures typically occur secondary to forced extension or flexion of the head and neck during a fall or collision. Associated fractures of the atlas occur in 10% to 15% of cases. Odontoid fractures are the most common cervical fracture in patients younger than 8 years or older than 70 years.

20. How are odontoid fractures classified?

The Anderson and D'Alonzo classification (Fig. 55-5) is widely accepted and is based on the location of the fracture line:

• Type 1: Stable avulsion fracture occurring at the tip of the odontoid. This must be differentiated from an avulsion fracture associated with AOD or os odontoideum

• Type 2: Unstable transverse fracture involving the cortical bone of the waist of the odontoid

• Type 3: Unstable fracture extending into the cancellous portion of the C2 vertebral body

Important fracture variables with potential therapeutic implications include segmental comminution, fracture displacement, and fracture obliquity. A more precise distinction between Type 2 and Type 3 fractures has been proposed. Type 2 fractures lack involvement of the superior articular facets of C2, whereas Type 3 fractures involve the superior articular facet.

21. How are odontoid fractures treated?

Type 1 fractures are treated with a cervical collar. It is important to evaluate the craniocervical junction to rule out concomitant ligamentous injuries.

Type 2 fractures are associated with a high incidence of nonunion (15%-85%). Risk factors associated with nonunion include initial fracture displacement greater than 4 mm, patient age older than 50 years, posteriorly displaced fractures, angulation greater than 10°, and inappropriate initial treatment. Treatment of Type 2 fractures is determined by a variety of factors including initial fracture displacement, presence of associated cervical fractures, fracture comminution, fracture obliquity, and bone quality. Nondisplaced or minimally displaced fractures may be treated with a halo vest or rigid collar for 8 to 12 weeks. Maintenance of fracture reduction is checked with lateral cervical spine radiographs taken in both the recumbent and upright positions. Anterior screw fixation or posterior C1-C2 fusion is performed for displaced fractures and for patients who are unable to tolerate halo immobilization. Treatment with benign neglect consisting of a soft neck collar has been suggested for geriatric patients too feeble to tolerate attempts at definitive care.

Type 3 fractures are reduced with skeletal traction as needed and externally immobilized. Severely comminuted or unstable fracture patterns may require posterior fusion and screw-rod fixation.

22. What options exist for surgical stabilization of odontoid fractures?

1. Anterior screw fixation: Single- or double-screw fixation can be performed for patients with transverse or posterior oblique odontoid fractures (fracture line courses from anterior superior to posterior inferior). Prerequisites include a fracture that is less than 3 weeks old and a patient with reasonable bone quality. Certain factors, such as a large body habitus, may preclude this form of treatment. There is also controversy with respect to anterior screw fixation in the elderly, secondary to swallowing difficulties encountered postoperatively. Contraindications to anterior screw fixation include fractures that course from anterior inferior to posterior superior (parallel to screw trajectory) and fractures with significant comminution

2. Posterior fusion with wires or cables: For patients with an intact C1 ring, posterior wire or cable fixation can be successful. Substantial limitations to this technique include insufficient biomechanical stiffness in rotation and the possibility of undesirable C1 posterior translation. With modern day techniques, stand-alone wiring is rarely utilized

3. Transarticular screw fixation: Placement of a small fragment screw from the midpoint of the inferior articular processes of C2 across the pars of C2 into the lateral mass of C1 provides excellent biomechanical stiffness and leads to a very high rate of bony union. Risks associated with this technique include iatrogenic injury to the vertebral artery as it passes laterally to the vertebral body of C2. Preoperative planning including CT evaluation to assess screw trajectory in relation to the vertebral artery and meticulous surgical technique are important for successful execution of this procedure

4. Posterior C1-C2 screw-rod fixation: This technique utilizes lateral mass screws for C1 fixation and pedicle screws for C2 fixation. Alternatively, C2 screw fixation can engage the laminae on either side. The screws are connected by rods along each side of the spine. Biomechanically, these segmental fixation constructs have similar biomechanical fixation strength compared with transarticular screw constructs

23. What is the usual mechanism of injury for a hangman's fracture?

The term hangman's fracture was originally used to describe the C2 fracture dislocation that occurred when criminals were treated by judicial hanging. A radiographically similar injury to the second cervical vertebra occurs as a result of motor vehicle trauma and is more appropriately termed traumatic spondylolisthesis of the axis. This fracture results in disruption of the bony bridge between the inferior and superior articular processes of the C2 segment. The fracture may be accompanied by injury to the C2-C3 disc, as well as disruption of the posterior ligaments between C2 and C3. Concurrent soft tissue injuries heavily influence fracture stability and treatment.

Type 1

((Cfe^

Jr-55.

fed. f

Type 2 ft

fl

(fe

Type 3

A

Figure 55-5. Anderson and D'Alonzo's classification of odontoid fractures. Type 1 fractures involve the tip of the odontoid process and are stable. Type 2 fractures penetrate the base of the odontoid. Type 3 fractures extend into the body of C2. (From Browner BD, Jupiter JB, Levine AM, editors. Skeletal Trauma. Philadelphia: Saunders; 1998.)

Figure 55-5. Anderson and D'Alonzo's classification of odontoid fractures. Type 1 fractures involve the tip of the odontoid process and are stable. Type 2 fractures penetrate the base of the odontoid. Type 3 fractures extend into the body of C2. (From Browner BD, Jupiter JB, Levine AM, editors. Skeletal Trauma. Philadelphia: Saunders; 1998.)

24. How is traumatic spondylolisthesis of the axis classified?

The Effendi classification, as modified by Levine and Edwards, is widely accepted (Fig. 55-6):

• Type I injuries consist of a fracture through the neural arch with no angulation and up to 3 mm of displacement

• Type II fractures have both significant angulation and fracture displacement (>3 mm)

• Type IIA injuries show minimal displacement but are associated with severe angulation as a result of a flexion-distraction injury mechanism. This injury may not be recognized until a radiograph is obtained in traction

• Type III injuries combine severe angulation and displacement with a unilateral or bilateral facet dislocation between C2 and C3

There is a low incidence of spinal cord injury with type I, II, and IIA injuries but a high incidence of spinal cord injury with type III injuries.

Eismont and Starr have described an atypical type of hangman's fracture (subsequently classified as type IA) in which on one side there is the typical location of fracture but the line of the fracture then cuts obliquely across the body in a similar pattern to a type III odontoid fracture. This injury type has a higher than usual rate of spinal cord injury. In the typical traumatic spondylolisthesis fracture patterns, the vertebral body displaces anteriorly and the corresponding posterior elements displace posteriorly, resulting in increased space for the spinal cord. In the atypical type IA variant, the circumference of the spinal canal is unchanged and bone or hematoma may result in cord compression and neurologic injury.

Type l Type ll Type ll-A Type Ill

Figure 55-6. Types of traumatic spondylolisthesis of the axis. (From Leventhal MR. Fractures, dislocations and fracture-dislocations of the spine. In: Crenshaw AH, editor. Campbell's Operative Orthopaedics. 8th ed. St. Louis: Mosby-Year Book; 1992.)

25. How is traumatic spondylolisthesis of the axis treated?

Treatment is based on the fracture type:

• Type I injuries can be treated in a rigid neck collar

• Type II injuries are usually treated with traction, followed by immobilization in a halo or a rigid collar. If significant disruption of the C2-C3 disc exists, surgical stabilization may be considered to avoid morbidity associated with prolonged traction and halo immobilization

• Type IIA injuries are usually treated by closed reduction by positioning in extension, followed by halo immobilization. If significant disruption of the C2-C3 disc exists, surgical stabilization is considered

• Type III injuries require surgical treatment for reduction of the facet dislocation and surgical stabilization

26. What are the surgical treatment options for unstable traumatic spondylolisthesis of the axis?

Unstable type II and IIA fractures not amenable to treatment with nonoperative means can be surgically stabilized by placement of C2 transpedicular screws or anterior C2-C3 plating. Type III fractures require open reduction of the facet dislocation and stabilization of the dislocation with lateral mass fixation to C3 and either C2 pedicle screws or fixation to C1. The fracture of the neural arch can be treated by placement of C2 transpedicular screws.

Key Points

1. The craniocervical junction consists of the osseous, ligamentous, and neurovascular structures that extend from the skull base to C2.

2. Injuries to the craniocervical junction are associated with a significant likelihood of death.

3. Nonoperative treatment options include recumbent skeletal traction, cervical orthoses, and halo immobilization.

4. Direct fracture osteosynthesis is an option for surgical treatment of select type 2 odontoid fractures and C2 pars interarticularis fractures.

5. Open reduction and stable internal fixation is indicated for unstable craniocervical injury patterns.

Websites

Internal decapitation: survival after head to neck dissociation injuries: http://www.medscape.com/viewarticle/577910_3 Measurement techniques for upper cervical spine injuries: http://www.medscape.com/viewarticle/555355 Upper cervical trauma: http://fhs.mcmaster.ca/surgery/documents/upper_cervical.pdf

BiBLiOGRAPHY

1. Anderson LD, D'Alonzo RT. Fractures of the odontoid process of the axis. J Bone Joint Surg 1974;56A:1663-74.

2. Bellabarba C, Mirza SK, West GA, et al. Diagnosis and treatment of craniocervical dislocation in a series of 17 consecutive survivors during an 8-year period. J Neurosurgery Spine 2006;4:429-40.

3. Bellabarba C, Mirza SK, Chapman JR. Injuries to the craniocervical junction. In: Bucholz RW, Heckman JD, Court-Brown CM, editors. Rockwood & Green's Fractures in Adults. 6th ed. Philadelphia: Lippincott Williams & Wilkins; 2006.

4. Chutkan NB, King AG, Harris MB. Odontoid fractures: Evaluation and management. J Am Acad Orthop Surg 1997;5:199-204.

5. Dickman CA, Greene KA, Sonntag VK. Injuries involving the transverse atlantal ligament: Classification and treatment guidelines based upon experience with 39 injuries. Neurosurgery 1996;38:44-50.

6. Dvorak MF, Johnson MG, Boyd M, et al. Long-term health-related quality of life outcomes following Jefferson-type burst fractures of the atlas. J Neurosurg Spine 2005;2:411-17.

7. Grauer JN, Shafi B, Hilibrand AS, et al. Proposal of a modified treatment-oriented classification of odontoid fractures. Spine J 2005; 5:123-9.

8. Greene KA, Dickman CA, Marciano FF, et al. Acute axis fractures: Analysis of management and outcome in 340 consecutive cases. Spine 1997;22:1843-52.

9. Levine AM, Edwards CC. Fractures of the Atlas. J Bone Joint Surg 1991;73A:680-91.

10. Levine AM, Edwards CC. The management of traumatic spondylolisthesis of the axis. J Bone Joint Surg Am 1985;67(2):217-26.

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