The craniocervical junction is comprised of C1 (atlas) and C2 (axis). The occipito-atlantal and atlantoaxial articulations provide 50% of the flexion and rotation in the cervical spine, respectively. Due to their high degree of motion, these bony segments are also the most often injured in adults. C1 fractures should be promptly identified and treated in all patients although they rarely require surgery. Any bony fracture of the atlas merits a thorough examination of the ligamentous structures between O-C1 and C1-C2, which in comparison often confer a poor prognosis.
C1 fractures generally occur due to an axial loading mechanism, which can be combined with flexion/extension or rotation to create the typical fracture patterns. The area is subject to high stresses from a large moment arm of the cranium.
Atlas fractures occur secondary to trauma (80% result from motor vehicle collisions) in most cases. Atlas fractures account for 3-13% of all cervical fractures, and 1-3% of all spinal injuries. They make up 25% of all injuries in the craniocervical spine.
The average age of patients with an atlas fracture is 64 years. These injuries have a bimodal distribution, with individuals 20-30 and those between 80–84 years most at risk. Older patients have a higher association with concomitant axis fractures. The incidence in the geriatric population continues to rise.
The most common type of atlas fracture is an isolated anterior or posterior arch fracture, which occurs in conjunction with other subaxial spine fractures in most cases. The second most common type involves isolated burst fractures without neurologic dysfunction. Associated neurologic and vascular injuries are uncommon but can have devastating consequences.
50% of atlas fractures involve a concomitant spine fracture, and there is also a 40% association with axis fractures.
Relevant anatomy of atlas
Atlas has a unique anatomy, in that it sits just inferior to occiput; and through the articulations with C2 and condyles of the occipital bone joins the skull with the cervical spine. C1 is devoid of a vertebral body. It contains the anterior and posterior arches, which encircle the spinal cord (posteriorly) and the odontoid process (anteriorly). The arches are joined by lateral masses on either side, whose superior and inferior articular surfaces take part in the occipito-cervical and atlantoaxial joints, respectively. The atlantoaxial joint is highly mobile and is stabilized by the anterior atlantoaxial ligament (between the anterior ring of atlas and C2), transverse ligament (posterior to odontoid process) and posterior atlantoaxial ligament (between the posterior ring of atlas and C2). Among these ligaments, the transverse ligament contributes the most to the C1-C2 articulation.
Levine Classification of Atlas Fractures
Type 1: Isolated bony apophysis (transverse process) fracture
Type 2: Isolated posterior arch fracture
Type 3: Isolated anterior arch fracture
Type 4: Comminuted fracture or lateral mass fracture
Type 5: Bilateral burst fracture (AKA Jefferson Fracture)
Gehweiler classification of Atlas Fractures
Type 1: Isolated anterior arch fracture
Type 2: Isolated, predominately bilateral, posterior ring fracture
Type 3: Combined injury of anterior and posterior arch (“classic Jefferson-fracture”)
Type 3a: Stable injuries (TAL intact)
Type 3b: Unstable injuries (TAL ruptured)
Type 4: Lateral mass fractures
Type 5: Isolated fractures of the C1 transverse process
Dickman classification of transverse atlantal ligament (TAL) injuries
Type 1: Intra-ligamentous rupture
Type 1a: Central lesion
Type 1b: Lesion close to the lateral mass
Type 2: Bony avulsion injuries
Type 2a: Isolated bony avulsion
Type 2b: Bony avulsion associated with lateral mass fractures
Most atlas fractures result from significant trauma. History will often reveal an axial load injury to the cranium, including a dive into shallow water, a football tackle, or a motor vehicle collision with blunt cranial trauma. However, different patient populations such as osteoporotic patients or patients with neuromuscular diseases may be at increased risk.
Any cervical trauma exam must initiate with an assessment of the ABC’s: airway, breathing, circulation. If tracheal intubation is required, it should be done with manual in-line stabilization to avoid any displacement of fractures/dislocations.
A thorough neurologic evaluation must be performed, starting with the Glasgow Coma Scale. All other traumatic injuries to other organs must be evaluated initially according to the Advanced Trauma Life Support (ATLS) protocol.
The physical exam must be detailed; one should take note of any axial neck pain or external signs of trauma to the cervical spine. A thorough neurologic exam should be performed including cranial nerves, complete upper and lower extremity sensory and motor examination. In patients who may demonstrate signs of neurologic shock, a rectal exam and bulbocavernosus reflex should be tested.
C1 fractures usually present with axial neck pain with no evidence of neurologic dysfunction. When involved, cranial nerves in the medulla and pons are at risk due to their caudal location: cranial nerves VI to XII may result in their respective palsies. Fractures involving the C1/C2 transverse foramina can cause blunt vertebral artery injury (BVAI) resulting in basilar insufficiency; a through cranial neurologic exam should be performed. ROM should not be permitted prior to radiographic clearance.
Radiographic evaluation of the C1 ring may be difficult due to interference from the occiput but are necessary. Views should include an anterior-posterior, lateral, and open-mouth odontoid.
Important radiographic parameters
Atlanto-Dens Interval (ADI): The distance measured on the lateral cervical spine radiograph between the posterior cortex of the anterior arch of the atlas and the anterior cortex of the dens.
Lateral mass displacement (aka Rule of Spence): The sum of lateral mass displacement on the open-mouth odontoid view has been described as prognostic of a transverse ligament injury.
> 7 mm sum of lateral mass displacement = Transverse ligament rupture (8.1mm with radiographic magnification per Heller 1993)
CT scans are recommended if possible as they will provide better delineation of the fracture pattern in multiple planes. They can also provide a good visualization of bony avulsion injuries of the transverse ligaments (on axial cuts).
MRI is indicated any time there is potential for transverse ligament injury or evidence of neurologic dysfunction concerning cord involvement.
A CT angiogram may be required if there is a concern for vertebral artery injury.
High-quality evidence regarding treatment of atlas fractures is lacking, and controversy still exists regarding surgical versus non-operative management in many cases.
Overall, the main factors to consider in the treatment of atlas fractures are: 1) is there an isolated fracture? and 2) is the transverse ligament intact?
If the fracture is isolated and the transverse ligament is intact, bony atlas fractures can be treated with strict immobilization in a halo brace versus a cervical collar. Bony avulsions of the transverse ligament are controversial, with advocates for both surgical stabilization versus non-operative immobilization. Halo immobilization is not recommended in the elderly population due to a high rate of complications.
Surgical instrumentation can fixate to C1-2 or occiput to C2 in directed circumstances.
Several options exist for C1-C2 fixation.
C1 lateral mass screws and C2 pedicle screws or C2 pars screws are a fixation option. The decision for pars or pedicle screw fixation is both surgeon and anatomy dependent. The surgeon must consider and measure the diameter of the C2 pedicle in order to judge the appropriateness of instrumentation through the anatomy. Other options include C2 laminar screw constructs. This can be an independent surgical option with biomechanical advance but also a bailout technique. C2 transarticular screws represent a multiple level instrumentation option. This provides stable fixation access through a single screw on each side of the construct, however, it provides the highest rate of possible vertebral artery injury. A historical option and a bailout option is a fusion of C1-2 through laminar wiring. The overarching purpose of all instrumentation techniques is the fundamental need for fusion with decortication and allograft/autograft considerations.
Concerns regarding the loss of cervical motion have raised interest in C1 osteosynthesis as an option for C1 fracture management. Transoral anterior (Ruf et al), posterior-only or combined posterior plus anterior (Böhm et al) osteosynthesis options have been described for these fractures; nevertheless there are only a few case series on each of these surgical techniques.
In the pediatric population, one should be careful to distinguish fractures of C1 vertebra from unfused ossification centers. Three ossification centers develop in an immature atlas, one for the anterior ring and one for each posterior neural arch. These centers appear at one year of age. The connection between anterior and posterior arches is composed of neurocentral synchondrosis, which fuses at 7 years of age. The posterior arch usually closes by three years of age. These unfused ossification centers can mimic fractures and should be borne in mind whenever children under six years present with cervical spine trauma.
The overall prognosis of C1 fractures is favorable and in a majority of the occasions, conservative management provides adequate fracture healing. These fractures are associated with a very low incidence of neurological deficit, as the fracture fragments tend to be outwardly displaced rather than encroach the neural canal. The spinal canal is also very spacious at this level. Associated bony and ligament (transverse ligament) injuries predominantly determine the prognosis and healing of these fractures.
After the application of a halo, regular x-rays are required to ensure that the fracture is healing. The halo is often required for 8-16 weeks, depending on the rate of healing. Once the halo is removed, a collar is placed, and the patient needs to be enrolled in a rehabilitation program to regain muscle and strength.
In general, a majority of the C1 fractures are managed conservatively with some form of immobilization. But the patients should be aware of the need for regular followup as advised by the medical personnel and the possibility of needing surgical intervention if the conservative treatment fails. Smoking can prevent fracture healing and should be strictly avoided. Strict compliance to the use of the collar is also recommended for fracture healing.
C1 fractures are relatively common following trauma and involve patients of all ages. This fracture can be life-threatening, and all personnel in the emergency department or the trauma team must be aware of its presentation and management. While a neurosurgeon usually manages the primary pathology, the aftercare is generally performed by the neurosurgery nurses and therapists. When promptly treated, the isolated C1 fracture has a good outcome. Small case series reveals that even after anterior plate fixation, the outcomes are good with patients achieving a return to their pre-injury status. However, the outcomes in the elderly and young males are guarded. This population often suffers from a multiorgan injury, requires intubation and admission to the ICU. Even when discharged, some of these individuals have residual neurological deficits.
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