The C1 vertebra (atlas) is a closed ring. A fracture of a closed ring necessarily results in at least two areas of ring disruption. These disruptions are customarily accompanied by a spread of the C1 ring fragments as a result of the axial loading mechanism of this injury and the weight of the head.  Jefferson originally described this type of C1 fracture in 1920.  Accordingly, the term Jefferson fracture is also used to denote burst fractures of the ring of C1.
In addition to anteroposterior (AP) and lateral views, radiographs of the upper cervical spine include the open-mouth view. This view may identify spreading or widening of the lateral masses or asymmetry of the separation of the odontoid from the lateral masses, which, in an appropriately centered radiograph, may be consistent with spreading of the C1 ring or a C1 fracture. Increased overhang of the lateral masses over the C2 facet totaling more than 6.9 mm suggests a fracture with disruption of the transverse odontoid ligament that may otherwise constrain displacement.
Fractures of the ring of C1 may be associated with an odontoid fracture; thus, the combination of the two fractures should be considered. Furthermore, congenital anomalies of the arch (eg, agenesis of the posterior ring) may be present. Anterior subluxation of C1 on C2 may be present and, if so, often indicates a disruption of the transverse odontoid ligament.
The principal treatment is with a halo and vest or cast, which remains an effective current treatment for many of these fractures.
The upper cervical spine is defined by the two most cephalad cervical vertebrae, C1 (the atlas) and C2 (the axis). This region is distinct in anatomic shape and is more mobile than the lower cervical spine, the subaxial cervical spine. The occipital condyles of the head (or the globe) rest upon the lateral masses of C1 (the atlas). These articular facets allow most of the flexion and extension of the head on the neck as the occipital condyles articulate on the atlas. [3, 4, 5, 6]
The ring of C1 has no vertebral body; the vertebral body that would correspond to C1 is connected or contiguous with the vertebral body of C2 and projects up as the dens (the tooth), also known as the odontoid of C2. Most of the lateral rotation of the neck actually occurs at the C1-2 junction; the remaining motion of the cervical spine is distributed among the subaxial spine vertebral motion segments as a fractional amount (~7%) per level and is less in total than the C1-C2 lateral rotation.
This area of the upper cervical spine is extremely mobile, and its stability is dependent on ligamentous structures. In unresponsive patients or those who are unable to report symptoms or pain, a C1 fracture or an occipital cervical dislocation must be excluded by radiographic screening. Also, displacement of the C1 ring may occur if the capsule or ligaments are disrupted, even without a C1 fracture; hence, the head may be displaced on the neck, and the atlas may also rotate around the odontoid or sustain a fracture of the dens.
The care of any fracture requires attention to the joint above and below. This cervical complex has often been treated as two separable articulations, C0-1 and C1-2, but the three-unit occipitoatlantoaxial complex (C0-C1-C2) articulation is much more functionally relevant.
The significance is the proximity to the brain, brainstem, and upper cervical spinal cord, but that is contrasted with the very significant motion that occurs in this area. Although patients are routinely asked to flex and extend their necks to determine range of motion, some of the motion observed is between the occiput and the atlas, and as the patient rotates laterally, at least 50% of that motion is atlantoaxial.
The stability of the injury depends on the ligaments between the bony structures. On the frontal view, the projecting occipital condyles are supported by the lateral masses (observed as wedges, narrow medially and expanding laterally), resting on the corresponding superior articular surface of C2. Consequently, the lateral masses provide inherent stability because of this bony shape and also illustrate the extent of the instability when this bony structure is disrupted, particularly when these wedges displace laterally.
The projecting condyles of the occiput are stabilized with the occipitoatlantal capsule, as well as anterior and posterior atlanto-occipital membranes. The ligamentum nuchae is a significant stabilizing structure; its specific relevance to the atlanto-occipital axial complex is controversial but should be considered. Connections from the occiput to the axis are the tectorium membrane and the alar and apical ligaments, which do not appear to be bulky enough to be independently significant restraints.
The dentate ligaments (ie, the alar ligament and the apical ligaments) attach to the dorsal lateral surface of the dens and run obliquely to the medial surfaces of the occipital condyles. In 1974, Anderson and D'Alonzo classified a type 1 odontoid fracture as an avulsion fracture of the odontoid tip caused by the apical ligament, suggesting that these ligaments impart a significant degree of stability.  Modifications aimed at expanding the Anderson-D'Alonzo classification have been proposed.  The Roy-Camille system has also been used to classify odontoid fractures. 
The transverse ligament goes from the medial surface of one side of the atlas to the other side and essentially constrains the axis to rotate around the odontoid in a closed ring of bone and the transverse ligament. As a consequence, the atlas can displace and embarrass the brainstem and spinal cord if this ligament ruptures or if an associated fracture of the odontoid is present as a result of this specific anatomic arrangement.
The ring of C1 is a structural member of the cervical spine. Because it is a ring and because fracture results in disruption of this ring, more than one location is affected.
The fragments have a propensity to shift laterally, both from the weight of the head and from the muscular contraction acting through this articulation; thus, occipital condylar support for the head is lost. The absence of the rigid bony structure and the lack of interconnection or interrelation of the attached ligamentous structures meet the definition of instability, particularly in that the bony protective function of C1 for the neural elements is lost.
Vertebral artery injuries have been reported as a result of C1 fractures, especially with atlanto-occipital dislocations; small excursions of displacement can be fatal. In addition, vertebral artery injuries can occur and have been reported in the absence of severe trauma as a result of cervical traction, chiropractic manipulation, overhead work, or yoga exercises. Hyperextension is customarily accompanied by rotation; when this is not limited by normal restraints, it becomes excessive, severely diminishing blood flow through the vertebral arteries.
This diminished blood flow is a particular problem in the posterior inferior cerebellar artery and may result in Wallenberg syndrome, which is characterized by ipsilateral loss of cranial nerves V, IX, X, and XI with cerebellar ataxia.
Horner syndrome may occur and, in some cases, may involve contralateral loss of pain and temperature sensation; involvement can extend up from a lateral medullary infarct and spread to the basilar superior cerebellar or the inferior cerebral artery, leading to sudden death, quadriplegia, and the locked-in syndrome, in which quadriplegia occurs with loss of lower cranial nerves and only eye-blinking is possible.
The Jefferson fracture most commonly occurs as the result of axial loading on the head through the occiput, which leads to a burst-type fracture of C1. Diving is the most frequent cause of this fracture, when it results from striking the head on an obstacle in shallow water; hence, the national program "Feet first, first time" (North American Spine Society, 2005) provides a motto for diving in unknown waters or shallow collections of water and has been an effective deterrent. 
The next most frequent cause of this fracture is being thrown up against the roof of a motor vehicle, a car or bus, or even an aircraft, with the forces being distributed to the body through the neck. The third most frequent cause of these injuries is falls onto the head, except in toddlers, who are predisposed to injury from falls because of their disproportionate head size. 
Less frequently, when a significant rotatory force is exerted, an atlanto-occipital junction dislocation may occur, or the force may also be dissipated through the odontoid as an associated fracture.
Fractures of the atlas account for 25% of atlantoaxial complex bony injuries, 10% of cervical spine injuries, and 2% of all spine injuries. Injury to the cervical spine occurs infrequently in pediatric populations, and although C1 represents only 1-2% of pediatric trauma and 2-10% of all cervical injuries in this population, the associated mortality is 16%.
Patients with Jefferson fractures are expected to heal and have an excellent prognosis for resumption of activity in the absence of associated injuries. Any surgical stabilization severely restricts the motion of the head, because the occipitoatlantoaxial complex represents over 50% of the motion of the head on the trunk.
Platzer et al studied nine patients (average age, 54 years) who underwent anterior plate fixation of an odontoid fracture because of unsuitability for anterior screw fixation.  After plate fixation, eight of the nine returned to their preinjury activity level and were satisfied with the treatment; one reported chronic pain and decreased cervical spine motion. Bony fusion was achieved in all patients; reduction or fixation failed in two. These findings suggested that anterior plate fixation may be a practical option for odontoid fractures requiring additional stabilization.
Al Eissa et al performed a retrospective review of 17 patients with isolated C1 and C2 fractures who experienced significant airway compromise.  Older age and male gender were found to be significant risk factors. Most patients also exhibited prevertebral swelling, significant degenerative changes, and significant fracture displacement. Of the 17 patients, 12 required intubation and admission to the intensive care unit; four died. The findings suggested that all patients with isolated C1 and C2 fractures should be assessed for potential airway compromise.