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The unique features of C2 anatomy and its articulations make assessment of its pathology challenging. The odontoid process (dens) is a vertical projection that lies just posterior to the anterior arch of C1, has ligamentous attachments to the skull base, and articulates with C1 (atlas).
The C1/C2 articulation (ie, atlantoaxial articulation) is made up of 3 joints, including the central atlantoaxial joint and the paired lateral atlantoaxial joints. These joints allow for rotation of C1 on C2. The transverse ligament of the atlas stabilizes the central atlantoaxial joint, and together with the odontoid process, acts as a restraint against horizontal displacement of the atlas. The dentate ligament attaches the apex of the odontoid process to the clivus and the paired alar ligaments, which originate from the transverse ligament and attach to the anterolateral rim of the foramen magnum. These ligaments provide for rotational and translational stability. The lateral atlantoaxial joints articulate at the superior articular facets of C2 and the inferior articular facets of C1. C2 also is composed of inferior facets, pedicles, transverse processes, and a spinal process.
Odontoid fractures
Incidence of odontoid fractures approaches 15% of all C-spine fractures. Usually, these fractures are secondary to MVAs or falls. When an odontoid fracture is suspected, it is important to rule out concomitant associated C-spine injuries. For example, C1 anterior ring fractures are not an uncommon finding, and a prevertebral soft-tissue shadow of more than 10 mm on plain films is highly suggestive of such a fracture. Anderson and D'Alonzo classified odontoid fractures based on the anatomic location of the fracture.
Type I fracture
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A type I fracture (less than 5% of cases) is an oblique fracture through the upper part of the odontoid process.
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This type of fracture occasionally is associated with gross instability due to traction forces applied to, and subsequent injury of, the apical and/or alar ligaments.
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This is an avulsion injury to the tip of the odontoid and is usually stable.
Type II fracture:
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A type II fracture (more than 60% of cases) is a fracture occurring at the base of the odontoid as it attaches to the body of C2.
Type III fracture
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A type III fracture (30% of cases) occurs when the fracture line extends through the body of the axis.
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The fracture line can extend laterally into the superior articular facet for the atlas.
Another type of odontoid process fracture is a vertical fracture through the odontoid process and body of the axis (less than 5% of cases). This type of fracture often is considered a variant of a traumatic spondylolisthesis of C2, which is discussed below.
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Treatment for type I fractures is hard-collar immobilization for 6-8 weeks, which usually is quite successful. Type II fractures can be managed conservatively or surgically. Treatment options include halo immobilization, internal fixation (odontoid screw fixation), and posterior atlantoaxial arthrodesis. Arthrodesis can be accomplished by C1/C2 transarticular crew fixation, interlaminar clamps, or wiring techniques such as the Gallie or the Brooks method. Management with the halo vest usually is considered if the initial dens displacement is less than 5 mm, the reduction is performed within one week of the injury and is able to be maintained, and the patient is younger than 60 years. During immobilization, alignment is assessed to ensure that reduction is maintained. Displacement of less than 20% is acceptable. The halo vest is in place from 12-16 weeks and the fusion rate is over 90%.
Wiring techniques, such as Gallie or Brooks methods, offer a high fusion rate (about 95%); however, the posterior arch needs to be intact and a halo vest must be worn postoperatively. Transarticular screw fixation provides a high fusion rate and the posterior arch need not be intact. Although the posterior surgical fusion techniques provide high fusion success rates, these do so at the expense of cervical rotation. Generally up to 50% of rotation is lost with these techniques.
Nonunion, malunion, and pseudarthrosis formation are potential major complications. Factors affecting this are amount and position of displacement, degree of angulation, ability to obtain and hold a reduced fracture, age of the patient, and tolerance to halo immobilization. However, some reports have demonstrated nonunion rates approaching 80% in certain subsets of patients. In a recent paper, Shilpakar and McLaughlin looked at all treatment options and associated rates of complications. Based on a meta-analysis, they concluded that type II fractures are best managed with odontoid screw fixation.
Anterior odontoid single screw fixation is noted to preserve normal rotation at C1/C2, provide immediate stability, and obviate the need for postoperative halo immobilization. Furthermore, rates of malunion, nonunion, and pseudarthrosis formation are very low. There are limitations to this approach, namely, the age of the fracture and the patient's body habitus. If the fracture is more than 4 weeks old or if the patient possesses a short neck and barrel-shaped chest, consider an alternative treatment approach such as transarticular screw fixation or Brooks sublaminar fusion.
Type III fractures are treated with halo immobilization, odontoid screw fixation, or C1/C2 arthrodesis. Deep, displaced, or angulated fractures are treated with closed reduction and halo thoracic immobilization. Uncomplicated shallow type III fractures are treated with odontoid screw fixation. Nonunion and malunion are potential complications. The vertical type of odontoid process fractures is addressed in the treatment section of Traumatic spondylolisthesis.
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