Indications
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C1-2 posterior cervical fusion is indicated in patients with odontoid fractures that cannot be repaired with an odontoid screw, including type II odontoid fractures associated with fractures of the atlantoaxial joint, type II odontoid fractures with oblique fractures in the sagittal plane that preclude odontoid screw placement, type II odontoid fractures with significant irreducible displacement that may not heal with immobilization (and are too displaced to place an odontoid screw), type II odontoid fractures with an associated Jefferson fracture, and type II odontoid fractures with a ruptured transverse ligament.
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In addition, patients with a cervicothoracic kyphosis or a very large barrel chest may be unable to be fixated with anterior odontoid screw placement (inability to achieve the trajectory required for odontoid screw placement) and are usually treated with a posterior C1-2 stabilization procedure.
Nonhealed Odontoid Fractures (Types II and III)
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Patients initially treated with immobilization who develop a pseudarthrosis are not ideal candidates for subsequent attempts at anterior odontoid screw fixation because of the material from the pseudarthrosis occupying the fracture line, which often prevents contact of the fracture surfaces.
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Type III odontoid fractures with atlantoaxial joint fracture combinations and type III odontoid fractures with associated Jefferson fracture are also unstable and may be treated with a posterior C1-2 stabilization procedure.
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Patients may also have ligamentous laxity and have resultant C1-2 instability. Ligamentous instability of C1-2 is identified with measurements of the atlantodental interval on flexion and extension views. Normally, this interval should not exceed 2 to 3 mm in adults. When the atlantodental interval exceeds 5 mm in nonrheumatoid patients and when it exceeds 7 to 8 mm in rheumatoid patients, there is instability of the C1-2 complex, and posterior C1-2 fixation is indicated.
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Atlantoaxial rotatory dislocations are also an indication for C1-2 fixation. This problem can often be treated via a posterior reduction and fusion approach (see Fig. 55-2 ).
Figure 55-2
A midline incision is made extending from the suboccipital area to the spinous process of C3. C2-3 facet joints are exposed (but not violated), and the dorsal arch of C1 is exposed laterally. The vertebral artery may be exposed in the vertebral groove on the superior aspect of the C1 arch (arterial sulci of C1). We review preoperative imaging to ensure that the vertebral artery does not have an aberrant course. The C2 nerve root is identified and is typically mobilized inferiorly. Bipolar cautery and hemostatic agents such as Surgifoam (Baxter, Deerfield, IL) are used to control bleeding from the venous plexus surrounding the C2 nerve root.
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Congenital malformations of C2 (i.e., os odontoideum and odontoid agenesis), degenerative diseases, inflammatory diseases (rheumatoid arthritis), tumors, and infections (osteomyelitis) can also result in instability of the atlantoaxial complex requiring C1-2 fixation.
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Postsurgical dynamic instability relating to odontoidectomy or C1 and C2 laminectomies with or without removal of adjoining facets is another indication for posterior C1-C2 fixation.
Planning and positioning
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Preoperative magnetic resonance imaging (MRI) and computed tomography (CT) are useful to evaluate any anatomic or physical constraints that may limit the placement of C1 lateral mass screws. Preoperative imaging may reveal substantial cranial-cervical settling (often seen in rheumatoid patients), and the C1 ring may be partially dislocated into the foramen magnum. In such cases, traction may be needed to expose the posterior C1 lateral mass.
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MRI helps delineate the course of the vertebral arteries, and in cases with aberrant vertebral arteries, magnetic resonance angiography or CT angiography may also be ordered.
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To minimize the risk of vertebral artery injury with C1 and C2 screw placement, we routinely perform CT of the cervical spine (often with three-dimensional remodeling sequences to identify an anomalous vertebral artery course, integrity of the bone at the intended site of screw fixation, or an unacceptably small C2 pars).
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Preoperatively, patients with C1-2 pathology are often immobilized using a rigid collar or halo. In the operating room, intubation is often best performed in a neutral supine position using a fiberoptic scope. We prefer to use somatosensory evoked potentials (SSEPs) and motor evoked potentials (MEPs) during surgery to monitor neurologic function. In patients with severe myelopathy, we prefer to obtain baseline SSEP and MEP signal after intubation while the patient is still supine.
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The choice of anesthetic agents is crucial when evoked potentials are monitored. Long-acting paralytic agents cannot be used in these cases because they blunt MEPs. Likewise, nitrous oxide cannot be used because it blunts SSEPs. One minimum alveolar concentration of vapor can also blunt evoked potentials. Consequently, we prefer to induce anesthesia by using propofol (2 to 3 mg/kg) along with a short-acting or medium-acting paralytic (rocuronium), and we maintain a propofol infusion throughout the case. After induction, we prefer to use 50% of the minimum alveolar concentration of vapor (i.e., isoflurane) and remifentanil (0.1 to 0.25 mg/kg/min) as a narcotic infusion. This combination is least likely to affect SSEPs and MEPs. Large-bore intravenous lines or a central line often is inserted to assist with fluid resuscitation. In adults, we typically use an arterial line and keep the mean arterial pressure around 90 mm Hg or more to prevent spinal cord ischemia.
