Indications
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Occipitocervical fusion is performed for craniovertebral junction (CVJ) instability resulting from various etiologies.
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Posttraumatic: Atlantooccipital dislocation, complex fractures involving CVJ, unstable odontoid fractures with incompetence of the posterior ring of C1
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Acquired cranial settling secondary to infectious or inflammatory disease: Rheumatoid arthritis, ankylosing spondylitis, Down syndrome, inflammatory bowel disease–associated arthropathy, pseudogout, ossification of posterior longitudinal ligament, chronic Grisel syndrome, CVJ tuberculosis, CVJ osteomyelitis
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Neoplastic: Primary tumors of CVJ such as chordomas, chondromas, and osteoblastomas; metastatic CVJ disease
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Congenital or developmental: Anterior and posterior bifid arches of C1, congenital basilar invagination, Chiari malformation–associated basilar invagination, absent occipital condyles or absent C1 lateral masses, os odontoideum, unilateral atlas assimilation with chronic occipitocervical rotatory subluxation
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Iatrogenic: Unstable craniocervical junction after transoral or endonasal endoscopic CVJ decompression, C1-2 pseudarthrosis, suboccipital craniectomy for Chiari malformation, extreme lateral transcondylar approach
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Planning and positioning
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Halo vest immobilization is applied before surgery in cases of posttraumatic occipitocervical instability such as atlantooccipital dislocation and after transoral or endonasal endoscopic CVJ decompression. Halo vest immobilization is maintained through prone positioning in surgery for occipitocervical fusion.
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Preoperative evaluation includes high-resolution computed tomography (CT) of the head, cervical spine, and CVJ with sagittal and coronal reconstructions. Careful attention is paid to the thickness of the midline occipital keel for measuring appropriate occipital screw lengths. CT angiography is obtained to evaluate the course of the vertebral arteries and to assess contraindications to C1 or C2 screw placement. Magnetic resonance imaging (MRI) of the cervical spine is obtained where indicated to document the presence of spinal cord signal change, to determine whether the CVJ requires decompression, to assess for the presence of subaxial instability, and to evaluate the degree of CVJ pannus in cases of inflammatory disease such as rheumatoid arthritis. Basilar invagination can be confirmed by drawing imaginary lines on reconstructed CT of the CVJ.
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We use sagittal CT reconstructions to calculate the basion-dens interval and basion-axial interval, which have been shown to provide the most accurate assessment of atlantooccipital dislocation. Atlantooccipital dislocation should be suspected when the basion-dens interval or basion-axial interval is greater than 12 mm.
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Basilar invagination is present if the odontoid intersects the Wackenheim clival-canal line, which is drawn along the posterior surface of the clivus. It is also present if more than one third of the length of the odontoid lies above the Chamberlain line, which is drawn from the hard palate to the posterior margin of the foramen magnum.
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Transarticular C1-2 fixation is commonly contraindicated by aberrant vertebral artery anatomy, and a medial vertebral artery course may preclude C2 pedicle and pars screw placement.
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We perform occipitocervical fixation by connecting an occipital plate by rods to an atlantoaxial Harms construct (bilateral C1 lateral mass screws connected to C2 pars or pedicle screws). We prefer to use the Mountaineer occipitocervical instrumentation system (DePuy Spine, Inc, Raynham, MA).
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We perform continuous neurophysiologic monitoring with cortical somatosensory evoked potentials (SSEPs) and obtain prepositioning baseline SSEPs in all cases.
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An arterial line is placed, and the anesthesiologist is asked to maintain a mean arterial pressure of 90 mm Hg in all cases with preexisting myelopathy, cervical spinal cord compression, and cervical spinal cord signal abnormality on MRI. Patients are typed and crossed with red blood cells available in the operating room should a need for immediate transfusion arise.
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A persistent change in SSEPs independent of mean arterial pressure after positioning the patient prone prompts us to return the patient to a supine position and awaken the patient for neurologic examination.
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We encourage fiberoptic-assisted intubation to reduce the chance of spinal cord injury in cases with preexisting cervical cord compression and severe instability.
