2 Anterior Odontoid Resection 1. Decompression of the neuraxis at the craniovertebral junction. 2. Correction of an anterior irreducible pathologic entity causing significant distortion of the cervicomedullary junction. 3. Ventral access to the craniovertebral space-occupying lesions (extradural and rarely intradural tumors). The factors that influence specific anterior odontoid resection are whether the bony abnormality can be removed or reduced to its normal position according to the etiology of the lesion and the direction and the biomechanics of the compression. It is axiomatic that the spinal cord or neuraxis be decompressed in the direction from which it is compromised; therefore, anterior compression at the cervicomedullary area should be accessed ventrally. Decompression must be considered in every case where there is significant compression, prior to craniovertebral fixation or stabilization. The apparently stable lesion in this area may not be stable under special circumstances (anesthesia, skull traction, decompression, and removal of some of the compressive material). Thus, it is essential to repeat studies postoperatively to ascertain if a fixation in addition to decompression is required. The patients’ presentation varies widely. The symptoms can be nonspecific and difficult to localize. In spite of these variations, however, there is usually the following: 1. Neck pain, particularly suboccipital pain with reflection behind the ear (irritation of C2 root), especially when spinal instability superimposes on the compressive signs and symptoms. 2. Deformity (limitation of neck movements, shorter and/or twisted neck, torticollis). 3. Progressive spastic quadriparesis (stiff legs) with wasting of the small muscles of the hands. The latter sign should alert the examiner as to the possibility of craniovertebral junction pathology. 4. Lower brainstem signs (slight change in voice, occasional difficulty in swallowing, or recurrent episodes of aspiration pneumonitis). These patients with bulbar signs, when asleep, display abnormal respiratory patterns. These radiographs are important in assessing every patient with a craniocervical junction abnormality, abnormal mobility of any joint from the occiput down to lower subaxial levels, particularly in cases of rheumatoid atlantoaxial subluxation. In addition, calcification, expansion, erosion, or destruction of bony anatomy will be seen. The value of this diagnostic modality, especially in sagittal reconstruction, to outline bony abnormalities is very useful. Measurement of atlantoaxial subluxation and of the canal diameter of the atlas can be easily obtained. While McGregor’s line was important in the past, we do not use these measurements now. This mode of imaging provides the most exquisite soft tissue detail, including the quality of the cervicomedullary junction (the presence of spinal cord atrophy preoperatively is a bad prognostic sign). MRI with gadolinium enhancement and MR angiography are only occasionally used to outline the vertebral arteries and to ensure that there is a competent circle of Willis. Navigational systems use dynamic referencing technology to establish a computer map between all locations on the preoperative images and the corresponding anatomic locations in the surgical field. Using the registration probe, the surgeon selects diverse points over the surgical anatomy to approximate the surgical space surface. These modern navigational systems enable the surgical team to operate with more confidence, speed, and accuracy, resulting in better surgical efficacy. Indications: Surgical decompression via transoral odontoidectomy of the cervicomedullary junction is required in patients with irreducible ventral pathology. 1. Irreducible atlantoaxial subluxation causing significant neuraxial distortion. 2. Significant soft tissue mass (the pannus). 3. Translocation of the odontoid peg with extensive lateral mass erosion. 1. Basilar invagination (congenital or acquired due to bone softening conditions). 2. Atlantoaxial subluxation and pseudotumor (Down syndrome, spondyloepiphyseal dysplasia, Ehlers-Danlos syndrome, Morquio-Brailsford disease). 3. Basilar impression, in-folded skull base (Klippel-Feil anomaly, osteogenesis imperfecta). 4. Posttraumatic deformities (undetected or untreated odontoid peg fracture). 1. Chordomas and chondrosarcomas of the clivus and C1–C2 complex. 2. Metastatic depositions of the C1–C2 complex. 3. Osteoblastomas. 4. Abscesses. 5. Pseudotumors of the transverse ligaments and foreign bodies removal. Contraindications: Ventral access for pure intradural lesions (e.g., foramen magnum meningiomas). A far lateral approach allows the access and better visualization of the entire pathology with surrounding anatomic structures. 6. Extradural tumors that do not arise from the midline, but may invade the clivus and C1–C2 complex, require some dissection from the front (far lateral approach is more suitable for these cases). 7. Surgery may not be of benefit in long-term bed-bound rheumatoid patients (Ranawat III B). 1. Midline ventral access to odontoid peg has the advantage that there are no important vessels or nerves sagittally. 2. Midline clival lesions with extension to the odontoid process or C1–C2 complex distort neurovascular structures around their lateral boundaries. 3. The spinal cord or the medulla is decompressed in the direction from which it is compromised.
Goals of Surgical Treatment
Diagnosis
Preoperative Neuroradiologic Investigations
Plain Radiographs of the Craniocervical Area (Flexion and Extension)
Computed Tomography (CT; Thin Slice 1.2 mm) and Three-Dimensional Reconstruction of Images to Visualize the Bone
Magnetic Resonance Imaging (MRI)
Neural Navigation (Fig. 2–1)
Rheumatoid Arthritis
Anterior Bony Decompression
Ventral Access to Extradural Lesions in the Foramen Magnum
Advantages and Disadvantages
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