Chapter 18 Anterior Approaches to the Craniovertebral Junction
Anterior approaches are considered the best means for removing ventral lesions at the craniovertebral junction (CVJ). These approaches provide direct access to the lesions and allow for the graft to be positioned after decompression. These approaches can be used to reach tumors of the clivus and upper cervical spines. They can be classified as the transoral transpharyngeal approach, the translabiomandibular transpharyngeal approach, and the transoral approach with maxillotomies according to the cephalocaudal extent of the exposure (Fig. 18-1).1 The standard transoral procedure allows the exposure of the midline between the C1 and the C2–3 intervertebral disc.2,3 The exposure may be extended in a cephalad direction by dividing the soft and hard palate to allow access to the foramen magnum and the lower clivus and even farther to the sphenoid sinus with an extended open door maxillotomy.
However, this procedure has limitations for laterally extending tumors, may increase the risk of meningitis, and lacks vertebral artery control. The transoral approach is contraindicated when there is significant dural invasion of tumor mass or when large dural resection is required.4
This approach exposes the inferior one-third of the clivus down to the C2 vertebral body. The entrance to the oral cavity must provide a working distance of 2.5–3 cm between the upper and lower teeth.2 A tracheostomy is performed when the operation involves the high nasopharynx and the craniocervical junction. An operation at the atlas and axis levels may not require a tracheostomy. The patient is placed supine with the neck in slight extension. The mouth is kept open with a Dingman or Crockard retractor.
The soft palate can be split at the midline to extend superior exposure (Figs. 18-2 and 18-3). The incision should be full thickness and extended to the junction of the hard and soft palates. If the lesion is located in the lower clivus, simply dividing the soft palate may be sufficient, but if the upper clivus needs to be exposed, removal of the bony plate is required.
The anterior C1 arch can be palpated through the posterior pharyngeal wall and verified with fluoroscopy. An initial incision is made on the midline through the mucosa of the posterior pharynx (Fig. 18-4). The anatomical landmark for the midline is the anterior tubercle of C1.
The mucosa layer is dissected separately from the prevertebral fascia investing the underlying muscle layer (Fig. 18-5). The longus capitis and longus colli muscles are subperiosteally elevated, and the arch of C1 is identified.
The longus and longus capitis muscles are originated from the basiocciput, and the origin site should be stripped off for exposure of the lower clivus (Fig. 18-6). Beneath the muscle layer, the anterior longitudinal ligament (ALL) is firmly attached to vertebral body surface. The ALL has a pointed attachment to the anterior tubercle of the atlas that widens as it descends to its insertion at the sacrum.5 The ALL is divided with electrocautery and reflected with a sharp dissect to complete the exposure of the C1 arch.
The mucoperiosteal layers are spread with a self-retaining retractor to a maximum of 1.5 cm to either side (Fig. 18-7). The anatomical landmark of the lateral limit of the exposure is the pharyngeal recess. Usually the maximal transverse exposure is provided at the level of the C1 arch, and the transverse length of exposure is lessened at the C2 level.
The vertebral artery is located 1.4 cm lateral from the midline at the C2 level, 2.2–2.4 cm at C1, and 1.1–1.4 cm at the foramen magnum level. Knowledge of the exact location of the midline minimizes vertebral artery injury.
The tumor is resected intralesionally, being debulked from the central to peripheral portion. In the case of chordoma, the mass is layered off from the inside out. Its margin is defined from the normal tissue. Tumors arising in this region are quite soft and well-circumscribed and may well be removed with relative ease using rongeurs, an ultrasonic aspirator, or a laser (Fig. 18-8).
On occasion it may be necessary to resect the odontoid. This is usually required for cases of basilar impression or rheumatoid arthritis. Odontoidectomy also is required when the dens is involved with the tumor. After the anterior arch of the C1 is removed, the odontoid is grasped with Crockard forceps and removed with a drill.
The apical and alar ligaments are identified at the apex of the odontoid and divided using electrocautery. The remnant of the odontoid can be removed with a 1- or 2-mm Kerrison punch. The dense tectorial membrane and cruciate ligament, which lie posterior to the dens, protect the dura from injury (Fig. 18-10).
In the event of a dural tear, primary closure should be attempted. If impossible, the defect is repaired with fascial graft. When watertight closure is not possible, the fascia patch is applied on the defect site; fat graft is packed into the corpectomy site. Surgicel or Gelfoam is added. The pharyngeal mucosa and longus colli muscle should be closed in two layers using interrupted 3/0 absorbable sutures, one layer for muscle and one for mucosa.
For soft palate closure, a three-layered closure is crucial and helps to decrease the risk of fistulization. The nasopharyngeal mucosa, muscle layer, and oral mucosa are reapproximated separately (Fig. 18-11).