Transmaxillary and Transmandibular Approaches to the Clivus and Upper Cervical Spine

Overview

The clivus, craniovertebral junction (CVJ), and ventral upper cervical spine are relatively inaccessible, and surgical approaches to these structures are intimidating; however, a wide range of pathologic lesions can affect this region. Because of the capaciousness of the cervicomedullary cisterns, lesions often encompass a large mass of compressive tissue before they produce neurologic symptoms. Lesions may also affect the vertebrobasilar arterial circulation and cerebrospinal fluid (CSF) circulation, adding to the complexity of the presenting symptomatology.

Whereas the clinical significance of abnormalities at the CVJ has been recognized since the early studies of basilar invagination by Chamberlain in 1939, the treatment of such abnormalities consisted almost exclusively of posterior decompression, until Menezes introduced the transoral approach for ventral decompression in 1977. This traditional transoral approach is well suited to access medial extradural lesions of the CVJ. Several modifications have been described that enhance the transoral exposure to allow access to lesions that extend to the upper lateral clivus or upper cervical spine; these include transmaxillary and transmandibular approaches.

These “extended transoral approaches” require knowledge of skull base anatomy and a multidisciplinary team to achieve entry and a good cosmetic reconstruction. The selection of the best approach is determined by factors such as location and nature of the lesion as well as individual patient anatomic variations. Generally, transmaxillary approaches expand the exposure rostrally to the sphenoid sinus and upper lateral clivus; transmandibular approaches expand the exposure caudally to C4–C5. In this chapter, we will discuss the various transmaxillary and transmandibular techniques as expanding maneuvers to the transoral approach.

Anatomy Review

Bony and Ligamentous Anatomy

The CVJ consists of the foramen magnum, atlas, and axis. The occipital bone surrounds the foramen magnum, and it can be divided into a squamosal part located posteriorly, a basal part located anteriorly, and paired condylar parts located laterally. The clivus, which includes the basal part, is a plate of bone that extends forward and upward at a 45-degree angle. It joins the sphenoid bone at the sphenooccipital synchondrosis. Along the superior surface, the clivus is separated from the petrous temporal bone laterally by the petroclival fissure. At the inferior surface is the pharyngeal tubercle, which gives attachment to the fibrous pharyngeal raphe. The oval-shaped occipital condyles articulate with the atlas. Above the condyle is the hypoglossal canal, and along the medial surface is the tubercle that forms the point of attachment for the alar ligament.

The occipital bone and the atlas are joined by the atlantooccipital joints and by the anterior and posterior atlantooccipital membranes, attached superiorly to the anterior edge of the foramen magnum, inferiorly to the edge of the anterior arch of the atlas, and laterally to the atlantooccipital joint capsule. The atlas, C1, forms a ring composed of two lateral masses connected by an anterior and posterior arch. At the midline along the anterior arch is the anterior tubercle. Along the medial surface of each lateral mass is a small tubercle for the attachment of the transverse ligament. The transverse process extends laterally from the lateral mass. Between the transverse process and the lateral mass is the transverse foramen, which contains the vertebral artery.

The atlas (C1) and axis (C2) articulate at four synovial joints: two median joints on the front and back of the dens and two lateral joints between the articular facets. These two vertebrae are joined by the cruciform ligament, the anterior and posterior longitudinal ligaments, and the joint capsules between the opposing articular facets. The cruciform ligament has transverse and vertical parts. The transverse part, or transverse ligament, is a thick band that arches across the ring of the atlas behind the dens. As it crosses the dens, ligamentous bands are directed upward to the clivus and downward to the body of the axis.

The axis (C2) is distinguished by the odontoid process (dens), which projects upward from the body. The dens and body are flanked by the facets, which join the pedicles of C2 posteriorly and articulate with the inferior facets of the atlas. The transverse processes are small and transmit the vertebral artery in a superolateral direction to allow for lateral deviation as the artery ascends from C2 to C1. Four fibrous bands connect the axis to the occipital bone: the tectorial membrane, the paired alar ligaments, and the apical ligament. The tectorial membrane is a rostral extension of the posterior longitudinal ligament, which attaches to the upper surface of the occipital bone in front of the foramen magnum. The alar ligaments arise from the upper part of the dens and attach to the medial surface of the occipital condyles. The apical ligament extends from the tip of the dens to the anterior margin of the foramen magnum.

Muscular Anatomy

The sternocleidomastoid muscle divides the neck into an anterior and posterior triangle. Within the anterior triangle, the platysma is outermost and extends from the face to the pectoralis and deltoid fascia. Deep to the platysma are the suprahyoid and infrahyoid muscles. The anterior vertebral muscles insert on the clivus; this group of muscles includes the longus colli, longus capitis, rectus capitis anterior, and rectus capitis lateralis. These muscles are embedded in the cervical fascia, which can be divided into superficial and deep layers: the superficial fascia invests the platysma, and the carotid sheath is a condensation of the deep fascia that invests the common and internal carotid arteries, jugular vein, and vagus nerve.

Relevant Neurovascular Anatomy

The blood supply to the craniocervical complex is via the vertebral arteries that form an arcade around the dens and the external carotid artery’s occipital branches. Anterior surgical approaches to this region may encounter other branches from the external carotid in association with major nerves. The inferior alveolar nerve, a branch of the mandibular division of the trigeminal nerve, supplies sensation to the jaw and mandibular teeth. This nerve runs with the inferior alveolar artery, a branch of the internal maxillary artery. The nerve exits the mandible anterolaterally at the mental foramen.

The lingual nerve also arises from the mandibular branch of the trigeminal nerve and courses together with the lingual artery, a branch of the external carotid artery, and the lingual vein, which drains into internal jugular vein. The lingual nerve courses anteriorly along the lateral aspect of the hyoglossus and genioglossus muscles of the tongue. Distally it dives into the anterior tongue base. The chorda tympani nerve, a branch of the facial nerve, joins the lingual nerve and runs in its sheath. The hypoglossal nerve supplies all muscles of the tongue except the palatoglossus and descends from the medulla and exits the skull via the hypoglossal canal. From the hypoglossal canal, located just above the occipital condyle, the nerve passes between the internal jugular vein and the internal and external carotid arteries. It then turns anteriorly to enter the tongue and, like the lingual nerve, courses along the lateral aspect of the genioglossus.

Indications and Contraindications

The basic expanding maneuvers to the transoral approach to access the clivus and upper cervical spine are outlined below ( Fig. 3-1 ):

1.

Transmaxillary approach
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  Unilateral Le Fort I osteotomy with palatal split
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  Bilateral Le Fort I osteotomies with downfracture of the maxilla ( drop-down maxillotomy )
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  Bilateral Le Fort I osteotomies with palatal split
2.

Transmandibular approach
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  Mandibulotomy
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  Mandibulotomy with midline glossotomy
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  Mandibular swing-transcervical approach
3.

Combined transmaxillary and transmandibular approach

Like the transoral approach, these extended approaches are primarily utilized for ventral access to extradural lesions of the CVJ ( Fig. 3-2 ). Whereas the transoral approach is capable of addressing midline pathology, expanding maneuvers allow for better access to the lateral clivus. They also increase the exposure to include the sella, upper and lateral clivus, and upper cervical spine to C4–C5. Generally, transmaxillary approaches are used to extend the upper limits, and transmandibular approaches are used to extend the lower limits. However, jaw-splitting approaches also extend rostral access by allowing surgeons to drop their hands more inferiorly. In addition to allowing ventral access to the lesion, the ideal approach should take into consideration the particular pathologic entity. More aggressive tumors, such as chordomas and chondrosarcomas, benefit from en bloc resections that require more extensive exposure to access the entire lesion. Benign tumors (e.g., schwannomas and neurofibromas), as well as degenerative processes, are amenable to piecemeal resection via more restricted approaches, thereby minimizing the associated morbidity.