Transoral Approach to the Craniocervical Junction and Upper Cervical Spine

Overview

Pathology of the craniocervical junction remains technically difficult to access, but because of advancements in instruments, the operating microscope, endoscopic techniques, and materials to repair skull base defects, these lesions have become more accessible, and it has become safer to provide treatment. Ventral pathology located anterior to the spinal cord and cervicomedullary junction is the most difficult to access. Historically, pathology in this location was often treated by a posterior or posterolateral decompression followed by a fusion if needed. This indirectly addressed the compressive component on the spinal cord or cervicomedullary junction, whereas it only minimally addressed the pathology itself, if this was addressed at all. The transoral approach and its variations were developed specifically as ventral or anterior approaches to the cervical spine and craniocervical junction.

Anterior approaches were first developed to reach pathology anterior to the spinal cord and then were extended to access pathology anterior or ventral to the lower brainstem and cervicomedullary junction. A truly anterior approach to this region offers the advantage of the shortest and most direct route and a straight-on view of the pathology without manipulation of the spinal cord or brainstem. The transoral approach was first described in 1909, when Kanaval used it to extract a bullet that had been lodged between the foramen magnum and the anterior arch of C1. The approach was later popularized by Scoville and Sherman in the 1950s for platybasia repair and by Fang and Ong in the 1960s for infectious etiology. Complications were common and included vertebral artery injuries, cerebrospinal fluid (CSF) leaks, and meningitis. The transoral approach did not gain popularity until the 1980s and early 1990s, after refinements in technique by Menezes and Crockard showed that the approach could be done safely with less morbidity than previous attempts.

Approaches to the craniocervical junction are divided into anterior and posterior approaches. Anterior approaches include the transoral approach and its variations and transfacial approaches. Anterolateral approaches include the high cervical retropharyngeal and posterior approaches, which include the standard midline posterior approach and the far-lateral approach and its variations (these are all described in separate chapters). The transoral approach provides the most direct access to midline ventral pathology but is limited in its lateral access. The transoral approach has a few variations that add rostral or caudal exposure to the standard approach. To better classify these approaches, the classic or standard transoral approach has been described with and without palatal splitting. The classic transoral approach is performed with sparing of the soft palate only where it is elevated; this commonly allows access from the anterior foramen magnum to the C2–C3 disk space. To gain more rostral access than that allowed by the standard transoral approach, the soft palate alone or the combination of the soft and hard palate can be split; this is termed the transpalatal approach , which will provide access to the lower third of the clivus. If further rostral or caudal exposure is needed than is available from the standard transoral or transpalatal approaches, extended transoral approaches may be used. Rostral exposure is provided by the addition of the transmaxillary or “open-door” maxillotomy procedure; this provides access to the sphenoethmoid recesses, the entire clivus, and to areas as caudal as the C2–C3 disk space. This approach was designed for severe basilar invagination to allow for clival resection. Further caudal access can be gained by the addition of a transmandibular splitting (labiomandibular) approach with or without a transglottic or tongue-splitting variation (labioglossomandibular). This will provide caudal access to the C3 and C4 vertebral bodies in most patients.

Anatomy

The anatomy of the craniocervical junction and upper cervical spine involves a complex arrangement of bony, vascular, and ligamentous anatomy. Through its paired occipital condyles, the clivus articulates with the superior articular facets of the atlas (C1). The occipital condyles are located lateral to the anterior half of the foramen magnum. This atlantooccipital joint provides about 50% of the head and neck flexion and extension. The atlas articulates through its inferior articular facets with the superior articular facets of the axis (C2). The atlantoaxial joint provides the majority of the rotation of the head, up to about 45 degrees in either direction. The anatomy of both C1 and C2 are dramatically different than the rest of the subaxial cervical spine. The important functional movements allowed by the anatomy of the craniocervical junction are held tightly together by a complex ligamentous network.

C1 is a unique bone of the cervical spine because it lacks a vertebral body and spinous process and takes the shape of a ring ( Fig. 2-1 ). At the lateral aspects of the ring are the two lateral masses that form the superior and inferior articular processes. The superior articular processes articulate with the occipital condyles of the clivus, and the inferior articular processes articulate with the superior articular processes of C2. The lateral masses each support a transverse process, which contains the transverse foramen that carries the vertebral artery from the transverse foramen of C2 inferiorly. The C1 ring is formed by an anterior arch and a posterior arch; these are separated by the lateral masses; the posterior arch has an indentation just behind the lateral masses called the sulcus arteriosis. As the vertebral artery enters the C1 transverse foramen, it takes a slightly posterior course along the sulcus arteriosis of the posterior arch before turning medially then anteriorly and superiorly to enter the dura.

C2 is also a unique bone of the cervical spine. Although it appears more like the rest of the cervical spine than the atlas, because it contains a vertebral body and spinous process, the axis is identifiable by its rostrally pointing peglike projection from the vertebral body, called the odontoid process ( Fig. 2-2 ). It also has lateral masses that contain the superior and inferior articular processes, which articulate with the atlas and C3, respectively; the superior articular processes are located more anteriorly than the inferior articular processes. Transverse foramina carry the vertebral arteries and span laterally off each of the lateral masses. The odontoid process has multiple impressions that show where the supporting ligaments attach or traverse.

The odontoid process is secured to the C1 ring and clivus by a series of ligaments that provide most of the strength of the atlantooccipital and atlantoaxial structures. The atlas and axis are held together by the anterior longitudinal ligament (ALL) and the posterior longitudinal ligament (PLL), the synovial joints of the articular processes, and the cruciform ligament. The cruciform ligament has a transverse portion, called the transverse atlantal ligament, and two vertical portions. The anterior arch of the C1 ring articulates posteriorly with the odontoid process of C2, and the transverse atlantal ligament holds the odontoid process against the anterior ring of C1. The transverse ligament attaches to small tubercles on each of the medial aspects of the C1 lateral masses, spanning from one lateral mass to the other while wrapping around the posterior surface of the odontoid process at its base with the vertebral body. The vertical portions of the cruciform ligament form from the transverse ligament as it crosses the odontoid; one travels superiorly, the other inferiorly ( Fig. 2-3 ).

Four structures attach the axis to the occipital bone: the tectorial membrane, apical ligament, and paired alar ligaments. The tectorial membrane is the cranial extension of the PLL; it is continuous with the PLL inferiorly from the posterior body of C2 and extends rostrally behind the odontoid process to attach at the inside of the foramen magnum at its anterior margin. The apical ligame nt is a single, midline structure that spans from the superiormost apex of the odontoid process to the anterior margin of the foramen magnum. The alar ligaments are paired bands that attach at either side or the tip of the odontoid and span superolaterally to insert onto the medial surface of each occipital condyle ( Fig. 2-4 ).

Midline anatomic structures are extremely important to identify when using any anterior approach, such as a transoral approach. In the most rostral exposure, the posterior nasal septum will attach at the sphenoid face in the midline at the vomer. The pharyngeal tubercle on the lower clivus is where the superior pharyngeal constrictor muscles attach ( Fig. 2-5, C ). In the middle of the anterior arch of C1 is the anterior atlantal tubercle, where the ALL attaches (see Fig. 2-1 ).