Lesions involving the clivus represent a challenge to neurosurgeons due to its depth and close relationship with vital structures. The localization of chordomas at the middle of the skull base makes surgical access to these tumors difficult, and the patterns of spread of various skull base chordomas many times preclude the use of a single surgical approach.1 In this chapter, the ventral approaches to the clivus applied to the resection of skull base chordomas are described. A review of the surgical anatomy of the midface is presented preceding the description and discussion of the surgical technique of the transoral approach, the transmaxillary approaches (Le Fort I osteotomy, unilateral paramedian maxillotomy with preservation of soft palate, and the Le Fort I osteotomy with midline splitting of the hard and soft palates), the extended transsphenoidal approach, the transcolumellar approach, and the midfacial translocation.
14.2 Anatomical Background
Anatomical knowledge is the foundation of surgical practice, and achievement of the clival region through an anterior approach requires the identification of several structures in order to avoid damage during the surgical procedure and to maximize the advantages of this exposure. Those structures will be described according to the compartment they belong: soft tissue and muscular, osseous, arterial, venous, and neural.
14.2.1 Soft Tissue and Muscular Relationships
For the surgical techniques described here, it is important to understand the limits of the oral and nasal cavities and their anatomical relationships. The inferior limit of the oral cavity is formed mainly of soft tissues, including a muscular diaphragm and the tongue. The lateral walls are muscular and merge anteriorly with the lips, surrounding the oral fissure. 1,2 The oral cavity is superiorly limited by the hard and soft palates, which separate it from the nasal cavity. The soft palate is a mobile, fibromuscular fold suspended from the hard palate posteriorly and ending in the uvula. It separates partially the nasopharynx from the oropharynx. 1,2,3 The posterior pharyngeal wall lies anterior to the cervical vertebrae and connects to the occipital bone anterior to the foramen magnum. 2,3,4,5 Behind the posterior pharyngeal wall, we can identify the pairs of longus capitis muscles superiorly, attached to the clivus, and longus colli inferiorly, connected to the medium anterior tubercle of the anterior arch of the atlas (C1) 4,5,6 ( ▶ Fig. 14.1 a).
Fig. 14.1 (a) The soft palate (SP) is elevated and the posterior pharyngeal wall (PPhW) opened, revealing the longus capitis (LCaM) superiorly and longus colli (LCoM) muscles inferiorly. (b) The sphenoid sinus (SS) and its limits: the anterior cranial fossa (ACF) superiorly, maxillary sinus (Max) and orbit (O) laterally, and the hard palate (HP) inferiorly and the relationship between the internal carotid artery (ICA) and the sinus walls. (c) The nasal cavity opened and the anterior wall of the sphenoid sinus removed, showing an asymmetric cavity divided by a bony septum (white arrow). Immediately inferior to the sinus is the clivus (C). (d) From inferior to superior: the body of C2 (C2) and the vertebral arteries (white arrows), the base of the odontoid process (OP), the anterior arch of C1 (C1), and the lower clivus (C).
14.2.2 Osseous Relationships
The palatine processes of the maxillae and the horizontal plates of the palatine bones form the hard palate. Superior to it is the nasal cavity, located between the ethmoid bones above and the maxillae, palatine bones, and sphenoid pterygoid process bellow. 1,2,3,7 It is limited above by the anterior cranial fossa, laterally by the orbit and the maxillary sinus, and below by the hard palate, and it is divided sagittally by the nasal septum 3,8 ( ▶ Fig. 14.1 b). Laterally, the nasal cavity has three medially directed projections: the superior, middle, and inferior nasal conchae, each one with its corresponding nasal meatus, located just below. 1,2,6,8 The medial wall of the maxillary sinus is limited by the middle and inferior nasal meatus and the inferior nasal concha. The maxillary sinus communicates with the middle nasal meatus through an opening located in the medial wall just below the roof of the sinus. 3,8 The pterygopalatine fossa is situated just outside the lateral wall of the nasal cavity, between the posterior wall of the maxillary sinus anteriorly and the pterygoid process posteriorly. It communicates laterally with the infratemporal fossa through the pterygomaxillary fissure and medially with the nasal cavity via the sphenopalatine foramen. 2,5,7,8 The nasal cavity is connected to the nasopharynx by the posterior nasal apertures and to the sphenoid sinus by the sphenoid ostium, located above and behind the superior nasal conchae. 3,7,8 The sphenoid sinus is an asymmetric cavity inside the sphenoid body separated by a bony septum. The superior wall of the sinus is part of the anterior and middle floors of the skull base and is related to the pituitary gland medially and the cavernous sinus on its lateral portion. 3,7,8,9 The anterior wall is connected to the perpendicular plate of the ethmoid and vomer in the midline, as well as the lateral masses of the ethmoid on each side. 3,7,8 The floor of the sinus forms the dome of the choanae and of the nasopharynx. The lateral walls are thin bony layers and can be divided into two areas: an anterior orbital area and the posterior cranial area, which is related to the cavernous portion of the carotid artery and the optic and maxillary nerves. 1,2,3,8 Posterior and inferior to the cranial area, we can identify the clivus ( ▶ Fig. 14.1 c). It extends 45 mm from the dorsum sella to the foramen magnum and 27.9 mm from the base of the vomer to the basion. At the level of the foramen lacerum, the width of the clivus is 22.5 mm, and at the jugular foramen it is 42.7 mm. At the level of the hypoglossal canal, the maximum width of exposure varies between 36 and 46 mm. The thickness of the clivus at the level of the foramen magnum ranges from 1.5 to 5.8 mm, and at the junction of the vomer with the clivus it is 18.3 mm. 10 Inferior to the clivus is the atlas (C1), which differs from the other vertebrae in that it has no body or spinous process. In the place of a usual vertebral body is the dens (odontoid process) of the axis (C2). 1,4,6 C1 and C2 are connected by the cruciform ligament, the anterior and posterior longitudinal ligaments, and the articular capsules surrounding the joints between the opposing articular facets on the lateral masses 1,4,5,6( ▶ Fig. 14.1d).
14.2.3 Arterial Relationships
The vertebral artery (VA) ascends through the transverse processes of the upper six cervical vertebrae. In this segment, the artery can be seen in the anterior aspect between the transverse process. Between C3 and C2, the VA can be identified in the lateral limit of the anterior transoral approach before it turns to the posterior aspect of the lateral masses of the atlas ( ▶ Fig. 14.1 d), enters the dura behind the occipital condyles, ascends through the foramen magnum to the front of the medulla, and both vertebral arteries join to form the basilar artery at the pontomedullary junction. 1,3,11 The branches arising from the vertebral artery in the region are the posterior spinal, anterior spinal, posterior inferior cerebellar (PICA), and anterior and posterior meningeal arteries 1,4,11 ( ▶ Fig. 14.2 a). The basilar artery begins in the area of the pontomedullary sulcus by the junction of the two vertebral arteries and courses upward in the prepontine cistern in a shallow groove on the surface of the pons. The distal segment reaches the interpeduncular cistern at about the level of the dorsum sellae where it divides into two posterior cerebral arteries 1,4,11,12 ( ▶ Fig. 14.2 b). The internal carotid artery rests directly against the lateral surface of the body of the sphenoid bone, and its course is marked by a groove in the bone, the carotid sulcus, that defines the course of the intracavernous portion of the carotid artery. 3,4,8,9 The bone separating the artery and the sphenoid sinus is thinner over the anterior than the posterior part of the carotid prominence and is thinnest over the part of the artery just below the tuberculum sellae 3 ( ▶ Fig. 14.2 b). As described by Rhoton in 20033, a layer of bone less than 0.5 mm thick separates the artery and sinus in nearly 90% of sinuses, and areas of absence of bone between the artery and the sinus are present in nearly 10%. The major source of bleeding during surgery in this area is the maxillary artery. It is the larger of the two terminal branches of the external carotid artery. 1 The maxillary artery enters the pterygopalatine fossa by passing through the pterygomaxillary fissure, then sends an inferior branch, the great palatine artery, which courses downwards through the lateral wall of the maxillary sinus to the posterolateral angle of the hard palate. 1,4,5
Fig. 14.2 (a) Removal of the anterior arch of C1, body of C2, the dens, and the clivus: vertebral arteries (VA) and branches, the basilar artery (BA), anterior inferior cerebellar artery (AICA), and the VI cranial nerve (VI) just superior to it; the lower medulla blending into the upper spinal cord; the basilar and vertebral venous plexus composing the main drainage complex of the region (white arrows). (b) The optic nerves (ON) protruding into the superolateral portion of the sphenoid sinus, separated from the sinus mucosa by a thin bony layer (white arrow); the internal carotid artery (ICA) immediately bellow the optic nerve; the basilar artery (BA) and its terminal branches (superior cerebellar [SCA] and posterior cerebral arteries [PCA]); the pituitary stalk (PitS). (c) The epidural venous plexus surrounding the clival area: the cavernous sinus superiorly (CS), the basilar venous plexus (BVP) on the upper clivus, and the vertebral venous plexus (VVP) posterior to C1 and C2.
14.2.4 Venous Relationships
The inferior petrosal sinuses run along the petroclival fissure and connect above with the basilar sinus and below with the jugular bulb. 3,4,5 The basilar venous plexus, located between the layers of the dura mater on the upper clivus, is composed of interconnecting venous channels that join the inferior petrosal sinuses laterally, the cavernous sinuses superiorly, and the marginal sinus and epidural venous plexus inferiorly 4,5,6,9 ( ▶ Fig. 14.2 c). The inferior petrosal sinus, as it enters the petrosal part of the jugular foramen, forms a plexiform connection with the venous plexus of the hypoglossal canal, the inferior petroclival vein, and tributaries from the vertebral venous plexus and posterior condylar emissary vein. It drains into the medial aspect of the jugular bulb through one or two openings in the venous walls between the glossopharyngeal and vagus nerves or into the internal jugular vein below the extracranial orifice 1,3,4,5,9 ( ▶ Fig. 14.2 a).
14.2.5 Neural Relationships
On the lateral wall of the sphenoid sinus, below the sella, a prominent bulge is frequently found that corresponds to the maxillary segment of the trigeminal nerve, just peripheral to the foramen rotundum. 3,8 The optic canals protrude into the superolateral portion of the sphenoid sinus ( ▶ Fig. 14.2 b). The superior orbital fissure produces a smooth, wide prominence in the midlateral wall below the optic canals, and sometimes there are areas where no bone separates the optic sheath and sinus mucosa. 2,3,8 Removing the mucosa and bone from the lateral wall of the sinus exposes the dura mater covering the medial surface of the cavernous sinus and optic canals. Opening this dura exposes the carotid arteries and optic and trigeminal nerves within the sinus 3,8,9 ( ▶ Fig. 14.2 b). The lower medulla blends indistinguishably into the upper spinal cord at the level of the C1 nerve roots. 1,4 The anterior surface of the medulla is formed by the medullary pyramids, which face the clivus, the anterior edge of the foramen magnum, and the rostral part of the odontoid process. 1,4,6 The abducens nerve arises at the lower margin of the pons and passes above, below, or is split into two bundles by the anterior inferior cerebellar artery (AICA) ( ▶ Fig. 14.2 a). It passes upward in the prepontine cistern and turns forward at the upper border of the petrous apex, where it pierces the dura to enter the posterior part of the cavernous sinus. 1,4,11,12
14.3 Surgical Planning
All patients with skull base chordomas that will be submitted to an anterior approach should have a preoperatory cranial magnetic resonance imaging (MRI) as well as a computerized tomography (CT) with bony study. Intraoperative monitoring of cranial nerves VI to XII, brainstem evoked potentials (BSEPs), and somatosensory evoked potentials (SSEPs) is very useful. Neuronavigation also constitutes an important tool while approaching those tumors, and endoscopic view improves considerably the exposition. In cases with intramural tumor extension, lateral to Dorello’s canal, the anterior approaches are not indicated. We also do not recommend the use of those approaches for lesions that need a wide dural opening, due to the difficulty of closure and the high risk of cerebrospinal fluid (CSF) leak, despite the use of local flaps or dural substitutes. In such cases, we prefer the transcondylar or the extended middle fossa approaches.
14.4 Surgical Techniques
14.4.1 The Transoral Approach
The patient is placed in a spine position, with a neutral alignment of the head. In cases where the hard palate needs to be opened, such as superior tumor extension, the head can be slightly extended. The placement of a nasoenteric or nasogastric feeding tube before surgery is very important, followed by the insertion of two Foley catheters, one in each external nasal aperture, exteriorizing them through the mouth for soft palate elevation (in cases where a soft palate incision is not planned) ( ▶ Fig. 14.3 a). After the retractor is allocated, displacing the tongue inferiorly and the soft palate superiorly, the mucosa of the posterior pharyngeal wall is infiltrated with lidocaine 2% with epinephrine through all the incision extension. The incision is in the midline, carried from the inferior border of C2 to the nasopharynx. The mucosa is then retracted laterally, exposing the longus colli muscles, which are detached and displaced laterally, revealing the bony structures ( ▶ Fig. 14.3 b). At this point, it is possible to identify the anterior aspect of C1 and C2 along with the atlantoaxial joint and the base of the odontoid process, the anterior border of the foramen magnum, and the lower clivus ( ▶ Fig. 14.3 d). Care must be taken when dissecting laterally at the level of C2 body to avoid damage to the vertebral arteries. The next step is the bony work. Generally, it starts with the removal of the anterior arch of C1, as wide as necessary, to expose the dens ( ▶ Fig. 14.3 c). The removal of the dens begins with a drilling inside the bone, until a very thin shell of bone is obtained. The removal of the tip is the most difficult step due to the ligamentous complex it is attached to, being necessary to cut those ligaments. After bone removal, the craniocervical dura mater can be visualized. Closure is carried out after careful hemostasis. The longus colli muscle is approximated, and the posterior pharyngeal wall is closed watertight.
Fig. 14.3 (a) The Foley catheters (F) elevating the soft palate; surgical field exposed (*). (b) The longus colli muscles (LCoM) displaced laterally revealing the bony structures (BS). (c) The anterior arch of C1 removed showing the odontoid process (OP) and the craniocervical dura mater (DM).