13 Transodontoid Approach

10.1055/b-0039-172575

13 Transodontoid Approach

Francesco Doglietto, Francesco Belotti, Andrea Bolzoni Villaret, Marco Ravanelli

Being parallel to Kassam’s line, ¹ which is considered the inferior anatomical boundary of transnasal endoscopic surgery, the transodontoid route is the most caudal corridor among sagittal approaches. This route exploits the nasal cavities and nasopharynx to expose the anterior portion of the craniocervical junction. ² ^, ³ The most relevant indications for the transodontoid approach include bulbomedullary compression caused by basilar invagination, os odontoideum, tumors (especially chordomas), irreducible atlantoaxial subluxation, rheumatoid arthritis pannus (especially if severe and/or unresolved by posterior fixation), or other rarer conditions. ³ – ⁸

The first phases of the dissection consist of progressively detaching the prevertebral muscles, ligaments, and fasciae to expose the craniocervical junction. The main bony landmarks to get oriented within this area are the lower clival border and the anterior arch of the atlas. Two variants of the transodontoid approach are illustrated in this chapter. ⁹ ^, ¹⁰ The first variant is performed by removing the cranial portion of the anterior arch of the atlas and the caudal portion of the lower clivus, thus enabling us to reach and resect the apex of the odontoid process. The second variant includes complete removal of both the anterior arch of the atlas and the odontoid process. The former variant allows the reader to understand the boundary between the transclival approach through the lower clivus and the transodontoid approach. As demonstrated by large clinical series, this concept is particularly important in view of the frequent need to adapt the approach according to the extent of clival or craniocervical lesions. ¹¹ Notably, the removal of bony and ligamentous components of the craniocervical junction causes variable grades of joint instability, which can require craniocervical fixation. ¹¹ ^, ¹²

The anatomy of ligaments and membranes of the craniocervical junction is exceedingly complex. The laboratory setting enables the analysis of each ligament by taking advantage of high magnification and absence of bleeding. Being frequently aimed to decompress the spinal cord and/or medulla oblongata, the transodontoid approach usually does not include a transdural extension. Nevertheless, dural resection is required when tumors of the craniocervical area invade or arise from the dura. ⁴ The transdural view through the transodontoid corridor faces the caudal portion of the medulla oblongata and the first, second, and cranial portion of the third neuromeres of the spinal cord with related nerves and vessels, which can be exposed after accurately removing the rhomboid arachnoid membrane.

**Fig. 13.1 Sagittal view of the transnasal route toward the craniocervical junction**. This illustration shows the route toward the craniocervical junction via the nasal cavity.

**Fig. 13.2 Coronal view of the craniocervical junction**. This coronal cadaver cut passes through the craniocervical junction and shows the basic anatomy of its bony boundaries and adjacent structures. Ar, anterior arch of the atlas; Ax, axis (body); CCJ, craniocervical junction (soft tissues); IJV, internal jugular vein; LMAt, lateral mass of the atlas; LMAx, lateral mass of the axis; LoC, lower clivus; OP, odontoid process; phICA, parapharyngeal tract of the internal carotid artery; VA, vertebral artery.

**Fig. 13.3 Multiplanar CT of the craniocervical junction**. The panel includes a coronal **(a)**, sagittal **(b)**, and axial CT image **(c)** passing through the craniocervical junction. The bony component of the craniocervical junction is made up by occipital condyles (OCo), anterior (Ar) and posterior arches of the atlas, lateral masses of the atlas (LMAt), the axis (Ax), and its odontoid process (OP). All these bony structures lie posterior to the passage from the nasopharynx to the oropharynx. CR, clival recess; HaP, hard palate; HyC, hypoglossal canal; JuT, jugular tubercle; LoC, lower clivus; LPP, lateral pterygoid process; NaP, nasopharyngeal posterior wall; NaV, nasopharyngeal vault; SoP, soft palate; StyP, styloid process.

**Fig. 13.4 Sagittal MRI anatomy of the craniocervical junction**. The panel is formed by sagittal T2-weighted **(a)**, CISS (constructive interference in steady state) **(b)**, and contrast-enhanced T1-weighted fat-saturated images **(c)** passing through the craniocervical junction. The soft-tissue component of the craniocervical junction includes, from anterior to posterior, the pharyngeal raphe (PhR), anterior longitudinal ligament (ALL), anterior atlanto-occipital membrane (AAOM), apical ligament (ApL), superior crus of the cruciform ligament (SCCL), and tectorial membrane (TMen). This complex system of ligaments and membranes anchors the lower clivus (LoC) and occipital condyles to the atlas and axis. VI, abducens nerve; Ar, anterior arch of the atlas; ASA, anterior spinal artery; BaP, basilar plexus; MC, midclivus; MOb, medulla oblongata; NaP, nasopharyngeal posterior wall; OP, odontoid process; Po, pons; SCo, spinal cord; VBJ, vertebrobasilar junction.

**Fig. 13.5 Coronal MRI anatomy of the craniocervical junction**. The panel includes two CISS (constructive interference in steady state) MRI axial images passing through the lower portion of occipital condyles **(a)** and through the atlas and odontoid process **(b)**. The connective space between the odontoid process (OP), occipital condyles (OCo), and lower clivus is filled by fat tissue, ligaments, and a venous plexus. From anterior to posterior, the systems of ligaments and membrane is formed by the anterior atlanto-occipital membrane (AAOM), apical ligament (ApL), superior crus of the cruciform ligament (SCCL), alar ligament (AL), and tectorial membrane (TMen). More inferiorly, the transverse ligaments (TrL) serve as the horizontal portion of the cruciform ligament, extending from the odontoid process to the lateral masses of the atlas (LMAt). LoCM, longus capitis muscle; SCo, spinal cord; VA, vertebral artery.

**Fig. 13.6 Coronal MRI anatomy of the craniocervical junction**. The panel includes three MRI coronal images (two CISS [constructive interference in steady state] MRI scan, top and bottom images, and one T1-weighted contrast-enhanced fat-saturated scan, middle image). The vertebral artery (VA) enters the intradural space through a venous plexus that is defined suboccipital cavernous sinus (SCS), almost at the level of the first cervical nerve (C1). Both the vertebral arteries merge in the vertebrobasilar junction (VBJ), which lies in front of the passage between the pons (Po) and the medulla oblongata (MOb). The anterior spinal artery (ASA) arises from small branches coming from the vertebral arteries. The anterior inferior cerebellar artery (AICA) and the posterior inferior cerebellar artery (PICA) arise from the basilar artery (BA) and the VA, respectively. XI, spinal accessory nerve; C2, second cervical nerve.

Endoscopic Dissection

Nasal Phase

Paraseptal sphenoidotomy.
Transrostral sphenoidotomy.
Facultative: Expanded transrostral sphenoidotomy.
Posterior septectomy.

Skull Base Phase

Transclival-transodontoid approach

Step 1: Incision of the posterior wall of the nasopharynx.
Step 2: Removal of the prevertebral fascia.
Step 3: Partial removal of the longus capitis muscle and removal of the pharyngeal raphe.
Step 4: Removal of the anterior longitudinal ligament.
Step 5: Removal of the anterior atlanto-occipital membrane.
Step 6: Removal of the apical ligament.
Step 7: Partial removal of the anterior arch of the atlas.
Step 8: Partial removal of the lower third of the clivus and removal of the superior crus of the cruciform ligament.
Step 9: Incision of the tectorial membrane.
Step 10: Superior odontoidectomy.