16 Microsurgical Anatomy of the Anterior Skull Base Through a Cranial View
Abstract
Skull base surgery as a field has witnessed an evolution through the three anatomical eras of neurosurgery, namely, osseous, vascular, and now neural, under the guiding principles of microsurgical anatomy and the incremental development of optics.
In this anatomic review, we will explain the microsurgical anatomy of the anterior skull base as seen through the endoscopic endonasal view by way of organizing it with our newly devised circumferential radial corridor system.
The anterior endocranial surface is formed by the ethmoid, sphenoid, and frontal bones, and is divided into medial and lateral segments. The medial segment is perpendicular to the cribriform plate, with the ethmoid bone located anteriorly and the planum, limbus, and tuberculum sellar of the sphenoid body located posteriorly. The lateral segment consists of the orbit and optic canal, and is formed by the frontal bone and the lesser wing of the sphenoid bone, which blends medially into the anterior clinoid process.
Microsurgical anatomy of the anterior skull base, especially through the endoscopic endonasal view, is complex, and its organization can be daunting. The development of three-vector circumferential-radial corridor system was founded in an effort to consolidate the vast “laundry list” of conventional surgical approaches, and organize the relevant soft tissue, osseous, vascular, and neural structures into a more conceptual, simplified framework.
Keywords: Keywords: skull base, anterior fossa, paranasal sinuses, orbit, cranial nerve
16.1 Bony Anatomy1 , 2 , 3 , 4
Skull base has three fossae. The anterior is limited posteriorly by the sphenoid ridges and chiasmatic sulcus; middle fossa goes until the level of petrous ridges and dorsum sellae; and the posterior fossa goes to the back of skull in the infratentorial region.
Anterior cranial fossa is composed by the frontal, ethmoid, and sphenoid bones (Fig. 16.1). It can be divided into medial and lateral portions; the medial one presents a slight depression in comparison with the lateral part. Medially it roofs the nasal cavity and it is formed in its anterior two-thirds by the cribriform plate and crista galli of ethmoid bone, and in the posterior third by the planum sphenoidale of sphenoid bone, which roofs sphenoid sinus. Crista galli corresponds to the anterior attachment of falx (Fig. 16.2).
Fig. 16.1 Bony anatomy of anterior skull base. (a, b) Superior view depicting the floor of anterior cranial fossa. (c) Posterolateral view depicting optic canals related to anterior clinoid processes, and also demonstrating optic strut. Middle clinoid process is present in this demonstration and is connected to anterior clinoid by an osseous bridge, creating a canal that may surround internal carotid artery. Planum sphenoidale corresponds to the roof of sphenoid sinus and cribriform plate, and crista galli relates to nasal cavity. (d) Posterior view demonstrating sphenoid bone (blue), ethmoid bone (green), and region of frontal bone, mainly its orbital plates (gray). ACP, anterior clinoid process; Crib. Plat., cribriform plate; Crist. Gal., crista galli; Dors. Sellae, dorsum sellae; Foram. Rot., foramen rotundum; Mid. Clin. Proc., middle clinoid process; Opt. Can., optic canal; Opt. Strut. optic strut; Osseous Brid., osseous bridge; Plan. Sphen., planum sphenoidale; Post. Clin. Proc., posterior clinoid process; Sup. Orb. Fis., superior orbital fissure; Tuberc. Sel., tuberculum sellae.
Fig. 16.2 Demonstration of neurovascular structures related to anterior fossa floor. (a) Axial superior view of anterior fossa, demonstrating orbit on the left side after removal of orbital roof, and frontal lobe in the right side. (b) Superior view depicting both orbits after removal of frontal lobe, showing periorbita in the left side and underlying muscles and nerves in the right side. (c) Sagittal view demonstrating the left orbit without roof and lateral orbital wall, as well as the right cerebral hemisphere, depicting neurovascular relationships with anterior fossa floor. (d) Anterior fossa floor depicting orbits bilaterally and roof of nasal cavity in the middle, exposing crista galli and olfactory bulb anteriorly and sphenoid sinus posteriorly. ACA, anterior cerebral artery; CN, cranial nerve; Crist. Gal., crista galli; Fr. N., frontal nerve; Front. Lobe, frontal lobe; gg, gasserian ganglion; ICA, internal carotid artery; Lac. Gland, lacrimal gland; Lev. M., levator muscle; Nasoc. N., nasociliary nerve; Opht. A., ophthalmic artery; Sup. Obliq. M. Tend., superior oblique muscle tendon; V1, V2, V3, first, second, and third divisions of fifth nerve.
Laterally, the anterior fossa floor roofs the orbit and superior orbital fissure and comprises the orbital plates of frontal bone anteriorly and lesser wings of sphenoid bone posteriorly. The posterior aspect of lesser sphenoid wings corresponds to sphenoid ridges; medially they blend with anterior clinoid processes (ACPs) and in the middle between both ACPs there is the chiasmatic sulcus, just above tuberculum sellae (Fig. 16.3).
Fig. 16.3 Posterior aspect of anterior fossa floor. (a) Both the anterior clinoid processes are demonstrated, without dura in the right side. Anterior clinoid corresponds to the medial aspect of sphenoid ridges and has a medial attachment to the roof of optic canal as well as to optic strut. Sphenoid ridges continue medially through chiasmatic sulcus, presenting tuberculum sellae just below it. (b) Removal of central part of anterior clinoid to demonstrate the underlying internal carotid artery. The ethmoid and sphenoid sinus mucosa are demonstrated. (c) Sphenoid and ethmoid sinuses are exposed. The anterior clinoid process had its central portion removed and the clinoidal segment of internal carotid artery is demonstrated. Third and fourth cranial nerves entering into the roof of cavernous sinus are demonstrated, along with the fifth nerve running to Meckel’s cave. ACP, anterior clinoid process; Chiasm. sulc., chiasmatic sulcus; CN, cranial nerve; Dors. Sel., dorsum sellae; Ethmoid. Sin, ethmoid sinus; Falc. lig., falciform ligament; ICA, internal carotid artery; ON, optic nerve; Sphen. Sin, sphenoid sinus; Tuberc. sel., tuberculum sellae.
The ACP has three bony attachments, which must be removed when performing an anterior clinoidectomy. One is the attachment to lesser sphenoid wing, the other to orbital roof, and below there is the optic strut, which separates the optic nerve medially from the superior orbital fissure laterally (Fig. 16.4). It is important to have in mind the critical neurovascular structures related to the ACP: inferolaterally runs the third nerve on the upper aspect of superior orbital fissure, inferomedially, there is the optic nerve and anteroinferiorly, the internal carotid artery (ICA).
Fig. 16.4 Superior orbital fissure dissection after removal of anterior clinoid process and lesser sphenoid wing. (a, b) Lateral view of obit and superior orbital fissure (SOF). The third and fourth cranial nerves run from the roof of cavernous sinus to orbit, through SOF. The fourth nerve crosses over the third close to SOF and ends in the superior oblique muscle. The nasociliary nerve is observed from the region where it branches from V1 to the anterior medial aspect of orbit to give rise to the ethmoidal nerves, crossing above optic nerve. The lacrimal nerve is also demonstrated, running lateral to optic nerve until it reaches lacrimal gland. The superior division of third nerve is also identified, as well as the abducens nerve reaching the lateral rectus muscle. (c) Closer view of superior orbital fissure, with elevation of V1 to demonstrate the sixth nerve running underneath it from Dorello’s canal to SOF. Distal aspect of tentorium is demonstrated; part of the roof of cavernous sinus and the third and trochlear nerves are exposed, depicting the crossing of fourth nerve over oculomotor close to SOF. As the anterior clinoid process is removed, the region of optic strut is exposed, showing the relationship between sphenoid sinus and clinoidal segment of internal carotid artery–the latter involved by the carotid collar between proximal and distal dural rings. The cavernous, clinoidal, and ophthalmic segments of internal carotid artery are demonstrated. Distal dural ring is exposed, initiating the first subarachnoid segment of carotid distally to it. (d) Superior view of orbit and SOF. ACA, anterior cerebral artery; Cav, Clin and Optht ICA, cavernous, clinoidal, and ophthalmic internal carotid artery; ddr, distal dural ring; Fr. N., frontal nerve; Lac. G., lacrimal gland; Lac. N., lacrimal nerve; Lat. Rec. M., lateral rectus muscle; MCA, middle cerebral artery; Med. Rec. M., medial rectus muscle; Mot. root CN V, motor root of fifth cranial nerve; Nasoc. N., nasociliary nerve; Olfact. Bulb, olfactory nerve; ON, optic nerve; Opht. A., ophthalmic artery; Opht. V., superior ophthalmic vein; Sphen. Sin, sphenoid sinus; Sup. Obl. M., superior oblique muscle; Sup. Rec. M., superior rectus muscle; Supraorb. N., supraorbital nerve; Supratroch. N., supratrochlear nerve; Tent, tentorium; III, oculomotor nerve; IV, trochlear nerve; V1, V2, and V3, first, second, and third divisions of trigeminal nerve.
The ACP has on average 7.25 to 7.7 mm of width and 5.4 mm of thickness in its base, and 9.65 to 10.31 mm of anteroposterior length approximately.5 , 6 The distance between the anterior and posterior clinoids is about 5.4 mm and between both ACP tips is 24.1 mm.5 , 6 ACP is composed by a shell of cortical bone surrounding inner spongy bone, and cases of ACP pneumatization can occur in up to 25.5% of cases, including situations of bilateral pneumatization.6 Other important anatomical variation to be considered is the carotid-clinoidal foramen, a bony bridge between anterior and middle clinoid processes that can encircle ICA, and even provoke an inadvertent injury to the artery; the foramen is present in about 14.2% of cases (Fig. 16.1). Also a bony bridge between the anterior and posterior clinoid processes can be present in 14.4% of cases.5 , 6 Preoperative planning is important, because at least one anatomical variation is found in about 38.7% of ACPs.6
Anteriorly, frontal bone contains the frontal sinuses that can vary in size and extend even downward to the anterior fossa floor. Frontal bone has an indentation between its orbital plates that gives room to cribriform plate and crista galli, called of ethmoidal notch. The medial aspect of each orbital plate, which corresponds to the borders of this indentation, roofs the ethmoidal air cells. There is a foramen in front of cribriform plate, the foramen caecum, which just gives passage to an emissary vein. The cribriform plate has two grooves where the olfactory bulbs rest. Posteriorly, the frontal bone articulates with both lesser and greater sphenoid wings.
Planum sphenoidale is the bony area just posterior to crista galli and cribriform plate, and has about 20 mm of anteroposterior length until the limbus of sphenoid posteriorly, which is located just above the chiasmatic sulcus5 (Fig. 16.4). This limbus is separated from tuberculum sellae by about 6.7 mm.5 It is important to mention that meningiomas arising from the median anterior fossa floor can usually present attachment to the olfactory groove, planum sphenoidale, or tuberculum sellae.
Below the cribriform plate and crista galli, the ethmoid bone provides a great part of nasal cavity roof and lateral walls (Fig. 16.5). In the middle just underneath crista galli, the perpendicular plate of ethmoid bone descends joining inferiorly with vomer to construct the bony nasal septum. Inferolaterally to the cribriform plate and roofed by the medial aspect of orbital plates of frontal bone, there are multiple air cells that correspond to the ethmoid sinus. The lamina papyracea, which is the ethmoid’s orbital plate, consists in the lateral wall of these air cells and separates them from orbit. Below the air cells and protruding from lateral wall to the inner space of ethmoid bone, which corresponds to nasal cavity, there is the superior turbinate and middle turbinates, both belonging to ethmoid. The lateral wall of ethmoid bone descends and joins the perpendicular plate of palatine bone posteriorly and maxilla anteriorly to complete the lateral wall of nasal cavity; the inferior turbinate is a separate bone that articulates with the inferior aspect of this wall and protrudes to the nasal cavity. In this way, superiorly the nasal cavity relates to the medial wall of orbit, separated from it by the ethmoid sinus and lamina papyracea; and inferiorly it is separated from the medial wall of maxillary sinus by the middle and inferior turbinates arising from the lateral wall of nasal cavity. Finally, the floor of nasal cavity is formed anteriorly by the palatine process of maxilla and posteriorly by the horizontal plate of palatine bone. Posteriorly, the nasal cavity communicates with nasopharynx through the choanae, each one bordered medially by vomer, inferiorly by the horizontal plate of palatine bone, laterally by the medial pterygoid process, and superiorly by inferior aspect of sphenoid conchae, which correspond to the walls of sphenoid sinus.
The frontal and maxillary sinus as well as the anterior part of ethmoidal air cells drain into the hiatus semilunaris, and the middle ethmoidal air cells drain through the ethmoidal bulla, both below the middle turbinate.7 The sphenoid sinus drains through its ostium over the superior turbinate, and the posterior ethmoidal air cells drain below it. The nasolacrimal duct empties into inferior nasal meatus, below the inferior turbinate.
The sphenoid sinus can be accessed through the floor of anterior fossa by opening the planum sphenoidale, which corresponds to its roof, and sometimes the sinus presents septa inside it (Fig. 16.1 and Fig. 16.5). There are several structures surrounding the sphenoid sinus that provide osseous impressions. Inside the sinus it is possible to observe the sellar floor in the middle, superolaterally the optic nerves, and laterally the cavernous carotids. In the outside lower aspect of sphenoid sinus run the sphenopalatine arteries, and through the lateral aspect of the sinus floor run the vidian nerves. Following these nerves posteriorly it is possible to reach the terminal portion of the petrous carotid. On the outside surface, more inferolaterally to the vidian nerves runs the third division of trigeminal nerve (V3) and superolaterally in the lateral wall runs V2, between the anterior loop of the carotid artery and the vidian canal. The inferoposterior aspect of the sinus corresponds to clivus, and removing this bone leads to the basilar venous plexus. This can allow access to lower basilar and vertebral arteries, close to the vertebrobasilar junction, in front of lower pons and upper medulla oblongata, anatomically being possible to even expose the lower cranial nerves.7
