Anterior skull base approaches often give the most direct access to midline skull base lesions. An understanding of the anatomy is important for transnasal, transsphenoidal, and transoral approaches and for approaching the medial cavernous sinus.
Key Steps
Step 1. Nose and nasal cavity, paranasal sinuses (Fig. 1.1)
Step 2. Cavernous sinus from the medial direction (Fig. 1.36)
Step 3. Anatomy for a transoral approach (Fig. 1.52)
Illustrated Steps with Commentary
Fig. 1.1 (Step 1) Cartilaginous framework of the nose.The major or greater alar cartilage is the thin flexible lattice that forms the anterior, medial, and lateral walls of the nares. The septal cartilage separates the two nares. (GAC, greater alar cartilage; LC, lateral crus of the greater alar cartilage; MC, medial crus of the greater alar cartilage; SNC, septal nasal cartilage)Fig. 1.2 Lateral nasal cartilage.The lateral nasal cartilage is the flat extension of the nasal bone that bridges the gap between the nasal bone and the lesser and greater alar cartilages. The septal cartilage separates the left and right lateral nasal cartilages, and the left and right greater alar cartilages. (GAC, greater alar cartilage; LNC, lateral nasal cartilage; SNC, septal nasal cartilage)Fig. 1.3 Depressor septi nasi muscle.The depressor septi nasi muscle originates from the incisive fossa of the maxilla and inserts into posterior ala and septal cartilage. The lateral nasal cartilage is the flat extension of the nasal bone that bridges the gap between the nasal bone and the lesser and greater alar cartilages. The septal cartilage separates the left and right lateral nasal cartilages, and the left and right greater alar cartilages. (DSNM, depressor septi nasi muscle; GAC, greater alar cartilage)Fig. 1.4 Medial palpebral ligament.The medial palpebral ligament originates from the frontal process of the maxilla, divides in half, and inserts into the upper and lower tarsus. Injury to this ligament can result in a cosmetically significant asymmetry of the palpebral fissure. (FNS, frontonasal suture; FPMx, frontal process of the maxilla; MPL, medial palpebral ligament; NMS, nasomaxillary suture)Fig. 1.5 Subperichondrial dissection plane between the septal nasal cartilage and the perichondrium.In the transseptal approach to the sphenoid sinus the mucosa is separated from the septal nasal cartilage, the vomer, and the perpendicular plate of the ethmoid. The mucosa is raised from anterior to posterior. (ANS, anterior nasal spine; M, mucosa; MPL, medial palpebral ligament; PC, perichondrium; SNC, septal nasal cartilage)Fig. 1.6 Incisive foramen.The nasopalatine artery (a branch of the sphenopalatine artery) runs in a groove in the vomer along with the nasopalatine nerve and exits through the incisive canal in the maxilla to anastomose with the greater palatine artery. The nasopalatine nerve communicates with the greater palatine nerves. (ANS, anterior nasal spine; IF, incisive foramen; M, mucosa; PC, perichondrium; SNC, septal nasal cartilage)Fig. 1.7 The nasopalatine artery and nerve are surrounded by mucosa as they exit through the incisive foramen.The incisive foramen is demonstrated at the tip of the blunt probe. (ANS, anterior nasal spine; IF, incisive foramen; SNC, septal nasal cartilage)Fig. 1.8 The septal cartilage articulates with the palatine process of the maxilla.articulation between the septal cartilage and the palatine process of the maxilla is dislocated to the contralateral side in the microscopic transnasal approach to the sphenoid sinus. In the endoscopic approach the nasal septum can be left in place. (ANS, anterior nasal spine; SNC, septal nasal cartilage)Fig. 1.9 Bony nasal septum.The anterior edges of the vomer and perpendicular plate of the ethmoid are seen after the nasal septal cartilage is detached. (BNS, bony nasal septum; SNC, septal nasal cartilage)Fig. 1.10 Inferior and middle turbinates (nasal concha) and nasal septum.The middle and inferior turbinates differ in shape. The inferior turbinate is comma shaped and the middle turbinate is teardrop shaped. The safest method for the operating surgeon to find the middle turbinate is to begin at the nasal floor and identify each more superior turbinate in succession. (ANS, anterior nasal spine; IT, inferior turbinate; MT, middle turbinate)Fig. 1.11 Fossa for the lacrimal sac.The fossa for the lacrimal sac sits over the suture between the lacrimal bone and the frontal process of the maxilla. (FLS, fossa for the lacrimal sac; FPMx, frontal process of the maxilla; M, mucosa; Orb, orbit; SON, supraorbital notch [foramen]; ZFF, zygomatico-facial foramen)Fig. 1.12 Nasal septum covered by the perichondrium, periosteum, and mucosa.A groove on the nasal septum marks the location of our initial mucosal incision when performing an endonasal transsphenoidal procedure. This is a modified transseptal approach in which mucosal flaps are raised over the posterior 1 cm of the nasal septum. After the incision is made, a posterior mucosal flap is stripped from the medial bony septum in the subperiosteal plane. When the body of the sphenoid bone is reached, the dissection is continued laterally across the face of the sphenoid bone to the sphenoid ostia. This flap is used to cover the sphenoid sinus opening at the end of the case. (ES, ethmoid sinus; FS, frontal sinus; M, mucosa; SS, sphenoid sinus)Fig. 1.13 Mucosa and perichondrium.The mucosa and perichondrium form distinct layers over the septal nasal cartilage. (M, mucosa; PC, perichondrium)Fig. 1.14 Variation of the nasal septum: protrusion of the upper edge of the vomer.The nasal septum may be irregular or fenestrated. These irregularities may obstruct the surgeon’s view of the posterior nasal septum. (PerE, perpendicular plate of the ethmoid; SS, sphenoid sinus)Fig. 1.15 Filaments of the olfactory nerve.Filaments of the olfactory nerve (arrow) penetrate the ethmoid bone through the cribriform plate. These filaments can be damaged by stripping the mucosa off the roof of the nasal cavity. (ES, ethmoid sinus; FS, frontal sinus; M, mucosa; SS, sphenoid sinus)Fig. 1.16 Nasopalatine nerve passing into the incisive foramen (canal).The nasopalatine nerve (arrows) is a distal branch of the maxillary nerve. The posterior superior nasal branches enter the nasal cavity through the sphenopalatine foramen. (ANS, anterior nasal spine; IF, incisive foramen; M, mucosa)Fig. 1.17 Nasal septum.The nasal septum is formed by the vomer, the septal nasal cartilage, and the perpendicular plate of the ethmoid. (ANS, anterior nasal spine; CVo, cuneiform part of the vomer; ES, ethmoid sinus; FS, frontal sinus; PerE, perpendicular plate of the ethmoid; PPM, palatine process of the maxilla; SNC, septal nasal cartilage; SS, sphenoid sinus; Vo, vomer)Fig. 1.18 Surgical trajectory for the transnasal transsphenoidal approach to the sella turcica.The speculum is advanced between the nasal septum and middle turbinate. The mucosa of the nasal septum is incised 1 cm anterior to the body of the sphenoid bone. As noted in Figure 1.12, this mucosal flap covers the sphenoid sinus after the operation is completed. Once the nasal septum is displaced contralateral to the surgical approach, the nasal mucosa is stripped from the contralateral posterior 1 cm of the nasal septum and anterior body of the sphenoid. (ANS, anterior nasal spine; ES, ethmoid sinus; FS, frontal sinus; PerE, perpendicular plate of the ethmoid; PPM, palatine process of the maxilla; SNC, septal nasal cartilage; SS, sphenoid sinus; Vo, vomer)Fig. 1.19 Natural ostium of the sphenoid sinus.The natural ostium of the sphenoid sinus is seen to enter the nasal cavity at the level of the superior half of the middle turbinate. (Ch, choana; ES, ethmoid sinus; IT, inferior turbinate; MT, middle turbinate; OSS, ostium of the sphenoid sinus; PPM, palatine process of the maxilla; SS, sphenoid sinus; ST, superior turbinate; Vo, vomer)Fig. 1.20 Communication of the anterior ethmoidal cells, frontal, and maxillary sinuses.The maxillary sinus drains under the middle turbinate into the hiatus semilunaris. At the anterior end of the hiatus is a canal, which drains the frontal sinus. (Ch, choana; ES, ethmoid sinus; IT, inferior turbinate; MT, middle turbinate; PPM, palatine process of the maxilla; SS, sphenoid sinus; Vo, vomer)Fig. 1.21 Location of the sphenopalatine foramen.The sphenopalatine foramen is a canal through the edge of the palatine bone that articulates with the body of the sphenoid bone. The posterior superior nasal branches of the maxillary nerve and the sphenopalatine artery enter the nasal cavity through that foramen. Bleeding from the sphenopalatine artery is easily controlled if the surgeon knows the anatomy of that vessel. (Ch, choana; ES, ethmoid sinus; ET, eustachian tube; MT, middle turbinate; OSS, ostium of the sphenoid sinus; SpC, sphenoidal crest; SPF, sphenopalatine foramen; SS, sphenoid sinus; ST, superior turbinate)Fig. 1.22 Magnification of Fig. 1.21.Magnified view of the natural ostium to the sphenoid sinus and the sphenopalatine foramen. (MT, middle turbinate; OSS, ostium of the sphenoid sinus; SpC, sphenoidal crest; SPF, sphenopalatine foramen; ST, superior turbinate)Fig. 1.23 Sphenopalatine artery.The sphenopalatine artery and the posterior superior nasal branches of the maxillary nerve enter the nasal cavity through the sphenopalatine foramen. (MT, middle turbinate; SPA, sphenopalatine artery; SS, sphenoid sinus; ST, superior turbinate)Fig. 1.24 Pharyngeal opening of the eustachian tube.The pharyngeal ostium of the eustachian tube is bound posteriorly by the torus tubarius, a protrusion in the mucosa overlying the cartilaginous auditory canal and the salpingopharyngeal fold overlying the salpingopharyngeus muscle. (ET, eustachian tube; MT, middle turbinate; ST, superior turbinate; TT, torus tubarius)Fig. 1.25 Inferior turbinate (inferior nasal concha).The inferior turbinate is a separate bone, which articulates from anterior to posterior with the maxilla, lacrimal, ethmoid, and palatine bones. When looking into the nares, the surgeon becomes oriented by locating the inferior turbinate as the closest turbinate to the nasal floor. (IT, inferior turbinate)Fig. 1.26 Middle turbinate (middle nasal concha).The middle turbinate is the termination of the lateral mass of the ethmoid bone. The surgeon finds this important landmark by angling the nasal speculum up from the inferior turbinate. (MT, middle turbinate)Fig. 1.27 Superior turbinate (superior nasal concha).A junction of the superior turbinate and middle turbinate is a landmark for the anterior border of the sphenoid sinus. (MT, middle turbinate; ST, superior turbinate)Fig. 1.28 Natural ostium of the sphenoid sinus.The natural ostium of the sphenoid sinus, which communicates the nasal cavity with the sphenoid sinus in the sphenoethmoidal recess, can be seen by retracting the middle concha laterally. The natural ostium enters the sphenoid sinus halfway between the sinus roof and floor. (OSS, ostium of the sphenoid sinus)Fig. 1.29 Upper arch of the choana (posterior nasal aperture).Behind the inferior turbinate within the nasal cavity the choana or opening to the pharynx is seen. A characteristic fold in the mucosa lying under the attachment of the ala vomer to the body of the sphenoid bone marks the upper end of the choana. The ala vomer underlies the medial portion of the upper arch of the posterior nasal aperture and the medial pterygoid process underlies the lateral arch. (UAC, upper arch of the choana)Fig. 1.30 Cuneiform part of the vomer.The ala is the horizontal portion of the vomer bone, which articulates with the sphenoid bone, the sphenoid process of the palatine bone, and the vaginal process of the medial pterygoid. The ala is evident as a thickening in the posterior inferior aspect of the bony septum. (CVo, cuneiform part of the vomer)Fig. 1.31 Suture between the body of the sphenoid and the vomer.During a transsphenoidal procedure, the vomer is broken anterior to its articulation with the sphenoid bone and dislocated away from the surgeon to enlarge the surgical corridor. The spine of the sphenoid bone and the overlying ala vomer form an excellent landmark for the midline, should the surgeon become disoriented during the procedure. (AVo, ala of vomer; S, sphenoid bone)Fig. 1.32 The ala of the vomer and the ostia of the sphenoid sinus.The relationship of the natural ostia of the sphenoid sinus to the articulation between the vomer and the sphenoid bone is demonstrated. (AVo, ala of vomer; CVo, cuneiform part of the vomer; S, sphenoid bone)Fig. 1.33 Sphenopalatine foramen.The sphenopalatine foramen transmits the nasopalatine nerve, one of the branches of the maxillary division of the trigeminal nerve and the sphenopalatine artery. In this figure, the surgeon is shown the relationship between the sphenopalatine fissure, the sphenopalatine foramen, the nasal cavity, and the sphenoid sinus. (AVo, ala of vomer; Ch, choana; CVo, cuneiform part of the vomer; ION, infraorbital nerve; NPN, nasopalatine nerve; OSS, ostium of the sphenoid sinus; PN, greater and lesser palatine nerves; PPF, pterygopalatine fossa; PPG, pterygopalatine ganglion; PSA, posterior superior alveolar branch; S, sphenoid bone; SPF, sphenopalatine foramen; UAC, upper arch of the choana; V2, maxillary nerve )Fig. 1.34 Pharyngeal branches emerging from the pterygopalatine ganglion.The nerves within the pterygopalatine fissure are demonstrated. The superior nasal branch enters the sphenopalatine foramen. The greater and lesser palatine nerves travel inferiorly to the greater and lesser palatine foramena. The posterior superior alveolar nerve enters the posterior alveolar canal. The pharyngeal branch of the maxillary nerve passes through the pharyngeal canal along with a branch of the maxillary artery. (NPN, nasopalatine nerve; PN, greater and lesser palatine nerves; PPF, pterygopalatine fossa; PPG, pterygopalatine ganglion; PSA, posterior superior alveolar branch; UAC, upper arch of the choana)Fig. 1.35 Opening of the sphenoid sinus.The anterior wall of the sphenoid sinus is opened revealing the anterior wall of the sella turcica. The surgeon should note the pattern of the intrasinus septa on preoperative imaging and identify those septa as the sinus is opened. This will avoid an incomplete view of the sella turcica. The surgeon should also keep in mind that the nasal speculum points across the sphenoid sinus instead of straight posteriorly. Thus, if the approach is through the left nostril, the surgeon will be looking toward the right side of the sella turcica. (CL, clivus; CP (3), carotid prominence formed by the C3 portion of the internal carotid artery; CP (5), carotid prominence formed by the C5 portion of the internal carotid artery; SF, sella floor; TS, tuberculum sellae)Fig. 1.36 (Step 2) Basilar venous plexus.Removing the posterior wall of the sphenoid sinus reveals the basilar venous complex, the C3 portion of the carotid artery, the dura of the sella turcica, and the medial cavernous sinus. There is usually an intercavernous dural sinus within the superior aspect of the dura and a large intracavernous sinus inferiorly. Bleeding from these sinuses can be controlled by packing the venous sinuses with small pieces of oxidized cellulose and bipolar cautery. Raising the head of the bed lowers the venous pressure. (BP, basilar plexus; C3, C3 portion of the internal carotid artery; C5, C5 portion of the internal carotid artery; DCL, dura of the clivus; DSF, dura of the sella floor)Fig. 1.37 Removal of the dura from the anterior pituitary fossa and anterior cavernous sinus.This overview shows the relationship of the C3 segment of the carotid artery to the pituitary gland. (C2, C2 [axis]; C3, C3 portion of the internal carotid artery; C4, C4 portion of the internal carotid artery; C5, C5 portion of the internal carotid artery; OA, ophthalmic artery; ON, optic nerve; PG, pituitary gland; St, stalk)Fig. 1.38 The C5 segment of the carotid arteries are seen inferior to the cavernous carotid arteries.The C5 segment of the carotid artery frequently indents the lateral walls of the sphenoid sinus. (C2, C2 [axis]; C3, C3 portion of the internal carotid artery; C4, C4 portion of the internal carotid artery; C5, C5 portion of the internal carotid artery; Ch, choana; CL, clivus; ES, ethmoid sinus; IOF, infraorbital fissure; ION, infraorbital nerve; MxS, maxillary sinus; ON, optic nerve; Orb, orbit; PG, pituitary gland; PN, greater and lesser palatine nerves; PPF, pterygopalatine fossa; SOF, superior orbital fissure; SPF, sphenopalatine foramen; St, stalk; V2, maxillary nerve )Fig. 1.39 Right internal carotid artery passing into the cavernous sinus.The C5 portion of the carotid artery lies adjacent to the wall of the sphenoid sinus before passing into the cavernous sinus. The lateral wall of the nasal cavity, the nasopharynx, and the medial cavernous sinus are exposed. An anatomical relationship of these structures is well documented. (C2, C2 [axis]; C3, C3 portion of the internal carotid artery; C4, C4 portion of the internal carotid artery; C5, C5 portion of the internal carotid artery; ET, eustachian tube; IT, inferior turbinate; MT, middle turbinate; ON, optic nerve; PG, pituitary gland; ST, superior turbinate; V2, maxillary nerve ; VI, abducens nerve)Fig. 1.40 Opening of the posterior ethmoid cells into the nasal cavity.The posterior ethmoid air cells communicate with the nasal cavity above the superior turbinate. (MT, middle turbinate; SS, sphenoid sinus; ST, superior turbinate)Fig. 1.41 Right carotid prominence.A bulge of bone, the carotid prominence, is seen on the lateral wall of the sphenoid sinus. Above the prominence is a bulge marking the optic canal. In individual cases there may be a dehiscence of this bone over these structures. (CP (3), carotid prominence formed by C3 portion of the internal carotid artery; SF, sella floor; ST, superior turbinate)Fig. 1.42 Medial wall of the cavernous sinus on the right side.Removal of the lateral wall of the sphenoid sinus reveals the cavernous sinus. The medial wall of the cavernous sinus and the C5 portion of the carotid artery can be seen once the lateral wall of the sphenoid sinus is removed. (C3, C3 portion of the internal carotid artery; C5, C5 portion of the internal carotid artery; DSF, dura of the sella floor; ON, optic nerve; VI, abducens nerve)Fig. 1.43 Extradural portion of the abducens nerve.The abducens nerve is seen after it exits Dorello’s canal and enters the cavernous sinus. (C5, C5 portion of the internal carotid artery; DCL, dura of the clivus; VI, abducens nerve)Fig. 1.44 An intercavernous sinus running between the osteal dura and dura propria on the sella floor.The interconnections (arrows) that run between the leaves of dura in front and under the pituitary gland can cause troublesome bleeding during transsphenoidal surgery. Pushing small bits of oxidized cellulose into these venous channels will help clear the surgical field of blood. (C3, C3 portion of the internal carotid artery; C5, C5 portion of the internal carotid artery; ON, optic nerve)Fig. 1.45 Right cavernous sinus.Removing bone with a diamond drill anterior to the carotid artery will open the bony canals containing V2 and the optic nerve. (C3, C3 portion of the internal carotid artery; C5, C5 portion of the internal carotid artery; DDR, distal dural ring; IOV, infraorbital vein; ON, optic nerve; PG, pituitary gland; V2, maxillary nerve ; VI, abducens nerve)Fig. 1.46 Common tendinous ring (common annular tendon).Further bony removal above V2 will open the apex of the orbit. The annular tendon gives origin to the muscles of the ocular cone. (C3, C3 portion of the internal carotid artery; CTR, common tendinous ring; IOV, infraorbital vein; MR, medial rectus muscle; OA, ophthalmic artery; ON, optic nerve)Fig. 1.47 Internal carotid sympathetic plexus.The carotid sympathetic plexus emanates from the superior cervical sympathetic ganglion and ascends with the internal carotid artery. Several communicating branches from the plexus exist. A communicating branch to the abducens nerve is exposed in this figure. (C3, C3 portion of the internal carotid artery; C4, C4 portion of the internal carotid artery; C5, C5 portion of the internal carotid artery; CTR, common tendinous ring; DR, dural ring; OA, ophthalmic artery; ON, optic nerve; PG, pituitary gland; SpP, sympathetic plexus; V2, maxillary nerve [second division of the trigeminal nerve]; VI, abducens nerve)Fig. 1.48 Meningohypophyseal trunk.The meningohypophyseal trunk originates from the junction of C5 and C4. It generally gives off an arterial branch to the clivus, the tentorium, and the posterior pituitary. The dorsal meningeal and tentorial arteries are seen in this dissection. (C3, C3 portion of the internal carotid artery; C4, C4 portion of the internal carotid artery; C5, C5 portion of the internal carotid artery; MHT, meningohypophyseal trunk; V2, maxillary nerve ; VI, abducens nerve)Fig. 1.49 Superior and inferior petrosal sinuses.The inferior and superior petrosal sinuses enter the cavernous sinus. The abducens nerve passes through the inferior petrosal sinus. (C3, C3 portion of the internal carotid artery; C4, C4 portion of the internal carotid artery; C5, C5 portion of the internal carotid artery; DCL, dura of the clivus; IPS, inferior petrosal sinus; ON, optic nerve; PG, pituitary gland; PSL, petrosphenoidal ligament; SPS, superior petrosal sinus; V2, maxillary nerve ; VI, abducens nerve)Fig. 1.50 Distal and proximal dural rings.The intraclinoidal portion of the carotid artery passes through the proximal and distal dural rings. (C3, C3 portion of the internal carotid artery; C4, C4 portion of the internal carotid artery; CTR, common tendinous ring; DDR, distal dural ring; MR, medial rectus muscle; OA, ophthalmic artery; ON, optic nerve; PDR, proximal dural ring; PG, pituitary gland; VI, abducens nerve)Fig. 1.51 Lateral wall of the cavernous sinus viewed from the medial direction.The oculomotor, trochlear, abducens, and ophthalmic nerves pass lateral to the carotid artery in the oculocarotid membrane. (C3, C3 portion of the internal carotid artery; C5, C5 portion of the internal carotid artery; CTR, common tendinous ring; III, oculomotor nerve; IPS, inferior petrosal sinus; IV, trochlear nerve; OA, ophthalmic artery; ON, optic nerve; PSL, petrosphenoidal ligament; SOV, superior orbital vein; SpP, sympathetic plexus; V1, ophthalmic nerve [first division of the trigeminal nerve]; V2, maxillary nerve ; VI, abducens nerve)Fig. 1.52 (Step 3) Posterior and superior walls of the oral cavity.In the transoral approach, the surgeon depresses the tongue and views the posterior mucosa of the oropharynx. (HP, hard palate; OPx, oropharynx; SP, soft palate; UV, uvula)Fig. 1.53 Mucosa of the nasopharnyx and oropharynx.The mucosa of naso- and oropharynx is exposed after removal of the soft palate. In surgery, a midline incision is made and thick flaps, including the mucosa and underlying muscle, are retracted laterally. The mucosa and soft palate have been removed in this cadaver dissection. At the time of surgery, the soft palate is usually retracted superiorly. Future superior exposure can be obtained by splitting the soft palate in the midline. This maneuver usually results in some incomplete separation of the oropharnyx and nasal pharynx at the time of swallowing. (ET, eustachian tube; HP, hard palate; NPx, nasopharynx; OPx, oropharynx)Fig. 1.54 Eustachian tube on the right side.The pharyngeal ostium of the eustacian tube is demonstrated. This landmark allows the exposure of the pharynx seen from a transoral approach to be compared with the exposure of the pharynx seen from a transnasal approach. (ET, eustachian tube; HP, hard palate; NPx, nasopharynx; OPx, oropharynx)Fig. 1.55 Longus capitis and longus colli muscle.The longus capiti are paired muscles that run from the inferior occipital bone to the anterior tubercle of the transverse processes of C3-6. They overlie the longus colli at this level, which runs from the anterior tubercle of the anterior arch of C1 to the anterior tubercle of the lateral masses of C3-5. (LClM, longus colli muscle; LCpM, longus capitis muscle)Fig. 1.56 Removal of the right longus capitis muscle.Removing the longus capitis muscle on the right reveals the right longus colli muscle and the C1-C2 facet. (C1F, C1 facet; C2F, C2 facet; CL, clivus; ICA, internal carotid artery; LClM, longus colli muscle; LCpM, longus capitis muscle; RCAM, rectus capitis anterior muscle; Vo, vomer)Fig. 1.57 Rectus capitis anterior muscle.The rectus capitis anterior muscle lies under the longus capitis muscle. It is a short muscle that arises from the anterior tubercle of the transverse process of C1 and inserts into the inferior margin of the occipital bone. (CL, clivus; RCAM, rectus capitis anterior muscle)Fig. 1.58 Occipital condyle, C1 condyle, and condylar emissary vein on the right side.Laterally we see an emissary vein coming from the condyle and the vertebral arteries passing from the transverse process of C1 behind the condyle. The high cervical internal carotid artery, just proximal to the carotid canal, is also seen in front of the condyle. (aC1, anterior arch of the C1 [atlas]; C1C, C1 condyle; C1F, C1 facet; C2F, C2 facet; CL, clivus; ICA, internal carotid artery; LClM, longus colli muscle; OcC, occipital condyle)Fig. 1.59 Anterior atlanto-occipital membrane.The anterior atlanto-occipital membrane attaches the anterior arch of the atlas to the occipital bone. (CL, clivus)Fig. 1.60 Exposure of pars basilaris of the occipital bone, atlas, and axis.The anterior muscles are removed from the base of the occiput through C2. The right vertebral artery is seen passing between the transverse processes of C2 and C3 and between the transverse process of C1 and the atlanto-occipital membrane. The artery may take a serpiginous course between the transverse process of C1 and the atlanto-occipital membrane. (aC1, anterior arch of the C1 [atlas]; C1C, C1 condyle; C1F, C1 facet; C2, C2 [axis]; C2F, C2 facet; CL, clivus; Ds, dens; ICA, internal carotid artery; VA, vertebral artery)Fig. 1.61 Drilling of the anterior arch of the atlas.The anterior arch of C1 can be removed using a Kerrison rongeur. The authors prefer to use a drill to remove the anterior arch of C1 in layers. (aC1, anterior arch of the C1 [atlas]; C1F, C1 facet; C2, C2 [axis]; C2F, C2 facet; CL, clivus; Ds, dens; Vo, vomer)Fig. 1.62 Dens and related ligaments.The dens is attached to the occipital bone by the apical and alar ligaments. Behind the dens lies the cruciate ligament. The apical ligament makes removal of the tip of the dens problematic. The authors prefer to drill the dens from the top down. Once the dens is detached from the body of C2 it is difficult to grasp or drill. (AlL, alar ligament; ApL, apical ligament; C1F, C1 facet; C2, C2 [axis]; C2F, C2 facet; CL, clivus; Ds, dens)Fig. 1.63 Tectorial membrane and cruciate ligament.The dens is removed to reveal the cruciate ligament and tectorial membrane, which lie anterior to the dura mater. (C1C, C1 condyle; C1F, C1 facet; C2, C2 [axis]; C2F, C2 facet; CL, clivus; CrL, cruciate ligament; OcC, occipital condyle)Fig. 1.64 Venous plexus anterior to the tectorial membrane.Removal of the cruciate ligament reveals a venous plexus anterior to the tectorial membrane. (C1C, C1 condyle; C1F, C1 facet; C2, C2 [axis]; C2F, C2 facet; CL, clivus; ICA, internal carotid artery; OcC, occipital condyle; TcM, tectorial membrane; TL, transverse ligament)Fig. 1.65 Tectorial membrane.The tectorial membrane is just anterior to the dura of the upper spinal canal. (C1C, C1 condyle; C1F, C1 facet; C2, C2 [axis]; C2F, C2 facet; CL, clivus; OcC, occipital condyle; TcM, tectorial membrane)Fig. 1.66 Tectorial membrane and dura mater.Opening the tectorial membrane reveals the dura matter. (C1C, C1 condyle; C1F, C1 facet; C2, C2 [axis]; C2F, C2 facet; CL, clivus; D, dura mater; OcC, occipital condyle; TcM, tectorial membrane)Fig. 1.67 Dura mater.The dura mater is exposed over the axis after the anterior arch of the atlas, the dens, the apical, alar, and cruciate ligaments, and the tectorial membrane are removed. (C1C, C1 condyle; C1F, C1 facet; C2, C2 [axis]; C2F, C2 facet; CL, clivus; D, dura mater; OcC, occipital condyle)Fig. 1.68 Upper spinal cord.The upper spinal cord and anterior spinal artery are seen once the upper spinal dura is opened. (C1C, C1 condyle; C1F, C1 facet; C2, C2 [axis]; C2F, C2 facet; CL, clivus; OcC, occipital condyle; Sp, spinal cord)
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