Surgical Anatomy for Endoscopic Sphenoethmoidectomy

SURGICAL ANATOMY FOR ENDOSCOPIC SPHENOETHMOIDECTOMY Prof. Dr. irfan YORULMAZ Prof. Dr. Babiir KUCUK Ankara University Faculty of Medicine Department of Ear, Nose and Throat - Ankara, Turkey

Prof. Dr. Alaittin ELHAN Prof. Dr. Jbrahim TEKDEMJR Ankara University Faculty of Medicine Department of Anatomy - Ankara, Turkey

Contents

Surgical Anatomy Contents Preface Middle meatus Hiatus semilunaris superior Maxillary sinus Maxillary sinus, natural ostium Infraorbital cell (Haller cell) Uncinate process and Uncinectomy

5 6 7 7 10 10 11 12

Frontal recess Middle turbinate, basal lamella Posterior ethmoid Onodi cell Sphenoid sinus Internal carotid artery Cavernous sinus Suggested reading

15 21 22. 23 24 25 26 27

Terminology Index

43 45

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Surgical Anatomy for Endoscopic Sphenoethmoidectomy

Preface The endoscopic sinus surgeon should be well aware of the anatomy of all paranasal sinus structures and their variations - among the most divergent structures of the human body - always keeping in mind that they are adjacent to vulnerable structures such as the anterior fossa dura, the orbit, the optic nerve and the internal carotid artery. Such knowledge of anatomy is the most crucial factor directly affecting the surgeon's skills, the outcomes of treatment, and the potential to prevent and master complications. Endoscopic sinus surgery is a procedure which should be performed by strictly following the principle of identifying the next anatomical landmark before removing the present one. There is no artificial model which might be used for practical training of surgical anatomical approaches to the complex and varying paranasal sinuses. This objective can only be achieved by performing anatomic dissections on fresh cadavers. The figures depicted in this booklet and in the CDROM enclosed were produced during sphenoethmoidectomy dissection performed on the left side of a fresh cadaver, proceeding from front to rear. They may serve as a guide to basic endoscopic dissection in the afore-mentioned anatomical areas. The procedure and order followed are not specific to a particular surgical technique. The information presented in this booklet aims to introduce the main anatomical structures to those who are just starting or who plan to learn endoscopic sinus surgery through anatomic dissections, and is also meant to describe the methods used for this purpose. The readers are suggested to use this guide after having familiarized themselves with the basic anatomy and terminology of the nasal cavity and paranasal sinuses. Prof. Dr. T. Metin ONERCi, President of the Congress XX ERS & XXIII ISIAN, Istanbul, Turkey, 2004 Hacettepe University Faculty of Medicine, Department of Ear Nose Throat and Head and Neck Surgery

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Surgical Anatomy for Endoscopic Sphenoethmoidectomy

Middle nasal meatus CD video: 0' 24" Once the middle turbinate is medialized with a Freer elevator, the middle nasal meatus should be inspected to assess (a) the shape and size of the middle turbinate, (b) the ethmoid bulla, and (c) the free margin of the uncinate process and the line of attachment to the frontal maxillary process. During medialization of the middle turbinate its vertical portion, which attaches to the cribriform plate, is susceptible to fracture involving the risk of subsequent cerebrospinal fluid leakage. Therefore, it is of utmost importance, that the middle turbinate be not medialized with undue force. Figure 1 View of the middle nasal meatus entrance (0° telescope). OK middle nasal turbinate UP uncinate process EB ethmoid bulla

Hiatus semilunaris superior CD video: 0 ' 5 0 " Viewing towards the anterosuperior section of the middle nasal meatus one can identify the opening at the medial end which normally extends to the frontal recess. The frontal recess always continues with the mucosa on the lateral surface of the middle turbinate and is located anteriorly to the frontal wall of the ethmoid bulla and communicates with the ethmoid infundibulum. The location of the frontal recess may vary depending on the number and volume of the pneumatized cavities in the frontal bone, the nasal bone, the lacrimal bone and the agger nasi located in front of the ethmoid infundibulum, and may not always be visible through the middle nasal meatus. Figure 2 View of the upper section of the middle nasal meatus. (30° telescope) MT middle turbinate UP uncinate process 1 frontal recess 2 hiatus semilunaris superior

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Surgical Anatomy for Endoscopic Sphenoethmoidectomy

The drainage opening of the hiatus semilunaris superior, visible from the conchal sinus (the space between the middle turbinate and the medial wall of the ethmoid bulla) communicates with the lateral sinus posterior to the ethmoid bulla.

Figure 2.1 Localization of the lateral sinus (axial cross-section) EB ethmoid bulla UP uncinate process Modified with kind permission of: KAMEL, R: Endoscopic anatomy of the lateral nasal wall, ostiomeatal complex and anterior skull base: A step-by-step guide. Tuttlingen, Endo-Press™, © 2002.

The lateral sinus is formed by two portions, the suprabullar and the retrobullar recesses. The ethmoid bulla is located inferior to the suprabullar recess which forms the anterior and superior parts of the lateral sinus, and the fovea ethmoidalis (ethmoid roof) is located above it. The retrobullar recess which forms the posterior and inferior parts of the lateral sinus has the ethmoid bulla in anterior and the oblique portion of the middle turbinate basal lamella in posterior location.

Figure 2.2 Localization of the suprabullar and retrobullar recesses (sagittal cross-section) AEA anterior ethmoid artery.

When the ethmoid bulla is attached to the ethmoid roof superiorly there is no suprabullar recess; when the bulla attaches to the basal lamella of the middle turbinate inferoposteriorly there is no retrobullar recess.

Surgical Anatomy for Endoscopic Sphenoethmoidectomy

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Hiatus semilunaris inferior CD video: 11 10" The posterior nasal fontanelle is visible in the area between the inferior margin of the ethmoid bulla and the horizontally oriented inferior margin of the uncinate process when viewed laterally from the hiatus semilunaris inferior, the latter forming an opening between the middle turbinate and the lateral nasal wall at the inferior part of the middle meatus. If an opening becomes visible in this area before the inferior half of the uncinate process is resected, this always belongs to the accessory maxillary ostium. The natural maxillary ostium lies lateral to the uncinate process and comes only into sight after resection of the uncinate process. The ethmoid infundibulum, a three-dimensional cavity, is located lateral to the hiatus semilunaris inferior, a two-dimensional opening between the free margin of the uncinate process and ethmoid bulla.

Figure 3 View of the inferior middle meatus (hiatus semilunaris inferior) through a 30° telescope. EB ethmoid bulla UP inferior margin of the uncinate process * posterior nasal fontanelle

Neighbouring structures of the ethmoid infundibulum are as follows: • the frontal wall of ethmoid bulla posteriorly, • the uncinate process and the frontal maxillary process anteriorly, • the lamina papyracea and the posterior nasal fontanelle laterally, • hiatus semilunaris inferior and the uncinate process medially.

Figures 3.1, 3.2 Localization of hiatus semilunaris inferior and the ethmoid infundibulum (coronal cross-section) Schematic figure 3.2 modified with permission of: KAMEL, R: Endoscopic anatomy of the lateral nasal wall, ostiomeatal complex and anterior skull base: A step-by-step guide. Tuttlingen, Endo-Press™, 2002.

Surgical Anatomy for Endoscopic Sphenoethmoidectomy

Maxillary sinus CD video: 31 05" The prominence that runs from posterior to anterior along the superior maxillary sinus wall belongs to the infraorbital nerve. It comes into view when the maxillary sinus cavity is inspected with a 30° telescope which is inserted through a maxillary sinus trocar placed in the canine fossa. The natural maxillary sinus ostium which normally has an elliptic shape should become visible in a medial and superior location.

Figure 4 View of the maxillary sinus cavity with a 0° telescope which is inserted through a maxillary sinus trocar placed in the canine fossa. * infraorbital nerve (superoposterior wall) .... natural maxillary sinus ostium (medial wall)

Natural maxillary sinus ostium CD video: 3' 30" Once the direction of view of the telescope's front lens points towards the natural ostium medially, the middle meatus, the side of the uncinate process facing the hiatus semilunaris and the inferior margin of the ethmoid bulla can be partially seen.

Figure 5 The natural ostium as viewed through a 30° telescope placed in the maxillary sinus cavity. UP uncinate process EB ethmoid bulla * middle nasal meatus

Surgical Anatomy for Endoscopic Sphenoethmoidectomy

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Natural maxillary sinus ostium CD video: 12' 50" The mucosal lining of the hook-shaped uncinate process faces the middle meatus medially, and the ethmoid infundibulum laterally. The bony portion is attached to the middle turbinate and lamina papyracea superiorly and to the posterior part of the middle turbinate and the palatine bone interiorly. The natural maxillary sinus ostium is located lateral to the uncinate process, at the level where the superior vertical segment of the uncinate process merges with the inferior horizontal segment. The maxillary sinus ostium becomes visible when the point of insertion of the vertical and horizontal portions of the uncinate process is opened with a backbiting forceps and the remnant of the inferior horizontal segment is resected with a sharp curette, Stammberger antrum punch, or back-biting forceps. The anterosuperior margin of the maxillary sinus ostium is of elliptic shape and located more laterally; its posteroinferior margin lies more medially, so when viewed through the 0° telescope a projection of the maxillary sinus ostium becomes visible. A 30° or 45° telescope must be used to view the ostium and maxillary sinus cavity at a right angle.

Figure 6 View of the natural maxillary sinus ostium from the nasal cavity (0° telescope) UP uncinate process lateral side mucosa; EB ethmoid bulla; * maxillary sinus ostium

Infraorbital ethmoid cell (Haller cell) CD video: 15' 00" Haller cells, formed by pneumatization of the lacrimal bone developing towards the orbital floor, may narrow the maxillary ostium, and account for persistent or recurrent sinusitis if overlooked and not opened.

Figure 7 Intracavitary view of the maxillary sinus medial wall (30° telescope) * Haller cell The maxillary sinus ostium seeker advances from the nasal cavity towards the natural ostium.

The bulge that narrows the maxillary sinus ostium from superior and posterior, marked with (*) in Fig. 7, belongs to a small Haller cell. The tip of the ostium seeker, as viewed from the natural ostium, is inserted through the middle meatus and advanced towards the ostium. •^ Figure 7.1

Coronal CT scan of a Haller cell (infraorbital ethmoid cell).

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Surgical Anatomy for Endoscopic Sphenoethmoidectomy

Infraorbital cell (Haller cell) CD video: 15' 50" Once the 30° telescope is located in the maxillary sinus cavity the prominent Haller cell can be identified and incised below the inferior margin of the ethmoid bulla at the posterosuperior margin of the natural ostium.

Figure 8 Transnasal view of the maxillary sinus ostium and infraorbital cell using a 30° telescope. * Haller cell arrow natural ostium of maxillary sinus

Uncinate process and Uncinectomy CD video: 17' 10" Resection of the uncinate process (uncinectomy) may be performed by choosing one of the following approaches:

Figure 9 Resection of the superior end of the uncinate process. UP superior end of the uncinate process • use of a through-cutting nasal forceps.

Using the sickle knife: Initially the uncinate process is incised with the sickle knife and then outfractured by use of Blakesley forceps, which is the usual approach for total uncinectomy. The procedure has certain disadvantages: • When the uncinate process is very close to the lamina papyracea, the sickle knife might superiorly fracture the lamina papyracea and the periorbita. • In the event of a pneumatized uncinate process the incision might not reach as far as the ethmoid. • If the incision, made at the inferior end of the horizontal portion of the uncinate process, is not carried backwards posteriorly to a sufficient extent, the remaining portion may impair vision of the maxillary sinus ostium.

Surgical Anatomy for Endoscopic Sphenoethmoidectomy

Figure 9.1 Uncinectomy incision with the sickle knife.

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Figure 9.2 CT image of the uncinate process located close to the lamina papyracea.

• If the incision is made too far anteriorly, the anterior portion of the maxilla - a thick bone - is encountered which in turn may impede the attempt of incising the uncinate process. • If the incision is made too close to the free margin of the uncinate process, uncinate remnants may obstruct vision of the foremost anterior ethmoid cells and agger nasi cells which need to be resected. Using the miniature backbiting nasal forceps: Once the first uncinate process incision has been made at the insertion point of the vertical and horizontal segments the maxillary sinus ostium should come into view; the lower half of the uncinate process is resected with a sharp curette, Stammberger antrum punch, or back-biting nasal forceps. The upper half of the uncinate process may be initially preserved to identify the frontal recess and its neighbouring ethmoid cells and finally resected in a controlled manner with a curette or through-cutting forceps.

Figure 9.3 Uncinate process incision with a miniature back-biting forceps.

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Surgical Anatomy for Endoscopic Sphenoethmoidectomy

Terminal recess CD video: 21' 10" If the uncinate process bends laterally and inserts onto the lamina papyracea a blind end called terminal recess (recessus terminalis) appears lateral to the superior attachment of the uncinate process following uncinectomy. The terminal recess is surrounded by agger nasi cells anteriorly, frontal cells posteriorly, and superiorly, and lacrimal cells posteriorly and interiorly.

Figure 10 View of the superior end of the uncinate process after dissection of the mucosa on the lateral side. UP Superior attachment of the uncinate process to the lamina papyracea * terminal recess ** lacrimal cell of the ethmoid infundibulum

Ethmoid cells CD video: 22' 10" Preservation of the superior attachment of the uncinate process is essential for identification of this important landmark and the adjacent cavities in the superior part of the ethmoid infundibulum. At this stage the lacrimal cells, frontal cells, agger nasi cells and the terminal recess located lateral to the uncinate process should be identified and dissected. Preservation of the superior attachment of the uncinate process also facilitates identification of the frontal recess, which is usually medial and posterior to the superior insertion of the uncinate process, and at a higher level. Figure 11 Ethmoid cells anterior to the infundibulum are dissected before the superior end of the uncinate process is completely resected. 1 frontal recess 2 hiatus semilunaris superior 3 terminal recess 4 and 5 lacrimal cells of the ethmoid infundibulum MT middle turbinate UP uncinate process EB ethmoid bulla

Surgical Anatomy for Endoscopic Sphenoethmoidectomy

Frontal recess CD video: 24'30" The medial line of the frontal recess lies on the continuation of the lateral wall of the middle turbinate. In the event of a well-pneumatized frontal bone, frontal cells can be found between the superior end of the uncinate process and the frontal recess. However, there are no cells between the frontal recess and the lateral margin of the middle turbinate, and the recess is consistently anterior to the superior insertion of the anterior wall of the ethmoid bulla. Excision of the ethmoid bulla is usually not necessary for identification of the frontal recess; the frontal recess can be traced by following the lateral portion of the middle turbinate, the anterior wall of the ethmoid bulla and the superior end of the uncinate process. The opening, superior to the anterior wall of the ethmoid bulla, lateral to the superior insertion of the bulla, posterior to the superior insertion of the uncinate process, and lateral to the frontal recess communicates with the frontal bulla which constitutes the roof of the frontal cells. The frontal bulla has a blind end at the floor of the frontal bone and should not be confused with the frontal recess.

Figure 12 Cavities medial and lateral to the superior end of the uncinate process. FR frontal recess UP superior insertion of the uncinate process EB anterior wall of the ethmoid bulla * terminal recess ** lacrimal cell of the frontal recess * * * opening towards the frontal bulla

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Surgical Anatomy for Endoscopic Sphenoethmoidectomy

Figure 12.1 The topographical relation between the frontal recess and the ethmoid cells

Variations of the agger nasi cells, nasal cells, frontal cells and mucosal alterations in the course of chronic sinusitis may compromise identification of the frontal recess. In such cases detailed analysis of the frontal recess area in preoperative CT cross-sections proves useful. Assessment of the CT scans should start by finding the coronal cross-section along the frontal recess. In coronal CT-scans the frontal recess is usually visible on the cross-section where the frontal sinus ends. In the 2 mm cross-sections shown above the right frontal recess presents in cross-sections nos. 7

and 6. Looking at the anterior cross-sections, there are two ethmoid cells in cross-sections nos. 5 and 4, and the one which is located more anterior and inferior continues in cross-sections nos. 3 and 2. This suggests that the two cells need to be resected before gaining access to the frontal recess during surgery. The left frontal recess is shown in cross-sections nos. 6 and 5. It becomes evident that the cells which appear to be two in number on the left side in crosssection no. 5 are in fact three in number in crosssections nos. 4 and 3.

Surgical Anatomy for Endoscopic Sphenoethmoidectomy

Frontal recess Frontal bulla CD video: 25' 25" Three different points of insertion of the superior end of the uncinate process can be found: the middle turbinate, lamina papyracea and the skull base. Any of these three types of insertion does not always exist alone, the uncinate process might be attached to more than one point. In the example illustrated in Fig. 13, the uncinate process delineates the frontal recess anteriorly by attaching to the middle turbinate medially (1), the frontal bulla anteriorly by attaching to the lamina papyracea laterally (2) and separates these two openings by also attaching to the superior end of the anterior wall of the bulla in the midline (3).

Figure 13 Points of insertion of the superior end of the uncinate process MT middle turbinate FR frontal recess EB ethmoid bulla * opening to the frontal bulla 1 point of insertion I: Uncinate process - middle tubinate 2 point of insertion II: Uncinate process - lamina papyracea 3 point of insertion III: Uncinate process - ethmoid bulla

Figure 13.1

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Surgical Anatomy for Endoscopic Sphenoethmoidectomy

Ethmoid bulla Basal lamella CD video: 31' 40"

Figure 14 Endoscopic aspect after dissection of the anterior wall of the ethmoid bulla (30° telescope). MT middle turbinate EB anterior wall of the ethmoid bulla HSS hiatus semilunaris superior BL basal lamella (oblique section) * frontal recess ** frontal bulla * * * suprabullar recess

The ethmoid bulla is the largest and the most interiorly located among the anterior ethmoid cells. The presence of a small bulla increases the risk of damaging the lamina papyracea during dissection; the safest entrance point to minimize such risk is the inferomedial part of the bulla. The bulla cavity can be exposed with a Blakesley forceps or a sharp curette. The superior insertion should then be followed towards the skull base after elevating the medial wall. Once inside the bulla cavity the basal lamella of the middle turbinate can be identified by viewing backwards. The basal lamella of the middle turbinate is divided into two parts as oblique and horizontal. The horizontal portion must always form the floor of the ethmoidectomy cavity and must not be damaged. Resection of the horizontal portion might cause problems from two points: • Static stability of the middle turbinate is impaired, which might may induce formation of synechia between the middle turbinate and lateral nasal wall or necrosis of the middle turbinate postoperatively; • Bleeding from branches of the sphenopalatine artery may be triggered. The shape of the oblique portion may be variable depending on the degree of pneumatization of the anterior and posterior ethmoid cells. When pneumatization of the suprabullar recess and anterior ethmoid cells superiorly is more pronounced than in posterior ethmoid cells, the insertion of the basal lamella to the skull base superiorly is located posterior to the inferior part of the oblique segment. When the retrobullar recess is more voluminous than the posterior ethmoid cells, the inferior part of the basal lamella assumes a more posterior position and the superior attachment to the skull base assumes a more anterior position.

Figure 14.1

Figure 14.2

Figure 14.3

Surgical Anatomy for Endoscopic Sphenoethmoidectomy

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Anterior ethmoid CD video: 35' 10" When the lamellae between the anterior wall of the ethmoid bulla and the superior attachment of the basal lamella of the middle turbinate are elevated access to the following cavities anterior to the skull base is gained: from anterior to posterior, the frontal recess and hiatus semilunaris superior medially, and in lateral position the frontal bulla, the superior boundary of the suprabullar recess and the superior boundary of an anterior ethmoid cell.

Figure 15 View of the roof of the anterior ethmoid after elevation of the anterior wall of the ethmoid bulla (30° telescope) MT middle turbinate LP lamina papyracea 1 frontal recess 2 hiatus semilunaris superior 3 frontal bulla 4 superior boundary of suprabullar recess - ethmoid fovea 5 anterior ethmoid cell

Supraorbital ethmoid cell Ethmoid fovea CD video: 35' 22" The frontal bulla, lateral to the frontal recess constitutes the roof of the frontal cells and may be pneumatized towards the orbital roof; in such cases a supraorbital cell can be seen. The anterior ethmoid artery passes along the anterior ethmoid roof (fovea ethmoidalis) located between the basal lamella of the middle turbinate and the anterior ethmoid crest forming the posterior margin of the frontal recess. The anterior ethmoid artery, usually passing through a thin bony canal, enters the ethmoid roof laterally from the lamina papyracea and extends anteromedially towards the middle turbinate. The anterior ethmoid artery divides the ethmoid fovea into two sub-zones in this way: the area in front of the artery extending up to the anterior ethmoid crest called the anterior ethmoid fovea, and the area posterior to the artery extending up to the point of attachment of the basal lamella oblique segment to the skull base superiorly called the posterior ethmoid fovea. The anterior ethmoid artery limits the anterior fovea fovea posteriorly, not the frontal recess.

Figure 16 Close-up view of the anterior ethmoid roof (30° telescope) * frontal bulla ** anterior ethmoid fovea * * * posterior ethmoid fovea FR frontal recess AEA anterior ethmoid artery arrow supraorbital cell

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Surgical Anatomy for Endoscopic Sphenoethmoidectomy

Supraorbital ethmoid cell Ethmoid fovea CD video: 38' 00" The cavity lateral to the anterior ethmoid roof delineated by the anterior ethmoid artery posteriorly is a supraorbital cell, located superomedially to the orbit and posterior to the frontal bulla.

Figure 17 Identification of a supraorbital cell (30° telescope) * frontal recess ** frontal bulla * * * anterior ethmoid roof arrow supraorbital ethmoid cell AEA anterior ethmoid artery

Frontal sinus Ethmoid fovea CD video: 40' 20" Once the bony lamellae adjacent to the frontal recess are completely removed the frontal sinus lumen can be viewed through its ostium or canal. The exposed cavity is confirmed to be in fact the frontal sinus by the presence of a prominent posterior wall and incomplete septations. The vertical lamella of the middle turbinate is attached to the cribriform plate of the ethmoid sinus medial to the fovea. This is the predilection site where the skull base is the weakest and where cerebrospinal fluid leaks occur most frequently due to anterior fossa dura injuries. Figure 18 View of the frontal sinus after the septa between the frontal recess, the frontal bulla and fovea ethmoidalis are removed (45° telescope) FS frontal sinus MT middle concha LP lamina papyracea * frontal bulla ** anterior ethmoid crest EF ethmoid fovea (anterior) arrows highest risk of dura injury

Surgical Anatomy for Endoscopic Sphenoethmoidectomy

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Pneumatization of the nasal bone CD video: 41' 05" Nasal cells may form as a result of pneumatization anterior to the frontal recess and posterior to the superior insertion of the uncinate process. Such cells may cause chronic or recurrent frontal sinusitis if not dissected.

Figure 19 Nasal sinus cells anterior and medial to the frontal recess. 1 frontal recess 2 frontal bulla 3 anterior ethmoid roof UP superior end of the uncinate process * nasal sinus cell

Middle turbinate basal lamella CD video: 43' 00" Access to the posterior ethmoid cells is achieved once the oblique portion of the basal lamella of the middle turbinate has been reached. The safest point for the initial dissection of the basal lamella is the inferomedial portion, as is the case with the anterior wall of the ethmoid bulla.

Figure 20 Oblique portion of the basal lamella of the middle turbinate. MT middle turbinate LP lamina papyracea BL basal lamella (oblique portion)

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Surgical Anatomy for Endoscopic Sphenoethmoidectomy

Posterior ethmoid CD video: 44' 10" Once the superior portion of the basal lamella is removed and the skull base comes into view, the posterior ethmoid artery travelling from lateral to medial almost parallel to the coronal plane of the skull base can be identified on average 12 mm behind the anterior ethmoid artery.

Figure 21 View of the posterior ethmoid fovea when the superior section of the basal lamella is dissected. EF ethmoid fovea LP lamina papyracea BL basal lamella (oblique portion) of the middle turbinate AEA anterior ethmoid artery PEA posterior ethmoid artery

Posterior ethmoid CD video: 46' 45" The posterior ethmoid artery travels in a thin bony canal at the skull base, like the anterior ethmoid artery. The artery is usually accompanied by the ethmoid nerve in the same bony canal but, as seen in the example above, the posterior ethmoid artery and posterior ethmoid nerve may also run separately.

Figure 22 View of the posterior ethmoid fovea once the opening in the superior section of the basal lamella is enlarged. PEA posterior ethmoid artery PEN posterior ethmoid nerve

Surgical Anatomy for Endoscopic Sphenoethmoidectomy

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Sphenoethmoid recess CD video: 48' 35" The anterior wall of the sphenoid sinus can be identified by several methods once ethmoidectomy is complete: • The anterior wall of the sphenoid sinus lies inferomedial to the ethmoidectomy cavity and at a distance of 6.5 cm on average from the anterior maxillary crest in an adult patient. • The anterior wall of the sphenoid sinus lies 1 cm anterior and superior to the superior boundary of the choana. The anterior wall of the sphenoid sinus may be identified by assessing the depth from inside the ethmoidectomy cavity after measuring the depth of the choana. • The sphenoethmoid recess lies superolateral to the vertical crus - formed by the basal lamella of the middle turbinate (2) - of an imaginary "Y" formed by the posterior nasal fontanelle, the horizontal portion of the basal lamella of the middle turbinate and the orbital floor. The sphenoid sinus lies medial to the sphenoethmoid recess.

Figure 23 View from the middle meatus after enlargement of the posterior nasal fontanelle posteriorly. MS maxillary sinus 1 posterior nasal fontanelle 2 basal lamella (horizontal portion) of the middle turbinate 3 orbital floor

Onodi cell CD video: 49' 35" A pneumatized posterior ethmoid cell superolateral to the sphenoid sinus is termed Onodi cell or sphenoethmoid cell. The optic nerve or even the internal carotid artery may run through the Onodi cell. An Onodi cell may be confirmed if pneumatization extends lateral to the sphenoid sinus in axial computed tomography cross-sections at the level of the sphenoid sinus. From pneumatization in anterior clinoid processes and lateral to the optic nerve in coronal computed tomography cross-sections (*) one may conclude the existence of an Onodi cell.

Figure 24 The presence of an Onodi cell may be confirmed while attempting to localize the sphenoethmoid recess in the course of posterior ethmoidectomy * attachment of the basal lamella of the middle turbinate to the skull base LP lamina papyracea PEA posterior ethmoid artery PEN posterior ethmoid nerve arrow Onodi cell Figure 24.1

Figure 24.2

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Surgical Anatomy for Endoscopic Sphenoethmoidectomy

Onodi cell CD video: 49' 35" Care should be taken when operating on the superolateral section of the posterior ethmoidectomy cavity in the presence of an Onodi cell. No structure should be resected unless the one situated behind the Onodi cell can be clearly identified. Iatrogenic optic nerve damage during endoscopic sinus surgery most frequently occurs within the Onodi cell and when the sphenoid sinus is sought in posterior position to this cell. In the presence of an Onodi cell the anterior wall of the sphenoid sinus lies to the anterior, medial and inferior of it. Taking into account that the orbital apex lies 1-8 mm (5 mm on average) behind the posterior ethmoid artery can be helpful in locating the optic nerve. Figure 25 View of the optic nerve after dissection of the anterior wall of the Onodi cell * Onodi cell PEA posterior ethmoid artery SS sphenoid sinus (anterior wall) n. 2 optic nerve

Sphenoid sinus CD video: 51' 15" After the anterior wall of the sphenoid sinus is dissected and the opening enlarged, the lamella separating the sphenoid sinus and the Onodi cell attaching to the optic nerve come into view. The lamella segment adjacent to the optic nerve should not be resected.

Figure 26 View of the Onodi cell and sphenoid sinus cavity after dissection of the anterior wall of the sphenoid sinus. 1 Onodi cell 2 sphenoid sinus LP lamina papyracea n. 2 optic nerve

Surgical Anatomy for Endoscopic Sphenoethmoidectomy

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Sphenoid sinus CD video: 51' 15" The prominences of the sella and internal carotid artery can almost always be identified when the sphenoid sinus cavity is inspected. In Fig. 27, the optic nerve prominence is not at the expected site of the sphenoid sinus superolateral wall since it is located within the Onodi cell. In addition to the optic nerve and internal carotid artery, the prominences of the maxillary nerve at the lateral wall and the vidian nerve at the inferior wall of the sphenoid sinus can occasionally be observed.

Figure 27 Posterior and lateral walls of the sphenoid sinus S sphenoid sinus septum ICA prominence of the internal carotid artery * sella prominence

Internal carotid artery CD video: 52' 25" The internal carotid artery has been demonstrated by opening the posterolateral wall of the sphenoid sinus to expose structures which are associated with the risk of complications.

Figure 28 Internal carotid artery (ICA) viewed after the posterior bony wall of the sphenoid sinus has been dissected. ICA internal carotid artery * sella prominence

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Surgical Anatomy for Endoscopic Sphenoethmoidectomy

Cavernous sinus CD video: 52'40" When the internal carotid artery is retracted medially, the veins of the cavernous sinus, the suspensory ligaments of the internal carotid artery and the abducent nerve superolateral^ come into view.

Figure 29 The cavernous sinus comes into view once the internal carotid artery is retracted medially. ICA internal carotid artery n.6 abducent nerve

Optic nerve CD video: 53' 10" The risk of nerve damage during endoscopic sinus surgery dissections is reduced when the optic nerve is completely isolated and its course clearly identified. When total sphenoidectomy is complete, the lamina papyracea is removed and the orbital apex can be identified without the risk of damaging the orbital periosteum, as in endoscopic optic nerve decompression. The bony canal of the medial wall of the optic nerve is resected with a diamond burr or thin dissector and the Zinn's ring ligaments extending perpendicular to the longitudinal axis of the nerve at the level of the orbital apex are identified. Figure 30 Optic nerve decompression after removal of the lamina papyracea n.2 optic nerve \\\ Zinn's ring ligaments