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At the level of Posterior ethmoidal cells
Sub-contents
- Scanograms, CT, Cadaver section and representative line diagrams.
- Check list.
- Surgical Importance.
- Anatomy
- Posterior ethmoidal cells.
- Sphenoethmoidal cells or Onodi cells.
- Onodi cell relationship to the Optic nerve.
Fig. 116: Scanogram at the level of posterior ethmoids.
Fig. 117: Coronal Section CT Scan at the level of posterior ethmoids.
Fig. 118: Representative line diagram of Coronal Section at the level of posterior ethmoids.
Fig. 119: Coronal Section in Cadaver at the level of posterior ethmoids.
Fig. 119a: Correlation of Coronal Section in Cadaver, CT and representative line diagrams
at the level of posterior ethmoids.
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Check list
- Nasal septum : Any deviation or spurs or posterior widening
- Height of the skull base (From roof of maxillary sinus to the skull base)
- Lamina Papyracca: Any dehisccnce
- Presence ofOnodi cells (Look for horizontal septa)
- Middle turbinate : Presence, superior attachment and any variants
- Presence of superior or supreme turbinatc and any variants
- Presence of accessory maxillary ostium
- Presence of Haller's cells
- Presence of Lateral sinus |
Surgical importance
Before resecting the bulla ethmoidalis, identify the mavillary sinus ostium. Bulla helps in localising the ostium. If the bulla is small or underdeveloped, enter anterninferior and medial part to avoid injury to lamina papyracea. Opening of the frontal sinus is anterior to ethmoid crest and the anterior ethmoid artery is posterior to the ethmoid crest. Therefore, it is safe to follow anterior wall of the bulla to reach the frontal recess.
Haller's cells are thought to cause chronic sinusitis by impinging on the ostium of the maxillary sinus and the infundibulum by inhibiting ciliary function, leading to obstruction of the ostium.
Study the course and attachment of middle turhinate in all sections. Because olits niimeroiK variations, it may be difficult to identify the basal lamella radiologically or during surgery. Resection of the posterior 2/3rds ol'the middle turbinatc may lead to instability of rest of the segment. Even perforation into the basal lamella to get access info posterior cthmoids or sphenoid can lead to middle turbinatc instability.
It may be difficult or impossible to introduce endoscope into the middle meatus when a turbinate sinus is present.
Look for Onodi cells in both axial and coronal sections. In coronal sections, a horizontal septum in the sphenoid indicates presence of Onodi cells.
Keep in mind that the skull base becomes lower as the sections proceed posteriorly. |
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Posterior ethmoidal cells
The size and shape of the posterior ethmoid depends on the position and course of basal lamella, and of its encroachment by the anterior ethmoid cells and posteriorly by the sphenoid.The number of cells varies from 1 to 7. They are larger and fewer in number thin the anterior ethmoid cells. These cells open posteriorly, above the ground lamella into the superior, and occasionally the supreme, meatus. Occasionally one or more cells may open directly into the sphenoid sinus.
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Sphenoethmoid cells or Onodi cells
Incidence is high in Asians. Posterior ethmoid ce!l that extends beyond the sphenoid, as much as 1.5 cm beyond its anterior wall. Stammberger categorized the Onodi cell as a posterior ethmoid cell presenting with an optic canal bulge. When present, it has an intimate spatial relationship with the optic nerve and/or internal carotid artery.
Fig. 120: Axial Section: Showing Onodi cell. OC: Onodi cell, ON: Optic nerve and SS: Sphenoid sinus.
Fig. 121: Axial Section: Showing Onodi cell with disease process in the Onodi cell (Mucosal thickning). OC: Onodi cell, ON: Optic nerve and SS: Sphenoid sinus.
Fig.122 : Coronal Section: Showing Onodi cell: At sphenoid level, if there is horizontal septa, the cells superior the horizontal septa are Onodi cells and the one inferior to it are sphenoid cells.Notice bilateral optic nerve bulge into the onodi cells and actually Optic canals are are dehiscent. OC: Onodi cell, ON: Optic nerve, SS: Sphenoid sinus and HS: Horizontal septa.
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Optic nerve and Sphenoethmoid cells relationship
The optic nerve and Sphenoethmoid cells relationship can be divided into three groups, depending on the relationship of annulus of Zinn to the ethmoids, a critical landmark where these two structures come in contact:
The groups are:
1 .Indirect: When the ethmoid sinus terminates anterior to annulus of Zinn
2. Direct Onodi+ve: When the ethmoid sinus extend posteriorly beyond annulus of Zinn with visible Optic nerve bulge into the ethmoids.
3. Direct Onodi-ve: Direct Onodi relationship without a optic canal bulge.
The thickness of the bony lamina separating the optic nerve from the ethmoids varies from 0.10 to 0.52 mm. Patients with Direct Onodi+ve relationship are at more risk for injury during surgery.
Fig. 123, 124, 125.
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