Anatomy of the Orbit and Paranasal Sinuses

Eyelids


The upper and lower eyelids are anatomically distinct, though analogous. Surgical approaches to the orbit may incorporate various portions of the eyelids, including the upper eyelid crease, lower eyelid conjunctiva, and lateral canthal region ( ▶ Fig. 1.1).



Detailed sagittal view of the eyelid as it relates to the eye. Note the orbital septum arises from the orbital rim, serving as a landmark dividing the preseptal and orbital space. (Used with permissio


Fig. 1.1 Detailed sagittal view of the eyelid as it relates to the eye. Note the orbital septum arises from the orbital rim, serving as a landmark dividing the preseptal and orbital space. (Used with permission from Gilroy AM, ed. Anatomy: An Essential Textbook. 2nd ed. New York, NY: Thieme; 2017: 485.)



Both the upper and lower eyelids can be conceptually and physically divided into the anterior and posterior lamellae.


 Anterior Lamellae


In both the upper and lower eyelids, the anterior lamellae are composed of skin and orbicularis oculi muscle. Eyelid skin is unique in that there is no underlying subcutaneous fat. The orbicularis muscle is a protractor innervated by cranial nerve VII, allowing for involuntary and voluntary eyelid closure. A continuation of the orbicularis muscle through to the margin of the eyelid is named the muscle of Riolan, more commonly referred to as the “gray line” due to its color. 1


The eyelid crease is formed from attachments between the anterior and posterior lamellae, namely, fine attachments between the septum, levator aponeurosis, and skin. 1 The varying location of the eyelid crease among ethnic groups is due to differences in anatomic attachment location, but in all cases, an eyelid crease approach is an aesthetically pleasing way to access superior orbital anatomy, including the superomedial orbit.


 Posterior Lamellae


The posterior lamellae differ in the upper and lower eyelids. Both have tarsal plates, a septum, fat pads, retractor muscles that open the eyelids, and conjunctiva. With the exception of the conjunctiva, each of these components is analogous but unique when comparing the upper and lower eyelids.


The tarsus is a dense connective tissue plate, providing structural support to the eyelid. The upper eyelid tarsus measures approximately 10 mm in height, as compared to the lower eyelid tarsus, which is 4 mm. Each tarsal plate is approximately 1 mm thick, and has tapered ends. They are attached to the periorbita through canthal tendons medially and laterally. The medial canthal tendon is split anteriorly and posteriorly to bridge the lacrimal crests, hugging the lacrimal sac, as well as superiorly and inferiorly to reach the respective eyelids. The lateral canthal tendon arises from the lateral orbital tubercle, splitting into superior and inferior components prior to reaching the upper and lower eyelids. 1 Each set of tendons may need to be breached in an external approach to the orbit.


The septum is a thin set of tissue layers arising from the periosteal arcus marginalis at the bony orbital rim. Preservation of the septum prevents fat prolapse into the surgical field. There are two fat pads located between the septum and the levator aponeurosis of the upper eyelid, and three between the septum and the capsulopalpebral fascia of the lower eyelid ( ▶ Fig. 1.2). 1



View of the nasolacrimal system in relation to the eyelids and the nose. While the puncta are located on the medial eyelid margin, the lacrimal sac sits more posteriorly between the split medial canth


Fig. 1.2 View of the nasolacrimal system in relation to the eyelids and the nose. While the puncta are located on the medial eyelid margin, the lacrimal sac sits more posteriorly between the split medial canthal tendons. The nasolacrimal duct runs inferiorly in a posteromedial direction, opening in the inferior meatus of the nose. (Used with permission from Gilroy AM, ed. Anatomy: An Essential Textbook. 2nd ed. New York, NY: Thieme; 2017: 486.)



There are two retractor muscles in each of the eyelids. In the upper eyelid, these are the levator palpebrae superioris muscle, including its aponeurotic insertion on the anterior tarsal face, and the more posterior Muller’s muscle, also known as the superior tarsal muscle, which inserts at the superior border of the tarsus. Whitnall’s ligament is about 4 cm anterior to the orbital apex, and marks the transition of the levator from a horizontal vector posteriorly to a vertical retracting vector anteriorly. Muller’s muscle originates around the area of Whitnall’s ligament. In the lower eyelid, the retractor muscles are named the capsulopalpebral fascia and the inferior tarsal muscle. The capsulopalpebral fascia originates along the inferior rectus muscle and wraps around the inferior oblique muscle as it is traced anteriorly. As the capsulopalpebral fascial arms fuse anterior to the inferior oblique muscle, they form a fibrous condensation called Lockwood’s ligament. Muller’s muscle and the inferior tarsal muscle are sympathetically innervated, whereas the levator palpebrae superioris muscle and capsulopalpebral fascia are innervated by cranial nerve III. 1


The conjunctiva is a nonkeratinizing, squamous epithelial layer coating the most posterior aspect of each eyelid, traveling to create superior and inferior fornices between the eyelids and the globe, and finally covering the ocular surface. The transconjunctival surgical approach in the lower eyelid allows for excellent access to the orbital floor and medial orbital wall. 1


 Eyelid Margin


The eyelid margin is the terminating platform of the upper and lower eyelids. The anterior and posterior lamellar classification system continues in this region. The gray line denotes the orbicularis muscle, allowing for the anatomic distinction between the lamellae. The cilia, or eyelashes, arise anterior to the gray line and exit along the anterior margin, while the oil-excreting meibomian glands are embedded within the tarsus itself, which is posterior to the gray line. 1 Surgical approaches disrupting the eyelid margin require careful realignment of the margin tissue.


 Vascular System


The eyelids are highly vascular. The arterial supply is indirectly derived from both the internal and external carotid branches. The external carotid artery supplies the angular and temporal arteries, and the internal carotid artery supplies the ophthalmic artery with its multiple branches. The two arterial supplies anastomose via the marginal arcades tracking the eyelid margins, and the upper eyelid peripheral arcade, seated just above the tarsus between the levator aponeurosis and Muller’s muscle. Since arterial bleeds can track behind the septum and cause retrobulbar hemorrhage, surgeons must pay close attention to hemostasis during any surgical manipulation behind the septum. 1


Venous drainage from the anterior lamellae exits along the facial veins, whereas the posterior lamellae drain into the orbital veins. Patterns of lymphatic drainage of the eyelids are variable, but are generally divided into medial and lateral eyelids; the medial eyelids drain into the submandibular lymph nodes, whereas the lateral eyelids drain into the preauricular lymph nodes. 1


1.1.2 Nasolacrimal System


The nasolacrimal system serves to drain the tear film that coats and protects the eye. It has a number of components, each of which must work in order for the system as a whole to function. Disruption or blockage of any component can result in tearing or infection ( ▶ Fig. 1.2).


 Puncta


In both the upper and lower eyelids, a round hole, the punctum, can be found in the medial-most aspect of the eyelid. Eyelid opening and closing cause both negative and positive pressure forces, pushing tears into the puncta and through the canalicular system. The inferior punctum is believed to allow for the majority of tear drainage. 2 For this reason, surgeons may choose the upper punctum for fiberoptic light source visualization of the lacrimal sac during endoscopic dacryocystorhinostomy.


 Canaliculi


Each punctum is contiguous with a canaliculus, which is lined with nonkeratinized squamous epithelium. The canaliculi first travel vertically for 2 mm prior to an 8-mm horizontal path. Understanding this anatomy is crucial when attempting to intubate the canalicular system so as to prevent false passages. The majority of upper and lower canaliculi meet in a common canaliculus prior to reaching the lacrimal sac. The valve of Rosenmüller helps prevent lacrimal sac reflux at this point. 2 Wide marsupialization of this valve into the nasal cavity is critical to the success of endoscopic dacryocystorhinostomy.


 Lacrimal Sac


The lacrimal sac sits in the orbit between the two arms of the medial canthal tendon, which attach to the anterior and posterior lacrimal crests on either side of the lacrimal sac fossa. The lacrimal sac is approximately 13 to 15 mm in length and is contiguous with the nasolacrimal duct. Its fundus rises approximately 3 to 5 mm above the medial canthal tendon. 2 Dacryocystorhinostomy anastomoses the lacrimal sac with the nasal mucosa of the lateral nasal wall of the middle meatus, bypassing an obstructed nasolacrimal duct.


 Nasolacrimal Duct


The nasolacrimal duct is approximately 15 mm in length and opens into the nose under the inferior turbinate. The valve of Hasner lies at the inferior end and is often a cause of congenital nasolacrimal obstruction. 2


The lacrimal sac and nasolacrimal duct follow the bony anatomy of their respective sites, traveling inferiorly in a posterolateral direction. 2


1.1.3 Orbit


The orbit is composed of soft tissues and bones. The orbit serves to protect and animate the eye, as well as to provide sensation and vascularity to the periocular tissues.


 Bony Anatomy


The bony orbit is considered to have a floor, roof, medial wall, and lateral wall ( ▶ Fig. 1.3). The height of the skeletal entrance is 35 mm and the width is 40 mm. The orbital volume is approximately 30 cm3. 3



Medial (a) and lateral (b) views of the bony orbit. The superior orbital fissure is formed in the space between the greater and lesser wings of the sphenoid. The optic canal lies medial to the superio


Fig. 1.3 Medial (a) and lateral (b) views of the bony orbit. The superior orbital fissure is formed in the space between the greater and lesser wings of the sphenoid. The optic canal lies medial to the superior orbital fissure, in the lesser wing of the sphenoid. (Part b used with permission from Schuenke M, Schulte E, Schumacher U, eds. Head and Neuroanatomy: Thieme Atlas of Anatomy. New York, NY: Thieme 2007:14)



The orbital roof is composed of the frontal bone and the lesser wing of the sphenoid bone. It houses the lacrimal gland fossa superotemporally. The trochlea is found 5 mm behind the superonasal orbital rim ( ▶ Fig. 1.4). The superior oblique’s mechanism of action is formed through its trochlear attachment at this point. Additionally, the supraorbital notch or foramen can be found superomedially on the orbital rim; the supraorbital neovascular bundle exits the orbit at this point. 3



A superior view of the orbit, with the roof and periorbita selectively removed, and the levator palpebrae superioris and superior rectus muscles transected and reflected. Note the location of the troc


Fig. 1.4 A superior view of the orbit, with the roof and periorbita selectively removed, and the levator palpebrae superioris and superior rectus muscles transected and reflected. Note the location of the trochlea in the anterior superomedial orbit, the medial course of the superior oblique and medial rectus muscles, the medial course of the superior ophthalmic vein and branches of the ophthalmic artery, and the ease of accidental approach to the optic nerve in the posteromedial orbit. (Used with permission from Gilroy AM, MacPherson B, Ross L, Atlas of Anatomy. 2nd ed. New York, NY: Thieme; 2012: 543.)



The medial wall is composed of the lesser wing of the sphenoid, ethmoid, lacrimal, and maxillary bones, and is 45 mm in anteroposterior length. The frontoethmoidal suture is a landmark for the general region of the cribriform plate. The anterior and posterior ethmoidal arteries are found approximately 25 and 35 mm posterior to the orbital rim, respectively, serving as important landmarks of distance to critical orbital apex structures; additionally, surgical manipulation superior to these arteries may lead to intracranial penetration. 3


The orbital floor is composed of the zygomatic, maxillary, and palatine bones. As it ends at the pterygopalatine fossa, rather than the orbital apex, it is the shortest of the four walls. The orbital process of the palatine bone may obstruct medial access to the orbital apex and is typically drilled away during an endoscopic approach. The infraorbital groove and canal are found along the medial aspect of the floor and carry cranial nerve V2. 3 This location is of significance in orbital floor fracture repair and decompression surgery.


Finally, the lateral wall is composed of the zygomatic and greater wing of the sphenoid bones. 3 The zygomaticotemporal and zygomaticofacial neurovascular bundles pierce this strongest of the orbital walls and are significant landmarks in lateral orbital wall decompressions.


The superior orbital fissure is found between the greater and lesser wings of the sphenoid bone, and carries cranial nerves III, IV, V1, and VI, sympathetic nerve fibers, and the superior ophthalmic vein. The inferior orbital fissure carries the inferior ophthalmic vein and a branch of cranial nerve V2 ( ▶ Fig. 1.5). 3



(a) The orbital apex and extraocular muscle origins are shown. Note the four rectus muscles, superior oblique muscle, and levator palpebrae superioris muscle arising from the apex; all but the levator


Fig. 1.5 (a) The orbital apex and extraocular muscle origins are shown. Note the four rectus muscles, superior oblique muscle, and levator palpebrae superioris muscle arising from the apex; all but the levator palpebrae superioris muscles are attached to the annulus of Zinn. The optic nerve enters the optic canal through the annulus of Zinn. In contrast, the inferior oblique muscle arises from the more anteromedial orbit. (b) With further resection of the muscles, the superior and inferior ophthalmic veins are found traversing the superior and inferior orbital fissures, respectively. In the superior orbital fissure region superolateral to the annulus of Zinn, cranial nerve IV and the lacrimal and frontal branches of V1 enter the orbit. Cranial nerves II, III, the nasociliary branch of V1, and VI enter the orbit through the annulus of Zinn. (Used with permission from Gilroy AM, MacPherson B, Ross L, Atlas of Anatomy. 2nd ed. New York, NY: Thieme; 2012: 501, 542).

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Feb 25, 2020 | Posted by in OTOLARYNGOLOGY | Comments Off on Anatomy of the Orbit and Paranasal Sinuses
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