Microsurgical Anatomy of the Orbit

12 Microsurgical Anatomy of the Orbit

Carolina Martins, Alvaro Campero, Marcelo Valença, Izabel Eugênia Costa e Silva, and Albert Rhoton Jr.†

Abstract

The orbit and its contents are frequently the site of several diseases that may occur in different corners of the orbital cavity. Lesions inside the orbital cavity can present restricted to this area or extend inferiorly to the maxillary sinus, medially to the ethmoid sinus, laterally to the zygomatic and infratemporal fossa, or posteriorly and laterally to the cavernous sinus. In this chapter, we describe a very useful way to reach the lateral compartment of the orbit, with or without extension to the cavernous sinus, highlighting the anatomical details and surgical nuances.

Keywords: Keywords: anatomy, cavernous sinus, optic nerve, orbit

12.1 Historical Account

A thorough review of the historical contributions to the current understanding of the microsurgical anatomy of the orbit are out of the scope of this chapter. Nevertheless, landmark contributions have been highlighted and are recommended for those interested in this aspect.1 ^, ² ^, ³ ^, ⁴ ^, ⁵ ^, ⁶ ^, ⁷ ^, ⁸ ^, ⁹ ^, ¹⁰

12.2 The Rule of Seven of the Orbit

The orbits are paired structures, located on the anterior part of the face and protected by the eyelids ( Fig. 12.1a, b). Each orbit can be compared to a tiny jewel box that has very precious contents; all carefully wrapped in fat tissue. They can also be compared to a main room, to which access is gathered through a pre-chamber, the cavernous sinus.

Fig. 12.1 (a) The orbits are paired structures, located on the anterior part of the face and protected by the eyelids. The skin and subcutaneous tissue have been removed on the left, to expose the orbicularis oculi muscle. The orbicularis oculi surround the orbital rim, extends into the eyelids, and encircle the upper part of the nasolacrimal duct. (b–d) Understanding the bony orbit is the first step in successfully choosing and performing an orbital approach. Seven bones form each orbit and they are arranged to form a four-sided pyramid with a posterior apex and anterior base as shown in red dots (c). This pyramid, however, is not straight, but displays a laterally tilted axis (black outline in c and d). Although simple, this fact constitutes the basis of the human stereoscopic vision and allows for understanding the location of orbital foramina. (e) In the orbit, all openings are arranged around the base, apex, or between the orbital walls. Along the base are the infraorbital (light blue) and supraorbital (dark blue) canals, the zygomatico-orbital foramen (gray), and the zygomaticofacial foramina (black); between the roof and the lateral wall are the superior orbital fissure (purple) and the lacrimal foramen (orange); between the roof and medial wall are the optic (yellow), and anterior and posterior ethmoidal (red) canals; between the lateral wall and floor is the inferior orbital fissure (green); and between the medial wall and the floor is the cranial opening of the nasolacrimal duct (pink). (f) The seven bones forming the orbit have been highlighted in color: frontal (blue), ethmoid (brown), lacrimal (grey), sphenoid (green), zygomatic (pink), palatine (red), and maxilla (yellow). In three of the four orbital walls, these bones are arranged in pairs, with the exception of the medial wall.

Morphologically, each orbit is a four-sided pyramid with a posterior apex and anterior base and a medially tilted axis ( Fig. 12.1c, d). Although simple, this fact constitutes the basis of the human stereoscopic vision and allows for understanding the location of orbital foramina.

In the orbit, all openings are arranged around the base, apex, or between the orbital walls ( Fig. 12.1d, e). Along the base are the infraorbital and supraorbital canals and the zygomaticofacial foramina; between the roof and the lateral wall are the superior orbital fissure and the lacrimal foramen; between the roof and medial wall are the optic, and anterior and posterior ethmoidal canals; between the lateral wall and floor is the inferior orbital fissure; and between the medial wall and orbital floor is the cranial opening of the nasolacrimal duct.

Another anatomical characteristic of the orbit is that structures are arranged in groups of seven: there are seven bones, seven intraorbital muscles, and seven nerves in the orbit.

12.3 Orbital Bones

The seven bones forming the orbit are: frontal, ethmoid, lacrimal, sphenoid, zygomatic, palatine, and maxilla ( Fig. 12.1f and Fig. 12.2). In three out of the four orbital walls, these bones are arranged in pairs, with the exception of the medial wall.

Fig. 12.2 Skull bones are added to gradually rebuild the bony orbit, starting along the medial orbital wall: (a) The ethmoid. The quadrangular orbital plate of the ethmoid constitutes the center of the medial orbital wall and separates the orbit from the medially located nasal cavity. The ethmoidal air cells can be seen through this paper-thin bony wall, which covers the middle and posterior ethmoidal air cells. (b) The frontal bone. The orbital plate of the ethmoid articulates superiorly with the medial edge of the orbital plate of the frontal bone. The anterior and posterior ethmoidal notches that exist in both plates, when combined, form the anterior and posterior ethmoidal canals. (c) Parasagittal cut through the right orbit, showing the combination of the orbital plates of the frontal (blue) and ethmoid (brown) to form the ethmoidal canals (red). These canals transmit the ethmoidal branches of the nasociliary nerve of the ophthalmic division of the trigeminal nerve and the branches of the ophthalmic artery that pass further to supply the sinus mucosa and the dura mater of the frontal pole and falx. The ethmoidal canals are useful landmarks to the anterior cranial base and orbital contents. (d) Maxillae. Inferiorly, the ethmoid articulates with the orbital surface of the maxilla. The posterior border of this surface, the maxillary lip, is the anterior border of the inferior orbital fissure. (e) Ethmoid, frontal bone, and maxilla are combined. At the most anterior portion of the orbit, the presence of the frontal process of the maxilla, forming most of the medial rim of the orbit, determines the presence of a gap, which is filled in by the upper portion of the lacrimal bone. (f) Left lacrimal bone—nasal surface. The upper part of the lacrimal bone is a thin plate of bone located between the anterior edge of the ethmoid and the frontal process of the maxilla. It consists of a curved plate of bone which delimits the anteromedial two-thirds of cranial opening of the nasolacrimal duct.

Fig. 12.2 (Continued) (g) In the articulated skull, the lacrimal bone can be recognized as a curved plate of bone which delimits the anteromedial two-thirds of cranial opening of the nasolacrimal duct. The opening of the nasolacrimal duct is located at the most anterior part of the junction between the medial wall and orbital floor. (h) Posteriorly, the ethmoid articulates with the body of the sphenoid, therefore completing the medial wall of the orbit. (i) The sphenoid contributes to the bony formation of the orbit through its body, and lesser and greater wings. The lesser wings combine with the orbital plates of the frontal bone to form the orbital roof. (j) The greater sphenoid wings, laterally directed, join the orbital surface of the zygomatic bone to form the lateral orbital wall. (k) The zygomatic bone—lateral surface. (l) The zygomatic bone—orbital surface.

Fig. 12.2 (Continued) (m) The lateral wall of the orbit is formed by the combination of the orbital surfaces of the greater sphenoid wing (green) and the zygomatic bone (pink). (n) An axial cut has been made through the right orbit, removing its roof. The greater wing of the sphenoid (green) is, on the endocranial side, the anterior limit of the middle fossa. The zygomatic bone (pink), in contrast, forms the anterior limit of the temporal fossa, where the temporal muscle is located. This anatomical fact is the basis of the lateral orbital approaches, in which by displacing the temporal bone and performing a pure zygomatic osteotomy, orbital lesions can be reached, without the need of a combined, cranio-orbital approach. (o) The sphenoid (green), maxillae (yellow), and zygomatic bones (pink) have been combined. The combination of the greater sphenoid wing and the zygomatic bone forms the posterior lip of the inferior orbital fissure. (p) The anterior lip of the inferior orbital fissure is formed by the orbital surface of the maxilla and the orbital process of the palatine bone (red), as they pair to form the orbital floor. (q) The palatine bones have a horizontal part, which forms the posterior part of the hard palate, and a vertical part (perpendicular plate). The perpendicular plate has a posteromedial process, directed to the sphenoid body, the sphenoid process, and an orbital process, directed anterolaterally, consisting of a single air cell.

(Continued) (r, s) The palatine bones face posteriorly the pterygoid processes of the sphenoid bone, across the pterygomaxillary fissure, while the orbital process (red) abuts along the floor of the orbit, between the maxilla and the ethmoid. (t) The inferior orbital fissure is located along the lateral wall and the floor of the orbit. It has broad anterior and posterior lips and narrow medial and lateral ends. (u) The inferior orbital fissure is an important surgical landmark in the orbit. Through its medial part, the orbit communicates with the pterygopalatine fossa and through this, with the nasal cavity. Laterally the fissure brings the orbit in contact with the temporal and infratemporal fossae. The lateral part of the fissure is filled with smooth muscle and fat tissue, making it a suitable point to bear the bony cuts needed to remove part of the roof and lateral walls with corresponding orbital rim, as in the cranio-orbito-zygomatic approaches (inset).

The largest component of the medial wall is the ethmoid ( Fig. 12.2a). The quadrangular orbital plate of the ethmoid constitutes the center of the medial orbital wall and separates the orbit from the medially located nasal cavity. The orbital plate of the ethmoid articulates superiorly with the medial edge of the orbital plate of the frontal bone ( Fig. 12.2b, c). The anterior and posterior ethmoidal notches that exist in both plates, when combined, form the anterior and posterior ethmoidal canals ( Fig. 12.2c). These canals transmit the ethmoidal branches of the nasociliary nerve of the ophthalmic division of the trigeminal nerve and the branches of the ophthalmic artery that pass further to supply the sinus mucosa and the dura mater of the frontal pole and falx.

The cranial openings of the ethmoidal canals are related with the anterior and posterior limits of the ethmoidal cribriform plate and help in dividing the nasal cavity roof and anterior fossa floor into frontal, cribriform, and planum areas. The ethmoidal canals also help in dividing the orbital area into bulbar, retrobulbar, and apical parts (Fig. 12.3). This anatomical fact is useful during intracranial exploration of the anterior fossa¹ and endonasal approaches to the anterior fossa and orbit.

Fig. 12.3 (a) The roof and lateral wall of the left orbit have been removed. Beyond the optic canal and superior orbital fissure, the bony orbit is covered by an extension of the cranial dura mater, the periorbit. The periorbit or orbital dura envelops and holds the intraorbital structures, especially the orbital fat. (b) Except for the orbicularis oculi muscle ( Fig. 12.1a), located along the base of the orbital pyramid, there are seven intraorbital muscles: levator palpebrae, superior, inferior, lateral and medial rectus, and superior and inferior oblique muscles. Of these, only the inferior oblique muscle is attached to the medial orbital wall. The four recti and the superior oblique muscles attach along the orbital apex in and around a common annular tendon or fibrous ring, the annulus of Zinn. (c) Left bony orbit. The annulus inserts along the orbital apex, encircling the orbital opening of the optic canal and the central part of the superior orbital fissure. Structures coursing through the annulus are the optic nerve (II) and ophthalmic artery (OA), the oculomotor (III) and abducens (VI) nerves, and the nasociliary part (NC) of the ophthalmic division of the trigeminal nerve. The trochlear nerve (IV), and the frontal (F) and lacrimal (L) divisions of the trigeminal nerve course outside the annulus, on the lateral sector of the superior orbital fissure. (d) Endoscopic view into the right bony orbit. Anatomical and topographical relationships along the orbital apex are paramount to microscopic and endoscopic approaches. During endoscopy, structures are seen as in a “fish-eye aspect” with a broad angle of view and perspective. (e) The cranial openings of the ethmoidal canals are related with the anterior and posterior limits of the ethmoidal cribriform plate. The rule of seven helps keep in mind estimated distances: the anterior ethmoidal canal is on average 21 mm from the medial orbital edge. The posterior ethmoidal canal is around 14 mm posterior to the anterior canal, and the optic canal is on average 7 mm posterior to the posterior canal. (f) The cranial openings of the ethmoidal canals may help in dividing the nasal cavity roof and anterior fossa floor into frontal, cribriform, and planum areas. The ethmoidal canals might help in dividing the orbital area into bulbar, retrobulbar, and apical parts. This anatomical fact can be useful during intracranial exploration of the anterior fossa and endonasal approaches to the anterior fossa and orbit.

Inferiorly, the ethmoid articulates with the orbital plate of the maxilla ( Fig. 12.2d). At the most anterior portion of the orbit, the presence of the frontal process of the maxilla, forming most of the medial rim of the orbit, determines the presence of a gap, which is filled in by the upper portion of the lacrimal bone ( Fig. 12.2e, f). The upper part of the lacrimal bone is therefore located between the anterior edge of the ethmoid and the frontal process of the maxilla. It consists of a curved plate of bone which delimits the anteromedial two-thirds of cranial opening of the nasolacrimal duct ( Fig. 12.2g).

Posteriorly, the ethmoid articulates with the body of the sphenoid, completing the medial wall of the orbit ( Fig. 12.2h).

In fact, the sphenoid contributes to the bony formation of the orbit through its body, and lesser and greater wings. The lesser wings combine with the orbital plates of the frontal bone to form the orbital roof ( Fig. 12.2I) and the greater wings—laterally directed—join the orbital surface of the zygomatic bone to form the lateral orbital wall ( Fig. 12.2j–l).

Understanding the bony formation of the lateral wall of the orbit is important from the surgical standpoint ( Fig. 12.2m, n). The greater wing of the sphenoid faces the orbit, along the exocranial side and is the anterior limit of the middle fossa, on the endocranial surface. The zygomatic bone, in contrast, has no cerebral surface. The zygoma faces the orbit and—through its opposite surface—forms the anterior limit of the temporal fossa, where the temporal muscle is located. This anatomical fact is the basis of the lateral orbital approaches, in which by displacing the temporal bone and performing a pure zygomatic osteotomy, orbital lesions can be reached, without the need of a combined, cranio-orbital approach.

The combination of the greater sphenoid wing and the zygomatic bone also form the posterior lip of the inferior orbital fissure ( Fig. 12.2o). The anterior lip of this fissure is formed mostly by the orbital plate of the maxilla and the anteromedial part is formed by the orbital process of the palatine bone ( Fig. 12.2p).

The palatine bones ( Fig. 12.2q) have a horizontal part, which form the posterior part of the hard palate and a vertical part. The vertical part has a posteromedial process, directed to the sphenoid body—the sphenoid process—and an orbital process, directed anterolaterally, consisting of a single air cell and abutting along the floor of the orbit, between the maxilla and the ethmoid. The palatine bone thus pairs with the maxilla to form the orbital floor. They face posteriorly, across the pterygomaxillary fissure, the pterygoid processes of the sphenoid bone ( Fig. 12.2r, s).

The inferior orbital fissure is an important surgical landmark in the orbit. Through the most medial part of this fissure, the orbit communicates with the pterygopalatine fossa and through this, with the nasal cavity ( Fig. 12.2t, u). Laterally the fissure brings the orbit in contact with the temporal and infratemporal fossae. The lateral part of the fissure is filled only with smooth muscle and adipose tissue, making it a suitable point to bear the bony cuts needed to remove part of the roof and lateral walls with corresponding orbital rim, as in the cranio-orbito-zygomatic approaches ( Fig. 12.2u).

Beyond the optic canal and superior orbital fissure, the bony orbit is covered by an extension of the cranial dura mater, the periorbit ( Fig. 12.3a). The periorbit or orbital dura envelops and holds the intraorbital structures, especially the orbital fat. This structure also protects the orbital contents from tumoral invasion and inflammation.

The presence of the periorbit allows classifying the orbital lesions into intradural—when deep to the periorbit—and extradural—when located between the periorbit and the bony orbit.

12.4 Orbital Muscles and Vessels

Except for the sphincter oculi muscle, located along the base of the orbital pyramid, there are seven intraorbital muscles ( Fig. 12.3b). Of these, only the inferior oblique muscle is attached to the medial orbital wall. The four recti and the superior oblique muscles attach along the orbital apex in and around a common annular tendon or fibrous ring, the annulus of Zinn ( Fig. 12.3c, d). The superior oblique muscle passes through the trochlea, a round tendon attached to the trochlear fossa of the frontal bone on the supero-medial part of the orbit. Lesion to the trochlea occurring with fractures of medial orbital wall may result in downward gaze dysfunction and diplopia.

The annulus inserts along the orbital apex, encircling the orbital opening of the optic canal and the central part of the superior orbital fissure ( Fig. 12.3c, d, e, f). Structures coursing through the annulus are the optic nerve and ophthalmic artery, the oculomotor and abducens nerves, and the ophthalmic division of the trigeminal nerve. The trochlear nerve, and the frontal and lacrimal divisions of the trigeminal nerve course outside the annulus.

The ophthalmic artery is the major supply to the orbit ( Fig. 12.4a). It is a branch of the supraclinoid portion of the internal carotid artery, in most cases. This vessel follows the optic nerve in the optic canal and orbit and is responsible for the supply of orbital structures, including optic and lacrimal apparatus, muscles, and nerves. The origin of the ophthalmic artery is usually medial to the anterior clinoid process, below the optic nerve ( Fig. 12.4b). At the optic canal the artery passes lateral to nerve ( Fig. 12.4c). This anatomical fact must be remembered during opening of the falciform ligament after removal of the anterior clinoid, to prevent iatrogenic lesion of the ophthalmic artery. Approximately 8% of ophthalmic arteries arise in the cavernous sinus rather than in the subarachnoid space.² The ophthalmic artery may also arise as duplicate arteries of nearly equal size.³ ^, ⁴ It may also infrequently arise as a branch of the middle meningeal artery.⁵