Topographical Anatomy of the Orbit

1.1 Introduction

1.2 Shape, Borders, Orifices, and Topographical Relations

1.3 Protective Apparatus of the Eye

1.4 Contents of the Orbit

1.5 Topography of Pathways Located Postseptally

1 Topographical Anatomy of the Orbit

A. Schmiedl

1.1 Introduction

The orbit is the bony basis for suspension and positioning of the eyeball (bulb of the eye; bulbus oculi) and its adnexa. Any soft tissue of the visual apparatus is located inside the orbit. It is characterized by much content in a very limited bony compartment, and by particularly thin bony borders toward the paranasal sinuses. It is also a thoroughfare of pathways to and from the face. Due to the presence of numerous blood vessels in this field (i.e., 10 branches of the ophthalmic artery, two large venous vessels along with their afferent vessels), significant hemorrhage can occur during operative procedures. Furthermore, five cranial nerves along with their branches, and vegetative nerve fibers are located inside the orbit, which can be threatened or compromised by surgery.

1.2 Shape, Borders, Orifices, and Topographical Relations

The orbit is located underneath the anterior cranial fossa (fossa cranii anterior, ▶ Fig. 1.1) and is shaped like a four-sided pyramid whose base opens to the face (orbital aditus, ▶ Fig. 1.2). Its axis proceeds from the pyramid basis (anterior) backward medially to the optic nerve canal at to the root of the lesser wing of the sphenoid bone (apex of the pyramid). As the apexes of both orbits are located medially, the longitudinal axis of both orbits are directed exteriorly, and the medial and lateral wall form an angle to each other of about 45° ( ▶ Fig. 1.1, ▶ Fig. 1.2). The average dimensions of the orbit ¹ demonstrate that there is not much space ( ▶ Table 1.1). The orbit has its largest extension 1 cm behind the anterior orbital rim. ² The distance between the reverse side of the eyeball and the entrance of the optic nerve canal is about 18 mm. ²

1.2.1 Bony Orbital Walls

The orbital aditus is limited by an orbital ridge (orbital margin), which is formed by three bones. The upper orbital ridge (supra-orbital margin) is constituted by the frontal bone; the lower orbital ridge (infra-orbital margin) by the zygomatic bone (laterally) and the maxilla (medially). Altogether the entire bony orbital walls consist of seven different bones ( ▶ Fig. 1.2). ^{4,​ 5,​ 6}

The major part of the orbital roof is formed by the concave vaulted orbital part of the frontal bone ( ▶ Fig. 1.1a). The posterior 1.5-cm wide flattened part of the roof is shaped by the minor wing of the sphenoid bone. Normally both bones are fairly strong and do not fracture following violent trauma to the orbit. Anteriorly in the upper part of the orbit and medially is located a small fossa for the roller cartilage (trochlea), which serves as a fulcrum for the tendon of the superior obliquus muscle (see ▶ Fig. 1.15). The large lacrimal fossa lies anteriorly and laterally in the orbit ( ▶ Fig. 1.2).

The major wing of the sphenoid bone forms the major part of the lateral orbital wall. Anteriorly the zygomatic bone and the zygomatic process of the frontal bone are also involved in forming the bony wall. The zygomatic bone thickens caudally, forms the anterior vault of the inferior orbital fissure ( ▶ Fig. 1.3), and divides the orbit from the subcutaneous adipose tissue of the cheeks. ¹ A small protuberance (Whitnall tubercle) is located about 4 to 5 mm behind the lateral orbital ridge. ⁷ The lateral palpebral ligament, the Lockwood-ligament, and the control band of the lateral rectus muscle (Check ligament) along with parts of the aponeurosis of the superior palpebral levator muscle are connected with the Whitnall-tubercle. ¹ The lateral orbital wall is anteriorly fairly thick and protects the orbit from lateral traumatic violence. ¹ The posterior part, however, is significantly thinner and can therefore serve as an approach for performing lateral orbitotomy ¹ ( ▶ Fig. 1.3). The temporalis muscle, which fills the temporal fossa underneath the zygomatic arch, is located at the posterior and lateral surface of the lateral orbital wall. Adjacent to the lateral orbital wall is the temporal squama, which forms the lateral wall of the medial cranial fossa. The distance between the lateral border of the orbit, which is suitable for osteotomy, and the border of the medial cranial fossa is only 12 to 13 mm in men, and 7 to 8 mm in women. ⁸

The wall that forms the orbital floor is very short, shaped as an equilateral triangle, and consists mainly of the 0.5-mm-thick orbital surface of the maxilla. The orbital surface of the zygomatic bone (lateral anteriorly) and the orbital process of the palatine bone (posteriorly) play minor parts in shaping the orbital floor.

The medial orbital wall is formed by the frontal process of the maxillary bone, by the lacrimal bone, by the minor wing of the sphenoid bone, and by the orbital lamina of the ethmoid bone (synonyms: lamina papyracea; lamina orbitalis). The bony wall that divides the orbit from the posterior ethmoidal cells (cellulae ethmoidales posteriores) is quite thin, only about 0.3 mm thick ( ▶ Fig. 1.4). However, the honeycomb-like structure of the pneumatized ethmoidal cells has a stabilizing effect, so that fractures occur less in the medial wall than in the slightly thicker orbital floor over the maxillary sinus. ¹ On the other hand, the lamina papyracea can easily be affected by inflammations or by tumors of sinogenic origin, and can easily be destroyed during surgical procedures in this area. ¹ The orbit is frequently affected by inflammations in the ethmoid cells. The roof of the ethmoid sinus is formed by the bone junction between the orbital lamina of the ethmoid bone and the orbital part of the frontal bone. The anterior cranial fossa along with the frontal lobe of the brain, which is covered by dura, is located above this bone junction. The medial wall exhibits anteriorly a deep cavity (lacrimal sac fossa) that contains the sacculus lacrimalis (lacrimal sac). This cavity is limited anteriorly by the infra-orbital margin and the anterior crista of the maxilla, and posteriorly by the posterior lacrimal crista of the lacrimal bone ( ▶ Fig. 1.2). The orbital apex is shaped by the sphenoid and the palatine bones.

1.2.2 Orbital Foramina

The bones of the orbital wall contain several foramina that open to the middle cranial fossa, the infratemporal fossa or the pterygopalatine fossa, or to the face and support various different pathways ( ▶ Table 1.2; ▶ Fig. 1.2). ^{4,​ 5,​ 6}

The foramen of the optic nerve canal (canalis opticus) is located in the orbital apex of the minor wing of the sphenoid bone. The optic nerve canal is separated from the superior orbital fissure by the lower root of the minor wing of the sphenoid bone. This thin bony lamella also limits the optic nerve canal laterally. ¹ The ophthalmic artery and the optic nerve pass through the optic nerve canal from the medial cranial fossa into the orbit. The optic nerve enters the orbit with an angle of 45° from the midline ( ▶ Fig. 1.1). Glioma of the optic nerve and aneurysms of the ophthalmic artery frequently lead to a dilatation of the canal. The superior orbital fissure (fissura orbitalis superior) is located beneath the optic nerve canal between the major wing, the minor wing, and the corpus of the sphenoid bone ( ▶ Fig. 1.1, ▶ Fig. 1.2). The superior orbital fissure is large medially and slim laterally. Parts of the fissure that do not contain pathways are covered with and closed by connective tissue, which can contain smooth muscle cells. The fissure is divided into three sections according to the origin of the intra-orbital muscles: (1) the medial section, which is closed; (2) the oculomotor nerve, the nasociliar nerve, and the abducens nerve pass through the intermediate section, which is framed by the outer eyeball muscles; (3) the superior ophthalmic vein, the frontal nerve, and the lacrimal nerve, along with the trochlear nerve, pass through the narrower lateral section of the fissure ( ▶ Fig. 1.5).

The inferior orbital fissure (fissura orbitalis inferior) is a cleft about 20 mm wide and is located posteriorly between major wing of the sphenoid bone and the maxilla. It maintains the connection between the orbit and the infratemporal fossa and pterygopalatine fossa. It is closed by connective tissue, which contains smooth muscle cells, except at the points of passage for branches of the inferior ophthalmic vein coming from the pterygoid plexus, infra-orbital nerve, zygomatic nerve, and smaller branches of the pterygopalatine ganglion. The inferior orbital fossa extends more anteriorly than the superior orbital fissure, and terminates 20 mm ahead of the anterior orbital ridge ¹ ( ▶ Fig. 1.5).

The posterior ethmoid foramen (foramen ethmoidale posterius) is located at the posterior upper ridge of the lamina papyracea of the ethmoid bone. This foramen is entered by the posterior ethmoid artery and vein along with the posterior ethmoid nerve, which proceed to the ethmoid cells. The anterior ethmoid foramen (foramen ethmoidale anterius) is located at the anterior upper ridge of the orbital lamina. The anterior ethmoid artery and vein along with the anterior ethmoid nerve are leaving the orbit through this foramen. Both foramina are located at the bone junction between the orbital lamina of the ethmoid bone and the orbital part of the frontal bone, approximately 24 to 25 mm dorsal to the anterior ridge of the lacrimal bone ¹ ( ▶ Fig. 1.2).

The supra-orbital margin of the orbital aditus has two variable notches or foramina: the frontal incisure/foramen, and the supra-orbital incisures/foramen, with branches of the ophthalmic nerve and supra-orbital artery passing through ( ▶ Fig. 1.2).

The zygomatico-orbital foramen, through which the zygomatic nerve enters, is located at the orbital surface of the zygomatic bone. The canal opens exteriorly into the zygomaticofacial foramen and zygomaticotemporal foramen, through which the correspondingly named sensory terminal branches of the zygomatic nerve along with similar named vessels pass to the temporal region and cheek. These pathways might be injured during lateral orbitotomy, when the temporal muscle is removed.

The posterior part of the orbital floor contains the infra-orbital sulcus, which extends anteriorly as the infra-orbital canal to the infra-orbital foramen, through which the infra-orbital nerve and the infra-orbital artery reach the face ( ▶ Fig. 1.2).

1.2.3 Topographic Relations of the Orbit

Due to its topographical position in the head, the orbit has close relations to several areas of the face ( ▶ Fig. 1.4). The anterior cranial fossa with the frontal lobe of the brain (telencephalon), which is covered by dura, is part of the orbital roof. The orbit has close relations to the medial cranial fossa, which contains the dura-covered temporal lobe of the telencephalon, via the optic nerve canal and superior orbital fissure. The inferior orbital fissure connects the orbit to the pterygopalatine and infratemporal fossae ( ▶ Fig. 1.3). The orbital floor, the orbital roof, and the medial wall separate the orbit from the paranasal sinuses with distinctly thin bony walls (caudal medially is the maxillary sinus; cranial anteriorly is the frontal sinus; medially is the ethmoid sinus) ( ▶ Fig. 1.4). However, it should be considered that the paranasal sinuses develop during youth and adolescence and reach their final extension almost in adulthood. Furthermore, the orbit has close relations to the nasal cavity (cavitas nasi) ( ▶ Fig. 1.4). Inflammations and tumors of the orbit can infiltrate and affect the neighboring regions and vice versa. Close interdisciplinary cooperation is required for considering their management.

1.3 Protective Apparatus of the Eye

1.3.1 Orbital Septum

The orbital septum (septum orbitale) ( ▶ Fig. 1.6) is a thin plate consisting of collagen and elastic connective tissue. It represents a physical barrier against pathogens and stabilizes the postseptal adipose tissue in the orbit. ¹ The septum is fixed at the upper and lower orbital ridges, surrounds the eyeball, and extends to the upper lid plate (tarsus). It inserts medially at the lacrimal bone. It joints laterally with the lateral palpebral ligament and the superior levator palpebrae muscle aponeurosis. ¹ The septum inserts at the lower lid at the lower ridge of the tarsus. It gets thinner medially, separates from the medial palpebral ligament, and extends at the back side of the smooth superior tarsal muscle behind the lacrimal sac to the lacrimal bone. Preseptally the septum is covered by the orbicularis oculi muscle and the skin.

1.3.2 Eyelids

Macro anatomy and blood supply

The eyebrows, hair-bearing elongated regions of skin that protect the eye from humidity and sweat, are located at the upper orbital rim ( ▶ Fig. 1.7). The eyelids (palpebrae) are mobile ectodermal folds that enable protection from excess light and dazzle and preserve the cornea from exsiccation, mechanical damage, and—via the opticofacial winking reflex—from foreign body impact on or into the eye. The lacrimal fluid is dispensed over the cornea with every closure of the eyelid. The eyebrow separates the upper lid from the frontal skin; the zygomatic lid sulcus separates the lower lid from the skin of the cheek. Laterally and medially both lids are connected to each other via the lateral and medial palpebral commissure. Thus both ends of the palpebral fissures (rima palpebrarum) are encircled at the inner and outer canthi. While the lateral canthus (angulus oculi lateralis) is acute-angled, the medial canthus is more roundly shaped and encircles a mucosal island, the lacrimal caruncula ( ▶ Fig. 1.7). The eyelid skin is very soft, contains only little adipose tissue, and is mobile. Due to the soft structure of the subcutaneous tissue of the eyelids, inflammations, hemorrhage, or fluid retention can lead to significant swelling, so that the palpebral fissure cannot be opened. Due to their rectangular cross-section ¹ the margins of the eyelids form an anterior limbus or margin and a posterior limbus (limbus anterior palpebrae; limbus posterior palpebrae). A number of eyelashes (cilia), which are arranged in 3 to 4 rows, originate from the margins of the eyelid between anterior and posterior limbus and spread protectively over the palpebral fissure. The external eyelid surface (facies anterior palpebrae) of both canthi transitions to the posterior surface (facies posterior palpebrae), which is covered by the conjunctiva. The conjunctiva at the reverse side of the eyelids (conjuctiva palpebrae) transitions to the eyeball conjunctiva at its fornix region (fornix conjunctiva superior and inferior). Thus, a pocket is formed, which is called conjunctival sac (saccus conjunctivalis). The conjunctiva at the palpebral fold is called the conjunctiva fornicis. The semi-oval lid plates (tarsi) are the fibrous tissue bases for the eyelids and give them form and stability. A constitutional or age-related relaxation of the orbital septum, which extends into the tarsus, can cause a prolapse of parts of intraorbital adipose tissue with consecutive visible protrusion of the upper lids and impairment of the aesthetic appearance of the eye. ¹ The lid plate of the upper lid (superior tarsus) is about 10 mm high; the lid plate of the lower lid (inferior tarsus) is about 5 mm high. Both lid plates are about 25 mm wide and 1 mm thick. They are elastic, suspended with a medial and lateral palpebral ligament on both sides of the orbital aditus ¹ ( ▶ Fig. 1.6). The medial ligament encircles the lacrimal sac.

Arterial blood supply of the eyelids comes from the area of distribution of the internal and external carotid arteries. At the orbital aditus can be found some thin branches of the lacrimal artery as well as from the lacrimal nerve. Branches of the supra-orbital artery and vein along with branches of the supra-orbital, supratrochlear, and infratrochlear nerve course medially. The vessels accomplish a vascular corona anterior to the septum, which encircles the orbital entrance ( ▶ Fig. 1.6). Branches of the lateral and medial palpebral artery become curved anteriorly of the septum to the eyelids and form the arcus palpebralis superior and inferior for the blood supply of the eyelids and conjunctival tunica ( ▶ Fig. 1.6). The anterior ciliary arteries are also involved in the blood supply of the conjunctiva. Venous drainage occurs via the superior and inferior ophthalmic veins to the cavernous sinus.

Locomotor System of the Eyelids: Lid Muscles

Two striped and two unstriped (smooth) muscles control the voluntary and involuntary eyelid movements.

Striped Muscles

The orbicularis oculi muscle is located anterior to both the orbital septum and the lid plate ( ▶ Fig. 1.8). It is innervated by the facial nerve. The orbital part of this muscle encircles the orbital margin and effects a voluntary closure of the eye. The external sheet of each eyelid contains muscular tracts of the palpebral part of the orbicularis oculi muscle (Riolan muscle). The lid closure is released by contraction of the palpebral part. It can be performed voluntarily, but in most cases it proceeds involuntarily. The lacrimal part of this muscle, on the other hand, encircles the lacrimal canaliculi and is necessary for lacrimation. Muscle fibers of the levator palpebrae superioris muscle, which are innervated by the oculomotor nerve, course from the minor wing of the sphenoid bone thoroughly through the orbit to the upper lid, where its tendon fans in two layers: one deep layer, which inserts at the upper lid plate, and the superficial layers, which course between the orbicularis muscle fibers into the skin of the lid. Paresis of the orbicularis muscle leads to an incomplete eyelid closure (lagophthalmus). Voluntary eyelid closure in these cases leads to an upward movement of the eyeball when the outer eye muscles are unimpaired—hence, the white sclera is visible (Bell phenomenon).

Unstriped Muscles

Smooth muscles are located in both lid plates, which are innervated by the sympathetic nervous system. The superior tarsal muscle courses from the lower surface of the superior levator palpebrae muscle to the upper rim of the lid plate, the inferior tarsal muscle to the lower lid plate. Both muscles, so-called “Müller eyelid lifting muscle,” lead to an extension of the lid plate during contraction. ¹

Palpebral Limbus and Palpebral Glands

The lid rim (palpebral limbus) is blunt-shaped anteriorly and sharp-edged posteriorly. Excretory ducts of large sebaceous glands (sebaceous ciliary glands [Zeis glands]) open to hair funnels of the anterior lid rim for greasing the cilia. Close to the cilia are also located the sudoriferous glands, apocrine sweat glands, (Moll glands). About 30 tubuloalveolar holocrine glands are located in the lamina propria of the upper lid, about 20 of these glands are located in the lower lid (tarsal glands; meibomian glands), which open with their own excretory ducts at the posterior lid rim. ⁴ Ectropionization of the upper lid exposes the tarsal glands as yellowish spotted lines. Their sebaceous-like secretion is extracted by contraction of the Riolan muscle for lubricating the lid rim and preventing lacrimal overflow. Diseases of the palpebral glands can present as a chalazion, which is characterized by a painless small node in the lid due to an obstruction of the excretory duct of a palpebral gland, or as a hordeolum, which is a painful inflammation of a palpebral gland.

1.3.3 Lacrimal Apparatus

Parts of the lacrimal apparatus, which consists of the lacrimal gland and the lacrimal ducts, are located in the bony orbit. The lacrimal gland is found superior to the lateral canthus in the anterior upper outer orbital quadrant above the eyeball in the frontal bone recess behind the orbital septum (lacrimal gland fossa). The lacrimal gland has dimensions of ca. 20 mm × 12 mm × 5 mm and is divided by the superior levator palpebrae muscle into a larger orbital part and a smaller palpebral part, which lies at the transition zone between conjunctiva and eyelid. ¹ However, this division is not complete, as both parts are connected by a parenchymatous bridge dorsal to the muscle. The upper part of the gland is limited anteriorly by the orbital septum and preaponeurotic adipose tissue, dorsally by the intraorbital adipose tissue, and laterally by the frontal bone. The lower part of the lacrimal gland is located underneath the aponeurosis of the superior levator palpebrae muscle in the subaponeurotic Jones space. ¹ The palpebral part is medially separated from the conjunctiva by the superior tarsal muscle. Several smaller glands (Krause and Wolfring glands) are distributed over the upper lid conjunctiva. Tear fluid reaches the superior conjunctival fornix via 1 to 5 excretory ducts in the orbital part, and 6 to 8 smaller excretory ducts in the palpebral part. The tear fluid is distributed evenly over the entire cornea by blinking; it runs from temporal to nasal into the medial canthus and collects in the lacrimal lacus, which is located in the inferior conjunctival fornix. The openings of the lacrimal canaliculi are prominent to the lacrimal lacus as upper and lower, ca. 0.25 mm and 0.3 mm, roundish, oval or slitlike lacrimal points. ⁵ These lacrimal points are located on a small peak at the medial inner surface of the upper and lower lid rims (lacrimal papilla), ⁴ which are visible when the eyelid is elevated. The tear fluid is cleared from the lacrimal points in the upper and lower lids into the lacrimal sac via upper and lower hooklike curved lacrimal canaliculI, ca. 10 mm long ( ▶ Fig. 1.7). The lacrimal canaliculi course 2 mm vertically and then, after widening (ampulla of the lacrimal canalicula), ca. 8 mm horizontally. The vertical part is surrounded by fibers of the orbicular oculi muscle (pars lacrimalis), the contraction of which facilitates the drainage. In about 70% of cases, a junction occurs of the two canaliculi, which course parallel to the lid rim, into a common canal 1 to 2 mm long, which then terminates in the lacrimal sac, which is limited by both medial crura of the palpebral ligament. A cupula of the lacrimal sac surpasses the upper lacrimal canaliculus by about 3 mm (fornix sacculi lacrimalis. ⁹

The lacrimal sac is about 12 mm × 6 mm × 5 mm³ and lies—covered by the periorbita—in a depression of the medial bony orbital wall (lacrimal sac fossa), which is formed by the posterior lacrimal crista of the lacrimal bone and the anterior lacrimal crista of the maxillary bone ( ▶ Fig. 1.2). The bone in the lacrimal sac fossa region is significantly thin, so that it is reasonable to perform endonasal lacrimal sac surgery (dacryocystorhinostomy) medially to the sac. Erectile tissue, which is suitable for tear drainage, encircles the lacrimal sac. In case of emotions or foreign bodies penetrating the conjunctival sac, filling of the blood vessels in the cavernous tissue occurs, which leads to congestion of the nasolacrimal duct with subsequent tear drainage over the cheeks.

The ca. 15 mm long and 5 to 6 mm wide nasolacrimal duct is a bony canal that is coated by pseudostratified columnar epithelium with goblet cells. The walls of the canal comprise the maxillary bone, the lacrimal bone, and the inferior nasal concha. ⁴ This canal courses caudally, laterally and posteriorly and terminates in the inferior nasal meatus. Thus, it projects on a line connecting the medial canthus and the first premolar tooth of the upper jaw. Probing of the nasolacrimal duct is performed in this direction. A reflux of the tear fluid is prevented by multiple mucosal folds with valvelike function. The first valve is the Rosenmüller valve, which is located at the entrance of the lacrimal sac to the common canaliculus; the second valve is the Krause valve, which is located between lacrimal sac and the entrance of the nasolacrimal duct; and finally the Hasner valve develops at the opening of the nasolacrimal duct into the inferior nasal meatus. The Hasner valve has a check function, preventing air penetration into the duct during sneezing. ³ Capillary forces in the canaliculi support tear drainage. Furthermore, a widening of the vertical parts with intensification of a suction effect is caused by lid opening. Every day about 500 mL of tear fluid is produced, which is stimulated by parasympathetic nerve fibers originating in the facial nerve. Lack of lid closure leads to a drying of the cornea, as the tear fluid cannot be distributed equally over the cornea.

In conclusion the tear fluid constitutes a precorneal fluid layer that consists of (1) an outer “oily” mucinous layer, which is produced by the meibomian glands; (2) a waterlike intermediate layer, which is produced by the lacrimal glands; and (3) an inner mucinous layer, produced by the goblet cells in the conjunctival tunica.

1.4 Contents of the Orbit

1.4.1 Classification

According to topographical and functional criteria, the orbit can be divided into several regions, which have also significant relevance for clinical purposes ( ▶ Fig. 1.9).

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