Residency · Residency · Oral Maxillofacial Surgery

Anatomy of the Orbit and Periorbital Region

Overview

The orbit is a pyramidal bony cavity that houses the globe, extraocular muscles, cranial nerves, blood vessels, and orbital fat. OMFS surgeons must understand orbital anatomy thoroughly for managing orbital floor fractures, zygomaticomaxillary complex fractures, Le Fort II and III injuries, orbital tumors, and decompression procedures. The orbit lies at the junction of the cranial and facial skeleton, making it a central structure in craniofacial surgery.

Bony Anatomy

Orbital Walls

Roof (Superior Wall)

The orbital roof is formed by the orbital plate of the frontal bone and the lesser wing of the sphenoid. It contains the fossa for the lacrimal gland in its anterolateral aspect and the trochlear fossa anteromedially. The bone here is thin, and frontal sinus pneumatization extends into the roof. Fractures of the orbital roof may cause CSF leak, pneumocephalus, or frontal lobe injury.

Floor (Inferior Wall)

The orbital floor is formed by the orbital plate of the maxilla, the zygomatic bone, and the orbital process of the palatine bone. It is thinnest in the posteromedial zone, measuring approximately 0.5 mm. The infraorbital groove and canal traverse the floor, transmitting the infraorbital nerve and artery. The floor is the most commonly fractured orbital wall due to blowout injuries, with fractures typically occurring medial and posterior to the infraorbital canal.

Medial Wall

The medial wall is formed by the ethmoid bone (lamina papyracea), lacrimal bone, maxillary frontal process, and lesser wing of the sphenoid. The lamina papyracea is paper-thin at only 0.2-0.4 mm and is frequently involved in orbital fractures. The medial wall contains the anterior and posterior ethmoidal foramina, which transmit the ethmoidal arteries and nerves. These foramina follow the "24-12-6 rule": the anterior ethmoidal foramen lies approximately 24 mm posterior to the anterior lacrimal crest, the posterior ethmoidal foramen is approximately 12 mm posterior to the anterior ethmoidal foramen, and the optic canal is approximately 6 mm posterior to the posterior ethmoidal foramen.

Lateral Wall

The lateral wall is formed by the zygomatic bone and the greater wing of the sphenoid. It is the thickest and strongest of the four orbital walls and separates the orbit from the temporal fossa anteriorly and the middle cranial fossa posteriorly.

Orbital Margins

The superior orbital margin is formed by the frontal bone and contains the supraorbital notch or foramen. The inferior margin is formed by the maxilla and zygomatic bone. The medial margin is composed of the frontal bone, lacrimal bone, and the frontal process of the maxilla. The lateral margin is formed by the zygomatic bone and the zygomatic process of the frontal bone.

Orbital Fissures and Foramina

Superior Orbital Fissure

The superior orbital fissure lies between the greater and lesser wings of the sphenoid. It transmits cranial nerves III, IV, V1 (specifically the frontal, lacrimal, and nasociliary branches), and VI, along with the superior ophthalmic vein and sympathetic fibers. Superior orbital fissure syndrome involves all of these structures and presents with ophthalmoplegia, ptosis, a fixed dilated pupil, and forehead numbness.

Inferior Orbital Fissure

The inferior orbital fissure lies between the greater wing of the sphenoid and the maxilla/zygomatic bone. It transmits the infraorbital nerve and artery (V2), the zygomatic nerve, the inferior ophthalmic vein, and branches to the pterygopalatine ganglion. It communicates with the infratemporal and pterygopalatine fossae.

Optic Canal

The optic canal passes through the lesser wing of the sphenoid and transmits the optic nerve (CN II) and the ophthalmic artery. It is 5-12 mm in length and lies approximately 40-45 mm from the anterior orbital rim. Orbital apex syndrome combines the features of superior orbital fissure syndrome with optic nerve involvement, resulting in loss of vision.

Orbital Contents

Globe

The globe measures approximately 24 mm in diameter and is supported by Tenon capsule, a fascial sheath. The normal axial globe position is approximately 2 mm posterior to a line connecting the lateral orbital rim to the medial orbital rim.

Extraocular Muscles

Four rectus muscles (superior, inferior, medial, and lateral) originate from the annulus of Zinn, or common tendinous ring, at the orbital apex. Two oblique muscles complement these: the superior oblique (which acts through the trochlea) and the inferior oblique (which originates from the orbital floor near the lacrimal fossa). The inferior rectus is the muscle most commonly entrapped in orbital floor fractures.

Innervation of Extraocular Muscles

The oculomotor nerve (CN III) innervates the superior, inferior, and medial rectus muscles, as well as the inferior oblique and the levator palpebrae superioris. The trochlear nerve (CN IV) innervates the superior oblique, and the abducens nerve (CN VI) innervates the lateral rectus. The classic mnemonic is LR6-SO4-All the rest CN III.

Orbital Fat

Orbital fat is divided into intraconal (within the muscle cone) and extraconal compartments. Herniation of orbital fat into the maxillary sinus is a hallmark CT finding of a blowout fracture. Orbital volume changes of as little as 1-2 mL can produce clinically noticeable enophthalmos.

Lacrimal System

The lacrimal gland is located in the lacrimal fossa of the frontal bone in the superolateral orbit. The lacrimal drainage pathway consists of the puncta, canaliculi, lacrimal sac (located in the lacrimal fossa of the medial wall), and the nasolacrimal duct, which drains into the inferior meatus of the nose. The lacrimal system is at risk during naso-orbito-ethmoid fracture repair and medial wall fracture management.

Periorbital Soft Tissue Anatomy

Eyelid Layers (Anterior to Posterior)

The eyelid is composed of eight distinct layers from anterior to posterior: skin (the thinnest skin in the body), orbicularis oculi muscle (innervated by CN VII), orbital septum (continuous with the periosteum at the orbital rim), orbital fat pads, levator palpebrae superioris aponeurosis in the upper lid or capsulopalpebral fascia in the lower lid, Muller muscle (a smooth muscle with sympathetic innervation), the tarsus (a dense connective tissue plate), and conjunctiva.

Orbital Septum

The orbital septum is a fibrous membrane that arises from the arcus marginalis, the periosteal attachment at the orbital rim. It serves as the barrier between the preseptal space and the orbit proper. This distinction is clinically critical: preseptal cellulitis, an infection anterior to the septum, is relatively benign, whereas orbital (postseptal) cellulitis, an infection posterior to the septum, is an ophthalmic emergency carrying risks of vision loss and cavernous sinus thrombosis.

Medial and Lateral Canthal Tendons

The medial canthal tendon attaches to the anterior and posterior lacrimal crests of the medial orbital wall via three limbs (anterior, posterior, and superior). The posterior limb is the strongest attachment and is responsible for maintaining the position of the medial canthus against the globe. Telecanthus, an increased intercanthal distance resulting from MCT avulsion or displacement, is a key finding in naso-orbito-ethmoid fractures. The normal intercanthal distance is approximately 30-35 mm, roughly equal to the width of one eye.

Surgical Approaches to the Orbit

Subciliary Approach

The subciliary approach uses an incision placed 2-3 mm below the lash line of the lower eyelid. Dissection proceeds through skin, orbicularis oculi, and the orbital septum to reach the infraorbital rim. This approach offers direct access to the orbital floor and infraorbital rim with a generally good cosmetic result, though it carries a 1-20% risk of ectropion, as well as risks of scleral show and lower lid retraction.

Subtarsal (Mid-Lid) Approach

The subtarsal approach places the incision in a natural skin crease 5-7 mm below the lash line. It carries a lower risk of ectropion compared with the subciliary approach and provides adequate access, though some patients may develop a visible scar.

Transconjunctival Approach

The transconjunctival approach uses an incision through the conjunctiva, either in a preseptal or retroseptal plane. It leaves no external scar and carries a low risk of ectropion. It can be combined with a lateral canthotomy for increased exposure. The preseptal approach dissects anterior to the orbital septum and keeps orbital fat contained but is more technically challenging. The retroseptal approach dissects posterior to the septum and is technically easier but may result in orbital fat obscuring the field. This is the preferred approach for orbital floor fractures at many centers.

Transcaruncular Approach

The transcaruncular approach provides access to the medial orbital wall through the caruncle and plica semilunaris. It allows repair of medial wall fractures without an external incision. Dissection proceeds posterior to the Horner muscle and lacrimal sac.

Coronal (Bicoronal) Approach

The coronal approach provides wide exposure of the upper and middle face, including the orbital roof, lateral wall, and zygomatic arch. It is used for frontal sinus fractures, orbital roof fractures, naso-orbito-ethmoid fractures, and extensive craniofacial procedures.

Existing Laceration

Traumatic lacerations in the periorbital region can be used for surgical access when they are appropriately located.

Orbital Floor Fractures

Mechanism

Two theories explain orbital floor blowout fractures. The hydraulic theory proposes that a direct blow to the globe transmits pressure to the weakest wall (the floor or medial wall). The buckling theory proposes that force transmitted to the infraorbital rim causes the thin orbital floor to buckle. Most fractures occur posteromedial to the infraorbital canal.

Indications for Repair

Surgical repair is indicated for enophthalmos greater than 2 mm (or when predicted based on CT volumetric analysis), diplopia with a positive forced duction test indicating muscle or soft tissue entrapment, or a large fracture involving more than 50% of the orbital floor on CT. A special scenario is the "white-eyed" blowout fracture in children: this trapdoor-type fracture entraps muscle and triggers the oculocardiac reflex (bradycardia, nausea), making it a surgical emergency.

Implant Materials

Titanium mesh is the most commonly used material; it is rigid, customizable, and radiopaque. Porous polyethylene (Medpor) allows fibrovascular ingrowth and good tissue integration but is difficult to remove once incorporated. Resorbable plates made of polylactic or polyglycolic acid are available but should be avoided in large defects. Autogenous bone (split calvarial bone or iliac crest) remains the gold standard for biological compatibility, though it carries donor site morbidity. Patient-specific implants designed from CT data provide precise anatomical fit.

Timing of Repair

Repair is traditionally performed 1-2 weeks after injury to allow swelling to resolve. The critical exception is pediatric trapdoor fractures with muscle entrapment, which require urgent repair within 24-48 hours to prevent ischemic muscle necrosis.

<image>Anterior view of the bony orbit with the four walls labeled (roof, floor, medial wall, lateral wall) and their constituent bones color-coded. The superior orbital fissure, inferior orbital fissure, and optic canal are shown at the orbital apex. The infraorbital groove on the orbital floor and the location of the anterior and posterior ethmoidal foramina on the medial wall are marked with distances from the anterior lacrimal crest (24-12-6 rule).</image>

<image>Sagittal cross-section through the lower eyelid and orbital floor showing the layered anatomy: skin, orbicularis oculi, orbital septum, orbital fat, capsulopalpebral fascia, tarsus, and conjunctiva. The transconjunctival incision plane (retroseptal) is indicated with a dashed line, and the infraorbital rim and orbital floor with a displaced fracture fragment and herniated orbital fat are depicted below.</image>

<image>Coronal CT scan reconstruction showing a unilateral orbital floor blowout fracture with herniation of orbital contents (inferior rectus muscle and orbital fat) into the maxillary sinus. The contralateral normal orbit is shown for comparison, with orbital volume measurements annotated. The infraorbital nerve within the infraorbital canal is labeled on both sides.</image>

Clinical Pearls

The posteromedial orbital floor is the thinnest area and the most common site of blowout fractures. The 24-12-6 rule is essential for navigating the medial orbital wall safely: the anterior ethmoidal foramen sits 24 mm from the anterior lacrimal crest, the posterior ethmoidal foramen sits 12 mm further posterior, and the optic canal lies 6 mm beyond that. Forced duction testing under anesthesia is the gold standard for diagnosing mechanical restriction of eye movement from entrapment. In children, a "white-eyed" blowout fracture with minimal swelling but nausea, vomiting, and restricted upgaze represents a surgical emergency due to trapdoor muscle entrapment. Enophthalmos may not be apparent acutely because of periorbital edema; CT-based volume assessment showing a volume increase greater than 1.5 mL predicts late enophthalmos. The transconjunctival approach has the lowest rate of ectropion and is preferred for isolated floor fractures. A complete ophthalmic examination (visual acuity, pupil reactivity, extraocular movements, forced duction) must always be performed before any orbital repair. Retrobulbar hemorrhage is a surgical emergency requiring immediate lateral canthotomy and inferior cantholysis to decompress the orbit and preserve vision.

References

• Ellis E III, Zide MF. Surgical Approaches to the Facial Skeleton. 2nd ed. Lippincott Williams & Wilkins; 2006.
• Burnstine MA. Clinical recommendations for repair of isolated orbital floor fractures. Ophthalmology. 2002;109(7):1207-1210.
• Bansagi ZC, Meyer DR. Internal orbital fractures in the pediatric age group: characterization and management. Ophthalmology. 2000;107(5):829-836.
• Turvey TA, Golden BA. Orbital anatomy for the surgeon. Oral Maxillofac Surg Clin North Am. 2012;24(4):525-536.
• Boyette JR, Pemberton JD, Bonilla-Velez J. Management of orbital fractures: challenges and solutions. Clin Ophthalmol. 2015;9:2127-2137.

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