Functional and Surgical Anatomy of the Orbit
Nicholas Ramey, MD
The globe, eyelids, and lacrimal drainage structures are discussed elsewhere.
Bones — the orbit is composed of the frontal, sphenoid, maxillary, lacrimal, palatine, ethmoid, and zygomatic bones (Figure 4.1). The general shape of the bony orbit is that of a pear with its widest portion located just posterior to its opening anteriorly and tapering posteriorly. It is composed of four walls meeting at the orbital apex.
Orbital roof (Figure 4.1A) — composed of the frontal and sphenoid (lesser wing) bones. The orbital roof makes up the floor of the anterior cranial fossa. Anteriorly, the roof is bordered by a variably sized air-filled frontal sinus cavity. It measures 45 to 50 mm from supraorbital notch to optic canal.
Supraorbital notch (foramen in 25%-50%) transmits the supraorbital nerve, and it is found along the medial one-third of the superior orbital rim about 25 to 27 mm from the midline.
Lacrimal fossa is a bony depression/concavity in the superolateral anterior orbit, just posterior to the orbital rim.
Orbital floor (Figure 4.1B) — horizontal wall composed of the maxillary, zygomatic, and palatine bones. It extends from the maxillary-ethmoid buttress medially to the inferior orbital fissure (IOF) laterally, and from the inferior orbital rim anteriorly to the posterior wall of the maxillary sinus posteriorly. It measures 35 and 40 mm in the anterior-posterior dimension.
Infraorbital canal runs anteroposteriorly in the central or centrolateral aspect of the floor, transmitting the infraorbital nerve.
IOF joins the superior orbital fissure (SOF) at the apex and is contiguous with the intracranial foramen rotundum.
Pterygopalatine fossa occupies the space posterior to the maxillary sinus. It sits lateral to the nasal cavity, communicating with it via the sphenopalatine foramen.
Medial wall (Figure 4.1C) — vertical wall composed of the maxillary, lacrimal, ethmoid, and sphenoid bones. The wall averages 42 mm in the anterior-posterior dimension, measured from the optic canal to the anterior lacrimal crest.
FIGURE 4.1. Bony anatomy of the orbit. A. Orbital roof. B. Orbital floor.
FIGURE 4.1. (continued) C. Medial orbital wall. D, Lateral orbital wall.
FIGURE 4.1. (continued) E, Orbital apex.
The lacrimal sac fossa is bounded anteriorly by the anterior lacrimal crest. The maxillary bone makes up this portion of the orbital rim, as well as most of the lacrimal sac fossa; the lacrimal bone makes up the remaining posterior portion of the fossa, including the posterior lacrimal crest. The anterior lacrimal crest is the medial continuation of the inferior orbital rim, and the posterior lacrimal crest is the inferior continuation of the superior orbital rim.
The lamina papyracea is formed by the ethmoid bone, extending posteriorly from the posterior lacrimal crest. At the apex, a short span of the sphenoid body forms the posterior portion of the medial wall as it approaches the optic canal.
The anterior and posterior ethmoidal foramina lie along the frontoethmoidal suture line at 22 and 33 mm posterior to the anterior lacrimal crest, respectively. The optic canal is situated 6 to 8 mm posterior to the posterior ethmoidal foramen.
Lateral wall (Figure 4.1D) — composed of the zygomatic and sphenoid (greater wing) bones. Bounded superiorly and posteriorly by the SOF and inferiorly by the IOF. Measures 47 mm from orbital rim to the SOF. The zygomaticosphenoid suture, at 10 mm posterior to the rim, is the thinnest part of the wall. The sphenoid bone transitions from thinner compact bone anteriorly to thicker cancellous bone posteriorly, beginning at 20 mm posterior to the rim.
The zygomaticotemporal and zygomaticofacial foramina perforate the wall laterally and inferiorly, several millimeters posterior to the rim.
The cranio-orbital foramen is located along the frontosphenoid suture 30 mm posterior to the rim.
Orbital apex (Figure 4.1E) — the sphenoid bone constitutes the majority of the orbital apex.
Medially, the optic foramen and canal are formed by two bony struts connecting the lesser wing to the body of the sphenoid bone. The optic canal measures 5 to 10 mm in length, with a variable diameter of 5 to 10 mm.
The SOF is a gap between the greater and lesser sphenoid wings, situated lateral and inferior to the optic foramen.
The IOF is situated at the posterior third of the orbital floor and separates the greater sphenoid wing of the lateral orbital wall from the maxillary and palatine bones of the orbital floor.
Soft tissues
Annulus of Zinn (Figure 4.2) — tendinous ring, contiguous with periorbita of orbital apex. It encircles the optic foramen and separates the SOF into extra- and intraconal portions. The ring also separates the optic foramen from the posterior ethmoidal foramen along the medial orbital wall. The four rectus muscles originate from their respective positions on the ring.
FIGURE 4.2. Annulus of Zinn and associated neurovascular structures.
Tenon’s capsule — envelope of fibrous connective tissue surrounding the globe. Anteriorly, adherent to the episclera about 2 mm posterior to the limbus. Moving posteriorly, Tenon’s capsule becomes loosely adherent to the episclera to allow smooth sliding of the globe with movements. Extraocular muscles pierce the capsule posterior to the globe’s equator on their way to their insertions.
Extraocular muscles
Recti — average about 41 mm long, 8 mm wide, and 3 to 4 mm thick at midpoint. Each rectus muscle runs roughly parallel with its associated orbital wall. At the apex, the muscle origins are fused together at the fibrous annulus of Zinn and are closely associated with each orbital wall. At the level of the rectus insertions, the distance between each muscle and the adjacent wall reaches 7 to 8 mm.
Levator palpebrae superioris originates from the periorbita just superior to the annulus of Zinn. It fans out horizontally as it traverses the superior orbit just superior to the superior rectus. The muscle transitions to aponeurosis just anterior to Whitnall’s ligament, then turns inferiorly to insert on the anterior surface of the tarsal plate.
Superior oblique muscle originates from the periorbita superomedial to the annulus of Zinn. It traverses the superomedial orbit and transitions to tendon as it approaches the trochlea. The trochlea is a cartilaginous saddle-shaped structure adherent to the anterior superomedial periorbita. It redirects the posterior pulling vector of the muscle to infraduction, adduction, and incyclotorsion of the globe.
Inferior oblique muscle originates on the maxillary bone, just lateral to the nasolacrimal duct entrance, about 1 mm posterior to the orbital rim. It pierces Tenon’s capsule within a few millimeters of its origin, coursing posterolaterally for a total length of 37 mm. As it travels inferior to the inferior rectus muscle, the two muscles’ fascial sheaths fuse to form the central part of Lockwood’s suspensory ligament.
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