Orbital Bony Anatomy

The bony orbit not only lies close to other compartments in the facial skeleton but also contains several foramina through which critical neurovascular structures pass¹ (Fig. 50-1). The orbital roof is formed by the frontal bone and lesser wing of sphenoid; its superior surface is the floor of the anterior cranial fossa. The roof also lies below the frontal sinus. The orbital plate of the maxillary bone forms most of the orbital floor, in addition to the roof of the maxillary sinus; the palatine and zygomatic bones also contribute to the floor. Medially, the medial wall, also known as the lamina papyracea, forms a thin wall separating the orbit from the ethmoid sinus anteriorly and the sphenoid sinus more posteriorly, a structure through which most endoscopic approaches to the orbit can be directed. It is comprised of four bones: the maxillary, lacrimal, ethmoid, and lesser wing of sphenoid. The medial wall of the orbit also contains the foramina through which the anterior and posterior ethmoidal arteries pass. The lateral orbital wall is covered externally by the temporalis muscle and formed by the frontosphenoid process of the zygomatic bone, in addition to the greater wing of the sphenoid. The apex of the orbit is directed in a medial oblique direction and contains three critical foramina: optic canal, superior orbital fissure, and inferior orbital fissure. The optic canal bridges the intracranial space and orbit inferomedial to the anterior clinoid process while it is bordered laterally by the lesser wing of the sphenoid. The superior orbital fissure is bordered laterally by the greater wing of the sphenoid.

FIGURE 50-1 Integrity of the bony orbit and the size and shape of its fissures and foramina can be defined by plain skull radiographs and CT bone windows. The frontal, ethmoidal, and maxillary sinuses are shown diagrammatically. Attention is directed to one of the two roots of the lesser wing of the sphenoid, which lies beneath the anterior clinoid and forms the lateral margin of the optic canal in the medial margin of the superior orbital fissure.

Muscle Cone and Annulus of Zinn

The annulus of Zinn serves as the origin of six of the seven extraocular muscles (Fig. 50-2). Superiorly, the superior rectus arises from the annulus, which at this point is fused with the dura of the optic nerve. The levator palpebrae arises medial and superior to the superior rectus muscle but remains intimately associated with it. More medial and inferior to this are the origins of the medial rectus and superior oblique muscles. Although it is firmly fused to the optic nerve dorsally, the annulus of Zinn loops widely around the nerve laterally and inferiorly, giving rise to the lateral rectus muscle, in addition to the inferior rectus. The space between the insertion sites of these two muscles is known as the oculomotor foramen. Based on this arrangement, there are evident portals of entry of neurovascular structures into the orbit: the optic canal and the superior orbital fissure.

FIGURE 50-2 The anulus of Zinn is a fibrous band giving rise to the origins of six of the seven extraocular muscles. This fibrous tissue is in continuity with the dural sheath of the optic nerve. The two heads of the lateral rectus loop around that portion of the superior orbital fissure known as the oculomotor foramen.

Optic Nerve and Orbital Nerves

The optic nerve, throughout its entire course from the chiasm to sclera, is covered with a pial membrane (providing vascular supply) and associated subarachnoid space (Fig. 50-3). As the optic nerve enters the optic canal, the intracranial dura splits, with the outer leaf forming the orbital periosteum and the inner leaf remaining with the optic nerve. The superior orbital fissure contains the remaining cranial nerves that enter the orbital compartment. The trochlear nerve and the frontal and lacrimal branches of the trigeminal nerve enter through the superior orbital fissure above the extraocular muscles and annulus of Zinn. The remaining nerves—the oculomotor nerve (superior and inferior divisions) and the abducens nerve—pass through the superior orbital fissure and annulus of Zinn.

FIGURE 50-3 Partial obliteration of the subarachnoid space of the optic nerve at the anulus of Zinn is shown, and the medial origin of the levator muscle is evident. The ophthalmic artery (OA) enters the orbit through the optic canal. The trochlear (CN IV), frontalis (CN V), and lacrimal (LA) nerves and the ophthalmic vein (OV) enter through the superior orbital fissure and thus lie within the periorbita but outside the muscle cone, whereas the superior division of the oculomotor nerve (Sup. CN III), the abducens nerve (CN VI), the nasociliary nerve (NN), and the inferior division of the oculomotor nerve (Inf. CN III) enter the muscle cone through the oculomotor foramen and lie within the muscle cone.

Because of this arrangement, it is evident that the optic nerve can be approached directly through the medial compartment, between the medial rectus and the levator muscles, without fear of injury to the nerve supply of any extraocular muscle (Fig. 50-4).

FIGURE 50-4 The nerve supply to the extraocular muscles is shown entering through the oculomotor foramen. A medial superior approach to the optic nerve between the lateral and the medial rectus muscles provides direct access, with minimal chance of injury to this nerve supply to the extraocular muscles.