Extraocular Muscles and Orbital Fat
The extraocular muscles are responsible for the movement of the eye and for synchronous movements of the eyelids. All of the extraocular muscles, except the inferior oblique muscle, originate at the orbital apex and travel anteriorly to insert onto the eye or eyelid. The 4 rectus muscles (superior, medial, lateral, and inferior) originate at the annulus of Zinn. The levator palpebrae superioris muscle arises above the annulus on the lesser wing of the sphenoid bone. The superior oblique muscle originates slightly medial to the levator muscle origin and travels anteriorly through the trochlea on the superomedial orbital rim, where it turns posterolaterally and inserts on the globe beneath the superior rectus muscle. The inferior oblique muscle originates in the anterior orbital floor lateral to the lacrimal sac and travels posterolaterally within the lower eyelid retractors to insert inferolateral to the macula.
In the anterior portion of the orbit, the rectus muscles are connected by a membrane known as the intermuscular septum. When viewed in the coronal plane, this membrane forms a ring that divides the orbital fat into the intraconal fat (central surgical space) and the extraconal fat (peripheral surgical space). These anatomic designations are helpful for describing the location of a mass on magnetic resonance imaging (MRI) or a computed tomography (CT) scan. A knowledge of these spaces helps direct the surgical dissection to the mass.
The orbit is further divided by many fine fibrous septa that unite and support the globe, optic nerve, and extraocular muscles (Fig 1-5). Accidental or surgical orbital trauma can disrupt this supporting system and contribute to globe displacement and restriction. In some cases of diplopia after a fracture, restriction of eye movement is caused by the entrapment of the orbital connective tissue rather than by the muscles themselves.
The motor innervation of the extraocular muscles arises from cranial nerves III, IV, and VI. The superior rectus and levator muscles are supplied by the superior division of CN III (oculomotor nerve). The inferior rectus, medial rectus, and inferior oblique muscles are supplied by the inferior division of CN III. The lateral rectus is supplied by CN VI (abducens nerve). The cranial nerves to the rectus muscles enter the orbit posteriorly through the superior orbital fissure and travel through the intraconal fat to enter the intraconal surface of the muscles at the junction of the posterior third and anterior two-thirds. Cranial nerve IV (trochlear nerve) crosses over the levator muscle and innervates the superior oblique on the superior surface at its posterior third. The nerve to the inferior oblique muscle travels anteriorly on the lateral aspect of the inferior rectus to enter the muscle on its posterior surface.
Figure 1-5 Cross section of the orbit at mid-orbit and at the widest extent of the extraocular muscles. Note the pink-stained fibrous tissue septa in the intraconal space.
(Modified with permission from Dutton JJ. Atlas of Clinical and Surgical Orbital Anatomy. Philadelphia: Saunders; 1994:151.)
Excerpted from BCSC 2020-2021 series: Section 10 - Glaucoma. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.