CHAPTER 3
Anatomy of the Extraocular Muscles
Origin, Course, Insertion, Innervation, and Action of the
Extraocular Muscles
There are 7 extraocular muscles (EOMs) in the human eye: the 4 rectus muscles (lateral, medial, superior, and inferior), the 2 oblique muscles, and the levator palpebrae superioris muscle. Figure 3-1 shows an anterior view of the EOMs and their relationships to one another. Cranial nerve (CN) VI (abducens) innervates the lateral rectus muscle; CN IV (trochlear), the superior oblique muscle; and CN III (oculomotor), the levator palpebrae, superior rectus, medial rectus, inferior rectus, and inferior oblique muscles. CN III has an upper and a lower division: the upper division supplies the levator palpebrae and superior rectus muscles; the lower division supplies the medial rectus, inferior rectus, and inferior oblique muscles. The parasympathetic innervation of the sphincter pupillae and ciliary muscle travels with the branch of the lower division of CN III that supplies the inferior oblique muscle. BCSC Section 5, Neuro-Ophthalmology, discusses the ocular motor nerves in more detail, and Section 2, Fundamentals and Principles of Ophthalmology, extensively illustrates the anatomical structures mentioned in this chapter.
Figure 3-1 Extraocular muscles, frontal composite view, left eye. (Reproduced with permission from Dutton JJ. Atlas of Clinical and
Surgical Orbital Anatomy. Philadelphia: Saunders; 1994:23.)
When the eye is directed straight ahead and the head is also straight, the eye is said to be in primary
position. The primary action of a muscle is its major effect on the position of the eye when the muscle contracts while the eye is in primary position. The secondary and tertiary actions of a muscle are the additional effects on the position of the eye in primary position (see also Chapter 5 and Table 5-1). The globe usually can be moved approximately 50° in each direction from primary position. Under normal viewing circumstances, however, the eyes move only about 15°–20° from primary position before head movement occurs.
Table 3-1 summarizes the characteristics of the EOMs.
Table 3-1
Horizontal Rectus Muscles
The horizontal rectus muscles are the medial and lateral rectus muscles. Both arise from the annulus of Zinn. The medial rectus muscle courses along the medial orbital wall. The proximity of the medial rectus muscle to the medial orbital wall means that the medial rectus can be injured during ethmoid sinus surgery. The lateral rectus muscle courses along the lateral orbital wall.
Vertical Rectus Muscles
The vertical rectus muscles are the superior and inferior rectus muscles. The superior rectus muscle originates from the annulus of Zinn and courses anteriorly, upward over the eyeball, and laterally, forming an angle of 23° with the visual axis or the midplane of the eye in primary position (Fig 3-2; see also Chapter 5, Fig 5-4). The inferior rectus muscle also arises from the annulus of Zinn, and it then courses anteriorly, downward, and laterally along the floor of the orbit, forming an angle of 23° with the visual axis or midplane of the eye in primary position (see Chapter 5, Fig 5-5).
Figure 3-2 The extrinsic muscles of the right eyeball in primary position, seen from above. Note that only the origin and insertion of the inferior oblique muscle are visible in this view. (Modified with permission from Yanoff M, Duker J, eds. Ophthalmology. 2nd
ed. London: Mosby; 2004:549.)
Oblique Muscles
The superior oblique muscle originates from the orbital apex, above the annulus of Zinn, and passes anteriorly and upward along the superomedial wall of the orbit. The muscle becomes tendinous before passing through the trochlea, a cartilaginous saddle attached to the frontal bone in the superior nasal orbit. The combination of the trochlea and the superior oblique tendon is known as the tendon– trochlea complex. A bursalike cleft separates the trochlea from the loose fibrovascular sheath
surrounding the tendon (Fig 3-3). The discrete fibers of the tendon telescope as they move through the trochlea, the central fibers moving farther than the peripheral ones. The function of the trochlea is to redirect the tendon inferiorly, posteriorly, and laterally, with the tendon forming an angle of 51° with the visual axis or midplane of the eye in primary position (see Chapter 5, Fig 5-6). The tendon penetrates the Tenon capsule 2 mm nasally and 5 mm posteriorly to the nasal insertion of the superior rectus muscle. Passing under the superior rectus muscle, the tendon inserts posterior to the equator in the superotemporal quadrant of the eyeball, almost or entirely laterally to the midvertical plane or center of rotation.
Figure 3-3 Components of the trochlea. (Modified with permission from Helveston EM, Merriam WW, Ellis FD, et al. The trochlea: a study of the anatomy and physiology. Ophthalmology. 1982;89:124–133.)
The inferior oblique muscle originates from the periosteum of the maxillary bone, just posterior to the orbital rim and lateral to the orifice of the lacrimal fossa. It courses laterally, superiorly, and posteriorly, going inferior to the inferior rectus muscle and inserting under the lateral rectus muscle in the posterolateral portion of the globe, in the area of the macula. The inferior oblique muscle forms an angle of 51° with the visual axis or midplane of the eye in primary position (see Chapter 5, Fig 5-7). A stiff neurofibrovascular bundle (NFVB) containing the nerve to the inferior oblique runs anteriorly, along the lateral border of the inferior rectus muscle, to the myoneural junction. Most inferior oblique muscles have a single belly, but approximately 10% have 2 bellies; in rare cases, there are 3.
DeAngelis DD, Kraft SP. The double-bellied inferior oblique muscle: clinical correlates. J AAPOS. 2001;5(2):76–81.
Stager DR. Costenbader lecture. Anatomy and surgery of the inferior oblique muscle: recent findings. J AAPOS.