Ординатура / Офтальмология / Английские материалы / Handbook of Pediatric Strabismus and Amblyopia_Wright, Spiegel, Thompson_2006

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FIGURE 2-7A–C. Diagram of secondary actions of the medial rectus when the eye rotates up or down. These secondary actions also relate to the lateral rectus. (A) Globe rotated superiorly; now the medial rectus acts as an elevator in addition to its adduction or horizontal function. (B) In the center part of the figure, the medial rectus is a pure adductor. (C) Globe rotated down; in this position, the medial rectus acts as a depressor and an adductor.

muscles. Remember, the secondary vertical functions of the horizontal rectus muscles occur only when the eye is rotated vertically off primary position.

MEDIAL RECTUS MUSCLE

The medial rectus muscle is innervated by the lower division of the oculomotor nerve (third cranial nerve) and, in primary position, is a pure adductor. The medial rectus is uniquely diminutive. It has the shortest arc of scleral contact (6 mm) and the shortest tendon length of the rectus muscles (4 mm). The inferior oblique muscle actually has the shortest tendon (1 mm) of the extraocular muscles, but it is not a rectus muscle. (Be careful; this could be the basis of a trick question.) Of the extraocular muscles, the medial rectus inserts closest to the limbus and is therefore susceptible to insult during anterior segment surgical procedures. Inadvertent removal of the medial rectus muscle is a well-known complication of pterygium removal. The medial rectus is also unique, as it is the only rectus muscle without fascial connections to an adjacent oblique muscle. This lack of oblique muscle connection makes the medial rectus the most difficult to surgically retrieve if lost. Once disinserted, the medial rectus is free to retract completely off the globe into the orbital fat, making retrieval extremely difficult and, in some cases, almost impossible.

LATERAL RECTUS MUSCLE

The lateral rectus muscle is innervated by the sixth cranial nerve and is a pure abductor. In direct contrast to the medial rectus muscle, the lateral rectus has the longest tendon (8 mm) and the longest arc of scleral contact (10 mm) of the rectus muscles. Be careful, the “longest” cited above refers to only rectus muscles, as the superior oblique tendon has the longest arc of contact and tendon length of all the extraocular muscles. (This could be the source of another trick question.) The long arc of contact occurs because the lateral rectus muscle initially has a divergent course following the lateral wall of the orbit. Then, in the anterior orbit, it turns nasally, wrapping around the globe to its scleral insertion point (see Fig. 2-6). This temporal to nasal wrap around the globe accounts for the long arc of contact. The inferior border of the lateral rectus muscle courses above the inferior oblique insertion, and there are connective tissue bands connecting the lateral rectus muscle to the inferior oblique muscle.13 This is an important anatomic relationship, because a lost lateral rectus

Their secondary action is adduction, and it occurs because the vertical rectus muscles pull the front of the eye nasal to the visual axis. Tertiary actions are torsional, consisting of intorsion for the superior rectus muscle and extorsion for the inferior rectus muscle. These secondary and tertiary muscle actions are dependent on eye position. If the eye is abducted 23°, for example, the muscle and visual axes will be in line, and the vertical rectus muscles lose their secondary and tertiary actions, leaving only their vertical actions (Fig. 2-8B). In this position of 23° abduction, the superior rectus acts purely as an elevator, and the inferior rectus purely as a depressor. With further abduction past 23°, the secondary and tertiary actions of the vertical rectus muscles return, but they are different. The secondary action for both vertical rectus muscles becomes abduction, and the tertiary functions reverse, becoming extorsion for the superior rectus and intorsion for the inferior rectus muscle (Fig. 2-8C).

SUPERIOR RECTUS MUSCLE

The upper division of the oculomotor nerve innervates the superior rectus muscle. It is the major elevator of the eye, and its actions include supraduction (primary), adduction (secondary), and intorsion (tertiary). The superior rectus muscle overlies the superior oblique tendon and has connective tissue connections to the superior oblique tendon below and the levator palpebrae muscle above (Fig. 2-9). This anatomic relationship to the levator palpebrae is important because a large superior rectus recession can cause upper lid retraction and lid fissure widening. On the other hand, a superior rectus resection pulls the upper lid down, resulting in lid fissure narrowing. Lid fissure changes associated with superior rectus surgery can be minimized by surgically removing the fascial connections between the levator and the superior rectus muscles.

INFERIOR RECTUS MUSCLE

The inferior rectus muscle is innervated by the lower division of the oculomotor nerve and is the principal depressor of the eye. Actions of the inferior rectus muscle include infraduction (primary), adduction (secondary), and extorsion (tertiary). The inferior rectus is sandwiched between the inferior oblique below

FIGURE 2-9. Diagram of the eye and orbit from a top view looking down on the superior rectus (SR) muscle. Note that the superior rectus muscle overlies the superior oblique (SO). T, temporal; N, nasal.

and the sclera above (Fig. 2-10). The fascial connection between the inferior rectus muscle, the inferior oblique muscle, and the lower lid retractors (capsulopalpebral fascia) is termed Lockwood’s ligament (Fig. 2-11).17 These fascial connections are responsible for the eyelid changes that often occur after inferior rectus surgery. An inferior rectus recession results in lower lid retraction with lid fissure widening, and a resection causes lid advancement with lid fissure narrowing. If the inferior rectus is inadvertently disinserted or lost during surgery, these connections will hold the inferior rectus to the inferior oblique and keep it from retracting posteriorly. The surgeon who is in search of a lost inferior rectus muscle can usually find it lying between the inferior oblique and sclera.

FIGURE 2-12. Diagram of the superior oblique (SO) muscle and tendon. The functional muscle axis extends from the trochlea to the superior oblique insertion. The muscle axis is 54° nasal to the visual axis.

OBLIQUE MUSCLES

Like the vertical rectus muscles, the oblique muscles have primary, secondary and tertiary actions. In the case of the oblique muscles, this is because the functional muscle axis is approximately 50° nasal to the visual axis, and the insertion extends posterior to the equator of the eye (Figs. 2-12, 2-13). By

FIGURE 2-11. Diagram of the relationship between the inferior rectus, inferior oblique, lower lid retractors, and Lockwood’s ligament. The inferior tarsal muscle (ITM) courses from the posterior border of the tarsus toward the inferior oblique muscle. It then passes between the inferior oblique muscle and the inferior rectus muscle to insert at the capsulopalpebral head (CPH). The CPH extends posteriorly to connect the inferior oblique to the inferior rectus muscle. The capsulopalpebral fascia (CPF) is the anterior extension of the CPH and courses from the inferior oblique anteriorly to the tarsus along with the ITM. “Lockwood’s ligament” (Lockwood’s lig.) consists of these fascial attachments that connect the lower lid, inferior rectus, and inferior oblique muscles.

FIGURE 2-13. Diagram of the inferior oblique (IO) from a view from below. The inferior oblique muscle axis is 51° nasal to the visual axis.

comparing Figures 2-12 and 2-13, one can see that the oblique muscles have an almost identical functional course with both muscle axes at approximately 50°. The posterior muscle–scleral insertion gives the oblique muscles their seemingly paradoxical vertical functions, with the superior oblique being a depressor and the inferior oblique an elevator. The oblique muscles have no anterior ciliary blood supply, and they do not contribute to the anterior segment circulation. Remember that the “oblique muscles always course below the corresponding vertical rectus muscle” (Fig. 2-14).

SUPERIOR OBLIQUE MUSCLE

The primary action of the superior oblique muscle is intorsion, but it also acts as a depressor (secondary) and an abductor (tertiary). Depression and abduction occur as the back of the eye is pulled up and in toward the trochlea. The superior oblique