Surgery on the rectus muscle would be straightforward but for two anatomical issues. First, the tendon of the lateral rectus muscle is both thin and long. This makes the tendon more prone to being split with a muscle hook as the muscle is being isolated and it makes passage of a suture to secure the muscle insertion more difficult. In addition to having a tendon that is longer and thinner than those of the other rectus muscles, the lateral rectus muscle typically has only a single anterior ciliary artery. These features can be helpful in confirming that the correct muscle has been isolated if the surgeon becomes disoriented during surgery. Second, the inferior oblique muscle inserts into the sclera approximately 10 mm posterior to the limbus beneath the inferior aspect of the lateral rectus muscle. Loose attachments between the fascial sheaths of these two muscles are usually present. The inferior oblique muscle is often inadvertently hooked along with the lateral rectus muscle when isolation of the lateral rectus muscle is attempted. This can occur during attempts to isolate the lateral rectus muscle from its superior or inferior border, but is more likely to occur when attempting to isolate the lateral rectus muscle from its inferior border. This complication is more likely to occur when the lateral rectus muscle has been previously recessed. The inferior oblique muscle can occasionally be isolated instead of the lateral rectus muscle when attempting to isolate the muscle from its inferior border. This is more likely to occur in eyes with previous surgery and an extensive amount of scarring. Failure to recognize that the inferior oblique muscle has been inadvertently hooked can result in a significant postoperative motility disturbance that is of a restrictive nature [22] (Chap. 25).

1.9  Anatomy of Individual Extraocular Muscles

Surgery on the inferior rectus muscle would be rather straightforward if not for the presence of a number of important surrounding structures. The midpoint of the insertion inserts an average of 6.8 mm posterior to the anterior limbus [18] (^>Fig. 1.12). The temporal border of the tendon inserts approximately 2.5 mm more posteriorly than the nasal border of the insertion [18], a fact which should be considered when recessing or resecting the muscle. When hooking the inferior rectus muscle, the surgeon should avoid passing the hook too deeply into the orbit. A vortex vein can typically be seen posteriorly near both the medial and lateral borders of the inferior rectus muscle (^>Fig. 1.19). Disturbance of a vortex vein can result in considerable bleeding, rendering continuation of surgery hazardous (Chap. 24).

The fascial sheath of the inferior rectus muscle is typically thicker than that associated with the other rectus muscles. The inferior rectus muscle is intimately associated with the inferior oblique muscle and Lockwood’s ligament through these fascial attachments. The orbital aspect of the fascial sheath of the inferior rectus muscle forms part of Lockwood’s ligament. Because of these firm attachments to Lockwood’s ligament, large recessions or resections of the inferior rectus muscle can produce unwanted retraction and advancement of the lower eyelid, respectively. Techniques to minimize these undesired changes in eyelid position are reviewed in Chap. 26. Many surgeons more generously dissect the fascial attachment associated with the inferior rectus muscle to minimize changes in eyelid position. Intrusion into a large fat pad that is closely associated with the inferior rectus muscle can occur during dissection of these fascial tissues.

The firm attachments of the inferior rectus muscle to Lockwood’s ligament and to the inferior oblique muscle, which crosses inferior to the inferior rectus muscle, limit the tendency of the inferior rectus muscle to retract into the posterior orbit when detached from the sclera. These attachments are invaluable in helping to identify a lost or traumatically detached inferior rectus muscle (Chap. 23).

Surgery on the superior rectus muscle is complicated by the presence of adjacent vascular, muscular, and tendinous structures. If a fornix conjunctival incision is planned to surgically access the muscle, it is generally placed in the superotemporal quadrant, to avoid disturbing the superior oblique tendon and its trochlea. The central portion of the tendon of the superior rectus muscle inserts approximately 7.9 mm from the anterior corneal limbus. The insertion is curved relative to the limbus. The temporal border of the tendon inserts almost 3 mm more posteriorly than the nasal border [18], a fact which should be considered when recessing or resecting the muscle. It is the only rectus extraocular muscle that inserts posterior to the ora serrata. As such, a perforation of the eye wall near the insertion of the superior rectus muscle may enter the retina. A vortex vein can usually be found posteriorly near the border of the superior rectus muscle nasally and temporally. Vortex veins are infrequently damaged during surgery on the superior rectus muscle. Compared to the inferiorly located vortex veins, they are more difficult to inadvertently hook or otherwise manipulate when attempting to isolate the superior rectus muscle because they are somewhat protected by the superior oblique tendon.

The fascial sheath surrounding the superior rectus muscle is thicker on its orbital surface. Here, there are relatively firm attachments between the sheath of the superior rectus muscle and the sheath of the levator muscle of the upper eyelid. ­These firm attachments between the superior rectus and levator muscles are in large part responsible for upper eyelid retraction and advancement following superior rectus muscle recession and resection, respectively (Chap. 26). These unwanted alterations of upper eyelid position are most likely to occur with recessions or resections of greater than 5 mm. The global aspect of the muscle sheath of the superior rectus muscle is attached to the superior oblique tendon through relatively tenuous connections (^>Fig. 1.20). If the close relationship between these two structures is not recognized, the superior oblique tendon can sometimes be inadvertently hooked, along with the superior rectus muscle tendon. If unrecognized, the superior oblique tendon may be sutured along with the superior rectus tendon and recessed or resected, producing unexpected torsional and vertical misalignment following surgery. Because the insertion of the superior rectus muscle is relatively far from the limbus, there is a tendency to pass the initial muscle hook more posteriorly when attempting to isolate the muscle insertion. This increases the risk of capturing a portion of the superior oblique tendon at the same time. We recommend that the strabismus surgeon who may be less accustom to operating on the superior rectus muscle inspect the insertion site once the muscle is isolated to make certain that the superior oblique tendon is not engaged on the hook.