Compared to the frequency of surgery on the other extraocular muscles, surgery is performed relatively infrequently on the superior oblique. Only the tendinous portion of the superior oblique, that portion distal to the trochlea, is manipulated surgically. The tendon runs posteriorly and temporally from the trochlea, which is located in the anteromedial orbit, to insert well behind the equator of the globe. The tendon travels inferior to the superior rectus muscle, crossing under the superior rectus muscle beginning 5–10 mm posterior to the insertion of the superior rectus muscle into the sclera. Adhesions between the sheath of the superior oblique tendon and the sheath of the superior rectus muscle often must be severed to complete surgery on the superior oblique tendon. The capsule of the tendon is continuous with the episcleral space at its insertion and is continuous with Tenon’s capsule surrounds the tendon (^>Fig. 1.10). During surgery, this capsule should be disturbed as little as possible. This is especially true of surgery performed to insert a superior oblique tendon expander [23]. Disruption of the capsule surrounding the superior oblique tendon is a potential cause of secondary restrictive strabismus following superior oblique tendon expander surgery (Chap. 25).

The anterior portion of the superior oblique tendon insertion may underlie the superior rectus muscle temporally. The thin tendinous insertion is broad and the most posterior portion of the tendon lies only 8–10 mm from the sheath of the optic nerve. The thinness, width, and posterior nature of the insertion of the superior oblique tendon combine to make surgical isolation of the tendon difficult (^>Fig. 1.20). The tendon is easily split with a muscle hook, and isolation of only a portion of the superior oblique tendon can easily occur. If unrecognized, the surgical procedure may have a minimal and/or unpredictable effect on ocular alignment. While it is optimal to isolate the superior oblique tendon under direct visualization, it is often not possible to clearly visualize the more posterior fibers of the tendon during this process. After hooking the tendon, the proximal fibers can be visually inspected to assure that there is not a bundle of remaining tendinous fibers running posteriorly toward the insertion, having been excluded from the hook. Superior oblique traction testing (Chap. 8) can also be useful in confirming that the entire superior oblique tendon has been surgically disinserted. The posterior edge of the superior oblique tendon insertion lies between and slightly anterior to two vortex veins (^>Fig. 1.19). These vortex veins, particularly the temporal one, can be easily damaged during surgery on the superior oblique tendon.

Surgery on the inferior oblique muscle is generally conducted in the inferotemporal quadrant, though some recently described procedures involve manipulation of the muscle in the inferonasal quadrant [24]. The inferior oblique muscle is the only extraocular muscle that does not have a tendinous portion distally. The muscular insertion of the inferior oblique is located temporally beneath the inferior border of the lateral rectus muscle. A dual insertion is said to be present in almost 11% of inferior oblique muscles [25]. Failure to recognize a dual insertion can result in minimal and/or unpredictable effects on ocular alignment following surgery. The capsule of the inferior oblique muscle is relatively thick and fine attachments between its capsule and that of the lateral rectus muscle are generally present near the insertion of the inferior oblique muscle (^>Fig. 1.21). A large orbital fat pad in the inferotemporal quadrant of the orbit can be easily disturbed during surgery on the inferior oblique muscle. This usually occurs during the isolation and dissection of the belly of the inferior oblique muscle, and can result in intraoperative bleeding and intrusion of orbital fat into the operative field, both of which can hinder the visualization necessary to complete surgery and can result in the development of a restrictive strabismus postoperatively.

The inferior oblique muscle is usually identified surgically as it courses across the inferotemporal quadrant, approximately 15 mm from the limbus. Unlike the other extraocular muscles, it is not surgically identified in its resting position on the globe, but rather is retracted inferiorly and identified as it courses within Tenon’s capsule (^>Fig. 1.22). The muscle is isolated by passing a hook posterior to the belly of the muscle and retracting the muscle anteriorly. It is during this process, that the surgeon is most likely to encounter surrounding orbital fat and may encounter a vortex vein located in the inferotemporal quadrant near the lateral border of the inferior rectus muscle. These complications can be minimized by attention to careful surgical technique as reviewed in Chap. 11. We recommend that the surgeon directly visualizes the posterior border of the inferior oblique muscle and the nearby vortex vein prior to attempting to isolate the muscle on a hook.

The effective insertion of the inferior oblique muscle is not at the medial orbital wall where the anatomical insertion is located, but instead is at the neurovascular bundle which enters the inferior oblique muscle near the temporal border of the inferior rectus muscle [20]. The neurovascular bundle is usually not visualized or disturbed during standard surgery on the inferior oblique muscle. An exception is denervation and extirpation of the inferior oblique muscle, which requires transection of the neurovascular bundle (Chap. 11). This bundle is generally best palpated with a hemostat, a maneuver that facilitates its visual identification.

Fig. 1.22. Identification of the inferior oblique muscle (arrow) in Tenon’s fascia as it is retracted inferiorly. Note the presence of a vortex vein (asterisk) near the posterior border of the muscle