Transposition of an extraocular muscle shifts its insertion to change its mode of action. Transposition procedures are usually used to treat deviations resulting from cranial nerve palsies but can also be useful to treat a secondary deviation along with a larger primary deviation during unilateral strabismus surgery. The effect of a transposition may be enhanced with the use of augmentation sutures, which shift the muscle belly (and pulley) concurrently with the insertional shift. Augmentation sutures may also decrease an unwanted torsional effect from insertional shift. Resection of a muscle prior to transposition is another method to enhance the effect of transposition.
Split muscle transposition procedures reduce the risk of anterior segment ischemia by allowing half the insertion of a rectus muscle and its ciliary vasculature to remain undisturbed, but split muscle procedures may lead to excessive fibrosis or reduction of effect by scar migration back toward the original insertion. Single muscle transpositions are reported to produce results comparable to double balanced transpositions of antagonist muscles, with variable risk of postoperative torsion. Concurrent use of botulinum toxin injected into a fibrotic muscle antagonist to the palsied muscle is another method to enhance transposition surgery while minimizing the risk of anterior segment ischemia.
Transposition of an eye muscle is defined as a shifting of its insertional position in order to change its mode of action. 1 This is distinguished from standard strabismus surgery, in which a muscle’s action is enhanced or weakened, while maintaining its normal direction of action. For example, horizontal transposition of a vertical rectus muscle confers a horizontal action to that muscle, and vertical transposition of a horizontal rectus muscle will give it a vertical action (Video 35.1). This is to be distinguished from correction of muscle belly displacement, which also may have a profound impact on the muscle’s mode of action (Chapter 19, Chapter 30). Transposition procedures are required when there is no intrinsic action of a rectus muscle into its normal direction of action. The most common uses are to treat complete third and sixth cranial nerve palsies, Duane’s syndrome, and unrecoverable lost muscles. Transposition may also be helpful to correct a secondary deviation simultaneously with a larger primary deviation when surgery needs to be confined to one eye. For example, a blind eye with a large exotropia and a smaller hypotropia could be treated with normal horizontal rectus muscle surgery combined with simultaneous superior transposition of both horizontal muscles (Video 35.2).
Insertional transposition may cause a significant torsional effect, which may be used to one’s advantage when torsion is present preoperatively. Combining insertional transposition with simultaneous muscle belly displacement (augmentation suture) may cancel out the torsional effect of insertional transposition, since the torsional effect of insertional transposition is opposite the effect of muscle belly repositioning of the same muscle in the same direction (Chapter 11, Table 11.1). Simultaneous transposition of two antagonist vertical or horizontal rectus muscles cancels out the torsional effects of each, provided each heals the same distance from the recipient muscle. (The use of nonabsorbable sutures helps prevent asymmetrical healing.) Concurrent recession or resection of only one of the two muscles in a combined transposition can also alter the torsional balance (Video 35.2).
Nonabsorbable sutures are recommended for all transpositions, as these are “active” procedures (Section 5.3.2).
Transposition of the inferior and superior oblique muscles also alters their actions and can be creatively used to correct deviations.
35.2 Full Rectus Muscle Transposition
35.2.1 Combined Superior and Inferior Rectus Muscle Transposition
In this section, the superior rectus (SR) muscle temporal transposition component is presented, but the technique applies equally to the inferior rectus (IR) muscle and to nasal transpositions of the SR and/or IR.
Begin the surgery by applying a marking pen dot to the limbus at the 12 o’clock and 6 o’clock positions, “Holmes’ Dots,” to allow one to monitor (and reverse) any surgically induced torsion, especially with posterior fixation sutures. 2
After disinsertion, bunch up the SR and suture it to the upper pole of the lateral rectus (LR) muscle. This maneuver may allow a better chance of obtaining a symmetrical result of combined SR and IR transposition (Fig. 35‑1, Fig. 35‑2). Other surgeons prefer to spread out the insertion, by suturing the nasal corner of the cut edge of the SR muscle to the (old) temporal edge of the SR insertion, and the temporal corner of the cut edge of the SR to the upper pole of the LR insertion. This is usually and incorrectly referred to as resuturing to the “spiral of Tillaux.” The temporal edge of the SR insertion in situ (before disinsertion) is typically several millimeters further from the limbus than is the upper pole of the LR insertion to which it is sutured, so this technique does not adhere to the spiral (Fig. 35‑3).
Another method of full rectus muscle transposition is to keep the transposed muscle’s insertion spread out on the suture while maintaining its normal width. The nasal pole is sutured adjacent to the insertion of the palsied LR, and the temporal pole just above the LR muscle a corresponding distance from the insertion. This method is technically more challenging, but it produces a larger temporal displacement, and is the preferred method of author IL (Fig. 35‑4, Video 35.1).