Strabismus Surgery Kenneth W

11 Strabismus Surgery Kenneth W. Wright and Pauline Hong his chapter discusses various strabismus surgery procedures Tand how they work. When a muscle contracts, it produces a force that rotates the globe. The rotational force that moves an eye is directly proportional to the length of the moment arm (m) (Fig. 11-1A) and the force of the muscle contraction (F) (Fig. 11-1B). Rotational force ϭ m ϫ F where m ϭ moment arm and F ϭ muscle force. Strabismus surgery corrects ocular misalignment by at least four different mechanisms: slackening a muscle (i.e., recession), tightening a muscle (i.e., resection or plication), reducing the length of the moment arm (i.e., Faden), or changing the vector of the muscle force by moving the muscle’s insertion site (i.e., transposition). MUSCLE RECESSION A muscle recession moves the muscle insertion closer to the muscle’s origin (Fig. 11-2), creating muscle slack. This muscle slack reduces muscle strength per Starling’s length–tension curve but does not significantly change the moment arm when the eye is in primary position (Fig. 11-3). The arc of contact of the rectus muscles wrapping around the globe to insert anterior to the equator of the eye allows for large recessions of the rectus muscles without significantly changing the moment arm. Figure 11-3 shows a 7.0-mm recession of the medial and lateral rectus muscles. Note there is no change in the moment arm with these large recessions. Thus, the effect of a recession on eye position is determined by the amount of muscle slack created.1a The 388 chapter 11: strabismus surgery 389 FIGURE 11-1A,B. (A) Diagram of the horizontal rectus muscles shows the relationship of the moment arm (m) to the muscle axis and center of rotation. The moment arm intersects the center of rotation and is per- pendicular to the muscle axis. The longer the moment arm, the greater the rotational force. (B) Starling’s length–tension curve. The relationship of a muscle’s force is proportional to the tension on the muscle. More tension on a muscle increases muscle force and slackening a muscle reduces its force. Note that the relationship is exponential, not linear: toward the end of the curve, a small amount of slackening produces a dis- proportionately large amount of muscle weakening. AB C FIGURE 11-2A–C. Drawing of rectus muscle recession (shaded muscle). The effect of the recession is greatest when the eye rotates toward the recessed muscle. (A) The eye rotates toward the recessed muscle, causing the recessed muscle to tighten, therefore reducing muscle slack. (B) A rectus muscle resection resulting in muscle slack. (C) The eye rotates toward the recessed muscle, and the muscle and the muscle slack increase. 390 handbook of pediatric strabismus and amblyopia 5.5 7.0 m MR FIGURE 11-3. Medial rectus muscle recession. Diagram shows normal insertion at 5.5 mm posterior to the limbus and a 7.0-mm medial rectus recession. In primary position, the moment arm (m) has not changed, so the effect of the recession is to create muscle slack rather than to change the moment arm. amount of muscle slack is most accurately determined by measuring the recession from the muscle insertion.8 Note the exponential character of the length–tension curve, as there is a precipitous loss of muscle force at the end of the curve when muscle slack is increased (see Fig. 11-1B); this is why even small, inadvertent inaccuracies of large recessions (Ͼ6– 7mm) can cause dramatic changes in muscle force and result in an unfavorable outcome. Technical mistakes, such as allow- ing central muscle sag and not properly securing the muscle, can lead to large overcorrections. For example, each 0.5mm of bilateral medial rectus recessions up to a recession of 5.5mm chapter 11: strabismus surgery 391 will correct approximately 5 prism diopters (PD) of esotropia. However, for recessions greater than 5.5 mm, each additional 0.5mm of recession results in 10 prism diopters of correction (see chart on inside cover). Thus, an overrecession of only 1.0mm on a planned 6.0-mm bilateral medial rectus recession would result in a 20-prism diopter overcorrection. Figure 11-4 shows the proper rectus muscle recession, with the muscle well secured and no central muscle sag. The best way to prevent central muscle sag is to broadly splay the new insertion so it is approximately the same width as the original insertion. A rectus muscle recession has its greatest effect in the field of action of the muscle. Figure 11-2 shows that muscle slack increases when the eye rotates toward the recessed muscle, thus reducing the rotational force on gaze toward the recessed muscle. In contrast, eye rotation away from the recessed muscle causes muscle slack to be reduced. In addition, on rotation away from the recessed muscle, the recessed muscle is inhibited (Sherrington’s law), minimizing the effect of the recession in this gaze. For example, a right medial rectus recession will produce an incomitant strabismus, with an exodeviation in primary position and a larger exodeviation in leftgaze with very little exodeviation in rightgaze. Induced incomitance can correct incomitant strabismus. If a patient has a small esotropia in primary position and a large esotropia in leftgaze, a right medial rectus recession would reduce the incomitance. Comitant strabismus, on the other hand, is best treated with bilateral sym- metrical surgery. Recessions are routinely performed on rectus muscles but can also be performed on oblique muscles. Inferior oblique muscle recession is a popular procedure for weakening the inferior oblique muscle. Recession of the superior oblique tendon has also been described. It not only slackens the superior oblique tendon but also changes the function of the muscle. A recession of the superior oblique tendon collapses the normally broad insertion and moves the new insertion nasal and anterior to the globe’s equator. This alteration changes the function of the superior oblique muscle and can result in unpredictable outcomes, including postoperative limitation of depression. A more con- trolled way of slackening the superior oblique tendon without changing the functional mechanics of the tendon insertion is a tendon-lengthening procedure, such as the Wright silicone tendon expander. 392 handbook of pediatric strabismus and amblyopia FIGURE 11-4A,B. (A) Drawing of rectus muscle recession with the muscle secured to sclera at the recession point posterior to the original insertion. Note that the new insertion is almost as wide as the original scleral insertion, and the new insertion is parallel to the original insertion. There is no central muscle sag. (B) Companion photograph shows a rectus muscle recession with no central sag because the new insertion is splayed as wide as the original insertion. chapter 11: strabismus surgery 393 Hang-Back Technique A hang-back recession suspends the muscle back, posterior to the scleral insertion, with a suture (Fig. 11-5). This technique has the advantage of excellent exposure and relatively easy needle passes through the thick anterior sclera. On the other hand, hang-back recessions are potentially less accurate than a fixed recession. Small to medium-sized hang-back recessions of 3 to 6 mm tend to result in overcorrections because they have inherent central muscle sag (Fig. 11.5). On the other hand, large hang-back recessions, over 6mm, tend to produce undercorrec- tions because an otherwise normal muscle will not consistently retract more than 6 to 7mm posterior to the insertion. The surgeon experienced with adjustable suture surgery knows it is difficult to recess a rectus muscle more than 6mm using an adjustable hang-back suture. Large hang-back recessions are FIGURE 11-5. Hang-back recession. The suture is passed through sclera at the original insertion and the muscle is suspended posteriorly to achieve the recession. Inset: Note the caliper is measuring the planned recession; however, the muscle is overrecessed because of central sag. Central sag occurs because the new insertion is lax and not splayed as widely as the original insertion. 394 handbook of pediatric strabismus and amblyopia possible if the muscle is tight and contracted, as in the case of thyroid-associated strabismus, congenital fibrosis syndrome, or a slipped muscle. Indications for hang-back recessions include a recession over a retinal buckle, recession over an area of scleral ectasia, or large recessions, of a tight contracted muscle, if posterior exposure is difficult. However, for routine strabismus surgery, the author (K.W.W.) prefers the fixed recession so the muscle is secured at the desired recession point. Adjustable Suture Technique Adjustable suture techniques allow movement of the muscle position after surgery when the patient is fully awake and the anesthesia has dissipated (Fig. 11-6). Unlike fixed sutures, the adjustable suture technique allows for fine-tuning of ocular alignment in the immediate postoperative period. The adjustable suture procedure is usually performed on recessions in two stages: in the first stage, surgery is performed under either local or general anesthesia, and the muscle is placed on a suture in such a way that the muscle position can be adjusted later. The second stage, or adjustment phase, is performed when the patient is fully awake or after the local anesthetic has worn off (5h for lidocaine) and the muscle function has returned to normal. In this phase, the muscle is adjusted to properly align the eyes and then permanently tied in place. The adjustment procedure must be performed within 24 to 48h after the initial surgery while the muscle is still freely mobile. Later adjust- ments have not been recommended because the muscle rapidly adheres to the globe.

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