First described by Parks and Bloom [1] in 1979, the slipped muscle occurs after inadequate suturing of muscle: the muscle capsule is attached to sclera, but the muscle itself retracts posteriorly. Ductions are limited but relatively preserved, as compared with a lost muscle (Fig. 21.1a). Plager and Parks [2] described the crucial finding of the muscle recoiled posteriorly within the capsule, found at the Tenon’s capsule penetration site. They also described specific intraoperative findings of a translucent, “empty, friable avascular capsule attached to the globe” (Fig. 21.1b). A slipped muscle occurs immediately, with limited ductions usually identified within a day or two after strabismus surgery, but the slippage can continue over time. Treatment is retrieval, excision of the capsule attachment, and advancement of the muscle to the intended surgical site with a nonabsorbable suture. Often the antagonist is contracted and tight, adding a restrictive component. If the antagonist is tight, it should be recessed.

The lost muscle has long been a dreaded complication of periocular surgery, including pterygium surgery, retinal detachment surgery, and strabismus surgery. A lost rectus muscle can occur when a muscle is surgically removed from the globe, or it can be associated with the “Pulled in Two Syndrome” (PITS), when the muscle is torn in two by being pulled on too hard. PITS is more common in elderly patients, and can occur with relatively minimal muscle traction. The hallmark of a lost muscle is severely limited ductions and palpebral fissure widening on attempted gaze in the direction of the lost muscle. The treatment is exploration and retrieval of the lost muscle. If the muscle cannot be found immediately during the primary surgery, it is often best to wait 2 weeks to perform the secondary exploration surgery. By waiting to reoperate, the swelling subsides and a fibrous scar usually forms, which extends from the scleral insertion to the site of the lost muscle. This scar can be traced posteriorly to help find the lost muscle (see Figs. 21.2, 21.3, 21.4, 21.5, 21.6, 21.7, 21.8, and 21.9). Once the lost muscle is found, it should be secured and reattached with nonabsorbable suture. The antagonist to the lost muscle is usually contracted and tight. If such tightness is diagnosed tight by forced ductions, recess the tight antagonist. If the lost muscle cannot be found, a rectus muscle transposition is indicated, with a large recession of the antagonist muscle.

Stretched scar is a lengthening of the muscle-to-sclera fibrous attachment that occurs several weeks to months after strabismus surgery. The result is posterior retraction of the muscle and reduced muscle function. In 1999, Ludwig [3] first described this phenomenon as a common cause of unfavorable outcomes after strabismus surgery. Using an animal model, Ludwig showed that muscles sutured with an absorbable suture had a significantly higher rate of stretched scar than those sutured with a nonabsorbable suture. Ludwig hypothesized that the mechanism of stretched scar is weakening of the absorbable suture before the muscle-to-sclera healing is complete. As with slipped muscle, patients with stretched scar have an amorphous, fibrous band connecting the muscle to the globe, but the tissue is thicker with a stretched scar (see Figs. 21.10 and 21.11). Also similar to a slipped muscle, ductions are limited, but they are less limited than with a lost muscle. In contrast to a slipped muscle, which occurs immediately, stretched scar occurs late, at least 4–6 weeks after surgery. Treatment is excision of the scar and advancement using a nonabsorbable suture to secure the muscle to the intended insertion site (see Figs. 21.12, 21.13, and 21.14). Add a recession of the antagonist muscle if it is tight on forced duction testing.