Slipped and lost muscles and other tales of the unexpected. Philip Knapp Lecture.

Loss of a rectus muscle may occur as a rare complication of strabismus surgery. In addition, extraocular muscles may become traumatically detached from the globe when they rupture or are transected as the result of an injury or during the course of retinal detachment or paranasal sinus or orbital surgery. Although the clinical features of a slipped muscle may resemble those of a lost muscle, the findings at the time of reoperation are distinct. Also the etiology, and therefore the prevention, of a slipped muscle differs from that of a lost muscle. It is likely that slipped muscles and even some lost muscles are underdiagnosed and represent a significant cause of unexpected overcorrection or undercorrection. Unless the displaced muscle is appropriately advanced, it can be extremely difficult to correct the associated strabismus, yet locating and repairing either a lost or slipped muscle can be challenging and is by no means always successful. Current concepts pertaining to the etiology, recognition, and management of slipped and lost muscles will be discussed.