Anatomic variations of the inferior oblique muscle: a potential cause of failed inferior oblique weakening surgery

PURPOSE: To document the variations in normal anatomy that occur at the insertion of the inferior oblique muscle and in the vicinity of its surgical capture site (10 to 12 mm from the insertion). METHODS: One hundred intact cadaver orbits with no history of eye muscle or orbital disorders during life were carefully dissected to expose the entire length of the inferior oblique muscle. The number of divisions of muscle at the insertion, total width of the muscle belly, and variations in anatomy 10 and 12 mm from the insertion were recorded. RESULTS: Seventeen (17%) of the 100 inferior oblique muscles had multiple divisions at the insertion. Eight muscles (8%) had two bellies at 10 or 12 mm from the insertion. Among these eight, four had two distinct (bifid) bellies extending to the insertion, and four had dehiscences in the muscle. The mean muscle width among these eight specimens was 0.5 and 0.7 mm larger than the mean width of the other 92 specimens at the 10 mm and 12 mm positions, respectively. Neither difference was significant at the .05 level. CONCLUSIONS: Multiple insertions were found in 17% of inferior oblique muscles examined; duplications of the inferior oblique muscle at the surgical capture site were found in 8%. These duplications may account for some cases of recurrence or persistence of inferior oblique overaction after weakening surgery, owing to inadvertent incomplete capture of the muscle during surgery.