Insertional pattern of the inferior oblique muscle

We aimed to define the anatomy and anatomic variations of the inferior oblique muscle (IO) and classify its insertional pattern with photographs and drawings.

Cohort study.

This study included 60 intact orbits of 30 (17 male and 13 female) embalmed adult cadavers. The inferior oblique muscle was dissected microanatomically from the origin to the insertion by using a dissection microscope. For each specimen, we photographed the insertional pattern of the muscle. A Vernier caliper was used to measure the width and length of the muscle at the insertion and from the division to the insertion, respectively.

We classified the inferior oblique muscle into four groups according to the insertional pattern. In the first group, the muscle was formed of only one belly ("single," five eyes, 8.3%). In the second group, the muscle had a main and a secondary belly ("double," 30 eyes, 50%). In the third group, the muscle had three bellies ("triple," 16 eyes, 26.6%). In the fourth group, the muscle had more than three bellies ("multiple," 9 eyes, 15%).

We found double, triple, and multiple muscle bellies in 91.7% of 60 cadaveric eyes. Ocular surgeons who do not frequently perform inferior oblique surgery should carefully examine the inferior temporal quadrant of the sclera to avoid missing portions of the inferior oblique muscle at the time of surgery.