Extraocular muscle insertions relative to the fovea and optic nerve: Humans and rhesus macaque

PURPOSE. To identify extraocular anatomic relationships of muscle insertions relative to the fovea and the optic nerve. METHODS. Thirty-eight human eye bank eyes and 10 rhesus macaque (Macaca mulatta) eyes were measured. Ten human volunteers were used to determine the horizontal rectus muscle-to-globe apposition in primary, left, and right gaze. RESULTS. External globe measurements (human/rhesus; mm ± SD) from the temporal border of the optic nerve (ON) to the center of the fovea (F) were 3-7 ± 0.6 and 2.6 ± 0.2; F to the posterior border of the inferior oblique (IO) insertion, 2.5 ± 0.8 and 0.5 ± 0.4; ON to the posterior border of the IO, 5.6 ± 0.9 and 2.8 ± 0.3; horizontal axial plane (H) of the eye, defined by the long posterior ciliary artery, to the IO, 2.0 ± 0.8 and 0.5 ± 0.4; and H to F, 1.0 ± 0.6 and 0.4 ± 0.3, respectively. The IO insertion formed an arc, inferior to H, with an anterior-to-posterior cord insertion width of 9.2 ± 0.7 and 7.7 ± 0.3. The IO angle of insertion (θ) was 30° in 84% (32/38) and 0° in 16% (6/38) of human eyes and 25 to 30° in all rhesus. In 20 human volunteers, from the ON to the apex of lateral rectus globe apposition was 13.9 ± 1.1 in primary, 17.2 ± 1.9 in lateral, and 9.3 ± 1.7 in medial gaze. CONCLUSIONS. The fovea is located mostly superior and slightly posterior to the posterior border of the IO insertion. Topographic relationships of the extraocular muscles relative to the fovea are essential for the design of extraocular drug delivery systems.