An anatomic study of retrobulbar needle path length.

One hundred twenty human orbits were measured to determine orbital depth and the position of orbital structures in relation to the length of needles used for retrobulbar anesthesia. The distance from the inferior temporal orbital rim to the nasal entrance of the optic foramen ranged from 42 to 54 mm. Taking into account the size of the optic nerve, the common 38-mm (1.5-in) retrobulbar needle could perforate the optic nerve where it is fixed in front of the foramen in over 11% of the population. The authors recommend that needles be placed less than 31 mm (1.25 in) behind the orbital rim for both retro- and peribulbar anesthesia.