Anatomic considerations in upper eyelid retraction.

A series of anatomically correct orbital drawings are created to examine the major integrating factors that determine upper eyelid position on the eye. The principal upper eyelid force planes can be identified as (1) the eyelid vertical retractor plane slightly divergent from the orbital axis, (2) the eyelid protractor plane nearly tangent to the orbital aperture, and (3) the upper eyelid tarsalligamentous plane swinging from within the horizontal orbital rims. These forces are governed by variation in orbital size and shape, globe size and position, and the length-tension characteristics of the eyelid retractors, protractors, and lamellae. Enhanced lateral upper eyelid retraction occurs in part because the eye in primary position looks away from the orbital axis and projects more lateral sclera. When the eye gazes laterally, coincident with the orbital axis, enhanced lateral upper eyelid retraction disappears.