Ocular Translation Due to Gravitational Acceleration.

Purpose: We asked if human globe position in the orbit is influenced by gravitational head orientation. Methods: In 11 adult volunteers, surface coil magnetic resonance imaging (MRI) in 2 mm thick axial and quasi-coronal planes with a centered visual target was performed in supine position that aligned the cranial midline along the gravity vector, and was repeated in right and left decubitus positions that oriented gravity orthogonally. From coronal MRI, we computed globe centroid positions relative to the whole head. Results: In decubitus posture relative to supine, the upper eye shifted 0.85 ± 0.55 mm anteriorly and 0.44 ± 0.49 mm medially (P < 0.005 for both), without shifting superiorly. The lower eye translated anteriorly by 0.73 ± 0.59 mm (P < 10-4), 0.36 ± 0.56 mm laterally (P < 0.02), and 0.25 ± 0.50 mm superiorly (P < 0.03). Vector magnitude of translation was significantly non-zero at 1.13 ± 0.55 mm for upper eyes and 1.16 ± 0.59 mm for lower eyes (P < 10-7). Conclusions: In supine position, gravity causes enophthalmos: a 0.73 to 0.85 mm shift in anteroposterior eye position that is significant relative to intraorbital optic nerve length. When in lateral decubitus position, the higher (upward) eye shifts nasally and anteriorly, so its translation is oblique to the orbital axis. The lower (downward) tilted eye shifts temporally and anteriorly along the orbital axis. Thus, the lower eye translation elongates the nerve by about threefold more than for the upper eye. Gravity therefore has an effect on the human eye that should be considered when modeling the effect of eye rotation on the mechanical state of the optic nerve and other orbital tissues.