Location of glaucoma drainage devices relative to the optic nerve

Background: Limited data are available to guide optimal positioning of glaucoma drainage devices (GDD) in relation to the limbus and optic nerve. The authors aim to provide guidelines for appropriate and safe GDD implantation. Method: The optimal positioning of five different GDD were evaluated using necropsy eyes of varying axial lengths. The dependent variable that was measured was the maximum distance that a GDD could be placed posterior to the limbus while remaining 2 mm away from the optic nerve. Results: The average maximum distance posterior to the limbus of the anterior plate edge ranged between 9.0-15.0 mm in the superotemporal quadrant for the GDD tested. The distances for superonasal, inferonasal, and inferotemporal quadrants ranged between 8.0-14.0 mm, 9.0-14.0 mm, and 11.0-17.0 mm, respectively. The Molteno device could be placed most posteriorly while remaining 2 mm away from the nerve. The Ahmed FP7 and S2 were the least amenable to posterior placement before encroaching on the 2 mm limit. Conclusion: The maximum distance that a GDD can be placed posterior to the limbus, before encroachment around the optic nerve, varies between different devices and quadrants of placement. Taking a measurement of the exact distance of the plate from the limbus during GDD surgery is recommended.