Transscleral flow of aqueous humor: an in vitro experiment

The minimal scleral flap thickness to ensure transscleral flow following a trabeculectomy has never been determined. The present study was designed to determine, in vitro, the critical scleral flap thickness that allows transscleral flow.

The apparatus consisted of two horizontal glass chambers (A and B) connected to each other by a customized scleral disc holder. High-pressure chamber A (at 25 mm Hg) was filled with sodium pertechnetate (99mTc) labeled normal saline and low-pressure chamber B (at 5 mm Hg) with normal saline. Transscleral flow of labeled normal saline from high-pressure chamber A via varying thickness scleral discs to low-pressure chamber B was observed over 16 hours using a gamma camera. Computer analysis was performed on the obtained images.

Transscleral flow of labeled saline was observed only with scleral discs 0.5 mm thick or less.

If the findings hold true for the in vivo situation, aqueous humor may reach the subconjunctival space following trabeculectomy via the transscleral route, provided scleral flap thickness is less than 0.5 mm. In addition, normal uveoscleral aqueous outflow may occur across sclera less than 0.5 mm thick, e.g., posterior to extraocular muscle insertions.