Fig. 20. Nonpenetrating filtration surgery. Nonpenetrating filtration surgery consists of fashioning dual scleral flaps, a superficial and deep flap. The superficial flap is typically 300 microns thick and must be dissected at least 1 mm into clear cornea. Fashioning of the deep flap (600 microns thick) reveals the underlying limbal anatomy with the circumferential white fibers of the scleral spur (SS) and more anteriorly, the blue zone with its corneal limit of Schwalbe's line. Schlemm's canal is located directly anterior to the SS. The trabecular meshwork is found anterior to Schlemm's canal and extends to Schwalbe's line. The trabeculodescemetic membrane (TDM) extends from Schwalbe's line anteriorly into clear cornea for 1 mm. The deep flap is removed at the time of surgery creating a potential space that functions as a collecting cavern for the potential egress of aqueous. Filtration occurs mainly internally through Schlemm's canal (SC) with successful deep sclerectomy with viscocanalostomy (DSVC), and externally into the subconjunctival space with deep sclerectomy with collagen implant (DSCI). The superficial flap guards externally and the TDM guards internally from overfiltration.