There is a number of variations for this surgical procedure, but essentially, all filtering operations share the same basic mechanism of action and general surgical principles. Trabeculectomy is currently the most frequently performed surgical procedure for glaucoma. Trabeculectomy is a safe and effective procedure in that it has a high success rate. In this surgery, the surgeon aims to allow aqueous humor to bypass the trabecular meshwork into the subconjunctival space. This would ensure an optimum intraocular pressure (IOP) while maintaining the anatomy of the globe that is indicated by prevention of shallowing of the anterior chamber (Pederson, 1996, chap 18).
Intraocular pressure (IOP) is determined by the balance between aqueous humor production and outflow. There are diurnal variations in aqueous humor production, but it has been observed that alterations in IOP usually result from a variation in the resistance to aqueous outflow. For normal clinical purposes, IOP can be defined, thus, as pressure which does not lead to glaucomatous damage of the optic nerve head. Aqueous outflow occurs through both conventional and unconventional routes. In the former, aqueous moves through the trabecular meshwork and into Schlemm's canal, and from there to episcleral veins via collector channels that traverse the limbal sclera (Seah et al., 1995, 73-79).
Conventional aqueous humor outflow begins with the trabecular meshwork. ...
IOP is determined primarily by the resistance to aqueous humor outflow. In the normal eye, this resistance appears to be dictated by several factors (Toris and Pederson, 1987, 477-481). These factors may be extrinsic or intrinsic to the aqueous humor outflow pathways. Extrinsic factors include IOP and ciliary muscle contraction. Intrinsic factors include both the direct activity of trabecular meshwork cells and the indirect effects of cellular activity, such as in maintaining the ECM of both the trabecular meshwork and the ciliary body, which then provides resistance to outflow. The measurement of intraocular pressure (IOP) is an essential part of the ocular examination. Lowering IOP is currently the only widely accepted method of preventing glaucomatous optic nerve damage or retarding its progression once present (Khaw, Shah, and Elkington, 2004, 97-99).
Studies have suggested that aqueous in the filtering bleb usually filters through the conjunctiva and mixes with the tear film or is absorbed by vascular or perivascular conjunctival tissue. A filtering procedure may control IOP even in the absence of an apparent filtering bleb. This is more common when the fistula is covered by a partial-thickness scleral flap performed in trabeculectomy. In these cases, the suggested mechanism of drainage of aqueous to accomplish reduction in IOP is different. These include flow of aqueous through lymphatic vessels near the scarred margins of the surgical area, atypical and newly incorporated aqueous veins, and normal aqueous veins. The aqueous drainage route beneath the scleral flap is preserved following development of filtering bleb in trabeculectomy (Budenz, Chen, and Weaver, 1999, 1014-1019).