Modification of the glaucoma drainage implant to prevent early postoperative hypertension and hypotony: a laboratory study.

Hypotony or hypertension in the early postoperative period following implantation of a nonvalved seton such as the Molteno tube is a common problem. We conducted a laboratory investigation evaluating two modifications of the silicone drainage tube. One involved a longitudinally-oriented, pressure-sensitive slit-valve combined with an absorbable occluding ligature interposed between the valve and the episcleral plate. Experiments examining the relationship between slit length, opening pressure, and flow rate were performed. The second modification involves focally constricting the lumen of the tube with an external ligature in order to reduce flow rates. Our results indicate that a slit-valve length of 2.0 mm appears to provide a reliable opening pressure of around 10 mm Hg, with relatively high flow when pressures exceed opening pressure. Focally constricting the lumen of the tube, however, was shown to be both unpredictable and unsatisfactory for reducing the flow of fluid to a range consistent with steady state aqueous production.