Postoperative Pressure Regulation in Glaucoma Shunt Surgery: Focal Tube Constriction is Not the Answer

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Purpose:To evaluate, in a laboratory setting, the accuracy and the clinical use of focal tube constriction as a means to regulate intraocular pressure after glaucoma drainage implant surgery.Materials and Methods:A silicone tube identical to the one used in the Baerveldt and other glaucoma drainage implants was connected to a syringe-pump delivering a continuous flow of demineralized water at a rate of 2.5 microliters per minute. Focal constriction of the tube was obtained by a ring made of a shape-memory metal alloy designed for this purpose. After complete occlusion of the tube lumen by crimping the ring with calibrated pliers, the ring was opened in a stepwise manner by heating it with an argon laser beam. In a second experiment the tube lumen was constricted by placing the tube between the jaws of a micrometer.Results:In both experiments a stepwise lowering of the pressure could be obtained. The resulting pressure levels, however, lacked consistency and predictability to such an extent that clinical application is not feasible.Conclusion:It is not possible to regulate pressure in a reliable and predictable way merely by constricting the tube lumen of glaucoma drainage implants.

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