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Gabriel T. Chong, Yasushi Kato, Esdras Arrieta-Quintero, Francisco Fantes; In Vitro Gravimetric Flow Model to Evaluate the Use of Fenestrations in Glaucoma Drainage Implants. Invest. Ophthalmol. Vis. Sci. 2011;52(14):2647.
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Because there is limited knowledge regarding the effectiveness of fenestrations to prevent intraocular pressure elevation after non-valved glaucoma drainage implant (GDI) surgery, a gravimetric flow evaluation system was used to study various types of fenestrations in a silicone tube-based in vitro model.
Fenestrations (varied by number or suture material used) were tested on 2 cm silicone tubes with an internal lumen of 300 microns (in groups of two). Tubes were sealed on one end with silicone glue to simulate ligated GDI tubes (2 tubes were left open without fenestrations as controls). The open end of the tube was connected to a 16-gauge needle that was connected to a 3-way stopcock. Water with a known pressure (electronic pressure gauge measurement) flowed from an adjustable, elevated reservoir via plastic tubing, exiting the silicone tube into a water-filled vial resting on a digital scale (measuring to the ten thousandth gram). Weight readings were recorded at a set time interval. Flow rate (µL/min) was calculated by change in weight (1 g of water equaling 1 mL) over change in time.
Flow rate through control tubes was 2.88 mL/min (2880 µL/min) at a pressure of 20 mmHg. No flow was noted through the sealed silicone tubes with 1 or 3 fenestrations up to a pressure of >70 mmHg. When these fenestrated tubes were bent, flow was noted at 20 mmHg and varied (0.7-9 µL/min) depending on the direction and amount of "bend." At a pressure of 20 mmHg, tubes with 10-0 monofilament nylon suture in the fenestration had non-linear flow rates ranging from 1-11 µL/min, and tubes with 8-0 polyglactin braided suture had non-linear flow rates ranging from 4-33 µL/min.
Direction and degree of bending appear to have a great effect on flow and may explain the variable clinical outcomes with fenestration use. Without bending the tube, simple slit-like fenestrations likely act as self-sealing valves and offer no ability to release pressure at physiological levels. Leaving behind suture material in a fenestration may offer a more predictable release of aqueous through a wick-like effect to lower IOP.
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