Performance of a new, low-volume, high-surface area aqueous shunt in normal rabbit eyes

Purpose: This experimental study was conducted to report perfusion characte ristics of small diameter, cylindrical aqueous shunts in normal rabbit eyes and to test the hypothesis that decreasing bleb diameter would decrease ca psular fibrosis, as evidenced by a thinner capsule forming around the impla nt. These two properties increase hydraulic conductivity of the fibrous mem brane forming around the device, resulting in a more effective filtering sh unt. Methods: Cylindrical latex tubes with the distal portion of the sidewall re moved were implanted under the conjunctiva. The proximal, intact end of tub ing was inserted into the anterior chamber and ligated to prevent hypotony. The Ligature was released after 1 week to inflate the bleb. Animals were a gain anesthetized at 6 or 12 weeks after Ligature release and in vivo perfu sion experiments conducted using a miniperfusion system and a water manomet er. Perfusion of the implant with latex microspheres was performed before t he animals were killed. Capsule diameters were measured in situ using calip ers under a dissecting microscope after excision of orbital tissues and bef ore fixation for histologic study. Membrane hydraulic conductivity (outflow per unit membrane area at unitary pressure gradient, mu L/min/mm(2)/mmHg) was calculated using data obtained juring perfusion experiments and compare d with results of other studies. Capsule thickness was measured histologica lly. Results: Cylindrical filtration membranes with thin (15-20 mu m) capsules f ormed around latex implants. Bleb diameters consistently measured 1 mm at a ll points along their length before formaldehyde fixation. Hydraulic conduc tivity was measured and found to be eight times higher than that reported f or capsules around conventional implants. Decreased diameter (16:1 for Baer veldt and 13:1 for Molteno implants) produced a proportional decrease in su rface tension on the wall. This resulted in decreased capsule thickness, fr om 222 mu m for Baerveldt implants in rabbit eyes and from 150 mu m for Mol teno implants in monkey eyes to less than 20 mu m for the shunting device s tudied here. Conclusion: As a consequence of Laplace's law, reduction in bleb diameter r educes surface tension on the bleb, reducing capsular fibrosis and conseque ntly capsule thickness, thus increasing hydraulic conductivity. Increased h ydraulic conductivity increases the effectiveness of the filtering surface. Cylindrical geometry allows reduced bleb volume vet maintains total surfac e area that is proportional to the length of the implant, which is unlimite d and customizable for each eye by simply cutting the length of implant nee ded.