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

This experimental study was conducted to report perfusion characteristics of small diameter, cylindrical aqueous shunts in normal rabbit eyes and to test the hypothesis that decreasing bleb diameter would decrease capsular fibrosis, as evidenced by a thinner capsule forming around the implant. These two properties increase hydraulic conductivity of the fibrous membrane forming around the device, resulting in a more effective filtering shunt.

Cylindrical latex tubes with the distal portion of the sidewall removed were implanted under the conjunctiva. The proximal, intact end of tubing was inserted into the anterior chamber and ligated to prevent hypotony. The ligature was released after 1 week to inflate the bleb. Animals were again anesthetized at 6 or 12 weeks after ligature release and in vivo perfusion experiments conducted using a miniperfusion system and a water manometer. Perfusion of the implant with latex microspheres was performed before the animals were killed. Capsule diameters were measured in situ using calipers under a dissecting microscope after excision of orbital tissues and before fixation for histologic study. Membrane hydraulic conductivity (outflow per unit membrane area at unitary pressure gradient, microL/min/mm2/mmHg) was calculated using data obtained during perfusion experiments and compared with results of other studies. Capsule thickness was measured histologically.

Cylindrical filtration membranes with thin (15-20 microm) capsules formed around latex implants. Bleb diameters consistently measured 1 mm at all points along their length before formaldehyde fixation. Hydraulic conductivity was measured and found to be eight times higher than that reported for capsules around conventional implants. Decreased diameter (16:1 for Baerveldt and 13:1 for Molteno implants) produced a proportional decrease in surface tension on the wall. This resulted in decreased capsule thickness, from 222 microm for Baerveldt implants in rabbit eyes and from 150 microm for Molteno implants in monkey eyes to less than 20 microm for the shunting device studied here.

As a consequence of Laplace's law, reduction in bleb diameter reduces surface tension on the bleb, reducing capsular fibrosis and consequently capsule thickness, thus increasing hydraulic conductivity. Increased hydraulic conductivity increases the effectiveness of the filtering surface. Cylindrical geometry allows reduced bleb volume yet maintains total surface area that is proportional to the length of the implant, which is unlimited and customizable for each eye by simply cutting the length of implant needed.