Evaluation of Dimensional and Flow Properties of ExPress Glaucoma Drainage Devices

ExPress devices are available as P50 and P200 models, the numbers related to their luminal diameters in microm. We compared their Poiseuille's Law-based theoretical resistance values with experimental values and correlated these with their luminal dimensions derived from electron microscopy.

Scanning electron microscopy was performed on P50 and P200 devices. Bench-top flow studies were performed to find the resistances of the devices. Devices were also incorporated into a perfused, ex vivo porcine sclera model to test and compare their control of pressure, with and without overlying scleral flaps, and with trabeculectomies.

The luminal dimensions of the P200 device were 206.4±3.3 and 204.5±0.9 microm at the subconjunctival space and anterior chamber ends, respectively. Those of the P50 device were 205.0±5.8 and 206.9±3.7 microm, respectively. There were no significant differences between the P200 and P50 devices (all P>0.05). The resistances of the P200 and P50 devices were 0.010±0.001 and 0.054±0.002 mm Hg/microL/min, respectively (P<0.05). Equilibrium pressures with overlying scleral flaps were 17.81±3.30 mm Hg for the P50, 17.31±4.24 mm Hg for the P200, and 16.28±6.67 mm Hg for trabeculectomies (P=0.850).

The luminal diameters of both devices are externally similar. The effective luminal diameter of the P50 is much larger than 50 microm. Both devices have low resistance values, making them unlikely to prevent hypotony on their own. They lead to similar equilibrium pressures as the trabeculectomy procedure when inserted under the scleral flap.