Surface area of glaucoma implants and perfusion flow rates in rabbit eyes

These experiments were designed to analyze the relationship between glaucoma drainage implant surface area and the physiological function of the surrounding encapsulation.

Three sizes of Baerveldt implants were studied. Commercially available 200 mm devices were trimmed to reduce surface area to 100 and 50 mm (one side), respectively. Five samples of each size of device were studied 3 weeks after implantation in normal rabbit eyes by perfusing the drain tubes in vivo using a micromanometric system allowing precise control of flow rates. Additional eyes were analyzed at 12 weeks. Resistance to flow was calculated using Poiseuille's equation after at least three different flow rate readings for each implant, and a linear regression line was plotted for each eye. Flow rates at the pressures of 10, 15, 20, and 25 mm Hg were standardized by slope calculation and mean flow rate values for each size of implant compared statistically. Calculated flow per unit area (hydraulic conductivity) was calculated for each sized implant.

The perfusion flow tests demonstrated statistically significant differences for the values of resistance to flow and flow through the implants for the three surface areas tested. The 200 mm implants had higher flow rates and lower resistance values. A statistically significant inverse correlation was found between the surface area of the implant and the resistance to flow (p = 0.0002). A statistically significant direct correlation was also found between the surface area of the implant and the values of flow rates (p = 0.0002) through the capsules. Hydraulic conductivity of the capsules was virtually identical for all three sizes of implants tested.

The results demonstrate a direct relationship between the surface area of glaucoma implants and the filtering capacity of their surrounding capsules.