The experimental setup is shown in
Figure 2. It is similar, with slight modifications, to what has been previously described for flow testing of glaucoma implants.
12 A syringe pump (New Era syringe pump model NE-4000; New Era Pump Systems, Inc., Farmingdale, NY, USA) housing a 500-μL syringe was connected to a 23-gauge stainless steel blunt tipped dispensing needle onto which implants would be placed. An Omegadyne differential pressure transducer (model MDWU001V10T3C0T1A1CE; Omega Co., Stamford, CT, USA) was connected upstream to the devices to be tested. Experiments were conducted in distilled water at 21°C, where the viscosity is 0.9778 centipoise (cP). The viscosity of water at 37°C is 0.6904 cP. Because the viscosity of water is temperature-dependent, all pressure measurements were converted to account for this at physiologic temperature (37°C) by dividing by 1.416 (0.9778/0.6904) where stated. The viscosity of AH at 36°C is only 2% higher than the viscosity of water at the same temperature (Vass C, et al.
IOVS 2004;45:ARVO E-Abstract 5030). This was not corrected because the difference is negligible. Once the syringe pump reached the desired flow rate (25 μL/min, 50 μL/min, and 74 μL/min), it was left to equilibrate until a steady-state pressure (there was no change in pressure over 5 minutes) was measured. The steady-state pressures at various flow rates were measured. Linear regression was used to extrapolate steady-state pressure at physiologic flow rates (2.5 μL/min) through the Ex-Press and Baerveldt devices. There was such little resistance to flow (essentially 0 mm Hg) when steady-state pressures were measured at physiologic flow rates that measuring steady-state pressure at higher flow rates was performed so that pressures could be detected for these two devices. Because pressures at these flow rates are linearly related, linear regression is a valid method for obtaining these results. The XEN implant measurements occurred at actual near-physiologic flow rates (1, 2, 5, and 10 μL/min) becauase measured pressures were above 0 mm Hg.
Initial steady-state pressure of the experimental setup was measured without a device on the needle. This value was then subtracted from the steady-state pressure measured for each device for every flow rate, and these adjusted values were reported in this study. Dye was placed in the distilled water solution, and flow was observed using a microscope to ensure that the system did not leak when devices were tested.
The Ex-Press P-50 was placed on the end of 10 mm of silicone tubing (300-μm internal diameter) in order to provide a water-tight seal directing flow only through the Ex-Press. The Baerveldt tubing was removed from the plate, and 10 mm of tubing was attached to the needle. Flow testing was also performed with a single 5-0 monofilament nylon suture as well as with a single 4-0 silk suture spanning the entire lumen of the tubing to simulate the surgical modification instituted by some surgeons. The XEN implant was directly attached to the end of an implant fixation block. The implant fixation block is a stainless steel block with a Luer fitting on one end and a small-diameter hole on the other end and with a passage connecting the two. The diameter of the hole is based on the outside diameter of the implant used and is selected to be approximately 10 μm smaller than that of the implant. When the implant hydrates, it swells to form a very light press-fit with the hole, thus forming a watertight seal.