Experimental flow studies in glaucoma drainage device development.

AIMS: (I) To examine whether small holes produced by 248 nm excimer laser ablation in a polymer substrate could consistently produce a pressure drop in the desired target range (5-15 mm Hg) at physiological aqueous flow rates for use as an internal flow restrictor in a glaucoma drainage device, and (ii) to investigate whether external leakage could be reduced in comparison with conventional tube and plate glaucoma drainage devices by redesigning the exterior cross sectional shape of the portion contained within the sclerocorneal tunnel.
METHODS: Single holes with target diameters of 10 microm, 15 microm, 20 microm, and 25 microm were drilled using a 248 nm excimer laser in sample discs (n=6 at each diameter) punched from a 75 microm thick polyimide sheet. Sample discs were tested in a flow rig designed to measure the pressure drop across the discs. Using filtered, degassed water at a flow rate of 1.4 microl/min repeated flow measurements were taken (n=6) for each disc. After flow testing, all discs were imaged using a scanning electron microscope and the dimensions of each hole were derived using image analysis software. In the external leakage study, corneoscleral buttons (n=13) were prepared from cadaver pig eyes and mounted on an artificial anterior chamber infused with Tyrode solution. After the pressure had stabilised, standard occluded silicone tube implants were inserted through 23 gauge needle stab incisions at the limbus. These were compared against prototype PMMA implants with a novel shape profile inserted through 1.15 mm width microvitreoretinal (MVR) stab incisions at the limbus. The infusion rate was maintained and a second pressure measurement was taken when the pressure had stabilised. The difference between the first and second pressure measurement was then compared, as an index of external leakage.
RESULTS: Ablated tubes were found to have a near perfect circular outline on both the entry and exit side. The observed pressure drops across the ablated sample discs at each target diameter were as follows: 10 microm, mean 25.66 (SD 4.9) mm Hg; 15 microm, 6.7 (1.15); 20 microm, 1.66 (1.07); and 25 microm, <0.1 mm Hg. A strong correlation was observed between observed pressure drops and those predicted by Poiseuille's formula (R(2) =0.996). Target ablations of 15 microm diameter produced tubes that consistently achieved a pressure drop within the desired range (5-15 mm Hg). In the external leakage study, preinsertion pressures (mm Hg; mean (SD)) were 19.00 (4.3) (conventional method) and 20.00 (3.9) (new technique with PMMA prototypes). Post-insertion pressures were significantly reduced (10.40 (7.7); p<0.01) for the conventional technique and were essentially unchanged for the new technique (18.80 (4.9); p>0.1).
CONCLUSIONS: It was shown that it is possible, in principle, to control the dimensions of a manufactured tubular lumen in a glaucoma drainage device accurately enough to provide consistent protection from hypotony in the early period after glaucoma filtration surgery. By redesigning the external profile of glaucoma drainage device and incision technique, it was also shown that it is possible to eliminate uncontrolled external leakage.