Intraocular pressure control of a novel glaucoma drainage device - in vitro and in vivo studies

Abstract

Aim: To evaluate the intraocular pressure (IOP) control of an artificial trabeculum drainage system (ATDS), a newly designed glaucoma drainage device, and postoperative complications in normal rabbit eyes.

Methods: Pressure drops in air and fluid of 30 ATDS were measured after being connected to a closed manometric system. Twenty of them were then chosen and implanted randomly into the eyes of 20 rabbits. Postoperative slit-lamp, gonioscopic examination and IOP measurements were recorded periodically. Ultrasound biomicroscopy and B-scan ultrasonography were also used to observe the complications. Eyes were enucleated on day 60.

Results: Pressure drops of 4.6-9.4 mm Hg were obtained at physiological aqueous flow rates in the tests in vitro. The average postoperative IOP of the experimental eyes (11.6-12.8 mm Hg) was lower than the controls significantly (P<0.05) at each time point. Complications of hemorrhage (n=1), cellulosic exudation (two cases) and local iris congestion (two cases) were observed. The lumina of the devices were devoid of obstructions in all specimens examined and a thin fibrous capsule was found around the endplate.

Conclusion: ATDS reduce IOP effectively. However, further studies on the structure are needed to reduce complications.

Keywords: aqueous humor; drainage device; intraocular pressure; outflow; rabbit.

Figure 1. Simulated construction of ATDS

A: The medical grade PUR plate, with a spherical undersurface, has an area of 162.2 mm². All the evections lying on the posterior plate are designed after analyzing the contact surface between Tenon's capsule and plate using finite element analysis (FEA). Each one has its optimal height, bulk and location to sustain Tenon's capsule and keep the largest drainage surface; B: Location of ATDS.

Figure 2. Structure of T-shaped tube

The T-shaped tube has an outer diameter of 600 µm and an inner diameter of 300 µm. Many micropores distributing to horizontal tube except the surface connecting with the perpendicular tube (A) are designed to increase the drainage surface and decrease the opportunity of tube obstructed by fibrin clot or iris. When the plate is pulled back, the 5 mm long horizontal tube will curve and match the AC angle (B) to decrease tube movement, prevent extrusion and also block the incision under scleral flap, which consequently, could avoid severe hypotony caused by peritubular filtration.

Figure 3. Pressure drops of PCS at different flow rates

Pressure drop produced by PCM had a linear correlation with flow rate at a range of 0.6-48.0 µL/min. The pressure drop across straight tube versus flow rate is from Poiseuille's formula: pressure drop=128nlQ/πd⁴, where n=aqueous viscosity=1.03×10⁻³ NS/m²; l=length=25.7 mm in this study; Q=aqueous flow rate, d=diameter (metres).

Figure 4. Flow rig for measuring pressure drop

Pressure drop were tested in air and water respectively. Repeated flow measurements were taken (n=3), and each device was measured during a 10min interval while a constant flow of fluid was pumped into the system.

Figure 5. Main procedures of ATDS implanted surgery

Arrow shows the T-shaped tube, and asterisk indicates the plate of ATDS.

Figure 6. Observed pressure drop versus predicted

Fitting linear formula between 0.6 and 48.0 µL/min of the three plots: predicted pressure drop, y=442x-1.2532, R²=1; observed pressure drop in air, y=473.11x-52.539, R²=0.9992; observed pressure drop in BSS, y=417.66x+72.411, R²=0.9998.

Figure 7. IOP change of bilateral eyes

A stable reduction of mean IOP was obtained after it dropped slowly to the average of 11.6 mm Hg 3d after surgery in the ATDS-implanted eyes, and there was no significant difference between consecutive study points.

Figure 8. Bleb appearance of ATDS-implanted eyes

A: Bleb on postoperative day 3. It was thin, lucent and diffused with slight hyperemia. Only the front part was available; B: Bleb on postoperative day 14. Bleb was thick, transparent ivory white and localized surround the plate.

Figure 9. Hemorrhage of anterior chamber

A: Slight hemorrhage on postoperative day 2. The T-shaped tube was filled with blood; B: Major part of hemorrhage was absorbed on postoperative day 7, but corneal edema still existed.

Figure 10. Ultrasound biomicroscopy images

The T-shaped tube was hyperechoic in UBM image, with band-shaped sound absorption beneath.

Figure 11. Fibrous capsule in an enucleated eye

A: The cornea was transparent, the wound of the limbus healed well; B: The plate was encapsulated by a pink, thin and tenacious fibrous layer including the undersurface connected with the eyeball, and it was easy to separate.