A model to measure surgical outflow using a two-tubed experimental glaucoma drainage device.

Abstract

Purpose: Engineering modelling suggests that the key determinant of success in glaucoma surgery is the hydraulic conductivity of the bleb or peri-implant capsule. However, previous experimental models of glaucoma surgery relied on indirect measures of surgical success such as bleb appearance or intraocular pressure in animals with normal trabecular outflow. We wished to develop a method to directly measure fluid outflow into rabbit implant capsules after experimental glaucoma surgery. We present a refinement and extension of the model we reported recently in IOVS.

Methods: An experimental glaucoma drainage device (GDD) was developed with similar physical dimensions to the Paediatric Molteno implant, except with two tubes. Implants were fabricated using a 3D-printer and tubes attached with medical-grade silicone adhesive. Devices were implanted into the right eyes of 17 New Zealand White rabbits, with one tube inserted into the anterior chamber and the other occluded and left under the conjunctiva. After one (n=5) or four (n=5) weeks, the anterior chamber tube was tied off and the second tube cannulated and connected to a pressure transducer-controlled syringe pump. Outflow into capsules at 12mmHg was measured for 5 minutes. In another group (n=7), there was no connection to the anterior chamber and capsule outflow was measured four weeks after surgery without aqueous exposure.

Results: There was a significant decline in capsule outflow between one and four weeks after surgery, from 2.46 +/- 0.80 to 0.67 +/- 0.16 μL/min at 12mmHg (mean + SD). However, four-week post-surgery capsules without aqueous drainage demonstrated greater outflow than capsules with fluid exposure, at 3.00 +/- 2.10 μL/min at 12mmHg. One-way ANOVA: p<0.05.

Conclusions: Two-tubed experimental GDDs allow for accurate and simplified recording of the outflow into capsules, the main determinant of success in glaucoma filtration surgery. The conductivity of implant capsules declines significantly by four weeks after surgery in the rabbit, however this does not occur in the absence of aqueous drainage. These results provide direct evidence that aqueous drainage itself is problematic to maintaining the function of glaucoma surgery in the rabbit. Additionally, the use of a 3D-printer allows for prototyping implants with differing physical characteristics, and by testing their efficacy one can adopt an informed design process.

Keywords: 765 wound healing • 427 aqueous • 557 inflammation