In Vitro Fluid Dynamics of the Ahmed Glaucoma Valve Modified with Expanded Polytetrafluoroethylene

Abstract

Purpose: Long-term intraocular pressure reduction by glaucoma drainage devices (GDDs) is often limited by the fibrotic capsule that forms around them. Prior work demonstrates that modifying a GDD with a porous membrane promotes a vascularized and more permeable capsule. This work examines the in vitro fluid dynamics of the Ahmed valve after enclosing the outflow tract with a porous membrane of expanded polytetrafluoroethylene (ePTFE).

Materials and Methods: The control and modified Ahmed implants (termed porous retrofitted implant with modified enclosure or PRIME-Ahmed) were submerged in saline and gelatin and perfused in a system that monitored flow (Q) and pressure (P). Flow rates of 1–50 μl/min were applied and steady state pressure recorded. Resistance was calculated by dividing pressure by flow.

Results: Modifying the Ahmed valve implant outflow with expanded ePTFE increased pressure and resistance. Pressure at a flow of 2 μl/min was increased in the PRIME-Ahmed (11.6 ± 1.5 mm Hg) relative to the control implant (6.5 ± 1.2 mm Hg). Resistance at a flow of 2 μl/min was increased in the PRIME-Ahmed (5.8 ± 0.8 mm Hg/μl/min) when compared to the control implant (3.2 ± 0.6 mm Hg/μl/min).

Conclusions: Modifying the outflow tract of the Ahmed valve with a porous membrane adds resistance that decreases with increasing flow. The Ahmed valve implant behaves as a variable resistor. It is partially open at low pressures and provides reduced resistance at physiologic flow rates.

KEYWORDS::

• Ahmed valve
• Expanded polytetrafluoroethylene
• Fluid dynamics
• Foreign body response
• Glaucoma implant

ACKNOWLEDGMENTS

The authors acknowledge that this study was supported in part by NIH R43 EY015587, the Ruth W. Morrow Glaucoma Research Fund, and a grant from New World Medical, Inc., Rancho Cucamonga, CA.

Declaration of interest: The authors have no intellectual property protection for the ePTFE-enclosed drainage devices, and they have no financial interest in New World Medical.