title: In vitro fluid dynamics of the Ahmed glaucoma valve modified with expanded polytetrafluoroethylene.
abstract: OBJECTIVE
  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).
  
  
  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.
authors.affiliation: Department of Ophthalmology, Duke University Medical Center, Durham, North Carolina, USA.
authors.forename: Francis Char
authors.initials: FC
authors.lastname: DeCroos
authors.forename: Yuji
authors.initials: Y
authors.lastname: Kondo
authors.forename: Daniel
authors.initials: D
authors.lastname: Mordes
authors.forename: Maria Regina
authors.initials: MR
authors.lastname: Lee
authors.forename: Sameer
authors.initials: S
authors.lastname: Ahmad
authors.forename: Sanjay
authors.initials: S
authors.lastname: Asrani
authors.forename: R Rand
authors.initials: RR
authors.lastname: Allingham
authors.forename: Kevin C
authors.initials: KC
authors.lastname: Olbrich
authors.forename: Bruce
authors.initials: B
authors.lastname: Klitzman
mesh_terms: D005780:Gelatin / Q000378:metabolism; D020327:Glaucoma Drainage Implants; D057446:Hydrodynamics; D066298:In Vitro Techniques; D011138:Polytetrafluoroethylene / Q000737:chemistry*; D011312:Pressure; D012965:Sodium Chloride / Q000378:metabolism; D014160:Transducers, Pressure
chemical_list: D012965:Sodium Chloride; D005780:Gelatin; D011138:Polytetrafluoroethylene