title: Evaluation of Dimensional and Flow Properties of ExPress Glaucoma Drainage Devices.
abstract: OBJECTIVE
  ExPress devices are available as P50 and P200 models, the numbers related to their luminal diameters in μm. We compared their Poiseuille's Law-based theoretical resistance values with experimental values and correlated these with their luminal dimensions derived from electron microscopy.
  
  
  METHODS
  Scanning electron microscopy was performed on P50 and P200 devices. Bench-top flow studies were performed to find the resistances of the devices. Devices were also incorporated into a perfused, ex vivo porcine sclera model to test and compare their control of pressure, with and without overlying scleral flaps, and with trabeculectomies.
  
  
  RESULTS
  The luminal dimensions of the P200 device were 206.4±3.3 and 204.5±0.9 μm at the subconjunctival space and anterior chamber ends, respectively. Those of the P50 device were 205.0±5.8 and 206.9±3.7 μm, respectively. There were no significant differences between the P200 and P50 devices (all P>0.05). The resistances of the P200 and P50 devices were 0.010±0.001 and 0.054±0.002 mm Hg/μL/min, respectively (P<0.05). Equilibrium pressures with overlying scleral flaps were 17.81±3.30 mm Hg for the P50, 17.31±4.24 mm Hg for the P200, and 16.28±6.67 mm Hg for trabeculectomies (P=0.850).
  
  
  CONCLUSIONS
  The luminal diameters of both devices are externally similar. The effective luminal diameter of the P50 is much larger than 50 μm. Both devices have low resistance values, making them unlikely to prevent hypotony on their own. They lead to similar equilibrium pressures as the trabeculectomy procedure when inserted under the scleral flap.
authors.affiliation: *Department of Ophthalmology, University of Malaya, Kuala Lumpur, Malaysia †UCL Department of Mechanical Engineering §UCL School of Pharmacy ‡NIHR Biomedical Research Centre at Moorfields Eye Hospital NHS Foundation Trust and UCL Institute of Ophthalmology, London, UK.
authors.forename: Amir
authors.initials: A
authors.lastname: Samsudin
authors.forename: Ian
authors.initials: I
authors.lastname: Eames
authors.forename: Steve
authors.initials: S
authors.lastname: Brocchini
authors.forename: Peng Tee
authors.initials: PT
authors.lastname: Khaw
mesh_terms: D000818:Animals; D001082:Aqueous Humor / Q000502:physiology*; D004195:Disease Models, Animal; D018463:Filtering Surgery / Q000295:instrumentation*; D005901:Glaucoma / Q000503:physiopathology / Q000601:surgery; D020327:Glaucoma Drainage Implants; D007429:Intraocular Pressure / Q000502:physiology; D008855:Microscopy, Electron, Scanning; D010477:Perfusion; D011312:Pressure; D012590:Sclera; D013524:Surgical Flaps; D034421:Sus scrofa