title: Does the surface property of a disposable applanation tonometer account for its underestimation of intraocular pressure when compared with the Goldmann tonometer?
abstract: BACKGROUND
  Disposable tonometers are increasingly being adopted partly because of concerns over the transmission of variant Creutzfeldt-Jakob disease and partly for convenience. Recently, we have found one such tonometer (Tonojet by Luneau Ophthalmologie, France) underestimated the intraocular pressure (IOP).
  
  
  METHODS
  We hypothesized that this underestimation was caused by a difference in the surface property of the tonometers. A tensiometer was used to measure the suction force resulting from interfacial tension between a solution of lignocaine and fluorescein and the tonometers.
  
  
  RESULTS
  The results showed that the suction force was significantly greater for the Goldmann compared to the Tonojet.
  
  
  CONCLUSIONS
  The magnitude of this force was too small to account for the difference in IOP measurements. The Tonojet was less hydrophilic than the Goldmann, and the contact angle of the fluid was therefore greater. For a given tear film, less hydrophilic tonometers will tend to have thicker mires, and this may lead to underestimation of the IOP. When such disposable tonometers are used, it is recommended care should be taken to reject readings from thick mires.
authors.affiliation: St. Paul's Ophthalmology Unit, Royal Liverpool University Hospital, Prescot Street, Liverpool, L7 8XP, UK.
authors.forename: Sarah F
authors.initials: SF
authors.lastname: Osborne
authors.forename: Rachel
authors.initials: R
authors.lastname: Williams
authors.forename: Mark
authors.initials: M
authors.lastname: Batterbury
authors.forename: David
authors.initials: D
authors.lastname: Wong
mesh_terms: D003951:Diagnostic Errors; D004209:Disposable Equipment; D019793:Fluorescein / Q000737:chemistry; D007429:Intraocular Pressure / Q000502:physiology*; D008012:Lidocaine / Q000737:chemistry; D015203:Reproducibility of Results; D013499:Surface Properties; D013500:Surface Tension; D014065:Tonometry, Ocular / Q000295:instrumentation*
chemical_list: D008012:Lidocaine; D019793:Fluorescein