title: [Metrological controls for impression tonometers - ensuring quality standards in ophthalmology].
abstract: BACKGROUND
  Impression tonometers are not only used for fast measurements of intraocular pressure on supine patients but also during surgery on the human eye. These mechanically very delicate devices have to be checked by metrological controls according to the German Medical Devices Act ("Medizinproduktegeset") and the German Ordinance on Operators of Medical Devices ("Medizinprodukte-Betreiberverordnung") every two years. These controls are carried out by independent testing agencies and are mandatory for all ophthalmologists in Germany using impression tonometers. For this purpose, special test devices have been developed by Physikalisch-Technische Bundesanstalt, Germany's national metrology institute. A large number of these test devices have been in use at private test agencies and verification offices since 1975. PROCEDURE AND METHOD: Eighteen test devices for impression tonometers from private test agencies, verification offices, or the Physikalisch-Technische Bundesanstalt were investigated. The instruments were checked for their measurement accuracy and their suitability to perform metrological controls. An impression tonometer of the Schiötz design served as reference instrument. The test devices were investigated with respect to parameters like tonometer mass, mass of the plunger-lever-pointer system, plunger displacement, or correlation of the curvature between foot plate and plunger. The findings were analysed by statistic methods.
  
  
  RESULTS
  The main outcome of the present work is the conclusion that the investigated test devices are generally suitable to perform metrological controls on impression tonometers. For certain parameters some instruments reached the error tolerances, but the majority was inside permissible limits. For one parameter (effective mass of plunger-lever-pointer system for scale value 10) the reference tonometer itself was not within permissible tolerances. This was correctly determined by all test devices. Failures of test devices due to damage or maladjustment were not observed.
  
  
  CONCLUSIONS
  Test devices for impression tonometers are mechanically delicate instruments. They have to be recalibrated at fixed intervals in order to remain suitable for metrological controls. If regularly maintained and recalibrated at least every three years, even devices with an age of 20 years or more are still absolutely fit for this purpose.
authors.affiliation: Physikalisch-Technische Bundesanstalt, AG 8.13, Berlin. thomas.schwenteck@ptb.de
authors.forename: T
authors.initials: T
authors.lastname: Schwenteck
authors.forename: H-J
authors.initials: HJ
authors.lastname: Thiemich
mesh_terms: D004867:Equipment Design; D019544:Equipment Failure Analysis / Q000592:standards*; D005858:Germany; D008365:Manometry / Q000295:instrumentation* / Q000592:standards*; D009885:Ophthalmology / Q000592:standards*; D011785:Quality Assurance, Health Care / Q000592:standards*; D015203:Reproducibility of Results; D012680:Sensitivity and Specificity