Abstract

SA.01.05

Progresses for the technical calibration of modern tonometers

Joerg Draeger1, Thomas Schwenteck2
1Klinik und Poliklinik für Augenheilkunde, Universitätsklinikum Hamburg-Eppendorf, Hamburg; 2Fachlaboratorium: Druck und Strömungsmessung in der Medizin, Physikalisch-Technische Bundesanstalt, Berlin

Objective
Only precise calibration of modern tonometers guarantees reliability of measurement assessed for clinical diagnosis or follow-up of the glaucomas. The correctness for the measurement of intraocular pressure, the fundamental measurement parameter in ophthalmology, leads to right estimation and correct assessment of the health state of human eye. The measurement accuracy connected with the knowledge of measurement uncertainty of a tonometer  characterise the measurement quality of modern tonometers.
Design:
The calibration of human eye tonometers is in the most cases performed by clinical comparison measurements with a reference tonometer. The purpose of this comparison is the exact determination between the measured values of a reference tonometer and the tonometer to be calibrated (called test tonometer). Before carrying out the clinical comparison measurements the reference tonometer has to be calibrated by means of  special test apparatuses.  
Methods
The international standard of tonometers ISO 8612 is the current basis to control the calibration of modern tonometers. This standard applies two different methods for data analysis between measurement results of reference and test tonometer, the differences method and the total method of least squares. The clinical calibration state of tonometers is transfered to tonometers of the same design by means of test apparatuses. These test apparatuses own a feedback to physical unities, for instance the mass, the length, the  force, the pressure. By applying these test apparatuses the clinical calibration state of a test tonometer is conserved.
Main outcome measure: A new procedure is introduced and performed for the description of the mathematical dependence between the measurements events of reference and test tonometer. The application of the new method is  presented, elucidated, and assessed.
Results
The  new performed method contains the advantages of both mathematical procedures of the international standard ISO 8612 with a better representation of measurements results by applying of one proceeding. The new method is applicable to calibrate modern tonometers of completely different design, e. g. non-contact tonometers, rebound tonometers, impedance tonometers, contur tonometers, and so on.
Conclusions
The correct calibration by clinical comparison measurements of is the fundamental basis for the  measurement quality and measurement stability of modern tonometers. The correct  measurement of intraocular pressure on human eye guarantees to apply right medical treatment methods in diagnostics and therapy.  

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