[Comparison of dynamic contour tonometry versus Goldmann applanation tonometry according to the International Ocular Tonometer Standards ISO 8612 in glaucoma patients]. / Vergleich der dynamischen Konturtonometrie vs. Goldmann-Applanationstonometrie nach dem internationalen Standard für Augentonometer (ISO 8612) in einem Glaukomkollektiv.

INTRODUCTION: The aim of this study was to compare the dynamic contour tonometry PASCAL® (DCT) versus the Goldmann applanation tonometry (GAT) in a glaucoma population and to analyse the correlation with the central corneal thickness (CCT). PATIENTS/METHODS: 191 eyes of 107 Caucasian glaucoma patients (62 female, 45 male) were included: 3 repeated GAT and dynamic contour tonometry (DCT) measurements (quality factor 1-2) were analysed. CCT was measured by ultrasound pachymetry. Criteria of ISO 8612 were fulfilled: 3 groups of IOP measurements were formed: group 1: 7-16 mmHg, group 2: 16-23 mmHg and group 3: > 23 mmHg, each including at least 40 eyes. Only 5.0 % outliers per area of intraocular pressure were permitted in a tolerance of ± 5.0 mmHg. RESULTS: Data of 191 eyes (mean CCT 553 µm) were analysed. GAT (19.68 mmHg ± 7.56 mmHg) showed good correlation to DCT (20.54 ± 8.21 mmHg) (r = 0.770, p < 0.001). Mean difference DCT-GAT was 0.86 ± 2.45 mmHg. In regard to the criteria of ISO 8612, the number of outliers were: group 1: (n = 68) 29.4 %, group 2: (n = 62) 41.9 %, and group 3: (n = 61) 31.2 %. No correlation was shown between CCT vs. GAT (r = 0.184, p = 0.057) and CCT vs. DCT (r = 0.177, p = 0.09), respectively. DISCUSSION: In conclusion, despite good correlation to GAT measurements DCT does not fulfil the ISO 8612 reference criteria in glaucoma patients. DCT and GAT did not show correlation to CCT.

[How does central cornea thickness influence intraocular pressure during applanation and contour tonometry?]. / Wie beeinflusst die zentrale Hornhautdicke den intraokularen Druck bei der Applanations- und Konturtonometrie?

BACKGROUND: Golmann applanation tonometry represents a well-established procedure for measuring intraocular pressure (IOP). This implies the necessity of an accurate measurement of IOP with the reference tonometer. One example is the contour tonometer Pascal with a measuring probe, adapted to the cornea geometry, for measuring the IOP and the ocular pulse amplitude. There is controversy of how strongly corneal thickness affects the measurement of IOP. We thus analysed, for a number of eyes, the correlation of IOP, as measured by two types of applanation tonometers and one contour tonometer and the central corneal thickness. METHODS: In all 158 patient eyes were investigated in a clinical comparison of applanation tonometers AT 870 and Ocuton-A. The study was performed by a trained ophthalmologist and the comparison was in accordance with international standard ISO 8612. In addition, the corneal thickness in the vertex was repeatedly determined using an Oculus Pentacam. The potential effect of central corneal thickness on the IOP as measured by the mentioned tonometers was statistically evaluated by rank correlation analysis. RESULTS: We found that the measured IOP values for the three investigated tonometers were not normally distributed. The central corneal thickness values, in contrast, measured on 158 eyes by means of an ultrasound pachymeter and additionally on 235 eyes by the Pentacam, obeyed a Gaussian distribution. For the correlation analysis of both parameters the Spearman linear rank correlation coefficient (r) was considered. We found a very weak (|r| < 0.2) correlation between central corneal thickness and IOP for all 3 tonometers. The softness of the correlation is also illustrated by a large standard deviation of the regression line. A comparison of the different devices for corneal-thickness measurements shows less variance and a smaller variation coefficient when the ultrasoundpachymeter AL-1000 is used. CONCLUSIONS: The measured values for IOP are only very weakly correlated to the central corneal thickness. For the 3 tonometer types studied there is no need to correct the indicated pressure values according to the central corneal thickness of the investigated eye. Clinical comparisons according to the ISO 8612 standard between a tonometer under test and a reference Goldmann applanation tonometer are always a time-consuming procedure. Additional measures to determine the central corneal thickness of every investigated eyes are dispensable.

[Metrological controls for impression tonometers - ensuring quality standards in ophthalmology]. / Messtechnische Kontrollen für Impressionstonometer - eine Qualitätsgarantie in der Augenheilkunde.

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. CONCLUSION: 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.

[The safety of metrological control of applanation tonometers by technical investigations on transfer standards]. / Die Sicherung der messtechnischen Kontrolle von Applanationstonometern durch technische Untersuchungen an Transfernormalen.

BACKGROUND: According to the German medical devices act and German ordinance on operators of medical devices, human eye tonometers have to be retested every two years. Independent verification laboratories carry out the metrological controls for tonometers which are used by ophthalmogists. Applanation tonometers are frequently used for the accurate measurement of intraocular pressure. For metrological controls these measurement devices are retested by means of test devices (transfer standards) used by verification offices and by private test agencies. PROCEDURE AND METHOD: 25 test devices for applanation tonometers (15 cross-beam balances and 10 force transducer devices), applied for metrological controls by verification offices and by private test agencies, were checked by means of comparative measurements with regard to their quality of measurement and their handling. An applanation tonometer of the type AT 870 (manufacturer: Haag-Streit, Switzerland) served as reference device. Within a measurement range from 9.81 mN to 78.45 mN, upward and downward measurements were performed for the mechanical force of applanation tonometers and for the determination of the reverse span. RESULTS: The investigations for all test devices of applanation tonometers show the compliance within permissible error limits in the observed measuring range for the reverse span with the tonometer AT 870. Some instruments exceed the permissible error limits for test devices for up- and downward force measurements. A comparison concerning the measuring accuracy of two different types of test devices (cross-beam balance and force transducer device) shows that the force transducers tend to have smaller measurement uncertainties. CONCLUSION: The high metrological demands on test devices for applanation tonometers for the measurement of mechanical force, a prerequisite for accurate calibration and adjustment of this tonometer type, require much experience and special training of the operator handling these standards, in order to minimize subjective error sources. Schoolings with practical trainings for user are recommended to minimize these errors. At least every three years the test devices have to be maintained and recalibrated, in order to guarantee proper function and accuracy of the measurement.

[Maintenance of measuring quality of transfer standards for the metrological control of air impulse tonometers]. / Wahrung der Messgüte von Transfernormalen für die messtechnische Kontrolle von Luftimpulstonometern.

BACKGROUND: According to the German Medical Devices Act ("Medizinproduktegesetz") from 2.8.1994 and the German Directive for Operators of Medical Devices ("Medizinprodukte-Betreiberverordnung") from 29.6.1998, human eye tonometers in Germany have to be checked by metrological controls every two years. Every ophthalmologist using such instruments in his medical practice has to ensure they are metrologically checked by independent testing agencies. One class of tonometers frequently used by ophthalmologists is the air impulse tonometer. PTB has designed a test device for this particular type of tonometer. A small series of these devices has been built and are in use by verification offices and private test agencies since 1984. PROCEDURE AND METHOD: At a seminar hosted by the "Deutsche Akademie für Metrologie" in 2005 the accuracy of 14 test devices for air impulse tonometers was checked in a comparative set of measurements. All test devices are in use for metrological control by verification offices or private test agencies. In the comparison a clinically calibrated air impulse tonometer "XPERT NCT PLUS" (manufactured by Leica in Buffalo, USA) was used as a reference instrument Three simulated intraocular pressure values, covering the full range of the tonometer (0 mmHg to 60 mmHg), had to be set on each test device and measured with the tonometer. For every device and every pre-set pressure value, a minimum of ten usable measurements were performed and statistically analysed. RESULTS: In this comparative measurement the averaged readings given by the reference tonometer were generally in good agreement with the pre-set reference values of 15.0 mmHg, 30.0 mmHg, and 45.0 mmHg on the testing devices. Only in the 15 mmHg case did two out of 13 test devices give a result slightly out of the permissible limits. One device had to be eliminated from the test due to a defect bearing. CONCLUSION: With only two exceptions, the results for all test devices agreed with the nominal values within permissible tolerances. These two exceptions may have been caused by slight misalignments between the respective test devices and the tonometer. In summary, it can be concluded that even 20-year-old test devices, if maintained and re-calibrated by the Physikalisch-Technische Bundesanstalt at least tri-annually, are still well suited for metrological control measurements.

[New trends in tonometry]. / Neue Trends in der Tonometrie.

BACKGROUND: Though the knowledge about the risk of elevated intraocular pressure for pathogenesis for development and progress of glaucoma is more than 400 years old. It took another 300 years before by means of tonometry this elevation could be quantified. After discussion of the key words it is our aim to demonstrate the latest trends in tonometry, specifically by use of modern electronics for instrument design. MATERIALS: Goldmann was the founder of the "applanation tonometry", measuring the corelation of applanated area and applied force. Here, the next step was the development of instruments, measuring independent of position or gravity to allow application also at the lying patient. Electronic area detection was the next step to avoid examinor's error. Also important was the introduction of automatic UV-desinfection to avoid contamination of the other eye or other patients. Other tonometers control the force applied by an inductive method, which also allows application independent of position. Airpuff-tonometry uses completely different measuring principles - the resulting change of corneal curvature indicates the intraocular pressure. The impedance principle is applied even through the closed eye lid. The shock waves resulting shall be measured. Direct application of a surface sensor incorporated into a concave surface contact body shall allow direct measurement of intraocular pressure independent of corneal thickness, curvature, astigmatism. This method has been tried for more than 20 years. Also exposition of the eye to count waves of different frequency was an early attempt. Here also change of reflectivity is used to calculate intraocular pressure. CONCLUSIONS: Even after use of modern electronic technology the precision of many of these new devices does not lead to better results. This specifically is shown by modern calibration methods.

[Clinical evaluation of a new tonometer on the basis of international standard ISO 8612]. / Klinische Evaluierung eines neuen Tonometers auf der Basis des internationalen Standards für Augentonometer ISO 8612.

BACKGROUND: There is a continuous process of development of new tonometer types following different measurement principles on the tonometer market. A new tonometer working on the dynamic contour measurement principle is currently available for ophthalmologists. This device has special properties, since it determines the intraocular pressure as well as the ocular pulse amplitude. So far, clinical comparison measurements exist between the new tonometer Pascal and the applanation tonometer by Goldmann albeit without any reference to a standard. Besides, these measurements cover only a small pressure range between 7 mmHg and 23 mmHg which is not sufficient for medical diagnosis. PATIENTS AND METHODS: This new tonometer was investigated by clinical comparison measurements according to the currently valid international standard for human eye tonometers (ISO 8612). The tested tonometer determines the intraocular pressure without fluorescein. In an ophthalmogical hospital in Berlin the clinical comparison measurements were carried out by ophthalmologists on 127 patients according to the methods and criteria of the above-mentioned standard. A calibrated AT 870 from the Haag-Streit company served as reference tonometer. RESULTS: The different cornea surface geometries of individual eyes, the variation of tear liquid volume, and also the different elastic properties of the human eye due to a large variation in rigidity led to measurements of intraocular pressure by this tonometer with an excessively high number of outliers p (i) (i = 1, ..., 3) per group (p (1) = 16.28 %, p (2) = 8.85 %, p (3) = 15.69 %, 5.0 % outliers per group only are permitted for the differences method), which do not correspond to the requirements of the international standard for tonometers ISO 8612. This is also shown by application of total method of least squares of regression line (slope a = 0.964, interception b = 2.868 mmHg, standard deviation s (y) = 2.628 mmHg). CONCLUSIONS: The test of conformity of results of clinical comparison measurements to the international standard for tonometers ISO 8612 is the target of this work. The measurement results of the investigated tonometer are analysed and discussed according to this standard. Statements about the medical application of this medical measurement device for ophthalmologists are presented. According to the presented investigations the requirements of this international standard are not fulfilled by the tonometer Pascal.

[Clinical Testing of New Tonometers on the Basis of the International Standard for Human eye Tonometers ISO 8612]. / Klinische Erprobung neuer Augentonometer auf der Basis des internationalen Standards für Augentonometer ISO 8612.

There is a continuous process of development of new tonometer types following different measurement principles. The impedance measurement method, using the elasticity of the eyeball, allows tonometry even through the eyelid. Two new tonometers based on the impedance principle were investigated by clinical comparison measurements according to the international standard for human eye tonometers ( ISO 8612). The tested tonometers determine the intraocular pressure without topical anaesthesia. The different elastic properties of an individual eyelid and also the different elastic properties of the human eye due to a large variation in rigidity cause serious problems for these measurements. Statistical properties of the two investigated tonometers are presented and discussed. Statements and conclusions about the medical application of these measurement devices for ophthalmologists are made.