Impression Tonometry Research Articles

Mathematical Modeling of Applanation and Impression Tonometry after Surgical Correction of Vision

Abstract—Mathematical modeling of applanation and impression tonometry (according to Maklakoff and Schiotz, respectively) was performed for the eye with artificially induced strong inhomogeneity of the elastic properties of the cornea. Such inhomogeneity occurs as a result of surgical correction of vision. Corneal weakening was considered for either the apical area (correction of myopia) or the peripheral annular zone lying close to the sclera (correction of hyperopia). A two-component model of the eyeball was used, where the cornea is represented by a momentless, deformable, linearly elastic surface and the scleral area is represented by an elastic element that responds to changes in intraocular pressure by changing its volume. Within this representation, the mechanical properties of the cornea are characterized by effective stiffness, which is a parameter that contains both the elastic properties of the cornea material and the corneal thickness. The relationships employed in the standard processing of tonometry data, as well as simulation results obtained previously for the case of a homogeneous cornea, were tested for applicability to operated eyes with significant inhomogeneity of the elastic properties of the cornea. The bias caused by inhomogeneity was found to be the most significant for Schiotz tonometry with an apical weakening and a rather high intraocular pressure. For Maklakoff tonometry, inhomogeneity exerted a less significant effect on the calculation of the true pressure in either type of corneal weakening.

Theoretical basis for estimating the elastic characteristics of individual living human eyes

AbstractPurpose To develop a method for estimating the effective elastic parameters of an individual eye from the data obtainable in various clinical tests with a static load applied.Methods Our mathematical model that represents the fibrous coat as a system consisting of a soft elastic shell (cornea) and an elastic element characterized by a single elastic constant (sclera and tissues surrounding it) is used for calculating the mechanical behavior of the eyeball during different loading procedures. The calculations were performed for two types of external mechanical loading corresponding to applanation and impression tonometry, as well as for inflating the eye by liquid. The presence in the model of only two important mechanical constants makes it possible to identify them by comparing the data of different tests and then to find the effective elastic characteristics important in diagnostics.Results A technique of determining the elastic constants of the model from the standard diagnostic tests is developed. This technique is applied to estimating the average elastic characteristics of the human eyeball in vivo basing on the data available from the literature.Conclusion Our results form a basis for obtaining in standard clinical tests representative individual mechanical characteristics of the living eye. This makes it possible to calculate effective (integral) elastic parameters needed in processing a specific test from the data obtained from other tests, for example, to estimate the volume rigidity from elastometry data. We can now correctly interpret the term “rigidity”: in different tests it is commonly used for characteristics that may differ substantially.

Vergleich der Applanations- und Impressionstonometrie in der zentralafrikanischen Bantu-Bevölkerung

Glaucoma is the main reason for irreversible blindness in the Central African population. Because of the high prevalence, the rapid progression and the limited possibilities of diagnosis and treatment in Africa, glaucoma leads more often to blindness, compared with other parts of the world. The irreversible character of the glaucoma makes it necessary to recognise this disease as soon as possible. Beside the appearance of the optic nerve and visual field defects, tonometry is considered as an essential part of the diagnosis of glaucoma. This study compares the Shiotz tonometer with applanation tonometry to find out whether the Shiotz tonometer presents similar results in the African population. In the time between October 2001 and July 2002 in the Democratic Republic of Congo 2020 eyes of 1027 persons were examined. Beside other ophthalmological examinations the intraocular pressure were taken by applanation (Perkins tonometer) and impression tonometry (Shiotz tonometer). A difference of more than 2 mmHg between the two methods was found in 26.5 % of the examined eyes. Among the first 1000 eyes this difference was found in 40 % of the eyes, while among the second part of the examined eyes only in 13.2 % did a difference appear. Both methods showed a good correlation with k = 0.96 (p < 0.01). For short- or long-sighted eyes, a difference between both methods was found more often than for normal eyes. Among the long-sighted eyes the impression tonometry found higher values for the intraocular pressure more often than among the short-sighted eyes. But altogether the applanation tonometry showed higher values more often for short- as well as for long-sighted eyes. There were no correlations between the age of the examined persons and the difference between the two methods. Altogether it can be said that for screening impression tonometry shows a satisfactory correlation with applanation tonometry in the hands of a well-trained ophthalmologist, who has done at least 1000 examinations. Differences between the two methods are mainly found for long- or short-sighted eyes.

Measuring intraocular pressure in keratoconus. Effect of the changed biomechanics

Keratoconus is associated with changes in the corneal structure, such as defects of Bowman's layer, a decrease of corneal thickness etc. They result in alterations of some of the biomechanical parameters of the cornea, namely, rigidity and elasticity. The present study was performed to examine how impression tonometry and applanation tonometry for determination of intraocular pressure (IOP) are affected by the changed biomechanical parameters associated with keratoconus. We examined 20 normal subjects (40 eyes) and 17 keratoconus patients (25 eyes). The corneal thickness was measured by ultrasound pachymetry in the corneal center. In the keratoconus patients, an additional measurement was made at the conus peak. The corneal curvature was determined using the TMS keratoscope. The IOP measurements were made with the Schiötz tonometer (10 g). For comparison, additional IOP measurements in the corneal center and, in the keratoconus group, on the conus peak were made with the applanation tonometer. The normal subjects had a central corneal thickness of 548 +/- 30 microns, compared to 505 +/- 42 microns in the corneal center and 425 +/- 41 microns on the conus peak in keratoconus patients. The average corneal curvature was 43.3 +/- 1.8 D in the normal subjects and 47.8 +/- 4.1 D in keratoconus patients. Applanation tonometry produced results on 11.33 +/- 1.43 mm Hg in the normal group (corneal center) compared to values of 12.00 +/- 2.55 mm Hg (corneal center) and 7.30 +/- 1.95 mm Hg (conus peak) in the keratoconus cohort. The coefficient of rigidity was 0.0236 +/- 0.0026 microliter-1 in the normal subjects, compared to 0.0173 +/- 0.0050 microliter-1 in the keratoconus patients. The morphological changes associated with keratoconus may cause tonometry errors.

Impression Tonometry of Chelonian Eggs (Reptilia, Testudines), a Guide to Internal Pressure and Membrane Phenomena

The weighted SchiOtz tonometer, an instrument used in ophthalmology for estimation of intraocular pressure through measurement of ocular tension, was used in studies on internal pressure and shell-membrane characteristics of freshwater turtle eggs of five species. The descriptive value of graphic profiles of egg tension portrayed by tonometry was demonstrated. Correlated tonometric and manometric studies on the same eggs provided a means of giving pressure values to tonometer readings, and of differentiating structural and pressure factors in tonometer measurement of egg tension. Impression tonometry is suggested as a technique for both field and laboratory monitoring of physical changes in reptilian eggs.

Model analysis of impression tonometry and tonography

Experimental equipment: Model spherical eyeballs and corneas have been constructed of rubber with stress-strain characteristics similar to those of selera. A model tonometer has simulated a Schiotz tonometer with a 7·5 g weight. Three sizes of ball corresponded with eyes of 22, 24 and 26 mm axial lengths (dimensions commonly found in emmetropia): three corneal curvature scorresponded to radii of 7·17, 7·75 and 8·23 mm (a wider range than would be expected in emmetropia—ametropias have been included). Tonometry: The pressure rise produced by a tonometer is greatest in the smallest ball and least in the largest ball, probably because of differences in volume—or surface area, if nonspherical shapes are considered. Rather surprisingly, indentation by the plunger at any given pressure (corneas the same) is least for the smallest ball and greatest for the largest ball, presumably because the steeply curved (small) ball will provide more support for its cornea than a ball with a flatter curve (i.e. larger). Both these effects, i.e. volume and curvature of wall of eyeball excluding cornea, would tend to be additive in producing erroneous estimates of intraocular pressure by Schiotz tonometry, e.g. a smaller-than-normal eyeball would give an erroneously high estimate. A steep cornea provides more support for the plunger of the tonometer than a flat one so that an erroneously high pressure reading from the former and a low one from the latter would be obtained. Since a steep cornea tends to be associated with a small ball, its error will be additive to that produced by the effect of volume and wall curvature. These effects, especially of total volume, must be important factors in “ocular rigidity”, although others probably exist. Applanation tonometry is hardly affected by them. Tonography: A large variation exists between eyes in normal and glaucoma patients, and the dividing line between these two groups is not very clear. The change in volume of the model seleral envelope, due more to distortion (“ squashing ”) than expansion-contraction , has been shown to be much greater than that attributable to corneal indentation. If standard pressure-indentation-volume curves calibrated on “normal” (model) eyeballs and a Friedenwald nomogram are used, a smaller than average eyeball will appear to have a greater outflow facility than in reality it has, and a larger eyeball will appear to have a smaller, other things being equal; the reason is that a given “volume out” will be associated with a greater fall in pressure and therefore a greater increase in indentation in the former than in the latter. The chances of an erroneous conclusion from tonography because of variations in eye volume would be less if the weight of the footplate were reduced and if its curvature were less flat, i.e. more like that of the cornea, or if changes in pressure were eliminated during indentation (isotonography).

The corneal indentation and intraocular pressure in impression tonometry

Die vom Stempel des Impressionstonometers erzeugte Hornhautdelle wird im optischen Schnitt erstmals dargestellt und photographisch registriert. Das Verhalten der Hornhaut wird im Bereich der Tonometerskalenwerte 2–10 untersucht und die Strecke b — i.e. die Distanz zwischen Stempelwand und jener Stelle, wo sich die eingedellte Hornhaut der Fusplatte anlegt — gemessen. Moses u. Hahns Befunde, wonach nur jene Skalenwerte verwertbar sind, bei welchen b groser ist als die Breite des zwischen Stempel und Fusplatte vorhandenen Spaltes, werden durch die Ergebnisse dieser Untersuchungen weitgehend bestatigt.

Continuous determination of intraocular pressure in the rat

A method has been developed allowing the continuous determination and recording of intraocular pressure in the anaesthetized rat. The method is based on the principle of impression tonometry. This method yielded relative values for the intraocular pressure, which were recorded simultaneously with the mean arterial pressure. Accordingly, the procedure described seems suitable for the pharmacological investigation of drugs that may influence both intraocular and blood pressure

The coefficient of scleral rigidity in normal and glaucomatous eyes.

Within recent years the early diagnosis of glaucoma has made considerable strides even though the management and social consequences of this disease remain as always a challenge to ophthalmology. The tonometer remains one of the most important tools in the early diagnosis of glaucoma, both as a means of measuring the intraocular pressure, as well as a means of carrying out tonography. In conjunction with scotometry, it gives the best indication of the success of medical control of conservative therapy. It has been realized for some considerable time1that all forms of impression tonometry with the Schiotz, McLean, or Gradle tonometers are far from reliable guides of the intraocular pressure, even when accurately standardized, because the indentation is influenced by variations in the scleral rigidity of normal human eyes and to a lesser degree by differences in corneal curvature. It has also been appreciated that tonography is influenced by

Impression tonometry and the effect of eye volume variation.

Impression tonometry and the effect of eye volume variation.

TONOMETRIC STANDARDIZATION

There are two ways whereby the intraocular pressure may be measured : manometry and tonometry. Of the two the former alone attains any degree of scientific accuracy. Manometry, however, being a clinically inapplicable method, suitable only for the laboratory, it has been necessary to devise an instrument for estimating the intraocular pressure by interpretation of measurements taken of the impressibility of the globe. This is the method of impression tonometry. It cannot be too strongly emphasized that the tonometer is not an instrument of accuracy in recording an absolute measure of the intraocular pressure. The tension of an eye is in part due to the pressure of its contents, but it is not identical with this pressure nor does it vary absolutely with it. Actually it is the impressibility of the eyeball which is measured by the tonometer. From this measure is deduced the tension of the eye, and the absolute