Classifications

• • A—HUMAN NECESSITIES
  • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
  • A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
  • A61B3/00—Apparatus for testing the eyes; Instruments for examining the eyes
  • A61B3/10—Objective types, i.e. instruments for examining the eyes independent of the patients' perceptions or reactions
  • A61B3/16—Objective types, i.e. instruments for examining the eyes independent of the patients' perceptions or reactions for measuring intraocular pressure, e.g. tonometers

Definitions

• the invention relates to an applanation tonometer, in particular for applanation tonometry according to Goldmann, according to the preamble of patent claim 1.
• the internal pressure in the eye can be determined with sufficient accuracy by flattening the cornea by means of a pressure body and measuring the force required to achieve a certain flattening area.
• the diameter of the 15 flattened surface should be 3.06 mm, since with this diameter the intrinsic rigidity of the cornea and the capillary adhesion of the tear fluid on the corneal surface are precisely balanced, so that the force required for flattening is maintained in relation to the flattening area corresponds to a pressure of just 20 dm in the eye.
• the diameter of the flattened surface was determined visually using a scale in the observation beam path.
• a surface sensor for the applied surface which uses the resistance change of an electrode arrangement due to the wetting with tear fluid to determine the size of the surface.
• the pressure body is designed as a prism, which is illuminated by a light source.
• the entrance and exit faces of the prism are formed such that the illumination light is totally reflected at the locations where the Druckkör- 20 P e r rests on the cornea, while the illumination light at the points at which the pressing body does not lie, not is reflected.
• the intensity of the total reflected light is measured with a detector, the output signal of which is a measure of the size of the applied 2 . 5 area is used.
• the sensors mentioned above for the applied surface - as has been recognized according to the invention - have a systematic measurement error due to the adherence of tear fluid to the pressure body: due to capillary adhesion, tear fluid also adheres to the side of the applied surface, so that with the known sensors not the size of the actually applied area, but an enlarged 10th area compared to this area.
• the invention is based on the object of specifying an application tonometer in which an exact determination of the pressure prevailing in the eye is possible.
• the flattened area of the cornea 5 is measured with an area sensor. Since the applied surface and the liquid ring surrounding it are measured with the surface sensor, the size of the liquid ring is determined according to the invention before the actual applanation measurement. This information is obtained - as TX - has also been recognized according to the invention - from the output signal of the area sensor with "zero applanation", i.e. the moment the pressure body "just” touches the corneal surface. With the size of the liquid ring determined in this way, the measurement result for the applied surface is then corrected during the measurement, i.e. essentially subtracts the area of the surrounding liquid ring from the measured area. This means that regardless of the specific surface wetting, the actual applanation diameter is always obtained, which allows the application of the IMBERT-FICK law.
• the force with which the flattening is caused is determined during the measurement. This determination of the actually acting force can either be carried out by evaluating the specific data of the drive used or by using a force measuring device.
• capacitive area- 0 * sensor is utilized, the good electrical conductivity in water acceptance range cornea.
• the applanation surface serves as a plate of a capacitor, against which a metal plate coated with a dielectric is pressed as a sensor surface and as a pressure body. This creates
• the measuring device 30 can be an optical sensor for determining the flattened area. This optical sensor uses the different refraction and reflection properties when touching media with different refractive index.
• the beam guidance in the measuring head can be designed in this way be, as described for example in DE-OS 26 43 879. Furthermore, it can also be designed in such a way that where the cornea or tear fluid is present as an aqueous medium, the rays are broken away from the plane of incidence away from the cornea, while the surface areas reflect totally against air. In any case, the size of the wetted sensor surface can be deduced from the difference between incident and reflected light, which is measured with an intensity sensor. Furthermore, it is also possible to work with a spatially resolving sensor, for example to directly determine the diameter of the applied surface.
• a further possibility for the area sensor is characterized in claim 6: a multiplicity of conductor tracks, which are connected to high-resistance input connections of the control unit, are applied to the sensor surface, which lies against the cornea.
• the comparatively good conductive cornea or the tear fluid briefly infer the directly adjacent conductor tracks to a potential that is applied, for example, to the forehead support of the tonometer.
• the control unit "counts" the short-circuited conductor tracks.
• This surface sensor also has the advantage over the sensor known from DE-PS 20 40 238 that individual fluctuations in the resistance of the cornea have no influence on the measurement result.
• the drives characterized in claims 7 and 8 not only offer the possibility to apply the forces required to achieve the desired application area in the range of about 1 to 5 ponds simply and quickly — so that no falsifications of the measurement result can occur — but allow them also in a simple way to determine the applied force from the feed current.
• a particularly advantageous embodiment of the tonometer according to the invention is characterized.
• an axial force transducer is used, which is in contact with the surface sensor, so that the instantaneous pressure can be determined with the application of any force.
• FIG. 1 a part of a tonometer according to the invention, namely a capacitive area sensor
• FIG. 2 part of a tonometer according to the invention namely a "potential line area sensor”.
• FIG. 1 shows part of an application tonometer according to the invention, namely a pressure body designed as a surface sensor.
• the pressure body has an electrically conductive plate 1, which is coated with a dielectric 2.
• the plate 1, the dielectric 2 and the cornea 3 of the applied eye form a capacitor, the capacitance C of which is measured with the measuring circuit indicated schematically in FIG. 1.
• This measuring circuit can be closed, for example, via the forehead support of the tonometer.
• the capacitance of the capacitor is a measure of the applied area and the tear fluid ring 4 surrounding this area.
• the size of the wetting of the sensor surface is the capacitance of the capacitor formed with zero applanation, i.e. measured at the moment when the sensor surface just touches the cornea 3 without flattening it.
• the control unit (not shown) can at any time correct the applanation area measured at a certain force by the instantaneous proportion of the liquid ring 4 surrounding the actual applanation area, since the width b of the liquid ring 4 from the "zero measurement" is known.
• the applanation tonometer according to the invention is not only suitable for measurements according to Goldmann, in which the force is determined which is required to achieve a flattened surface with a diameter of 3.06 mm, but also for other measurements , for example for measurement in 10 of veterinary medicine, where other diameters of the applied area are required, or for measurements in human medicine in which force / applanation area characteristics are recorded.
• FIG. 2 shows an embodiment of the surface sensor of an applanation tomometer according to the invention.
• the pressure body not shown in detail, has a large number of conductor tracks 10 to 10 on its contact surface, which are connected to inputs of a control unit 11.
• An analog or digital circuit or a computer can be used as the control unit.
• it is particularly advantageous to use a single-chip microprocessor which, in particular when a device without a drive for the applanation force is used, enables the design of a compact and lightweight device which is only of pen size.
• the applanation tonometer according to the invention can also be integrated into a slit lamp or an observation magnifier.
• devices can of course also be used in which a position-independent drive for the application force, for example a moving coil or a Voice coil drive is used.
• a position-independent drive for the application force for example a moving coil or a Voice coil drive is used.
• These drives have the advantage that a rapid increase in power can be achieved with them.
• 25 force applied to the drive can be determined via the feed current.

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• Life Sciences & Earth Sciences (AREA)
• Health & Medical Sciences (AREA)
• Medical Informatics (AREA)
• Biophysics (AREA)
• Ophthalmology & Optometry (AREA)
• Engineering & Computer Science (AREA)
• Biomedical Technology (AREA)
• Heart & Thoracic Surgery (AREA)
• Physics & Mathematics (AREA)
• Molecular Biology (AREA)
• Surgery (AREA)
• Animal Behavior & Ethology (AREA)
• General Health & Medical Sciences (AREA)
• Public Health (AREA)
• Veterinary Medicine (AREA)
• Eye Examination Apparatus (AREA)

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Citations (6)

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• 1985
  • 1985-09-21 DE DE19853533693 patent/DE3533693A1/de active Granted
• 1986
  • 1986-09-22 WO PCT/DE1986/000387 patent/WO1987001572A1/de unknown
  • 1986-09-22 EP EP19860905229 patent/EP0238522A1/de active Pending

Patent Citations (6)

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