Automatic applanation tonometer

• US 4,621,644 A
• Filed: 04/13/1984
• Issued: 11/11/1986
• Est. Priority Date: 04/13/1984
• Status: Expired due to Fees

• Current Assignee: George J. Eilers
• Original Assignee: George J. Eilers
• Inventors: Eilers, George J.
• Primary Examiner(s): Coven, Edward M.
• Application Number:
  US06/599,968
• Time in Patent Office: 942 Days
• Field of Search: 128/652, 128/645, 128/748, 128/639, 128/774, 324/61 R, 324/61 P, 324/72.5
• US Class Current: 600/405
• CPC Class Codes: A61B 3/16 for measuring intraocular pressure, e.g. tonometers

Abstract

A portion of the cornea or similar flexible membrane is flattened by urging against the cornea a footplate with a measurable force and the magnitude of the applanated area of the cornea is sensed and compared to a threshold which corresponds to an area equal to a selected reference area of the planar footplate contacting the cornea. When the selected threshold is reached the force urging the footplate against the cornea is processed for scaling by another signal proportional to the reference area. In another embodiment signals proportional to both the applied force and the applanated area are digitized for averaging and/or storage and the two parameter functional dependence of area on the force or a selected region thereof is displayed and recorded for analysis.

Claims

1. An applanation tonometer for ascertaining the internal pressure of a flexible body containing a fluid, said body subject to curvature due to said internal pressure, comprising

(a) a footplate comprising a planar portion for bearing against said flexible body, a shaft for supporting said footplate and a guard electrode spaced apart from said footplate and substantially surrounding said footplate,

(b) a housing comprising means for supporting said footplate, said footplate capable of displacement toward said body whereby a portion of the surface of said flexible body is flattened against said footplate,

(c) force sensing means for ascertaining the magnitude of the force exerted by said flexible body against said footplate and for generating a force proportional,

(d) area signal generating means for developing an area proportional signal responsive to the area of said flexible body in contact with said footplate comprising oscillator means for driving both said footplate and said guard electrode from the same oscillating potential,

(e) comparator means responsive to said area proportional signal for establishing the equality or excess of said area proportional signal with respect to a threshold value of selectable magnitude and for generating a trigger signal representative of an area magnitude, said signal indicative of condition of equality or excess of said area proportional signal with respect to said threshold value,

(f) scaling means responsive to said trigger signal for dividing the instantaneous signal magnitude of said force proportional signal with a representation of said area proportional signal to obtain a pressure value, and

(g) display means to indicate said pressure value.

2. The applanation tonometer of claim 1 wherein said means for supporting said footplate comprises spring means for coupling said shaft to said housing.

3. The applanation tonometer of claim 2 wherein said area signal generating means comprises means for measuring the capacitive reactance of said footplate in series with said flexible body.

4. The applanation tonometer of claim 2 wherein said force sensing means comprises a linear variable differential transformer mechanically coupled to said shaft.

5. The applanation tonometer of claim 2 wherein said force sensing means comprises a piezo electric transducer mechanically coupled to said shaft.

6. The applanation tonometer of claim 2 wherein said force sensing means comprises variable reluctance transducer means to generate a signal responsive to the displacement of said footplate.

7. The applanation tonometer of claim 2 wherein said spring means comprise a plurality of flat metallic members and means for securing peripheral portions thereof to said housing, said shaft bonded to a central portion of each said member.

8. The applanation tonometer of claim 7 wherein said housing comprises an insulating material, the bond between said shaft and said springs means is electrically conducting and said footplate is in electrical contact with at least one said spring means.

9. The applanation tonometer of claim 7 wherein said footplate is insulated from said shaft, said shaft is a hollow metallic body, and said footplate is in electrical contact with a conductor supported within said shaft.

10. The method of measuring the internal pressure within a flexible body containing a fluid, the walls of said body curved outwardly due to said pressure, comprising the steps of

(a) applying a force against said outwardly curved walls to deform a portion thereof against a footplate to conform to a planar surface portion,

(b) sensing the area of said planar surface portion, comprising the steps of applying an oscillatory potential of selected waveform to said footplate, whereby an oscillatory current flows through said planar surface portion while maintaining an equipotential surface region substantially surrounding said footplate by applying an oscillating potential of the same selected waveform to said equipotential surface and generating an area proportional signal in response to the magnitude of said oscillatory current,

(c) generating a trigger signal upon detecting a preselected relative magnitude of said area proportional signal with a selected value,

(d) measuring the magnitude of the force resulting from step (a) said step of measuring concurrent with the detecting of said preselected relative magnitude,

(e) combining the force resulting from step (a) with said preselected relative magnitude of said area proportional signal to obtain the ratio of said force of said area, and

(f) displaying said ratio.

11. The method of studying conditions of the eye comprising the steps of

(a) applying a mechanical force to flatten a portion of the surface of the cornea,

(b) deriving a first signal proportional to said applied force,

(c) obtaining a second signal proportional to the area of the flattened portion achieved by step (a),

(d) changing the magnitude of said mechanical force while repeatedly sampling the instantaneous values of each said first and second signals to obtain a set of said values for establishing the functional relationship of the quantities represented by said signals,

(e) combining said first signal with said second signal to obtain the ratio of said first to said second signal for each said sample whereby an applanation function is obtained.

12. Apparatus for ascertaining the pressure internal to a flexible fluid filled body, comprising

(a) probe means disposed for support in a vertical aspect by said body, said probe means comprising a flat member for contacting said body and flattening a portion of the surface of said body in proportion to the gravitational loading by said flat member,

(b) area sensing means for generating a signal proportional to the magnitude of said flattened area and comprising first means for coupling an oscillatory current to said body through said flat member, current measuring means for measuring said oscillatory current, guard means substantially surrounding both said flat member and at least a portion of said current measuring means, and guard potential means for applying to said guard means an oscillatory potential of identical waveform as said oscillatory current and having a magnitude substantially equal to the potential obtaining at said flat member,

(c) scaling means operative upon said area proportional signal for obtaining a pressure signal indicative of said hydrostatic pressure, and

(d) display means for indicating a representation of said pressure signal.

13. The apparatus of claim 12 wherein said flat member further comprises a thin dielectric layer on the surface thereof for contacting said body and said area sensing means comprises means responsive to capacitive reactance arising from said contact between said flat member and said flexible body in an electrical circuit which includes said flat member and said flexible body.

14. In a corneal applanation tonometer,

(a) footplate means for applanating the subject cornea, comprising an applanating disk having a dielectric layer disposed on one side thereof, and footplate support means for supporting said footplate in contact with the cornea,

(b) guard electrode means substantially surrounding said footplate means and spaced apart therefrom for providing a controlled equipotential surface thereabout,

(c) oscillator means providing an oscillatory current coupled to said guard ring through a first impedance and in turn coupled to said footplate through a second impedance and thence through said dielectric layer to the cornea,

(d) differential amplifier having respective inputs coupled to said footplate means and said guard electrode for obtaining a difference signal proportional to the oscillating potential difference between said footplate and said guard electrode,

(e) phase sensitive detector for developing a d.c. signal from the phase related difference signal and oscillator means, whereby said d.c. signal represents the capacitive impedance coupling of said cornea and said footplate, and

(f) force sensing means for generating a force proportional signal in response to the force applied by the cornea against said footplate,

(g) discriminator means for comparing said d.c. signal with a reference d.c. potential and deriving a trigger signal upon the equality of said reference potential and said d.c. signal, and

(h) display means responsive to said trigger signal for displaying a representation of said force proportional signal, whereby said force proportional signal expresses a measure of corneal pressure relative to said capcitive impedance value.