Method and Apparatus for Determining Intra Ocular Pressure of an Eye

• US 20070173713A1
• Filed: 04/05/2007
• Published: 07/26/2007
• Est. Priority Date: 06/25/2002
• Status: Abandoned Application

• Current Assignee: Falck Medical, Inc.
• Original Assignee: Falck Medical, Inc.
• Inventors: Falck, Robert, Falck, Francis
• Application Number:
  US11/697,006
• Publication Number:
  US 20070173713A1
• Time in Patent Office: Days
• US Class Current: 600/405
• CPC Class Codes: A61B 3/16 for measuring intraocular pressure, e.g. tonometers
• CPC Class Codes: A61B 5/02216 Ophthalmodynamometers

Abstract

A tonometer, which uses an applanation prism that reflects light inversely related to an applanated area of the cornea, determines intraocular pressure (IOP) by making initial contact with the cornea, and then pressing the prism against the eye through a predetermined departure range from the initial contact value. The departure can add a predetermined prism pressing force beyond the initial contact value or can increase prism pressing force sufficiently to produce a predetermined increase in the applanated area. From the change in applanated area and prism pressing force required, the microprocessor can be programmed to determine IOP in several ways that do not suffer inaccuracy from variations in tear volume and corneal thickness and curvature in different eyes being examined.

Claims

1. A method of measuring intraocular pressure (IOP) of an eye, the method using a tonometer having a programmed microprocessor controlling an actuator to press a prism with a variable and determinable force against a cornea of the eye while a source directs light to reflect from an applanation surface of the prism to a detector producing a detected light signal received by the microprocessor, which calculates IOP, the method being implemented by the microprocessor controlling the tonometer so that the method comprises:

operating the tonometer to recognize a change in the detected light signal from a pre-contact value occurring before the prism contacts the eye to an initial contact value occurring upon initial contact between the cornea and the applanation surface, the initial contact value representing an initial applanation area that varies from one eye to another based on specific tear volume and biomechanical properties of the cornea of the eye being examined;

operating the actuator to press the prism applanation surface against the cornea with increasing force applied during a time interval so that the detected light signal changes as an applanated area increases from the initial contact value by a predetermined amount; and

determining IOP from a ratio of increased prism pressing force needed to increase the applanated area from the initial contact value by the predetermined amount.

2. A tonometer operated by the method of claim 1.

3. The method of claim 1 wherein the recognized change in the detected light signal upon initial contact is a reduction from the pre-contact value to the initial contact value.

4. The method of claim 1 including mounting the prism on a counterbalanced arm so that the prism is free to move as the prism is moved into the initial contact.

5. The method of claim 1 wherein the actuator prism pressing force is divided into predetermined increments of force applied at predetermined intervals of time.

6. The method of claim 5 including using the time intervals as a measure of increased prism pressing force.

7. The method of claim 1 including subtracting the initial contact value from the increased applanation area before determining IOP.

8. A tonometer operated by the method of claim 7.

9. A tonometer operated by the method of claim 1 including considering predetermined initial steps of actuator force as part of the initial contact value.

10. A tonometer comprising:

an applanation prism mounted on a rotationally counterbalanced arm;

an electromagnetic actuator arranged to move the arm in response to a microprocessor to press the prism against a cornea of an eye being examined;

the prism and the counterbalanced arm being arranged so that upon initial contact of an applanation surface of the prism with the cornea of the eye being examined, the arm is free to move, and the electromagnetic actuator does not press the prism against the cornea;

the microprocessor being programmed to press the prism against the cornea after the initial contact is made to produce a predetermined departure from the initial contact;

when the departure is a predetermined increase in applanation area, then the microprocessor is programmed to determine intraocular pressure from the prism pressing forces required to reach the predetermined increase in applanation area; and

when the departure is a predetermined application of prism pressing force by the electromagnetic actuator, then the microprocessor is programmed to determine intraocular pressure from the difference between an applanation area of the initial contact and applanation areas occurring during the predetermined application of prism pressing force.

11. The tonometer of claim 10 wherein the microprocessor is programmed to subtract an initial contact signal from signal values reached during the predetermined departures before determining intraocular pressure.

12. The tonometer of claim 10 wherein the actuator presses the prism against the cornea in predetermined incremental increases in force applied at predetermined time intervals so that a total applied force and a total elapsed time are equivalent.

13. The tonometer of claim 10 wherein the microprocessor, in determining the intraocular pressure, is programmed to consider predetermined initial steps of actuator force as part of the initial contact.

14. A method of measuring intraocular pressure (IOP) of an eye by using an applanation tonometer having a prism with an applanation surface, a source directing light into the prism to be incident on the applanation surface, and a detector detecting light reflected from the applanation surface, the method comprising:

mounting the prism on a rotationally counterbalanced arm that is free to move upon initial contact of the applanation surface with the cornea, surface tension of tears in the eye supplying a force to pull the freely movable prism against the eye to applanate an initial contact area whose size is a function of the tears and of biomechanical properties of the cornea of the eye;

applying an electromagnetic force to the prism arm to press the prism against the eye after the initial contact to enlarge the applanated area; and

determining IOP by using at least one of the following;

a) the prism pressing force needed to enlarge the initial applanated area by a predetermined amount; and

b) an increase in applanated area caused by a predetermined application of the electromagnetic force.

15. A tonometer operated by the method of claim 14.

16. The method of claim 14 wherein the IOP is determined by the electromagnetic force divided by the applanation area.

17. The method of claim 14 wherein the electromagnetic force is applied in predetermined force increments at predetermined time intervals so that prism pressing force varies directly with elapsed time.

18. A method of measuring intraocular pressure (IOP) of an eye using an applanation tonometer having a prism with an applanation surface mounted on a pivot arm, a source directing light into the prism to be incident on the applanation surface, and a detector detecting light reflected from the applanation surface, the method comprising:

counterbalancing the pivot arm so that surface tension of tears is the only force pressing the applanation surface against the cornea as the applanation surface makes initial contact with the cornea;

arranging the prism, the counterbalanced pivot arm, the source, and the detector so that light reflected from the applanation surface diminishes from a pre-contact value to an initial contact value representing an initial applanation area occurring upon initial contact of the prism applanation surface with the eye;

applying electromagnetic force to the pivot arm to press the prism against the eye after the initial contact to enlarge the initial applanation area by a predetermined amount; and

determining IOP by the ratio of the enlarged applanation area and the electromagnetic prism pressing force required to enlarge the applanated area to the predetermined amount.

19. A tonometer operated by the method of claim 18.

20. The method of claim 18 including applying the electromagnetic prism pressing force in predetermined force increments at predetermined time intervals to make the force increments and the time intervals equivalent.

21. The method of claim 20 including determining the prism pressing force by the number of time intervals used in applying the prism pressing force.

22. The method of claim 20 including subtracting the initial contact value from the enlarged applanated area.

23. A method of measuring intraocular pressure (IOP) of an eye by using an applanation tonometer having a prism with an applanation surface, a source directing light into the prism to be incident on the applanation surface, and a detector detecting light reflected from the applanation surface, the method comprising:

bringing the applanation surface of the prism into contact with the cornea of the eye so that surface tension of tears in the eye being examined pulls the applanation surface against the cornea, and the surface tension is resisted by corneal thickness and curvature of the eye being examined to produce an initial contact applanation area;

pressing the prism against the cornea to increase a size of the initial contact applanation area by a predetermined amount; and

determining IOP from the prism pressing force to increase the applanated area by the predetermined amount.

24. A tonometer operated by the method of claim 23.

25. The method of claim 23 including mounting the prism on a counterbalanced arm so that the prism is free to move as the prism is moved into the initial contact.