Surgery – Diagnostic testing – Testing aqueous humor pressure or related condition
Reexamination Certificate
2001-10-05
2004-01-06
Hindenburg, Max F. (Department: 3736)
Surgery
Diagnostic testing
Testing aqueous humor pressure or related condition
C600S402000, C600S400000
Reexamination Certificate
active
06673014
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to methods and apparatuses for measuring the intraocular pressure of an eye, and more particularly to tonometer methods and apparatuses for performing this measurement without touching the eye itself.
BACKGROUND OF THE INVENTION
Normal pressure within the human eye ranges between about 10 mm Hg and about 20 mm Hg above atmospheric pressure, which ranges between 700 mm Hg and 800 mm Hg at sea level, and which is nominally around 760 mm Hg at zero degrees Celsius. The eye pressure above atmospheric pressure is formally called intraocular pressure, or IOP. The intraocular pressure varies during the time of day by an amount of 3 mm Hg to 4 mm Hg, generally being highest in the morning. It also varies during the course of the year, generally being highest in the winter.
Glaucoma is a disease whereby peripheral vision is lost first, and it is related to elevation of the pressure within the eye to values higher than 21 mm Hg above atmospheric pressure. Such elevated pressure, over long duration, can cause blindness. Glaucoma affects as much as 2% to 3% of the population over the age of 40, and is a leading cause of blindness. The disease can be treated, but not cured, by application of one of a number of drug-therapy regimes. These regimes usually last for the rest of the patient's life, and require close monitoring and frequent eye-pressure measurements. In cases when the drug treatment is inadequate, laser or incisional surgery may be tried.
Instruments for measuring eye pressure are called tonometers. They are typically not portable, and could be quite expensive. Moreover, they need to be operated by a doctor or trained technician while the patient holds a fixed position with respect to the instrument. To date, there has not been a commercially successful tonometer which can be operated by the patient alone and that is portable and inexpensive, (although it should be mentioned that in April 2001 a new eyelid-contacting tonometer has been introduced, operating on the claimed experimental effect that when an object is pushed against the eye there will be a faint light halo appearing in the eye at the point in time when the external pressure at the area of contact equals the intraocular pressure at that moment.) The consequence of this is that frequent measurements of the patient's eyes are typically not made in order to determine the full range over which the patient's IOP varies. Because of this, doctors end up using a less than optimal application of the drug-therapy regimes since they do not have enough measurement data to fine-tune the regimes. In addition, a doctor may fail to suspect and diagnose a patient's glaucoma because the tonometer measurement may have been taken at a time when the patient's IOP was at its lowest point in the range during the measurement.
Most of the tonometers described above operate by pressing an area of the eye by a known force and then measuring the resulting displacement, or by pressing the area of the eye by a known displacement and measuring the force required to do so. The former approach may be conducted by an “air-puff” tonometer, which blows a puff of air toward the eye at a known force. Either of the above approaches may be conducted by a contact tonometer, which has a plunger that physically contacts the eye. Air-puff tonometers are uncomfortable, and contact tonometers require that the patient's eyes be anesthetized.
To address these problems, much research work has been done in the area of vibration tonometers. These tonometers apply vibrations to the eye, such as by a loud speaker or by a vibrating element contacted to the eyelid, vary the frequency of vibrations to find the maximum amplitude vibration of the eye (called the “resonance point”), and compute the IOP based on the frequency of maximum amplitude vibration. These tonometers are based on the assumption that the human eye can be modeled as a spherical body of water held together by the surface tension of the water (the so-called “water drop” model). Such a body of water has a plurality of vibratory modes n=1, 2, 3, . . . , each of which has a corresponding natural frequency, or resonant frequency f
n,res
, at which the surface vibrations of the water drop are at maximums. The value of each resonant frequency depends upon the difference in pressure, &Dgr;p, between the interior of the water body and the external atmosphere, as provided by the following equation:
f
n
,
res
=
λ
n
·
Δ
p
/
ρ
π
·
a
where:
&lgr;
n
is the eigenvalue of the n-th mode, having approximate values of 1.0, 1.94, 3.0, and 4.18 for value of n=2 through n=5;
&rgr; is the fluid density;
&agr; is the radius of the sphere, and
&pgr; is a constant equal to the ratio of the circumference of a circle to its diameter (3.14159. . . ).
As applied to the eye, the pressure difference &Dgr;p has been equated to the eye's intraocular pressure, as the IOP is defined as the pressure in the eye that is above atmospheric pressure. Many prior art approaches have used the above to model the eye.
However, it is important to note that the water-drop model predicts a zero value for each resonant frequency at 0 mm Hg of intraocular pressure. That is to say that at 0 mm Hg, f
1,res
=0, f
2,res
=0, f
3,res
=0, f
4,res
=0, etc.
As indicated by U.S. Pat. No. 5,865,742 to Massie (Non-Contact Tonometer), the use of this model for the measurement of intraocular pressure (IOP) has not met with success. The following quote from U.S. Pat. No. 5,865,742 points to some reasons for lack of success:
“One additional type is the vibration tonometer, first patented in the 1960's (U.S. Pat. Nos. 3,192,765 and 3,882,718). In this device, it is proposed that the response of the eye to a vibrational excitation will be a measure of the IOP. The proposed exciters include very low-frequency sound and mechanical plungers. However, it is likely that the vibrational frequencies of the eye are affected by many factors not related to the IOP. It is, in fact, expected that the actual resonance spectrum of the eye would be dictated more by the connective tissue than by the IOP. All of these factors may be the reason why no commercial use of the vibration tonometer has been disclosed even though its development has been attempted” (Massey patent, column2, lines 50 to 62).
A thorough theoretical background going beyond the simple water balloon model is provided by “A Nonlinear Modal Frequency Response Analysis of the Pre-stressed Human Eye by the Finite Element Method” by K. C. Henderson (submitted in partial fulfillment of the requirements for the degree Master of Science, University of Rochester, 1995) with experimental results described in a concurrent associated thesis for the same degree at the same university: “Intraocular Pressure Measurement Using Resonance Detection” by K. S. Bhella.
While vibration tonomoters offer the possibility of inexpensive and convenient measurement tools, they have not met with successful implementation, and consequently have not met with commercial success. The present invention is directed to providing a vibration tonometer that does not touch the surface of the eye and that provides accurate and reliable results, and which is affordable by home users.
SUMMARY OF THE INVENTION
In making their invention, the inventors have recognized that the “resonant frequencies” computed by the water-drop model do not account for the damping by the surrounding tissue and connective muscles, and that the frequencies computed by the model are, in reality, undamped natural frequencies that do not take into account the damping. The inventors have further determined that nearly all of the prior art vibration tonometers have measured each water-drop “resonant frequency” of the eye by finding a frequency at which an area of the eye's sclera undergoes maximum vibratory displacement when excited by an excitation source, and that this resonant frequenc
Badehi Avner Pierre
Glazer Arieh
Klein Raphael
Foreman Jonathan
Itonix, Inc.
Sheppard Mullin Richter & Hampton LLP
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