LOAD SENSING APPLANATION TONOMETER
Field of the Invention
The present invention pertains to medical diagnostic systems, and more
particularly to an improved applanation tonometer with greater reliability in measuring the
intraocular pressure of an eye.
Background of the Invention
Tonometers are known in the art and are widely used to facilitate indirect
measurement of the intraocular pressure of a person's eye. The ability to obtain
measurements of intraocular pressure has proved important for detecting diseases, such as
glaucoma, or other ailments of the eye.
Various types of tonometers have been developed and may be classified,
generally, as non-contact tonometers, indentation tonometers and applanation tonometers.
Non-contact tonometers generally utilize a puff of air applied to the cornea of an eye in
conjunction with shining a light beam onto the eye. As the cornea becomes flattened by the
puff of air, the light intensity reflected from the cornea of the eye varies and is detected by an
optical device. Non-contact type tonometers are generally considered by medical
professionals to be less accurate than contact-type tonometers and thus are primarily used for
initial screening purposes.
While non-contact type tonometers do not actually touch the eye, indentation
tonometers and applanation tonometers include members that are actually brought into
physical contact with the cornea of an eye to depress a portion of the cornea while obtaining a
measurement of force. An indentation tonometer utilizes a weighted plunger to apply a known
force to the cornea while measuring the deformation produced. An applanation tonometer
measures the force required to flatten a predetermined area of the cornea. By far the most
widely used mechanical tonometer is the Goldmann applanation tonometer, described in U.S.
Patent No. 3,070,997. Using the Goldmann applanation tonometer, the cornea of an eye
being tested is flattened with a probe having a 3.06 mm diameter flat area. Once the cornea
has been flattened to the 3.06 mm diameter, the force required to flatten the cornea is
measured and used to calculate the internal pressure of the eye.
While the Goldmann applanation tonometer is widely used and familiar to
practitioners, it also suffers several drawbacks. For example, the Goldmann-type tonometer
is a complex mechanical system comprising internal weights, springs and bearings which must
be periodically calibrated to ensure accurate measurements of intraocular eye pressure. This
drawback is heightened by the fact that there are relatively few persons who have the
resources and technical capability for performing these periodic calibrations. Furthermore, the
Goldmann applanation tonometer is susceptible to error due to improper balancing or level
orientation of the tonometer with respect to the microscope and is sensitive to interference
from electromagnetic fields. Accordingly, there is thus a need for an applanation tonometer
which overcomes drawbacks of the prior art such as those described above.
Summary of the Invention
The present invention overcomes these drawbacks by incorporating into an
applanation tonometer a force sensor that is configured to sense the force applied by the
probe to the cornea of the patient's eye, and to generate a signal corresponding to the sensed
force. This inventive applanation tonometer has an applanation probe containing a prism
system that, when viewed through a microscope assembly, generates a visual indication when
a desired area of the cornea of a patient's eye has been flattened by the probe. This inventive
applanation tonometer can be used with a conventional microscope assembly, such as a
biomicroscope slit lamp, in a manner that is familiar to and widely accepted by medical
practitioners, to measure the intraocular pressure of a patient's eye. Advantageously, the
applanation tonometer of the present invention permits measuring the intraocular pressure of a
patient's eye without the need for the complex mechanical calibrations of internal weights,
springs, and bearings of prior art devices. Accordingly, the applanation tonometer ensures
robust performance without the need for precise balancing and orientation of the device.
Furthermore, the need to have specially trained technicians periodically calibrate the system
by tediously adjusting these internal components is eliminated. Instead, the force sensor of the
present invention may easily be "zeroed-out" by users as required.
The signal generated by the force sensor may be transmitted to a display for
indication of the measured intraocular pressure, and/or it may be transmitted to a data storage
device configured to receive and store information for individual patients. The display may be
part of the applanation tonometer, or it may be a stand alone unit. Alternatively, the display
may be configured as a heads-up display that can be viewed by a user through the
microscope assembly while applanating the patient's eye with the probe.
hi one aspect of the invention, the applanation tonometer includes a housing,
and the applanation probe is supported on one end of a support arm movably coupled to the
housing. As the probe is brought into contact with the cornea of the patient's eye, the
opposite end of the support arm engages the force sensor, whereby force applied to the
patient's eye is sensed by the force sensor.
In another aspect of the invention, the support arm is coupled to a rotatable
knob configured to move the support arm, and thus the applanation probe supported on the
support arm. The probe may be advanced to applanate the cornea by rotating the knob.
In yet another aspect of the invention, an instrument for measuring the
intraocular pressure of a patient's eye comprises a stand, a microscope supported on the
stand, a slit lamp coupled to the stand, and an applanation tonometer having a prism system
and force sensor, as described above.
In another aspect of the invention, a method of measuring intraocular pressure
of an eye includes applanating the cornea of the eye, viewing the applanated cornea through a
microscope, sensing the force required to applanate the cornea, sending a signal
corresponding to the sensed force to an output device, and displaying a value at the output
device that corresponds to the sensed force.
The features and objectives of the present invention will become more readily
understood from the following Detailed Description and the accompanying drawings.
Brief Description of the Drawings
FIG. 1 is a perspective view of a biomicroscope slit lamp including an
applanation tonometer in accordance with a preferred embodiment of the invention.
FIG. 2 illustrates the tonometer of FIG. 1 during applanation of a subject's
eye.
FIG. 3 is a side view of the exemplary applanation tonometer of FIG. 1.
FIGS. 4 A - 4C are schematic illustrations depicting an applanated cornea
viewed through the applanation tonometer of the present invention.
FIG. 5 is a partial cross sectional view of the applanation tonometer of FIG.
3, taken along line 5-5.
FIGS. 6 A - 6B are cross sectional views of the applanation tonometer of
FIG. 5 taken along line 6-6 and illustrating operation of the applanation tonometer.
FIGS. 7 A - 7B are cross sectional views of the applanation tonometer of
FIG. 5, taken along line 7-7 and further illustrating operation of the applanation tonometer.
FIGS. 8A-8B are views depicting an exemplary display for the applanation
tonometer of FIG. 1.
FIG. 9 is an exemplary electrical schematic for an applanation tonometer
constructed in accordance with a preferred embodiment of the invention.
Detailed Description of the Preferred Embodiment
FIG. 1 shows a microscope assembly 10 configured to facilitate the
examination of a patient's eye 12, as known in the art. The microscope assembly 10
generally comprises a base 14 which is securely mounted to a table 16, or other secure
structure, and a patient support 18 extending upward from the base 14. A microscope 20 is
pivotally supported to the base 14 and is shown in FIG. 1 as a biomicroscope, having dual
eyepieces 21 for viewing the anatomy of a patient's eye 12. It will be recognized, however,
that the microscope 20 may alternatively comprise only a single eyepiece. A chin rest 22 and
a head brace 24 are coupled to the patient support 18, in a known manner, whereby a patient
26 may position his or her chin on the chin rest 22 and bring their head forward so that the
forehead rests against the head brace 24 to position the patient 26 for observation through the
microscope 20. The microscope assembly 10 further includes a slit lamp 28 pivotally
mounted to the base 14, whereby the slit lamp 28 maybe swung into a position for directing a
beam of light into the patient's eye 12, as known in the art.
FIG. 1 shows that the microscope assembly 10 also includes an applanation
tonometer 30 of the present invention. The applanation tonometer 30 is pivotally mounted to
the microscope assembly 10 by a mount fixture 32 and support arm 34 pivotally coupled to
the microscope 20. Alternatively, the exemplary applanation tonometer 30 may be pivotally
mounted to an arm (not shown) coupled to the base 14 of the microscope assembly 10.
With the patient 26 in position, the applanation tonometer 30 and microscope
20 may be moved forward such that a probe 36 of the applanation tonometer 30 may be
brought into contact with the cornea of the patient's eye 12, as depicted in FIG. 2. FIG. 2
further illustrates use of the slit lamp 28 to direct a beam of light into the patient's eye 12 while
applanating the cornea with the probe 36 of the tonometer 30, all while viewing the
applanated cornea through the microscope 20. Just prior to contact with the cornea, the
positions of the microscope 20 and tonometer 30 are firmly secured and the probe 36 is
advanced to applanate the cornea of the patient's eye 12 using knobs 38 provided on the
applanation tonometer 30, as depicted in FIGS. 2 and 3. The probe 36 of the applanation
tonometer 30 includes a prism system 40 configured to provide an indication when the cornea
of the patient has been applanated to a desired area. This prism system 40 is well known in
the art and disclosed in U.S. Patent No. 3,070,997 to Papritz et al. An exemplary prism
system 40 is part no. 0900.2236A, available from Haag-Streit AG, Berne, Switzerland.
FIGS. 4A - 4C illustrate the image created by the prism system 40 while
applanating the patient's eye 12, as viewed through the eyepiece 21 of the microscope 20.
The prism system 40 creates two semicircular arcs 42a, 42b which translate relative to one
another while the probe 36 is advanced to applanate the patient's eye 12. When the probe
36 has applanated the desired area of the cornea, ends 44a, 44b of the respective arcs 42a,
42b coincide to produce a sinusoidal image as depicted in FIG. 4A. When the applanation
tonometer 30 has flattened an area of the cornea greater than the desired area, the ends 44a,
44b of the arcs 42a, 42b are offset as depicted in FIG. 4B. Similarly, when the applanation
probe 36 flattens an area of the cornea less than the desired area, the arcs 42a, 42b are offset
as depicted in FIG. 4C. Accordingly, a user may adjust the position of the probe 36 to
obtain the desired applanation area by turning the knob 38 on the tonometer 30 and viewing
the applanated cornea through the microscope 20 and probe 36 until the ends 44a, 44b of the
respective arcs 42a, 42b coincide as depicted in FIG. 4A. At this point, the applanation
probe 36 has flattened the cornea to the desired area for measurement of intraocular pressure.
Advantageously, the applanation tonometer 30 is configured to sense the
force of the probe 36 against the applanated cornea and to provide to the user a signal
representative of the sensed force, hi one embodiment, the signal is transmitted to a display
46 that is configured to indicate a value corresponding to the force sensed by the applanation
tonometer 30, as depicted in FIGS. 8A-8B. a preferred embodiment, the display 46
indicates a value representative of the intraocular pressure of the eye 12. The display 46 may
be mounted to the microscope assembly 10, such as on the applanation tonometer, for
example, as shown in FIGS. 8A-8B, or may be provided on a separate unit. Alternatively,
the display 6 may be configured as a heads up display which is viewable through the
microscope 20, whereby a user may readily observe the indicated value corresponding to the
force of the applanation probe 36 against the cornea, while permitting the user to continue to
observe the applanated cornea of the patient 26.
FIGS. 5-7 show the construction and operation of the applanation tonometer
30. With particular reference to FIG. 5, the applanation tonometer 30 includes a housing 50
having opposing side walls 52a, 52b, front and rear walls 54, 56, and opposing top and
bottom walls 58a, 58b. The side walls 52a, 52b, front and rear walls 54, 56, and top and
bottom walls 58a, 58b of the housing 50 are secured to and enclose a frame 60 for supporting
the interior components of the applanation tonometer 30. The frame 60 has a generally
rectangular shape defined by opposing side plates 62a, 62b and upper and lower plates 64a,
64b. A probe arm 66 extends through a hole formed through the upper plate 64a and top
wall 58a of the housing 50 and supports the applanation probe 36 at a distal end 68. A cap
69 disposed on probe arm 66 covers the hole formed through top wall 58a of housing 50.
The proximal end 70 of the probe arm 66 is coupled to a horizontal shaft 72 extending
between the side plates 62a, 62b of the frame 60. The shaft 72 is coupled at its ends 74 to
the side plates 62a, 62b of the frame 60 such that the shaft 72 may be rotated about its
longitudinal axis. The ends 74 of the shaft 72 are supported by ball bearings 76 that are press
fit into apertures formed in the respective side plates 62a, 62b of the frame 60. In this
configuration, the probe arm 66 may pivot about the shaft 72 to permit movement of the
applanation probe 36 in a direction substantially transverse to the longitudinal axis of the shaft
72.
A balance arm 80 having a movable balance weight 82 is coupled to the shaft
72 such that the balance weight 82 may be positioned along the balance arm 80 to counteract
the moment offeree created by the mass of the applanation probe 36 at the distal end 68 of
the probe arm 66. The applanation tonometer 30 is configured to limit the motion of the
probe 36 to provide controlled applanation of the patient's eye 12. As shown in FIGS. 5, 6A
and 6B, a limit adjustment member 84 is coupled to the shaft 72 by a collar 85 and extends
generally rearwardly through an aperture 86 formed through a transverse rib 88 extending
between the side plates 62a, 62b of the frame 60. Set screws 90a, 90b are installed through
tapped holes 92a, 92b in the frame 60 proximate the limit aperture 86, whereby movement of
the limit adjustment member 84 in the aperture 86 may be controlled by adjusting the set
screws 90a, 90b to provide a desired clearance between the limit adjustment member 84 and
the set screws 90a, 90b. In this manner, rotation of shaft 72, and thus movement of
applanation probe 36 on probe arm 66, is limited over a desired range.
The applanation tonometer 30 further includes a force sensor 100, such as
sensor model No. GSO-100 available from Transducer Technologies, Temecula, CA,
disposed within the housing 50 and configured to sense a force applied to the applanation
probe 36. The force sensor 100 is secured to a sensor support 102 slidably coupled to the
bottom plate 64b of the frame 60 by a slider mechanism 104, such as Linear Slide No. Y-
LMS-156, available from Parker Hannifin Corp., Daedal Division, Irwin, PA.
The slider mechanism 104 is configured to constrain the sensor support 102
for linear movement between the front wall 54 and rear wall 56 of the housing 30, as depicted
by arrow 106 in FIGS. 7A and 7B. The sensor support 102 cooperates with a cam 108 that
is threadably coupled to a horizontally positioned threaded rod 110 extending between side
plates 62a, 62b and coupled to adjustment knobs 38 at its distal ends. Threaded rod 110
rotates in bearings (not shown) installed through apertures in side plates 62a5 62b. The cam
108 has a bearing 112 positioned to contact a follower surface 114 of the sensor support 102
whereby rotation of the knobs 38 causes the cam 108 to move along the threaded rod 110,
between side plates 62a, 62b, as indicated by directional arrow 116. A torsion spring 118 is
disposed within the housing 50 and has a first spring arm 120a coupled to the lower plate 64b
and a second spring arm 120b engaging the force sensor 100 whereby the spring 118 biases
the sensor support 102 and force sensor 100 in a direction toward front wall 54.
Accordingly, the spring 118 urges the follower surface 114 of the sensor support 102 into
engagement with the bearing 112 on the cam 108.
With particular reference to FIGS. 6A-6B and 7A-7B, as knobs 38 are
manipulated to cause the cam 108 to move to the left in FIGS. 7A, 7B, as illustrated by arrow
116, bearing 112 on the cam 108 engages the follower surface 114 and causes the sensor
support 102 to move in a direction toward the rear wall 56 against the bias force of the spring
118. As shown in FIGS. 6A-6B, as the sensor support 102 moves toward the rear wall 56,
an actuating member 122 of the force sensor 100 engages the proximal end 70 of the probe
arm 66, thereby causing the probe arm 66 to pivot about the horizontal shaft 72. As the
probe arm 66 pivots about horizontal shaft 72, the probe 36 is brought into contact with the
cornea of the patient's eye 12 and maybe advanced to applanate an area of the patient's eye,
limited by the limit adjustment member 84 which also pivots about horizontal shaft 72. While
the probe 36 applanates the patient's eye 12, the force required to applanate the patient's eye
12 is transferred through the probe arm 66 and applied to the actuating member 122 of the
force sensor 100. The force sensor 100 generates a signal related to the force applied to the
cornea of the patient's eye 12. Advantageously, the signal maybe transmitted from the force
sensor 100 to the display 46 to indicate a value related to the force applied to the cornea of
the patient's eye 12. As shown in the figures, the force sensor 100 transmits the signal to the
display 46 via a wire. However, it will be recognized that the signal may be transmitted by
other structure or devices, such as by radio frequency or infrared transmission paths to the
display 46.
FIGS. 8 A and 8B show an exemplary display 46 for indicating a value related
to the force of the probe 36 that is required to applanate the patient's eye 12 to a desired
area. The display 46 is mounted to the housing 50 of the applanation tonometer 30. The
display 46 is coupled to an electronic circuit 48 disposed within the housing 50 and depicted
by hidden lines in FIG. 8B. The electronic circuit 48 controls the format of the value indicated
on the display 46. For example, the circuit 48 may be used to cause the display 46 to
indicate the intraocular pressure of the patient's eye in millimeters of mercury, based on the
signal received from the force sensor 100. As shown, the display 46 is pivotally coupled to
the housing 50, so that it may be adjustably positioned for convenient viewing by a user of the
applanation tonometer 30.
FIG. 9 shows an exemplary electrical schematic 130 for the applanation
tonometer 30. hi FIG. 9, force sensor 100 has a power supply 132, which may be external
or may be incorporated internally, as a battery for example. Force sensor 100 communicates
with an electric circuit 48, which generally comprises an amplifier 134, an analog-to-digital
(A/D) converter 136, and a microprocessor 138. Signals produced by the force sensor 100,
in response to engagement of applanation probe 36 with the cornea of a patient's eye 12, are
sent to the amplifier 134, A/D converter 136 and microprocessor 138 for conditioning into a
signal that can be received and used by various peripheral devices. For example, the
conditioned signal from the electronic circuit 48 may be transmitted to the display 46 for
indication of intraocular pressure to a user, as described above. Alternatively, the conditioned
signal maybe transmitted to a separate heads-up display 140 or a patient data interface 142
that is configured to receive and store data from the applanation tonometer that is related to
the intraocular pressure for a given patient 26.
Advantageously, the applanation tonometer 30 of the present invention may
be used in a manner which is familiar to and well accepted by medical practitioners who are
familiar with the well-known Goldmann tonometer. But this invention also provides a
convenient display of the force required to applanate a patient's eye 12. Thus, the invention
uses the same paradigm that is currently most familiar to medical practitioners, but provides
significant advantages over prior conventional devices. More specifically, the applanation
tonometer 30 of the present invention overcomes drawbacks of prior art tonometers by
eliminating the complex internal mechanisms utilized in those devices and replacing them with a
force sensor 100 which provides a digital signal. Accordingly, the applanation tonometer 30
of the present invention is less sensitive to the effects of gravity or electromagnetic fields and
can be readily autocalibrated by users, such a by depressing a reset button 144 (FIGS. 8 A,
8B), to ensure accurate operation. Moreover, a conventional applanation tonometer may be
directly replaced by an applanation tonometer of the present invention, without the need for
additional modification of equipment and without the need for practitioners to learn how to use
a new type of device.
While the present invention has been illustrated by the description of the
various embodiments thereof, and while the embodiments have been described in
considerable detail, it is not intended to restrict or in any way limit the scope of the appended
claims to such detail. Additional advantages and modifications will readily appear to those
skilled in the art. The invention in its broader aspects is therefore not limited to the specific
details, representative apparatus and methods and illustrative examples shown and described.
Accordingly, departures may be made from such details without departing from the scope or
spirit of Applicant's general inventive concept.