WO2017208131A1

WO2017208131A1 - Device and method for the measurement of corneal thickness and intraocular pressure

Info

Publication number: WO2017208131A1
Application number: PCT/IB2017/053146
Authority: WO
Inventors: Gualtiero Regini
Original assignee: Gualtiero Regini
Priority date: 2016-05-30
Filing date: 2017-05-29
Publication date: 2017-12-07
Also published as: ITUA20163905A1

Abstract

A device for the measurement of corneal thickness and intraocular pressure has a console (12) that includes a control board (81), a touch screen display (13), a data processing board with a microprocessor (82) and, connected to the console (12) by means of a cable (27), a hand-piece (20) having a body (33), a removable tip (22) including an elastic interface means (23) in contact with an ultrasound transducer (21) carried by the hand-piece body (33). The device further comprise a calibrator (16), that includes a pressure sensor (15) and a relevant analog to digital converter (14) and provides a calibration curve (60) in which applied pressures are related to axial compressions of the elastic interface means (23), the ultrasound transducer (21) alone providing both the measurement of corneal thickness and the measurement of intraocular pressure. Further, a relevant method is described.

Description

DEVICE AND METHOD FOR THE MEASUREMENT OF CORNEAL THICKNESS AND INTRAOCULAR PRESSURE

Field of the invention

The present invention relates to a device and method for the measurement of corneal thickness and intraocular pressure.

Prior art

Since many years, ophthalmologists and eye care professionals use measurement instruments for checking patients intraocular pressure (IOP) in order to diagnose pathologies associated to a high ocular pressure as for example glaucoma, as well as for examinations before and after ocular surgery, in order to evaluate if the intra-ocular pressure in its normal range. In the past the most used tool for measuring IOP has been the Goldmann applanation tonometer. A light pressure on the patient's cornea is applied with this tool, until applanation is achieved in a contact zone having a diameter of 3.06 mm. Such a tool needs a certain skill set for a visual, even if subjective, evaluation of a correct applanation with the aid of a slit lamp, reason for which the tool is not easily transportable; furthermore, the patient must be in erect position and it is necessary to repeat the measure more times to have a good precision. The Goldmann tonometer, still very diffused, has been accompanied along the years by other types of contact tonometers, among which can be cited JP2004041372 (A) in the name of Matsumura, that uses a central force sensor and other peripheral force sensors that come into contact with the cornea at the applanation.

At the state of the art, the most accurate IOP measures are achieved with the above said techniques of corneal applanation; however, said measures have the main defect of depending on the mechanical strength of an average thickness cornea. Nevertheless, it is evident that the cornea flattening under the effect of a known external force depends not only on IOP, but also on the hardness of the cornea itself and this is well correlated to the central corneal thickness, called corneal apex. A central corneal thickness out of the normal range may therefore remarkably degrade the IOP measurement precision, as well documented by the publication: "The importance of central corneal thickness measurements and decision making in general ophthalmology clinics: a masse observational study" Ashish A Patwardhan et al, BMC Ophthalmology 2008, 8:1. This document shows that the error can reach +/- 6 mm-Hg with respect to the normal IOP that is typically 15 or 16 mm-Hg, with very important diagnostic consequences in case of a not treated glaucoma.

For what said above, the determination of the corneal thickness is one of the determinant elements for a precise IOP measurement. For this reason, the tonometry is nowadays preceded or followed by a pachymeter measure of the central cornea thickness. The pachymetry can be performed either with sophisticated and expensive techniques, entirely optical as the coherent light tomography OCT, or with the ultrasound technique, much more economical and transportable, consisting of a simple probe dimensioned like a pen, for which reason the ultrasound pachymetry is by far the most used method for the measurement of the corneal thickness in the clinical practise in order to make a map of the corneal thicknesses in the centre as well as in a plurality of points of the cornea. In this case, the corneal thickness measure is obtained, as known, by means of ultrasound pulses emitted by the transducer and reflected by the external and internal corneal surfaces. The corneal thickness can be accurately calculated by measuring a delay between emitted ultrasound waves and received ultrasound waves, provided that the ultrasound wave velocity in that tissue is known.

The frequent and complementary use of the tonometer and the pachymeter has led to solutions that could combine the functionalities of the two instruments and could provide a measure of IOP that is corrected on the basis of the corneal thickness by means of a single apparatus called tono- pachymeter. A tono-pachymeter is described in US2007123769 (Al). Inside the tip of said apparatus there are both a pressure sensor and an ultrasound transducer for the corneal thickness measurement.

A drawback of the apparatus described in US2007123769 (Al) is a certain construction complexity due to the presence of both the force sensor and the ultrasound transducer with a central hole, all of them being located in a tip of small size.

Another drawback of this apparatus is that the pachymeter measure does not correspond to the measure in the central contact point, such a point being occupied by the pressure sensor; rather this measure corresponds to an average value along a para-central ring. Therefore, said apparatus, even if usable in order to provide an "average" corneal thickness aimed to the correction of IOP, is not able to replace ultrasound pachymeters that, at the state of the art, have the feature of a beam focused at the tip centre, on a small corneal surface.

Another drawback is that the direct contact with the cornea occurs through a rigid tip that, even if protected by a thin membrane, may result potentially dangerous in case of unadvised movements of the patient's eye, of the patient or of the operator.

Summary of the invention

An object of the present invention is to make a tono-pachymeter for an integrated and automatic measurement of IOP and of the corneal thickness on the central contact point, that is portable and neither requires any specific skill for its use, nor a subjective evaluation of correct applanation.

Another object of the present invention is to make easier the manufacture and to reduce the cost thereof. Further another object of the present invention is to use a soft interface means for the contact with the cornea, i.e. not dangerous in case of unadvised movements of the patient's eye, of the patient or of the operator.

The present invention therefore describes a method and a device that is of small dimensions, easily transportable, acting as a tono-pachymeter, thanks to which both the corneal thickness measure and the ocular pressure measure are obtained by a single ultrasound transducer and an elastic interface means. In particular, the present device allows to eliminate the pressure sensor that is present inside the tip of other contact type tono-pachymeters, thus making easier and more economical the hand-piece manufacture. The absence of the pressure sensor at the tip centre, allows the measurement of the corneal thickness exactly on the contact point and not on a para-central ring; furthermore, the tip can be replaced after every use to prevent cross infections, as there is not a pressure sensor inside and the tip materials have a low cost.

The device according to the invention has a console that is provided with a pressure sensor. The device according to the invention has also a calibration function; however, once defined the physical-mechanical features and the elasticity module of the elastic interface means, this calibration function is not necessarily used before each measurement unless the physical-mechanical features of the tip or of the elastic interface means therein change.

The components of the tono-pachymeter according to the invention are: The console that includes a control board, a touch screen display, a microprocessor based processing board and a calibrator having a pressure sensor and, wired to the console by means of a cable, a ultrasound hand-piece having a body, a removable tip which includes an elastic interface means in contact with an ultrasound transducer carried by the hand-piece body. The microprocessor processing board includes a process software for calibration, measurement, calculation and visualization of IOP and of the corneal thickness, according to a method consisting on the following steps: a calibration step that is performed by approaching the ultrasound hand-piece tip to a pressure sensor and applying a pressure on the elastic interface means, while the console acquires a series of measures that relate pressure measures by the calibrator with axial compressions of the elastic interface means as measured by means of the ultrasound transducer, thus determining a calibration curve interpolating said series of measures;

a measurement step that is performed by approaching the ultrasound hand-piece tip close to the patient's eye up to compress the cornea and the elastic interface means, while the console records a series of ultrasound measures of the corneal thickness and a series of ultrasound measures of the axial deformation of the elastic interface means up to detection of the corneal applanation time, where an abrupt slope change of said axial deformation occurs.

a calculation step to convert said axial deformation measured at time, into a pressure measure by means of the calibration curve, and

a visualization step on the touch screen display of said corneal thickness and intraocular pressure at the corneal applanation time.

Brief description of the drawings

Figure 1 is a perspective view of the tono-pachymeter device according to the present invention;

Figure 2 is a schematic partial longitudinal cross-section view of the ultrasound hand-piece structure of the tono-pachymeter device in Figure 1;

Figure 3 is a side view of the ultrasound hand-piece in Figure 2 being far from the patient's eye, and a graph of amplitude versus time of the relevant ultrasound A-scan echoes in air; Figure 4 is the ultrasound hand-piece in Figure 3 in contact with the patient's eye;

Figure 5 is a graph related to the measurement step, representing the time in x-axis and the length in y-axis;

Figure 6 is a graph related to the calibration step, representing the pressure in x-axis and the length in y-axis;

Figure 7 is a longitudinal cross-section view of an embodiment of a tip with a protection cap of the ultrasound hand-piece in Figure 2;

Figure 8 is the block diagram of a console of the tono-pachymeter device according to the invention;

Figure 9 is a side view showing an implementation of the ultrasound handpiece in Figure 4 with a motorised linear guide;

Figure 10 is a longitudinal cross-section of a variant of the ultrasound handpiece in Figure 2 with a build-in accelerometer.

Description of embodiments of the invention

The perspective view of Figure 1 and the block diagram of Figure 8 depict the device of the present invention in its preferred embodiment.

The device comprises an ultrasound hand-piece 20, connected by a cable 27 to a console 12 comprising a microprocessor data processing board 82 connected to a touch-screen graphic display 13. The console 12 also comprises a control board 81 connected to the ultrasound hand-piece 20. The control board 81 generates hand-piece transmitting pulses, receives reflected ultrasound echoes, amplifies these echoes, detects their amplitudes and delays, and communicates these data to the microprocessor data processing board 82 that converts said delays into distances and thickness. The console 12 also comprises a calibrator 16 including a pressure sensor 15 of a piezo- resistive type with a full scale preferably between 7 kPa and 14 kPa, equivalent respectively to 50 and 100 mm-Hg. This range is suitable for small pressure measurements; the range in the eye is typically 10-30 mm Hg. The pressure sensor 15 is interfaced to an analog to digital converter 14 with resolution preferably between 16 and 24 bit. The digital converter 14 is connected to said data processing board 82.

A set of fast keys 17, connected to board 82, are dedicated to main functions among which a measurement start key and a calibration start key.

Figure 2 shows the ultrasound hand-piece 20 in a partial longitudinal section along the movement x-axis. The ultrasound hand-piece 20, having a cylindrical shape, has dimension similar to a fountain pen, to be easily graspable. The ultrasound hand-piece 20 comprises a body 33 which includes an ultrasound transducer 21 with the relevant shield 211. The ultrasound transducer 21 is preferably centred between 20 MHz and 50 MHz, focused to a distance between 6 and 15 mm, with a focal surface less than 1 mm at -6 db, and it is connected to the console 12 by a cable 27. The ultrasound hand-piece 20 further comprises a tip 22 and, inside the tip, an elastic interface means 23, preferably of cylindrical form, length between 6 and 15 mm, corresponding to the same focal length of the transducer 21. The preferential elastic interface means 23 is made of an elastomer as, for example, Aqualene by Olympus, that is an optimal ultrasound conductor, has an acoustic velocity about 1590 m/s similar to that of the corneal tissue (1640 m/s). This elastomer is soft, elastically deformable and has a low cost that is compatible with a disposable use. Provided on tip 22 is a housing 29 for the mounting of a complimentary elastic cap. According a preferred embodiment, said tip 22 is of a removable type, by a female thread 25 adapted to receive a male thread 26 of the handpiece body 33. Therefore, the tip 22 can be unscrewed from the hand-piece body 33 and replaced after use, in order to avoid cross infections. For mounting the tip, it is sufficient to screw the female thread 25 to the male thread 26 of the hand-piece body 33, in order to put in direct contact the elastic interface means 23 with the face of the ultrasound transducer 21. The application of the hand-piece pressure over the cornea through the elastic interface means 23 causes axial length shortening and radial expansion 24 of the same elastic interface means, expansion being indicated by dot lines that occurs freely thanks to a cavity 28 provided inside the tip 22.

Figure 3 shows the ultrasound hand-piece 20 in a side view, before touching the patient's cornea 31 and schematically shows a graph 30 of amplitude versus time, known as A-scan trace, of the ultrasound echoes, detected by the ultrasound transducer 21. The peak at time tO ps occurs when the ultrasound transducer 21 emits the pulse, the peak at time tl is the one relative to the reflection by the end of the elastic interface means 23.

Figure 4 shows the ultrasound hand-piece 20 in a side view, when the patient's cornea 31 is flattened due to the effect of a pressure applied by the elastic interface means 23 and shows the A-Scan trace 40 of the ultrasound echoes detected by means of the ultrasound transducer 21. The peak at time tO ps occurs when the ultrasound transducer 21 emits the pulse, the peak at time t2 is the one relative to the anterior corneal surface 31; the peak at time t3 is the one relative to the posterior corneal surface 32.

Figure 5 shows the points of the curves 50 and 51, recorded by the console 12 during the measurement step; x-axis shows the measurement time T in seconds starting from T=T0 that is the start of test; at T=Tc the first contact with the cornea takes place, and at T=Ta the corneal applanation occurs; at T=Ts the measurement stops.

Figure 6 shows the points of a calibration curve 60, recorded by the console 12 during the calibration step, when the elastic interface means of the ultrasound hand-piece 20 is applied to the calibrator 16. Y-axis indicates the compression of the elastic interface means 23 in mm, and x-axis indicates the pressure in mm-Hg detected by the pressure sensor 15 of the calibrator 16. The above mentioned figures illustrate in an exemplifying mode the device that will be described in its preferred embodiment with more details. Now it is important to start by saying that the technology of measurement of corneal thickness by ultrasounds with a focused beam in A-Scan mode is well known by designers of ultrasound instruments and pachymeters and does not need here more detailed explanations. As known, in order to perform such kind of measure, the console 12 has the feature to set the velocity V of ultrasounds in the cornea and to evaluate the corneal thickness Sc according to the formula Sc= 1/2 V*(t3-t2) that is relevant, in the present example, to times t3 and t2 of the A-scan trace 40.

In the calibration step the console 12 emits pulses with a period preferably between 1 ms and 10 ms, respectively equal to 1000 and 100 measures per second; for each emitted pulse, the console 12, by means of the ultrasound transducer 21, performs the measurement of the compression of the elastic interface means 23 and at the same time acquires the measure of the pressure applied to the pressure sensor 15 of the calibrator 16, as illustrated by the curve 60. The operator, by means of the tip 22 of the ultrasound hand-piece 20, applies a light pressure to the pressure sensor 15. The pressure is gradually increased with the time by the operator, until he/she hears an acoustic signal of calibration end; this indicates that the maximum pressure of the calibration range Pmax and then the relevant maximum compression Cmax of the elastic interface means 23 is reached. The duration of the calibration depends on the operator's speed and is generally between 0.5 s and 2 s. The operator performs the measurement step by approaching the tip 22 of the ultrasound hand-piece 20 to the patient's eye, with a velocity as uniform as possible, until touching the cornea and a little further, that is until the operator hears the acoustic signal of measurement end; this indicates that the IOP measure and the corneal thickness measure have been completed, as will be explained later. During the measurement, as during calibration, the console 12 emits pulses with a period preferably between 1 ms and 10 ms, respectively equal to 1000 and 100 measures per second; for each emitted pulse, the console 12, by the ultrasound transducer 21, takes the measurement of the elastic interface means 23, of the corneal thickness, if present, and of time T elapsed from the Start key activation, that is time T=T0 as depicted by the graph in figure 5. At time T=T0, being the elastic interface means 23 into air, compression is equal to zero; at time Tc a first contact with the cornea occurs, still with compression equal to zero; During the time between Tc e Ta the elastic interface means compression quickly increases, up to balance the ocular pressure. After time Ta, the cornea, by moving backwards, just opposes a small elastic resistance and this explains the weak slope of tract 51 with respect to the slope of the previous tract 50.

During the measurement step the console software detects the above mentioned slope change between the sequence of points of curves 50 and 51 that corresponds to the condition of corneal applanation. Therefore, it is possible to determine the time Ta when said change occurs; thus the measurement terminates at time Ts, just subsequent to Ta.

On the next calculation step, once known the Ta time value, it is possible to find the corresponding compression Ca value as shown in Figure 5. On the basis of the calibration curve 60 previously calculated, it is now possible to determine the pressure value Po measured by the pressure sensor 15, that corresponds to said compression Ca, as shown in Figure 6. In the corneal applanation condition, said pressure Po balances and is approximately equal to the intraocular pressure, i.e. Po~IOP.

The calculation ends with the visualization of the corneal thickness, of the measured IOP and of the IOP corrected in function of the central corneal thickness, as, for example, the table 1 in the publication of Ashish A Patwardhan et al. cited above.

With reference to the graph shown in Figure 5, an example of a process software which implements measurement, calculation and visualization of the measures, is articulated into the following seven steps A, B, C, D, E, F, G: Step A: after pushing the Start key, at time TO, an acquisition of a two- dimensional vector V (T, C) begins, with a period of 2 ms, where T=time in s, and C= axial compression of the elastic interface means in mm.

Step B: Starting from time Tc, when the posterior corneal echo appears, the software calculates the corneal thickness Sc by averaging the relevant measures acquired every 2 ms;

Step C: the derivative of vector V (T, C) as represented by curves 50 and 51, is analysed in function of time T and the time Ta is found where said derivative abruptly changes its value in the corneal applanation condition;

Step D: if both the above steps B and C are not completed within the maximum time of 2 seconds, the acquisition stops and an error signal is emitted, otherwise:

Step E: the measure of the compression Ca of the elastic interface means which corresponds to time Ta is found inside vector V (T, C).

Step F: by evaluating the calibration curve 60 previously saved, the pressure Po that corresponds to the compression Ca is found.

Step G: a sound of measurement end is emitted and the following results are visualized: pressure Po; mean corneal thickness Sc and IOP corrected in function of the thickness Sc.

In brief, the gold standard for the measurement of the IOP is the Goldmann tonometer, which measures the pressure applied to flatten a corneal surface with a diameter of 3.06 mm. The present invention allows to execute in accurate and automatic way, without any specific operator skill and without any other instrument, the measurement of the corneal thickness and of the pressure required to flatten a corneal surface with a diameter preferably equal to that one of the Goldmann tonometer, and provides an IOP measure comparable with the reference standard. A feature of the present device is the absence of the pressure sensor inside the hand-piece, whose function is implemented by the ultrasound transducer itself by the combined use of an elastic interface means and a tip with a cavity that allows the elastic deformation of said elastic interface means. The above described solution makes easier and more economical the manufacture thereof, provides the measure of the corneal thickness exactly on the contact point and not over a para-central ring; furthermore, the solution allows the use of a corneal contact means that is soft, not dangerous, inexpensive and removable after every use to avoid cross infections.

In another embodiment of the present invention represented in Figure 7 as an illustrative and not limiting example, collocated on the tip 22 is a cap 70, made of a thin rubber membrane, like polyurethane for example, with a thickness preferably inferior to 0.5 mm, in contact with the elastic interface means 23. This cap, provided with an elastic ring 71 for the fastening to the tip housing 29, has the aim to prevent the transmission of cross infections without any significant attenuation of the passage of the ultrasounds. According to this embodiment, it is no longer necessary to replace the tip 22 after an exam, but only the cap 70. The thickness of the cap 70 may be measured during the calibration and can be subtracted during the measurement of the corneal thickness, without affecting the pachymetry precision.

Another embodiment of the present invention, represented in Figure 9 as an illustrative and not limiting example, comprises a motorised linear actuator 90, including a linear guide 94, a screw nut slide 91, a screw shaft 92 joined to the axis of a motor 93 automatically controlled by the board 82 of the console 12 by means of a connecting cable 95. According to this embodiment the ultrasound hand-piece 20 secured to the slide 91 is moved along the x-axis towards the patient's eye with a constant velocity, is stopped after the corneal applanation and removed during the measurement step. The actuator 90 can be mounted, for example, on a common support for slit lamp, by means of the threaded holes 96 and the relevant screws. In another embodiment of the present invention illustrated in Figure 10 as an illustrative and not limiting example, the use of an accelerometer 34 inside the hand-piece 20 is provided, having an axis oriented like the x-axis of the handpiece, i.e. of the movement thereof towards the examined eye. Said accelerometer 34, made by one among the many available integrated microcircuits like for example LIS3DH by ST Microelectronics, is connected to the control board 81 of the console 12 with wires 35 that pass through the cable 27, and monitors the measurement reliability by controlling that, during the hand-piece movement, there are no accelerations outside a desired range, for example from 0.1 to 0.0 g, so that the curve 50 in Figure 5 is linearly ascending and monotonic; this control prevents the possibility that the slope change between the curves 50 and 51 is due to an abrupt handling deceleration rather than to the corneal applanation condition. In case of accelerations or decelerations outside said range, the measurement is invalidated with a signal and an error message on the display.

Claims

1. A device for the measurement of corneal thickness and intraocular pressure comprising a console (12) that includes a control board (81), a touch screen display (13), a data processing board with a microprocessor (82) and, connected to the console (12) by means of a cable (27), a handpiece (20) having a body (33), a removable tip (22) including an elastic interface means (23) in contact with an ultrasound transducer (21) carried by the hand-piece body (33), characterized in that the device further comprise a calibrator (16), that includes a pressure sensor (15) and a relevant analog to digital converter (14) and provides a calibration curve (60) in which applied pressures are related to axial compressions of the elastic interface means (23), the ultrasound transducer (21) alone providing both the measurement of corneal thickness and the measurement of intraocular pressure.

2. The device according to claim 1, wherein the elastic interface means (23) inside the removable tip (22) has a cylindrical shape and a length corresponding to the focal length of the ultrasound transducer (21), and passes across a cavity (28) of the removable tip (22).

3. The device according to claim 1, wherein the removable tip (22) is covered by a cap (70) made of a rubber membrane having a thickness preferably smaller than 0.5 mm.

4. The device according to claim 1, wherein the hand-piece (20) is moved along a longitudinal axis (x) in the direction of a patient's eye under test by means of a linear motorized actuator (90) that is driven by a motor (93) controlled by the console (12) through a cable (95).

5. The device according to claim 1, wherein provided inside the handpiece body (33) is an accelerometer (34) that is connected to the console (12) through the cable (27) and has an axis oriented like the longitudinal axis (x) of the hand-piece (20).

6. A method for the measurement of corneal thickness and intraocular pressure by means of a device comprising a console (12) that includes a control board (81), a touch screen display (13), a data processing board with a microprocessor (82), a calibrator (16) comprising a pressure sensor (15) and a relevant analog to digital converter (14) and, connected to the console (12) by means of a cable (27), a hand-piece (20) having a body (33), a removable tip (22) including an elastic interface means (23) in contact with an ultrasound transducer (21) carried by the hand-piece body (33), characterized by:

a calibration step that is performed by approaching the hand-piece tip (22) to a pressure sensor (15) and applying a pressure on the elastic interface means (23), while the console (12) acquires a series of measures that relate pressure measures by the calibrator (16) with axial compressions of the elastic interface means (23) as measured by means of the ultrasound transducer (21), thus determining a calibration curve (60) interpolating said series of measures;

a measurement step that is performed by approaching the handpiece tip (22) close to the patient's eye up to compress the cornea and the elastic interface means (23), while the console (12) records a series of ultrasound measures of the corneal thickness and a series of ultrasound measures (50, 51) of the axial deformation of the elastic interface means (23) up to detection of the corneal applanation time Ta, where an abrupt slope change of said axial deformation occurs.

a calculation step to convert said axial deformation measured at time Ta, into a pressure measure by means of the calibration curve (60), and a visualization step on the touch screen display (13) of said corneal thickness and intraocular pressure at the corneal applanation time Ta.

7. The method according to claim 6, wherein acoustic tones are produced for warning about both an end of the measurement step and an end of the calibration step.

8. The method according to claim 6, wherein the console (12) calculates and visualizes on the touch screen display (13) the intraocular pressure corrected according to a function of the central corneal thickness.

9. The method according to claim 6, wherein, during the measurement step, a constant speed forward movement, a stop and backward movement of the hand-piece (20) are done by a linear motorized actuator (90) automatically controlled by the console (12).

10. The method according to claim 6, wherein, during the measurement step, a control is made by an accelerometer (34) that is placed inside the hand-piece (33) and has an axis oriented like the longitudinal axis (x) of the hand-piece (20), such a control checking that there is no acceleration outside a pre-defined range.