Patent Application
20070123768
The present invention relates generally to ophthalmic instruments, systems and methods. More specifically, the present invention relates to instruments, systems and methods by which tonometry and pachymetry measurements are conducted.
Goldmann applanation tonometry is a well known technique in the ophthalmic field to measure a patient's intraocular pressure (IOP) which is used as a diagnostic tool for determining eye disorders, such as glaucoma. However, conventional applanation tonometry assumes that each patient has a standard central corneal thickness. Since corneal thickness has a direct impact on the tonometry measurement, the assumption of a standardized corneal thickness means that conventional applanation tonometry can only at best provide a close approximation of the actual IOP from one patient to another.
Goldmann and Schmidt noted the probable relationship of central corneal thickness (CCT) to intraocular pressure (IOP) measurement when they introduced the applanation tonometer in 1956. Multiple reports have confirmed the correlation between increased CCT and increased measured IOP in adults, and a single published report notes a similar finding in children. The Ocular Hypertension Treatment Study highlighted decreased CCT as a predictive factor for progression to glaucoma and other studies have supported the importance of CCT as a contributor to the risk of glaucomatous progression. In addition, black adults demonstrate thinner corneas than whites, and also show increased risk of visual loss from glaucoma.
Both under-diagnosis and over-diagnosis of glaucoma can occur as a result of false intraocular pressure readings. Patients with thinner corneas may register normal intraocular pressures when in fact the pressure within the eye may be high, causing damage to the optic nerve. The reverse also is true in that patients with corneas thicker than normal may have high intraocular pressure readings, yet have normal intraocular pressure and still be treated inappropriately for glaucoma. Accurate and reproducible intraocular pressure readings are not only important for diagnosis of glaucoma but for monitoring the effects of drugs, and laser or incisional surgery. The inability of tonometry alone to detect accurately the true intraocular pressure hampers both detection and treatment. The need for accurate IOP measurements regardless of corneal thickness has also become more important in recent years with the popular practice of surgically altering a patient's cornea for purpose of vision correction. More specifically, patients who have undergone photorefractive keratotomy (PRK) or laser in situ keratomileusis (LASIK) procedures have thinner corneas. These surgically thinned corneas may therefore produce inaccurate and/or misleading IOP readings using current methods, which could lead to an inability to detect glaucoma and loss of sight.
In addition, and perhaps of even more clinical significance is the recent clinical data suggesting that central corneal thickness may be an independent risk factor for glaucomatous damage, even when the IOP has been ‘corrected’ for the measured central corneal thickness. Hence those eyes with thinner than average corneas, seem to be at increased risk for optic nerve damage compared to those with thicker corneas.
For the reasons noted above, it has recently been accepted practice in the ophthalmic field to determine separately the corneal thickness of a patient (e.g., using an ophthalmic pachymeter such as disclosed in U.S. Pat. No. 5,512,966 to Snook1) in conjunction with conventional applanation tonometry (e.g., using an applanation tonometer such as disclosed in U.S. Pat. Nos. 5,355,884 to Bennett and 4,987,899 to Brown). The tonometer reading is then corrected based on the measured corneal thickness using conventional correction algorithms so as to arrive at a more accurate IOP determination. It has also been proposed in U.S. Pat. No. 6,113,542 to Hyman et al to combine a conventional applanation tonometer with an optical pachymeter having respective separate tonometer and pachymeter probes in a single instrument by which the tonometer signal may be modified using correction algorithms stored in a microprocessor based on the corneal thickness determined previously by the optical pachymeter. A device has also been proposed in U.S. Pat. No. 6,083,161 to O'Donnell, Jr. whereby applanation is done with an ultrasonic transducer which measures the corneal thickness at an exact point of applanation to thereby allow for the simultaneous determination of both applanation pressure and corneal thickness.
1 All cited patents and written publications are expressly incorporated fully hereinto by reference.
While the techniques employed presently in the art may be satisfactory to improve the accuracy of IOP measurements, there still exists a need for improvement. For example, it would be highly desirable if instruments and methods could be provided which enable ophthalmic tonometry and pachymetry to be conducted simultaneously using a common optical signal path. The importance of a single instrument capable of measuring simultaneously both IOP and central corneal thickness goes beyond convenience. Currently, both of these measurements require that something touch the surface of the anesthetized cornea. While this is an inconvenience to cooperative patients, there are many children and some adults who would greatly benefit by having only one instrumentation of their corneas. Each time an instrument touches the cornea, there is a small disturbance of the surface corneal epithelium, and a topical anesthetic must be placed, which lasts only a few minutes, and which itself can interfere with the blinking response and cause irregularity and even damage to the corneal surface in vulnerable subjects. Therefore, obtaining all needed measurement (i.e., the IOP and central corneal thickness) by a single ‘touch’ to the cornea, poses significant benefit for the patient as well as the eye care provider.
Also, the current algorithms are empirically derived from the average characteristics of a group of test subjects, and therefore result in only an approximation of the IOP correction needed for varying corneal thickness. A more desirable instrument would separate the influence of each cornea on each IOP measurement by an analytic algorithm.
It is towards fulfilling such needs that the present invention is directed.
Broadly, the present invention is embodied in ophthalmic instruments, systems and methods which enable contact tonometry and optical pachymetry measurements to be conducted simultaneously. According to the present invention, therefore, ophthalmic instruments, systems and methods are provided whereby a patient's intraocular pressure (IOP) and corneal thickness may be determined accurately by a single path optical signal.
In especially preferred forms, the present invention is embodied in ophthalmic instruments whereby contact tonometry and optical pachymetry measurements are obtained simultaneously (i.e., both measurements are obtained only during a single corneal applanation). Preferably, both the tonometry and pachymetry measurements are obtained optically using a common optical signal path. Alternatively, a tonometer tip associated with a conventional Goldmann contact tonometer employing a pressure sensor to determine applanation pressure may be modified to include optical pachymetry according to the present invention.
According to one embodiment of the present invention ophthalmic instruments are provided which include an applanation tonometer for determining an intraocular pressure measurement, and an optical pachymeter for determining a corneal thickness measurement. The optical pachymeter and tonometer are integral so as to simultaneously generate respective output signals indicative of the intraocular pressure and corneal thickness measurements in response to a single corneal applanation.
In other preferred embodiments of the present invention, both the tonometry and pachymetry measurements are obtained optically using a common optical signal path. Such ophthalmic instruments of the present invention will therefore most preferably include a reference surface at a distal end of the instrument, an applanation plate spaced from the reference surface, and a compliant mount for mounting the applanation plate for resilient displacements relative to the reference surface. The instrument most preferably has a housing such that the reference surface is located at a fixed position at one end of the housing, and the applanation plate is mounted to that one end of the housing in coaxial spaced alignment relative to the reference surface by means of the compliant mount. If desired, the housing may have a handle and a substantially orthogonal head piece containing suitable optic mirrors which allow visible light to pass therethrough so that the procedure can be viewed and/or photographed.
The applanation plate is most preferably circular so that the compliant mount defines an annular mounting region for mounting the applanation plate relative to the reference surface. In this regard, the mounting region is continuous or discontinuous. Preferably, the compliant mount comprises an elastomeric structure, such as an elastomeric O-ring (in which case the annular mounting region is continuous) or a plurality of elastomeric posts circumferentially arranged to define an annular mounting region (in which case the annular mounting region is discontinuous). However, the compliant mount may also be in the form of a plurality of compression springs arranged circumferentially about the applanation plate.
A preferred system for conducting simultaneous tonometry and pachymetry measurements will include an ophthalmic instrument having a reference surface at a distal end of the instrument, an applanation plate spaced from the reference surface; and a compliant mount for mounting the applanation plate for resilient displacements relative to the reference surface, and an interferometer for optical connection to the instrument. The interferometer will most preferably have a source of light to be supplied to the instrument, and a spectrometer for receiving reflected light from the instrument and generating a signal indicative of a patient's intraocular pressure and corneal thickness. A microprocessor is preferably provided to receive the signal generated by the interferometer and determine a patient's intraocular pressure corrected for the patient's corneal thickness.
In use, the ophthalmic instrument of this invention may be brought into alignment with a patient's cornea and advanced toward the cornea so as to cause contact between the cornea and the applanation plate thereof. Continued advancement of the ophthalmic instrument relative to the cornea will cause the applanation plate to be displaced resiliently parallel to and toward the reference surface of the instrument while the cornea is progressively “flattened”. Such advancement of the ophthalmic instrument continues until the cornea is flattened or scanned across this surface sufficiently against the applanation plate. A light beam (preferably annular although another shaped beam or multiple beams could be utilized) may thus be directed toward the flattened cornea so that light may be reflected therefrom. A signal may thus be generated from the reflected light which contains data yielding a patient's intraocular pressure and corneal thickness. In such a manner, therefore, a measurement of the patient's intraocular pressure corrected by the patient's corneal thickness may be obtained.
The measurements and data obtained optically by means of the present invention may also be advantageously used to determine IOP and/or corneal biomechanical properties. For example, the present invention is capable of determining the distance of several points on the cornea from an applanating surface over a range from first corneal contact to full corneal applanation. From such distance data, one can derive factors such as corneal curvature and the applanation area/diameter. Differences of distance to the applanating surface among such points provides a measure of the probes alignment and may thus also be used to drive indicators for the user to assess the quality of the readings. A table with multiple entries of force generated (by the combined effects of IOP and corneal bending) versus applanation diameter can be analyzed to derive an expression of the form F=f1(d)+f2(d), where F is the measured force and d is the applanation diameter. Function f1 is of order 1, and represents the cornea component of force, function f2 is of order 2 and represents IOP component; the internal coefficients required to generate the best fit to the data points in the table indicate the true correction for the force from cornea (which primarily varies with thickness), and the true IOP. Mathematical transformations known to those skilled in the art can be performed with the data from this technology to generate other expressions, e.g., functions of applanation area rather than diameter, and to apply calibration corrections.
These and other aspects and advantages will become more apparent after careful consideration is given to the following detailed description of the preferred exemplary embodiments thereof.
Reference will hereinafter be made to the accompanying drawings, wherein like reference numerals throughout the various FIGURES denote like structural elements, and wherein;
Accompanying
The housing 12 includes an annular stop 12-1 located proximally of the cylindrical mounting sleeve 17. A compliant annular mount 20 is thereby positioned between the mounting sleeve 17 and the stop 12-1 to allow the applanation plate 18 to be capable of parallel resilient displacements towards and away from the reference surface 16.
Since the applanation plate 18 is the forwardmost structure of the instrument 10, it is thereby adapted to being brought into physical contact with the surface of a patient's cornea C. (see
Preferably, the compliant mount 20 is formed of an elastomeric material, such as silicone rubber or the like. Other suitable compliant mounting means may also be envisioned, such as compression springs with mechanical restriction to ensure parallelism in movement of the applanation plate using for example the exterior mounting sleeve 17 depicted in
The applanation plate 18 is most preferably an optically transparent material that has, or may be made to have, at least about 5% reflectivity at the light wavelengths of the source. Thin films or coatings may thus be provided on the applanation plate 18 and/or reference surface 16 in accordance with well known optical techniques so as to impart desired reflectivity properties. Single crystal sapphire with thickness between 100 and 200 microns is presently preferred as the material from which the applanation plate 18 is constructed though different glasses, acrylics, or other crystals could be used. The applanation plate 18 is also sufficiently rigid so as to remain planar in response to a wide range of intraocular pressure conditions that may be encountered.
In especially preferred embodiments, the applanation plate 18 has a circular geometry as depicted in
Referring again to
The output signal 28a from the spectrometer 28 is connected to a microprocessor 32 (e.g., a personal computer) by presently preferred means of a conventional USB connection (not shown). Microprocessor 32 stores the algorithms for converting the interferograms data provided by the spectrometer signal 28a into a corrected IOP measurement. Most preferably, the interferometer 24 embodies the principles disclosed more completely in Izatt et al, “Novel Noncontact Optical Pachymeter”, SPIE Ophthalmic Technologies XV Conference, Photonics West, Jan. 22-23, 2005 and Fercher et al, “Measurement of Intraocular Distances by Backscattering Spectral Interferometry”, Optics Communications 117, 43-48 (1995).
Accompanying
The interferograms generated by the spectrometer 28 corresponding to the instrument conditions depicted in
As the cornea C is flattened, the distance between peaks 20 and 30 trends toward zero when there is no longer any annular separation distance between the applanation plate 18 and cornea C. As shown in
It will be appreciated that the actual distance that the applanation plate 18 is displaced from its normal or rest condition will depend on the particular form and material of the compliant mount 20 which can vary from mount to mount or even with the particular instrument's temperature or age. Thus, for any compliant mount 20, the actual displacement distance that the applanation plate 18 moves towards the reference surface 16 will be a function of the magnitude of compression force that is exerted against the applanation plate 18 which those skilled in this art may determine empirically by standard calibration testing before each use. Thus, following such empirical determination of the relationship of the displacement distance and the pressure force for a given compliant mount configuration and/or material, the microprocessor 32 may be provided with an algorithm or look-up table. The displacement distance of the applanation plate 18 relative to the reference surface 16 which is determined by the spectrometer 28 may therefore be converted into a pressure force against the applanation plate 18. This pressure force against the applanation plate 18 will thus correspond to a patient's intraocular pressure condition uncorrected by corneal thickness-that is, an IOP measurement that corresponds to conventional Goldmann-type applanation tonometers. It may be useful to employ an annular beam which is larger than the desired applanation diameter, in which case, peaks 20 and 30 do not merge to peak 21, but remain separate (
Accompanying
The embodiments described above employ optical means for simultaneously obtaining both tonometry and pachymetry measurements. However, according to the present invention conventional tonometers could be modified so that optical pachymetry measurements could be obtained simultaneously with conventional tonometry measurements using standard electromechanical pressure sensors. Such an embodiment of the present invention is depicted in accompanying
The optical fiber 76 is coupled operatively to an interferometer 84 having characteristics similar to the interferometer 24 described previously. The interferometer 84 will thus output a signal via line 86 which is indicative of the thickness of the patient's cornea C. Thus, simultaneously with corneal applanation by the applanation surface 70-1 and the generation of the tonometry signal 82, the interferometer 84 will output a pachymetry signal via line 86. These simultaneously generated tonometry and pachymetry signals 82, 86, respectively, are received by microprocessor 90 (e.g., a personal computer) which converts the data signals into a corrected IOP measurement using algorithms according to the techniques described previously.
It is entirely conceivable that the instruments and systems described fully herein, while being especially suited for the simultaneous measurement of a patient's IOP and corneal thickness, could be employed to determine such measurements separately. Thus, if desired, the instruments and systems described herein could be employed so as to determine separately one of the IOP and corneal thickness if that were deemed desirable. Thus, the microprocessor could be configured to provide a readout of the patient's IOP (e.g., as determined by the displacement distance of the applanation plate 18, in which case the IOP measurement would be commensurate with conventional Goldmann-type tonometer readings at a full applanation diameter of 3.06 mm) and/or a readout of the patient's corneal thickness. In other words, while it may be very desirable to conduct simultaneous measurements of both IOP and corneal thickness, the instruments and systems are sufficiently flexible to permit separate measurement determinations if desired.
Therefore, while the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not to be limited to the disclosed embodiment, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.
