WO2004021940A1

WO2004021940A1 - System and method for measuring tissue thickness

Info

Publication number: WO2004021940A1
Application number: PCT/US2003/028170
Authority: WO
Inventors: Alexander Dybbs
Original assignee: Alexander Dybbs
Priority date: 2002-09-09
Filing date: 2003-09-09
Publication date: 2004-03-18
Also published as: AU2003268559A1

Abstract

A method of directly measuring tissue thickness includes providing an ultrasonic measuring device (22) and a disc-shape biocompatible acoustic material (30). The tissue to be measured is interposed between the ultrasonic measuring device and the acoustic material such that ultrasonic sound waves will pass through the tissue and reflect off the acoustic material. The ultrasonic measuring device receives the reflected sound waves and outputs a thickness value, and the acoustic material can be removed.

Description

SYSTEM AND METHOD FOR MEASURING TISSUE THICKNESS

The benefit of the filing date of U.S. Provisional Application No. 60/409,524, filed September 9, 2002 is hereby claimed, and the entire disclosure therein is hereby incorporated by reference.

Field of the Invention A system and method for measuring tissue thickness, and, more particularly, an ultrasonic system and method for measuring the thickness of corneal tissue, such as corneal tissue during ophthalmic surgery, including laser- assisted in situ keratomileusis (LASIK).

Background of the Invention Various methods and/or systems are used to accurately measure the thickness of a section of tissue. Ultrasonic devices, particularly those of the type generally referred to as pachometers, are commonly used to measure the thickness of the cornea or a portion thereof for ophthalmic surgical procedures, for example.

One such procedure is laser-assisted in situ keratomileusis (commonly referred to as LASIK), in which a surgeon uses a microkeratome 10 mounted on an eye 12 to cut a lamellar resection 14 from the cornea 16 as shown in FIG. 1 , either completely severing the resection from the cornea, creating a severed

"cap", or leaving a hinged flap of corneal tissue that can be folded over and out of the way. The surgeon uses an excimer laser to ablate the exposed stromal tissue to correct for vision problems such as myopia, astigmatism and hyperopia. The surgeon then replaces the corneal resection. Ophthalmologists generally agree that the corneal thickness remaining after the creation of the flap and the laser ablation should be at least approximately two hundred fifty microns. This minimum thickness is necessary to avoid complications such as ectasia, in which the cornea takes on a conical shape with high radii of curvature due to the intra-ocular pressure and the structural weakening of the cornea from the LASIK procedure.

The microkeratome generally is intended to provide a lamellar resection of a specific and uniform thickness, generally about one hundred sixty microns, for example. Thus, it is important to know the thickness of the corneal tissue prior to ablation. This is particularly important for cases of high myopes where the amount of tissue removed by the ablation step generally needs to be large. To date, the thickness of the lamellar resection has been measured indirectly. Intraoperatively, the surgeon measures the total corneal thickness prior to surgery, uses a microkeratome to cut a lamellar resection (the aforementioned cap or flap), and measures the thickness of the remaining corneal tissue. The surgeon then subtracts the corneal thickness after removing the lamellar resection from the preoperative corneal thickness to determine the thickness of the lamellar resection. The surgeon uses this information to determine whether the desired amount of ablation is acceptable. Unfortunately, this technique has not produced consistent accurate results.

Summary of the Invention The present invention provides a system and method for directly measuring tissue thickness. For example, during LASIK, before the ablation step the surgeon places a piece of material (referred to herein as a "button") under the corneal resection and uses a pachometer to directly measure the thickness of the tissue using ultrasound. The button has a significantly different acoustic impedance than the corneal tissue to reflect the ultrasonic sound waves back through the tissue to the probe.

More particularly, the present invention provides a method of measuring tissue thickness that includes the following steps: providing an ultrasonic measuring device, providing a material having an acoustic impedance that is substantially different from the tissue being measured, interposing the tissue between the ultrasonic measuring device and the material, and measuring the thickness of the tissue based on acoustic reflections.

The present invention also provides, in combination, a tissue-thickness ultrasonic measuring device and a reflecting button made of a material having an acoustic impedance that is substantially different than that of the tissue where thickness is being measured.

Preferably, the material is bio-compatible and may be selected from the group that includes a polystyrene film, such as Rexolite® from C-LEC Plastics, Inc., of Philadelphia, Pennsylvania, U.S.A., a polyester film, such as Mylar® from E. I. du Pont De Nemours and Company of Wilmington, Delaware, U.S.A., and an epoxy film, such as Master Bond® Epoxy, from Master Bond, Inc. of Hackensack, New Jersey, U.S.A. The material preferably is formed from a flexible sheet material. The thickness of the material is preferably about one quarter wavelength of the ultrasonic measuring device, which may mean, for example, about fifty-eight ten thousandths of a millimeter. Although an embodiment of the present invention is described with reference to measurement of corneal tissue, the button and the ultrasonic sensor also may be used to measure the thickness of other tissues. Generally, such a button is a thin wafer preferably of uniform thickness and circular shape.

The foregoing and other features of the invention are hereinafter fully described and particularly pointed out in the claims, the following description and annexed drawings setting forth in detail a certain illustrative embodiment of the invention, this embodiment being indicative, however, of but one of the various ways in which the principles of the invention may be employed.

Brief Description of the Drawings FIG. 1 is a schematic illustration of a portion of an eye and a portion of a microkeratome blade cutting a flap in the cornea for LASIK. FIG. 2 schematically illustrates a system provided by the present invention for measuring the thickness of a corneal flap.

FIG. 3A is an enlarged schematic illustration of an exemplary arrangement for measuring the thickness of a corneal flap.

FIG. 3B is an enlarged schematic illustration of an alternate arrangement for measuring the thickness of a corneal flap.

FIG. 4 is an enlarged schematic illustration of an ultrasonic tissue thickness measurement in accordance with the present invention.

FIG. 5 is a plan view of an exemplary ultrasonic reflector in accordance with the present invention. Detailed Description

The present invention provides a system and method that uses an ultrasonic sensor and a reflective "button" between which tissue can be interposed for measurement of the thickness of the tissue. For example, in LASIK the button can be placed between an exposed stromal bed and a resected flap to directly measure the thickness of the flap. The button is made of a material having an acoustic impedance that is substantially different from the acoustic impedance of the tissue to reflect the ultrasonic waves at the interface between the tissue and the button to allow the sensor to directly measure the thickness of the tissue, preferably about twice the acoustic impedance of the tissue being measured. In a preferred form, the button is a thin wafer of uniform thickness of less than about one millimeter, preferably less than about two hundred microns and more preferably about fifty-eight microns. The wafers may have a circular shape, although other shapes may be used. The term "button" is used interchangeably with the word "reflector."

Although the description herein generally refers to a flap being cut from the cornea for LASIK, other lamellar resections of the cornea also may be used in accordance with the present invention, including a completely severed section or cap that can be removed from the cornea. To facilitate the description, the lamellar resection will be referred to as a flap, a corneal resection attached to the cornea by a hinge at one side. The flap generally has a diameter of approximately nine millimeters and a thickness of about one hundred sixty microns to one hundred eighty microns (one hundred sixty to one hundred eighty ten-thousandths of a millimeter). An exemplary hinge has a width of about four millimeters.

Referring now to FIG. 2, a schematic illustration of an ophthalmic surgical system 20 is shown. The system 20 includes a sensor device 22, an excimer laser (not shown), a microkeratome 24 and associated controller 26, and a button 30, which is shown in the grip of a surgeon's tweezers 32. The sensor device 22 preferably includes an ultrasonic sensor and a probe 34 in communication with a controller 36. The illustrated controller 36 has a display 40 that provides an output related to the thickness of the tissue being measured by the probe. In general, an ultrasonic sensor device uses a piezoelectric crystal to generate an ultrasonic sound wave that is transmitted through a probe 34. The center frequency of the probe preferably is at least about fifty MHZ. An exemplary probe 34 has a diameter of about one and a half millimeters at its tip and focuses the sound wave at a point interiorly of the tissue. A measurement of the difference in reflected echoes can be used by circuitry or software to determine the thickness represented thereby. An exemplary ultrasonic probe and associated ultrasonic depth measuring circuitry is provided in a pachometer sold by Sonogage, Inc. of Cleveland, Ohio. For further information on use and operation of such probes, reference may be had to U.S. Patent No. 5,674,233, the entire disclosure of which is hereby incorporated herein by reference.

FIG. 2 also shows an eye 42 with a corneal flap 44 folded back to expose the stromal bed 46. The stromal tissue is exposed for a surgeon to ablate the surface with the laser. The microkeratome has already cut the flap (for example, as shown in FIG. 1 ) and the flap has been folded back over an outer portion of the eye to expose the stromal bed. The surgeon is preparing to place the button 30 underneath the corneal flap 44 for a measurement of its thickness using the ultrasonic probe 34.

The cornea 50 of the eye 42 has several layers which generally have substantially the same acoustic impedance that transmit the ultrasonic sound waves therethrough. Behind the cornea is an aqueous layer 52 which has a very different acoustic impedance, thereby causing the majority of the ultrasonic energy to be reflected. The reflection of the ultrasonic sound waves enables the surgeon to measure the thickness of the preoperative cornea and the thickness of the remaining cornea (from the exposed stromal bed 46) after removing the lamellar resection, such as the illustrated flap 44.

After the flap 44 is cut from the cornea 50 in a LASIK procedure, the thickness of the remaining cornea can be measured using the ultrasonic probe 34 in communication with the exposed stromal bed 46 to measure the remaining thickness of the cornea. Unfortunately, the formation of the flap 44 traumatizes the cornea 50 which swells in response. In addition, any fluid present at the exposed stromal bed 46 is rapidly absorbed. Accordingly, the ultrasonic measurements at the exposed stromal bed may be an inaccurate indication of the post-resection thickness of the cornea. This could lead to an incorrect conclusion that sufficient thickness exists to perform substantial excimer laser ablation while maintaining the recommended minimum post-resection corneal thickness of at least about two hundred fifty microns.

Preoperatively, the thickness of the cornea 50 is measured easily due to the different acoustic impedance between the corneal tissue and the aqueous layer 52 behind the cornea 50. However, when the flap 44 is severed from the cornea 50, the surgeon cannot obtain a direct measurement of the flap thickness because the acoustic impedance of the flap is similar to the acoustic impedance of the other exposed portions of the eye 42.

The button 30 has a significantly different impedance relative to the cornea 50 such that ultrasonic sound waves passing through the flap 44 reflect off the button and return to the probe 34. In particular, the acoustic impedance of the button 30 preferably is at least about twice the acoustic impedance of the corneal tissue in the flap 44 or other tissue being measured.

The button 30 preferably is formed of a flexible biocompatible sheet material that conforms to the shape of the corneal tissue of the flap 44 against which it is placed and thereby maintains contact with the tissue to reflect the acoustic waves and facilitate measurement of the thickness. The button has sufficient thickness to inhibit wrinkling of the reflective material as it is maneuvered into position. Exemplary materials for making the button include a sterile plastic, such as Rexolite® (a polymer available from C-LEC Plastics, Inc. of Beverly, New Jersey, USA), Mylar® (available from E. I. DuPont De Nemours and Company Corporation, of Wilmington, Delaware, USA), or a Master Bond® epoxy (available from Master Bond, Inc. Corporation of Hackensack, New Jersey, USA). The button can be made by forming the epoxy into a wafer or button of the desired shape which is then lapped to the desired thickness. Alternatively, a sheet may be purchased in different thicknesses and then die-punched to the desired dimensions of the button.

The thickness of the button 30 is preferably approximately one quarter of a wavelength emitted by the probe 34. An exemplary button 30 has a disc shape with a thickness of approximately fifty-eight ten-thousandths of a millimeter (about fifty-eight microns) and a length or diameter of at least eight millimeters, and preferably at least nine millimeters, approximately the width of the flap 44. However, the button 30 may cover an area much larger than the flap 44 to facilitate moving the button 30 into place. Although the button 30 is dimensioned to emulate the flap 44, a much larger or smaller button may be used. A larger button may be easier for the surgeon to manipulate. In addition, the button 30 may have different shapes such as square, circular, oval, rectangular, etc., and preferably has no holes in the vicinity where measurements will be taken. As shown in FIG. 5, the button 30 also can have a shape that provides a handle 45 on at least one side to facilitate gripping and manipulating the button. Such a handle also could be called a manipulating tab.

The button is positioned under the tissue so as to interpose the tissue between the button and the ultrasonic probe. For example, in FIG. 3A, an eye 42 is shown that has a flap 44 formed in the cornea 50. The button 30 has been placed on the exposed surface 46 of the stroma and the flap 44 has been folded over the button 30 (or the button 30 has been slid under the flap 44). The ultrasonic probe 34 is placed in communication with the outer surface of the corneal flap 44 to measure the thickness of the tissue between the probe 34 and the button 30. As shown in FIG. 3B, an alternative arrangement is shown with the button 30 placed over a portion of the eye 42 adjacent the hinge of the flap 44, and the flap 44 lays over the button 30. The probe 34 is then placed in communication with an inner surface of the flap 44 to measure the thickness of the tissue interposed between the probe 34 and the button 30 in the illustrated arrangement.

A thickness measurement is shown schematically in FIG. 4 with the paths of the ultrasonic sound wave 60 and reflected wave 62 shown for illustration purposes. The probe 30 is in communication with the flap 44 which in turn is sandwiched between the probe 34 and the button 30. The ultrasonic sound waves 60 and 62 pass through the tissue and reflect from the button 30 back to the probe 34, and the controller 36 (FIG. 2) determines and outputs a thickness measurement. The measurement of the thickness may be made near the center of the flap 44, since that is likely to be the thinnest portion of the flap 44.

A method of conducting ophthalmic surgery in accordance with the present invention can include the steps of measuring a pre-operative thickness of a cornea, resecting the cornea to produce the flap, placing a sterile button of high acoustic impedance under the resected flap, measuring the thickness of the flap ultrasonically, removing the sterile button, determining the thickness of the post- resection cornea by subtracting the flap thickness from the pre-operative corneal thickness, ablating the exposed stroma using an excimer laser if the post- resection cornea has a thickness of at least about two hundred fifty microns, and placing the flap over the exposed stroma. Once the flap thickness is known, the true thickness of the remaining portion of the cornea (post-resection) can be determined by subtraction from the pre-operative thickness of the cornea. If the remaining thickness of the cornea is less than two hundred fifty microns, the ablation can be avoided. The flap can be allowed to heal and the procedure attempted again at a later date.

While an embodiment of the present invention has been described with reference to LASIK, any tissue thickness that a surgeon would want to measure would be applicable, particularly when a surgeon would not want to measure the tissue thickness as a difference between pre- and post-procedural thicknesses. Although the invention has been shown and described with respect to certain illustrated embodiment, equivalent alterations and modifications will occur to others skilled in the art upon reading and understanding the specification and the annexed drawings. In particular regard to the various functions performed by the above described integers (components, assemblies, devices, compositions, etc.), the terms (including a reference to a "means") used to describe such integers are intended to correspond, unless otherwise indicated, to any integer which performs the specified function (i.e., that is functionally equivalent), even though not structurally equivalent to the disclosed structure which performs the function in the herein illustrated embodiment of the invention.

Claims

What is claimed is:

1. A method of measuring a tissue thickness comprising: interposing the tissue between an ultrasonic measuring device and a material having an impedance that is substantially different than the tissue being measured, and measuring the thickness of the interposed tissue based on acoustic reflections.

2. A method as set forth in claim 1 , wherein the material having an acoustic impedance that is substantially different than the tissue being measured has a wafer shape.

3. A method as set forth in claim 1 , wherein the button has a generally planar shape that is dimensionally configured for insertion between a corneal resection and an unresected portion of an eye.

3. A method as set forth in claim 1 , further comprising measuring a pre-operative tissue thickness and subtracting the thickness of the interposed tissue from the pre-operative thickness.

4. A method as set forth in claim 1 , wherein interposing includes interposing a corneal resection.

5. In combination, a tissue-thickness ultrasonic measuring device and an ultrasonic reflector made of a biocompatible material having an acoustic impedance that is substantially different than that of a tissue whose thickness is to be measured.

6. The combination as set forth in claim 5, wherein the reflector material includes a plastic.

7. The combination as set forth in claim 5, wherein the reflector material includes a material selected from a group that includes: a polystyrene film, a polyester film, and an epoxy film.

8. The combination as set forth in claim 5, wherein the ultrasonic reflector is formed from a flexible sheet material.

9. The combination as set forth in claim 5, wherein the ultrasonic reflector has a circular shape.

10. The combination as set forth in claim 5, wherein the ultrasonic reflector has a dimension greater than about eight millimeters.

11. The combination as set forth in claim 5, wherein the ultrasonic reflector has a size greater than a circle of about eight millimeters in diameter.

12. The combination as set forth in claim 11 , wherein the ultrasonic reflector has an approximately circular shape with a diameter of about nine millimeters.

13. The combination as set forth in claim 5, wherein the button has a thickness of about one quarter wavelength of the ultrasonic measuring device.

14. The combination as set forth in claim 5, wherein the ultrasonic reflector has a thickness of less than about one millimeter.

15. The combination as set forth in claim 5, wherein the ultrasonic reflector has a thickness of less than about two hundred microns.

16. The combination as set forth in claim 13, wherein the ultrasonic reflector has a thickness of about fifty-eight microns.

17. A method of supplying a plurality of ultrasonic reflectors for use in measuring tissue thickness, each reflector being a reflector as set forth in claim 5.