System and method for correcting higher order aberrations with changes in intrastromal biomechanical stress distributions

A method for correcting higher order aberrations in an eye requires Laser Induced Optical Breakdown (LIOB) of stromal tissue. In detail, the method identifies at least one volume of stromal tissue in the eye, with each volume defining a central axis parallel to the visual axis of the eye. Thereafter, a pulsed laser beam is focused to a focal spot in each volume of stromal tissue to cause LIOB of stromal tissue at the focal spot. Further, the focal spot is moved through the volume of stromal tissue to create a plurality of incisions centered about the respective central axis of the volume. As a result, a predetermined selective weakening of the stroma is caused for correction of the higher order aberration.

FIELD OF THE INVENTION

The present invention pertains generally to methods for performing intrastromal ophthalmic laser surgery. More particularly, the present invention pertains to laser surgery to correct higher order aberrations in an eye. The present invention is particularly, but not exclusively, useful as a method for correcting higher order aberrations in an eye wherein incisions centered about a plurality of axes parallel to the visual axis cause a predetermined selective weakening of the stroma via changes in intrastromal biomechanical stress distributions.

BACKGROUND OF THE INVENTION

The cornea of an eye has five (5) different identifiable layers of tissue. Proceeding in a posterior direction from the anterior surface of the cornea, these layers are: the epithelium; Bowman's capsule (membrane); the stroma; Descemet's membrane; and the endothelium. Behind the cornea is an aqueous-containing space called the anterior chamber. Importantly, pressure from the aqueous in the anterior chamber acts on the cornea with bio-mechanical consequences. Specifically, the aqueous in the anterior chamber of the eye exerts an intraocular pressure against the cornea. This creates stresses and strains that place the cornea under tension.

Structurally, the cornea of the eye has a thickness (T) that extends between the epithelium and the endothelium. Typically, “T” is approximately five hundred microns (T=500 μm). From a bio-mechanical perspective, Bowman's capsule and the stroma are the most important layers of the cornea. Within the cornea, Bowman's capsule is a relatively thin layer (e.g. 20 to 30 μm) that is located below the epithelium, within the anterior one hundred microns of the cornea. The stroma then comprises almost all of the remaining four hundred microns in the cornea. Further, the tissue of Bowman's capsule creates a relatively strong, elastic membrane that effectively resists forces in tension. On the other hand, the stroma comprises relatively weak connective tissue.

Bio-mechanically, Bowman's capsule and the stroma are both significantly influenced by the intraocular pressure that is exerted against the cornea by aqueous in the anterior chamber. In particular, this pressure is transferred from the anterior chamber, and through the stroma, to Bowman's membrane. It is known that how these forces are transmitted through the stroma will affect the shape of the cornea. Thus, by disrupting forces between interconnective tissue in the stroma, the overall force distribution in the cornea can be altered. Consequently, this altered force distribution will then act against Bowman's capsule. In response, the shape of Bowman's capsule is changed, and due to the elasticity and strength of Bowman's capsule, this change will directly influence the shape of the cornea.

It is well known that all of the different tissues of the cornea are susceptible to LIOB. Further, it is known that different tissues will respond differently to a laser beam, and that the orientation of tissue being subjected to LIOB may also affect how the tissue reacts to LIOB. With this in mind, the stroma needs to be specifically considered.

The stroma essentially comprises many lamellae that extend substantially parallel to the anterior surface of the eye. In the stroma, the lamellae are bonded together by a glue-like tissue that is inherently weaker than the lamellae themselves. Consequently, LIOB over layers parallel to the lamellae can be performed with less energy (e.g. 0.8 μJ) than the energy required for the LIOB over cuts that are oriented perpendicular to the lamellae (e.g. 1.2 μJ). It will be appreciated by the skilled artisan, however, that these energy levels are only exemplary. If tighter focusing optics can be used, the required energy levels will be appropriately lower. In any event, depending on the desired result, it may be desirable to make only cuts in the stroma. On the other hand, for some procedures it may be more desirable to make a combination of cuts and layers.

As implied above, reshaping of the cornea by weakening tissue in the stroma can be an effective way to provide refractive corrections that will improve a vision defect. Not all vision defects, however, are caused by aberrations that are symmetrical with respect to the visual axis. Indeed, the higher order aberrations are typically asymmetrical. Accordingly, it may be necessary to weaken tissue in volumes that are offset from the visual axis. With all of this in mind, and as intended for the present invention, refractive surgery is accomplished by making incisions in the stroma centered about axes parallel to the visual axis to induce a redistribution of bio-mechanical forces that will reshape the cornea.

In light of the above, it is an object of the present invention to provide methods for correcting higher order aberrations through changes in intrastromal biomechanical stress distributions for improvement of a patient's vision. Another object of the present invention is to provide methods for correcting higher order aberrations that require minimal LIOB of stromal tissue. Still another object of the present invention is to provide methods for performing ophthalmic laser surgery that create incisions having a same pattern at selected locations about the visual axis. Yet another object of the present invention is to provide methods for correcting higher order aberrations via ophthalmic laser surgery that are relatively easy to implement and comparatively cost effective.

SUMMARY OF THE INVENTION

In accordance with the present invention, methods for correcting higher order aberrations in an eye via intrastromal ophthalmic laser surgery are provided that cause the cornea to be reshaped under the influence of intrastromal bio-mechanical stress distributions. Importantly, for these methods, at least one volume of stromal tissue is identified for operation. Structurally, each operational volume extends posteriorly from about ten microns below Bowman's membrane to a substantial depth into the stroma that is about 150 microns from the endothelium. Further, each operational volume defines a central axis that is parallel to and located at a distance from the visual axis of the eye.

In general, the method of the present invention requires the use of a laser unit that is capable of generating a so-called pulsed, femtosecond laser beam. Stated differently, the duration of each pulse in the beam will approximately be less than one picosecond. When generated, this beam is focused onto a focal spot in the volume of stromal tissue. The well-known result of this is a Laser Induced Optical Breakdown (LIOB) of stromal tissue at the focal spot. In particular, and as intended for the present invention, movement of the focal spot within each volume of stromal tissue creates a plurality of incisions that are centered about the respective central axis of the volume. The purpose here is to cause a predetermined selective weakening of the stroma for correction of the higher order aberration. Preferably, each incision has a same pattern. For purposes of the present invention, “incision” may refer to a location of weakened or eliminated tissue along the path of the focal point.

In certain embodiments, various volumes of stromal tissue with corresponding central axes are identified. For each embodiment, the central axes are arranged equidistant from the visual axis. Geometrically, the respective incisions may form concentric cylinders that are centered on the respective central axis. Other incision shapes may, however, be used. For example, the incisions may be concentric cylinder sections centered on the central axis, or they may be rectangular cylinders centered on the central axis, or they may be crosses that are centered on the central axis. In certain embodiments, the incisions will each have a thickness of about two microns.

In accordance with the present invention, various procedures can be customized to treat identifiable refractive imperfections. Specifically, in addition to specific incisions alone, the present invention contemplates using combinations of various types of incisions. In each instance, the selection of incisions will depend on how the cornea needs to be reshaped.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring initially toFIG. 1, it will be seen that the present invention includes a laser unit10for generating a laser beam12. More specifically, the laser beam12is preferably a pulsed laser beam, and the laser unit10generates pulses for the beam12that are less than one picosecond in duration (i.e. they are femtosecond pulses). InFIG. 1, the laser beam12is shown being directed along the visual axis14and onto the cornea16of the eye. Also shown inFIG. 1is the anterior chamber18of the eye that is located immediately posterior to the cornea16. There is also a lens20that is located posterior to both the anterior chamber18and the sclera22.

InFIG. 2, five (5) different anatomical tissues of the cornea16are shown. The first of these, the epithelium24defines the anterior surface of the cornea16. Behind the epithelium24, and ordered in a posterior direction along the visual axis14, are Bowman's capsule (membrane)26, the stroma28, Descemet's membrane30and the endothelium32. Of these tissues, Bowman's capsule26and the stroma28are the most important for the present invention. Specifically, Bowman's capsule26is important because it is very elastic and has superior tensile strength. It therefore, contributes significantly to maintaining the general integrity of the cornea16.

For the methods of the present invention, Bowman's capsule26must not be compromised (i.e. weakened). On the other hand, the stroma28is intentionally weakened. In this case, the stroma28is important because it transfers intraocular pressure from the aqueous in the anterior chamber18to Bowman's membrane26. Any selective weakening of the stroma28will therefore alter the force distribution in the stroma28. Thus, as envisioned by the present invention, LIOB in the stroma28can be effectively used to alter the force distribution that is transferred through the stroma28, with a consequent reshaping of the cornea16. Bowman's capsule26will then provide structure for maintaining a reshaped cornea16that will effectively correct refractive imperfections.

While referring now toFIG. 2, it is to be appreciated that an important aspect of the present invention is the identification of operational volumes34which are defined in the stroma28. Although the operational volumes34are shown in cross-section inFIG. 2, they are actually three-dimensional, and extends from an anterior surface36that is located at a distance38below Bowman's capsule26, to a posterior surface40that is located at a distance41from the endothelium32. Both the anterior surface36and the posterior surface40essentially conform to the curvature of the stroma28. For a more exact location of the anterior surface36of the operational volumes, the distance38will be about ten microns. For the posterior surfaces40, the distance41will be about one-hundred-fifty microns.

InFIG. 3, incisions44a-44fare made in a plurality of operational volumes34a-34fas envisioned for the present invention. Although six different volumes34a-34fare shown inFIG. 3(alsoFIGS. 6D and 6E) it will be appreciated by the skilled artisan, this is only exemplary and presented here for purposes of disclosure. More specifically, for third order aberrations only three volumes34need to be identified. In any event, the exact number of volumes34, and their respective radial distances from the visual axis14for any specific higher order aberration can be ascertained from the well known Zernike polynomials. As shown, for each operational volume34a-34f, a plurality of incisions44′,44″ and44′″ are made, though there may be more or fewer incisions44, depending on the needs of the particular procedure. With this in mind, and for purposes of this disclosure, the plurality in a selected volume34will sometimes be collectively referred to as incisions44. Further, as shown inFIG. 3, six operational volumes have been identified. However, any number of operational volumes34may be used for the present invention.

As shown inFIG. 3, the exemplary incisions44for each operational volume34are made on respective cylindrical surfaces. Although the incisions44are shown as circular cylindrical surfaces, these surfaces may be oval. When the plurality of incisions44is made in the stroma28, it is absolutely essential that it be confined within the respective operational volume34. With this in mind, it is envisioned that incisions44will be made by a laser process using the laser unit10. And, that this process will result in Laser Induced Optical Breakdown (LIOB). Further, in the illustrated embodiment, it is important these cylindrical surfaces be concentric, and that they are centered on a respective central axis45a-45fdistanced from and parallel to the visual axis14.

Cross-referencingFIG. 3withFIGS. 4 and 5, it can be seen that each incision44has an anterior end46and a posterior end48. Further, the incisions44(i.e. the circular or oval cylindrical surfaces) have a spacing50between adjacent incisions44. Preferably, this spacing50is equal to approximately two hundred microns.FIG. 5also shows that the anterior ends46of respective individual incisions44can be displaced axially from each other by a distance52. Typically, this distance52will be around ten microns. Further, the innermost incision44(e.g. incision44′″ shown inFIG. 4) will be at a radial distance “rc” that will be about 1 millimeter from the central axis45. From another perspective,FIG. 6Ashows the incisions44centered on the central axis45to form a plurality of rings. In this other perspective, the incisions44collectively establish an inner radius “rci” and an outer radius “rco”. Preferably, each incision44will have a thickness of about two microns, and the energy required to make the incision44will be approximately 1.2 microJoules.

As an alternative to the incisions44disclosed above,FIG. 4indicates that only arc segments54may be used, if desired. Specifically, in all essential respects, the arc segments54are identical with the incisions44. The exception, however, is that they are confined within diametrically opposed arcs identified inFIGS. 4 and 6Bby the angle “α”. More specifically, the result is two sets of diametrically opposed arc segments54. Preferably, “α” is in a range between five degrees and one hundred and sixty degrees.

An alternate embodiment for the arc segments54are the arc segments54′ shown inFIG. 6C. There it will be seen that the arc segments54′ like the arc segments54are in diametrically opposed sets. The arc segments54′, however, are centered on respective axes (not shown) that are parallel to each other, and equidistant from the central axis45.

As an alternative to the incisions44disclosed above,FIG. 6Dindicates that incisions44may be created to form rectangular cylinders centered on the respective central axes45. Similarly,FIG. 6Eindicates that the incisions44may be created to form crosses centered on the respective central axes45. As shown inFIGS. 6D and 6E, the rectangular cylinders and crosses are also aligned with the visual axis14.

FIG. 7provides an overview of the bio-mechanical reaction of the cornea16when incisions44have been made in the operational volume34of the stroma28. As stated above, the incisions44are intended to weaken the stroma28. Consequently, once the incisions44have been made, the intraocular pressure (represented by arrow56) causes a change in the force distribution within the stroma28. This causes bulges58aand58bthat result in a change in shape from the original cornea16into a new configuration for cornea16′, represented by the dashed lines. As intended for the present invention, this results in refractive corrections for the cornea16that improves vision.

While the particular System and Method for Correcting Higher Order Aberrations with Changes in Intrastromal Biomechanical Stress Distributions as herein shown and disclosed in detail is fully capable of obtaining the objects and providing the advantages herein before stated, it is to be understood that it is merely illustrative of the presently preferred embodiments of the invention and that no limitations are intended to the details of construction or design herein shown other than as described in the appended claims.