Patent Application
20090299345
The present invention pertains generally to systems and methods for reshaping and structurally altering the cornea of an eye to improve a patient's vision. More particularly, the present invention pertains to systems and methods that employ a laser beam for different purposes in an ophthalmic surgical procedure. The present invention is particularly, but not exclusively, useful as a system and method for reshaping the cornea of an eye, wherein Laser Induced Optical Breakdown (LIOB) of selected LIOB stromal tissue is accomplished to reshape the eye, and wherein Permanent Structural Change (PSC) of selected PSC stromal tissue is accomplished to alter the cornea by changing the refractive index of the tissue selected for PSC.
As laser system capabilities have become better understood, the use of lasers to change the physical characteristics of a resilient transparent material, such as the cornea of an eye, has become a more acceptable surgical procedure for vision correction. For purposes of discussion, refractive surgical procedures that are particularly useful for ophthalmic surgery can generally be grouped into either of two categories. The procedures most commonly used at the present time are those that result in a reshaping of the cornea. In these procedures, tissue is selectively removed from the cornea. Consequently, the cornea changes its shape to achieve changes in the refractive properties of the cornea. Procedures in the other category, however, do not rely on tissue removal. Instead, they rely on structurally changing the corneal tissue to alter the refractive properties of the tissue. Though both type procedures can result in a predictable vision correction, they are quite different from each other. And, importantly, they require different laser beam energy configuration (i.e. volumetric power densities).
Reshaping a cornea by removing tissue from the cornea is typically accomplished by performing Laser Induced Optical Breakdown (LIOB) of tissue. Simply stated, LIOB results in the photoablation (i.e. destruction) of tissue. The consequence of this is that tissue is effectively removed from the cornea. Thus, the cornea is somehow weakened. Internal forces inside the eye (i.e. intraocular pressure, and biomechanical forces) will then reshape the cornea to achieve the desired optical effect. To achieve LIOB, however, the energy in the laser beam must be above the LIOB threshold of the tissue. Typically, the required energy level for LIOB will be above about 500 nJ. As implied above, LIOB is associated with procedures in the first (i.e. cornea reshaping) category.
For the second category of surgical procedures (i.e. tissue altering), corneal tissue is not removed, and the cornea is not reshaped. Instead, structural aspects of corneal tissue are physically altered. This alteration consequently changes the refractive index of the tissue. In comparison with LIOB, this requires much less energy in the laser beam (e.g. 25 nJ vis-à-vis 500 nJ). Importantly, in order to effectively alter the structure of tissue in the cornea, the energy level must be above a threshold level where the tissue is responsive to the laser beam. In this case, however, rather than photoablating tissue, as happens with LIOB, tissue is merely altered (i.e. melted into a semi-liquefied state) by heat from the laser beam. In this heating process, the visco-elastic properties of the tissue are altered. Specifically, the tissue's inter-molecular bonds are changed to create a structure having denser tissue, with a higher refractive index. This phenomenon was first recognized in U.S. Pat. No. 4,907,586 which issued to Bille et al. for an invention entitled “Method for Reshaping the Eye.” Hereinafter, in the context of the present invention, a surgical procedure wherein tissue is altered to change its refractive index is referred to as a Permanent Structural Change (PSC) procedure.
In light of the above, it is an object of the present invention to provide a system and method for reshaping and altering the cornea of an eye that employs LIOB and PSC as complementary surgical procedures for the correction of vision defects. Another object of the present invention is to provide a system and method for using PSC to enhance the surgical outcome obtained by LIOB. Still another object of the present invention is to provide a system and method for reshaping and altering the cornea of an eye that is easy to use, is simple to implement and is comparatively cost effective.
In accordance with the present invention, a system and method are provided for reshaping and altering the cornea of an eye to correct vision defects. More specifically, for the present invention laser beams having different energies are used for two different purposes. Essentially, these purposes differ from each other in the effect they have on stromal tissue in the cornea.
In a first mode of operation, a laser unit generates a first laser beam for the purpose of performing Laser Induced Optical Breakdown (LIOB) on selected tissue in the stroma. In a second mode of operation, the laser unit generates a second laser beam (e.g. reconfigures the first laser beam) for the purpose of causing a Permanent Structural Change (PSC) in selected tissue of the stroma. As envisioned for the present invention, LIOB is performed on stromal tissue to generally weaken selected portions of the stroma. This weakening of the tissue will then allow intraocular pressure and bio-mechanical forces in the eye to reshape the cornea. On the other hand, PSC is performed on stromal tissue with the intention of altering the selected tissue in a manner that will change its refractive index. Together, these procedures can be complementarily employed to correct vision defects.
For purposes of the present invention, the first laser beam and the second laser beam will each have pulses of approximately 500 fs duration, and each will have a pulse repetition rate of approximately 80 kHz. Further, the first laser beam and the second laser beam are each generated with a numerical aperture (N. A.) of 0.2, corresponding to a focal point spot size of approximately four microns. The primary difference between the laser beams will be that they operate at different energy levels. More specifically, the first laser beam is envisioned to operate at an energy level that is above the LIOB threshold of the selected LIOB tissue. Preferably, energy in the first laser beam will be greater than approximately 500 nJ. On the other hand, the second laser beam will operate at an energy level that is below the LIOB threshold, but above the Permanent Structural Change threshold of the selected PSC tissue. Preferably the energy in the second laser beam will be approximately 25 nJ. As envisioned by the present invention, the second laser beam can be configured from the first laser beam by using a beam splitter that will split the first laser beam into a plurality of different beams.
The novel features of this invention, as well as the invention itself, both as to its structure and its operation, will be best understood from the accompanying drawings, taken in conjunction with the accompanying description, in which similar reference characters refer to similar parts, and in which:
Referring initially to
As envisioned for the present invention, the laser unit 12 needs to be capable of generating a laser beam 16 having either an energy level (i.e. volumetric power density) of either around 500 nJ or around 25 nJ. This change in energy level can be accomplished in either of several ways. For one, the switch 14 can change operation of the laser unit 12 from one laser source to another. For another, the laser beam 16 having around 500 nJ of energy can be divided by a beam splitter (designated 14 with the switch) that will provide for a plurality of individual beams having respectively less energy.
Insofar as the different energy levels of the beam 16 are concerned, it is important that the laser unit 12 of system 10 be able to operate in a first mode with a beam energy that is above the Laser Induced Optical Breakdown (LIOB) threshold of tissue in the cornea 18 of an eye 20. As indicated above, this energy level is at, or above, approximately 500 nJ. On the other hand, the laser unit 12 must also be able to operate in a second mode where the beam energy is above the Permanent Structural Change (PSC) threshold for tissue in the cornea 18. As indicated above, this energy level is generally greater than, but preferably near, about 25 nJ.
For the present invention, it is envisioned that LIOB can be performed on selected tissue in the cornea 18 with a resultant refractive change of approximately 2 diopters. The present invention also envisions that PSC can be performed on selected tissue in the cornea 18 with a local resultant refractive change of approximately 0.5 diopters. In general, this will be so regardless wherein the focal point 22 of the laser beam 16 is located in the cornea 18. While it is recognized that PSC may be performed on a layer across the entire cornea 18, it may well be preferable to employ PSC more selectively. In any event, the present invention envisions the complementary employment of both LIOB and PSC.
An exemplary sequence for the employment of an LIOB/PSC procedure in accordance with the present invention is shown in
While the particular System and Method for Reshaping a Cornea Using a Combination of LIOB and Structural Change Procedures 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.
