Closed loop control for intrastromal wavefront-guided ablation

#The present invention pertains generally to laser systems that photoablate corneal tissue to improve the visual acuity of an eye. More particularly, the present invention pertains to laser systems and methods that perform intrastromal photoablation of tissue during corrective optical surgery. The present invention is particularly, but not exclusively, useful as a closed-loop control system to accurately and precisely maintain control of an ablation laser beam when bubbles form in the stroma, and introduce induced optical aberrations during a laser surgery.

#It is well known that the optical characteristics of an eye can be altered through laser surgery. For example, U.S. Pat. No. 6,050,687 which issued to Bille et al. for an invention entitled “Method and Apparatus for Measuring the Refractive Properties of the Human Eye,” and which is assigned to the same assignee as the present invention, discloses a laser system that can be used for such purposes. In any event, a consequence of photoablation, is that individual cells in the tissue are vaporized. Gas is, therefore, a product of photoablation. When a surgical laser procedure involves the superficial photoablation of tissue, the fact that such gases are created does not cause much of a problem. This is not the case, however, when internal tissue is photoablated.

#For specific surgical procedures that involve the intrastromal photoablation of corneal tissue, it is known that such photoablation results in the formation of tiny bubbles in the stroma. Further, it is known that the formation of these bubbles introduces induced aberrations that change the optical characteristics of the cornea. The reason for this change is essentially two-fold. First, the gas bubbles have a different refractive index than that of the surrounding stromal tissue. Second, and perhaps more important, the gas bubbles tend to deform the stroma and, thus, they alter its refractive effect on light passing through the cornea. In a controlled surgical procedure these induced aberrations must be accounted for.

#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:

#FIG. 1 is a schematic layout showing the interrelationships of components in a system for controlling the intrastromal photoablation of corneal tissue in an eye in accordance with the present invention;

#FIG. 2 is a functional representation of the wavefront analysis techniques used in the operation of the system of the present invention; and

#Referring initially to FIG. 1, a closed-loop system for intrastromal photoablation of corneal tissue in accordance with the present invention is shown and is generally designated 10. In detail, the components of system 10 include a source 12 for generating an ablation laser beam 14, and a source 16 for generating a diagnostic laser beam 18. Further, the system 10 includes an active, multi-facet mirror 20, a beam splitter 22 and a beam splitter 24. More particularly, the active mirror 20 is preferably of a type disclosed in U.S. Pat. No. 6,220,707 which issued to Bille for an invention entitled “Method for Programming an Active Mirror to Mimic a Wavefront” and which is assigned to the same assignee as the present invention. As shown, the active mirror 20 and the beam splitters 22 and 24 direct the diagnostic laser beam 18 from diagnostic laser source 16 toward an eye 26. Likewise, the beam splitters 22 and 24 are used to direct the ablation laser beam 14 from the ablation laser source 12 toward the eye 26.

#FIG. 1 also shows that the system 10 of the present invention includes a detector 28, a comparator 30 and a compensator 32. In particular, the detector 28 is preferably of a type commonly known as a Hartmann-Shack sensor. The comparator 30 and compensator 32 are electronic components known in the pertinent art that will perform the requisite functions for the system 10.

#Still referring to FIG. 1, it is to be appreciated and understood that during an intrastromal photoablation procedure, as performed by the system 10 of the present invention, the ablation laser beam 14 is focused (by optical components not shown) onto stromal tissue 34 in the cornea of the eye 26 for the purpose of accomplishing intrastromal photoablation. A consequence of this photoablation of the tissue 34 is the formation of gas bubbles 36 that introduce optical aberrations in the stromal tissue 34. At the same time, the diagnostic laser beam 18 is focused (by optical components not shown) to a focal spot 38 on the retina 40 of the eye 26. In this combination, control by the system 10 over the ablation laser beam 14 is actually accomplished using the reflected diagnostic laser beam 18′, as it is reflected through the stromal tissue 34 from the focal spot 38 on the retina 40 of eye 26.