1. Field of the Invention
This invention relates generally to the design of energy delivery system in laser photorefractive keratectomy. More particularly, this invention relates to the design of the energy delivery system in laser photorefractive keratectomy which utilizes continuous energy-delivery control means such as time-varying moving shutters or rotating diaphragms, or lens system, which may be controlled by a computer to continuously deliver the laser energy onto the surface of a cornea for correction of myopia, hyperopia, or astigmatism.
2. Description of the Prior Art
Conventional laser photorefractive keratectomy is limited by several technical difficulties due to the use of discrete aperture masks. On the one hand, the efficiency of conventional photorefractive keratectomy in applying laser beam to the cornea for correction of myopia, hyperopia, and astigmatism is limited by the number of discrete aperture masks on the aperture wheels and the time required for changing aperture masks and realigning the optical system. On the other hand, the result of the laser operation with the use of discrete masks will lead to ablation of cornea tissues in the form of discrete stepped rings. In order to achieve smoother laser ablation of the cornea, it is desirable to increase the number of masks in the aperture wheel. However, this would require more time consuming processes in changing the aperture masks and optical realignment thus make the entire process even slower and more expensive.
The diagram in FIG. 1A shows the optical design of a typical laser photorefractive keratectomy (PK) system 10. An excimer laser source 15 is used for applying the energy for removing different amount of cornea tissues depending on the corrections required for that specific operation. The extremely short pulse duration of the excimer laser (ten to twenty nanoseconds) would decrease the undesirable thermal effects on the cornea surface to a theoretically infinitesimal level because of the apparent lack of time for thermal diffusion. The output beam 20 from the excimer laser 15 may have irregularities including high energy spots, i.e., hot spots, which need to be processed before the laser beam can be applied to the cornea. A beam processing system 25 is used to smooth out and reshape the laser beam profile. The beam processing system 25 may include optical components such as shutters, mirrors, beam scrapers, anamorphic prisms, etc. After passes through the beam processing system 25, the laser beam 20 is transmitted to an image rotator-beam homogenizer 30 which homogenizes and converts the laser beam into a circular beam.
The photorefractive keratectomy (PK) system 10 also includes a myopia wheel 35, an astigmatisms wheel 40, a hyperopia wheel 45 and beam directing system 50 for directing the laser beam to the surface of a cornea 60 to be operated. The beam directing system may includes various optical components such as mirrors, safety shutters, zoom lenses, beam splitter, etc. to accurately direct the beam to the cornea 60. FIGS. 1B, and 1C show the myopia aperture wheel 35 and the hyperopia aperture wheel 45 respectively wherein the oversize apertures are shown for the purpose of illustration. The myopia aperture wheel 35 employs various decreasing-diameter apertures while the hyperopia aperture wheel 45 employs annular apertures of decreasing radial width. For the purpose of vision correction, the mechanism applied by the PK system 10 is to allow each position of the wheel to remain an appropriate number of pulses such that precise amount of ablazing energy may be delivered to the cornea 60 for removing a piece of cornea tissue for correcting the vision.
FIGS. 1D and 1E show the myopia correction profile and hyperopia correction profile respectively wherein the initial surface, the desired surface and the actual ablated surface resulting from the correction operation are shown. For myopia correction, more of the central tissue from the cornea 60 is removed in order to flatten the cornea curvature by a series of stepwise enlargements of the beam impacting on the cornea. The length of time that the aperture opening remains at each of these stations on the myopia wheel 35 is precisely calculated and controlled by a computer (not shown). In FIG. 1E, the operation employs aperture masks of progressive enlarging annuli such that greater amount of peripheral corneal tissues are removed in order to steepen the curvature of the cornea surface to correct the hyperopia refractive condition.
The stepwise profiles as that shown in FIGS. 1D and 1E clearly illustrate the undesirable effects caused by the use of these discrete aperture masks. Additionally, the efficiency of the operation is adversely affected by the time and processes required to change the aperture masks and the realignment of the optical system every time when a new mask is used. Since an optical system as that shown in FIG. 1A with the discrete aperture masks mounted on the aperture wheels, e.g., aperture wheels 35, 40 and 45, are commonly employed by the conventional PK systems marketed by Taunton Technologies, VISX, or Meditec, further improvements of the energy delivery system are till required to overcome this technical limitation.
Therefore, a need still exits in the art of laser energy delivery system in photorefractive keratectomy for a new technique to resolve the technical difficulties discussed above. Specifically, the energy delivery system must be able to eliminate the use of the discrete aperture masks in order to achieve smooth laser ablation on the cornea. Meanwhile, the new PK energy delivery system must be controlled with high degree of precision while shortening the surgery time and furthermore providing more operational flexibility thus capable of accommodating different excimer laser beam profiles.