This invention relates to improvements in photorefractive keratectomy (PRK).
In PRK, corneal tissue is removed in a controlled fashion to shape the surface of the cornea of a patient's eye to treat, e.g., myopia, hyperopia, presbyopia or astigmatism
The cornea comprises transparent avascular tissue that forms the anterior portion of the eye. The cornea functions as both a protective membrane and a "window" through which light passes as it proceeds to the retina. The transparency of the cornea is due to its uniform structure, avascularity, and deturgescence, which is the state of relative hydration of the corneal tissue.
A major proportion of the refractive power of the eye is determined by the curvature of the anterior surface of the cornea, so that changing the shape of the cornea offers a way to significantly reduce a refractive problem in the eye.
Various techniques have been proposed for shaping the cornea of a patient's eye. Some techniques include removing the epithelium, and then shaping the underlying Bowman's and stroma layers. In PRK, photoablation is employed using e.g., ultraviolet radiation from an excimer laser, e.g., at 193 nm wavelength, or infrared laser radiation that has a wavelength in the range of about 2.9 to 3.2 .mu.m.
In one technique, described in Marshall et al., U.S. Pat. No. 4,941,093 (assigned to Summit Technology Inc.), the shape and size of the area of the corneal surface which is irradiated by laser radiation is selected and controlled so that some areas of the corneal surface become more eroded than others and a desired corneal shape is achieved.
Another technique, described in Muller, U.S. Pat. No. 4,856,513 (assigned to Summit Technology Inc.), uses a laser and an erodible mask. The mask, with a predefined profile of resistance to erosion by laser radiation, is disposed between the laser and the corneal surface. A portion of the laser radiation is absorbed by the mask, while another portion is transmitted to the corneal surface in accordance with the mask profile, thereby enabling the selective photoablation of the corneal surface into a desired shape.
There are circumstances in which it is desired to accomplish the PRK treatment with the ablated zone larger than 5 mm in diameter. With such zones, under usual operating conditions it has been observed that the final surface achieved by the ablation process differs from the expected shape. Surface irregularity and significant refractive error have been observed post-operatively in the corneal topographies of some patients treated for PRK. These irregularities may lead to visual disturbances such as diplopia, blurred vision, and loss of Best Corrected Visual Acuity (i.e., the vision obtained with the best possible lens correction).
The PRK procedure has achieved a clinically accepted level. However the possibility of achieving even better results with larger ablation zones has been somewhat elusive. The present invention provides a new insight into conditions that can occur in PRK and provides techniques that address these conditions to enable enhanced predictability, stability, and safety of the procedure to be achieved.