Surgery – Instruments – Light application
Reexamination Certificate
1996-08-08
2001-09-25
Shay, David M. (Department: 3739)
Surgery
Instruments
Light application
C606S003000, C606S010000, C606S013000, C128S898000
Reexamination Certificate
active
06293938
ABSTRACT:
BACKGROUND
This invention relates to improvements in photo-refractive keratectomy (PRK).
PRK involves the removal of corneal tissue in a controlled fashion to shape the surface of the cornea to change the radius of curvature, or refractive power, of a patient's eye to treat e.g., myopia or hyperopia.
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. The average adult cornea is about 0.65 mm thick at the periphery, and about 0.54 mm thick in the center. From anterior to posterior, the cornea has the following five distinct layers: the epithelium, Bowman's membrane, the stroma, Descemet's membrane, and the endothelium. The present invention concerns the epithelium, Bowman's membrane, and the stroma. The epithelium consists of five or six layers of cells, and the underlying Bowman's membrane, a clear acellular layer, is a modified portion of the stroma. The corneal stroma accounts for about 90 percent of the corneal thickness.
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. The general technique involves removing the epithelium, and then shaping the underlying Bowman's and stroma layers, either surgically, or by using photoablation with e.g., ultraviolet radiation from an excimer laser or infrared laser radiation from an infrared laser operating at a wavelength of about 2.9-3.2
One technique, described in Muller, U.S. Pat. No. 4,856,513 (assigned to the present assignee), 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 selectively photoablating the corneal surface into a desired shape.
In another technique, described in Marshall et al., U.S. Pat. No. 4,941,093 (assigned to the present assignee), the shape and size of the area of the corneal surface irradiated by laser energy is selected and controlled so that some areas of the corneal surface become more eroded than others and a desired corneal shape is achieved.
While the PRK procedure has already reached a clinically accepted level, the possibility of achieving even better results has been somewhat elusive. The present invention provides a new level of 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.
SUMMARY
According to the invention it has been realized that under particular conditions various phenomena may occur.
It has been realized that photoablation by-products resulting during the PRK procedure can affect the accuracy and the predictability of the procedure. Under certain conditions, the plume of photoablation by-products that have left the corneal surface can non-uniformly redeposit onto the ablation area and thus affect the uniformity of the result. The plume of photoablation by-products, in the space above the corneal surface, can also non-uniformly affect the escape of further photoablation products from the surface. In addition to the plume effects, it has been realized that the hydration level of the corneal tissue during the PRK procedure can vary over the ablation area and likewise non-uniformly affect the PRK results.
One cause of non-uniform tissue removal is non-uniform redeposition of ablation by-products during the PRK procedure. Typical PRK procedures employ substantially flat (i.e., uniform) laser beam intensity profiles to shape the corneal surface. The inventors have discovered that under such conditions, the beam pulses impinging on the corneal surface can cause photoablation by-products in the form of a plume to redeposit onto the corneal surface in a manner causing non-uniform removal of material. The rapid expansion of the ablation products away from the surface of the eye caused by conventional PRK conditions may create a relatively decreased pressure in the space above a central portion of the ablation region at the eye surface and a relatively higher pressure in the space near the edges of the ablation region. Such conditions can produce a plume action that can cause non-uniform redeposition onto the ablation area. The particular manner in which the ablation products deposit onto the surface of the ablation area depends upon a number of factors. Under conventional PRK conditions, the ablation products may preferentially deposit in the central region of the ablation area, thereby creating a central island of excess material. Also, the deposited material can act as e.g., a mask, or radiation absorber, that interferes with further corneal tissue ablation. The non-uniform distribution of ablation products on the surface of the ablation area can cause a corresponding non-uniform shape to become transferred into the patient's cornea with each successive photoablative pulse.
Another cause of non-uniform tissue removal is plume interference with either beam delivery or escape of the ablation by-products from the surface of the ablation area. Under conventional PRK conditions, the dynamics of the plume of ablation by-products can cause less material to be removed from central portion of the ablation area than from the surrounding portion. For example, the region just above the central portion of the ablation area appears to provide a less effective escape path for gaseous ablation by-products than the region above the periphery of the ablation area.
The non-uniform removal of corneal material tends to cause a surgeon performing a PRK procedure to shape a patient's cornea in a manner outside of the planned correction, which is usually selected based upon a predetermined assumption of a uniform depth of ablation per pulse. Non-uniform shaping of the patient's eye could cause the patient discomfort and possibly a loss in visual acuity.
As mentioned above, the variation in the hydration level of the corneal tissue across the surface of the ablation area can also cause non-uniform ablation. The cornea includes a major proportion of fluid. The inventors have discovered that, as corneal tissue is ablated during a PRK procedure, fluids can accumulate in the ablated areas of the cornea and non-uniformly alter the photoablation sensitivity of the corneal tissue across the surface of the ablation area, and thereby cause non-uniform ablation of the cornea.
Thus the inventors have discovered that the PRK procedure may be further refined by novel steps that substantially control, or avoid, the above-mentioned phenomena.
In one general aspect, the invention features a PRK system for producing a desired refractive correction in an ablation area in the cornea of a patient's eye comprising a source of an initial beam of photoablating radiation capable of ablating the cornea of a patient's eye during a PRK procedure, and means for modifying a plume of ablation products resulting from photoablation of the patient's cornea, the modifying means being constructed and arranged to alter formation dynamics of the plume in a manner substantially preventing the plume from causing non-uniform ablation of the patient's cornea.
According to one aspect, the invention features a PRK system wherein the plume modifying means comprises means for removing material that leaves the surface of the ablation area in a manner substantially preventing a plume of ablation produ
D'Agati Mike
Friedman Marc
Harmon Troy
Klopotek Peter
Muller David F.
Engellenner Thomas J.
Nguyen Tram Anh T.
Nutter & McClennen & Fish LLP
Shay David M.
Summit Technology Inc.
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