Ablatement designed for dark adaptability

A system and method for performing a reshaping of a cornea of an eye for improved vision is presented. The system comprises a first appartus for determining dark adapted pupil size of an eye and a second apparatus for reshaping a cornea of the eye in an area approximately equal to the dark adapted pupil size as determined by the first apparatus. The method of the present invention involves dilating the pupil of an eye to its dark adapted size, determining the diameter of the dilated pupil, and ablating the cornea of the eye to match the dilated pupil size. The advantage to using such a system and method when reshaping a cornea of an eye is reduced halo effect or improved night vision.

FIELD OF INVENTION 
The invention relates generally to a laser system for corneal sculpting. 
More specifically, it relates to a system and method for determining the 
dark adapted pupil size of a patient and reshaping the cornea of the eye 
based on the dark adapted pupil size. 
BACKGROUND OF THE INVENTION 
Of the various components in the human eye, the cornea is the principal 
optical element for refracting incident light onto the retina in the form 
of a clear image. Photorefractive keratectomy (PRK) is a procedure which 
typically utilizes an excimer laser beam to vaporize "ablate" corneal 
tissue in a precise manner to correct for focussing deficiencies of the 
eye. An excimer laser is preferred for this procedure because pulsed 
ultraviolet ablation is predictable, discrete, and non-damaging to 
adjacent tissue. PRK generally involves mechanical removal of the 
epithelium or outer layer of the cornea to expose the Bowman's layer on 
the anterior surface of the stroma. Laser ablation usually begins at the 
Bowman's layer. The laser beam removes corneal tissue to varying depths as 
necessary for recontouring the anterior stroma. Afterward, the epithelium 
rapidly regrows and resurfaces the contoured area, resulting in an 
optically correct (or much more nearly so) cornea. In a variation of the 
procedure, a surface flap of the cornea is folded aside and the exposed 
surface of the cornea's stroma is ablated to the desired surface shape 
with the surface flap then being replaced. 
The specific region of the cornea involved in the refractive image 
formation will vary with the size of the pupil. Only a small central 
corneal region will refract light onto the cornea when the pupil is 
constricted under bright lighting conditions. Under dim lighting 
conditions, when the pupil is substantially dilated, a much larger corneal 
region is involved forming an image on the retina. This variation in pupil 
size can become an issue for a PRK patient if the diameter of the 
laser-treated corneal region ("the optical zone") is smaller than the 
dilated pupil diameter. When the ablated optical zone is smaller than the 
patient's dark adapted pupil size, the patient's night vision is affected. 
Typically, the patient's vision will be hazy or somewhat blurred, and the 
patient may perceive halos around bright lights. Approximately 20 percent 
of patients treated with a 5 mm optical zone have complained of such 
problems. This is a result of the pupil becoming larger than 5 mm as the 
pupil adapts for darkness. When a 6 mm optical zone is ablated, it is 
estimated that only 2 percent of patients complain of night-vision 
problems. 
One apparent strategy for avoiding such night-vision problems would be to 
treat all patients with an optical zone larger, much larger, than the 
maximum pupil diameter. However, there are several disadvantages to this 
approach. First, maximum pupil diameter varies from patient to patient. 
Second, the maximum depth of laser ablation and the total volume of tissue 
removed both increase with optical zone diameter. Such increases typically 
lead to more regression, that is, deterioration, of the refractive change 
as the cornea heals. In addition, the increased tissue volume to be 
removed necessitates a longer laser procedure. Variations in corneal 
ablation behavior over time due to hydration changes in the 
de-epithelialized tissue (known to occur) maybe degrade the accuracy of 
the ablative corneal reshaping for lengthy procedures. A much more 
desirable strategy, not advanced until the present invention, is to tailor 
the optical zone diameter in each treatment to the maximum pupil diameter 
of that patient. 
SUMMARY OF THE INVENTION 
Accordingly, it is an object of the present invention to provide a system 
and method for determining the-dark adapted pupil size of an eye. 
Another object of the present invention is to provide a system and method 
for determining the dark adapted pupil size of an eye as a tool in 
ophthalmic laser surgery to include corneal sculpting procedures. 
Yet another object of the present invention is to provide a system and 
method for determining the dark adapted pupil size of an eye that is 
surgically eye safe. 
Other objects and advantages of the present invention will become more 
obvious hereinafter in the specification and drawings. 
In accordance with the present invention, a system and method are provided 
for performing a reshaping of a cornea of an eye for improved vision. The 
system comprises a first apparatus for determining dark adapted pupil size 
of an eye; and a second apparatus for reshaping the cornea of the eye in 
an area approximately equal to the dark adapted pupil size as determined 
by the first apparatus. The preferable appartus for determining the dark 
adapted pupil size of an eye permits tracking the eye as ambient lighting 
is dimmed down to complete or near-complete darkness with an electrical 
control of a zoom mechanism. The zoom refers to adjusting the diameter of 
the spot pattern projected onto the eye. Feed back can be utilized to 
sense the return from the spots, and then to adjust the zoom so that the 
spot pattern diameter matches the pupil diameter. Laser ablation would 
then proceed over that circular region (maybe, plus a little outside). 
Alternatively, a video system with a circular cursor could be used in dim 
illumination to identify the position and diameter of the dilated pupil. 
The zoom mechanism has an electrical control for size adjustment of a beam 
optical radiation or a plurality of such radiation beams (either at a 
visible or infra-red wavelength) incident on a boundary coincident with 
the dark adapted pupil size of the eye. The first apparatus may also 
comprise delivery optics for focusing a plurality of optical radiation 
beams on a corresponding plurality of positions located on a boundary 
coincident with the pupil size of the eye to form a pattern. Zoom optics 
may be used for adjusting the pattern formed by the plurality of optical 
radiation beams incident on said corresponding plurality of positions. An 
optical receiving arrangement for detecting reflected energy from each of 
the plurality of positions is also employed to determine the dark adapted 
pupil size. The pattern formed by the plurality of optical radiation beams 
is equivalent to the dark adapted pupil size of the eye. The dark adapted 
pupil size is entered into a computer program for corneal sculpting. 
In performing laser reshaping of an eye for improved vision, the diameter 
of the optical zone ablated on the cornea is adjusted to match, or nearly 
match, the diameter of the dilated pupil. The ablation profile at the 
periphery of the optical zone may be tapered to form a smooth transition 
between treated and untreated portions of the eye. 
This patent application is copending with related PCT patent applications 
entitled, "Laser Sculpting Method and System", International publication 
number WO 95/28890; "Eye Movement Sensing Method and System", 
International publication number WO 95/28879; and "Laser Beam Delivery and 
Eye Tracking System", International publication number WO 95/28989 all of 
which were published on Nov. 2, 1995, and owned by a common assignee of 
subject PCT applications. The disclosures of these three applications all 
of which are based on United States patent applications, are incorporated 
herein by reference.

DETAILED DESCRIPTION OF THE INVENTION 
Referring now to the drawings, and more particularly to FIG. 1A, a block 
diagram of the system of the present invention is shown. The eye 10 is 
shown with an enlarged pupil 11 which has been adapted to its dark adapted 
size. As the lights are dimmed or the pupil is otherwise enlarged to a 
dark adapted pupil size such as by dilation by appropriate eye drops, the 
apparatus for determining dark adapted pupil size 12 emits at least one 
invisible beam of light onto the eye to form one or more spots for 
defining the size of the dark adapted pupil. The size of the spot is 
adjusted to match the size of the dark adapted pupil 12 by focussing on 
the pupil/iris boundary. The measured diameter of the spot size is then 
input into a program which is used to run the cornea shaping apparatus 13. 
The zone which is ablated under the corneal sculpting program is just a 
little larger in diameter than the dark diameter of the adapted pupil to 
allow for a smooth transition between the treated and untreated portion of 
the eye. 
FIG. 1B shows a preferred method for determining the diameter of the dark 
adapted pupil size 11 of an eye 10. In this method, four spots of light 
15, 16, 17 and 18 are positioned on the boundary between the pupil 11 and 
the iris 14. As the lights are dimmed in the room, the spots of light are 
used to track the pupil as it dilates. Any movement of the eye may also be 
tracked using the four spots of light as has been previously disclosed. 
The wavelength and power of the light spots can be outside the visible 
spectrum so as not to negate the dark adaptable pupil size or to interfere 
or obstruct the surgeon's view of the eye undergoing the surgical 
procedure or injure the subject eye. 
As a preferred embodiment, the apparatus for determining the dark adapted 
pupil size and the cornea shaping apparatus for carrying out the method of 
the invention are described in detail in the aforementioned copending PCT 
patent applications. However, for the purpose of the present description, 
the optical arrangement will be described briefly with the aid of the 
block diagram of FIG. 2. In FIG. 2, fiber optic bundle 123 is positioned 
at the working distance of microscope objective 1302. The numerical 
aperture of microscope objective 1302 is selected to be equal to the 
numerical aperture of fibers 116, 118, 120 and 122. Microscope objective 
1302 magnifies and collimates the incoming light. Zoom lens 1304 provides 
an additional magnification factor for further tunability and for 
determining the dark adapted pupil size. The zoom lens increases the spot 
size as the lights are turned down to examine the patient. Collimating 
lens 1306 has a focal length that is equal to its distance from the image 
of zoom lens 1304 such that its output is collimated. The focal length of 
imaging lens 1308 is the distance to the eye such that imaging lens 1308 
focuses the light as four sharp spots on the corneal surface of the eye at 
the boundary between the pupil and the iris. An optical receiving 
arrangement detects reflected energy from each of the light spots which is 
used to measure the diameter of the dark adapted pupil size. This size 
measurement is subsequently entered into a program for corneal sculpting. 
Once the size of the ablation zone is determined, the depth of ablation for 
each diopter of correction can be determined. For example, if the surgical 
method requires a 5 mm zone, ablation will only have to go down about 10 
microns deep for one diopter of correction. For a 6 mm zone, the ablation 
would go down about 13 to 15 microns deep for each diopter of correction. 
The treatment is customized for the needs of the individual, but a lower 
limit set for approximately 5.5 mm as determined from a statistical data 
base. Alternatively, the low end limit for the ablation diameter may be 
the measured diameter of the dark adapted pupil size of the individual. 
This method allows for customizing the ablation zone of the patient's dark 
adapted pupil size to eliminate the halo problem. In turn, this procedure 
may be done for all of the patients on a customized basis without having 
to ablate a large volume for patients who only need a smaller volume 
ablated if they have a smaller dark adapted pupil size. 
Although the invention has been described relative to a specific embodiment 
thereof, there are numerous variations and modifications that will be 
readily apparent to those skilled in the art in the light of the above 
teachings. It is therefore to be understood that, within the scope of the 
appended claims, the invention may be practiced other than as specifically 
described.