U.S. patent application number 09/951827 was filed with the patent office on 2002-03-07 for refraction correction with custom shaping by inner corneal tissue removal using a microjet beam.
This patent application is currently assigned to Medjet, Inc.. Invention is credited to Gordon, Eugene I..
Application Number | 20020029053 09/951827 |
Document ID | / |
Family ID | 26813770 |
Filed Date | 2002-03-07 |
United States Patent
Application |
20020029053 |
Kind Code |
A1 |
Gordon, Eugene I. |
March 7, 2002 |
Refraction correction with custom shaping by inner corneal tissue
removal using a microjet beam
Abstract
The present invention provides a new approach to reshaping of
the cornea, e.g., for refraction change, using multiple, displaced
planar cuts and a custom shaping template. Large refractive change
and/or substantial tissue removal can be obtained by a two-cut
approach to reshaping of the cornea to a desired shape using a
template or applanator. The process begins with a planar template
being applied to the cornea. The template includes one or more
moveable sections or cams positioned to provide an overall flat
contact surface with the cornea. Then, a first cut is made by a
water microjet producing a hinged flap. The first cut is parallel
to but displaced from the anterior cornea surface in contact with
the template. Then the template cam or cams are repositioned to
change the shape of the cornea surface in situ in preparation for
the second cut. The hinged flap is not moved; it remains in contact
with the stromal bed. The second cut is along the same path as the
first cut. However, since the cornea has been reshaped, the second
cut defines a separate cut line in the cornea. As a result of the
first and second cuts, a body of internal tissue defined by the
relative paths of the first and second cuts through the corneal
tissue can be removed from the cornea to thereby shape the cornea
and provide refractive correction. This enables a large range of
accurate refractive correction and/or therapeutic tissue removal by
directly controlling the geometry of the volume of tissue removed
from the interior of the cornea.
Inventors: |
Gordon, Eugene I.;
(Mountainside, NJ) |
Correspondence
Address: |
DARBY & DARBY P.C.
805 Third Avenue
New York
NY
10022
US
|
Assignee: |
Medjet, Inc.
|
Family ID: |
26813770 |
Appl. No.: |
09/951827 |
Filed: |
September 13, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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09951827 |
Sep 13, 2001 |
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09483687 |
Jan 14, 2000 |
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6312439 |
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60115966 |
Jan 15, 1999 |
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60137242 |
Jun 2, 1999 |
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Current U.S.
Class: |
606/167 |
Current CPC
Class: |
A61B 17/3203 20130101;
A61F 9/013 20130101; A61B 2017/306 20130101 |
Class at
Publication: |
606/167 |
International
Class: |
A61B 017/32 |
Claims
1. Apparatus for removing inner corneal tissue comprising: a vacuum
template adapted to contact an anterior surface of a cornea and
operable to produce at least two configurations thereby to maintain
the anterior surface of the cornea in at least two respective
shapes; a fluid beam producing device for producing a fluid beam;
and a beam scan guide for scanning said fluid beam across said
cornea.
2. Apparatus according to claim 1 wherein said vacuum template is
further adapted to engage said anterior surface of said cornea by
including a vacuum guard and a stationary template, wherein said
vacuum guard surrounds said stationary template and said vacuum
template further includes a gap between said vacuum guard and said
stationary template and a means for applying a vacuum to said gap,
thereby securing the cornea against the vacuum template.
3. Apparatus according to claim 1 wherein said vacuum template is
further adapted to engage said anterior surface of said cornea by
including a vacuum guard and a stationary template, wherein said
vacuum guard surrounds said stationary template in a snug fit, said
stationary template includes a plurality of grooves and said vacuum
template further includes a means for applying a vacuum to said
plurality of grooves, thereby securing the cornea against the
vacuum template.
4. Apparatus according to claim 1 and further comprising a control
unit for controlling the configurations produced by said vacuum
template.
5. Apparatus for removing inner corneal tissue comprising: a vacuum
template adapted to contact an anterior surface of a cornea, said
vacuum template including at least one piston which moves in a
direction perpendicular to the plane defined by the direction of
the fluid beam and the direction of scanning in order to produce a
plurality of configurations and, whereby the anterior surface of
the cornea is adaptable to a plurality of respective shapes
reflecting the plurality of configurations of said piston; a fluid
beam producing device for producing a fluid beam; and a beam scan
guide for scanning said fluid beam across said cornea.
6. Apparatus according to claim 5 wherein said vacuum template
includes a plurality of pistons and each of said plurality of
pistons is adaptable to move in a direction perpendicular to the
plane defined by the direction of the fluid beam and the direction
of scanning and to maintain contact with the anterior surface of
the cornea, and, for each of said plurality of pistons, the
direction of movement is opposite from the direction of movement of
at least one other of said plurality of pistons and the magnitude
of movement is different than the magnitude of movement of at least
one other of said plurality of pistons, whereby the anterior
surface of the cornea is changed to a custom shape reflecting the
custom shape of said plurality of pistons.
7. Apparatus according to claim 6 and further comprising a control
unit for controlling the movement of each of said plurality of
pistons in a direction perpendicular to the plane defined by the
direction of the fluid beam and the direction of scanning, to the
anterior surface of the cornea in situ.
8. Apparatus according to claim 6 wherein the cross-section of said
plurality of pistons forms a honeycomb.
9. Apparatus according to claim 6 wherein said plurality of pistons
includes gaps in between each of said plurality of pistons and
further including and a means for applying a vacuum to said gaps,
thereby securing the cornea against said plurality of pistons.
10. Apparatus according to claim 6 wherein said plurality of
pistons includes a deformable disc at one end for contacting the
anterior surface of the cornea and smoothing the height transitions
in between adjacent ones of said plurality of pistons.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This patent application is a divisional of U.S. patent
application Ser. No. 09/483,687, filed on Jan. 14, 2000, which
claims priority to U.S. Provisional Patent Application Nos.
60/115,966, filed on Jan. 15, 1999, and 60/137,242, filed on Jun.
2, 1999, all of which are incorporated by reference in their
entirety.
FIELD OF THE INVENTION
[0002] The present invention relates to refraction correction in
general and, more particularly, to refractive correction involving
a shaping template and a fluid microjet applied to the cornea to
cut tissue.
BACKGROUND OF THE INVENTION
[0003] Reshaping of the cornea for refractive vision correction has
been the object of various procedures, some of which have only been
recently developed. In one well known procedure, namely, radial
keratectomy (RK), the cornea is incised with radial cuts to flatten
the shape of the anterior surface of the cornea in order to correct
for myopia. This is a surgical procedure requiring a high degree of
skill and judgment for effective and safe implementation.
Additionally, even when such procedure is carried out properly,
myopia-corrective flattening may cause instabilities, such as a
gradual progression to hyperopia over time.
[0004] Another recently developed system uses an excimer laser to
remove corneal tissue by photo-thermal ablation rather than
cutting. In the latest version of this system, a sequence of
incident laser pulses with energy focused to a small spot moving
from point to point gradually removes tissue from the anterior
surface of the cornea. The local extent of tissue removal depends
on the number of laser pulses at the position on the cornea and
results in a new shape for the ablated surface. An eye tracker is
used in some versions to compensate for eye motion during the
lengthy scan period, e.g., tens of seconds. Laser spot scanners
utilize a bell-shaped laser energy distribution having a half-power
diameter of about 2 mm. It is likely that smaller laser spots could
be achieved, but stability of the eye or accuracy of the eye
tracker may limit the useful resolution. The pulsed delivery of
laser energy in spots and arbitrary spot positioning allows overlap
during scanning for smoothing. The equivalent spot density can be
high. Nevertheless, the achievable resolution or shaping detail is
limited by the spot size, since overlapped spots are not
independent. Also, the spatial frequency transfer function for
patterning effects the accuracy of the laser spot scanners. Based
on the 2 mm spot size, the distribution is probably flat out to a
cutoff spatial frequency of about 0.25 cycles/mm. Over a 6 mm
ablation zone, that implies only 1.5 cycles of shaping. It seems
barely enough for myopic correction. For hyperopic correction, a 9
mm zone would be required. It may not be enough for achieving super
acute vision where finer features need to be resolved.
[0005] This use of laser pulses for shaping the cornea, known as
photo-refractive keratectomy (PRK) is generally safe and effective.
However, there are several drawbacks to this method, including the
high cost of the equipment required for the PRK procedure. Another
drawback is the relatively high residual error factor (or lack of
emmetropia), often on the order of .+-.1.0 diopter more, as
compared to a typical error of less than .+-.0.25 diopter for
spectacles or contact lenses. In addition, laser ablation results
in a rough corneal surface. Furthermore, there are long term
effects relating to the physiology of the cornea and its
interaction with the laser during ablation, which may result in
subsequent gradual reversal of the correction and/or complications
due to wound healing and/or potential carcinogenic effects. Other
common side effects of PRK include haze, night-time glare and
reduced best-corrected visual acuity.
[0006] The cornea comprises a thin protective epithelium layer on
top of the Bowman's membrane or layer, which in turn covers the
major corneal stroma. While the epithelium is regenerative, the
Bowman's membrane is not. With ablative corneal tissue removal
procedures such as PRK, the epithelium and Bowman's membrane are
removed together with a portion of the stroma. Subsequently, the
epithelium regenerates on the exposed outer surface of the cornea
directly on the stroma because the Bowman's layer is not
regenerated. However, direct regrowth of the epithelium on the
stroma can cause an undesirable corneal haze which gradually
dissipates over time.
[0007] Both the RK and PRK methods described above have inherent
instabilities and error factors which make them generally
unsuitable for correction of myopia of more than -9 diopters. A
surgical procedure known as Automated Lamellar Keratoplasty (ALK)
preserves the Bowman membrane and has been used for corrections of
up to -20 diopters. In this procedure, in a first surgical step, a
blade micro-keratome is used to remove a uniform thickness button
or lenticule of corneal tissue which contains a portion of the
epithelium layer, the Bowman's membrane (intact) and a portion of
the stroma. The button or lenticule preferably remains "hinged" at
one point to the cornea. The hinged lenticule is then moved out of
the way and the stromal bed is surgically reshaped with the
micro-keratome by removal of a second unhinged lenticule to produce
the required refraction correction. Then, the hinged lenticule is
replaced on the stromal bed, providing good adherence and healing
of the stroma-stroma interface, preserving the Bowman's membrane,
and leaving the cornea substantially clear. It appears that the
stroma-stroma healing of the ALK procedure reduces, if not
eliminates, wound healing instabilities, making this procedure
suitable for large refractive corrections.
[0008] However, despite the advantage of retention of vision
clarity and healing stability, the ALK procedure is not favored
because it is complex and expensive, requires high surgical skills
and, depending on the surgeon's skill, is usually inaccurate and
may cause irregular astigmatism. Some of these problems may be
attributed to the viscous and generally unsupported nature of the
cornea, which may be enhanced by reflexive movements of the
patient, making the use of a scalpel or even a micro-keratome
difficult and inaccurate.
[0009] In view of the above, currently the most favored approach to
refraction correction is to produce a hinged flap with a blade
micro-keratome and then to reshape the exposed stromal bed using
PRK as described above. This procedure, commonly referred to as
LASIK, is less safe than conventional PRK and is used primarily
because of reduced short-term inconveniences, such as pain and
delay in return of visual acuity. The long term effects of LASIK
are similar to those of PRK.
[0010] Cleaving off a lenticule having a predetermined shape using
a microjet beam is also known in the art. Such a procedure is
described in U.S. Pat. No. 5,556,406 to Gordon et al., the entire
disclosure of which is incorporated herein by reference. In
practice, a number of different procedures using a microjet beam
have been applied for refraction correction.
[0011] In a procedure known as the HRK1, by Medjet Inc. (Edison,
N.J.), a lenticule having a desired shaped is removed by a microjet
beam. After this removal, epithelium growth on the remaining
stromal bed may change the optical properties of the cornea causing
inaccuracies in the refraction correction. This phenomenon is
similar to that described above with reference to PRK. Another
procedure using a microjet beam, known as HRK2, is similar to the
two-step ALK technique described above. In a first step, a microjet
beam cut is used to form a hinged flap in the cornea. The flap is
then moved to the side and a second cut is made with the microjet
beam, removing a lenticule of a predetermined shape for refractive
correction. Finally, the flap is replaced in its original position.
The results are similar to those of the ALK technique, but the use
of a water jet beam is safer and more accurate. This technique is
described in U.S. Pat. No. 5,556,406 to Gordon et al.
[0012] By investigating the interaction of a fluid microjet beam
with the cornea, the present inventors have discovered that a
single lamellar cut in the cornea can be used to remove inner
corneal tissue under a parallel flap. When the flap is placed back
on the cutting site, the resultant corneal surface is flattened
compared to the original surface topography.
[0013] In a procedure known as HRK3 by Medjet Inc., shaping of the
cornea by erosion and cutting a hinged flap are performed
simultaneously. According to experimental results, a surface cut by
fluid microjet cannot be distinguished, under microscopic
examination, from a surface cleaved by a micro-keratome. Shaped
erosion removal of tissue is also possible under certain scan
conditions. Experimental results also indicate that HRK tissue
removal can result in a spherical surface. The thickness of removed
tissue is less than or greater than the microjet beam diameter, as
required. However, based on experimental results, there seems to be
a practical limit to the thickness of tissue that may be removed by
a single beam scan and, thus, there is a limit to the refractive
change that may be achieved by this method. In general, erosion
tissue removal can be increased by reducing the scanning speed of
the microjet beam; however, substantial slowing of the scanning
speed results in poor or even unacceptable surface quality. This
technique is described in patent application Ser. No. 08/955,645,
filed Oct. 22, 1997, the entirety of which is incorporated by
reference. To achieve greater refraction correction by erosion
shaping, a multiscan technique has been used, wherein a high
accuracy scanning robot performs multiple scans in the same plane
for additional tissue removal by erosion. In this technique,
greater tissue removal can be achieved by cutting and, thus,
greater diopter correction. However, multiple scanning of the
microjet beam is similar to slow scanning of the beam and may
therefore result in poor surface quality.
[0014] Therefore, the rapid evolution of refractive surgery based
on the LASIK procedure and the increasing interest in the potential
of a surgical approach to achieve super acute vision has created an
interest in a surgical procedure which will allow accurate and high
resolution custom tissue removal. Improved refraction correction
results compared to the surgical procedures described above are
needed.
SUMMARY OF THE INVENTION
[0015] The present invention provides a new approach to reshaping
of the cornea, e.g., for refraction change, using multiple,
displaced planar cuts and a custom shaping template. In accordance
with an embodiment of the present invention, large refractive
change and/or substantial tissue removal can be obtained by a
two-cut approach to reshaping of the cornea to a desired shape
using a template or applanator. More particularly, a water microjet
is used to shape the cornea by two successive co-planar cuts in the
cornea. For example, the process begins with a planar template
being applied to the cornea. The template includes one or more
moveable sections or cams positioned to provide an overall flat
contact surface with the cornea. Then, a first cut is made by the
microjet producing a hinged flap. The first cut is parallel to but
displaced from the anterior cornea surface in contact with the
template. Then the template cam or cams are repositioned to change
the shape of the cornea surface in situ in preparation for the
second cut. The hinged flap is not moved; it remains in contact
with the stromal bed. The second cut is along the same path as the
first cut. However, since the cornea has been reshaped, the second
cut defines a separate cut line in the cornea. As a result of the
first and second cuts, a body of internal tissue defined by the
relative paths of the first and second cuts through the corneal
tissue can be removed from the cornea to thereby shape the cornea
and provide refractive correction. This enables a large range of
accurate refractive correction and/or therapeutic tissue removal by
directly controlling the geometry of the volume of tissue removed
from the interior of the cornea.
[0016] In accordance with one embodiment of the present invention,
a body of inner corneal tissue is removed by first and second
successive cuts in the cornea, the first cut producing a hinged
flap of corneal tissue and the second cut being made without
lifting the anterior flap of tissue and without otherwise moving,
repositioning and/or realigning the cornea between the first and
second cuts. The shape of the template is changed between the first
and second cuts. In the alternative, the microjet beam can be moved
but changing the shape of the template is easier than moving the
beam.
[0017] In accordance with a preferred embodiment of the invention,
a first lamellar cut is made by scanning a microjet beam across the
cornea while the cornea is subjected to a predefined planar
applanation by a template which maintains a flat shape of the
cornea. After completing the first cut, the microjet can be scanned
back to its starting position, typically with the microjet beam
deactivated and without lifting or otherwise moving the parallel
flap produced by the first lamellar cut. At this point, the
template shape can be changed to a predetermined configuration,
causing the anterior surface of the cornea to assume a new
predetermined shape. Then, the waterjet beam is reactivated and a
second scanning of the waterjet in the same plane is performed,
producing a second cut in the corneal tissue. Relative to the
corneal tissue, the second cut is displaced with respect to the
first cut and, thus, a body of tissue defined by the relative paths
of the first and second cuts is ejected during the second scanning
of the microjet. The ejection of such body of tissue, which
resembles a thin slab, has been viewed experimentally. The volume
of the ejected tissue is generally responsive to the difference in
shape of the anterior surface between the first and second cuts due
to a change in the template configuration and/or a difference in
position of the scanning beam relative to the cornea. By changing
the shape of the cornea between cuts, the technique of the present
invention can be used for refraction correction applications.
Alternatively, the order of the template positions described above
can be reversed such that the first template configuration is
non-planar and the second template configuration is planar.
Furthermore, by changing the scanning position or plane of the
microjet beam between cuts, the technique of the present invention
can be used for other ophthalmic application, such as removal of
defects in the cornea, without refraction correction. In this case,
a parallel slab is removed.
[0018] Another embodiment of the present invention further provides
a device for variably controlling the shape of the anterior surface
of the cornea. In an embodiment of the present invention, the
device includes a variable vacuum template to support the anterior
surface of the cornea in different shapes, while continuously
engaging the cornea. This device is capable of changing the shape
of the anterior surface of the cornea between the first and second
cuts of the technique described above. The shape is changed based
on the use of one or more cams or pistons as part of the template.
The cams can provide a flat surface for contact with the anterior
cornea surface for the first cut and a different shape (e.g., a
convex or concave shape relative to the slope of the corneal
surface etc.) for repositioning the anterior cornea surface for the
second cut.
[0019] In accordance with another feature of the invention, the
template shape has the following operating specifications: the
shaping resolution has a falloff spatial frequency of 0.5 cycles/mm
or approximately twice the resolution of prior art laser devices.
This implies that the linear density of controlled, independent
tissue incision zones should be at least 1 per mm. It should be
possible to remove no tissue at one point and to remove 100 .mu.m
of tissue at any adjacent point distant by 1 mm. As a result, a
smooth gradation is provided and the tissue incision zones fall
within particular diameter circles to accommodate particular
corrections. For example, a tissue incision zone falling within a 9
mm diameter circle accommodates hyperopic correction while a tissue
incision zone falling within a 6 mm diameter circle accommodates
myopic correction. In addition, the maximum thickness of the
lamella layer, the tissue removal aliquot, is set to .+-.2 microns
based on the following analysis: the photo-ablation depth of laser
shapers is not really under good control. In a given cornea, the
photo-ablation rate is dependent on many factors: the particular
cornea, the surface preparation, the thickness of the flap, the
level of hydration, the temperature, etc. Moreover, the pulse power
varies from pulse to pulse, perhaps as much as .+-.20%. Although
ambient temperature and humidity play a role, they are usually not
well controlled in the surgical suite. In terms of results, the
breadth of the distribution of initially achieved refractive
correction versus intended correction is greater than .+-.1
diopter. For myopia correction of a plano-convex volume having a
diameter of 6 mm, the refractive power is +1 diopter for each 13 mm
of thickness at the center of the volume. This suggests that the
tissue removal accuracy at the center of a 6 mm diameter circle is
not better than .+-.13 .mu.m. As a result, .+-.2 microns, about the
maximum thickness of a lamella layer, the tissue removal aliquot,
is the objective using the template. This would provide an accuracy
of .+-.{fraction (1/6)} diopter.
[0020] The preferred embodiment of the present invention uses a
liquid microjet as the scanned fluid beam for implementing the
refractive correction techniques using multiple displaced cuts. A
liquid microjet is described in U.S. Pat. No. 5,556,406.
BRIEF DESCRIPTION OF THE DRAWINGS
[0021] FIG. 1 is a schematic side view illustration of an
arrangement for removal of corneal tissue using a microjet beam and
a vacuum template in accordance with an embodiment of the present
invention;
[0022] FIG. 2A is a schematic, side view, cross-sectional
illustration of part of a cornea engaged by a substantially flat
vacuum template after cutting a substantially parallel inner slab
of corneal tissue using a microjet beam in accordance with the FIG.
1 embodiment of the present invention;
[0023] FIG. 2B is a schematic, side view, cross sectional
illustration of part of the cornea and the fixed vacuum template of
FIG. 2A showing cutting lines for removing the parallel slab of
corneal tissue in accordance with the FIG. 1 embodiment of the
present invention;
[0024] FIG. 3 is a schematic, side view, cross sectional
illustration of part of a cornea and a vacuum template configured
for cutting a slab of corneal tissue shaped for correction of
myopia in accordance with an alternative embodiment of the present
invention;
[0025] FIG. 4 is a schematic, side view, cross sectional
illustration of part of a cornea and a vacuum template configured
for cutting slabs of corneal tissue shaped for correction of
hyperopia and astigmatism in accordance with a second alternative
embodiment of the present invention;
[0026] FIG. 5 is a schematic side view illustration of an
arrangement for removal of corneal tissue using a microjet beam and
a vacuum template in accordance with a third alternative embodiment
of the present invention;
[0027] FIG. 6 is a schematic side view illustration of an
arrangement for removal of corneal tissue using a microjet beam and
a vacuum template including a piston or cam in position for a first
cut of the liquid microjet beam in accordance with the FIG. 5
embodiment of the present invention;
[0028] FIG. 7 is a schematic side view illustration of the FIG. 6
arrangement for removal of corneal tissue in position for a second
cut of the microjet beam in accordance with the FIG. 5 embodiment
of the present invention;
[0029] FIG. 8 is a schematic side view illustration of an
arrangement for removal of corneal tissue using a liquid microjet
beam and a vacuum template including multiple pistons or cams for
translation downward or upward relative to the anterior cornea
surface in accordance with a fourth alternative embodiment of the
present invention;
[0030] FIG. 9 is a schematic bottom view illustration of an
arrangement for removal of corneal tissue using a microjet beam and
a vacuum template including multiple pistons or cams according to a
fifth embodiment of the present invention;
[0031] FIG. 9A is a schematic cross sectional illustration of the
multiple pistons or cams of FIG. 9 according to the fifth
embodiment of the present invention;
[0032] FIG. 10 is a schematic side view illustration of the FIG. 9
embodiment where there are five pistons or cams and the corneal
tissue removal device is applied to a cornea for a second cut in
order to correct myopia according to the fifth embodiment of the
present invention;
[0033] FIG. 11 is a schematic side view illustration of the FIG. 9
embodiment where there are five pistons and the corneal tissue
removal device is applied to a cornea for a second cut in order to
correct hyperopia according to the fifth embodiment of the present
invention; and
[0034] FIG. 12 is a schematic side view illustration of the FIG. 9
embodiment where there are five pistons including a disc or
membrane at the contact point with the cornea and the corneal
tissue removal device is applied to the cornea for a second cut
according to the fifth embodiment of the present invention.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
[0035] FIG. 1 schematically illustrates a side-view cross section
of an arrangement for removing inner layer tissue from a cornea 10
using a microjet beam 12 and a vacuum template 20 in accordance
with an embodiment of the present invention. The general structure
and operation of the arrangement of FIG. 1, except for the specific
structure and operation of template 20, described in detail below,
are generally analogous to FIG. 4 of U.S. Pat. No. 5,556,406,
wherein similar elements perform similar functions, except as
described below. Vacuum template 20 may be electronically
controlled to provide a predetermined configuration which maintains
the anterior surface 22 of cornea 10 in a predetermined shape,
which may differ depending on the desired refraction correction
being performed, e.g., correction for myopia, hyperopia,
astigmatism and/or any other desired refraction correction
presently known or hereinafter discovered. In some embodiments of
the invention, to accommodate different configurations for cornea
10, template 20 includes a plurality of sub-templates (shown in
FIGS. 4 and 8-11), which may be movable relative to each other
and/or relative to cornea 10, to enable control of the physical
shape of template 20 in addition to or instead of controlling the
vacuum applied by the template.
[0036] As further shown in FIG. 1, the microjet cutting guide 5 is
positioned relative to the template 20 such that the microjet beam
12 is aligned and coincident with the intended plane to be cut by
such beam 12. The microjet cutting guide 5 is in the form of a ring
6, and the liquid microjet inlet 7 provides high-pressure liquid to
the beam 12 to exit at the liquid microjet outlet 4. Template 20 is
concentrically placed within the ring 6 and locked into position by
locking tabs 8 and 9. To ensure that the deformation is effective
in making the planar surface a true surface for cutting (i.e.,
wherein, after the cutting, the cornea relaxes into the desired
configuration), a vacuum is applied through the porous template to
cause the cornea surface 22 to conform closely to the template 20.
The vacuum can be maintained until at least the intended cut is
completed. The scan speed is preferably greater than 15 mm/sec to
avoid erosion. Slower scan speeds are within the scope of the
invention. The pump stagnation pressure is preferably high enough
for the microjet to cut a full flap that's approximately 9 mm at
maximum scan speed. An exemplary pressure is 20,000 psi with a beam
diameter of 33 .mu.m.
[0037] In accordance with the invention, as described in detail
below, microjet beam 12 is activated and scanned to produce two
cuts in cornea 10, such as cut 13, while vacuum template 20
maintains cornea 10 in a predetermined shape and position with
respect to beam 12 during each of the two cuts. When two cuts are
made at two different configurations, as described below, a slab of
inner corneal tissue of a predetermined shape is removed from
cornea 10.
[0038] FIGS. 2A and 2B schematically illustrate a method for
removal of inner corneal tissue using a microjet beam 12 in
accordance with one embodiment of the invention. In the method of
FIGS. 2A and 2B, a substantially flat template 20 is used whereby a
slab of tissue 30 (shown in FIG. 2B) having substantially parallel
surfaces is removed from cornea 10. Slab 30 is excised based on the
anterior surface 22 of the cornea 10 contacting the template and
the microjet beam (not shown) applying a first cut 36 followed by a
second cut 34. In contrast to multiple scanning techniques in which
the beam is used for erosion shaping (as shown, e.g., in U.S. Pat.
No. 5,964,775 to Gordon et al., which is incorporated in its
entirety into this specification), the method of the present
invention relies on first and second cuts, 36 and 34, respectively,
in FIGS. 2A and 2B, wherein the relative paths of cuts 34 and 36
are not coincidental by virtue of a template. Due to the
non-coincidental cuts, in contrast to prior art cut techniques, a
volume of corneal tissue not connected to adjacent tissue is
produced and at least one macroscopic piece (slab) 30 of inner
corneal tissue is ejected from cornea 10 as the second cut 34 is
completed. In the exemplary embodiment of FIGS. 2A and 2B, the two
cuts are substantially parallel and the plane of the second cut 34
is below the plane of the first cut 36, so that a substantially
parallel slab 30 having a width 6 is removed. Slab 30 forms a
shallow cavity 18 (shown in FIG. 2A) having substantially parallel
walls. Cavity 18 separates cornea 10 into an upper flap portion 14
and a lower stromal bed portion 16. Because a parallel flap is
removed, this procedure produces substantially no refractive
correction, in contrast to other procedures of the invention as
described below. The procedure of FIGS. 2A and 2B is thus useful,
inter alia, for removing internal defects in the cornea and other
ophthalmic applications for which no refractive correction is
necessary. The procedure of FIGS. 2A and 2B can be combined with
procedures as described below to produce refraction correction in
combination with other applications, such as removal of internal
defects, e.g., by removing a non-parallel slab of a predefined
thickness and shape.
[0039] In an embodiment of the present invention, the microjet beam
is scanned, e.g., by a scanner robot, at very high speed, for
example, 20 millimeters per second, with high accuracy. Thus, if
the position of the microjet beam is not changed, the physical
plane of the second cut is virtually identical to that of the first
cut. Cavity 18 may be produced by merely changing the plane of the
second microjet cut relative to the cornea. Alternatively, in
embodiments of the present invention as described below, the shape
and size of the removed inner corneal tissue are controlled, for
example, by controlling the configuration produced by vacuum
template 20.
[0040] To measure the effect of corneal shaping on the resultant
shape of the cornea after removal of the slab 30, the present
inventors used varying weights in earlier experiments, e.g., up to
a few hundred grams, on the support structure of the template 20
after performing the first cut 36 and before performing the second
cut 34. This displaced the template in the direction perpendicular
to its plane by small amounts without changing the plane of the
microjet cut. These experiments indicate that the relative
displacement between the first and second cut is very small (a
vertical displacement on the order of tens of microns of the plane
of the second cut 34 relative to the plane of the first cut 36 is
possible by a corresponding displacement of template 20; such
vertical displacement of template 20 (e.g., upwards) is indicated
by arrow 35 in FIG. 2B). Integral slabs are ejected after the
second cut.
[0041] In accordance with experiments performed by the present
inventors, an average central thickness of approximately 13 .mu.m
per diopter, for a removed plano-convex slab having a diameter of 6
mm, is required for typical refraction correction of myopia. This
thickness per diopter is generally proportional to the diameter
squared of the removed slab 30, as shown in FIG. 2B as the
thickness .delta. and the diameter d of the slab 30, where .delta.
is proportional to d.sup.2. After completion of the above described
procedure, cornea 10, including lower portion 16 and flap portion
14 thereon, is released from engagement with template 20, whereby
flap 14 assumes its normal position. At this point, the sphericity
of cornea 10 is substantially restored but the new spherical
surface assumes a different curvature which corresponds to the
desired refractive change. In addition, in order to obtain a
controlled refractive change, the central thickness .delta. (shown
in FIG. 2B) of the removed slab 30 must be accurately controlled,
for example, with a maximum error of a few micrometers. The
structure of the stroma and the physical processes related to
cutting of the cornea impose a theoretical limit on the dimensional
accuracy of the cutting, typically on the order of .+-.2 .mu.m.
[0042] FIG. 3 shows a template arrangement generally suitable for
use in correction of myopia. In this embodiment of the invention,
template arrangement includes a fixed, annular, vacuum template 42
and a movable, central vacuum template 48 to provide a desired
displacement between a first cut 56 and a second cut 54 made in a
cornea 50 having an anterior surface 52 which contacts template
elements 42 and 48. The illustrated template arrangement is merely
an example and various other arrangements may yield similar
results. For example, annular template element 42 may be movable
and central template element 48 may be fixed, or both template
elements may be movable to provide more flexibility in controlling
the shape of anterior surface 52 of cornea 50 and, thus, more
accurate refraction correction.
[0043] In the embodiment of FIG. 3, the position of central
template element 48 is accurately adjusted and controlled, using
means which are known in the art, while the plane of fixed template
element 42 remains unchanged between the first and second cuts.
During the scanning which forms first cut 56, performed prior to
the situation shown in FIG. 3, template elements 42 and 48 are set
to be substantially in the same plane and cornea 50 is divided into
a stromal bed portion 46 and a parallel, yet applanated, flap
portion 44. The boundary between these two parts of the cornea is
defined by the plane of the first cut 56.
[0044] After the first cut 56 is complete, central template element
48 is displaced upwards (anteriorly) a predetermined distance,
along the axis indicated by the upward arrow along axis 55. This
allows the interface between stromal bed portion 46 and flap
portion 44 to move upwards, as indicated by the broken line which
designates the displaced path of first cut 56. The interface is
displaced only in a predefined central area, due to the fixed
annular template element 42. The amount of upward extension is
responsive to the amount of upward displacement of moveable
template element 48. Because the cornea is applanated, the natural
direction of motion of the corneal tissue is upward when the
constraint of template element 48 is removed. Thus, when a second
cut 54 is performed in the same plane as the first cut 56, a slab
of stromal tissue 58 extending above the cutting plane in the
central region, is cleaved away.
[0045] Due to the speed of the scanning microjet beam during the
second cut, slab 58 is ejected from cornea 50 without requiring any
further steps. This surprising aspect of the present invention has
been determined experimentally, as described above. Thus, as the
second cut is performed, the slab of tissue 58 between the paths of
first cut 56 and second cut 54 is separated from stromal bed 46 and
is blown away by the scanning waterjet. In this manner, a shaped
interior section of tissue is removed from the stroma. This results
in controlled flattening of the cornea when the cornea including
the flap and the stromal bed resumes its natural disposition after
the template is removed. If the excised tissue 58 is designed to be
elliptical, correction for astigmatism is also possible.
[0046] In another embodiment of the invention (shown in FIGS.
9-12), a more precise shaping of the tissue to be excised may be
achieved by using a template consisting of multiple piston
elements, providing a curved template shape made up of a number of
elements. This may provide a smoother transition from the center of
cornea 50 to the edges and, thus, more precise and controlled
myopia correction.
[0047] By displacing moveable template 48 downwards (posteriorly)
along axis 55, after the first cut 56, a pre-shaped tissue may be
cleaved from the underside of flap portion 44 rather than from
stromal bed portion 46. In other embodiments of the present
invention, a series of two non-planar cuts can be performed (e.g.,
first cut 56 and second cut 59 shown in a dotted line in FIG. 3,
although the corresponding downward movement of the piston is shown
in FIG. 7), displacing the movable template upwards and then
downwards below its original position, or vice versa, to remove
tissue both from stromal bed portion 46 and the underside of flap
portion 44. The second cut 54 of the earlier embodiments is not
necessary in this embodiment. This allows creation of thick inner
cavities in cornea 50, for example, 200 .mu.m or more, without
excessive thinning of the stromal bed. Since the thickness and
shape of a microjet cut in accordance with the present invention is
adjustable, e.g., vernier adjustable, to a very high accuracy,
e.g., 1 .mu.m, the resultant cut can be controlled with great
accuracy, for example, 4 .mu.m or better. This enables removal of a
thick lamellar layer with a reproducibility of better than a 1/3
diopter, even with the simple two-template embodiment shown in FIG.
3. This estimation takes into consideration possible inaccuracies
in the scanning plane. By improving beam accuracy and template
resolution, refraction changes in accordance with the present
invention may be reproduced at even higher accuracies.
[0048] FIG. 4 schematically illustrates a multiple vacuum template
arrangement 60 adapted for removing a circular annulus of tissue
78, e.g., to correct hyperopia of a cornea 70. In the exemplary
arrangement of FIG. 4, three templates elements are used, namely a
fixed annular template 62, a middle moveable annular template 68
and a central fixed circular template 65 coplanar with the fixed
annular template 62. The central fixed template 65 is connected to
the template 62 in a cap arrangement at the top of the template 94.
In addition, moveable template 68 contacts the anterior surface 72
of the cornea 70. This provides an interface of template
arrangement 60 with the cornea surface 72 in a shape which
resembles the annulus 78 to be removed. If annulus 78 is designed
to be elliptical, correction for astigmatism is also possible. By
moving template 68 upward along the axis indicated by arrows 75, a
desired displacement is provided between a first cut 76 and a
second cut 74 in cornea 70. As in the preceding embodiments, upper
surface 72 of cornea 70 can be held by vacuum template arrangement
60 during the entire cutting process, i.e., the vacuum should not
be released between the first and second cuts, but may be released
if needed. The result of cuts 76 and 74 is an upper flap 64 and a
stromal bed portion 66. The space between elements 62, 68 and 65
can be used to provide additional vacuum for holding top surface 72
of cornea 70. In some embodiments of the invention, the function of
moveable template 68 is preformed by providing air pressure or
partial vacuum in a predetermined area to control the extension of
the tissue between cuts. Other aspects of hyperopia and or
astigmatism correction in accordance with the present invention are
generally analogous to those described above with reference to
myopia correction and FIG. 3.
[0049] FIG. 5 illustrates an arrangement for removal of corneal
tissue 91 using a liquid microjet beam cut line 92 and a flat
vacuum template 94 according to another embodiment of the
invention. The template 94 includes protective boundaries 96 also
called a "vacuum guard" (which can also be referred to as a "vacuum
trephine" or a traditional trephine cornea cutting tool with the
edge contacting the cornea rounded so that it functions as a
contact and support structure rather than a cutting structure) and
a stationary template 93. The vacuum guard can be in the form of a
ring 96 (which creates boundaries 96 in the side view illustration
of FIG. 5). The stationary template 93 is oriented inside of the
vacuum guard 96 but does not contact the vacuum guard 96. Rather,
in this embodiment, there is a gap 95 between the vacuum guard 96
and the stationary template 93. In addition, a vacuum is created
above the template 90 such that the gap 95 provides application of
the vacuum to the cornea 91 in order to form a strong holding force
so that the cornea anterior surface 116 conforms closely to the
template lower surface 97. In alternative embodiments of the
invention, gap 95 need not be provided between the vacuum guard 96
and the stationary template 93. Rather, the stationary template 93
can be porous so that a sufficient vacuum is supplied by the
template 93 to conform the cornea anterior surface 116 to the
template lower surface 97. Accordingly, the invention is not
dependent on the means of applying a vacuum to the cornea anterior
surface 116.
[0050] Exemplary dimensions for the template 94 components are as
follows: the diameter of the stationary template 116 is 9 mm and
the cam 114 diameter is 1 mm. In addition, the inner diameter of
the vacuum guard can be 9.2 mm, including a 0.1 mm gap around the
template to enable it to fit but to allow a vacuum to be created.
In further alternative embodiments, the stationary template can fit
snugly inside the vacuum guard. However, the stationary template
has grooves on its perimeter to provide vacuum channels.
[0051] The refractive correction procedure shown in FIGS. 5-7 is as
follows: the first microjet cut 92 produces a hinged flap 98. The
cornea 91 is applanated by the flat vacuum template 94 surrounded
by the vacuum guard 96. The microjet 101 can have a beam scanning
speed at a high value of 10-20 mm/second so that erosion associated
with a cut is minimized and the cut is clean. Therefore, each cut
can be completed in around 1 second such that the entire procedure
can be completed in several seconds. Typically, a beam diameter of
33 .mu.m at a stagnation pressure of 25 Kpsi is used. The cut plane
92 is parallel to the plane of the template 94 at an accurately set
distance in the range 150-250 .mu.m. The microjet beam begins the
cut 92 and block 100 ends the cut 92 abruptly so that the hinge 98
remains. Consider that the cut 92 is completed and the template 94,
scleral chuck (shown as 11 in FIG. 1; the scleral chuck allows the
cornea to be immobilized as a base for holding the template 20),
and the hinge 98 remain in place, the microjet beam 92 is turned
off, and the nozzle assembly (not shown) is scanned back to its
initial starting position. The scan 92 is repeated. In a
mechanically stable, precise apparatus, the beam scans exactly in
the plane of the first cut 92. It has been confirmed experimentally
that under such circumstances no additional cutting occurs.
[0052] FIGS. 6 and 7 illustrate an arrangement for removal of
corneal tissue 91 using a vacuum template 94 along with a piston or
cam 114. The cam 114 is oriented in the interior of the stationary
vacuum template 93, e.g., at the center of the stationary template
93. In this embodiment, the stationary vacuum template 93 assumes a
ring shape with the cam 114 as its center. Initially, the surface
of independently controlled cam 114 is coplanar with the stationary
planar template surface 116 of the vacuum template 94. This is not
essential for this technique to work although it is preferable. The
cross-sectional shape of the cam 114 can be circular (not shown in
this illustration). In alternative embodiments according to the
invention, several different shapes may be used. The gap between
the stationary template 93 and the cam 114 is small, just large
enough to support a vacuum. The plane of the first microjet cut 118
is set at a distance S below the plane of the template 94.
Therefore, S is the thickness of an upper flap 120 of the cornea 91
resulting from the first cut 118.
[0053] After the first cut 118, the cam 114 is translated downward
a distance .DELTA. into the anterior corneal surface (as shown in
FIG. 7). Under the cam 114, and only under the cam 114, the plane
of the initial cut 118 is pressed downward a distance,
.DELTA.-.delta., in which .delta./.DELTA.<<1 and .delta. is
the proximity correction as described below. The second cut 122 is
therefore displaced .DELTA.-.delta. from the initial cut 118. The
magnitude of 6 increases with increase in S and decreases with
increase in D, the diameter of the cam 114. The choice of S=200
.mu.m and D=1 mm probably makes .delta.<<.DELTA.. However, in
any cam array configuration, the change in .DELTA. from one cam to
an adjacent cam is usually small. This implies a larger effective
value of D, hence, .delta. is reduced in any case. Nevertheless, if
it is not negligible, its effect can be readily accommodated in any
actual array-shaping algorithm.
[0054] As depicted in FIG. 7, there is a slight rounding 124 of the
cut boundary 122 of the depressed interior volume under the cam
114. This rounding increases with S. When the second cut 122 is
made, the microjet (not shown in FIG. 7) cuts in the same plane as
the first cut 118, except for the section under the cam 114; the
second cut 122 defines a new parallel cut interface surface for the
depressed tissue volume. Following the cut 122, the tissue in this
volume is no longer connected to the stromal bed or the underside
of the upper flap 120. It is free and if it is thin, lamellar
fragments are ejected by the microjet. If the free section is thick
enough, it should have greater strength so that it comes out as a
single piece. This is observed experimentally for 9 mm sections of
100 .mu.m thickness which appear to be complete discs.
[0055] Shaping of the cornea as a result of the procedure
illustrated in FIGS. 6 and 7 will now be described. The template 94
is removed and the cornea 92 becomes unconstrained. However, it has
had tissue excised from the interface at the underside of the flap
120. The volume and shape of the excised tissue correspond almost
precisely to the volume defined by the extension of cam 114. The
smaller the value of S, the more closely the excised tissue volume
approximates the extended volume. With the flap 120 in place, this
excision will be reflected mostly as a relative depression in the
anterior surface of the cornea 126 rather than the posterior cornea
shape. In removal over a large area, the excised volume induces a
change in the anterior surface shape because the underlying stromal
bed is so much thicker than the flap and no tissue is removed from
the bed. Hence, the posterior surface of the cornea and the stromal
bed surface maintain essentially their original shape. The flap 120
deforms and fills in the excised volume. The same issue of
deformation arises in LASIK. It is the anterior surface that
changes because the photoablation layer is much closer to the
anterior surface. The essential difference is that in LASIK, the
tissue is removed from the stromal bed rather than the flap 120. It
has been reported that for large corrections, the thinning and
weakening of the stromal bed can lead to kerato-ectasia. This is
avoided in the microjet technique by virtue of the fact that the
tissue is removed from the underside of the flap. However, for
large corrections, it may be desirable to remove tissue from both
the underside of the flap and from the stromal bed.
[0056] In addition, the microjet cut 118 is always at the local
lamellar interface, since the mechanism of the microjet cut is to
strip away sections of lamellae. (The laser photoablation has the
same characteristic.) Hence, the plane of the microjet cut is
indeterminate to the thickness of the lamellae. The maximum
lamellar thickness is about 2 .mu.m, hence the associated thickness
ambiguity, .+-.2 .mu.m, is trivial.
[0057] In an alternative embodiment of the present invention, the
cam 114 can be recessed by raising it away from the anterior
surface rather than lowering it to achieve extension.
[0058] The result can be virtually identical to the FIGS. 6 and 7
procedure except that the excised tissue is removed from the
stromal bed 121 interface rather than from the underside of the
flap 120. All other considerations are the same. Since for
accuracy, it is desirable to minimize S, it is probably more
appropriate to remove tissue from the flap 120. The risk of ectasia
for large corrections is reduced. Hence, extending the cam 114 into
the anterior cornea surface is more desirable.
[0059] FIG. 8 is a schematic side view illustration of an
arrangement for removal of corneal tissue using a liquid microjet
beam and a vacuum template along with multiple pistons or cams (for
example, two cams 114 and 130 are shown). The cams 114 and 130 can
translate downward or upward relative to the anterior cornea
surface. Each of the cams 114 and 130 can operate in the same
manner as cam 114 to excise tissue from portions of the cornea 91.
For example, cam 130 can produce excisable tissue by a first cut
118 followed by a second cut 122 of the liquid microjet. In
addition, as in the FIG. 7 embodiment, cams 114 and 130 can recess
(not shown) in order to remove tissue from the stromal bed 121
rather than from the underside of the flap 120. In addition, in
alternative embodiments, the distances A of movement of cams 114
and 130 can be different. The operation of each cam 114 and 130
relative to the other does not limit the scope of the invention.
Rather, their operation depends on the refractive correction
objective of the procedure.
[0060] To complete the discussion of the basic technique, the
origin of the proximity effect is described next. The cornea 91,
while nominally incompressible because it is made up of mostly
water-based fluid, is actually locally compressible. When the
cornea 91 is applanated in a localized area, corneal fluid may move
laterally into the immediate region surrounding the applanation.
Thus, a localized edema (observed by the ophthalmologist when
pushing on the cornea) is created in the surrounding region. This
edema surrounding the local applanation region is the origin of the
observed light backscatter or haze seen when pressing on the
cornea. The result of the fluid motion away from the applanation is
a local thinning of the cornea 91 and a thickening in the
surrounding annulus. Thus, a motion downward of the anterior
surface into the cornea 91 is not reflected fully in an equivalent
downward motion of the posterior surface. This is also the case at
any intermediate plane. The closer to the anterior surface, the
smaller is the effect. The larger the diameter of the local
applanation region, the smaller this effect. The deviation from
one-to-one extension under a local applanation is called the
proximity effect. In an array, with respect to a given cam 114, an
adjacent cam 130 (shown in FIG. 8) or additional cams (as shown in
FIGS. 9-11) may applanate a similar amount, effectively increasing
the lateral extent of the applanation region and reducing the
proximity effect. In any case, this correction may be calculated
once the proximity effect is characterized. Hence, shaping
corrections can be applied if necessary. The mechanical aspects of
the cornea are not well characterized; accordingly, this analysis
is done empirically.
[0061] FIG. 9 is a schematic bottom view illustration of an
arrangement 140 for removal of corneal tissue using a microjet beam
(not shown) and a vacuum template 96 (shown and further described
in FIG. 9A) including multiple pistons or cams 150 according to a
fifth embodiment of the present invention. The vacuum template 151
is supported on a base plate 142 and the vacuum is provided by a
vacuum cable 144. The microjet 146 is positioned by use of a scan
guide 147 and can be powered by a linear motor 148. In addition,
the movement of the cams 150 in the vacuum template can be
controlled by a control cable 149. The FIG. 9 illustration is
exemplary of a device for implementing the corneal tissue removal
using one or more cams 150 which shape the anterior surface of the
cornea, a microjet 146 for cutting the corneal tissue and the
devices used to control the cams 150 and the microjet 146. The
devices 142, 144, 147, 148 and 149 may be conventional.
[0062] FIG. 9A is a schematic cross-sectional illustration of the
multiple pistons or cams for a vacuum template used in the
apparatus of FIG. 9. A multiplicity of pistons 150, hexagonal in
cross section, are placed in a honeycomb array to form the template
151 within the vacuum guard boundary 96. Small gaps 152 between
cams 150 allow access to the vacuum (not shown) above the template.
The cams 150 may be piezoelectric pistons which are under computer
control to provide any variety of shapes to custom shape the
anterior surface of the cornea. The exemplary structure of FIG. 9A
includes 11 rows of cams 150, those rows including from left to
right the following number of cams 150: 4, 7, 8, 8, 9, 10, 9, 10,
9, 8, 7 and 4. There are many ways to arrange the cam 150. The
length of the piezoelectric pistons 150 will vary depending upon
the material from which they are made, but can be several
centimeters long to achieve an extension of 100 .mu.m.
Magnetostriction may also work. The piezoelectric construction may
be a single, shaped rod of piezoelectric ceramic, or a stack of
alternating, reverse-polled, ceramic discs or other piezoelectric
materials etc. The latter construction allows low voltage operation
from integrated circuit drivers. One end of each piston is made
coplanar with the others. The other end of the piston is free and,
in the absence of applied voltage, is approximately coplanar with
the others. The invention does not require precise co-planarity.
Only the extension of the piston with voltage needs to be
accurately controlled. Exactly where the end of the piston starts
from is not so important. It might be desirable to program the
amplifiers of the driver array to ensure that extension versus
input voltage for each piston is the same. To ensure the utmost
accuracy, temperature can be determined and inputted. A simple
truth table built into the electronics will serve to produce
submicron accuracy for the template shape. Such truth tables are
known to those of ordinary skill in the art and will therefore not
be further described herein.
[0063] FIG. 10 is a schematic side view illustration of an
arrangement for removal of corneal tissue using a microjet beam and
a vacuum template 94 including the multiple cams 150 shown in FIG.
9 in position for a second cut of the liquid microjet beam for
correction of myopia. The template includes the vacuum guard 96 and
the stationary template 93 separated by gap 95. Five cams 150 are
shown in this embodiment. The cams 150 are extendable downward into
the anterior of the cornea 91. The shaded area indicates a volume
of excised tissue 154 such that a first cut 156 has already
occurred and the cams 150 are in position for a second cut 158,
which is shown in FIG. 10. The excised tissue 154 is a crescent
shaped (plano-convex) volume internal to the stroma from the
posterior side of the flap created by the first cut 156. When the
cornea 91 is allowed to resume its normal shape after the template
is removed and the flap is smoothed and flattened (juxtaposed)
against the stromal bed, the anterior corneal surface is flattened
relative to its original shape. The radius of curvature of the new
surface is greater than that of the original surface leading to a
reduction in refractive power. This corresponds to a correction for
myopia.
[0064] FIG. 11 is a schematic side view illustration of an
arrangement for removal of corneal tissue using a microjet beam and
a vacuum template 94 including the multiple cams 150 shown in FIG.
9 in position for a second cut of the liquid microjet beam for
correction of hyperopia. FIG. 11 includes the same components as
FIG. 10, but the position of the cams 150 in FIG. 11 is adjusted
such that custom shaping for correction of hyperopia is achieved.
As a result, the first and second cuts 166 and 168, respectively,
cause the resulting excised tissue volume 164 to differ in shape
from the volume 154 in FIG. 10. The tissue 164 is a plano-concave
volume taken from the posterior side of the flap. This leads to a
steepening of the anterior surface of the cornea 91 and an increase
in the radius of curvature. It increases refractive power and
corrects for hyperopia. Otherwise, FIG. 11 operates in the same
manner as FIG. 10. In general, FIGS. 10 and 11 illustrate how an
array of cams 150 can be used to create custom changes in the shape
of the cornea 91 for different purposes.
[0065] FIG. 12 is a schematic side view illustration of the FIG. 9
embodiment where there are five pistons 150 including a disc or
membrane 170 at the contact point with the cornea 91 and the
corneal tissue removal device is applied to the cornea for a second
cut. More particularly, the free end of the cam 150 array can be
covered with a thin, precision thickness, disc 170 made from a
flexible membrane that has micro-channels (not shown). The membrane
disc 170, with a serrated, perimetric boundary, fits inside the
vacuum guard directly against the cam array 150. The microchannels
in the membrane serve to create the vacuum interface surface. The
membrane serves as an insulating barrier between the cam array and
the cornea. The flexible nature of the membrane smooths the
transition between elements. The surgical nature of the function
makes it desirable that the membrane be a disposable element.
Examples of materials for the disc include sintered thin soft
metal, porous rubber, woven fabric etc.
[0066] Common vision errors can be corrected under computer control
of the cams 150. For example, a cylinder shape for the excised
volume achieved with a predetermined configuration of the cams 150
and the two cut approach could be used to correct astigmatism.
Accordingly, the present invention is not limited to the custom
shapes or types of correction shown herein. Rather, any shape
consistent with the number and density of cams 150 is attainable
with multiple pistons such that the present invention applies to
currently known and hereinafter discovered correction procedures
which involve altering the shape of the cornea. For example,
changing the shape without a change in central curvature would also
allow a change in the sphericity of the anterior surface.
Corrections for common lens aberrations such as coma and spherical
aberration become possible. This allows improvement of
best-corrected visual acuity if the nature of the necessary change
can be specified.
[0067] The procedures described above show both an elegant and
practical approach to achieve refractive change. In accordance with
the present invention, unlike prior art multiple cut ALK and HRK
techniques, there is no need to move the flap out of the way for
the second cut. Additionally, the refractive change in accordance
with the present invention is vernier adjustable. It has been shown
that this procedure is highly accurate and highly reproducible and
is automatically and accurately centered. The diameter of the slab
of inner corneal tissue that is removed need not be very large
because of the accurate centering enabled by the invention. The
boundary transition may be smooth, limiting scattering of light and
the glare effects it produces.
[0068] It will be appreciated by persons skilled in the art that
the present invention is not limited to the specific embodiments
described herein with reference to the accompanying drawing.
Rather, the scope of the present invention is limited only by the
following claims:
* * * * *