Note: Descriptions are shown in the official language in which they were submitted.
-1-
DEVICE FOR LASIK
The invention relates to an apparatus and a method for laser in-situ
keratomileusis (LASIK).
In refractive ophthalmological surgery the refractive properties and imaging
properties of the eye are changed by interventions in respect of the eye of a
patient for the purpose of correcting or alleviating sight defects. Known, in
particular, is the LASIK process, wherein the cornea of the eye is reshaped.
In
the conventional LASIK process, in a first step a flat corneal incision is
made with
a mechanical microkeratome, in order in this way to produce a so-called flap
which remains firmly connected to the cornea on one side, so that it can be
folded upwards in order to expose underlying corneal tissue (stroma). In the
exposed stroma the ablation - that is to say, the removal of tissue by means
of,
ordinarily, excimer-laser radiation - is then carried out, whereupon the flap
is
then folded back and heals up. In this process the epithelium remains largely
uninjured and the healing process takes place relatively quickly and in pain-
free
manner. In a conventional mechanical microkeratome a sharp blade oscillates.
For the purpose of cutting the flap, the mechanical microkeratome has recently
been increasingly replaced by laser radiation. The laser radiation is focused
below the surface of the cornea and guided on a trajectory, the power
densities
being so high that a continuous incision arises by virtue of photodisruptive
effects. In order to obtain the high power densities, short laser pulses
within
the femtosecond range are employed, for which reason this process is also
designated as fs LASIK.
In the case of the aforementioned fs LASIK, in the course of cutting the flap
a
dense succession of aligned (micro)disruptions occurs by reason of the highly
focused laser radiation pulses of high power density. Overall a two-
dimensional
continuous incision arises in the stroma of the cornea. Via the remaining
hinge
-2-
the flap is then folded aside, and the actual LASIK then takes place - that is
to
say, the ablation (removal) of corneal tissue in the open stroma in accordance
with a defined treatment program for resection of a defined ablation volume
for
the purpose of reshaping the cornea.
The production of the flap in the case of fs LASIK has a number of advantages
in
comparison with the use of conventional mechanical microkeratomes and is
therefore increasingly gaining acceptance. In the case of fs LASIK (sometimes
also designated as fs microkeratome), the depths of cut can be adhered to
exactly in the desired manner with very small fluctuations, and special
marginal
incisions with defined angles can also be formed which, in particular, bring
advantages with regard to the biomechanical stability of the flap that has
been
folded back.
However, sometimes in the course of the fs-LASIK incision side-effects
disturbing the patient may also occur in the form of the so-called rainbow-
glare
effect. This effect, which is felt to be annoying by some patients, consists
in the
perception of colour dispersions when observing certain structures and sharp
edges. The cause of this annoying rainbow-glare effect is the generation of a
type of grating structure in the incision surface for the production of the
flap that
has arisen by virtue of the photodisruption. The individual laser spots are
typically placed so regularly that regular two-dimensional gratings with
grating
constants within the pm range may arise at least in certain regions of the
incision, which may then continue to exist in the healed eye also after
conclusion
of the LASIK procedure and may then cause the known grating effect - that is
to
say, a colour-resolved dispersion in the sense described above. Frequently the
refractive ablation - that is to say, the targeted removal of corneal tissue
for the
purpose of reshaping the cornea with desired imaging properties - does also
bring about a removal of the aforementioned undesirable grating structures,
but
this is successful, as a rule, only in those regions of the cornea in which a
relatively 'large' amount of tissue is resected, whereas in those corneal
regions
-3-
in which the refractive ablation (that is to say, the removal of the desired
ablation volume for the purpose of correcting the imaging properties) does not
resect so much tissue the undesirable grating structures in the cornea
frequently
remain, for example in the central corneal region in the case of hyperopia
correction.
In EP-A 1 977 725 this problem is countered by the regularity of the spot
positions of the laser radiation in the course of producing the incision being
cancelled to such an extent that the undesirable regular grating structure
does
not arise. A stochastic 'wobbling' of the mirrors controlling the radiation is
implemented therein, in order to avoid the undesirable regular grating
structures
in the course of producing the flap incision. However, despite these
stochastic
fluctuations of the spot positions, it has to be guaranteed that the incision
passes through continuously and a sufficiently smooth surface in the exposed
stroma is guaranteed. In the aforementioned known process this requires a
very elaborate optimisation and control.
The object underlying the invention is to avoid the occurrence of the so-
called
rainbow-glare effect with means that are as simple as possible in the case of
fs
LASIK.
An apparatus according to the invention is equipped with
- a first laser radiation source for generating first laser radiation pulses
having a power density for bringing about disruption in corneal tissue,
- first means for guiding and shaping the first laser radiation pulses into
the
corneal tissue,
- a second laser radiation source for generating second laser radiation
pulses having a power density for bringing about ablation of corneal tissue,
- second means for guiding and shaping the second laser radiation pulses
in relation to the cornea,
-4-
a controller with a first treatment program for controlling the first means
and the first laser radiation pulses for the purpose of producing an incision
in the
cornea, and with
- a second treatment program for controlling the second means and the
second laser radiation pulses for the purpose of reshaping the cornea and
changing its imaging properties, wherein
- the first treatment program generates regular corneal surface structures
which cause a rainbow-glare effect in connection with the imaging properties
of
the cornea,
to characterized by
- a third treatment program which controls the second means and the
second laser radiation pulses for the purpose of removing the aforementioned
regular structures.
In the case of the apparatus described above, the aforementioned "regular
corneal surface structures", which bring about a rainbow-glare effect, are to
be
understood as being those structures which in the course of making the
incision
for the flap generate in undesirable manner a grating structure in the sense
described above and therefore in accordance with the invention are separately
removed or at least so greatly reduced with the treatment program which
subsequently brings about in the stroma the ablative shaping by resection of
the
so-called ablation volume that the aforementioned rainbow-glare effect
disappears. In this sense the ablation volume is to be understood as being
that
volume of the cornea which has been calculated in advance for the refractive
surgery in order to bring about the desired change in the imaging of the eye
overall. But, in accordance with the invention, going beyond that, a smoothing
is provided of the stromal surface that is exposed after the flap has been
folded
back in those regions in which undesirable grating structures have arisen in
the
course of the flap incision, this smoothing having nothing significant to do
with
the change in the refractive properties (imaging properties) of the eye.
-5-
The invention may also be described in such a way that in addition to the PRK -
that is to say, the photorefractive keratectomy (in the course of which as a
result
of reshaping of the cornea the imaging properties thereof are changed) a PTK
is
provided - that is to say, a phototherapeutic keratectomy, in the course of
which
relatively superficially situated defects, scars and other surface structures
are
removed. The latter procedure serves in the invention for removal of the
aforementioned grating structures on the surface of the exposed stroma.
This removal is effected in the course of the photoablation in a separate
step,
together with the refractive reshaping of the cornea for the purpose of
changing
the imaging properties thereof.
The patent claim as reproduced above differentiates first, second and third
treatment programs having the respectively specified functions. This
differentiation is to be understood as being functional - i.e. the three
stated
functions of the three treatment programs may be combined in a single
computer program, or the second and third treatment programs, which bring
about the ablative effects, may be combined with one another in a single
program.
In the case of a LASIK myopia treatment the cornea is normally flattened -
i.e.
the radius of curvature of the cornea is increased. This means that the
ablation
volume is situated mainly in the middle region of the cornea - that is to say,
around the optical axis - whereas in the outer regions of the cornea no tissue
or
only little tissue is resected. But, as a rule, the flap is cut over a very
wide
region of the cornea, so that in the case of treatment of myopia the grating
structures generated by the flap incision in the marginal regions of the
exposed
stroma cannot, under certain circumstances, be totally removed by the
subsequent resection of ablation volume from the stroma, whereby according to
the invention in these outer regions of the cornea close to the marginal flap
incision the risk of undesirable grating structures remaining in the corneal
issue
-6-
is particularly high and therefore according to the invention, in addition to
the
resection of the refractive ablation volume, a smoothing ablation is also
effected
in the marginal regions of the cornea.
On the other hand, in the case of LASIK treatment of hyperopia the ablation
volume is normally calculated in such a way that the radius of curvature of
the
cornea is reduced - that is to say, in the marginal regions of the cornea,
close to
the marginal flap incision, normally more corneal tissue is ablated than in
central, middle regions of the cornea. Therefore there is the risk that
without
the invention in the course of the hyperopia treatment undesirable grating
structures that have arisen in middle regions of the cornea in the course of
the
flap incision remain and in this way bring about a strong rainbow-glare
effect.
Therefore in the case of hyperopia treatment the invention provides that, over
and beyond the marginal ablation volume that is stipulated for the refractive
correction of the cornea, in addition a smoothing ablation is carried out for
the
purpose of removing the grating structures also in middle regions of the
cornea.
Exemplary embodiments of the invention will be elucidated in more detail in
the
following on the basis of the drawing. Shown are:
Fig. 1 schematically, an apparatus for implementing an fs LASIK;
Fig. 2 schematically, a section through the cornea of an eye for the purpose
of elucidating a myopia treatment;
Fig. 3 a schematic section through the cornea of an eye for the purpose of
elucidating a hyperopia treatment; and
Fig. 4 a section corresponding to Fig. 3, including the target surface striven
for in accordance with the invention.
-7-
Fig. 1 shows an apparatus for fs LASIK, wherein ordinarily two different
sources
of laser radiation are employed, namely a first laser radiation source for the
purpose of generating femtosecond pulses for the implementation of the flap
incision by photodisruption and a second laser radiation source for the
purpose
s of generating laser radiation pulses of another type having a lower power
density for the purpose of implementing the ablation of corneal tissue.
Ordinarily in the state of the art two different laser-radiation systems with
separate optical systems for beam shaping and beam guidance in relation to the
eye are provided for this purpose, which are alternatively (independently of
one
another) aligned in relation to the eye to be treated. In Fig. 1, however, the
two
systems have been represented in virtually combined manner for the sake of
simplicity.
A first laser radiation source 12 serves for generating femtosecond pulses 14
which are known as such in this technology and which have such a high power
density that after focusing in the interior of the cornea they bring about a
disruptive effect there. The means for shaping and guiding these first laser
radiation pulses 14 are indicated in Fig. 1 synoptically by reference symbol K
and
are known as such in the state of the art. Via a mirror 44 which is
transmitting
in respect of the first laser radiation pulses 14 this laser radiation is
guided in the
direction towards the eye 10. The eye 10 is fixed by means of a suction ring
16,
and an applanation lens 20 is lowered coaxially relative to the axis 18 of the
suction ring 16, downwards in the Figure, so that an interface unit 22 engages
in
a conical socket on the suction ring 16. By means of focusing optics 24, the
first
laser radiation pulses 14' for generating the flap incision in a manner known
as
such are focused into a previously calculated surface below the surface of the
cornea of the eye 10. The focusing optics 24 are guided in a mount 26. The
guidance is effected by means of a location sensor 28, the focusing optics 24
being suspended in freely hanging manner via a counterweight 30 and a rocker,
in order to enable a coupling, which places virtually no burden on the eye, of
the
interface unit 22 onto the eye 10. The suction ring 16 is fixed by means of
pipe
-8-
connections 34, 36, known as such, and vacuum pumps 38. The focusing optics
24 described above serve mainly for the focusing of the second laser radiation
pulses, described below, for the ablation. For the first laser radiation
pulses 14,
optical shaping means and guidance means for the radiation are provided which
are known as such in the state of the art and are indicated in Fig. 1 by
function
block K. as a result of which the control of the foci of the first laser
radiation
pulses in space and time is then also effected in known manner.
A second laser radiation source 46 serves for generating second laser
radiation
pulses 48 for the ablation. These second laser radiation pulses 48 are
directed
into the focusing optics 24 via mirrors (including scanner mirrors) 40, 42, 44
known as such. Details of this arrangement are elucidated in more detail in
international patent application PCT/EP2008/006962, which is included here in
full by reference.
A computer controller 50 controls all the controllable components of the
system,
the control connections being indicated in Fig. 1 by dashed lines.
In a memory 54 there are stored, in particular, a first treatment program 56a,
a
second treatment program 56b and a third treatment program 56c, which the
controller 50 can access alternatively. These three treatment programs will be
described in more detail below.
With the first treatment program 56a the computer controller 50 controls the
laser 12 and the first laser radiation pulses 14 generated thereby for the
purpose
of implementing the flap incision that has been described by means of
photodisruption. This is known as such in the state of the art, as is the
phenomenon that in the process undesirable grating structures in the above
sense are generated on the surface of the stroma that is exposed after the
flap
has been folded back.
-9-
For the subsequent ablation of corneal tissue for the purpose of implementing
the PRK, the computer controller 50 accesses the second treatment program
56b, so that an ablation volume that has been calculated in advance in known
manner is ablated out of the stroma of the cornea, in order to modify the
imaging properties of the cornea in desired manner.
The undesirable grating structures that are possibly generated when executing
the first treatment program 56a are then removed in a third step in accordance
with a third treatment program 56c (PTK). This is described in more detail on
the basis of Figs. 2 to 4.
Figs. 2 to 4 show the cornea 60 of an eye 10 schematically in section.
Represented are only the parts of the eye that are of greater interest here
(the
retina etc. have accordingly been omitted).
Shown, in addition to the cornea 60, are the crystalline lens 62 and the iris
64.
With the first treatment program 56a, by means of the first laser radiation
source 12 the flap incision of the fs LASIK is implemented in a manner known
as
such. In the process the undesirable grating structures, elucidated above,
arise,
which are indicated schematically in the Figures by reference symbol 68 - that
is
to say, depressions in the respectively exposed surface of the corneal tissue
having, by reason of their regular structure, the undesirable effects that
have
been described. This grating-like structure typically has a grating constant
within the pm range, with the consequences elucidated above with regard to
rainbow glare. Reference symbol 68 accordingly indicates a so-called
micrograting.
In the Figures the initial surface of the stroma which is exposed after the
flap 66
has been folded back is denoted by 70. According to Fig. 2, the micrograting
that is formed from the depressions 68 is accordingly distributed over the
entire
-10-
exposed surface of the stroma of the cornea. In the exemplary embodiment
according to Fig. 2, myopia is to be treated - that is to say, the radius of
curvature of the cornea after the treatment is to be increased, so the cornea
is
to be flattened. This is represented in Fig. 2 by the ablation volume 78
(closely
hatched) - that is to say, that volume of the cornea which is to be resected
by
ablation by means of the second laser radiation pulses 48. The target surface
that is striven for in this case is provided with reference symbol 72.
Consequently the ablation volume 78 in Fig. 2 is the closely hatched region
between the initial surface 70 and the target surface 72. In the case of
myopia
treatment, in the middle region of the cornea the depressions 68 forming the
micrograting therefore disappear almost by themselves, since sufficient
corneal
tissue is resected in order to cause the undesirable grating structure largely
to
disappear at the end. Therefore in the case of a typical myopia treatment it
is
not absolutely necessary to provide in this middle region of the cornea
separate
measures for smoothing the surface and for removing the undesirable grating
structure (although, in accordance with the invention, this is not excluded).
However, as Fig. 2 shows, in marginal regions 80 of the exposed stromal bed
the undesirable microstructure formed by the depressions 68 is largely
preserved even after implementation of the ablation, so that in marginal
regions
80 of the cornea - that is to say, close to the marginal flap incision 76 -
special
measures for removing the micrograting generated by the depressions 68 are
required. For this purpose the computer controller 50 has recourse to the
third
treatment program 56c, which guides the second laser radiation pulses 48 over
the exposed surface of the stroma in such a way that the surface is smoothed
also in the marginal regions 80. For this purpose, processes - known as such -
of phototherapeutic keratectomy (PTK) can be called upon - see, for example,
A.N. Kollias et al., Journal of Refractive Surgery, Vol. 23, September 2007,
pp 703-708, or Arch. Ophthalmology, Vol. 109, June 1991, pp 860-863 or
P. Vinciguerra, F. Camesasca, Journal of Refractive Surgery, Vol. 20, 2004,
pp 555-563. To this end, recourse may be had to the cited known processes of
PTK.
-11-
In modification of the exemplary embodiments described above, this smoothing
of the stromal surface that is exposed after the flap has been folded back may
also be implemented not by the third treatment program with laser radiation
but
rather by other PTK techniques, for example by a manual smoothing, for
example of the marginal regions 80 in the exemplary embodiment according to
Fig. 2, with application of suitable liquids (see the cited literature) and,
for
example, with a brush. In this variant of the invention the undesirable
micrograting is accordingly 'ground away' mechanically (without laser
radiation).
to After removal of the undesirable grating structure and folding-back of the
flap
66, it is normally ensured that no undesirable grating structure finally
remains in
the refractively amended corneal tissue. Microstructures possibly remaining on
the inside of the flap are not sufficient to form the undesirable
microstructure, or
do not come to be situated exactly above the original grating structures after
the
flap has been folded back, so that the smoothing measures described above with
respect to the stromal surface are sufficient. It has been proved
experimentally
that grating structures possibly remaining in the flap are less critical than
the
grating structures in the stroma, described above. This is explained by the
fact
that in the course of photodisruption the fs pulses in the direction of
propagation
of the radiation have a relatively sharp start. In the case of LIOB (laser-
induced
optical breakdown), therefore, the depressions remaining in the flap are
distinctly less pronounced (deep) than in the stromal bed. The depressions in
the flap are typically less deep than 5 pm, whereas the depressions in the
stromal bed are distinctly deeper and almost attain the Rayleigh length (15 pm
to 20 pm). Alternatively, the undesirable grating structures 68 in the flap 66
may also be removed mechanically in the manner described above.
Fig. 3 shows the treatment of a hyperopia with fs LASIK, parts corresponding
to
one another having been provided with identical reference symbols in all the
Figures. As already elucidated above, in the case of hyperopia treatment the
ablation volume 78' is situated mainly in the marginal regions of the exposed
-12-
stroma - that is to say, close to the marginal flap incision 76, this being
represented in Fig. 3 by the closely hatched regions that mark the ablation
volume 78'. As a result of removal of the ablation volume 78' by means of the
second laser radiation pulses 48, the undesirable grating structures 68 also
disappear in these regions, whereas in the middle region these structures are
largely preserved in the initial surface 70, as Fig. 3 represents. Therefore
in the
case of the hyperopia treatment according to Fig. 4 also in the middle region
82
of the stroma which is exposed after the flap 66 has been folded back a
smoothing is carried out to the effect that the microgratings 68 disappear and
a
smooth target surface 72 is obtained. The PTK techniques elucidated in more
detail above - that is to say, either the third treatment program 56c or even
other PTK smoothing techniques according to the literature cited above - serve
for this purpose.
If the third treatment program 56c is employed, then in particular in the
middle
region of the exposed stroma a layer between the initial surface 70 and the
target surface 72 is resected having a thickness of up to 10 pm, this being
capable of being implemented effectively with the excimer laser 46 via the
computer controller 50. A flap 66 is typically 100-160 pm thick.
In this connection the ablation serving for smoothing with the third treatment
program can be taken into account in the second treatment program which
brings about the refractive correction of the cornea - i.e. when calculating
the
ablation volume and accordingly when generating the second treatment program
for the photoablation it can be taken into account from the outset that a
uniform
resection of tissue occurs over the entire corneal surface or over selected
parts
of the corneal surface (in the case of Fig. 4, accordingly the middle region
82).
Analogous remarks apply to the myopia treatment according to Fig. 2, in which
over the entire exposed surface of the stroma or even parts thereof (such as,
in
particular, the marginal regions 80 according to Fig. 2) a smoothing resection
of
-13-
tissue occurs which is taken into account in the second treatment program for
the calculation of the refractive effect.
