U.S. patent application number 12/351784 was filed with the patent office on 2009-07-09 for photodisruptive laser fragmentation of tissue.
Invention is credited to Ferenc Raksi.
Application Number | 20090177189 12/351784 |
Document ID | / |
Family ID | 40845168 |
Filed Date | 2009-07-09 |
United States Patent
Application |
20090177189 |
Kind Code |
A1 |
Raksi; Ferenc |
July 9, 2009 |
Photodisruptive laser fragmentation of tissue
Abstract
A method of photodisruptive laser surgery includes selecting a
target region of a tissue for fragmentation, directing a beam of
laser pulses to the selected target region of the tissue, and
forming cells in the target region of the tissue by directing the
laser beam to generate cell boundaries. The cells can be arranged
in regular or irregular arrays. The cells can be generated in
parallel or successively, with cell sizes and laser parameters
which reduce the time of ophthalmic surgery considerably.
Inventors: |
Raksi; Ferenc; (Mission
Viejo, CA) |
Correspondence
Address: |
FISH & RICHARDSON, PC
P.O. BOX 1022
MINNEAPOLIS
MN
55440-1022
US
|
Family ID: |
40845168 |
Appl. No.: |
12/351784 |
Filed: |
January 9, 2009 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61020115 |
Jan 9, 2008 |
|
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|
Current U.S.
Class: |
606/4 |
Current CPC
Class: |
A61F 2009/0087 20130101;
A61F 9/008 20130101; A61F 9/00825 20130101; A61F 2009/00844
20130101; A61F 2009/00897 20130101; A61F 9/00736 20130101; A61B
2217/005 20130101 |
Class at
Publication: |
606/4 |
International
Class: |
A61F 9/008 20060101
A61F009/008; A61B 18/20 20060101 A61B018/20 |
Claims
1. A method of fragmenting biological tissue with a photodisruptive
laser, comprising the steps of: selecting a target region of the
tissue for fragmentation; directing a beam of laser pulses to the
selected target region of the tissue; and directing the laser beam
to generate cell boundaries in the selected target region of the
tissue to form cells in the selected target region.
2. The method of claim 1 wherein the tissue is a tissue of an
eye.
3. The method of claim 2, wherein the tissue is a crystalline lens
of the eye.
4. The method of claim 1, comprising: inserting an aspiration
needle into the selected target region; and removing fragmented
tissue from the selected target region already scanned by the laser
beam by using the aspiration needle.
5. The method of claim 4, the forming the cells comprising: forming
cells with size sufficiently small to pass through the aspiration
needle.
6. The method of claim 1, the forming the cells comprising: forming
the cells arranged in an array.
7. The method of claim 6, wherein the array is a regular array.
8. The method of claim 7, wherein the regular array is one of a
simple cubic lattice, a face centered lattice, a body centered
lattice, a hexagonal lattice, a bravais lattice, and a stack of two
dimensional lattices.
9. The method of claim 8, wherein the array is essentially a random
array.
10. The method of claim 1, the forming the cells comprising:
fragmenting the target tissue into cells of at least one of spheres
and polyhedra.
11. The method of claim 1, the forming the cells comprising:
scanning the laser beam to form multiple cells in parallel in a
layer.
12. The method of claim 1, the forming the cells comprising:
directing the laser beam to form individual cells successively.
13. The method of claim 1, the forming the cells comprising at
least one of: scanning the laser beam to form a cell array
progressing from a posterior to an anterior direction; and scanning
the laser beam to form a cell array progressing from an anterior to
a posterior direction.
14. The method of claim 1, the directing the laser beam to generate
cell boundaries comprising: generating the cell boundaries by
creating layers of bubbles in the selected target region of the
tissue.
15. The method of claim 14, the creating layers of bubbles
comprising at least one of: creating a layer of bubbles by applying
a laser beam with an essentially constant power; and creating a
layer of bubbles by applying a laser beam with a varying power.
16. The method of claim 1, wherein the directing the beam of laser
pulses comprises applying the laser pulses with a laser parameter
of at least one of: a pulse duration between 0.01 picosecond and 50
picoseconds; a repetition rate between 10 kiloHertz and 100
megaHertz; a pulse energy between 1 microjoule and 25 microjoule;
and a pulse target separation between 0.1 micron and 50
microns.
17. The method of claim 1, wherein the directing the beam of laser
pulses comprises applying the laser pulses with a laser parameter
based on at least one of: a preoperative measurement of structural
properties of the selected target region of the tissue; and an age
dependent algorithm.
18. The method of claim 1, comprising: applying additional laser
pulses to one or more locations outside the selected target region
of the tissue to create an opening for an additional procedure.
19. The method of claim 1, the method comprising: identifying a
surgical goal; and selecting laser parameters and method features
to achieve the identified surgical goal.
20. The method of claim 19, the surgical goal being an optimization
of one or more of: a speed of the method of fragmenting; a total
amount of energy applied to the eye during the fragmenting; and a
total number of generated bubbles.
21. The method of claim 19, the surgical goal being one or more of:
maximization of the speed of the method of fragmenting;
minimization of the total amount of energy applied to the eye
during the fragmenting; and minimization of the total number of
generated bubbles.
22. The method of claim 19, comprising: selecting laser parameters
and method features to achieve a total time of fragmentation of one
of: less than 2 minutes; less than 1 minute; and less than 30
seconds.
23. The method of claim 19, comprising: selecting laser parameters
and method features to achieve a ratio of a cell size to a bubble
size of one of: larger than 10; larger than 100; and larger than
1000.
24. A laser system for fragmenting biological tissue, comprising: a
pulsed laser to produce a laser beam of pulses; and a laser control
module to direct the laser beam to a selected target region of the
tissue; and to direct the laser beam to generate cell boundaries to
form cells in the selected target region of the tissue.
25. The laser system of claim 24, wherein the laser control module
is configured to form cells in a regular array.
26. The laser system of claim 24, the laser control module formed
to generate the laser pulses with laser parameters of at least one
of: a pulse duration between 0.01 and 50 picoseconds; a repetition
rate between 10 kHz and 100 megahertz; a pulse energy between 1
microjoule and 25 microjoule; and a pulse target separation between
0.1 micron and 50 microns.
27. A method of fragmenting a tissue in an eye with a
photodisruptive laser, comprising the steps of: selecting a target
region in the eye for fragmentation; and forming an array of cells
in the target region by directing a beam of laser pulses to
generate cell boundaries in the target region, with a cell size and
laser parameters of the laser beam such that the tissue
fragmentation requires a surgical time of less than two minutes,
whereas a volumetric tissue fragmentation of the same target region
with the same laser parameters would require a surgical time in
excess of two minutes.
28. The method of claim 27, wherein the laser parameters are at
least one of: a pulse duration between 0.01 and 50 picoseconds; a
repetition rate between 10 kHz and 100 megahertz; a pulse energy
between 1 microjoule and 25 microjoule; and a pulse target
separation between 0.1 micron and 50 microns; and the cell size is
between 1 microns and 50 microns.
Description
CROSS REFERENCE TO RELATED APPLICATION
[0001] This application claims benefit from and priority of
provisional application "Photodisruptive laser fragmentation of
tissue", Ser. No. 61/020,115, filed on Jan. 9, 2008, which
incorporated by reference in its entirety as part of the disclosure
of this application.
BACKGROUND
[0002] This application relates to laser surgery techniques and
systems for operating on eyes.
[0003] Laser light can be used to perform surgical operations on
various parts of an eye for vision correction and other medical
treatment. Techniques for performing such procedures with higher
efficiency may provide desired benefits.
SUMMARY
[0004] A method of photodisruptive laser surgery and corresponding
system are provided. In some implementations the method of
fragmenting biological tissue with a photodisruptive laser includes
selecting a target region of the tissue for fragmentation,
directing a beam of laser pulses to the selected target region of
the tissue, and forming cells in the target region of the tissue by
directing the laser beam to generate cell boundaries.
[0005] In some implementations the tissue is a tissue of an
eye.
[0006] In some implementations the tissue is a crystalline lens of
the eye.
[0007] In some implementations the method further includes
inserting an aspiration needle into the target region and removing
fragmented tissue from the target region already scanned by the
laser beam by using the aspiration needle.
[0008] In some implementations the forming the cells includes
forming cells with size sufficiently small to pass through the
aspiration needle.
[0009] In some implementations the forming the cells includes
forming the cells arranged in an array.
[0010] In some implementations the array is a regular array.
[0011] In some implementations the regular array is one of a simple
cubic lattice, a face centered lattice, a body centered lattice, a
hexagonal lattice, a Bravais lattice, and a stack of two
dimensional lattices.
[0012] In some implementations the array is essentially a random
array.
[0013] In some implementations the forming the cells includes
fragmenting the target tissue into cells of spheres or
polyhedra.
[0014] In some implementations the forming the cells includes
scanning the laser beam to form multiple cells in parallel in a
layer.
[0015] In some implementations the forming the cells includes
directing the laser beam to form individual cells successively.
[0016] In some implementations the forming the cells includes
scanning the laser beam to form a cell array progressing from a
posterior to an anterior direction, or scanning the laser beam to
form a cell array progressing from an anterior to a posterior
direction.
[0017] In some implementations the directing the laser beam to
generate cell boundaries includes generating the cell boundaries by
creating layers of bubbles in the target region of the tissue.
[0018] In some implementations the creating layers of bubbles
includes creating a layer of bubbles by applying a laser beam with
an essentially constant power, or with a varying power.
[0019] In some implementations the directing the beam of laser
pulses includes applying the laser pulses with a laser parameter of
at least one of: a pulse duration between 0.01 picosecond and 50
picoseconds, a repetition rate between 10 kiloHertz and 100
megaHertz, a pulse energy between 1 microjoule and 25 microjoule,
and a pulse target separation between 0.1 micron and 50
microns.
[0020] In some implementations the directing the beam of laser
pulses comprises applying the laser pulses with a laser parameter
based on a preoperative measurement of structural properties of the
target region of the tissue, or an age dependent algorithm.
[0021] In some implementations the method also includes applying
additional laser pulses to one or more locations outside the target
region of the tissue to create an opening for an additional
procedure.
[0022] In some implementations the method includes identifying a
surgical goal, and selecting laser parameters and method features
to achieve the identified surgical goal.
[0023] In some implementations the surgical goal is an optimization
of one or more of a speed of the method of fragmenting, a total
amount of energy applied to the eye during the fragmenting, and a
total number of generated bubbles.
[0024] In some implementation the surgical goal is one or more of:
maximization of the speed of the method of fragmenting,
minimization of the total amount of energy applied to the eye
during the fragmenting, and minimization of the total number of
generated bubbles.
[0025] In some implementations the method includes selecting laser
parameters and method features to achieve a total time of
fragmentation of one of less than 2 minutes, less than 1 minute,
and less than 30 seconds.
[0026] In some implementations the method includes selecting laser
parameters and method features to achieve a ratio of a cell size to
a bubble size of one of: larger than 10, larger than 100, and
larger than 1000.
[0027] In some implementations a laser system for fragmenting
biological tissue includes a pulsed laser to produce a laser beam
of pulses, and a laser control module to direct the laser beam to a
selected target region of the tissue and to direct the laser beam
to generate cell boundaries to form cells in the target region of
the tissue.
[0028] In some implementations the laser control module is
configured to form cells in a regular array.
[0029] In some implementations the laser control module formed to
generate the laser pulses with laser parameters of at least one of:
a pulse duration between 0.01 and 50 picoseconds, a repetition rate
between 10 kHz and 100 megahertz, a pulse energy between 1
microjoule and 25 microjoule, and a pulse target separation between
0.1 micron and 50 microns.
[0030] In some implementations a method of fragmenting a tissue in
an eye with a photodisruptive laser includes selecting a target
region in the eye for fragmentation, and forming an array of cells
in the target region by directing a beam of laser pulses to
generate cell boundaries in the target region, with a cell size and
laser parameters of the laser beam such that the tissue
fragmentation requires a surgical time of less than two minutes,
whereas a volumetric tissue fragmentation of the same target region
with the same laser parameters would require a surgical time in
excess of two minutes.
[0031] In some implementations the laser parameters are at least
one of a pulse duration between 0.01 and 50 picoseconds, a
repetition rate between 10 kHz and 100 MHz, a pulse energy between
1 microjoule and 25 microjoule, and a pulse target separation
between 0.1 micron and 50 microns, and the cell size is between 1
microns and 50 microns.
BRIEF DESCRIPTION OF THE DRAWINGS
[0032] FIGS. 1a-c illustrate a volumetric eye disruption
procedure.
[0033] FIG. 2 illustrates an aspiration step.
[0034] FIG. 3 illustrates an ophthalmic surgery system.
[0035] FIGS. 4a-b illustrate a regular cell array.
[0036] FIGS. 5a-d illustrate a layer-by-layer formation of a cell
array.
[0037] FIGS. 6a-b illustrate a formation of a cell array on a
cell-by-cell basis.
[0038] FIGS. 7a-c illustrate spherical and polyhedral cell
structures.
DETAILED DESCRIPTION
[0039] This application describes examples and implementations of
techniques and systems for laser surgery on the crystalline lens
via photodisruption caused by laser pulses. Various lens surgical
procedures for removal of the crystalline lens utilize various
techniques to break up the lens into small fragments that can be
removed from the eye through small incisions. These procedures may
use manual mechanical instruments, ultrasound, heated fluids or
lasers and tend to have significant drawbacks, including the need
to enter the eye with probes in order to accomplish the
fragmentation, and the limited precision associated with such lens
fragmentation techniques. Photodisruptive laser technology can
deliver laser pulses into the lens to optically fragment the lens
without the insertion of a probe and thus can offer the potential
for lens removal with improved control and efficiency.
Laser-induced photodisruption has been widely used in laser
ophthalmic surgery. In particular, Nd: YAG lasers have been
frequently used as the laser sources, including lens fragmentation
via laser induced photodisruption.
[0040] In a laser-induced lens fragmentation process, laser pulses
interact with the lens tissue to generate gas in form of cavitation
bubbles and decrease the lens tissue transparency. Because the
laser pulses are sequentially delivered into the lens, the
cavitation bubbles and reduced lens tissue transparency caused by
the initial laser pulses can obscure the optical path of subsequent
laser pulses and thus can interfere with the delivery of subsequent
laser pulses directed to additional target positions in the lens by
blocking, attenuating or scattering the subsequent laser pulses.
This effect can reduce the actual optical power level of the
subsequent laser pulses and thus adversely affect fragmentation at
the deeper locations in the lens. Some known laser-induced lens
fragmentation processes do not provide effective solutions to
address this technical issue.
[0041] Based on effects of distinct regional properties of the lens
and laser pulse parameters on spreading of gas produced during
photodisruption, techniques, apparatus and systems described in
this application can be used to effectively fragment the
crystalline lens to remove a portion of or the entirety of the lens
by using photodisruptive laser pulses with reduced interference
caused by laser-induced air bubbles in the eye during the
photodisruption process. The present methods and apparatus allow
fragmentation of the entire or significant portions of the
crystalline lens utilizing a photodisruptive laser delivered with
minimized interference from gas generated during photodisruption.
In addition to reduced gas generation the method allows the use of
significantly less total laser energy to treat the eye and reduces
potential undesired effects such as heat generated by the laser and
reduces overall procedure time. The removal of a portion of or the
entirety of the crystalline lens can be achieved via aspiration
with reduced or no need of other lens fragmentation or modification
modalities.
[0042] The crystalline lens has multiple optical functions in the
eye, including preservation of a transparent optical path and
dynamic focusing capability. The lens is a unique tissue in the
human body in that it continues to grow in size during gestation,
after birth and throughout life. Since new lens fiber cells are
added from a germinal center located on the equatorial periphery of
the lens, the oldest lens fibers are located centrally in the lens.
This region, called the lens nucleus, has been further subdivided
into embryonal, fetal and adult nuclear zones. While the lens
increases in diameter, it may also undergo compaction so that the
properties of the nucleus are different from the cortex (Freel et
al BMC Opthalmology 2003, 3:1). In addition, since lens fiber cells
undergo progressive loss of cytoplasmic elements and since there is
no blood supply or lymphatics to supply the inner zone of the lens,
it becomes progressively more difficult to preserve the optical
clarity and other properties (e.g., lens flexibility) that serve
the function of the lens. Of particular importance is the central
core of the lens, occupying the inner approximately 6-8 mm in
equatorial diameter and approximately 2-3.5 mm in axial diameter.
This region has been shown to have reduced permeability to and from
the metabolically active cortex and outer nucleus (Sweeney et al
Exp Eye res, 1998:67, 587-95). A correlation with this observation
is the progressive loss of transparency that is identified in the
most common type of cataract in the same region in patients, as
well as increases in lens stiffness that occur with age in a
gradient fashion from the peripheral to central portion of the lens
(Heys et al Molecular Vision 2004: 10:956-63). One result of such
changes is the development of presbyopia and cataract that increase
in severity and incidence with age.
[0043] The above identification of a central zone with different
transport, optical and biomechanical properties has significant
implications for fragmentation techniques with photodisruption,
because one significant limitation to various laser-based lens
fragmentation techniques is the uncontrolled spread of gas bubbles
that can occur during photodisruption that can reduce the
effectiveness of subsequent laser pulses in interacting with the
lens. The layered structure of the lens body at different locations
exhibits differing resistance to spread of cavitation bubble gas.
Additionally, the softer peripheral layers can be sufficiently soft
so as not to require photodisruption and/or significant
fragmentation prior to aspiration and removal. These softer, less
resistant peripheral layers however can allow the gas generated by
photodisruption to spread and block subsequent laser pulses that
are directed to fragment the harder central core. The precise
determination of regions of a lens that are more or less resistant
to the spread of cavitation bubble gas depends on individual
characteristics of each patient including the age of the patient.
The spread of gas can also be influenced by the particular laser
parameters and treatment pattern applied to the target.
[0044] The tissue of the lens can be treated in an essentially
uniform fashion. FIGS. 1a-c illustrate an example where a
photodistruptive laser system is operated to place laser pulses
essentially uniformly within a surgical region, e.g., the eye lens,
to be treated to allow aspiration and removal of the lens material.
One way to fill the volume with laser is to direct the laser pulses
with a scanner to form a bubble layer and fill the entire volume
with a multitude of layers, as shown in FIG. 1c.
[0045] FIG. 2 illustrates that after the laser treatment, an
aspiration device can be used to remove the disrupted lens
material.
[0046] In such procedures, the laser pulse characteristics, such as
their duration can range from 0.01 picoseconds (ps) to 50
picoseconds. The laser pulse energy, layer, line and spot
separations can be optimized to achieve the highest effectiveness
of breaking up the tissue wile minimizing the effects of gas
spreading, laser exposure and procedure time.
[0047] One determining factor of the laser pulse characteristics is
the need to avoid or minimize a triggering of an uncontrolled gas
spreading. Since the conditions and properties of the lenses can
vary from patient to patient, the threshold laser pulse parameters
to achieve this can vary as well. In some implementations, the
laser energy per pulse can range from 1 microjoule (.mu.J) to 25
.mu.J and the spatial pulse separation between two initial pulses
adjacent in space can fall within the range of 0.1 micron to 50
microns. The laser pulse duration can range from 0.01 picoseconds
to 50 picoseconds and the laser repetition rate from 10 kHz to 100
MHz.
[0048] The parameters of the laser pulses and the scan pattern can
be determined by various methods. For example, they can be based on
a preoperative measurement of the lens optical or structural
properties. The laser energy and the spot separation can also be
selected based on a preoperative measurement of lens optical or
structural properties and the use of an age-dependant algorithm.
The pulsed laser is operated to direct a sequence of laser pulses
to a target lens region of the lens to fragment the target lens
region. The laser pulses may also be directed to one or more
regions of the lens other than the target lens region, e.g.,
peripheral locations and/or the lens capsule, to create an opening
or incision in the lens. After the desired fragmentation and
incision are achieved, the laser pulses can be terminated and the
fragmented target lens region and any other selected portions of
the lens are removed from the lens body by aspiration.
[0049] The following sections describe techniques and laser systems
for applying laser pulses to surfaces and boundaries of cells of
predetermined size, shape and spatial distribution that differ from
the above described uniform volumetric distribution of the laser
pulses within the treated volume. Following such a laser treatment
the lens tissue can subsequently break up along the surfaces and
boundaries of the cells. The size of the cells or granules can be
determined to be small enough that they can easily be removed by
using e.g. an aspiration device. A typical aspiration device is a
needle attached to a suction pump. For example a 23 gauge needle
has an inner diameter of 0.34 mm. Cells smaller than the inner
diameter of the aspiration needle can pass through the needle
without clogging.
[0050] FIG. 3 illustrates a laser surgical system for performing
such a non-uniform laser distribution process. An optics module 310
can focus and direct the laser beam to a target lens 301. The
optics module 310 can include one or more lenses and may further
include one or more reflectors. A control actuator can be included
in the optics module 310 to adjust the focusing and the beam
direction in response to a beam control signal. A system control
module 320 can control both a pulsed laser 302 via a laser control
signal and the optics module 310 via the beam control signal. An
imaging device 330 may collect reflected or scattered light or
sound from the target lens 301 to capture image data via the target
lens 301. The captured image data can then be processed by the
laser system control module 320 to determine the placement of the
applied laser pulses. This control can be a dynamic alignment
process during the surgical process to ensure that the laser beam
is properly directed at each target position. The imaging device
330 can be implemented in various forms, including an optical
coherent tomography (OCT) device. In other implementations, an
ultrasound imaging device can also be used.
[0051] The system control module 320 may process image data from
the imaging device 330 that includes the position offset
information for the photodisruption byproducts in the target
region. Based on the offset information obtained from the image
data, the beam control signal can be generated to control the
optics module 310, which can adjust the laser beam in response. A
digital processing unit can be included in the system control
module 320 to perform various data processing functions for the
laser alignment and laser surgery. The digital processing unit can
be programmed to control the laser parameters of the initial laser
pulses and the additional laser pulses, laser beam scanning
direction from the posterior to anterior direction for the initial
laser pulses and the laser movement of the additional laser
pulses.
[0052] In one implementation, the pulsed laser 302 can be a high
repetition rate pulsed laser at a pulse repetition rate of
thousands of shots per second or higher with relatively low energy
per pulse. Such a laser can be operated to use relatively low
energy per pulse to localize the tissue effect caused by
laser-induced photodisruption. A form of this tissue effect is the
formation of cavitation bubbles. In some implementations, the
impacted tissue region can have a size of the order of microns or
tens of microns. This localized tissue effect can improve the
precision of the laser surgery and can be desirable in certain
ophthalmic surgical procedures. In one example of such surgery,
placement of many hundred, thousands or millions of contiguous or
near contiguous pulses, which may be separated by microns or tens
of microns, can be used to achieve certain desired surgical effect
placement. Such procedures using high repetition rate pulsed lasers
may require high precision in positioning each pulse in the target
region during surgery, both regarding their absolute position with
respect to a target location and their relative position with
respect to preceding pulses. For example, in some cases, subsequent
laser pulses may be required to be delivered next to each other
with an accuracy of a few microns, when the time between pulses
(the repetition rate) can be of the order of microseconds.
[0053] FIGS. 4a-b show an implementation of an ophthalmic surgical
procedure, during which laser spots (or bubbles) are generated to
form granules, the granules themselves forming a granule array. The
laser spots can be generated to form a regular spatial pattern of
the granules, as shown in FIG. 4b. Regularly spaced granules
utilize the laser pulses well, since they require a limited amount
of laser energy to break up a target region. Nevertheless, in other
implementations the granules may form an irregular or even random
array.
[0054] The cells can be packed next to one another. Creating the
side wall of one cell can simultaneously create the side of the
neighboring cell as well, making the process efficient. The design
of the individual cells and the cell pattern may be selected based
on the physical properties of the tissue to be treated. The bulk of
the lens consists of concentric layers of elongated fiber cells.
Cleavage of tissue parallel and perpendicular to the layers and
individual fibers is different. Therefore, in some implementations
a higher spot density and/or laser pulse energy can be used to form
cell boundaries which are perpendicular to layers and fibers.
[0055] During the formation of a particular spatial pattern,
different implementations of the present surgical method may
utilize different scanning paths. A regular pattern can be built
all at once or granule by granule. Which method to use may depend
on the particular laser scanner and it is a matter of optimization
to achieve higher precision and shorter procedure time.
[0056] In a particular implementation the granules, or cells, can
be cubes. With an analogy to crystal cell structures, this pattern
of cells can be described as a Simple Cubic (SC) crystal. Layers of
these cubes can be formed simultaneously, followed by repeating the
procedure in subsequent layers.
[0057] FIGS. 5a-d illustrate that in some implementations first a
bubble layer can be generated with the laser pulses to form the
bottom layer of the SC crystal, as shown in FIG. 5a. In some
implementations, this bottom layer can be essentially transverse,
or perpendicular, to an optical axis of the lens or the eye. As it
is known, an optical axis can be defined in several different ways,
with somewhat different outcomes. In what follows, the term
"optical axis" will be used to refer to an axis defined by any one
of these procedures, or even a compromise axis, defined as a
direction falling between differently defined axes.
[0058] FIG. 5b illustrates that subsequent to the formation of the
bottom layer, a regular array of cell walls can be generated. These
walls can be essentially parallel to the optical axis, formed with
a predetermined cell height.
[0059] FIGS. 5b and 5c illustrate that the scanner can raster, or
sweep, first in one direction (FIG. 5b) then in an orthogonal
direction (FIG. 5c).
[0060] FIG. 5d illustrates that a layer of cells can be completed
by placing a bubble-layer to form the top of the cells. This "top"
bubble-layer can then form the bottom bubble-layer for the next
layer of cells. The target volume can be filled up with a regular
array of granules/cell by repeating the steps 5a-d.
[0061] In some implementations, a smooth boundary layer can be
formed around the regular array of cells in the surgical target
region, partially, or in its entirety. Such a boundary layer can
provide a smooth surface without the raggedness of the edges of the
cell array.
[0062] In other implementations, the crystalline cell
array/structure can be oriented differently, the bottom layer
forming any angle with the optical axis of the eye. In yet other
implementations, the layers themselves can be somewhat curved, to
accommodate the natural curvature of the lens target region itself
or the natural curvature of the focal plane of the surgical system.
Such structures may not be entirely regular. They may contain
deformations or lattice defects. These defects can be formed
intentionally or may emerge during the creation of the cell
array.
[0063] FIGS. 6a-b illustrate an alternative implementation, where
complete cells of a layer can be formed individually one after the
other by controlling the scanner to form all walls of a single
cell, and to start forming the next cell only after the previous
cell is completed.
[0064] FIG. 6a illustrates that rows of cells can be formed first
to build a layer of cells.
[0065] FIG. 6b illustrates that subsequent layers can be formed on
top of already created layers, to fill a surgical target
volume.
[0066] The implementations which form the cells individually may
have the following features.
[0067] First, FIGS. 7a-c illustrate that the shape of the cells can
be different from the simplest shapes, like cubes. For example, the
cell array can have a hexagonal base to form a Simple Hexagonal
(SH) lattice. Or a target volume can be broken up into cells of a
different type, such as spheres (FIG. 7a), separated by
complementary small interstitial regions. Spherical cell shapes can
minimize the occurrence of clogging in the aspiration needle. The
type of the lattice, formed by the spherical cells can be selected
to optimize the ratio of the volume of the spheres to the volume of
the interstitial regions. These lattice types are known as "close
packed" crystals structures. Face Centered Cubic (FCC) and
Hexagonal Closest Packing (HCP) are two such structures.
[0068] In some implementations, the cells can have shapes which
approximate spheres, such as polyhedral shapes. In these
implementations, the polyhedral cells can form close packed
structures, analogously to the lattice of spheres.
[0069] FIG. 7b illustrates a Truncated Rhombic Dodecahedron, which
is an example of a polyhedron approximating a sphere.
[0070] FIG. 7c illustrates that truncated rhombic dodecahedrons can
be close packed to form a layer, and fill a target volume with a
stack of layers. When passing through the aspiration needle, these
polyhedra can more readily roll than cubes and the likelihood of
clogging is smaller.
[0071] Second, the laser scanner pattern can be optimized to speed
up completion of the whole pattern. Creating individual cells with
sizes of the order of 100 to 200 micrometers require small scanner
displacements and can be performed at higher scanner speed. Larger
scale moves of the scanner can be slower. The design of the scanner
can be optimized effectively for achieving the shortest overall
procedure time. Applying a combination of two different scanner
types, a fast small displacement scanner and a slow but larger
displacement scanner, can further optimize system performance.
Building cells individually is also consistent with modular
software design practices. A complete volumetric pattern can be
created from building blocks; cells, rows and layers of cells and
finally the complete pattern. An analogous approach can be
effective in constructing a software code for the scanner drivers
as well.
[0072] The above described or other patterns can be created by
proceeding from the posterior to the anterior side of the lens or
from the anterior to the posterior side. The former can be
advantageous to avoid blocking the laser beam by bubbles previously
formed in the target tissue. The latter may be preferable when a
cataract is present in the lens and penetration of laser light
through the cataract is already compromised. In that case
fragmentation of the anterior portion of the lens may be necessary
followed by aspiration of the treated portion and successive laser
treatment and aspiration of the deeper-lying parts of the lens
until the full volume is fragmented.
[0073] Additional features of implementations which fragment the
target tissue in a granular or cellular form include the
following.
[0074] 1) Reduction of the amount of gas bubbles formed in the eye
and thus reduction of the induced opacity of the tissue. Since a
similar degree of tissue disruption can be achieved by considerably
smaller number of bubbles in a granular/cellular procedure, this
aspect can increase the effectiveness of the laser treatment to a
substantial degree.
[0075] 2) Reduction of the number of pulses applied to the tissue,
which increases the speed of the procedure. Time is at a premium
during eye surgery, as after about two minutes patients sometimes
develop an increasing, hard-to-control eye movement. This can
necessitate the abandonment of the surgical procedure. Therefore,
if a new feature in a surgical procedure is capable of reducing a
time of the surgery from above two minutes to below two minutes,
that feature may increase the utility of the surgical procedure
qualitatively.
[0076] 3) Reduction of the required total energy applied to the
tissue, which reduces potential undesired side-effects related to
the exposure of the eye to laser light. In most procedures, a
substantial portion of the laser light passes through the surgical
region and continues its way to the retina. The retina being a
strongly light sensitive tissue itself, this transmitted surgical
laser light may damage the retina to an undesirable or unacceptable
degree. Thus, a reduction of the transmitted laser light can be an
advantageous feature.
[0077] To quantitatively assess the different implementations, a
comparison is provided for the amount of laser energy required for
treating a given volume, and for the number of laser pulses
required for volumetric and cellular fragmentation procedures.
TABLE-US-00001 TABLE 1 Number of Number of Ratio of pulses, Spot
Cube pulses per mm.sup.3 pulses per mm.sup.3 volumetric vs.
separation size volumetric granular granular, ratio (um) (um)
breakdown breakdown of procedure time 2 50 125000000 14400000 8.7 2
150 125000000 4933333 25.3 2 250 125000000 2976000 42.0 5 50
8000000 2160000 3.7 5 150 8000000 773333 10.3 5 250 8000000 470400
17.0 10 50 1000000 480000 2.1 10 150 1000000 186667 5.4 10 250
1000000 115200 8.7 20 50 125000 90000 1.4 20 150 125000 43333 2.9
20 250 125000 27600 4.5
[0078] Table 1 illustrates some results of the comparison,
contrasting tissue breakdown by performing a volumetric method and
by forming a Simple Cubic lattice of cells. Typically, the volume
of an individual gas bubbles is approximately proportional to the
energy of the femtosecond laser pulse which created the bubble.
This proportionality holds for energies not too close to the
threshold of producing plasma. Further, the individual pulses are
directed such that the gas bubbles touch each other. In the
volumetric method this translates to the spot, line, and layer
separation being approximately equal to one another, all being set
by the diameter of the bubbles. In the cellular implementation this
translates to the cell boundaries being formed by touching spheres.
It is noted that in practice, some overlap may be necessary for
both volumetric and cell-array methods.
[0079] Table 1 reports comparison results, varying the spot
separation from 2 microns to 20 microns and the cell size from 50
microns to 300 microns. The speed of the procedure was
characterized by the number of pulses needed for a given total
volume and the total energy needed.
[0080] In some implementations of the volumetric breakdown, the
total energy needed to break down the tissue in the target region
can be approximately independent of the size of the bubbles. This
energy is of the order of 1 Joule per cubic millimeter of target
volume. This relationship holds most accurately in the energy range
in which the volume of an individual bubble is proportional to the
laser pulse energy.
[0081] For implementations which form a lattice of cells, the speed
and the corresponding energy depends both on the size of the
individual bubbles and on the size of the cell. The speed increases
with increasing cell sizes and decreasing bubble sizes. This is the
result of the change in the volume to surface ratio of the cells as
a function of their size. This comparison is based on using a 23
gauge needle which has inner diameter of 340 micrometers. The
largest size cube which can enter the tube at the least favored
orientation, with its body diagonal perpendicular to the length of
the tube, is about 196 micrometers. Actual implementations may use
smaller grain sizes, such as 150 microns.
[0082] As Table 1 illustrates, methods based on forming
cell-lattices can exhibit an increase of speed by a factor of 2.9
to 25.3 over the speed of the volumetric method as the size of the
bubbles is varied. For a typical bubble size of 10 microns, the
increase of speed can be about 5.4-fold. The speed ratio increases
with decreasing bubble size. The improvement in the procedure time
is approximately a factor of 10 for 5 micron bubble size and 25 for
2 micron bubble sizes. These are quite significant improvements
over the volumetric method.
[0083] As mentioned above, the required total laser energy is
proportional to the total volume of gas produced, which is
proportional to the number of bubbles for bubbles of a fixed size.
Therefore, among methods which use the same average laser power and
create similarly sized bubbles, the procedure time is approximately
proportional to the number of bubbles created. Thus, the speed
improvement of the cell-array methods over the volumetric methods
is proportional to the ratio R of the total number of pulses, as
demonstrated in Table 1. Implementations with a different bubble
size in general require different laser average power, repetition
rate, and scanner speed setting. Nevertheless, it remains true for
implementations with all bubble sizes that granular/cellular
fragmentation decreases the total energy and time required compared
to volumetric fragmentation with the same bubble size. Thus
R = Volume_of _gas _produced _in _volumetric _fragmentation
Volume_of _gas _produced _in _granular _fragmentation Also , ( 1 )
R = Procedure_time _of _volumetric _fragmentation Procedure_time
_of _granular _fragmentation ( 2 ) ##EQU00001##
[0084] When the cell size is significantly larger than the bubble
size, R can be approximately proportional to the Volume/Surface
ratio of the cell. Since the volume of gas produced is
approximately proportional to the product of the area of the cell
boundaries multiplied with the bubble size, R is approximately
proportional to a ratio of a cell size to a bubble size.
R .varies. Cell_size Bubble_size . ( 3 ) ##EQU00002##
[0085] Granular fragmentation with the smallest bubble size
produces the smallest amount of gas in the target tissue and uses
the smallest amount of total laser energy. In some implementations
there can be a practical limit, how small a bubble size should be
used. In some implementations the bubbles are closely packed, and
the corresponding spot separation and line separation is nearly
equal to the bubble size when creating the surface of a cell
boundary. Although the laser parameters, average power, pulse
energy and repetition rate can be chosen over a wide enough range
to achieve the desired bubble size, spot and line separations, the
scanning system can be limited in its speed and acceleration to
generate a particular pattern. To keep the acceleration of the
scanner under control at turning points, in some implementations
the linear speed of progression of the bubble placement can be kept
smaller than a limiting value, v.sub.lim.
[0086] For a given granular pattern the total area of the cell
boundaries, A, and as demonstrated in Table 1. the total number of
pulses N per total volume are given for given bubble size. For
example, for a close packed area A=N*bubble_size.sup.2.
[0087] The total linear path of bubble placement s may equal
N*bubble_size, which is also the speed of progression of bubble
placement times the total procedure time, s=v*T. Therefore, in some
approaches, the product v*T*bubble_size may be approximately
constant.
[0088] In order to minimize the total amount of gas produced and
the total amount of laser energy used for a granular pattern, some
approaches minimize the bubble size by selecting the linear speed
of progression and the procedure time to their largest acceptable
value v.sub.lim and T.sub.max. Here T.sub.max is the maximum
procedure time tolerable by the clinical environment, less than 1
minute is acceptable, less than 30 seconds is desirable, dictated
mainly by the tolerance of the patient to keep still during the
procedure.
[0089] On the other hand, to minimize procedure time, the highest
linear speed v.sub.lim and the largest acceptable bubble size can
be selected. The largest acceptable bubble size is determined by
the amount of gas produced, pulse energy used, and by the required
precision of the surgery.
[0090] Some implementations of the surgical system maximize the
speed of scanner and lower the laser energy threshold for the
formation of cavitation bubbles, in order to minimize bubble size.
The surgeon has the choice to select particular parameters within
the limitation of the surgical system to optimize the parameters
for particular surgical outcomes. The decision may take into
account the size of the surgically affected area, tissue parameters
the age of the patient and other factors.
[0091] While this specification contains many specifics, these
should not be construed as limitations on the scope of an invention
or of what may be claimed, but, rather, as descriptions of features
specific to particular embodiments of the invention. Certain
features that are described in this specification in the context of
separate embodiments can also be implemented in combination in a
single embodiment. Conversely, various features that are described
in the context of a single embodiment can also be implemented in
multiple embodiments separately or in any suitable subcombination.
Moreover, although features may be described above as acting in
certain combinations and even initially claimed as such, one or
more features from a claimed combination can in some cases be
excised from the combination, and the claimed combination may be
directed to a subcombination or a variation of a
subcombination.
* * * * *