Transcript Wavefront-Guided (Customized) Excimer Laser Refractive Surgery

Customized Corneal Ablation
Customized LASIK & PRK will dominate in
next few years
Speedy recovery, good quality of vision
satisfactory outcome
Disadvantage of conventional refractive
surgery in some patients:
* Increase in HOA
* Reduction in visual quality
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Wavefront Customized Visual Correction
Ocular wavefront sensing:
Will be increasingly employed
Will become routine in vision
assessment
Wavefront customization is employed to
optimize any refractive surgery
procedure
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Wavefront Customized Visual Correction
(cont)
Future wavefront customized refractive
procedures
Implantation of optimized IOL’s e.g
Technis aspheric lens
Customized IOL’s preinsersion,
customized phakic IOL’s
Types of customization
Two main methods are available in refractive surgery:
1. Topography guided ablation
2. Wavefront guided ablation
According to recent statistics, approximately
55% of North American refractive surgeons
have wavefront analyzers in their practice and
routinely perform wavefront-guided ablations.
Definition of Wavefront
Wavefront-Guided (Customized)
Excimer Laser Refractive Surgery
 Definition: Wavefront-guided custom
ablation is used to correct higher-order
aberrations, in addition to spherocylindrical
correction.
Incidence in General population
10-15% have significant
higher-order aberration
Hard contact lenses
Corrects higher order aberrations
resulting from the cornea.
How to measure the higher order
optical aberration?
 By using a Wavefront analysis system called
aberrometry
Pupil and
Wavefront
Larger diameter leads to
a larger Wavefront error
Evaluation of vision quality
Compromised more at dim light during night
and represented by:
a) Double vision
b) Ghosting
c) Glare
d) Halos
e) Starbursts
d) Reduced contrast sensitivity
Quantification of magnitude
of the aberration
1)Usually by RMS wavefront error
2)It gives a rough estimate
Types of aberrations
 Low Order Aberration (LOA) : 1st & 2nd
order aberration
 High Order Aberration (HOA) : 3rd to 6th or
10th
How to evaluate the quantity of
optical system
• Root Mean Square ( RMS )
The RMS is SD of height (depth) of wavefront
relative to the reference at all the point in the
wavefront. RMS is very useful measure of optical
quality.
• Peak-To-Valley (PV)
Distance from the highest to lowest point on the
deformed wavefront relative to the reference
wavefront.
• RMS1 is qualitative presentation of 1st order
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aberration, RMS2 is due to 2nd order and etc.
RMSh represents the total RMS of HOA (including
3rd to 6th OA)
RMSg represents the total RMS of HOA and LOA
Total RMS increases with aging
LASIK increases RMS especially in subjects with
larger pupils
RMSh > 0.3 and > 20% of total RMS is clinically
significant and indicative for customized
ablation
Wavefront measuring devices
 Hartmann-shack style devices are the
most common used
Hartmann-shack style
1)A narrow beam of light is projected on to the retina
and the light reflected from the fovea passes through
the lens and the cornea and exists the eye
Customization can be based
on corneal topography or wavefront
measurements.
 Corneal topography guided ablation has been
attempted on patients with regular and irregular
astigmatism, decentered ablations, and central
islands. The irregular astigmatism group is more
challenging and may benefit most from corneal
topography guided ablation as the systems become
more refined.
Hartmann-shack aberrometer
 A low intensity laser beam is directed to retinal
surface
 Light rays from this laser spot are reflected back to
the front of the eye
Laser spot on retinal surface
light rays are reflected back to the front of the eye
Lens arrey
focuses these
light rays
Photodetector (charged coupled device,CCD)
Dots of light
Real eye with Aberrations
Considering refractive lens at corneal surface
Excimer etching by customized ablation
Converting wavefront to flat wavefront
Perfect focus on fovea
Ideal eye
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Zernike’s Polynomials
Zero order (no order)=axial symmetry, flat
wavefront
First order = linear aberration, tilting around a
horizontal (x) or vertical axis (y)
Second order = focus shift,
spherical defocus or astigmatism
Third order = corresponding to
coma, triangular astigmatism
Fourth order = spherical
aberration, complex patterns
Fifth-10th order = Irregular aberrations,
important when pupil is wide dilated.
10/14/2007
XVth Congress of Iranian Society of Ophthalmology
October 2005
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Wavefront guided customization
Goals
a) Spherocylinder correction
b) To correct the aberration that are induced by
conventional laser vision correction and pre-existing
aberration.
Requirment for wavefront
1) Efficient eye tracking
2) Good registeration system for eye
3) Small size laser spot
4) Sufficient corneal bed thickness
Challenges to wavefront
 Tear film abnormalities
 Miotic pupil : If severe may be difficult to measure and
provide information beyond the 3mm optical zone
and therefore require pharmacological dilation.
Optical effects of tear film disruption
Wavefront map with
cycloplegic or mydriatic
1) Has significant difference in pre-op refractive error
compared to subjective refraction
(Ablation profile algorithm)
1) After good aberration maps, Ablation profile
algorithm is needed
2) Should be designed to make the post-op higher order
aberrations as minimal as possible
3) Not only correcting the pre-op higher order aberration
but also preventing the induction of new higher order
aberrations.
Conversion to Ablation Profile:
Ablation profile is fundamentally inverse of wavefront
error map
Goal: Correction of refractive error and higher-order
aberrations
Pupil diameter at least 0.5mm
larger than scotopic
Limbal marking for cyclotorsion
detection
Wavefront measurement + corneal curvature +
biomechanics
ablation profile complex
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Registration
 Registration refers to the method by which a
customized ablation pattern based on preoperative
wavefront analysis from the aberrometer is
superimposed in the same position on the eye during
the laser treatment.
LADARWave and LADARVision
systems:
 Centration is established by the surgeon when the
patient fixates on the target light. The instrument
captures a video image of the eye, and the surgeon
defines the pupillary ring (red) and limbus ring
(yellow). These will be used by the LADARVision
tracker to follow the eye during application of the
customized ablation pattern.
Torsional alignment
 Torsional alignment refers to the need to compensate
for the cyclotorsion of the eye that occurs when the
patient lies down.
 The eye usually rotates less than 5 degrees, but in some
patients, postural cyclotorsion can be as much as 10 to
12 degrees.
Misalignment
1) A small misalignment in in the axis can have
significant impact on the results of procedure
2) A 5-7 degrees of cyclotorsion when changing from
sitting position to supine position. 50% of the visual
benefit of HOA is lost.
Bausch and lomb zyolptix system
has
1) Diagnostic
2) Treatment components for wavefront customized
corneal ablation
Diagnostic part of zioptix systems
(2172 laser)
In zy wave aberometer and ORBscan both machine
are integrated in one workstation.
It has Hartmann-shack sensor that measures up to 5th
order zernike term.
It measures sphere from +6.0 D to -12. D and 0 to 5 D
of cylender with pupil diameter from 2.5 to 8.6 mm.
Pupil during exam
1) Orbscan must be performed with an undilatd pupil
2) Wavefront meaurements are taken with a dilated
pupil.
Types of beam
Has a truncated gaussion 3 beam to achieve the benefits
of both beam types.
1
2
3
Laser beam sizes in Tech217z
1) The 2mm beam treat the majority of the refractive
errors.
2) The 1mm beam used for more specific ablation
pattern on the transition zone
Eye tracker
The system has 120 Hz active eye tracker with a passive
automatic shut-off system corrects for intraoperative
eye movement in every dimention including
cyclotortion and pupil shift
FDA investigation for outcomes for
wavefront guided lasik in myopia
for 3 separate laser platforms
1) LADAR vision
2) Technolus 217 z
for low to moderate myopia
3) visx
Wave front guided
89%
……..20/20
Conventional
72%
………….20/20
Environmental Conditions
 Control of environmental conditions during treatments is
important. In previous U.S. FDA Multi-Center Clinical Trials,
the room conditions were:
 Temperature ranged from 68ºF to 72ºF (20ºC to 22.2ºC)
 Relative humidity ranged from 40% to 45%
 Treatments performed at >24ºC were associated with
less accurate outcomes
 Stability of temperature and humidity is important
Future of Customized Corneal
Ablation
LASIK vs surface ablation
Each microkeratome induces specific “flap
only” aberration
(flap size, thickness)
Considering “flap aberrations” in total
treatment calculations
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Future of Customized Corneal Ablation …
cont.
Surface Ablations:
Show promising results with use of
immunomodulating agents
Better control of cellular and biochemical
reactions
Introduction of new drugs to better regulate
wound healing and refractive outcome
Gene therapy for better control of post laser
keratocyte activation and wound healing
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Multifocal Ablation
Presbyopia: Customized multifocal ablation
Aberrations may be induced when creating
multifocality
Potential loss of contrast sensitivity and quality
of visual function
Future results will be improved:
* Wavefront mapping, sophisticated eye
trackers and corneal registration
* Preoperative simulation of postop condition
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Laser Delivery Refinements
Correction of HOA needs
smaller spot delivery
requires 0.6-0.8mm spot size
>5th order
Smaller spot size needs faster and better eye
trackers
Smaller ablation depth per pulse provides
ideal correction profile for higher orders
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The Human Hair (125 microns)
Etched with an excimer laser
Courtesy of IBM
Effects of Laser Ablation
Non-thermal
Thermal
YAG
CO2
Excimer
Conclusion
Wavefront measurement devices and
consequently wavefront correction
procedures are still in process of
evolution
Achievement of “supervision”: with
advancement in current procedures
will not be a dream in near future
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Thanks for your attention