lasik the technology...

Transferring wavefront measurements to ablation profiles

Michael Mrochen PhDSwiss Federal Institut of Technology, Zurich

IROC Zurich

corneal ablationcorneal ablation

Calculationablation profile

Patient Eye

Centrationduring measurement

Calculation laserspot positions Centration

Eye Tracking

PerformanceLaser

photoablation

Biological responsewound healing

Wavefront sensing



Predictability of the refractive

outcome

Patient Eye



Eye Tracking

PerformanceLaser

photoablation


Wavefront sensing



Patient Eye



Eye Tracking

PerformanceLaser

photoablation


Wavefront sensing

• Accommodation• Tear Film• Refractive errors• Pupil Size• Opacity of media• Age

Patients expectationsPatients expectations

• Is it save and predictablil ?• Correction for far or near distance• monovision / presbyopia• high expectations on visual performance• Cost / service ratio

Preview !!Preview !!

See also:

Mirko Jankov (Poster session)“Can dry eye influence the wavefront measurement”

Takashi Fujikado (Sunday)“Wavefront sensing and the tear film”



Patient Eye



Eye Tracking

PerformanceLaser

photoablation


Wavefront sensing• Fixation Target• line of sight• pupil size



Patient Eye



Eye Tracking

PerformanceLaser

photoablation


Wavefront sensing

• Optical Setup• System calibration• Wavelength • Zernike calculation• Dynamic range of

sensor

∆Φ = 1DF

486

nm

d 5

88 n

m

C 6

56 n

m

Chromatic aberrationsChromatic aberrations

Wavefront sensorsusually work in thenear infrared

wave length > 750 nm


See also:

Larry Thibos (Hot Topics)“Does Chromatic Aberration Hinder or Help ?”

Wavefront sensingWavefront sensing

~ 1100 spots over a 7 mm pupil

~ 96 spots over a 7 mm pupil



Patient Eye



Eye Tracking

PerformanceLaser

photoablation


Wavefront sensing • Optical eye model • K-Readings• Topography of the cornea• Biometric data of the eye

• Subjective Refraction• Pupil size• corneal curvature• age• female / mail• patien expectations

20

211 )()( rrRry −−=

20

222 )()( rrRry −−=

Optical zone radius r0

−−=−=∆

21

11)1(RR

nDDD afterbefore

Ablation profile

( )( ) 0

20 0,

12)( rr

nrrDra ≤≤

−−∆

=

Ablation profilesAblation profiles

“…. the corneal topography information specifying corneal shape has very little effecton the desired ablation depth for an optimal refraction. “

Stanley Klein, J Opt. Soc. Am A (1999)


Wavefront – guided treatments

.

“…. the first surface of the cornea and internal optics partially compensate for each other's aberrations and produce an improved retinal image. “

“…it shows the limitation of corneal topography as a guide for new refractive procedures and provides a strong endorsement of the value of ocular wave-front sensing for those applications.”

Pablo Artal, J. Vis. (2001)


Total wavefront aberrations

Corneal wavefront aberrations

wavefront aberrations of the internal structures


Pupil diameter 6 mm

2 µm

2 µm

Converting wavefronts into corrections1st - order approximation !

Pupil diameter 6 mm

Wavefront inversion

2 µm

2 µm

Pupil diameter 6 mm

6 µm

6 µm

Ablations profil conversion

337.11

),(),( =−

= nnyxWyxa

Pupil diameter 6 mm

12 µm

6 µm

Ablations profile shift

2

4

6

8

10

2 4 6 8 10

abla

tion

dept

h [ µ

m]

verti

cal a

xis

[mm

]

horizontal axis [mm]

02.04.06.08.01012141618202224262830323436


Pre-OP Post-OP 6 months

Tota

l w

avef

ront

Wav

efro

nt o

f hi

gher

ord

ers

2

4

6

8

10

2 4 6 8 10

abla

tion

dept

h [ µ

m]

verti

cal a

xis

[mm

]


02.04.06.08.01012141618202224262830323436

2

4

6

8

10

2 4 6 8 10

abla

tion

dept

h [µ

m]

verti

cal a

xis

[mm

]


02.04.06.08.01012141618202224262830323436

Ablation profile used !

“classical” ablation profile


“… a ray that is less bent when it enters the eye is expected to intersect the lens much further away form the axis.“

“… the spherical aberration is expected to be much higher in the operated eye.”

Fabrice Manns, SPIE Ophthalmic Technologies XI (2001)


1f


1f 2f ′2f


“… because of individual interactions of the aberrations in the ocular components, a combination of corneal and total aberration measurements is critical to understand the individual outcomes, and by extension, to designing customized ablation algorithms.“

Susana Marcos, IOVS (2001)



See also:

Susana Marcos (Sunday)“From theoretical laser ablation profile design ro real outcomes: implications for optimized corneal refractive correction”

Steve Burns (Hot topics)“What’s better than a perfect optical correction ?”



Patient Eye



Eye Tracking

PerformanceLaser

photoablation


Wavefront sensing • Overlapping of spots• assumed ablation depth per pulse

• corneal shape factors• thermal heating

Tissue removal

10 100 10000.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

Abla

tion

dept

h [µ

m]

Radiant exposure [mJ/cm²)

Ablation threshold @ 193 nm 50 - 60 mJ/cm2

Threshold process

Principles of laser-tissue interactionPrinciples of laser-tissue interaction

Central ablation depthof a single laser spot

~ 0.5 microns

Ablation diameter of a single spot0.5 - 1.0 mm

Ablation threshold~ 50 mJ/cm2

Ablation diameter of a single spot0.5 - 1.0 mm

Planned ablation profile

Treatment zone

Optical zone

Laser pulses

Spot overlapping


Attempted profileon the cornea

Small laserbeam

Pulse ablationprofile

Largelaser beam

Achieved profile Achieved profile

Pulse ablationprofile


Ablation depth0.5µm 0.25µm 0.125µm

Puls

e di

amet

er1.

5mm

1.0m

m0.

5mmC7 = 0.5µm

Example Coma-like AberrationExample Coma-like Aberration

Ablation depth0.5µm 0.25µm 0.125µm

Puls

e di

amet

er1.

5mm

1.0m

m0.

5mmC23 = 0.25µm

Example 6th order astigmatismExample 6th order astigmatism

Reducing the spot diameter by a factor of 2 results in an increase of the treatment time by a factor of 4.

2)(~ diameterspottimetreatment

Treatment time !Treatment time !



Patient Eye



Eye Tracking

PerformanceLaser

photoablation


Wavefront sensing• Fixation Target• line of sight• pupil size• rough corneal surface

Difficulty 2Centration: A task with 6 degrees of freedom

The coordinate systems used in the measurement (M) and the treatment (T) have to coincide exactly!

• Horizontal shifts• Vertical shifts• Rotation around longitudinal axis (cyclotorsion)• Rotation around horizontal axis• Rotation around vertical axis• Z - distance

CentrationCentration

2 Types of centration errors:

• Systematic centration errors→ causing constant decentration

• Random (dynamic) centration errors → causing the ablation to be „smeared“


• Systematic centration errors

→ avoid with precise alignment techniques

• Random (dynamic) centration errors

→ avoid with active eye tracking


7 mm

3 mm

7 mm

3 mm

21 deg4 deg1 deg

29 deg6 deg3 degTorsional alignment

0.62 mm0.22 mm0.07 mm

0.85 mm0.41 mm0.21 mmLateralcentration

Same image quality

10th percentileof rms of normal

eyes

Diffraction limit

Treat 95% of normal eyes to:Required Accuracy




Patient Eye



Eye Tracking

PerformanceLaser

photoablation


Wavefront sensing • Sapling rate / Latency• Resolution• Pupil Size• Paraxial errors of entrance pupil

The eye moves during treatment

Eye - trackingEye - tracking

Position Control

ScannerDevice

Laser

Image Processing Ablation Image

TransferImage Acquisition

EyeTracker

Camera

x/yimage

α/βbeamsplitter

infraredillumination

Scannermirror

Eye

ablation beam

Latency

How does latency cause positioning errors?


Position Control

ScannerDevice

Laser


TransferImageAcquisition

EyeTracker

Camera


Position Control

ScannerDevice

Laser



EyeTracker

Camera

image


Position Control

ScannerDevice

Laser



EyeTracker

Camera

x/yimage


Position Control

ScannerDevice

Laser



EyeTracker

Camera

x/yimage

α/β


Eye Motion during Latency => Positioning Error

Latency

Positioning ErrorEye motion


-400

-200

0

200

400

-400 -200 0 200 400

Vertical deviation [µm]

Horizontal deviation [µm]

-400

-200

0

200

400

-400 -200 0 200 400



-400

-200

0

200

400

-400 -200 0 200 400



-400

-200

0

200

400

-400 -200 0 200 400



Notracking

0 ms latency 4 ms latency

8 ms latency

Positioning errors increase with increasing latency


SMI


More detailed information on the assumptions, stability, and outcomes of different scanning - spot laser parameters such as ablation depth, spot diameter, and eye-tracking latency

treatments are presented tomorrow by

Michael Bueeler



Patient Eye



Eye Tracking

PerformanceLaser

photoablation


Wavefront sensing • Energy stability• Beam profile• Scanning technology• Wavelength

Excimer laser Beam profiling / shaping

Beam delivery

Imaging / focusingoptics




Patient Eye



Eye Tracking

PerformanceLaser

photoablation


Wavefront sensing • Tissue absorption• Laser pulse duration• Tissue properties• Biomechanical

properties of tissue

Dissoziation and vaporisationVaporisation

Tissuez

da b l

Ft h

F0

~e- zα

Dissoziation


• Radiant exposure (fluence)• Absorption• Breaking of molecular bonds • Increase of temperature• Breaking of hydrogen bonds• Dissociation and vaporization• Tissue removal• Stress waves• Ablation plume dynamics


Tissue removal

10 100 10000.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4Ab

latio

n de

pth

[µm

]

Radiant exposure [mJ/cm²)

Ablation threshold @ 193 nm50 - 60 mJ/cm2

Threshold process


Spot cross-section

The illumination problemThe illumination problem

α

r x

y

z

Ae f f

0 1 2 3 4 50.70

0.75

0.80

0.85

0.90

0.95

1.00

F = 150mJ/cm²; R = 6.5 mm F = 150mJ/cm²; R = 7.0 mm F = 150mJ/cm²; R = 7.8 mm F = 150mJ/cm²; R = 8.3 mm

kor(r

)

radius r[mm]

Fluence losses

The illumination problemThe illumination problem



Patient Eye



Eye Tracking

PerformanceLaser

photoablation


Wavefront sensing• Epithelium smoothing• Flap / Hinge• Biomechanical changes• DLK••

IOP

Biomechanical effect Biomechanical effect

IOP

myopic shift !


Do we have a method for stiffening the cornea ?


Cross-linking by UV - light and riboflavin is able to increase

Young’s module of the corneaby a factor of 5


Transfering wavefronts onto the cornea includes complex

physical, optical, and biological assumtions

that are not fully understood or studied

SUMMARY SUMMARY



Further research isrequired to increase

Patient Eye



Eye Tracking

PerformanceLaser

photoablation


Wavefront sensing



the predictability of the refractive

outcomes

Patient Eye



Eye Tracking

PerformanceLaser

photoablation


Wavefront sensing

lasik the technology...

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