From theoretical laser ablation designto real outcomes: implications for

optimized corneal refractive surgery

Instituto de Optica, CSIC,Madrid, Spain

Wavefront Sensing Meeting. Whistler,Canada, Feb 04

Susana Marcos, PhD

Ablation pattern design

Laser energy delivery

Desired corneal shape?

ahcoi

CornealTotal

Pre-LASIK

Post-LASIK

RMS=0.54 mm RMS=0.35 mm

RMS=1.95 mm RMS=1.93 mm

Marcos et al. IOVS (2001)

Why corneal spherical aberration Why corneal spherical aberration //asphericity increasesasphericity increases afterafter standardstandardLASIK?LASIK?

¸Due to the design of the profile?

¸Due to discrepancies in the laserenergy delivery?

¸Due to corneal biomechanicaleffects?

Ablation pattern design

Theoretical predictions of the standardMunnerlyn pattern and its parabollicapproximation

ExperimentalPRE corneal height

Munnerlyn ablationdepth

SimulatedPOST corneal height

- =

Using actual topographic and clinical data from the patients

Laser energydelivery

Theoretical predictions of laser efficiencychanges across the cornea

Experimentalablation model inPMMA:

Flat and sphericalsurfaces

Theoreticalcomputations

Patients outcome

Evaluation of ablation profile

Contribution of laser efficiencychanges across the cornea

Effects of ablation profile, laserefficiency and corneal biomechanicson post-operative corneal shape

PMMA model

Subjects and Subjects and surgerysurgery

¸13 eyes from 7 subjects¸mean age: 28±5¸Spherical equivalent: -2.5 thru –13 D¸No ocular or retinal condition¸Flying spot Technolas 217-C (B&L surgical)¸Hansatome microkeratome¸Optical zones: 4.4-7 mm. Transition zone: 9 mm

Marcos et al. IOVS 2001

Computer surgeryComputer surgery

¸Pre-operative corneal topographies¸Munnerlyn and parabollic ablation patterns¸Clinical data of radii of curvature, correction andablation zone.¸Programmed in Matlab

Marcos, Cano, Barbero. JRS 2003; Cano, Barbero & Marcos, JOSA A 2004

PMMA PMMA model model and surgeryand surgery

¸Flat and spherical surfaces¸Radii of curvature:7-8 mm¸Different types (cast/extruded) of PMMA tested¸Spherical corrections from –3 D to –12 D¸Flying spot Technolas 217-C (B&L surgical)¸Optical zones: 4.4-7 mm. Transition zone: 9 mm

Experimental Experimental measurements measurements in in patientspatients

Corneal topography before (< 1month) and after (>1 month)LASIK refractive surgery

Corneal sphericalaberrationCorneal radius and

asphericity

Ray tracing

Computer surgeryComputer surgery: : ablation depthablation depth

y

R1R2S/2

R1= Initial radius of curvatureR2= Intended radius of curvatureS=Optical zoneD= dyopter change

Munnerlyn equationMunnerlyn equation

Parabollic approximationParabollic approximation

334

)(22 DSDr

rt +-=

0

0.02

0.04

0.06

0.08

0.1

0.12

1 0 3,5

60

r (mm)

abla

tio

n d

epth

(m

m)

-3,5

80

100

40

120

MunnerlynMunnerlyn andand parabolicparabolic approximationapproximation

Correction = - 4D

MUNNERLYNPARABOLIC

More tissueremoved

Less tissueremoved

Computer surgeryComputer surgery: : laser efficiency changeslaser efficiency changes

a

Reflection losses

Larger spot size in the periphery

Ka=1+1,14 ln[(cos a(1-R)] a = angle of incidenceR = reflection factor

Munnerlyn equation Munnerlyn equation + + laser efficiencylaser efficiency

t’Mun= tMun(y) K(ay)

Parabollic equation Parabollic equation + + laser efficiencylaser efficiency

t’Par= tPar(r) K(ar)

Mrochen & Seiler, 2001Jiménez et al., 2002

0

0.02

0.04

0.06

0.08

0.1

0.12

-3.5 0 3,5

60

r (mm)

-3,5

80

100

40

120

MunnerlynMunnerlyn withwith andand w/o w/o efficiency changesefficiency changesCorrection = - 4D

NO CONSIDERINGEFFICIENCY

CONSIDERINGEFFICIENCY

More tissueremoved

Less tissueremoved

abla

tio

n d

epth

(m

m)

Corneal shapes after Corneal shapes after ““computer surgerycomputer surgery””

Corneal topography before (< 1month) and after subtraction oftheoretical ablation patterns

Corneal sphericalaberration

Corneal radius andasphericity

Ray tracing

PREPOST

AsphericityAsphericity

-1

0

1

2

3

4

5

1 2 3 4 5 6 7 8 9 10 11 12 13-1

0

1

2

3

4

5

1 2 3 4 5 6 7 8 9 10 11 12 13

MUN

PMUN

-1

0

1

2

3

4

5

1 2 3 4 5 6 7 8 9 10 11 12 13

asp

her

icit

y

Eye #Cano, Barbero & Marcos. JOSA A 2004

0 4 8 12 16Pre-op spherical error (D)

-0.1

0.0

0.1

0.2

0.3

0.4

sphe

rica

l abe

rrat

i on

(mi c

rons

)Corneal spherical aberrationCorneal spherical aberration

F= 4.4 mm

Post real

Post Parabollic Munnerlyn

Post Munnerlyn

Cano, Barbero & Marcos. JOSA A 2004

-0,4

-0,2

0

0,2

0,4

0,6

0,8

1

1,2

Average Average asphericityasphericity

PRE

POST

Munnerlyn

ParabollicMunnerlyn

Munnerlyn+

efficiency

ParabollicMunnerlyn+ efficiency

Asp

heri

city

Cano, Barbero & Marcos. JOSA A 2004

Experimental Experimental measurements measurements in PMMA in PMMA surfacessurfacesFlat surfaces Contact profilometry

(Dektek 3000)

Correcting for tiltsApplying scale factor

2- D profiles Pre-op cornea+profile

AblatedAblated PMMA PMMA flat surfacesflat surfaces

Ablation profile

Surface location (mm)

Ab

lati

on

dep

th (

mm

)

-3 D

-6 D

Ablation depth

60 80 100 120 140 160Nominal ablation depth (microns)

0

20

40

60

80

Mea

sure

d ab

lati o

n d e

pth

(mi c

ron s

)

slope=0.4

Nominal (mm)

Mea

sure

d (

mm

)

Slope=0.4

AblatedAblated PMMA PMMA flatflat surfaces surfaces

Ablation profile

Surface location (mm)

Ab

lati

on

dep

th (

mm

)

-3 D

-6 DSlope=0.4

-12 -8 -4 0Nominal ablation depth (microns)

-6

-4

-2

0

Mea

sure

d ab

latio

n de

p th

( mic

rons

)

slope=0.39

M

easu

red

po

wer

ch

ang

e (D

)

Correction

Nominal correction (D)

-0,4

-0,2

0

0,2

0,4

0,6

0,8

1

1,2

Average Average asphericityasphericity

PRE

POST

Munnerlyn

Parabollic

Asp

heri

city Experimental

Flat PMMA profile*

(*) After application of scale factor

Experimental Experimental measurements measurements in PMMA in PMMA surfacessurfacesSpherical surfaces

Goodcorrespondence

3- D profiles

Post-operative“corneal”topography

Cornealtopography

Contact profilometry(Talysurf)

Problems

¸Inaccurate centerestimation

¸Inaccurate pre-opshape

PMMA PMMA spheres spheres data data processingprocessing

Decentration of topography measurement

Errors in ablation centration

Inaccuracy of corneal topographer at the periphery

¸Uses measured spherebefore ablation

¸Estimates center oftopography

¸Performs virtual rotationof ablation

PMMA PMMA spheres spheres data data processingprocessing

Custom software

Asymmetric in ablation zone

Symmetric withinablation zone

3- D profiles

-3 D

-6 D

Ablation depth

60 80 100 120 140 160Nominal ablation depth (microns)

0

20

40

60

80

Mea

sure

d ab

latio

n d e

p th

( mic

r on s

)

slope=0.397

Nominal (mm)

M

easu

red

(m

m)

Slope=0.4

AblatedAblated PMMA PMMA sphericalspherical surfaces surfaces

3- D profiles

-3 D

-6 D

AblatedAblated PMMA PMMA sphericaspherical surfacesl surfaces

-12 -8 -4 0Nominal correction (D)

-6

-4

-2

0

Mea

sure

d po

we r

cha

nge

(D)

Correction

Nominal correction (D) Mea

sure

d p

ow

er c

han

ge

(D)

Slope=0.4

-12 -8 -4 0Nominal correction (D)

-6

-4

-2

0

Mea

sure

d po

wer

ch a

n ge

(D)

3- D profiles

-3 D

-6 D

AblatedAblated PMMA PMMA sphericaspherical surfacesl surfaces

-12 -8 -4 0Nominal correction (D)

-6

-4

-2

0

Mea

sure

d po

wer

ch a

n ge

(D)

Correction

Nominal correction (D) Mea

sure

d p

ow

er c

han

ge

(D)

Flat surface

Spherical surface

0 5 10 15

-1

0

1

2

3

4

5A

sph

eric

ity

in c

orn

eaClinical data

Munnerlyn prof.

-1

0

1

2

3

4

5A

sph

eric

ity

in c

orn

ea Parabolic prof

-1

0

1

2

3

4

5A

sph

eric

ity

in c

orn

ea

Flat PMMA prof.*

-1

0

1

2

3

4

5A

sph

eric

ity

in c

orn

ea

Parabolic + eff.

Myopic correction (D)

AsphericityAsphericity

-1

0

1

2

3

4

5A

sph

eric

ity

in c

orn

ea

Spherical PMMA*

(*)After application of scale factor

Post-op PMMA sphere*

““CornealCorneal”” aberrationsaberrations

Post-op real cornea

*After alignment and application of scale factor

-6 D -5.5 D

ConclusionsConclusions

The Munnerlyn ablation profile itself should not increaseasphericity. A parabolic profile increases it slightly

Experiments on PMMA show a larger contribution of thelaser efficiency changes across the cornea to theincrease of asphericity than previously published

Ablation of PMMA spherical surfaces replicates thefindings on real corneas, leaving a small contribution tobiomechanical effects, once the scale ablation factor istaken into account

Relevant for customized corneal ablation

Funding

Grant CAM08.7/0010.1/2000Madrid Regional Government

Carl Zeiss, S.A., Spain

, Atlanta, GA

Grant BFM2002-02638Spanish Ministry of Science and Technology

Alcon Research Labs

www.vision.io.csic.es

Daniel Cano

Elena Garciade la Cera

PatriciaRosales

SergioBarbero

CarlosDorronsoro

LourdesLlorente

SusanaMarcos

VISUAL OPTICS & BIOPHOTONICS LAB. Instituto de Optica, CSIC

SergioOrtiz

www.vision.io.csic.es

0

1

2

0 5 10 15Myopic correction

in P

MM

A s

ph

ere

s

Munn + absorption (inPMMA)Parab + absorption (inPMMA)Ablated spheres NEW

60 80 100 120 140 160Nominal ablation depth (microns)

0

20

40

60

80

Mea

sure

d ab

latio

n de

p th

(mic

rons

)PMMA spheres

PMMA flat surf.