Excimer BeamConsiderations forWavefront Surgery

Kevin Liedel OD, MSfor

George Pettit MD, PhD

Alcon, Orlando, FL USA

Overview

Corneal ablation behavior Ablation efficiency Treatment fidelity Ablation byproducts

Ablation Behavior

Most fundamental consideration for laserablation is how much tissue does the laserremove per unit energy

Human hair ablated with 193 nm excimer

Ablation Behavior For organic materials there is a fairly consistent

relationship over a large fluence range betweenthe incident laser fluence and the ablation depthin the target:

=

THFFd ln1

α

d = Ablation depth per pulse

α = Material absorption coefficient

F = Incident laser fluence per pulse

FTH = Ablation threshold fluence

Example Ablation Behavior forPolyethylene Terephthalate

0

0.05

0.1

0.15

0.2

0 100 200 300 400 500 600 700 800

ArF Pulse Fluence (mJ/cm2)

Abl

atio

n D

epth

per

Pul

se (m

m)

Ablation Behavior for Cornea Measuring the ablation rate in cornea directly is very

difficult to do Based on composite results from many studies we

estimate the behavior to be:

This is consistent with results we see in clinicalstudies

( )

⋅= 2/60ln34.0

cmmJFmd µ

Ablation Efficiency

In performing laser surgery we want to beefficient with the laser beam (i.e., we want the“biggest bang for the buck”) Minimize corneal exposure to UV radiation Minimize procedure time

This means we want to operate at a laserfluence that provides high efficiency: Laser efficiency = Volume removed / pulse energy

Ablation Efficiency For a “top-hat” (uniform fluence) laser beam the

efficiency is:

For a Gaussian laser beam (useful for reasonsexplained below) the efficiency is:

FFF

Efficiency THTophat α

=

ln

P

TH

P

Gaussian FFF

Efficiencyα2

ln2

=

Ablation Efficiency Curves

0

50

100

150

200

250

0 200 400 600 800 1000

Peak Fluence (mJ/cm2)

Abl

atio

n E

ffici

ency

(pl/m

J)

GaussianTop Hat

Ablation Efficiency

It is interesting to note that commercialdevices typically operate near the peak of therelevant efficiency curve

This adds credence to the basic assumptionof the corneal ablation behavior

Ablation Efficiency Curves

0

50

100

150

200

250

0 200 400 600 800 1000

Peak Fluence (mJ/cm2)

Abl

atio

n E

ffici

ency

(pl/m

J)

GaussianTop Hat

Summit

VISX

WaveLight

Alcon

Why Gaussian?

Accurate ablations are possible with anyablation profile IF enough pulses are delivered to the right places

Highly overlapped Gaussian pulses yieldparticularly smooth treated beds

0

0.5

1

1.5

2

-4 -3 -2 -1 0 1 2 3 4

Position (mm)

Abl

atio

n D

epth

(mic

rons

)

Cummulative Effect

Individual Pulses

Why Gaussian?

In addition Gaussian pulses operating nearthe peak of the efficiency curve are minimallyaffected by the corneal curvature

Corneal Angle of Incidence Reflection losses are small under

realistic angles Corneal reflectivity goes down during

ablation Peak fluence decreases with

increasing angle of incidence Peak ablation depth goes down

However, irradiated areaincreases at the same time Ablation volume minimally affected if

Gaussian beam operates nearefficiency peak

Spot Size

Gaussian beam with ideal peak fluence canstill be of any diameter

A bigger diameter beam will completesurgery faster

A smaller diameter beam will be able toperform more complex corrections

What is the right diameter for realistic customsurgeries?

Higher Order Aberrations6.5 mm analysis diameter

Higher Order Aberrations For untreated eyes aberrations are trivial

beyond ~6th order In some cases ~8th order terms may be

significant Therefore treatment beam should be effective

to ~8th order Mathematical modeling and test ablations in

plastic indicate beam with FWHM diameter of0.75 mm or less is suitable

8th Order Treatment Simulation(6.5 mm OZ + 1.25 mm Blend)

Z 80 Z 88

Pre-Ablation

Post-Ablation

Ablation Byproducts

The ablation event produces 2 things Debris plume in the air above the eye Some heating of the corneal bulk

For accurate, safe surgery these cannotinterfere with the treatment

Ablation Plume

The ablation plume disperses on amillisecond timescale after the pulse

Corneal Heating

Ablation ByproductConsiderations

By moving the laser beam around the cornea,and only periodically visiting any one site, wecan avoid plume interference and preventexcessive heating

Doing this allows for Gaussian beam firing at>> 100 Hz

Operation at ~1 kHz or above may beproblematic

What is Achievable?

4 diopter ablation in plastic

Summary

Many excimer beam considerations forrefractive surgery Energy, profile, size, speed, byproducts etc.

Control of these parameters can result in ahighly efficient beam optimized for clinicalapplication