2. Excimer Laser Basics

transcript

EXCIMER LASER

BASICS

Original Author:

Professor Theo Seiler

Universitätsaugenklinik Dresden

Dresden, Germany

The Excimer Laser

Laser Fundamentals

ISRS GLORY Fast Track to LASIK

The Excimer Laser

• 193 nm (ArF) ultraviolet Laser• Primarily Non-Thermal

– 10 - 20 Degree C Temperature Rise

• No Collateral Tissue Damage• Submicron precision

Laser Ablation Of ALaser Ablation Of AHuman HairHuman Hair

The Excimer Laser

• Ablative Photodecomposition– High Energy (6.4 eV)– Irradiance

• Fluence• Measure Of Energy Density

– Tissue Ablation Threshold• 50 mJ/cm2

Plume After ImpactPlume After Impact

The Photoablation Process

laser light hits tissue

collegen molecule undergoes conformal change

bound water heats up

molecules break

fragments are expelled as plasma

10 psec10 psec

10 nsec10 nsec

2 2 μμsecsec

Physical Side Effects

• Thermal Damage *heat diffusion *condensation

•Mechanical Damage *direct damage *repulsive forces

• Actinic Damage *primary radiation *secondary radiation

Thermal Damages

The travel time of a heat front for 1 micron in corneal tissue is

approximately 5 μsec

Expellation of the 500° C hot ablation products is completed after

approximately 1 μsec

The excimer laser is a “cold laser“

Thermal Damages

The 500° C hot ablation may condensate at the ablation edges and form the

pseudomembrane

Physical Side Effects• Thermal Damage *heat diffusion *condensation

• Mechanical Damage *direct damage *repulsive forces

Mechanical Damages

•Direct Damage

Photoablation leads to keatocyte loss (cell death and apoptosis) 100μm deep. Repopulation within 6 weeks.

•Repulsive Forces

Photoablation products leave the corneal surface with suprasonic speed

Repulsive Forces

Mechanical Damages

•Fast Effects of Repulsion

Pressure transients of up to 100 bar travel through the eye and orbit

•Slow Effects of Repulsion

Trampolin oscillations of the cornea may lead to central steep islands

Physical Side Effects

• Thermal Damage *heat diffusion *condensation

• Mechanical Damage *direct damage *repulsive forces

Actinic Damages

•Primary Radiation

Due to the small penetration depth the 193 nm-light can not reach the cell nucleus (cytoplasmatic shielding)

•Secondary Radiation

Secondary radiation has longer wavelengths that can reach the DNA

Actinic Damages

Due to enzymatic repair the threshold of mutagenicity of UV-radiation is

approximately 10 μW/cm2

During a standard PRK, the secondary radiation reaches a level of

approximately 5 μW/cm2

Excimer Laser

• Beam Profile– Fluence

• Homogeneous Versus Gaussian

– “Hot Spots”

HomogeneousHomogeneous GaussianGaussian

Excimer LaserBroadbeam Lasers

• Energy Distribution – Masks, Diaphragms, Rotating Mirrors– “Smoothness”– Optical Zones - 2mm To 7mm

0.25 Micron Steps0.25 Micron Steps

Excimer LaserScanning Lasers

• 1-2 mm spot• Computer controlled delivery• Active/Passive tracking• Myopic, astigmatic, hyperopic, patterns• Custom patterns (topography linked)

– Irregular corneas– Wavefront guided– Prolate ablations

Excimer Lasers

Summit Apex Plus VISX Star S2

Excimer LasersScanning

Nidek EC 5000 LaserSight LSX

Technolas 217

Autonomous

Munnerlyn’s Formula

• Depth (mM)=Diopters/3 x O.Z. Squared• As Optical Zone forcreases for Diameter, The

Depth Of The Ablation Is Greater To Achieve Any Given Correction

Excimer LaserAblative Depth

Diameter & Depth Per DiopterDiameter & Depth Per Diopter

DiameterDiameter DepthDepth3.00 mm3.00 mm 3.0 microns3.0 microns4.00 mm4.00 mm 5.3 microns5.3 microns5.00 mm5.00 mm 8.3 microns8.3 microns6.00 mm6.00 mm 11.9 microns11.9 microns7.00 mm7.00 mm 16.2 microns16.2 microns8.00 mm8.00 mm 21.2 microns21.2 microns

Excimer LaserAblative Depth

Depth Versus CorrectionDepth Versus Correction10010090908080707060605050404030302020101000

0 -1 -2 -3 -4 -5 -6 -7 -8 -9 -100 -1 -2 -3 -4 -5 -6 -7 -8 -9 -10

Center DepthCenter DepthMicronsMicrons

Correction (Diopters)Correction (Diopters)

6 mm6 mm5 mm5 mm

4 mm4 mm

3 mm3 mm

Common LASIK Surgeon’s Error:“The excimer is a simple, reliable trouble-

free device”

• Although relatively simple from the outside, the excimer laser is an extremely complex device

Excimer Laser• Naturally produces “dirty”

beam– requires extensive

“cleaning”

• Optical elements damaged by far UV

• Can cause “cold spots”, other calibration problems

Excimer Calibration

• Calibration extremely critical– crude assay– surgeon needs to know technique

• Affected by gas mixture, state of optics, energy level, environment (humidity, temp, perfumes)– Control OR environment

• Remember that the refractive results are produced by the laser (the actual “refractive surgeon”)

Common Error #2Instrument Reliability

• Know the instrument• Participate in calibration

– multi-user center: good working relationship with laser technician

• Standardize environmental conditions in OR• Prepare yourself, patient, family for possible

cancellation (“Bataan Death March”)• Monitor your results (overRx, underRx, astigmatism)

Conclusions

None of the physical side effects of excimer photoablation of the cornea is really dangerous.

Endothelial cell damage due to pressure transients as well as resonance oscillations of the eye need to be further investigated.

Thank you for your kind attention!!!

2. Excimer Laser Basics

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