2. Excimer Laser Basics

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Transcript of 2. Excimer Laser Basics

EXCIMER LASER BASICSOriginal Author: Professor Theo Seiler Universittsaugenklinik Dresden Dresden, Germany

The Excimer LaserLaser Fundamentals

The Excimer Laser 193 nm (ArF) ultraviolet Laser Primarily Non-Thermal 10 - 20 Degree C Temperature Rise

No Collateral Tissue Damage Submicron precisionLaser Ablation Of A Human Hair

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The Excimer Laser Ablative Photodecomposition High Energy (6.4 eV) Irradiance Fluence Measure Of Energy Density

Tissue Ablation Threshold 50 mJ/cm2 Plume After ImpactISRS GLORY Fast Track to LASIK

The Photoablation Processlaser light hits tissue 10 psec collegen molecule undergoes conformal change bound water heats up 10 nsec molecules break 2 sec fragments are expelled as plasmaISRS GLORY Fast Track to LASIK

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Physical Side Effects Thermal Damage*heat diffusion *condensation

Mechanical Damage*direct damage *repulsive forces

Actinic Damage*primary radiation *secondary radiationISRS GLORY Fast Track to LASIK

Thermal DamagesThe travel time of a heat front for 1 micron in corneal tissue is approximately 5 sec Expellation of the 500 C hot ablation products is completed after approximately 1 sec

The excimer laser is a cold laserISRS GLORY Fast Track to LASIK

Thermal DamagesThe 500 C hot ablation may condensate at the ablation edges and form the pseudomembrane

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Physical Side Effects Thermal Damage*heat diffusion *condensation

Mechanical Damage*direct damage *repulsive forces

Actinic Damage*primary radiation *secondary radiationISRS GLORY Fast Track to LASIK

Mechanical Damages

Direct DamagePhotoablation leads to keatocyte loss (cell death and apoptosis) 100m deep. Repopulation within 6 weeks.

Repulsive ForcesPhotoablation products leave the corneal surface with suprasonic speed

Repulsive ForcesISRS GLORY Fast Track to LASIK

Mechanical Damages

Fast Effects of RepulsionPressure transients of up to 100 bar travel through the eye and orbit

Slow Effects of RepulsionTrampolin oscillations of the cornea may lead to central steep islands

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Thermal Damage *heat diffusion*condensation *repulsive forces

Physical Side Effects

Mechanical Damage *direct damage Actinic Damage*primary radiation *secondary radiationISRS GLORY Fast Track to LASIK

Actinic Damages

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

Secondary RadiationSecondary radiation has longer wavelengths that can reach the DNA

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Actinic DamagesDue to enzymatic repair the threshold of

mutagenicity of UV-radiation is approximately 10 W/cm2During a standard PRK, the secondary radiation reaches a level of

approximately 5 W/cm2

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Excimer Laser Beam Profile Fluence Homogeneous Versus Gaussian

Hot Spots

Homogeneous

GaussianISRS GLORY Fast Track to LASIK

Excimer LaserBroadbeam Lasers

Energy Distribution Masks, Diaphragms, Rotating Mirrors Smoothness Optical Zones - 2mm To 7mm

0.25 Micron StepsISRS GLORY Fast Track to LASIK

Excimer Laser Scanning Lasers 1-2 mm spot Computer controlled delivery Active/Passive tracking Myopic, astigmatic, hyperopic, patterns Custom patterns (topography linked) Irregular corneas Wavefront guided Prolate ablationsISRS GLORY Fast Track to LASIK

Excimer Lasers

Summit Apex Plus

VISX Star S2

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Excimer Lasers Scanning

Nidek EC 5000

LaserSight LSX

Autonomous

Technolas 217

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Munnerlyns 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 CorrectionISRS GLORY Fast Track to LASIK

Excimer LaserAblative DepthDiameter & Depth Per Diopter Diameter 3.00 mm 4.00 mm 5.00 mm 6.00 mm 7.00 mm 8.00 mm Depth 3.0 microns 5.3 microns 8.3 microns 11.9 microns 16.2 microns 21.2 micronsISRS GLORY Fast Track to LASIK

Excimer LaserAblative Depth6 mm

Depth Versus Correction100 90 80 Center Depth 70 60 Microns 50 40 30 20 10 0 0 -1 -2 5 mm 4 mm 3 mm

-3 -4 -5 -6 -7 -8 Correction (Diopters)

-9

-10

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Common LASIK Surgeons Error: The excimer is a simple, reliable trouble-free device

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

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Excimer Laser Naturally produces dirty beam requires extensive cleaning

Optical elements damaged by far UV Can cause cold spots, other calibration problems

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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)

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Common Error #2 Instrument 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, Fast Track to LASIK ISRS GLORY

ConclusionsNone 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.

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Thank you for your kind attention!!!

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