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EXCIMER LASER
BASICS
Original Author:
Professor Theo Seiler
Universitätsaugenklinik Dresden
Dresden, Germany
The Excimer Laser
Laser Fundamentals
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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
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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 ImpactPlume After Impact
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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
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Physical Side Effects
• Thermal Damage *heat diffusion *condensation
•Mechanical Damage *direct damage *repulsive forces
• Actinic Damage *primary radiation *secondary radiation
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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“
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Thermal Damages
The 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 radiation
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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
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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
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Physical Side Effects
• Thermal Damage *heat diffusion *condensation
• Mechanical Damage *direct damage *repulsive forces
• Actinic Damage *primary radiation *secondary radiation
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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
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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
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Excimer Laser
• Beam Profile– Fluence
• Homogeneous Versus Gaussian
– “Hot Spots”
HomogeneousHomogeneous GaussianGaussian
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Excimer LaserBroadbeam Lasers
• Energy Distribution – Masks, Diaphragms, Rotating Mirrors– “Smoothness”– Optical Zones - 2mm To 7mm
0.25 Micron Steps0.25 Micron Steps
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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
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Excimer Lasers
Summit Apex Plus VISX Star S2
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Excimer LasersScanning
Nidek EC 5000 LaserSight LSX
Technolas 217
Autonomous
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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
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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
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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
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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
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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 #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)
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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.
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Thank you for your kind attention!!!