EXCIMER LASER BASICS Original Author: Professor Theo Seiler Universitätsaugenklinik Dresden Dresden, Germany The Excimer Laser Laser Fundamentals 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 A Human Hair ISRS GLORY Fast Track to LASIK The Excimer Laser • Ablative Photodecomposition – High Energy (6.4 eV) – Irradiance • Fluence • Measure Of Energy Density – Tissue Ablation Threshold • 50 mJ/cm2 Plume After Impact ISRS GLORY Fast Track to LASIK The Photoablation Process laser light hits tissue 10 psec collegen molecule undergoes conformal change bound water heats up 10 nsec molecules break 2 μsec fragments are expelled as plasma ISRS GLORY Fast Track to LASIK + Physical Side Effects • Thermal Damage *heat diffusion *condensation •Mechanical Damage *direct damage *repulsive forces • Actinic Damage *primary radiation *secondary radiation ISRS GLORY Fast Track to LASIK 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“ ISRS GLORY Fast Track to LASIK Thermal Damages The 500° C hot ablation may condensate at the ablation edges and form the pseudomembrane ISRS GLORY Fast Track to LASIK Physical Side Effects • Thermal Damage *heat diffusion *condensation • Mechanical Damage *direct damage *repulsive forces • Actinic Damage *primary radiation *secondary radiation ISRS GLORY Fast Track to LASIK 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 ISRS GLORY Fast Track to LASIK 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 ISRS GLORY Fast Track to LASIK • Thermal Damage *heat diffusion *condensation *repulsive forces Physical Side Effects • Mechanical Damage *direct damage • Actinic Damage *primary radiation *secondary radiation ISRS GLORY Fast Track to LASIK 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 ISRS GLORY Fast Track to LASIK 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 ISRS GLORY Fast Track to LASIK Excimer Laser • Beam Profile – Fluence • Homogeneous Versus Gaussian – “Hot Spots” Homogeneous Gaussian ISRS GLORY Fast Track to LASIK Excimer Laser Broadbeam Lasers • Energy Distribution – Masks, Diaphragms, Rotating Mirrors – “Smoothness” – Optical Zones - 2mm To 7mm 0.25 Micron Steps ISRS 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 ablations ISRS GLORY Fast Track to LASIK Excimer Lasers Summit Apex Plus VISX Star S2 ISRS GLORY Fast Track to LASIK Excimer Lasers Scanning Nidek EC 5000 LaserSight LSX Autonomous Technolas 217 ISRS GLORY Fast Track to LASIK 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 ISRS GLORY Fast Track to LASIK Excimer Laser Ablative Depth Diameter & 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 microns ISRS GLORY Fast Track to LASIK Excimer Laser Ablative Depth 6 mm Depth Versus Correction 100 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 ISRS GLORY Fast Track to LASIK 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 ISRS GLORY Fast Track to LASIK Excimer Laser • Naturally produces “dirty” beam – requires extensive “cleaning” • Optical elements damaged by far UV • Can cause “cold spots”, other calibration problems ISRS GLORY Fast Track to LASIK 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”) ISRS GLORY Fast Track to LASIK 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 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. ISRS GLORY Fast Track to LASIK Thank you for your kind attention!!! ISRS GLORY Fast Track to LASIK