LASIK Lasers: Which Excimer
Laser Is Best?
LASIK and other forms of laser refractive surgery, such as PRK and LASEK, all use a highly specialized excimer laser to reshape the cornea and correct refractive errors including myopia (nearsightedness), hyperopia (farsightedness) and astigmatism.
Excimer lasers have revolutionized the field of laser eye surgery and over the decades have greatly increased the safety, efficacy and predictability of corneal refractive surgery.
Excimer lasers have the ability to remove, or "ablate," microscopic amounts of tissue from the cornea's underlying stromal layer with a very high degree of accuracy and without damaging the surrounding corneal tissue.
Several FDA-approved excimer lasers are on the market, but one is not necessarily "better" than another. The most suitable excimer laser for you will depend on your specific requirements, such as your degree of refractive error, the size of your pupils and the thickness of your cornea. Your eye surgeon will advise which excimer laser is best for you.
Most experts agree that your surgeon's skill and experience, and an accurate evaluation of whether you're a suitable LASIK candidate, are far more critical factors affecting final LASIK outcomes than subtle differences between excimer lasers.
How Do Excimer Lasers Work?
The excimer laser emits a cool beam of ultraviolet light of a specific wavelength (typically 193 nanometers) to precisely remove corneal tissue. When the surface of the cornea is reshaped in the right way, it allows light rays to focus properly onto the retina for clear vision.
The high-energy pulses of ultraviolet light penetrate only a tiny amount of the cornea and have the ability to remove as little as 0.25 microns of tissue at a time. (One micron is a thousandth of a millimeter.)
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An excimer laser corrects nearsightedness by flattening the cornea; it corrects farsightedness by making the cornea steeper. Also, astigmatism can be corrected by smoothing an irregular cornea into a more normal shape.
Excimer lasers are controlled by computer settings programmed to correct your specific refractive error. Your surgeon will program the excimer laser with the desired measurements in order to reshape your cornea and treat your prescription. The quantity and pattern of tissue removal are unique to each patient.
Most modern excimer lasers have automated eye-tracking systems that monitor eye movements and keep the laser beam on target during surgery. Studies have shown that eye trackers produce better outcomes and decrease LASIK complications compared with past lasers that did not use eye-tracking systems.
Types of Modern Excimer Lasers Used for LASIK
In the United States, all approved excimer lasers meet safety and effectiveness criteria established by the FDA. The main differences among LASIK lasers are the patterns they use to deliver the laser beam and track the eye during laser eye treatment. Certain people may be better off with one laser over another.
FDA-approved excimer lasers shown here are used in Abbott Medical Optical's iLASIK system with the Visx Star S4 IR (top) and Alcon's OptiLASIK system with the Allegretto Wave Eye-Q.
Spot scanning lasers. Spot scanning (or "flying-spot") lasers, which are the most common, use small-diameter laser beams (0.8 to 2 mm) scanned across the cornea to produce the treatment zone. This approach has the potential to produce the smoothest corneal treatments, to more readily allow customized treatments and to better treat irregular astigmatism.
Slit scanning lasers. Slit scanning lasers use relatively small beams linked to a rotational device with slit holes that enlarge. The laser beams scan across these holes during surgery, producing a gradually enlarging ablation zone. This approach produces a uniform beam that creates smoother treatments than older broad-beam lasers. But slit scanning lasers have the potential to cause a slightly greater risk of decentration and overcorrection unless an eye tracker is used, or with an inexperienced surgeon.
Wavefront-guided lasers. Many excimer lasers, whether spot or slit scanning, are connected to a device that detects and "maps" defects in the eye's optical system, based on how light travels through the eye. These wavefront devices produce a custom LASIK treatment that is unique to each patient. Both slit scanning and spot scanning lasers can be used for wavefront-guided treatments.
Pupil Size, Ablation Speed and Patient Comfort
In recent years, increasing evidence has indicated that larger pupil sizes may affect laser vision correction outcomes. If your pupil expands in low light beyond the diameter of the laser treatment zone on the cornea, you may experience vision problems such as glare and halos at night.
Some surgeons believe the diameter of the laser ablation should be at least as large as your pupil in dim light. If you have larger pupils, the type of excimer laser may be important in relation to how large the treatment zone (diameter) the laser is capable of creating. You should discuss this with your surgeon.
Treatment times also differ among lasers, ranging from 30 to 60 seconds or longer. You may consider that important in terms of your comfort as you undergo a procedure.
You also might want to ask whether your surgeon uses a femtosecond laser or a surgical instrument (microkeratome) to create the corneal flap in LASIK eye surgery and how these two approaches might differ in terms of your comfort. Many surgeons take opposing sides in the microkeratome vs. femtosecond LASIK debate.
What To Consider When Comparing Different LASIK Lasers
When evaluating lasers for LASIK, PRK or other corneal refractive surgery, you may be drawn to information gathered during FDA clinical trials leading up to approval. But you should keep these points in mind:
Today's results are often better than FDA data. By definition, FDA trials occur during the early period of a laser's life cycle. Manufacturers are allowed to and often do make technical improvements to the instruments, sometimes even while the lasers are still under investigation. One such improvement has been the introduction of wavefront LASIK technology, which delivers a more precise and customized refractive correction than earlier lasers.
The surgeon's technique evolves as well, and usually becomes more advanced than the technique used in the FDA trials. This, together with increasing surgeon experience over time, means that results in actual clinical practice often are better than the initial FDA data.
FDA data in one study cannot fairly be compared with FDA data in another study. Manufacturers go to the FDA with various study designs, which often have differing patient-enrollment criteria and endpoints. Although all studies must answer certain basic questions regarding safety and effectiveness, they are not designed to be compared with one another.
A true comparison of Laser A and Laser B would require randomized clinical trials, in which patients would be randomly assigned to receive surgery by one laser or the other over the same time period by the same surgeons. FDA trials don't do that although other, non-FDA studies sometimes do.
Studies don't cover every possibility. Even if a certain characteristic or condition appears to exclude you from treatment because of how the approval for the excimer laser is worded, your eye surgeon still may consider you a candidate.
Also, another laser with specific approval for your characteristic or condition may not necessarily do a better job.
Remember that studies have differing designs, and people with your characteristic or condition may have been excluded. Even if they were included in the study, results may have been inconclusive because of factors such as too few people or insufficient data for determining statistical or clinical significance.
Once the FDA approves a laser, your surgeon can use it any way he or she deems appropriate. This is true of all FDA-approved drugs and devices. This is commonly referred to as "off-label" use of an FDA-approved drug, device or laser. It's called a physician's practice-of-medicine prerogative, and there's nothing inherently wrong with it. It's 100 percent legal for physicians to use devices and medications in "off-label" mode.
In fact, most advances in medicine occur because of off-label uses of devices or medications. So this type of use can be critical in modern medicine.
For example, without off-label use of aspirin, no one ever would have known that aspirin (originally FDA-approved for pain control) is vital for reducing the risk of heart attacks.
Excimer Laser Features and Indications
In the following chart, OZ stands for optical zone, meaning the maximum treatment diameter that can be targeted effectively for correction with a specific laser. TZ stands for transition zone, which is the blend area beyond the OZ to the edge of the entire laser diameter.
|Alcon LADARVision 4000 & CustomCornea (laser plus wavefront device to guide laser)||Indications: myopia: up to -8.00 D with or without astigmatism (up to -4.00 D); hyperopia and hyperopic astigmatism: up to +5.00 D (near vision problems) and astigmatism causing distance vision problems up to -3.00 D
Type of Laser Beam: scanning spot (0.8 mm)
Optical Zone and Treatment Zone: OZ is 5.5 mm; TZ is 7.5 mm
FDA Approval Years: 2002 (myopia with or without astigmatism); 2006 (hyperopia and hyperopic astigmatism)
|Bausch + Lomb Technolas 217A and Technolas 217z Zyoptix (laser plus wavefront device to guide laser, approved 2003)
Advanced Control Eyetracking (ACE) (rotational eye tracking system approved 2009 for Technolas platforms)
|Indications: myopia: up to -12.00 D with or without astigmatism (up to -3.00 D); hyperopia: up to +4.00 D with or without astigmatism (up to +2.00 D)
Type of Laser Beam: scanning spot (2.0 mm)
Optical Zone and Treatment Zone: OZ is 6.0 mm; TZ is 7.0 mm
FDA Approval Years: 2000 (myopia from -1.00 to -7.00 D); 2002 (myopia up to -11.00 D); 2003 (hyperopia with or without mixed astigmatism)
|Carl Zeiss Meditec MEL 80||Indications: myopia: up to -7.00 D with or without astigmatism (up to -3.00 D); hyperopia: up to +5.00 D with or without astigmatism (up to +3.00 D)
Type of Laser Beam: scanning spot (0.7 mm); Gaussian profile with more energy applied centrally
Optical Zone and Treatment Zone: OZ is 6.0 to 7.0 mm; TZ is 7.7 to 8.9 mm
FDA Approval Years: 2006 (myopia with or without astigmatism); 2011 (hyperopia with or without astigmatism)
|Nidek EC-5000||Indications: myopia: -1.00 to -14.00 D with or without astigmatism (less than 4.00 D); hyperopia: +0.50 to +5.00 D and up to +2.00 D astigmatism
Type of Laser Beam: scanning slit (7.0 x 2.0 mm)
Optical Zone and Treatment Zone: OZ is 5.5 mm; TZ is 7.0 mm
FDA Approval Years: 2000 (myopia from -1.00 to -14.00 D); 2006 (hyperopia and hyperopic astigmatism)
|Visx Star S4 & WaveScan WaveFront System (laser plus wavefront device to guide laser)||Indications: myopia: up to -6.00 D with or without astigmatism (up to -3.00 D)
Type of Laser Beam: variable scanning spot beam (0.65 mm to 6.5 mm)
Optical Zone and Treatment Zone: OZ is 4.0 to 9.0 mm; TZ is 4.5 to 9.5 mm
FDA Approval Year: 2003
|Visx Star S4 IR & CustomVue (laser plus wavefront device to guide laser)||Indications: myopia: up to -6.00 D with or without astigmatism (up to -3.00 D); hyperopia: up to +3.00 D and up to +2.00 D of astigmatism; mixed astigmatism: up to 5.00 D
Type of Laser Beam: same as for S4
Optical Zone and Treatment Zone: OZ is 6.0 mm; TZ is 9.0 mm
FDA Approval Year: 2005
|WaveLight Allegretto Wave||Indications: myopia: up to -12.00 D with or without astigmatism (up to -6.00 D); hyperopia: up to +6.00 D with or without astigmatism (up to +5.00 D, not exceeding mean spherical equivalent or total refractive error of +6.00 D); mixed astigmatism: up to 6.00 D
Type of Laser Beam: scanning spot beam (0.95 mm) with emphasis on applying more energy centrally (Gaussian profile)
Optical Zone and Treatment Zone: OZ is 4.5 to 8.0 mm; TZ is 5.2 to 8.7 mm for spherical treatments, 7.0 to 9.0 mm for cylindrical and spherico-cylindrical treatments
FDA Approval Years: 2003 (myopia and hyperopia); 2006 (mixed astigmatism)
|WaveLight Allegretto Wave with Allegro Analyzer (laser plus wavefront device to guide laser)||Indications: myopia: up to -7.00 D with or without astigmatism (up to 3.00 D); mixed astigmatism: up to 6.00 D
Type of Laser Beam: same as for Allegretto Wave
Optical Zone and Treatment Zone: same as for Allegretto Wave
FDA Approval Years: 2006; 2007 (mixed astigmatism)
|Notes: "D" is an abbreviation for "diopters." While FDA approval is based on studies with these levels of diopters, individual doctors are free to use their own discretion in deciding what is best for their patients. For example, a doctor may choose to use a laser for a patient whose vision falls outside the above ranges or may decide against using any laser on a patient with -13.50 D of myopia, even though some are approved for up to -14.00 D. It is important for you to discuss risks and benefits with your doctor before consenting to undergo laser vision correction surgery.|
While specific excimer laser technology plays a key role, ultimately it is your surgeon's skill and experience and your suitability as a candidate that will be the most important factors affecting your LASIK outcome.
About the Author: Brian Boxer Wachler, MD, is an ophthalmologist and refractive surgeon at the Boxer Wachler Vision Institute in Beverly Hills, Calif. He has pioneered treatments for keratoconus, participated in many FDA clinical trials for new refractive surgery technologies and written several books. He is a member of All About Vision's editorial advisory board.
Aimee Surtenich also contributed to this article.
[Page updated May 2014]