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Like most modern technologies, Excimer lasers have seen significant improvements through the past 20 years. As a result, today’s Excimer lasers are far more sophisticated, safer and significantly more precise than previous generations.
The most significant technology changes with excimer lasers over the years include the following:
- Improved algorithms for defining the best optics for human vision
- Wavefront guided treatments
- Wavefront optimized treatments
- Topography-guided treatments
- Traditional or conventional spherocylindrical lens-based treatments (opens in new tab) (opens in new tab)
Active eye tracking
- Ability to compensate for cyclorotation of the eye during surgery
- Ability to compensate for the migration of the pupil centroid during surgery
- Ability to compensate for the adverse thermal and physiologic effects of laser energy
Best Human Optics
There is a significant difference in the quality of human vision when comparing wavefront, topographic, and traditional excimer laser treatments. It is also important to recognize that even within the broad category of wavefront-guided treatments (opens in new tab), not all wavefront-guided procedures are created equal. Accurately measuring and delivering wavefront-guided excimer laser treatments to the eye in a manner that respects the cornea’s physiology is complex. As a result, not all laser manufacturers can deliver the best wavefront technology.
Wavefront Guided versus Wavefront Optimized
When it comes to wavefront-guided technology, the VISX Star S4 IR from Johnson & Johnson Vision® is currently superior to any other technology. Because it uses a very high definition Hartmann-Shack system with over 1000 individual ‘lenslets’ combined with Fourier algorithms to define even the most minute wavefront distortions, it is significantly more precise and more accurate than every other wavefront system in the world, including the WaveLigtht Allegretto from Alcon. The newest iDesign wavefront aberrometer from Johnson & Johnson Vision® is now 5 times more precise than its previous-generation wavefront aberrometer (CustomVue), which in turn was 25 times more accurate than glasses and contact lenses. These aberrometry measurements of the patient’s visual system are processed and used to directly guide the creation of the new customized corneal profile by the VISX Star S4 IR excimer laser.
In the United States, compared to the VISX Star S4 IR, the second most popular excimer laser system is the WaveLight Allegretto manufactured by Alcon. In contrast to VISX, Alcon suggests their users avoid using its Tscherning’s aberrometer because it has very limited accuracy, a poor dynamic range and uses Zernike interpolation rather than Fourier transformation thereby losing measurement detail. For the vast majority of its treatments, the Alcon WaveLight Allegretto typically does not even use customized wavefront-guided technology, instead relying on a treatment known as “wavefront optimized”.
The term “wavefront optimized” is highly confusing for patients because it implies that these treatments are based on actual wavefront testing. However, it should be noted that an Alcon wavefront optimized treatment is never based on any wavefront diagnostic measurement from the patient’s eye. With the WaveLight system and its wavefront-optimized treatments, actual wavefront tests are simply not performed. Patients are treated using the same spherocylindrical measurements obtained from a subjective refraction with conventional glass lenses, the same method used for low-complexity traditional non-wavefront treatments.
The primary application of wavefront technology in the Alcon wavefront-optimized algorithm is to compensate for the ‘cosine effect’. The cosine effect refers to the development of excessive positive spherical aberration induced by virtue of less efficient photoablation occurring when the excimer beam strikes the corneal surface more tangentially towards the periphery. Alcon attempts to correct for the cosine problem by applying additional laser pulses to the peripheral cornea. However, because this is a one-size-fits-all approach, patients that present with negative spherical aberration preoperatively may have their aberrations made worse.
Although, on average, the wavefront-optimized approach is superior to the older broad-beam technology that did not have cosine compensation, it represents a very blunt-edged tool that fails to provide surgeons with the same level of personalized customization of the corneal treatment profile that can be delivered using a truly wavefront-guided approach. One fact not fully understood by many LASIK surgeons is that the Johnson & Johnson Vision®-VISX family of lasers has included cosine compensation in all its systems since 1991.
As a result, in the United States, patients who want the best visual result from LASIK have only one choice that delivers wavefront guided treatments. That system is the VISX Star S4 IR.
Topography Driven Algorithms
Topographically guided excimer laser treatments have become available in recent years. Although there are many challenges to developing a successful topographically driven excimer algorithm, the most challenging issue is the inaccuracy inherent in measuring corneal height data. Such errors are particularly large compared to wavefront measurements. Corneal topography or tomography devices available today are at best accurate to a few microns, particularly in the corneal periphery. This is because data interpolation errors inherent in the measurement process are compounded as one moves from the corneal center to the periphery. As a result, topography driven treatments demonstrate errors in the 0.25 diopter range. In contrast, wavefront measurements are accurate and reproducible at the sub-micron level which produces treatments errors that are 25 times smaller in magnitude than topographically derived measurements.
Traditional Spherocylindrical Lens Based Algorithms
Traditional or conventional excimer laser treatments are the oldest treatment algorithms and are based upon the patient’s subjective refraction. When the surgeon has you look through a series of spherical and cylindrical lenses housed in a device known as a “phoropter” at a Snellen letter chart and asks you which lens is better “lens 1 or lens 2” you are creating a subjective refraction. Such an approach has multiple challenges. First, as noted, it is highly “subjective”. The patient must provide correct responses or the final result will be invalid. In addition, the lens technique can only be performed with lenses in 0.25-diopter increments.
In contrast, wavefront measurements or point-spread function refractors can objectively quantify the refractive error of the human eye down to 0.01 diopters. In addition to its subjectivity and lack of precision, subjective refraction ignores aberrations that adversely affect visual quality, such as coma, trefoil, spherical aberration, tetrafoil, pentafoil, and higher-order astigmatism. The results of traditional excimer laser treatments are therefore known to be less accurate, to have a higher frequency of required surgical enhancement, and to have a higher frequency of side effects such as halos and glare.
Today, most traditional LASIK procedures are performed at LASIK discount centers because they are significantly less expensive. However, because wavefront guided treatments are more accurate and create fewer such unwanted optical side effects, at Will Vision and Laser Centers we only employ traditional excimer ablation algorithms in the rare event when accurate wavefront measurements are not available.
Active Eye Tracking
Will Vision and Laser Centers introduced active eye tracking to the Pacific Northwest in 2001. Prior to that time, accuracy of excimer laser pulse placement on the cornea during LASIK was entirely determined by the ability of a patient to fixate on a blinking target. As scary as this method was for traditional treatment algorithms, it is entirely unsuitable for advanced wavefront or topographically driven treatments.
Compensation for X, Y, and Z
Accurate placement of a laser pulse on the corneal surface is dependent upon multiple factors. First, the eye can rapidly move off target in eye motions known as “saccades”. Patient head or eye movement can also cause the entire eye to drift off-axis or move closer or further away from the laser delivery system.
The VISX Star S4 IR from Johnson & Johnson Vision® is the only laser system in the United States that uses 3-D eye tracking (X, Y, and Z axes), thereby compensating for all possible intraoperative eye-position errors. All other excimer laser system restrict their eye tracking systems to 2 dimensions (X and Y axes), and systematically ignore Z-axis induced eye position errors.
Cyclorotation Compensation
For highly complex wavefront or topographically methods the amount of corneal tissue to be removed at any point on the cornea is highly specific. If the eye cyclorotates (turns like the hands of an analog watch) during the excimer laser treatment, the cornea will no longer be positioned towards the laser delivery system in the same orientation as when the eye measurements were taken. As a result, a “point-to-point” referencing system is required to define the orientation of the cornea during the eye measurement and the excimer laser treatment, ensuring the orientation of the ocular surface is identical between events.
This point-to-point referencing is called cyclorotation compensation. Currently in the United States, the only excimer laser system capable of cyclorotation compensation is the VISX Star S4 IR from Johnson and Johnson Vision®. The “IR” stands for “iris registration” referring to the complex iris recognition technology used to accurately manage this challenge. The VISX computer system aligns the iris images captured during the measurement process to the iris image that is presented during the treatment event. The computer software then rotates the treatment to align itself to the measurement orientation. This realignment occurs actively during the laser photoablation process, thereby ensuring true intraoperative point-to-point eye registration.
Unfortunately, aside from the VISX Star S4 IR from Johnson & Johnson Vision®, no other US-based excimer system uses iris registration or any comparable technology. As a result, these other systems systematically ignore cyclotorsion-induced treatment errors. These errors are not insignificant. For example, a 5-degree cyclorotation will induce a 20% error in simple astigmatism correction. They can only be ignored at the risk of producing an inaccurate result and having an elevated rate of LASIK retreatment.
Compensation for Pupil Centroid Migration
All measurements of refractive error are referenced to the patient’s visual axis. The closest approximation to that axis is the center of the patient’s pupil, or “pupil centroid”. Just at the eye can cyclorotate, the size of the patient’s pupil is dynamic and depends upon the ambient light level, the state of eye focus, and the patient’s emotional state. Moreover, as the pupil dilates (or constricts), the pupil centroid moves. As a result, if we reference the excimer laser treatment to the “visual axis” defined by the pupil centroid, we have a moving target that frequently differs markedly between the measurement and treatment events.
Not surprisingly, the engineers at Johnson and Johnson Vision® include in the iris registration software the ability to reference the pupil centroid position during measurement and then realign the treatment to the actual pupil centroid found during treatment. Once again, no other laser system in the United States employs compensation for pupil centroid drift in its treatment algorithms. As a result, they systematically misalign their treatments, inducing unwanted higher-order aberrations such as coma.