Excimer Laser Treatment

When it comes to wavefront guided technology, the VISX Star S4 IR from Johnson and 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 and Johnson Vision® is now 5 times more precise than their previous generation wavefront aberrometer (CustomVue) which in turn was 25 times more accurate than glasses and contact lenses.These aberrometry measurements made on the patient’s visual system are processed and then used to directly guide the new customized corneal profile created 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, choosing to rely on a treatment known as “wavefront optimized”.

This “wavefront optimized” terminology 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 generated by a subjection refraction of the patient using conventional glass lenses which is the identical measurement method used for low complexity traditional non-wavefront treatments.

The primary application of wavefront technology in the Alcon wavefront optimized algorithm is compensation for what is known as 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 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 and Johnson Vision® -VISX family of lasers has included cosine compensation in all of their laser 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.

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 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 using lenses in 0.25 diopter increments.

In contrast, wavefront measurements or point spread function refractors can objectively define the refractive error of the human eye down to 0.01 diopters. In addition to the subjectivity and lack of precision, the subjective refraction ignores aberrations that adversely affect vision quality such as coma, trefoil, spherical aberration, tetrafoil, pentafoil and higher order astigmatism. The results of treatments based on traditional excimer laser treatments are therefore known to be less accurate, have higher frequencies of needed surgical enhancement and a higher frequency of demonstrating side effects such as halos and glare.

Today, most traditional Lasik is performed at Lasik discount centers because these procedures are significantly less expensive to perform. 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.