Wavefront Technology and Custom Vue Wavefront guided Ablation

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How Refractive (Wavefront) Errors Affect Your Vision

The cornea and lens of the eye focus rays of light by bending (or refracting) them to focus an image on the retina at the back of the eye, much like a camera focuses images onto film.

Figure 1: On the left is a diagram showing how the eye focuses light rays to create a sharp image on the retina. The corresponding wavefront map of an ideal eye is displayed on the right.

The above figure shows an ideal eye with no focusing imperfections. All of the rays of light traveling through the eye focus to a single point on the retina at the back of the eye.

In reality, all eyes have some degree of imperfections. One way to measure the focusing errors of an eye is to measure the wavefront of the eye. This can be done with an instrument like the VISX Wavefront System. The wavefront map is a picture of the rays of light as they travel through the eye. The WaveScan System measures the wavefront errors by measuring light as it reflects out of the eye with a camera sensor.

The wavefront of a perfect eye has a flat surface because all of the light rays travel uniformly through the eye, as shown in figure1. The wavefront image on the left is the total aberrations of the eye. The wavefront image on the right is just the higher order aberrations of the eye. Note that both images are near perfect planar surfaces. The wavefront of an eye with imperfections is curved or wavy because some light rays reach the retina before others, and some light rays strike different locations on the retina than others. Wavefront errors include both simple and complex focusing errors. The simple wavefront errors (2nd order), which can be corrected with curved lenses (e.g., glasses or contact lenses) are also called refractive errors and include myopia, hyperopia and astigmatism.

Nearsighted (or myopic) eyes bend light too much so that light rays focus to a single point in front of the retina. Things that are far away look blurry because the rays are spread apart instead of focused when they strike the retina.

Figure 2: On the left is a diagram of a nearsighted eye showing the light rays focusing in front of the retina. The corresponding wavefront of a myopic eye shows a curved wavefront surface. The height difference between center and edge is indicated by the change in color scale. This is a concave surface.

Astigmatism causes the rays of light entering through different parts of the eye to focus unequally so that they do not ever form a single spot. Some rays may focus on the retina, but other rays focus in front of the retina.

Figure 3: On the left is a diagram of an astigmatic eye showing rays of light that do not ever come to a focus at one point. The corresponding wavefront map for this eye shows a surface that is curved more in one direction than the other. This is a surface shaped like a potato chip.

Hyperopic (or farsighted) eyes bend light too little so that light rays focus to a single point in back of the retina. Things that are far away look blurry because the rays are spread apart instead of focused when they strike the retina.

Figure 4: On the left is a diagram of a farsighted eye showing the light rays focusing in back of the retina. The corresponding wavefront of a hyperopic eye shows a curved wavefront surface. The height difference between center and edge is indicated by the change in color scale. This is a convex surface.

The WaveScan System can also measure complex focusing errors (higher order aberrations) not previously accounted for with conventional LASIK or PRK. On the left in figure 4 is a map of the total wavefront errors and on the right is a map showing just the complex errors (higher order). The combination of simple and complex wavefront errors in any eye is unique. The CustomVue treatment is "custom" because it includes information from the WaveScan System that is more individualized than what an eye surgeon uses to program a non-custom treatment.

Figure 4: On the left is a wavefront map of all the total wavefront errors and on the right is a wavefront map showing only the complex errors (higher order).

One of the ways to quantify the amount of aberration is a number called RMS (root mean squared). RMS is simply the amplitude or difference between the advanced portions of the wavefront (red) the retarded portion of the wavefront (blue) expressed in microns. The wavefront map above left shows a RMS of 14.81microns. This is the total amount of aberrations that includes myopia and some astigmatism. When the second order aberrations (myopia and astigmatism) are subtracted out, the total amount of higher order aberration is .59 microns (wavefront map above right). In figure 5, the results of wavefront measurements on two human eyes are shown. The dependence of the root mean square (RMS) wavefront error on the diameter of the pupil and amount of myopia is plotted. The data show that the optical aberration of the human eye increases considerably with larger pupil sizes and amount of myopia. This is why it is important to measure pupil size in light and dark lighting conditions. This is why the WaveScan System also uses pupil size and will not capture an image unless the pupil dilates up to 4 mm. in the dark. All of this is taken into account when deciding weather wavefront-driven custom treatment is right for your eyes.

Some studies show that patients with an RMS of greater than 0.4 or 0.5 microns in their higher order aberrations achieve a significant benefit from wavefront-driven ablation. More recent studies show that most patients get an improvement in night vision and overall quality of vision compared to non wavefront-driven ablation no mater what the RMS values are.