Wavefront Technology and Custom Vue Wavefront guided Ablation

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The VISX Wavescan Wavefront System

Before your CustomVue PRK or LASIK treatment is programmed into the laser, you must have one or more WavePrint measurements taken by the WaveScan System. The WaveScan System is a tabletop diagnostic system that measures your eyes with a laser and special cameras 25 times more precisely than with standard methods. The VISX WaveScan Wavefront System is a Hartmann-Shack device named after the inventors.

The WaveScan System directs a very small round laser beam (red) into the eye and illuminates a very small round area on the retina (yellow)(see figure below).

The small laser spot on the retina acts as a point light source and the light propagates back out through the eye onto a special lenslet array and a video sensor (CCD chip or camera). As the light propagates out of the eye it is bent (refracted) and quite often distorted (aberrated) by the lens or the cornea before the round pencil of light reaches the lenslet array. The light propagated out of the eye is shaped round by the round pupil.

This is an eye (see above figure) with no aberration since the CCD image (extreme left) is made up of a regular lattice of image points and the wave front is planar or regular.

This is an eye (see above figure) with aberrations since the CCD image (extreme left) is made up of an irregular lattice of image points and the wave front is irregular. Note that the irregular lattice of image points is round. This is due to the round pupil.

The lenslet array consists of many micro-focusing lenses arranged with their centers in a square grid pattern. The spacing of the centers of the many micro-focusing lenses are all the same and of a known distance. The round pencil of light that propagates out of the eye falls onto the lenslet array and each micro-focusing lens. Each micro lens focuses the light onto the video sensor and forms a regular lattice of image points for a perfect plane wave of light emerging from a perfect unaberrated eye (see figure on the left below).

In the eye with a refractive error (myopia, astigmatism or hyperopia) and or other higher order aberrations, the condition of the lattice of image points is different. Each micro lens focuses the light onto the video sensor and forms an irregular lattice of image points from the aberrated wavefront of light emerging from an aberrated eye (see figure above right). The red focused spots are displaced from the regular grid of yellow points. The amount of displacement of each red spot is noted and the local wavefront slopes (first derivative in the x and y direction) and the local wavefront curvatures (second partial derivatives) are computed for each displaced red spot. The computer in the WaveScan System then constructs the entire wavefront shape from all the local wavefront slopes and local wavefront curvatures for the eye being scanned. This is done by a special mathematical smoothing function called Zernike polynomials. The computer then forms a wire diagram of the wavefront surface and then color-codes it in microns of deviation from the ideal planar shape. It can be displayed in two or three dimensions.

Cold colors (blue) represent photons that lag or are slowed up from a longer path length or more tissue to travel through. Hot colors (red) represent photons that are advanced or are sped up from a shorter path length or less tissue to travel through.

The shape of the wavefront of the eye is described by the Zernike polynomials and is broken up into low order aberrations and higher order aberrations. A combination of these vision aberrations (imperfections) are what makes your vision imperfections 100% unique. The low order aberrations are 2nd order and consist of defocus (myopia and hyperopia) and astigmatism. These are the aberrations that most people have and that are treated with conventional LASIK and PRK. The figures below are two and three-dimensional representations of the wavefronts from eyes that have pure myopia, hyperopia and astigmatism.

Myopia

Hyperopia

Astigmatism

The higher order aberrations are 3rd 4th 5th 6th…. order. These are the aberrations that some people have but in much smaller and varying amounts. Before wavefront technology, all we corrected was 2nd order aberrations. We defined the 2nd order aberrations by doing refractions in the examining lanes. An example is myopic astigmatism is -6.50-2.25x90. The first number is the amount of myopia. The second number is the amount of astigmatism and the last number is the orientation (axis) of the astigmatism. This fully defined the 2nd order aberrations and this is what was typed into the excimer lasers computer for conventional LASIK and PRK. We can define any order of aberration by the WaveScan System but can currently correct up to 6th order aberrations with the Excimer laser. Third order aberrations are trefoil and coma. The figures below are two-dimensional representations of the wavefronts from eyes that have trefoil and coma. Note that both coma and trefoil have an orientation or axis as well as axial symmetry.

 

Fourth order aberrations are tetrafoil, high order astigmatism and spherical. The figures below are two-dimensional representations of the wavefronts from eyes that have tetrafoil, high order astigmatism and spherical aberrations. Note that both tetrafoil and high order astigmatism have an orientation or axis as well as axial symmetry.

 

Fifth order aberrations are pentafoil, complex trefoil and complex coma. The figures below are two-dimensional representations of the wavefronts from eyes that have pentafoil, complex trefoil and complex coma. Note that pentafoil, complex trefoil and complex coma have an orientation or axis as well as axial symmetry.

 

Sixth order aberrations are hexafoil, complex tetrafoil, complex astigmatism and plus spherical. The figures below are two-dimensional representations of the wavefronts from eyes that have hexafoil, complex tetrafoil, complex astigmatism and plus spherical. Note that hexafoil, complex tetrafoil and complex astigmatism have an orientation or axis as well as axial symmetry.

Just like a fingerprint, each person's visual imperfections (optical aberrations) are 100% unique to each of their eyes. A combination of some of the above aberrations in a variable amount are what makes each person's vision imperfections 100% unique. When a person has one or more of the aberrations it can degrade the persons vision. The following is an example of just two of the more common higher order aberrations and what the degraded vision can look like. Blow the figures is the original target image.

As you can see, the original target point of light and the letter E that the person looks at are distorted (aberrated) with these vision imperfections. Point light sources like streetlights, automobile headlights, candlelight and stars appear with streaks of spurious light emerging from them. Street signs, letters on TV screens and billboard letters would be blurry or ghosted.