LASIK MD Laser Eye Surgery – Wavefront Analysis
Wavefront analysis is performed by an instrument called an aberometer, which can detect optical errors at a fine level. Wavefront technology assesses every ray of light that enters the eye and then determines what changes will produce the clearest image. Wavefront analyzers therefore precisely measure the overall refractive error of the entire eye, including any aberrations caused by the tear film, anterior and posterior cornea, lens, vitreous, and retina. Remember that corneal topography systems can define corneal irregularities, but they cannot detect aberrations in other parts of the eye.
The wavefront sensor measures the refraction to submicron levels, or about 0.01 D. When refraction is measured with today's conventional subjective tests, the accuracy is only within 0.25 to 0.50 D.
Hartmann-Shack Wavefront Analyzer: How Does It Really Work?
Light travels in flat sheets called wavefronts. The irregularities or aberrations in the cornea and the lens of the eye wrinkle the light waves and create wavefront errors or distortions as the light rays enter and exit the eye. This is the scientific principle that this technology uses.
The wavefront analyzer aims light rays from a single laser beam into the eye and focuses them on the retina. As they are reflected back out from the retina, these light rays are subjected to possible aberrations as they travel through the eye's optics.
If the eye has no irregularities, these light
rays will come out of the eye in a plane wavefront, or a straight
line. However, if the eye has irregularities, also called higher
order aberrations, the wavefront emerges not in a straight line,
but with a unique shape specific for that eye.
This wavefront of light then passes through a tiny array of lenses,
called the lenslet array, in the wavefront analyzer.
The analyzer measures wavefront deviation of the reflected light and the image created by the lenslet array is captured by a video camera. The wavefront maps created can be compared to a fingerprint of the eye.
In a normal eye, the video image shows small
dots of light in a symmetrical grid, aligned in a highly uniform
pattern. In an eye with significant aberrations, however, the dots
are blurred and the pattern appears distorted. The system then
compares the pattern seen in the eye being analyzed to an ideal
pattern with no optical aberrations to generate a series of
equations that describe the aberrations, called Zernike
polynomials, for that particular eye.
Higher-Order Aberrations
High-order aberrations are ones that cannot be corrected by simple spherocylindrical systems, such as spectacles or contact lenses. They are caused by minute misalignments of the eye's optical components and include, in order of visual significance, spherical aberration, coma, higher-order astigmatism, and others. Theoretically, an ablation that removes aberrations increases visual contrast and the spatial detail of images seen by the eye.
Zernike Polynomials
Zernike polynomials are numbers that describe wavefront aberrations. These numbers are used to generate an ablation pattern (treatment profile) for the excimer laser. A different number of Zernike terms can be included in the ablation design; some lasers use 12, others use 14.
