The development of wavefront technology has been central to driving recent improvements in the safety and improved quality of vision delivered by laser vision correction. The Wavefront CustomVue from AMO-VISX continues to be on the forefront of these advances. In order to understand how the CustomVue technology works it is helpful to learn some detail regarding the science of optics.
Why is My Vision Blurry?
Optical engineers have defined a number of ways that any optical system can be “out of focus”. With respect to human vision, generally they can be divided into three broad groups; spherical errors, astigmatism and complex higher order aberrations including spherical aberration(s), coma, trefoil, tetrafoil and high order astigmatism.
Prior to the development of the CustomVue technology, laser vision correction had only been able to treat nearsightedness, farsightedness, and astigmatism. These are considered “simple” or “lower order” refractive errors. These simple refractive errors are what glasses, contact lenses and traditional Lasik correct. With the development of CustomVue and the iDesign wavefront diagnostic systems, this is no longer true. When combined with advanced excimer lasers like the AMO-VISX Star S4 IR we are now capable of treating even the complex higher orders aberrations.
Nearsightedness and farsightedness are basic “spherical” errors. They are symmetrical spherical (“round” or “basketball” shaped) distortions that can be corrected for using simple glasses using symmetrical spherical lenses. Spherical errors are corrected by increasing or decreasing the radius of curvature of the “basketball” shaped lens.
Astigmatism occurs naturally in most eyes in varying degrees. Regular astigmatism is a symmetrical aberration that is caused by the optical system demonstrating a toric shape (“football” or “spoon” shape). A toric lens is a lens with a different optical power and focal length in two orientations perpendicular to each other. Glasses or contact lenses that correct astigmatism are therefore “toric” or football shaped.
Generally, astigmatism can be corrected with an excimer laser by flattening the radius of curvature of the steepest axis of the “football” and / or steepening axis of curvature of the opposite and perpendicular flattest axis. Similar to spherical errors, because the axes of a toric lens are perpendicular (at right angles to each other) this steepening and flattening is performed in a highly symmetrical manner.
Virtually all eyes demonstrate some degree of degraded vision due to complex aberrations. These aberrations include trefoil, coma, tetrafoil, pentafoil, hexafoil and spherical aberration(s). Unlike nearsightedness, farsightedness and astigmatism, these aberrations are generally not symmetrical and cannot be corrected for using standard glasses or contact lenses. Even in small amounts complex aberrations can cause visually significant degradation in overall clarity of best-corrected vision. This results in decreased visual acuity for the patient or may cause halos or glare around lights at night.
What is a Wavefront?
In order to measure all the aberrations present in the human visual system, it is not possible to simply rely upon routine refractions using spherical or astigmatism correcting lenses. The only method to measure higher order aberrations is with a wavefront aberrometer.
The Optical Principles of Wavefront
In a perfect eye, all light entering the pupil would come to an exact point of focus on the back of the eye at the retina. Conversely, in that same ideal eye, if we were to bounce a collimated laser beam off of the retina using it as a “mirror”, all of the light emanating from the pupil would form a perfect flat plane of photons located in front of the eye. These are the optical principles that define wavefront aberrometry. In reality, all eyes have some degrees of simple and complex aberrations. In this case the plane of light focused in front of the eye would be warped or distorted or, conversely, the light focused on the back of the eye would be smeared or smudged rather than being in a single point.
How a Tscherning’s Aberrometer Works
Wavefront systems can be used to measure how a flat plane of light projected through the pupil in an organized grid pattern becomes distributed on the retina. These are known as “ingoing” aberrometers and the Alcon Tscherning’s aberrometer uses this method. The degree of scattering and disorganization of the incoming grid of light defines what is known as a “point spread function”. Unfortunately in highly aberrated eyes the grid viewed on the retinal surface can become so disorganized that the resulting information is corrupted and misleading.
How the CustomVue Shack Hartmann Aberrometer Works
The CustomVue and iDesign systems measure how a beam of laser light that is reflected from the back of your retina comes into focus in space in front of the eye. This is known as “outgoing” aberrometry. The now distorted plane of light located in space in front of the eye is broken up into thousands of individual “pencils” of light using a Shack Hartmann “lenslet” array. The Shack Hartmann lenslets consist of microscopic lenses organized into a perfect grid or array. These tiny lenslets focus each individual pencil of light onto a comparable microscopic array of CCD microchips. Using the X and Y deviation of the focus of each pencil of light on each individual CCD microchip it is possible, using Fourier algorithms, to reconstruct the shape of the distorted wavefront. The Shack Hartmann system is much more resistant to crossing of the pencils of light in highly aberrated eyes than the Tscherning’s aberrometer and therefore is much more accurate in all eyes. It also has a very high dynamic range allowing it to capture wavefront data even from very optically challenged eyes.
Why Use the CustomVue System?
Once the wavefront shape is defined, sophisticated software algorithms are used to calculate the ideal slope of the cornea at each point on its surface so that a flat plane of light entering the eye would now come into a perfect central point of focus or have a perfect “point spread function”. This data guides the AMO-VISX Star S4 IR ablation (reshaping) of your cornea by the laser. Once this highly customized photoablation pattern is completed, the new corneal shape should bring light into proper focus with a markedly improved “point spread function”. Clear focus combined with minimal light scatter will produce the highest fidelity of vision with the fewest side effects of halo or glow.
The CustomVue and iDesign wavefront analyzers from AMO-VISX provide the most accurate wavefront measurements of any device available. In addition, in combination with the AMO-VISX Star S4 IR excimer laser system, it is possible to not only accurately measure but to precisely treat these aberrations.
If making the wavefront measurement sounded complicated, actually applying the new treatment profile to the cornea is equally challenging. The latter must be done using a complex system involving “iris registration” which provides perfect “point-to-point” referencing so that the planned treatment is properly aligned on the moving human eye as well as pupil centroid compensation to account for potential misalignment of the treatment off the patient’s visual axis (line of sight) caused by pupil size. In addition, highly accurate eye tracking must follow even small eye movements in all dimensions in order to ensure that each laser pulse lands on the correct location on the cornea. Moreover, all this must be done using a thermally neutral profile that respects the health of the corneal tissue thereby reducing localized changes in corneal shape caused by tissue swelling and edema from damaged cells or extracellular matrix.
When all of these factors are accounted for, a customized cornea treated using accurate wavefront aberrometry data whose optical system has been optimized for vision can demonstrate an unparalleled reduction in distortions. This dramatic improvement can mean the difference between just barely reading the small row of letters on the eye chart and reading it without effort. Many patients have reported an improvement in night vision.
So now that you understand how wavefront aberrometry and the CustomVue system works, you understand how no other competing technology can deliver the same quality of visual outcomes. So what are you waiting for? Come visit us today and See the Difference!