Wavefront Guided Lasik

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 iDesign wavefront technology from Johnson and Johnson Vision is the industry leader – by a lot. In order to fully understand how the iDesign wavefront technology works it is helpful to learn some detail regarding the science of optics.

What does Wavefront Measure that Glasses and Contacts Don’t?

Glasses and contact lenses are limited to the correction of nearsightedness, farsightedness and astigmatism. These are considered “simple” or “lower order” errors in focus. They correct poor vision by assuming that the eye is either perfectly round like a basketball or oval shaped like a football. In practice, eyes never follow these assumptions. As a result, glasses and contact lenses only approximate the actual lens needed to fully restore perfect vision. So even though a person may see 20/20 with glasses, most have unwanted visual noise in the form of night glare and blur caused by focusing error not fully corrected by these crude spectacles.

Prior to the development of wavefront technology, laser vision correction had only been able to treat these same simple focusing errors. However, with the development of CustomVue and the more recent iDesign wavefront diagnostic systems, this is no longer true. When combined with advanced excimer lasers like the Johnson and Johnson Vision Star S4 IR we are now capable of treating even complex higher orders aberrations and delivering vision solutions that are superior to that produced by glasses or contact lenses.

How Does Wavefront Aberrometry Work?

What is about to follow may be bit too complex for the average person. However, if you are a fan of “How Its Made” or have a passion for science, you will definitely want to read on and we’ll do our best to explain wavefront aberrometry in the simplest form possible.

Generally speaking, we typically consider complex optical errors when creating a camera or a telescope. In a camera, it is possible that the individual lens element could be out of alignment or created with gross imperfections. Similarly, in a telescope, lenses may be misaligned or in the case of astronomical telescopes, there may be atmospheric aberrations that distort incoming light. In the case of a celestial telescope such as the Hubble or the new James Webb deep space telescope, there may be errors in how the large mirror is positioned in space when the system unfolds after launch from earth.

As an aside, you may recall that when the Hubble telescope was first deployed into space, the images that it recorded were nearly useless because of an error in the mirror design. Fortunately, it was repaired using wavefront aberrometry technology. The same engineers that fixed the Hubble telescope using wavefront aberrometry techniques are now in charge of the development of NASA’s newest James Webb Deep Space Telescope launching in 2017. They are also the same engineers that developed the technology in the iDesign system used in our office.

Optical engineers, like those that repaired the Hubble and designed the iDesign system, 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. Don’t give up yet – we’ll explain what these terms mean as we go along.

Nearsightedness and Farsightedness – Spherical Errors

Nearsightedness and farsightedness are basic “spherical” focusing errors and virtually everyone that wears glasses has some degree of either nearsightedness or farsightedness. Spherical errors assume that the eye is round like a basketball. If the curvature of the “eye-ball” is too high (the surface curvature is too steep) then the eye is nearsighted. To correct such an eye with a laser we would need to flatten the curvature of the cornea to make it less steep. On the contrary, if the curvature of the “eye-ball” is too little (the surface curvature is too flat) then the eye is farsighted. To correct this eye with a laser we would need to make the cornea steeper.

Astigmatism

Virtually every patient struggles to understand astigmatism. Astigmatism occurs naturally in most every eye in varying degrees. Eyes that exhibit astigmatism are not shaped like a basketball but rather are shaped like football. In this case the football shaped eye would be corrected by a laser procedure that would make the eye more round – that might require flattening the steepest area of curvature or steeping the flattest area of curvature.

In a football, the steepest axis of curvature is 90 degrees to the flattest axis. As a result, the steepening and flattening using the laser are done in a highly symmetrical manner.

Complex Aberrations

Virtually all eyes demonstrate some degree of degraded vision due to complex or “higher order” aberrations. To visualize these complex aberrations imagine an eye that is football shaped but has some of the air let out so that is somewhat deformed – kind of a “deflate-gate” eyeball. Depending on how this slightly deflated football is deformed you can imagine a complicated laser pattern that would be required to make such an eyeball round.

As you can also imagine, it is impossible to create a pair of glasses or contact lenses that would completely correct this deflate-gate eyeball due to the lack of symmetry. Even in small amounts, complex aberrations can cause blurred vision and particularly create halos or glare around lights at night. A wavefront aberrometer is able to measure all of the vision irregularities created by this “deformed” eyeball and provides a map that can be used to create a complex laser treatment pattern. This laser treatment pattern can fix all of the vision irregularities caused by the deformed eyeball and deliver a very clear level of vision that is superior to that provided by glasses or contact lenses.

What is a Wavefront?

In order to measure all the visual aberrations present in the human visual system, it is not possible to simply rely upon lenses used in the “better one or better two” technique. The only method to measure higher order aberrations is with a wavefront aberrometer. So let’s examine what a wavefront is and how a wavefront aberrometer works.

The Optical Principles of Wavefront

In a perfect eye, if we held a photograph in front of the eye, all light coming off of that photograph and entering the pupil would come to an exact point of focus on the back of the eye at the retina. Although slightly more confusing, let’s also think about running this in process in reverse. Considering that same ideal eye, if we were to bounce a laser beam off of a single point of reflection on the retina, using it as a “mirror”, all of the light exiting from the pupil would form a perfect flat plane of photons located in front of that perfect eye in the same plane of the original photograph.

In reality, all eyes are shaped like our “deflate-gate” football and therefore have some degrees of simple and complex aberrations. In such cases, the light coming from our photograph in front of the eye that is focused on the back of the eye would be smeared or smudged rather than being in a single point and, conversely, the plane of light focused in front of the eye would be warped or distorted rather than falling on the plane of the photograph. In 3-D, the warped light visualized in front of the eye would be distorted in the shape of a potato chip – with each chip unique to that individual eye.

The wavefront aberrometer captures the 3-D profile of this potato chip-shaped wavefront and describes this shape using mathematical terms. Surgeons examine these shapes and group them into categories based on similar characteristics. For example, some shapes might look like a Mexican sombrero while other might look like a taco. Instead of names like “sombrero” or “taco” the surgeon will use more technical sounding names like “spherical aberration”, “coma”, “trefoil”, “tetrafoil” and “high order astigmatism” but the idea is the same.

The laser profile is then defined based upon the actual shape of this 3-D wavefront map. The laser pulses will be laid down in the shape of a “sombrero” or “taco” as defined by the unique wavefront map of that patient’s deflate-gate eyeball.

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. The now distorted 3-D potato chip shaped plane of light located in space in front of the eye is broken up into thousands of individual “arrows” 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 arrow of light onto a comparable microscopic array of CCD microchips. Using the X and Y deviation of the focus of each arrow 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 used in the iDesign device is highly resistant to crossing of the “arrows” of light in highly aberrated eyes and is recognized as being the most accurate in all eyes. In addition to the highest level of accuracy, the iDesign has a very high dynamic range allowing it to capture wavefront data even from very optically challenged eyes.

Why Use the iDesign System?

The best vision is created by an eye with the least amount of wavefront distortion. Once the 3-D potato chip 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 from our theoretical photograph would now come into a perfect central point of focus on the retina. This data guides the Johnson and Johnson Vision Star S4 IR laser to reshape your cornea to create this complex shape. Once this highly customized laser treatment pattern is completed, the new corneal shape should bring light into proper focus. The new clear focus will produce the best vision quality and have the least side effects such as halo or glow around lights.

It’s Even More Complicated Than You Think

If making the wavefront measurement sounded complicated, actually applying the new treatment profile to the cornea is equally challenging. That’s because the eye is moving and rotating and the position of the pupil is not stable during the treatment. Compensation for the eye and pupil movements are performed 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. In addition, highly accurate eye tracking follows even small eye movements in order to ensure that each laser pulse lands on the correct location on the cornea. All this must be done using a laser treatment shot pattern that does not increase the temperature of the cornea tissue and controls swelling that might adversely affect corneal shape postoperatively.

Summary

When all of these factors are accounted for, a cornea treated based upon wavefront aberrometry will demonstrate superior vision to that provided by glasses, contact lenses and non-wavefront guided laser treatments.

So now that you understand how wavefront aberrometry and the iDesign system works, you can now understand how no other competing technology can deliver the same quality of vision.
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