Wavefront Analysis

Gimbel Eye Centre strives to remain at the cutting edge of technology, both with the equipment that it uses, and how that equipment is used to improve the vision of our patients.

Wavefront Technology is a new tool available to ophthalmologists at Gimbel Eye Centre and can be used with both LASIK and PRK surgeries. Using a Wavefront sensor, also called an aberrometer, we are able to analyze the exact path or shape of light rays or Wavefront as light travels through the various structures of the eye.

Wavefront analysis is the method used to measure how the path of light (or Wavefront) is changed as it goes through the eye to the retina. A perfect Wavefront would be completely flat. In human eyes the Wavefront is changed by refractive errors such as nearsightedness, farsightedness and astigmatism and also by irregularities in the cornea, crystalline lens, and retina. These changes to the Wavefront are called Aberrations and affect the quality of vision. Total aberrations include all the changes to a Wavefront, including Lower Order aberrations and Higher Order aberrations Traditionally, doctors have been limited to measuring and treating Lower Order aberrations such as nearsightedness, farsightedness and astigmatism. Higher Order aberrations are more complex and cannot be corrected by traditional means such as glasses or contact lenses or by conventional refractive surgery such as LASIK or PRK. Higher order aberrations can cause such symptoms as glare, reduced contrast, doubling or reduced vision quality. The goal with customized laser treatment is to compensate for any aberrations in the eye by reshaping the cornea to achieve optimal visual outcome. While not everyone has significant higher order aberrations, every patient at Gimbel Eye Centre has Wavefront analysis done as part of their assessment to aid the surgeon in determining the best treatment plan.

How do we measure Wavefront errors?

Nidek's NAVEX platform is used at Gimbel Eye Centre for complete analysis and treatment of Wavefront errors. The OPD-Scan® is a time-based aberrometer that measures the optical path difference of light at an incredible 1,440 points in the pupil of the eye. Unique to Nidek's OPD-Scan is the integration of corneal topography and Wavefront sensing to provide a complete analysis with one exam, without the problem of trying to merge the data from the two tests after the fact. Once the data is collected, the information is analyzed with Final Fit® software, giving the surgeon control over the treatment plan yet allowing the accuracy of computers to determine the best laser treatment pattern for each individual eye. Final Fit® analysis separates the treatment pattern into spherical, cylindrical and irregularity components and each is treated as a separate component by the laser.

Once the appropriate parameters are set, laser shot data is sent to the Nidek EC5000 CX laser ready for patient treatment. With the EC5000 CX, the irregularity component of the total correction is treated first with 1mm spot size Multipoint ablation, up to 6 spots at a time. The remainder of the correction is done with scanning slit delivery which is fast, leaves a smooth stromal bed and can be programmed for aspherical ablations.

Nidek's NAVEX Platform

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Aspheric treatment to avoid inducing Wavefront errors

It is a known fact that conventional laser surgery can induce higher order aberrations such as spherical aberrations, causing decreased quality of vision even though the patient may be able to read 20/20 on the eye chart. Symptoms of higher order aberrations include ghosting, distortion, blurring, shadowing or reduced contrast. Combinations of these symptoms can subtly or significantly affect the quality of vision. A normal cornea, which has a curve that is steeper centrally and flatter peripherally, is called a Prolate shape. In most cases the cornea is flattened centrally after laser surgery and this changes the cornea from a Prolate shape to an Oblate shape meaning the cornea is flatter centrally than peripherally. An oblate cornea typically does not give the best quality of vision.

Aspheric treatment is a unique Nidek algorithm that treats the cornea in such a way that the natural Prolate shape is maintained as much as possible. This minimizes the introduction of spherical aberrations, providing a more natural and enjoyable quality of vision.

How light focuses with Aspheric vs. Spherical lens
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How the laser knows where to treat

It is very important for the laser to apply the energy to the right place on the cornea. There are two ways the NAVEX system provides for this. Torsion Error Detection (TED) is a process whereby the OPD-Scan takes an image of the patient's iris and transmits that image to the Nidek EC5000 CX laser. The laser then scans the patient's iris just prior to laser ablation to confirm that the eye is in the exact same position as when the OPD-Scan exam was done, adjusting for the natural rotation of the eye. This registration is very important, especially when dealing with accuracy of treatment measured in microns of tissue. Once the eye is aligned, the Nidek EC5000 CX laser's 200Hz Active Tracker® is activated to track and follow the eye's movements as the laser ablation is being done.

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Who is Zernike?

Fritz Zernike was a physicist who won a Nobel prize in physics in 1953 "... for his demonstration of the phase contrast method, especially for his invention of the phase contrast microscope ..." Zernike developed a set of mathematical functions to describe Wavefront. The Zernike polynomial catalog demonstrates the graphical representation of each of these functions. Each Zernike coefficient represents a specific Wavefront error and these are used to describe both lower and higher order aberrations of the eye. Lower order aberrations include Orders zero through two (Coefficients 0–5) and higher order aberrations are from Order three and above (Coefficents 6 and greater). When determining how much Wavefront error an eye has, it is the Root Mean Square (RMS) value of the Zernike coefficients that is used, and is shown as Total RMS. The RMS value can also be separated to show only Higher Order RMS or HO RMS.

Zernike Polynomial Catalog Click to view larger

Zernike Coefficients Example Click to view larger

OPD-scan Measuring Principle Click to view larger

Sample Wavefront Map Click to view larger

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NIDEK OPD Scan

Simultaneous acquisition of:

• Wavefront aberration coefficients
• Measures both Total and Higher Order Aberrations
• Corneal Topography
• Auto refraction
• Auto keratometry
• Point Spread Function
• Pupil diameter
• Fixation Offset

Final Fit Software

• Prepares laser shot data from OPD scan information

NIDEK EC5000CX Excimer laser

• Multipoint custom ablation
• Aspheric custom ablation
• Active Tracker® with Torsion Error Detection

Why the NAVEX System?

• Topography linked to aberrometry may not be as important with small aberrations as with large. However, registration of the Wavefront sensor (OPD) and the laser is important. NAVEX has this.
• Leaves a smooth stromal bed and can be programmed for aspherical ablations.