6. WAVEFRONT TECHNOLOGY



6-1. What is wavefront technology?

Wavefront technology is a method for analyzing waves of light as they are reflected from the inside of the eye to the outside. The importance of this technique for laser vision correction, is that it permits the measurement and treatment of very minute optical abnormalities of the eye.

6-2. How is the wavefront measured?

A dim beam of light is directed into the eye. It passes through the cornea, and then through the pupil. When it strikes the retina, it is reflected back out, first through the pupil and then, through the cornea. The beam of light is “altered” somewhat by the structures inside the eye. The exiting beam is picked up by sensors and projected onto a computer screen.

6-3. Is every person’s wavefront different?

Yes. A wavefront is like a fingerprint. Since no two eyes are identical, no two wavefronts are identical.

6-4. How is the wavefront analyzed?

There are three methods to analyze the optical system using wavefront technology. The methods carry the names of the scientists who described them. They are: The Hartmann-Shack, Tschernig, and Tracey systems.

6-5. What is the Hartmann-Shack method?

The Hartmann-Shack method analyzes light rays as they exit the eye. It is referred to as an “outgoing wavefront-sensing method.” The VISX laser uses this method. A low-intensity light is directed into the eye. As the reflected light exits the eye, the light waves pass through sensors, which function as tiny lenses, or “lenslets.” Each lenslet focuses its part of the light wave onto a closed circuit device (CCD) chip. It is the deviation of the spots from their ideal location that provides the information about the wavefront error. An analyzer reconstructs the wavefront and measures the shift in these points caused by the irregularities, or “aberrations,” inside the eye.

6-6. What is the Tschernig method?

The Tschernig method shines a 532 nm laser beam into the eye, through a mask, which breaks up the beam into 128 equidistant and parallel light rays. The system focuses these rays on the retina, and a computerized, low-light, closed circuit device (CCD) camera measures the deviation of the spots as they appear on the retina. The system reconstructs the wavefront as an image.

6-7. What is the Tracey method?

The Tracey method, also known as “ray-tracing,” measures the refractive power of the eye on a “point-by-point” basis. The device rapidly fires a series of very small parallel light beams, one at a time, through the pupil, and analyzes each beam as it is reflected. Since each beam is fired and analyzed separately, there is no “crisscrossing” of data points.

6-8. Why is wavefront technology important for LASIK?

It is believed that by correcting the imperfections that wavefront technology measures, we can improve the quality of vision. These imperfections in the optics of the eye result in subtle, or “higher-order,” optical aberrations. These aberrations are thought to cause subtle, but annoying effects, such as glare, halos, light sensitivity, and image distortion.

6-9. Which aberrations are the most important?

First and second-order aberrations, known as sphere and cylinder, are the most important, and routinely corrected with glasses or contact lenses. Higher-order aberrations (third-, fourth-, and fifth-order), known by such names as coma, trefoil, and spherical aberration, are more subtle, but can be corrected with the excimer laser using wavefront technology.

6-10. Does everyone have higher-order optical aberrations?

Everyone has some degree of higher order optical aberrations. A more important question is, “How much impact do these aberrations have on vision?” For most people, higher-order aberrations probably have little effect on vision. For others, they may interfere significantly with the quality of vision.

6-11. Will correction of higher-order optical aberrations by wavefront result in improved vision after LASIK?

Possibly. Wavefront can correct subtle irregularities in the visual system. For most people, these subtle irregularities do not interfere with vision, so correcting them is not important. But, for some people, even the slightest irregularity in the visual system interferes with vision. For this latter group, wavefront may create a better visual result.

6-12. Does wavefront add extra expense to LASIK?

Yes. Most laser centers charge an additional fee for wavefront corrections. This is in the range of $400 per eye.

6-13. What is the root mean square, or RMS, value?

The wavefront error can be defined in terms of its many components (sphere, cylinder, piston, tilt, coma, trefoil), each of which can be measured with a mathematical technique known as the “root mean square,” or RMS. The RMS takes all the wavefront elevations above and below a reference point, squares them, and then takes the root mean. It is a single number that gives a sense of the magnitude of the wavefront error, without saying anything about its nature. The higher the RMS value, the worse the quality of the image.

6-14. How much aberration does it take to interfere with vision?

If the root mean square of a particular aberration is over 0.3 microns, it is considered visually significant.

6-15. What is irregular astigmatism?

Irregular astigmatism is another name for abnormalities in the visual system that cannot be corrected with glasses. Glasses correct vision abnormalities by the science of traditional optics, which is a science of regular surfaces. Traditional optics uses spheres and cylinders to correct vision. The abnormality that remains after correction with spheres and cylinders is known as irregular astigmatism. Theoretically, wavefront technology should be able to correct irregular astigmatism,

6-16. Can wavefront abnormalities be expressed mathematically?

Yes. Wavefront abnormalities are expressed using mathematical formulas known as Zernike polynomials. These were first described a century ago by the Dutch mathematician and astronomer, Fritz Zernike. Zernike won the Nobel Prize for his invention of the phase contrast microscope. Zernike polynomials are based on circular geometry, and are well-suited for round apertures, like the pupil. They plot wavefront elevations above and below a reference plane. The plots can be added and layered in a simple fashion. This results in three-dimensional images showing the arrival of light at the pupil.

6-17. What are some examples of Zernike polynomials?

The Zernike system uses polar coordinates, and each order has an increasing number of solutions. The zero-order is called “piston.” It’s the simplest polynomial, and represents only an up or down displacement of the wavefront. The first-order polynomial is called “tilt.” Zernike’s second-order aberrations are known as “sphere” and “cylinder.” They can be corrected with glasses, contact lenses, or standard refractive surgery. Third- and fourth- and fifth-order aberrations, also known as “higher-order” aberrations, can degrade vision in subtle ways. For example, someone might see 20/20 on the eye chart in the doctor’s office, but have a lot of glare, halos, and distortion when driving at night.

6-18. Do higher-order aberrations have as much effect on vision as lower-order aberrations?

No. The higher the order of aberration, the less it contributes to image degradation in a normal eye. Above the fourth-order, aberrations have more mathematical than clinical meaning. They contribute to image degradation, but only in a minor way.

6-19. Where is the wavefront measured?

A wavefront is a wave of light that is reflected from the eye after shining a monochromatic light into the eye, and onto the center of the retina. The wave of light is typically measured at the pupil. If all the light particles, or photons, were to arrive at the pupil at exactly the same instant, the wavefront would be perfectly uniform, or flat. But since the inside of the eye reflects light in a nonuniform way, some photons reach the pupil a fraction of a second earlier, or later, than other photons. The light waves can be captured on a screen to produce an image. So, when we speak of a wavefront, we are really speaking of the front of a wave of light, reflected from the retina, and captured when it reaches the pupil.

6-20. How much do higher-order aberrations interfere with vision?

The importance of higher-order aberrations varies from one individual to the next. A widely quoted figure is that 87% of optical aberrations are “lower-order” and can be corrected by spheres and cylinders, that is, by glasses and contact lenses. Thirteen percent of optical aberrations are “higher-order,” and those are the ones that can be corrected with wavefront technology. Of course, these are averages. Some individuals have 95% lower-order aberrations, while others may have 60% lower-order aberrations. The person with 95% lower-order aberrations may not benefit from a wavefront correction, while the person with 60% higher-order aberrations would.

6-21. What are lower-order aberrations?

Lower-order aberrations are the simplest types of optical aberrations. There are three types: Tilt, sphere, and cylinder.

6-22. What is tilt?

Tilt describes the angular position of the lens. It moves the image up, down, left, or right.

6-23. What is sphere?

Sphere describes a symmetric curvature over the lens. It is the most important aberration for most people. Sphere is characterized by an identical curve in every direction, just like a baseball. Sphere is the main aberration we correct with glasses or contact lenses. If the sphere is a negative number, a person is myopic, or nearsighted. If the sphere is positive, a person is hyperopic, or farsighted.

6-24. What is cylinder?

Cylinder describes a symmetric curve of the lens at a certain angle, or axis. The curve at a specified axis is steeper or flatter than the curve 90 degrees away. A baseball has no cylinder, since it is perfectly spherical. It has the same curve in every axis. A football, however, has cylinder. It has a steep curve in one direction, and a flatter curve 90 degrees away. If an eye has cylinder, we can say that the eye has “astigmatism.”

6-25. Do all higher-order aberrations interfere with clear vision?

Probably not. We are just starting to understand the type of wavefront that provides the best quality of vision. It is by no means certain that a perfect plane or wavefront provides the highest quality visual performance. A certain amount of spherical aberration or coma may be desirable for certain aspects of vision such as depth of focus and peripheral vision. A major task ahead for the field of wavefront technology is to better understand the impact of these various higher-order aberrations on quality of vision.

6-26. What is the difference between a corneal topography map and a wavefront map?

Corneal topography, also known as computerized videokeratography, recognizes complex patterns of the front surface of the cornea. These maps provide measurements and analysis of only the front surface of the cornea. They are similar to topographic maps of land. Different colors are assigned to represent different elevations. Usually, the higher, or steeper, elevations are represented by “warm” colors, such as red and yellow. Lower, or flatter, elevations are represented by “cool” colors, like blues and greens.

6-27. What does wavefront technology measure?

Wavefront technology measures the optical path in its entirety and is not limited to any given refractive surface. Wavefront provides a more complete picture of the refractive characteristics of the eye, answering the question, “what happens to light as it travels through the eye?” Wavefront technology cannot diagnose the depth within the eye at which various aberrations are produced.

6-28. What are the major limitations of wavefront technology?

A major limitation of wavefront technology is that it requires a relatively clear optical system. Since it operates by assessing light as it travels through the eye, anything that blocks light will prevent an image from being formed.

6-29. What kinds of things would block light and interfere with the wavefront image?

Corneal scars and cataracts could interfere with the wavefront image.

6-30. Can wavefront technology be used to measure the glasses prescription?

Yes. Wavefront technology can measure the lower-order aberrations, sphere and cylinder, quite accurately. It can determine the refraction with the accuracy of 1/100th of a diopter. Commonly used autorefractors measure only within 1/4 of a diopters. It is possible that wavefront technology will one day replace currently used autorefractors to create more accurate glasses and contact lens prescriptions. But, we should keep in mind that opacities, such as corneal scars and cataracts, may sufficiently block light passage to prevent a clear image from reaching the sensor, and providing the information needed for glasses and contact lenses.

6-31. What is a point spread function?

Point spread function, PSF, is a wavefront map that shows how someone might see a point source of light, like a star. Lower and higher-order aberrations can distort the point of light, so that it spreads out, forming an irregularly-shaped blur. The point spread function map lets others see what a person with aberrations of the optical system sees.

6-32. What are “difference maps?”

Difference maps are wavefront maps that can be used to track changes of the eye over a period of time. They are used to highlight preoperative and postoperative wavefront patterns, and to follow wavefront changes over time, particularly as the eye heals after laser vision correction.

6-33. How does the ophthalmologist use wavefront data?

Wavefront data is often used to guide the ophthalmologist and patient in the choice between standard laser vision correction and wavefront-guided laser vision correction. If, for example, the higher-order aberrations represent less than 20% of the total aberrations, and the patient is happy with the way he or she sees with glasses, standard laser vision correction should produce excellent results. If, on the other hand, higher-order aberrations account for more than 20% of the total aberrations, the patient might be better served by wavefront-guided laser vision correction.

6-34. Do all people receiving laser vision correction benefit from wavefront corrections?

Probably not. People with fairly large refractive errors of -8 diopters and above, are usually good candidates for standard laser vision correction, regardless of the amount of higher-order aberrations. This is because higher-order aberrations comprise a very small percentage of their total optical aberrations. On the other hand, those patients with small refractive errors who have difficulty seeing 20/20 with their glasses, and those who have a lot of glare, halos, and blurring, should be evaluated with wavefront technology to see if they have a high percentage of higher order aberrations.

6-35. What does it mean if the wavefront map changes over time while the corneal topography map remains unchanged?

Changes in the wavefront map over time indicate a change in the optical pathway as light goes in and out of the eye. If the corneal topography remains unchanged, then the wavefront alteration is not caused by the cornea. The most likely reason for change in the wavefront would be the development of a cataract.

6-36. How does pupil size influence wavefront measurements?

Pupil size is very important when measuring the wavefront. We can only measure the wavefront of light going in and out of the pupil. The larger the pupil, the more aberrations enter the wavefront from the peripheral cornea. Corneal topography looks at the entire cornea, not just the central pupillary area. By evaluating both wavefront and corneal topography, the ophthalmologist can determine whether aberrations are coming from the cornea or from another part of the eye.

6-37. What is coma?

Coma is one of the two Zernike third-order aberrations. It consists of a bulge above the reference plane and a bulge below the reference plane, adjacent to each other but on opposite sides of the visual field. Put another way, it represents a nearsighted area across from a farsighted area. In a point spread function, coma may look like a comet – hence the name.

6-38. What is trefoil?

Trefoil is the other third-order aberration. The shape is reminiscent of a cylinder, but with three projections coming out of a center point, it looks like a pinwheel, or a Mercedes-Benz car emblem. This type of aberration probably explains why certain patients seem to have astigmatism in more than one axis.

6-39. What is spherical aberration?

Spherical aberration is the most common fourth-order aberration. It is sometimes called “the sombrero hat,” which its mathematical plot resembles. It is a significant contributor to blur.

6-40. Are all higher-order aberrations equally disruptive?

No. Some higher-order aberrations, like coma, trefoil, and spherical aberration are more important than others.

6-41. Can standard LASIK cause significant increases in higher-order aberrations?

Yes. Simply cutting a LASIK flap can create some corneal distortion and induce some higher-order optical aberrations. For this reason, it was originally thought that wavefront technology might be more applicable to PRK, rather than LASIK. Now that wavefront has been used with both procedures, it appears that patients with significant amounts of higher-order aberrations can benefit from wavefront treatments applied to both LASIK and PRK.

6-42. What is registration?

Registration refers to the process of defining the center of the laser treatment, and ensuring that the treatment is performed at this exact location. The patient’s line of sight is generally regarded as the ideal center of the treatment zone. Linking this point to the center of the pupil is the usual way to “register” this position.

6-43. Why is registration important?

Registration is important because it allows the surgeon to create the pattern of laser treatment exactly in the visual axis. The pattern of correction must be placed as precisely as possible over the pattern of the cornea that requires correction. This is particularly true for wavefront corrections. Alignment of the wavefront treatment is critical because the wavefront pattern is quite complex. To obtain excellent vision, the treatment must be placed exactly over the visual axis.

6-44. How is registration accomplished?

Registration is accomplished by special software in the laser that recognizes certain “landmarks” of the eye. The main landmarks are the pupil and the limbus, the junction between the white and colored part of the eye. In addition, the software recognizes the pattern of the iris, the colored tissue of the eye. When seen up close, the iris has many variations in color and texture. The laser captures an image of the iris, and superimposes the pattern of wavefront treatment over it. This exact positioning makes wavefront correction even more precise.

6-45. What is cyclotorsion?

Cyclotorsion refers to a clockwise or counterclockwise rotation of the eyes caused by contraction of the extraocular muscles. Laser surgeons are concerned about cyclotorsion because the eyes can sometimes rotate slightly from their original position before or during laser vision correction.

6-46. Why is cyclotorsion undesirable?

Cyclotorsion is undesirable because it changes the position, or axis, of the eye’s astigmatism. For example, if the astigmatism is measured preoperatively at 90 degrees, and once beneath the laser, the eye rotates 3 degrees, the axis of astigmatism has shifted to either 87 degrees or 103 degrees. If astigmatism treatment were carried out according to the original plan, treatment would be off by 3 degrees, and the visual result might be suboptimal.

6-47. How can cyclotorsion be avoided?

Cyclotorsion cannot be avoided, but it can be compensated for. This is done by placing ink marks on the white part of the eye, next to the cornea, to identify the 180-degree axis. This corresponds to the 3 and 9 o’clock position when we are facing the eye and looking at it like a clock. If, under the laser, we see that the marks have rotated so they are no longer in the 3 and 9 o’clock positions, we know we have experienced cyclotorsion.

6-48. What can we do about cyclotorsion?

If cyclotorsion occurs under the laser, the head can be rotated so the alignment marks are adjusted back to the 3 and 9 o’clock positions. Once this adjustment has been made, laser treatment can be carried out. Registration and capture of the iris architecture, the pupil, and the limbus, can automatically correct for cyclotorsion.

6-49. What is a “mesopic” pupil?

A mesopic pupil refers to a pupil that reflects the lighting conditions of daylight. The pupil is a hole, or aperture, in the center of the iris. It behaves like the diaphragm of a camera to control the amount of light that enters the eye. When light shines directly in the eye, the pupil “constricts,” or gets smaller. When light is dim, the pupil “dilates,” or gets larger.

6-50. What is a “scotopic” pupil?

A scotopic pupil refers to the state of the pupil when the light is dim, or when the pupil is measured in the dark. Scotopic pupils are larger than mesopic pupils. It is important to measure the scotopic pupil before LASIK. Ideally, the diameter of the laser treatment should be larger than the scotopic pupil. If this is the case, there will be less chance of glare at night. Glare can be caused when the outer edge of the laser treatment zone is within the scotopic pupil.

6-51. What is “hippus?”

Hippus is the continuous motion of the pupil, also known as “pupillary unrest.” Hippus is a natural quality of the pupil, and it is caused by very slight contraction and relaxation of the muscles in the iris. The importance for LASIK is that hippus makes the pupil harder to measure because of the slight, but constantly changing, pupil size. To overcome this problem, several measurements of pupil size can be made and averaged.

6-52. Are both pupils usually equal in size?

It is common to have slightly unequal pupils. This condition is referred to as anisocoria. A variation in pupil size between two eyes of the same individual may be as much as 2 mm and still considered normal. A difference greater than 2 mm is considered abnormal.

6-53. What factors, beside illumination, affect pupil size?

Pupil size may be affected by fatigue, exercise, medications, age, and eye color.