Clinicians, visual scientists, and opticians have known for centuries that retinal image quality is degraded by the eye's poor optical quality. For hundreds of years, glasses have been used to correct for the eye's basic refractive errors, namely defocus and later both defocus (sphere) and astigmatism (cylinder and axis). Contact lenses, on the other hand, have not been around for nearly as long as glasses. The soft contact lenses we enjoy today, which also correct for defocus and astigmatism, did not get their start until the late 1960s and early 1970s. Laser refractive surgery has recently emerged as another method that can successfully be used to correct for the eye's basic refractive errors. Learn more about the conventional vision correction methods in use today: glasses, contacts, and laser refractive surgery.

Unfortunately, the types of conventional glasses and contact lenses that are typically prescribed today to improve vision still only correct these two basic types of aberrations. Though it is well established that the eye suffers from many more higher order monochromatic aberrations that defocus and astigmatism, there has been relatively little work on correcting them until recently. In 1961, Smirnov, an early pioneer in the characterization of the eye's higher order aberrations, suggested that it would be possible to manufacture customized lenses to compensate for them in individual eyes. Recent accurate instruments for measuring the ocular aberrations are available, most notably the Shack-Hartmann wavefront sensor, first applied to the eye by Liang et al. (1994). Moreover, there are new techniques to correct higher order aberrations. Liang et al. (1997) showed that a deformable mirror in an adaptive optics system can correct the eye's higher order aberrations. This study was the first to demonstrate that the correction of higher order aberrations can lead to substantial improvements in visual performance in normal eyes. Presently the visual benefits of adaptive optics can only be obtained in the laboratory, but the success of the technique encourages the implementation of higher order correction in everyday vision through laser refractive surgery, intraocular lenses, or customized contact lenses.

Customized contact lenses

Several companies are now working to design and develop customized contact lenses that would correct for the eye's higher order aberrations, in addition to the typical correction of defocus and astigmatism (sphere and cylinder). These lenses would be tailored to correct each individual's own wave aberration and would have to be specially designed and fabricated for each patient. (This is due to the fact that the wave aberration can vary dramtically from person to person.) Before sculpting a patient's customized contact lens, the patient's wave aberration would have to be measured. Once the wavefront is calculated, the customized lens can be designed and fabricated such that it possesses the opposite wavefront error (when compared with that from the patient's eye) to counteract all of the eye's aberrations.

The operating principle for this type of lens is shown in the figure below. The top half of the figure shows how conventional contact lenses can still allow for some residual blurring of objects on the retina. Light from a distant object travels as a plane wave (in flat, parallel sheets) until it strikes the conventional contact lens that corrects the eye's defocus and astigmatism. However, since the lens cannot correct the eye's higher order aberrations, the image of this object on the retina is not perfect and can be slightly blurred (red oval), especially when the pupil is large (as at night).

The lower half of this figure shows the concept behind a customized contact lens that can correct for all of the eye's aberrations. Here, we have separated the customized lens from the eye for illustrative purposes only, to show how the wavefront would be corrected with this device. In practice, customized contact lenses would be worn directly on the eye, as normally done with conventional soft and hard contact lenses in the top half of the figure. Customized contact lenses would ideally produce a diffraction-limited, perfect image of a distant object on the retina. As in the figure, light from a distant object would travel as a plane wave (red, flat sheets of light) until striking the customized lens. The light emerging from this lens would have a wavefront shape that is exactly the opposite of that initially measured in the patient's eye. Therefore, the aberrated wavefront produced by the contact would be transformed into a perfect wavefront (denoted in blue) after traveling through the lens and cornea, yielding a perfect image on the retina (in focus, blue spot).

How close are we to realizing customized contacts that can truly correct for aberrations above and beyond defocus and astigmatism? Lathing and laser ablative technologies now exist that can create arbitrary surfaces on contact lenses, offering the possibility of truly customized contact lenses. However, the correction afforded by this type of contact will be limited by its tolerances as it rotates ans moves on the eye. Soft contact lenses have been reported to have maximum translations and rotations of around 0.6 mm and 6 degrees that are induced by blinking (Tomlinson et al., 1994) and mean translations of approximately 0.4 mm for a 30 degree down-gaze (Erickson and Robboy, 1985). Antonio Guirao has performed theoretical simulations of the potential impact of rotations and decentrations of a customized contact lens on image quality (2001). (These results are also applicable to any other customized correction technique, such as laser refractive surgery or adaptive optics). He found that, for these types of typical decentrations, a customized contact would offer, on average, a 1.5 - 2 fold improvement in visual performance over a standard correction of defocus and astigmatism. Therefore, a customized contact would need to be properly ballasted and fit to each eye in order to reap the largest benefits of a customized correction.

Laser refractive surgery

Another popular method that could ultimately provide higher order correction and is currently used to improve vision is laser refractive surgery, namely laser in-situ keratomileusis (LASIK). In this procedure, a microkeratome (shown on the left) is placed on the eye and is used as a precise knife to cut a thin corneal flap. The flap is then retracted, as shown in the rightmost picture below, and an excimer laser is used to ablate the underlying corneal tissue in order to achieve the desired correction. After the tissue is removed by the laser, the thin flap is laid back on top of the cornea as it quickly settles back into place. While conventional LASIK treatments can successfully correct defocus and astigmatism in the eye, they also typically introduce higher order aberrations, including large amounts of spherical aberration.

The following figure shows the wave aberration, point spread function, and retinal images for a normal eye and a post-operative LASIK eye over a large pupil diameter of 7 mm. The spherocylindrical refraction for this typical subject was 2.00 0.00. It is important to note that normal, typical subjects can have higher order aberrations, as seen in the top left image of the wave aberration. Even though this patient has normal vision and acuity, their wavefront is not flat and a moderate amount of higher order aberrations are present, mostly at the edge of the pupil (denoted by a higher density of contour lines). The top middle and rightmost images show the normal patient's point spread function and a convolved image of the letter E that subtends 1 degree of visual angle on the retina (roughly equivalent to the Snellen 20/200 line). You can see that there is a small amount of blur in the retinal image, but overall image quality is quite good.

The bottom three images were obtained from measurements on a post-operative patient who received a conventional LASIK procedure, also for a large 7 mm diameter pupil. The pre-operative spherocylindrical refraction for this patient was -3.00 0.00 and the attempted correction was -3.00 D over a large optical zone (>7 mm). The wave aberration over the central area of this patient's pupil is relatively flat and is similar to that of the typical eye shown above. However, the wave aberration in the post-LASIK eye becomes quite severe at the edge of the pupil (high density of contour lines) and shows a pattern that is indicative of spherical aberration. This, in fact, was the most predominant higher order aberration that was measured post-operatively in this patient and is typically one of the largest aberrations induced following a LASIK procedure (Oshika et al., 1999; Thibos & Hong, 1999; Williams et al., 2000). Spherical aberration tends to uniformly blur the retinal image and can be responsible for the halos sometimes reported by post-LASIK patients. This effect can be seen in this patient's point spread function, as well as the retinal image. An underlying halo and a ring are present in the patient's point spread function (bottom, middle image) and the letter E tends to be uniformly blurred in all directions (bottom, rightmost image). Therefore, for large pupil diameters, this post-op LASIK patient will see poorly when compared with the normal, unoperated eye shown above.

Perhaps a fairer comparison would be to look at the change in higher order aberrations within the same eye after a conventional LASIK procedure. This is shown in the figure below for a patient with a pupil size of 4.8 mm. This patient's pre-operative spherocylindrical refraction was -7.75 -2.00 x 57 and her wave aberration after a conventional correction of defocus and astigmatism (sphere and cylinder) is shown in the top, leftmost image (Pre-Op Best Corrected Wave Aberration). Even for a moderate pupil size of 4.8 mm, this patient still had a fair amount of higher order aberrations, dominated by trefoil and spherical aberrations. However, the pre-op point spread function is fairly tight and the retinal image of the letter E is only slightly blurred. In addition, all of this patient's higher order aberrations fell within the range of those expected from a normal population (Porter et al, 2001).

The bottom three images were measured in the same patient 4 months after receiving a conventional LASIK treatment. The attempted treatment was for a full correction (-7.75 -2.00 x 57) and the spherocylindrical refraction at this 4 month post-op visit was +0.25 -0.50 x 172. The wave aberration, shown in the bottom leftmost image, was measured when the patient was best corrected for defocus and astigmatism, as in the pre-op case. This allows us to directly assess the change in the eye's higher order aberrations and their impact on the point spread function and the retinal image. As in the pre-op case, the wave aberration over the central pupillary region is fairly smooth. The differences between the pre-op and post-op wavefronts become much more evident as one moves out towards the edge of the pupil. The density of contour lines becomes much higher in the post-op wave aberration, indicating that higher order aberrations have been induced by the procedure, namely large amounts of spherical aberration, coma, trefoil, and secondary astigmatism. This is evidenced in the point spread function, which is now much worse than the pre-op PSF. In addition, the retinal image now has some doubling in this patient's spherical aberration. This increase in higher order aberrations at the edge of the pupil in this patient is primarily caused by a steepening of the cornea near the transition zone of the ablation (Oshika et al., 1999). These aberrations would become even worse as the pupil diameter increases from a moderate value of 4.8 mm to a fully dilated size of 6 mm, severely impacting nighttime vision in this patient.

There is currently a major ongoing effort to refine laser refractive surgery to correct other defects besides conventional refractive errors (MacRae et al., 2000). The first groups who have performed wavefront guided customized ablation clinical trials are now starting to report preliminary data that are encouraging but are still tentative. Investigators are first trying to minimize the increase in higher order aberrations induced by conventional LASIK and then to reduce higher order aberrations below preoperative levels, especially in highly aberrated eyes. Several of the excimer laser manufacturers are already planning to change their ablation profiles from spherical to aspherical profiles to correct for spherical aberrations.

Ultimately, the visual benefit we could receive from attempts to correct higher order aberrations depends on two things. First, it depends on the relative importance of these aberrations in limiting human vision, and second, it depends on the finesse with which these aberrations can be corrected in everyday vision. There are a number of factors that limit the finest detail we can see, and an understanding of each factor is required to appreciate how much vision can be improved by correcting higher order aberrations, in addition to defocus and astigmatism (Williams et al., 2001; Charman, 2000). These include:

• Pupil size
• Chromatic aberration
• Changes in higher order aberrations with age
• Depth of field
• Photoreceptor sampling and neural factors
• The dependence of higher order aberrations on accommodative state
• Accommodative lag
• Rapid changes in the wave aberration with time
• Accuracy of centration of the correction
• Biomechanical effects of the cornea

Does this mean the quest to treat higher order aberrations is misguided? We feel it is not. The visual benefit in some eyes in the normal population is considerable. These eyes have large amounts of higher order aberrations just as some normal eyes have a large amount of astigmatism. In these patients we have found wavefront sensing to be a powerful tool in characterizing their specific optical abnormality, which was previously difficult to describe. The eyes that could potentially gain the most are those with large amounts of higher order aberration such as postoperative corneal transplant eyes or postoperative LASIK eyes with decentrations, central islands, or asymmetric ablations. If this latter group of difficult cases can be successfully treated with wavefront driven ablation, we will have taken a vital step in improving the safety and efficacy of excimer laser visual correction.

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Liang J., Williams D.R., & Miller D.T., (1994). Objective measurement of the wave aberrations of the human eye using a Shack-Hartmann wavefront sensor. Journal of the Optical Society of America A, 11, 1949-1957.

Tomlinson A., Ridder, III W.H., & Watanabe R., (1994). Blink-induced variations in visual performance with toric soft contact lenses. Optometry and Vision Science, 71, 545-549.

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Oshika T., Klyce S.D., Applegate R.A., Howland H.C., & El Danasoury M.A., (1999). Comparisons of corneal wavefront aberrations after photorefractice keratectomy and laser in situ keratomileusis. Archives of Ophthalmology, 127, 1-7.

Thibos L.N., & Hong X., (1999). Clinical applications of the Shack-Hartmann Aberrometer. Optometry and Vision Science, 76, 817-825.

Charman N., (2000). Ocular aberrations and supernormal vision. Optician, 220, 20-24.

Williams D.R., Yoon G.Y., Porter J., Guirao A., Hofer H., & Cox I., (2000). Visual benefit of correcting higher order aberrations of the eye. Jounral of Refractive Surgery, 16, 1-6.

MacRae S., Schwiegerling J., & Snyder R., (2000). Customized corneal ablation and super vision. Journal of Refractive Surgery, 16, S230-S235.

Guirao A., Williams D.R., & Cox I.G., (2001). Effect of rotation and translation on the expected benefit of an ideal method to correct the eye's higher order aberrations. Journal of the Optical Society of America A, 18, 1003-1015.

Porter J., Guirao A., Cox I.G., & Williams D.R., (2001). Monochromatic aberrations of the human eye in a large population. Journal of the Optical Society of America A, 18(8), 1793-1803.

Williams DR, Yoon G.Y., Guirao A., Hofer H., & Porter J., (2001). How far can we extend the limits of human vision. In S. MacRae, R. Krueger, & R.A. Applegate (eds.), Customized Corneal Ablation: The Quest for SuperVision (pp. 11-32). Thorofare, NJ: SLACK, Inc.

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