Researchers who have been studying the science of wavefront sensing and adaptive optics have known for some time now that correction of higher-order aberrations in the eye could offer patients better vision. Wavefront is now being incorporated into contact lenses and may one day offer patients the potential for 20/10 or even 20/8 visual acuity.

But is this so-called super vision a reality for all your contact lens patients? Here, well look at the benefits contact lens advances can offer your patients and help you set realistic expectations for them.

Aberrant Eyes
To understand the concept of super vision, lets review wavefront technology and optical aberrations.

Rays of light from a distant point source form a parallel bundle of rays. These can be converted to a wavefront by connecting perpendicular line segments on each ray at the same point in time. If the eye has no aberrations, they converge on a single point of the retina. If the eye has aberrations, the rays will be refracted at varying angles rather than focusing precisely on the back of the retina.

Aberrations fall into two groups:

Lower-order aberrations. These include sphere (myopia or hyperopia) and astigmatism. Lower-order aberrations constitute about 80% of the optical aberrations of the typical human eye needing an optical correction. These are measured by a standard refraction.

Higher-order aberrations. In the past, these were all lumped together as irregular astigmatism. Higher-order aberrations now include terms for coma and trefoil (third radial-order aberrations); spherical aberrations, secondary astigmatism and quadrafoil (fourth radial-order aberrations); and so on for as many types of aberration needed to describe all the refractive errors of the eye. In a typical normal eye, it generally takes about 22 additional terms beyond the sphere and astigmatism terms to describe the wavefront error.

These aberrations are typically displayed in a tree format. The most debilitating: the aberrations that fall in the central portion of the tree (e.g., spherical aberration, coma and secondary astigmatism), says Paul M. Karpecki, O.D., director of research for Moyes Eye Center in Kansas City, Mo. Coma and spherical aberration cause most night vision problems. By contrast, equal amounts of aberrations that are concentrated near the edge (as opposed to the center) of the pupil, namely trefoil, quadrafoil and pentafoil, do not affect vision as much.

Aberrations above the sixth order are not typically a significant problem in a normal population, but patients with abnormal corneas may have up to eighth-order aberrations, adds Geunyoung Yoon, Ph.D., of the Customized Vision Correction Laboratory at University of Rochester.

By measuring higher-order aberrations, you can seek to improve the remaining 20% of vision. Viable modes of correction then include refractive surgery, contact lenses and intraocular lenses.¹ Perhaps more importantly, help is available for patients (e.g., keratoconic patients) who had a normal visual experience in early life but now have highly aberrated eyes.

Baby Steps
Several manufacturers offer contact lenses designed to correct spherical aberration. While these aspheric lenses can offer improved vision vs. spherical lenses, they also have their limits, namely:

They are not customized to individual patients but based on population statistics. Any technology that is developed is going to have to be custom to the individual to be maximally effective, says Cristina Schnider, O.D., director of academic affairs for Vistakon.

They typically try to fix positive spherical aberration, the dominant aberration in the typical normal eye. If you have the typical amount of positive spherical aberration, [your patient] probably will see better wearing these lenses. But if you have negative spherical aberration and use these lenses, your patients vision will likely be worse, says Raymond A. Applegate, O.D., Ph.D., of the University of Houston College of Optometrys Visual Optics Institute.

They cant correct other higher-order aberrations. Richard Franz, O.D., vice president, professional relations, for Ocular Sciences, a CooperVision Company, points out that the aberrations that exist in a spherical contact lens/eye optical system come predominantly from the spherical aberration that is created by the contact lens itself. Correcting for this aberration is not specific to the eyes that will wear the lens, he says. But any truly spherical lens will generate spherical aberration and correcting this is beneficial, regardless of the optics of the eye.

True, these lenses cannot correct other higher-order aberrations. This could only be accomplished with a lens that neither rotates nor translates on the eye, Dr. Franz says. Additionally, the significant majority of aberration that exists in the eye is due to spherical aberration rather than other higher-order aberrations.

Despite their limits, existing contact lenses are a sign that technology will continually evolve toward a goal of super vision. Were taking baby steps, but were moving in the right direction, says Kirk Smick, O.D., of Morrow, Ga.

A bigger step: custom wavefront-designed contact lenses in which the optometrist measures each patients higher-order aberrations, then orders a contact lens specially designed to neutralize those aberrations. When the patient wears a custom lens, light from a distant object would pass through the contact lens with a wavefront shape that is exactly opposite the wavefront shape initially measured, correcting the patients eye.²

The concept of correction with customized contact lenses is similar to customized LASIK, but with three advantages:³ The effects of the lens are fully reversible. You can fit a different lens if the patient does not achieve the desired improvement or if the aberrations in the patients eye change due to aging or pathology. And, patients in whom refractive surgery is contraindicated can still wear customized contact lenses.

Patient Selection
Several companies and university research laboratories are working on customized wavefront-designed contact lenses. (See Making the Move to Custom Lenses) But not all patients are ideal candidates for these lenses. For a cornea without significant higher-order aberrations, wavefront-guided custom contact lenses are not necessarily a major advantage, says St. Louis optometrist Carmen Castellano.

David Williams, Ph.D., of the University of Rochesters Center for Visual Science, agrees. Some people have such good eyes that theres no point in trying to improve them with this super vision idea, he says.

Patients who will benefit include those who have:

Irregular corneas. This includes keratoconic patients, patients who have undergone penetrating keratoplasty or other surgical procedures with resultant irregular astigmatism, or patients with corneal trauma.

A patient who has irregular astigmatism isnt corrected optimally in glasses because these only correct the patients lower-order aberrations. But if you can measure the optical systems aberrations with a contact lens on the eye and find you have higher-order aberrations, a wavefront-guided customized contact lens can further improve the patients visual acuity and quality, says Ann Laurenzi, O.D., of Cole Eye Institute in Cleveland.

In keratoconic patients, the benefits to visual acuity and quality far outweigh the limitations of rotational and translational effects from customized contact lenses, Dr. Laurenzi says. Rotation and translation of the lens is smaller in the keratoconic eye due to the highly asymmetric cone shape of the cornea.³

Dr. Applegate agrees. You do not have to correct everything to provide significant benefit in highly aberrated eyes, he says. For example, the aberrations in a keratoconic patient are heavily weighed toward coma. Coma is reasonably tolerant to the small amounts of translation and rotation present in a prism-ballasted soft lens. That is, the correction does not require absolute registry to have a real benefit.

When higher-order aberrations are corrected with customized visual correction, patientsespecially keratoconic patients or patients who have spherical aberrations induced by laser refractive surgeryexperience improved contrast sensitivity and spatial vision over a range of distances, particularly when the pupils are large.⁴

Poor refractive surgery outcomes. Dont forget that LASIK can induce higher-order aberrations. People who have had refractive surgery are particularly challenged because man has messed up natures optics, says Vistakons Dr. Schnider.

Occupations (e.g., truck driver, police officer) that require superior night vision. These patients are more likely to be bothered by higher-order aberrations, says Dr. Karpecki. Other patients with high visual demands, such as athletes, may also benefit from custom correction.

Pupil sizes larger than 6mm. These patients have more spherical aberration in general.

Any inexplicable subjective visual complaints that arent corrected by standard refraction. For example, if a patient can see well during the day but has difficulty seeing at night, he may have a lot of higher-order aberrations, especially spherical aberration, Dr. Laurenzi says.

Making the Move to Custom Lenses Several companies and university research laboratories are at work on customized wavefront-designed contact lenses. Among them: Optical Connection Inc., San Jose, Calif. In 2003, Optical Connection acquired rights to several patents, which led to the development of the WaveTouchProcess. This manufacturing system can create contact lenses that correct higher-order aberrations, even for a seriously distorted eye, based on office aberrometry readings. Optical Connection recently entered into an alliance with Ophthonix Inc., which manufactures the Z-View Aberrometer, to produce iZon by Definition Wavefront-Guided Contact Lenses. Once iZon by Definition lenses are available, Kevin S. Bligh, senior vice president, sales and marketing for Optical Connection, envisions that the fitting process would work this way: You would position the patient at the Z-View, which is about the size of an autorefractor. The Z-View generates more than 11,000 points of information in a central pupillary area of about 6mm, measuring both lower- and higher-order aberrations. A color map describes the amount of higher-order aberrations, and a simulated Snellen chart shows how the patient sees when corrected to 20/20 and with all higher-order aberrations corrected. The process takes less than two minutes for both eyes. The Z-View Aberrometer yields a color map of the aberrations in each eye. Based on the patients aberration profile and visual demands, you would fit a pair of soft molded trial lenses, or predicate lenses, on the patients eye, ensure that lenses are stable, and take another set of readings. The Z-View determines where additional corrections are needed, then electronically transmits a bar code to the Optical Connections lab. The lab scans the barcode into the WaveTouchProcess and, within 48 hours, the patient has his or her new lenses. Optical Connections is conducting field studies, and expects iZon by Definition lenses to be available by the late second or third quarter of 2005, Mr. Bligh says.  SynergEyes Inc. (formerly Quarter Lambda Technologies Inc.), San Marco, Calif. The company is awaiting FDA approval for SynergEyes, its hybrid lens that features a gas permeable contact lens surrounded by a soft skirt. Jerry Legerton, O.D., the companys chief technology officer and co-inventor of SynergEyes, believes that rigid optics are ideal for a wavefront platform because any time you hydrate soft lens materials, variations result that could interfere with the lens ability to correct higher-order aberrations. Also, changes in the corneal topography or pre- and post-lens tear films cause significant changes in the aberration structure with soft lenses. Dr. Legerton says that the rigid portion of the lens will neutralize higher-order aberrations, some 70% of which are found on the front surface of the cornea, while the soft skirt will offer stability (rotational and translational) and comfort. Wave Contact Lens System, Ponte Vedra, Fla. The company uses corneal topography to calculate the aberrations on the corneal surface, then fabricates a lens that incorporates that aberration. Patients with abnormal corneas can achieve the most benefit from these lenses, says company president Scott Lewis. For example, keratoconic patients have improved from 20/40 to 20/25.  Customized Vision Correction Laboratory, University of Rochester. Geunyoung Yoon, Ph.D., and his colleagues have developed a Larger Dynamic Range Wavefront Sensor, which operates similar to Shack-Hartmann technology. Because patients with abnormal corneas have more higher-order aberrations with much larger magnitudes than normal patients, conventional wavefront sensors cannot measure their aberrations reliably, Dr. Yoon says. Adjacent spots tend to overlap or cross over. The Large Dynamic Range Wavefront Sensor, however, blocks every other lenslet so that each spot has twice the space for centroiding, resulting in twice the dynamic range without the loss of measurement sensitivity. With a grant from the National Institutes of Health, the laboratory is working on developing custom contact lenses, and collaborating with Bausch & Lomb to demonstrate the feasibility of these lenses. Visual Optics Institute, University of Houston. Raymond Applegate, O.D., Ph.D., and his group within the institute are investigating the use of ideal soft contact lenses to determine ideal parameters. Soft lenses increase wear time and comfort, he says, particularly in abnormal corneas.J.S.E.

Realistic Goals
Dr. Smick estimates that patients may see as much as 10 to 15% better than they could have without this technology. So, a patient who might otherwise see 20/25 with conventional contact lenses might see 20/20 with custom-designed lenses. I would hate to pin it down to a particular number, he says. But I would say that many patients can leave the doctors office planning to see better than they could have without the benefit of this technology.

Their better vision is due to less distortion from correcting the higher-order aberrations. Reducing the aberrations leads to increased contrast of the image, consequently improving contrast sensitivity. But that doesnt mean patients will leave your office with super vision, at least not at the present time. Its a possibility, but its a pie-in-the sky possibility at this time, Dr. Karpecki says. Thats due to several variables, including:

Lens dynamics. Studies have shown that excessive rotation and translation of contact lenses with respect to correct position on the eye are the main limitation for precise correction of the ocular aberrations.^3,5 So even if you could conceivably correct the higher-order aberrations, there still might be some inconsistencies in the visual quality, Dr. Castellano says.

Other researchers, however, have found that typical decentrations of a lens will only slightly reduce the optical benefits expected from an ideal correcting method.⁶

If you carve ideal optics on a contact lens, the better they align to the optical errors of the eye, the better the benefit, Dr. Applegate adds. Unlike spherocylindrical correction, the more complicated the aberrations structure, the better the registration between the optics of the eye and the optics of the correction have to be.

For the patient who has many higher-order aberrations, you need to consider how much movement is possible and how many aberrations you should correct. It becomes a whole series of trade-offs which need to be optimized, Dr. Applegate says.

With a soft lens, the typical goal is 0.5mm or less of movement on the cornea, says Sandra Zuccaro-Keyes, director of professional services for Optical Connection.

Tear film. An unstable or insufficient tear film dramatically degrades the optics, says Scranton, Pa., optometrist Joseph P. Shovlin. (See The Tear Film and Vision.)

The Tear Film and Vision Theres another reason to improve your patients tear film stability other than comfort: Tear film stability plays a role in patients overall quality of vision. Consider: Dry eye patients often complain of fluctuating vision, particularly in the afternoons. Tear break-up introduces significant optical changes in the eye that can result in degraded retinal image quality and reduced spatial vision. Complaints of fluctuating vision may be due to this decline in image quality that accompanies tear break-up.8 When patients complain of fluctuating vision, optometrist Kirk Smick immediately suspects dry eye. A poor tear film can alter measurements of higher- and even lower-order aberrations. A poor tear film sometimes affects capture of wavefront measurement and the tests accuracy, says Ann Laurenzi, O.D. Rapid tear break-up time may increase the number of higher-order aberrations in the eye. In one study, wavefront analysis of 20 dry eyes and 20 control eyes showed that total spherical-like and coma-like aberrations were significantly greater in dry eyes than in normal control eyes.9 Also keep in mind, every blink (typically less than 10 seconds apart) effectively creates a new optical surface. As long as the tear film remains stable, aberrations should remain small, says Raymond Applegate, O.D., Ph.D. Given these various effects of tear film stability, dry eye diagnosis and management becomes even more important. This whole concept of super vision begs the need for assessing tear film quality and movement, says optometrist Paul M. Karpecki. Researchers in Spain measured ocular aberrations in 15 eyes (15 patients). Total aberrations decreased on average by a factor of two to three times immediately after instillation of artificial tears, and the reduction was maintained after 10 minutes.10 The good news: There are strategies that already exist to improve the quality of the tear film, says Joseph P. Shovlin, O.D. These include lid hygiene, pharmaceutical treatments and, more recently, nutraceuticals. Manufacturers have introduced several products designed not only to care for contact lenses but also to help alleviate dryness and discomfort in contact lens wearers. Even solutions that maintain greater moisture are going to help if youre going to correct for the higher-order aberrations in contact lenses, Dr. Karpecki says.J.S.E.

Accommodation. Wavefront dynamics change with age, pupil diameter and even accommodation levels. As patients accommodate, their wavefront patterns will fluctuate somewhat, Dr. Shovlin says.

Point spread function (PSF). The PSF reveals how a point is imaged by an optical system. For example, if the patient has much coma, a light will appear to have a tail. If the patient has spherical aberration, he or she will notice a halo around the light. Several factors determine PSF, not all of which can be corrected with a contact lens. These include aberrations on the cornea and aberrations of the eyes crystalline lens, Dr. Karpecki says.

Space between photoreceptors in the retina. Imagine a picture in the newspaper, made up of individual pixels. The resolution wont be as good when the pixels are spaced far apart as when theyre packed more closely together. The same applies to the spacing of the photoreceptors in the retina, which is determined at birth. You have no control over the spacing, Dr. Karpecki says.

In other words, even if you took care of all the aberrations, but you have a big gap between each receptor, improving the optical image will increase the contrast but not the ability to resolve targets.

Cortical interpretation. The visual system has two major components: the optics that form the retinal image and the neural signal that translates the optical image. Both filter information. We can manipulate the optical side relatively easily, Dr. Applegate says. Manipulating the neural side is tougher. Not only does the neural system filter the spatial information much like a sieve; it also samples the retinal image much like the individual pixels of a digital camera sample the image formed by the camera lens. The sampling limit fundamentally limits the resolution of an imaging system.

The absolute sampling limit for the eye is somewhere between 20/8 and 20/10, depending on how densely packed the foveolar cones are. So, even if you correct all a patients higher-order aberrations, the brain may not be able to interpret the new images simply because it isnt used to this new way of seeing them and doesnt know how to interpret them, Dr. Karpecki says.

Also, by correcting all the higher-order aberrations with contact lenses, you may actually bring the lower-order aberrations of sphere and cylinder to unacceptable levels.⁷ In well-corrected eyes, the total amount of higher-order aberrations was equivalent to about 0.25D.⁷ Also, the aberration map of the eye may change over time.

A Way Off
That doesnt mean that so-called super vision will always be impossible to achieve and that patients will never see 20/8. Meanwhile, dont forget about improvements to existing contact lens technology, including silicone hydrogel materials, and advances in torics and presbyopic correction. It doesnt matter how well you correct higher-order aberrations. If you dont correct lower-order aberrations, it doesnt mean a thing, Dr. Karpecki says.

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