Optometry Catches the Wave
Optometry Catches the Wave
Wavefront technology is taking vision to new heights for spectacle and contact lens wearers.
By Desiree Ifft, Contributing Editor
IF YOU CAN'T measure it, you can't fix it. The field of organization management gave birth to this adage, but there isn't a more appropriate way to describe why wavefront technology is making such an impact in eye care. It has enabled refractive and cataract surgeons to provide their patients with better visual outcomes than had been possible in the past. And now, less than a decade after its first use in ophthalmology, optometrists are using the same technology to prescribe more precise vision correction with spectacles and contact lenses. This article provides a look at the latest developments and why wavefront technology likely will be part of your practice in the future.
Origins of Wavefront Technology
Astronomers devised wavefront technology several decades ago as a way to reduce distortions, or aberrations, that degrade the celestial images they view through telescopes. Light travels in waves, and the fronts of the waves form a flat surface, called a wavefront. If nothing were to interfere with the wavefront as it traveled through space, it would remain flat and result in a sharp image. However, the telescope lenses themselves as well as turbulence in the atmosphere distort the wavefront and degrade the image. Wavefront sensors are used to detect and measure the aberrations, which then can be cancelled out with adaptive optics. In astronomy, the adaptive optics are computer-controlled deformable mirrors, whose shape is changed to counteract the aberrations in real time, thus flattening the wavefront and sharpening the image.
When applied to the human eye, wavefront sensing reveals aberrations that aren't detectable by conventional means. The ocular wavefront can be distorted by imperfections in any part of the visual system, including the tear film, cornea, lens and vitreous. While traditional refraction methods allow detection and measurement of lower-order aberrations — myopia, hyperopia and astigmatism — wavefront sensing allows detection and measurement of higher-order aberrations, which include much more complex distortions, such as spherical aberration, coma and trefoil.
Lower-order aberrations blur vision, but higher-order aberrations can be responsible for additional visual disturbances, such as halos around lights or poor contrast sensitivity, depth perception or color vision. Therefore, reducing higher-order aberrations in addition to lower-order aberrations would theoretically result in the best possible quality of vision.1
Benefits in Refractive and Cataract Surgery
Wavefront technology's ability to detect higher-order aberrations revealed why some of the first patients to undergo LASIK experienced less than perfect results. In some cases, even patients who achieved 20/20 Snellen visual acuity postoperatively complained of problems with their vision, such as glare or halos or starbursts around lights, especially in low-light conditions. Their vision was perfect by traditional standards, yet their overall quality of vision was unsatisfactory. The culprit was higher-order aberrations, spherical aberration in particular, induced when the natural curvature of the cornea is altered.
Spherical aberration occurs when light strikes the periphery of a spherical optical device or surface. The peripheral light rays come into focus at different distances along the optical axis unlike light striking the center. The deviation degrades the resultant image.
The knowledge of induced aberrations spurred significant improvements in the LASIK procedure. New laser ablation patterns were developed. They apply not only spherocylindrical correction to the cornea but also ablate more sophisticated patterns based on each patient's wavefront data. Therefore, today's wavefront-guided procedures induce far fewer higher-order aberrations than conventional LASIK, producing excellent results for a greater number of patients.2
Similarly, wavefront technology led to the development of aspheric IOLs for cataract and refractive lens exchange patients. Their optical performance is based on the wavefront-based discovery that the eye functions best when the positive spherical aberration of the cornea is balanced by the negative spherical aberration of the crystalline lens. All standard IOLs are positive spherical aberration lenses. Therefore, replacing the crystalline lens with a standard IOL leaves the eye with too much positive spherical aberration, which can cause blur, a reduction in contrast sensitivity and glare and halos in the visual system. Aspheric IOLs don't add positive spherical aberration to the eye. Therefore, they induce far fewer unwanted optical side effects and provide better quality vision than their spherical predecessors.3 More recently, asphericity to help counteract higher-order aberrations also has been incorporated into multifocal IOL designs.
From the OR to the Exam Lane
The more detailed assessment of refractive status that wavefront technology provides is also proving useful for optometrists and their patients. It's being applied to spectacle and contact lens designs to increase their optical quality. In addition, it's being used to create more precise spectacle and contact lens prescriptions based on the aberrations unique to each eye. Both approaches make it possible for patients to experience a higher quality of vision. These developments, detailed below, signal some of the most significant changes in the art and science of refraction and optometric vision correction since the invention of the phoropter.
■ Spectacle lenses. Wavefront technology and advances in spectacle lens production methods now can be used in tandem to reduce optical aberrations inherent in various conventional spectacle lens designs. Wavefront technology can be used to identify the nature and location of higher-order lens aberrations. An innovation in lens surfacing, known as direct surfacing, digital surfacing or free form, makes it possible to apply wavefront design concepts to a lens. (Note that wavefront and free form are separate technologies. Not all free-form lenses have wavefront correction.)
Lenses produced by the free-form method are designed and surfaced with the assistance of computers. Each of the major lens manufacturers uses its own proprietary process, but most involve the key elements of a free-form generator and software to calculate point-by-point cutting instructions. Lenses are typically cut with a precision diamond tool rather than ground, and advanced polishing equipment is employed. Both the front and back surfaces can be used to optimize the optical quality of the lens and virtually any surface shape can be produced with precision. An optimized set of curves can be incorporated for each required lens power.
Wavefront technology has been especially integral to reducing aberrations in progressive addition lenses (PALs).
“Years ago, there was some recognition that when curvatures change across a PAL, higher-order aberrations are created,” explained Pete Hanlin, ABOM, LDO, technical marketing manager for Essilor of America. “That was pretty much impossible to measure, and even if you could measure it, there was no way to make the curves precise enough to remedy it. Once wavefront aberrometry became available, we were able to start defining where the aberrations were in the design. Then digital surfacing allowed the carryover of that information into the lens design.”
Essilor's Varilux Physio 360 PAL, for example, incorporates all of the latest wavefront analysis and production capabilities. Its anterior surface is wavefront optimized. Digital surfacing on the posterior surface ensures that the two surfaces work together to reduce distortions. “In addition to managing the lower-order aberration of unwanted astigmatism, we also can manage some of the finer aberrations such as coma,” Hanlin said. The end result for patients is a lens that provides improved clarity of vision at near, intermediate and distance.
■ Spectacle lenses for an eye's unique aberration profile. Ophthonix, based in Vista, Calif., has harnessed the power of wavefront technology to produce spectacle lenses that are customized for each patient based on his or her unique ocular aberrations. Its Z-View Aberrometer measures all second-order (lower) aberrations as well as third- through sixth-order (higher) aberrations that may be present in the eye. A proprietary algorithm analyzes the wavefront data to determine the best sphere-cylinder prescription for the lens. The prescription is sent to the Ophthonix lab, which uses free-form technology to transfer it onto the iZon lens. Clinicians can prescribe single-vision lenses, PALs and sun lenses.
Fine-tuning the spherocylindrical correction by taking into account how it's affected by higher-order aberrations allows patients to experience what many have described as “high-definition vision,” according to Ophthonix vice president of marketing Dennis Jarvis. Studies show that compared with conventional lenses prescribed by manifest refraction, iZon lenses provide better overall visual acuity, better performance in visually challenging conditions, including glare acuity, low and intermediate contrast and low light, and better performance on MNRead Critical Print Size.4
According to Jarvis, “It's important to emphasize that the iZon Lens is wavefront-guided, as opposed to wavefront-corrected.” The wavefront-guided strategy is preferred, Jarvis said, because the lens is optimized but not affected by shifts in the wearer's gaze angle or other temporary effects of higher-order aberrations.
Jacqueline Campisi, OD, was the first doctor in Connecticut to adopt the iZon Lens System. “I thought there had to be a way to help people who felt their vision was just not that good even while wearing their eyeglasses,” she said. “That led me to aberrometry.” Dr. Campisi said the Z-View Aberrometer is an outstanding refraction system. “I use it for every patient, and then I do my own refraction. Nine times out of 10, the patient prefers the Z-View prescription.”
The Z-View also has been a useful tool for early diagnosis of keratoconus, Dr. Campisi explained. Studies have shown that keratoconic eyes can have higher-order aberration levels five times higher than normal eyes.5 “Even when patients with undiagnosed keratoconus don't have a high level of astigmatism, the aberrometer detects ‘hot spots’ of higher-order aberrations,” she said. “I've seen these areas of significant aberration inferior on the cornea several times and investigated further with topography, which confirmed keratoconus.” (See “Detecting Keratoconus with Wavefront Aberrometry.”)
Marco Ophthalmics also offers a system that uses wavefront technology to provide patients with spherocylindrical spectacle lens prescriptions of unprecedented accuracy. In addition to pupillometry and keratometry, its 3-D Wave instrument performs corneal topography and measures lower- and higher-order aberrations using optical path difference technology (dynamic spatial skiascopy). Because changes in pupil size can cause changes in quality of vision, the instrument captures three lower-order refractions for each patient, at the center of the pupil, at the 3-mm zone and at the 5-mm zone. It simultaneously measures higher-order aberrations across the entire pupil. For a large percentage of patients, an increase in pupil size as small as 1 mm, which increases the retina's exposure to higher-order aberrations, changes the refractive power and axis of the eye.
In this patient, a 3-D Wave refraction at the pupil center and at the 3-mm and 5-mm zones, as well as the associated corneal topography reveal the potential for night vision problems. At the 5-mm zone, spherical refractive error increases by -.50D, and cylinder increases by -2.00D compared with the pupil center. The higher-order root mean square (RMS) value also increases with pupil size.
The information captured by the 3-D Wave allows the doctor to compare patients' lower-order refractive status at the different pupil sizes, which correspond to the photopic, mesopic and scotopic lighting conditions they experience in their daily lives. Furthermore, it reveals the effect higher-order aberrations have on vision at the various pupil sizes. When the information is transferred to the Marco Epic or TRS 5100 electronic phoropter, the patient can compare potential new prescriptions with his current prescription.
Based on the objective and subjective information, the doctor can choose a spectacle correction approach that provides the patient with the best functional vision for his or her specific needs. For example, patients whose photopic and mesopic/scotopic refractions differ noticeably may benefit from two separate prescriptions, one for light conditions and one for dark. In cases of a less noticeable difference, the doctor can prescribe a blend of the patient's small- and large-pupil refractive results to provide the best all around vision in a single prescription.
The various maps and calculations the 3-D Wave provides also make for a more efficient and productive exam, said John Warren, OD, who uses both the 3-D Wave and the TRS refraction system in his Racine, Wis., practice. “For example, the 3-D Wave refraction shows me the overall quality of vision of patients, whether they're likely to have night vision problems, and how well I can expect them to see with correction. The higher-order root mean square (RMS) values eliminate the type of situation where I spend 20 minutes refracting a patient and can't figure out why I can only bring them to 20/30. Also, if a patient should be correctable to 20/20 and isn't, I know to look for other problems, for instance with the retina.”
According to Marco product manager Mayah Shurbet, “There's a huge wow factor when patients realize that in less than 1 minute all of their data is collected. Networking the unit using viewing software allows the doctor to pull up the maps in the exam lane and show images to the patient. Patients are always impressed.”
Detecting Keratoconus With Wavefront Aberrometry
In the Connecticut practice of Jacqueline Campisi, OD, the Z-View Aberrometer, which is part of the Ophthonix iZon Lens System, has been a useful tool for early diagnosis of keratoconus, as the following two cases illustrate.
To demonstrate the benefits of the aberrometer for a TV program, Dr. Campisi performed a scan on a visiting TV crew member. The scan detected an elevated amount of astigmatism in the right eye and high levels of coma in both eyes. Topography confirmed bilateral keratoconus.
This individual had no history of spectacle or contact lens wear, but his vision became blurry in his right eye. In addition, he said the representation of the visual effects of coma, included on the wavefront printout, accurately reflected how he sees. Dr. Campisi advised him to consult an optometrist in his home town for a full workup and to inquire about an RGP lens fitting.
This patient reported that wearing spectacles made it difficult to do his job as a photographer. He also said the Frequency 55 soft contact lens (14.4 diameter, 8.7 base curve) on his right eye often shifted and glare bothered him. Aberrometry revealed high amounts of coma (red inferior zone), so Dr. Campisi performed topography and confirmed bilateral keratoconus.
Concerned about the cost of custom lens fitting, Dr. Campisi prescribed a Boston ES Magniclear Plus contact lens (−1.00 +1.75, 10.2 diameter, 7.75 base curve) for his right eye. On follow-up, he said the lens occasionally popped out of his eye so he only wears it during photo shoots. For his best visual acuity and comfort, he prefers the iZon spectacles Dr. Campisi also prescribed.
■ Contact lenses. Contact lens wearers are also benefiting from wavefront technology. Several manufacturers offer rigid or soft single-vision or multifocal lenses that incorporate asphericity to provide crisper vision. Some designs are aimed at correcting for a population average amount of higher-order aberrations; others take each eye's aberrations into account. Changing lens thickness for aberrated areas of the eye is one way to accomplish the latter.
With the same approach used for prescribing spectacle lenses, the Marco 3-D Wave and electronic refraction systems can be used to fine-tune rigid or soft contact lens prescriptions based on individual eye data.
Another of the newest options is individualized wavefront-corrected contact lenses from WaveTouch Technologies. According to Kevin Bligh, the California company's executive vice president, approximately 25 optometrists and ophthalmologists are currently prescribing the soft lenses. The design of the lenses is based on each eye's lower- and higher-order aberrations and intense attention is paid to the fit of the lens on the eye.
In a patented process, the eye's wavefront is measured first on its own and then again through a specially designed WaveTouch Acquisition lens. “The goal is to identify the lower- and higher-order aberrations present in each patient,” Bligh said. “These measurements are then retaken over the acquisition lens. This is important for two reasons. First, it allows us to compensate for aberrations that the lens itself causes. Second, not only is the patient's individualized correction calculated, but the location of the visual axis of the lens is optimized to the patient's visual axis.”
Two aberrometers have been certified for use in prescribing the WaveTouch lenses: the iTrace Combo from Tracey
Technologies and the 3-D Wave Analyzer from Marco. Doctors send the patient's measurements to the company, where they're converted into a manufacturing data point file that guides the production of a methafilcon A lens for quarterly replacement.
The lenses can provide the best possible vision for patients with various levels of higher-order aberrations, and can be a particularly helpful option for difficult-to-correct vision, such as in keratoconus and post-LASIK cases.6
Advanced Diagnostics on the Horizon
The wavefront-related innovations in spectacles and contact lenses that have occurred already will continue to progress as long as they're providing superior vision for patients, said Paul Karpecki, OD, FAAO, director of research in cornea and external disease at Koffler Vision Group in Lexington, Ky. He also said he expects wavefront technology to be used more and more in optometric practices for the diagnosis of conditions other than refractive error.
“Evidence is emerging that clinicians can use wavefront to objectively detect the visual effects of early cataracts7 (which can be difficult to do with our traditional tools), and whether surgery is needed. It's really beginning to look as if our old friend the phoropter eventually will be replaced by measurements of both lower- and higher-order aberrations. The increasing availability of quality, less expensive aberrometers, which are often being combined with other types of instrumentation, makes this even more likely.” nOD
- Yoon GY, Williams DR. Visual performance after correcting the monochromatic and chromatic aberrations of the eye. J Opt Soc Am A Opt Image Sci Vis. 2002;19:266–275.
- Schallhorn SC, Farjo AA, Huang D. Wavefront-guided LASIK for the correction of primary myopia and astigmatism a report by the American Academy of Ophthalmology. Ophthalmology. 2008;115:1249–1261.
- Dick HB. Recent developments in aspheric intraocular lenses. Curr Opin Ophthalmol. 2009;20:25–32.
- Seiple WH, Szlyk JP. Clinical investigation into the vision performance provided by the iZon spectacle lens system. Review of Optometry. October 2008:S1–S16.
- Pantanelli S, MacRae S, Jeong TM, Yoon G. Characterizing the wave aberration in eyes with keratoconus or penetrating keratoplasty using a high-dynamic range wavefront sensor. Ophthalmology. 2007;114:2013–2021.
- Meshel LG. Wavefront corrected contact lenses a new opportunity for low vision and hard to fit patients. Poster presented at the Global Specialty Lens Symposium, January 15–18, 2009, Las Vegas, Nev.
- Sachdev N, Ormonde SE, Sherwin T, McGhee CN. Higher-order aberrations of lenticular opacities. J Cataract Refract Surg. 2004;30:1642–1648.
Optometric Management, Issue: June 2009