FOCUS ON OPTICS
Improving the Quality of Our Patients' Vision: Aspheric Lens Designs
Aspheric contact lenses can decrease spherical aberrations and improve visual quality.
By Kathryn Richdale, OD, MS, FAAO
Lately, the contact lens community seems to be in a race to claim the top spot in comfort or oxygen permeability, with rials, wetting agents, and lens care products. While these are important factors in the overall health and satisfaction of contact lens wearers, it appears as though we're straying from our primary job as vision care providers: to provide good vision.
The need to get back to vision is heralded by our patients. Although comfort, convenience, and cosmesis are all important factors in meeting the demands of our patients, a recent survey of nearly 4,000 ametropes in the United States, Europe, and Asia found that a major factor in choosing eyecare products is one that provides the “best possible overall vision” (Market Probe Europe, December, 2009) (Figure 1).
Figure 1. Vision needs survey of 3,800 ametropes. Vision was ranked as the most important factor in selecting an optical correction.
An aberration is any error in, or departure from, a perfect optical system. The refractive error of the eye can be divided into lower-order and higher-order aberrations. Lower-order aberrations include myopic or hyperopic defocus and astigmatism and account for about 93 percent of the total error in a normal eye (Porter et al, 2001; Thibos et al, 2002). Higher-order aberrations include spherical aberration, coma, trefoil, and others. Once myopia or hyperopia and astigmatism are corrected, the higher-order aberrations become significant factors in visual quality (Thibos et al, 2002; Lindskoog Pettersson et al, 2008).
Spherical aberration is typically the largest of the higher-order aberrations (Thibos et al, 2002). Spherical aberration occurs because light rays coming through the periphery of the pupil are bent more or less compared to those through the center, which causes them to converge in front of or behind the retina (Figure 2). On average, healthy eyes have about 0.10 microns to 0.15 microns of positive spherical aberration (Thibos et al, 2002; Lindskoog Pettersson et al, 2008).
Figure 2. Spherical aberration: light rays from the periphery are not focused on the retina.
Unlike lower-order aberrations, spherical aberrations increase dramatically with pupil size (Charman, 1991). Spherical aberrations are also altered by age, surgery, and disease and can degrade optical quality (Porter et al, 2001; Lindskoog Pettersson et al, 2008; Maeda, 2009). In fact, there is a direct correlation between certain ocular aberrations and visual symptoms. Specifically, coma can induce monocular diplopia, and spherical aberrations can cause star-burst and glare symptoms (Maeda, 2009). The presence of glare not only elicits complaints of visual discomfort, but also decreases visual acuity, especially under low lighting or low-contrast conditions (Allen et al, 2008).
The use of aspheric optics can decrease spherical aberrations and improve visual quality. Several studies have demonstrated that the application of aspheric optics can improve low-contrast and mesopic acuity and decrease glare (Kershner, 2003; Mester et al, 2003; Dietze and Cox, 2004). Furthermore, patients have cited an improvement in the subjective quality of vision with an aspheric optical correction (Tran et al, 2005).
Contact Lens Optics and Vision
Not only are there inherent aberrations in the healthy eye, but traditional contact lenses can induce additional aberrations, with positive spherical lenses adding positive spherical aberration and negative lenses inducing negative spherical aberrations (Figure 3). On average, a −4.00D to −5.00D spherical contact lens will compensate for the 0.10 microns to 0.15 microns of positive spherical aberration found in the typical eye; however, any contact lens power outside of that range may induce excess aberrations and could degrade visual quality (Lindskoog Pettersson et al, 2008).
Figure 3. Spherical aberrations with plus (top) and minus (bottom) powered contact lenses.
A recent global study found that 39 percent of contact lens wearers complained of halos, 46 percent of glare, and 41 percent of blur or difficulty seeing fine details (Market Probe Europe, December 2009). These complaints may be due to uncorrected higher-order aberrations (Dietze and Cox, 2004; Allen et al, 2008). Fortunately, spherical aberration is relatively easy to correct as it is rotationally symmetric about the pupil, whereas other higher-order aberrations vary by orientation (Charman, 1991; Dietze and Cox, 2004).
There are a number of aspheric contact lens designs on the market today, in both soft and GP lens materials. Soft aspheric contact lenses are created by altering the shape and thickness of the lens from the center to the edge. Contact lenses can be designed to counteract the spherical aberrations of just the contact lens alone or the combination of the spherical aberrations from the contact lens and the eye. The latest advances in aspheric technology include more precise correction for each 0.25D step along the entire power range, and designs that account for the change in contact lens shape—and therefore optics—when the lens is placed on the eye. Bausch + Lomb's PureVision2 HD lenses are designed to reduce spherical aberrations across the entire profile of contact lens powers and also to compensate for the inherent spherical aberration of the average eye. A multi-center study of 171 established contact lens wearers found that, when patients were given a chance to try the PureVision2 HD aspheric contact lens for one week, 75 percent of patients stated that the aspheric design provided superior vision, and 77 percent reported a reduction in glare and halos (Millward Brown, 2010). Practitioners can educate themselves on the specific attributes of various designs by contacting their local representatives or visiting the company Web sites.
Providing Optimal Vision for Lens Patients
The methods typically used to measure patients' vision—high-contrast charts with ample illumination—test only a very small range of their full visual function. Because most practices don't have the time or equipment to test contrast sensitivity and higher-order aberrations, it's important to inquire about the presence of blur, glare, or halos and the general “quality” of the patient's vision.
It may be helpful to keep in mind which patients could be most affected by spherical aberrations and would thus be ideal candidates for an aspheric contact lens. Patients who have higher refractive errors, especially hyperopic refractive error, could benefit from aspheric lenses because traditional bi-spherical soft contact lenses induce greater amounts of spherical aberrations. It's important to remember that all aspheric lens designs are not equal; whereas some designs provide the same aberration control across all powers, others have modified optics for various lens powers. Patients who have large pupils may also benefit from aspheric optics, as the impact of aberrations increases with pupil sizes above 3mm. Lastly, inquire about patients' visual needs. Do they work or study in dim lecture halls or conference rooms? Are they involved in music or theater performance? Do they drive before dawn or after dusk? All of these factors could indicate that an aspheric contact lens could improve your patients' quality of vision.
There are a few key factors to keep in mind when fitting an aspheric contact lens. First, accurate centration is important for any contact lens fitting, but is especially critical for aspheric designs as decentration of aspheric optics could induce even greater higher-order aberrations. Second, lower-order aberrations have a much greater effect on vision and should always be fully corrected before minimizing higher-order aberrations. While previous reports suggested that aspheric lenses “mask” uncorrected cylinder, more recent studies demonstrated that aspheric optics do not correct astigmatism (Kollbaum and Bradley, 2005). A toric contact lens with an aspheric front surface would be a better choice for a patient who has at least 0.75D of astigmatism.
Finally, patients who have a vastly different spherical aberration profile compared to the population norm may actually experience worse vision with mass-produced aspheric lenses. Likewise, there are significant differences in the optical designs among various lens manufacturers. Most eyecare practices don't have an aberrometer available to measure a patient's unique aberration profile with each contact lens, so it's important to allow the patient to trial contact lenses in the “real world.” Educate patients on the advanced optical design of the lens and encourage them to compare the quality of their vision in their own environment (Figure 4).
Figure 4. Image with +0.15 microns of spherical aberration and a 6mm pupil (left) and with aberrations corrected (right).
It's our primary job, as eyecare practitioners, to inquire about, measure, and seek to improve our patients' vision. Visual function is not simply the ability to read high-contrast letters on a well-lit chart; today's patients desire the ability to read a smart phone while typing on a netbook and viewing a PowerPoint presentation in a dimly lighted room. But, thanks to advances in optical design, we can improve the quality of lens wearers' vision and keep pace with their growing visual demands. CLS
For references, please visit www.clspectrum.com/references.asp and click on document #184.
|Dr. Richdale is a senior research associate and clinical attending in the Contact Lens Service at The Ohio State University College of Optometry. She has received support for her research from industry, private and government grants including B+L and Ciba Vision contact lens companies.|