These Aren't Your Father's Contact Lenses
Researchers are exploring novel applications—both vision-related and non vision-related—for contact lenses. Here's a peek at the future.
By Pete S. Kollbaum, OD, PhD, FAAO
Seven billion people now inhabit the world in which we live. Approximately 3.5 billion of these people require vision correction; however, only about 120 million people wear contact lenses. Doing some simple math, we can easily see that tremendous growth opportunities exist in the contact lens market.
Some 3 billion people who need vision correction choose not to wear contact lenses for some reason. Can we, as contact lens practitioners and contact lens manufacturers, remove some of the obstacles that are preventing these people from wearing them? And suppose we could expand the functionality of contact lenses to make them useful even for people who do not require vision correction? Perhaps an additional 3 billion people would have the opportunity to use contact lenses.
This article briefly describes some of the ongoing research efforts aimed at expanding the use of contact lenses in people who require vision correction, as well as those who do not, opening up the potential growth in contact lens use by 98 percent.
VISION-RELATED GROWTH OPPORTUNITIES
Control of Myopia Progression
According to best estimates, by the year 2020, there will be between 1.6 billion and 2.5 billion people with myopia in the world (Holden et al, 2008). Some regions in Asia, such as Hong Kong, China, Singapore and Taiwan, are experiencing an "epidemic" of myopia (Rose et al, 2008), and it is clear that, regardless of the reason for this spike in myopia, a well-researched and validated prevention or treatment option for myopia progression must be found.
Although researchers have found that alignment-fitted GP lenses do not successfully control the progression of myopia in children (Walline et al, 2004), numerous pilot studies comparing children of similar age and baseline refractions in the United States (Rah et al, 2002; Walline et al, 2009) and Hong Kong (Cho et al, 2005) have shown orthokeratology GP lenses can reduce axial growth rate and spherical equivalent refraction.
Recent research suggests the peripheral retina may be important in controlling eye growth (Smith et al, 2007) and that peripheral retinal blur is a major factor leading to uncontrolled axial length growth (Mutti et al, 2000). Some potential for retardation of myopia has been shown with the use of currently available center-distance multizone lenses, initially designed to correct presbyopia (Aller and Wildsoet, 2008; Walline, 2011).
Additionally, a "dual-focus" lens designed with the specific aim of controlling myopia is currently available in Hong Kong. This lens, MiSight (CooperVision), has a center zone and several peripheral annular zones to correct distance vision, while alternate concentric rings induce myopic defocus. A study by the inventors of the MiSight lens design found that the lens significantly reduced the rate of axial length growth and refractive error change over a one-month period (Anstice, 2011). Longer-term studies are in progress.
A review of the patent literature suggests other companies are working on lens designs with myopia control capabilities (e.g., Holden and de la Jara, 2007).
One potential concern is that, like the decreased quality of vision sometimes noted with multizone lenses to correct presbyopia (e.g., Richdale et al, 2006), similar visual degradation may be experienced by children wearing multizone lenses to control myopia, which may hinder their daily performance or may make them not want to wear the lenses. A recent study of 24 young subjects (Meyer et al, 2010) has shown, however, that wearers of the newly developed dual-focus lenses specifically designed to control myopia progression have good measured visual acuity and good patient-reported vision quality that did not differ significantly from typical presbyopic multifocal corrections; however, low contrast acuity was slightly worse than optimal spectacle correction. Figure 1 shows the mean and 95 percent confidence intervals for the measured logMAR acuity and patient-reported vision for a group of subjects while wearing a traditional presbyopic design, the new dual-focus design and optimally corrected with spectacle lenses. Although promising, these results indicate multizone lenses, like multifocal lenses, may not be for everyone, but rather are most beneficial for patients most concerned about myopia progression, where any potential slight decrease in image quality is worth the gain achieved in controlling myopia progression.
Figure 1. (a) Mean binocular logMAR distance letter acuity and (b) patient-reported vision for typical multifocal, dual-focus and habitual lens correction.
Correction of Presbyopia
It is estimated that more than 1 billion people in the world have presbyopia (Holden et al, 2008), but fewer than 7 percent of these people wear contact lenses to correct their presbyopia. Despite the availability of alternative lens designs and potentially improved vision with GP lenses, most lenses being prescribed today are hydrogel or silicone hydrogel (Morgan et al, 2011). If we assume this preference trend will continue, this suggests that although new soft designs are released by contact lens manufacturers each year, there may be a barrier to the success of these designs. Typically, measured visual acuity with these lenses is quite good; however, patient-reported quality is sometimes decreased (Richdale et al, 2006). What this means is that, although a patient may be able to read 20/20 on the acuity chart, he may not be satisfied with the quality of his vision.
This altered letter quality may be a direct result of the lens design. For example, multifocal lenses or lenses with a gradient power change from the lens center to the periphery typically contain higher levels of spherical aberration. This results in multiple focused/defocused images being formed on the retina simultaneously. Figure 2a depicts an eye wearing a multifocal lens with positive spherical aberration, where the peripheral rays of light focus on the retina (dashed line) in front of the central rays of light. Similarly, in bifocal lenses or lenses with discrete power zones, there will always be focused light and light defocused by the amount of the lens add power. Figure 2b shows a two-zone bifocal lens, in which the center of the lens contains the near power. In this example, light passing through the central, near-powered section of the lens is focused on the retina (dashed line); however, the distant light passing through the peripheral, distance-powered annulus creates a blurred image on the retina. In both of these cases, the combinations of focused and defocused light create multiple images, which wearers of these lenses may report as ghosting or shadowing.
Figure 2. Ray diagrams of an eye wearing (a) a multifocal lens with positive spherical aberration and (b) a 2-zone bifocal lens.
Several different methods have been proposed to reduce ghosting with presbyopic designs yet avoid the dissimilar image quality between the two eyes common to monovision. For example, clinicians have long employed forms of modified monovision. Recently, adaptive optics systems have been used to induce specific combinations of optics or modified monovision in the two eyes. Specifically, researchers have found that a small amount of defocus or add power (e.g., +1.50D) combined with a small amount of spherical aberration (Zheleznyak et al, 2011) yields intermediate acuity that is superior to typical monovision.
At Indiana University, we have employed computational modeling, physical modeling and psychophysical testing to evaluate an alternative way to combine spherical aberration and defocus (Kollbaum et al, 2011). Our hypothesis is that if spherical aberration were always combined with defocus of the same sign, the contrast of the overall image would be decreased and, therefore, the noticeability of the ghosting would be decreased. Figure 3 depicts a sample distance-corrected patient viewing at distance while wearing a +2.00D center-distance bifocal contact lens. The image on the left represents what the patient would see if his eye had negative spherical aberration. Although the focused portion of the letters remain legible, the defocused or ghosted images immediately surrounding the letters are also quite visible. The image on the right represents what the patient would see if his eye had positive spherical aberration. Although the overall contrast of the image is slightly decreased, the visibility of the ghosted image is dramatically decreased. This work, combined with our other work, suggests that a strategy of adding negative spherical aberration to the distance zone of a lens and positive spherical aberration to the near zone of the lens may assure that defocus for both distance and near targets is always accompanied by the same sign of spherical aberration, reducing the visibility of the ghosted image.
Figure 3. Simulated retinal images of a distance-corrected eye wearing a +2.00D add center-distance bifocal lens. The eye on the left has negative spherical aberration; the eye on the right has positive spherical aberration.
NON-VISION-RELATED GROWTH OPPORTUNITIES
Drug delivery contact lenses
Ocular drug delivery has always been problematic, and current ophthalmic drug delivery depends greatly upon topical drops. Although eye drops account for 90 percent of all ophthalmic solutions used, we know that only 5 percent of the drug actually penetrates the cornea and reaches the ocular tissue (Gulsen and Chauhan, 2004). This poor penetration rate leads to the necessity for increased drug volume and the potential for systemic problems or side effects (Ciolino et al, 2009; Gulsen and Chauhan, 2004).
Although drug uptake and delivery associated with conventional soft contact lenses has proven inconsistent in many cases (Karlgard et al, 2003a; Karlgard et al, 2003b), recent advances in nano and polymer technology have opened the possibility of more consistent release of a drug from a soft contact lens. Some of the common uses for a vehicle of this nature include the administration of glaucoma, dry eye and topical antibiotic treatments.
In one technique, called microemulsion, the drug molecules can be encased within a protective hydrocarbon micelle, which is engineered to penetrate the corneal cells. These emulsified drug particles can then be loaded into a hydrogel soft contact lens and once the lens rests on the eye, the particles diffuse. The microemulsion technique increases membrane permeability, thus increasing the bioavailability of the drug and ultimately providing a sustained release of the drug into the deeper corneal layers. Because the rate of the drug release can be controlled, a uniform low-level dose of the drug is provided (Sahoo et al, 2008). Furthermore, recent research has shown that the additional infusion of fat-soluble vitamin E along with a drug into a soft contact lens can increase the diffusion time of the drug (Peng and Chauhan, 2011).
Although researchers have demonstrated preliminary success (Gulsen and Chauhan, 2004), commercial development of these lenses will require additional work. If these technologies come to fruition, however, they may replace traditional ophthalmic drops as a means of treatment for many ocular conditions in patients of all ages. Instead of prescribing a 6-month supply of glaucoma drops, for example, imagine prescribing a 6-month supply of contact lenses that will release a controlled dose of the medication all day, every day.
Photochromic Contact Lenses
Another example of a revolutionary advancement in polymer technology is the development of photochromic contact lenses. Using a microemulsion technique similar to what is used for ophthalmic lenses, researchers at the Institute of Bio-engineering and Nanotechnology (IBN) in Singapore claim to have developed the world's first photochromic soft contact lens.
The lens shown at the top of Figure 4 is a prototype lens (www.technologyreview.es/biomedicine/23922). IBN researchers are still pursuing commercialization of this technology, but if they are successful, these types of contact lenses would benefit patients who want improved sun protection but are not able to wear traditional sunglasses.
Figure 4. (Top) Prototype photochromic contact lens; (center and bottom) Triggerfish lens for continuous IOP measurement.
Contact Lenses for Diagnosis
Another area of untapped potential for contact lenses is their use as diagnostic devices, such as for use in glaucoma. Although glaucoma is a common cause of blindness, a definitive treatment for this disease has proven elusive. Traditionally, glaucoma therapy involves managing intraocular pressure (IOP) with medications administered topically, sometimes several times a day. Treatment is often hindered, however, when patients do not adhere to their therapy. The drug delivery contact lenses described above may help overcome this challenge.
Likewise, successful treatment hinges on the clinician's ability to accurately diagnose glaucoma and monitor the efficacy of therapy. For example, a patient's IOP may be within normal limits at the time of his office visit, leading the clinician to conclude the current treatment is successful (or that no treatment is necessary). At other times during the day, however, the patient's IOP may be much higher. Sensimed, a Swiss biotech company, has developed the Triggerfish lens, a single-use SiHy contact lens aimed at providing continuous IOP measurements (Figure 4 center and bottom). The lens is embedded with a microprocessor and a strain gauge that encircles its outer edge. When fluid accumulates in the eye, the diameter of the cornea changes, and that change is detected by the strain gauge. This information is then sent wirelessly to a receiver. The company has received safety approval for this lens in Europe, and approval is expected from the U.S. Food and Drug Administration by the end of this year.
Contact Lenses for All
We live in a rapidly changing world. As times change, we change how we view things, and new opportunities arise. I have briefly discussed a few newly available and developmental technologies that may lead to new commercial products, giving us opportunities as contact lens practitioners to expand our roles. These technologies include some that aim to improve vision and others that aim to capitalize on other uses for contact lenses. Finances aside, in theory, these new technologies have the potential to make virtually everyone a contact lens wearer and to make contact lenses drastically different from what they were just a few decades ago when our fathers (in the figurative sense for all and the literal sense for some) were fitting these lenses. CLS
For references, please visit www.clspectrum.com/references.asp and click on document #193.
|Dr. Kollbaum is an assistant professor at the Indiana University School of Optometry, where he teaches and performs translational research in the areas of contact lenses and clinical optics. He has received research funding from Ciba, CooperVision and Vistakon. He has filed a patent application regarding a technology discussed.|