The Quest for the Ultimate Contact Lens
CONTACT LENS ADVANCES
The Quest for the Ultimate Contact Lens
Although the quest continues, the industry has marked some important milestones.
|Dr. Draper joined Ciba Vision in May 2005 working in professional affairs and is currently is global manager on Dailies Total1 for Alcon Visioncare.|
|Dr. Perez-Gomez is head of Professional Affairs, Alcon Vision Care for EURMEA.|
|Dr. Giles is the associate director Clinical Applications for Alcon’s Surgical Department in Global Medical Affairs. He is responsible for managing and coordinating customer field training and support activity of Clinical Application Specialists.
By Mark Draper, BOptom, MCOptom, MBA; Inma Pérez-Gómez, PhD, MCOptom, FAAO, FBCLA; & Tim Giles, OD, MBA, FBCLA, FIACLE
Leonardo da Vinci sketched out some ideas based upon the ingenious but inelegant concept of placing the eye in contact with water in a bowl, taking perhaps the first step in the development of contact lenses. Fast-forwarding 500 years, we find that contact lens technology has made great strides, yet the quest for the ultimate contact lens continues.
Let’s take a look at some of the breakthrough technologies that have had a positive impact on the contact lens market in recent years and consider what will bring us ever closer to the so called “holy grail” of contact lenses: the successful merger of comfort, vision, health, and convenience, delivered to all contact lens wearers all of the time.
Seeking the Holy Grail
During the past 25 years, soft contact lenses have undergone dramatic advances as manufacturers strive to fulfill the ultimate desire of contact lens wearers: to feel as if they are wearing no lenses at all. The challenge has always been to meet the needs and expectations of lens wearers and eyecare professionals while avoiding the comfort issues that often cause patients to stop wearing their lenses. The motivation behind this ongoing quest is compelling, as the benefits of contact lens wear are well documented. For patients, the functional benefits of more natural, unimpeded vision without the inconvenience and annoyance of eyeglasses that slip down or fog up are complemented by emotional benefits, ranging from increased confidence for school children (Rah et al, 2010) to a more youthful appearance for presbyopes.
To achieve the objectives of comfort, vision, health, and convenience requires specific contact lens properties that may conflict with other objectives or ocular needs. For example, silicone hydrogel contact lens materials provide high oxygen transmissibility (Dk/t), which is beneficial for corneal health, but their hydrophobicity may interfere with lens wettability, which is a key factor in vision and comfort. Being able to wear contact lenses for an extended period is convenient, but extended wear comes with a risk of complications. The challenge is to optimize the desired features while minimizing the issues that they bring. Contact lens manufacturers continue to address these sometimes conflicting features.
With ocular health an overarching concern, the quest for the ideal contact lens has focused mainly on lens materials with high Dk/t to avoid corneal hypoxia. The knowledge that silicone has better oxygen transmissibility compared to some HEMA-based hydrogels led to the development of silicone hydrogel contact lens materials. Manufacturers have used various strategies to make silicone hydrogel materials wettable, comfortable, and deposit-resistant. Some silicone hydrogel lenses employ plasma-treated surfaces for improved wettability, while others incorporate internal wetting agents and polymer changes. Table 1 provides a list of the major spherical silicone hydrogel contact lenses.
The use of highly oxygen-permeable silicone hydrogel contact lens materials has significantly reduced the number of hypoxia-related complications in daily and extended wear. In fact, hypoxia-related conditions such as microcysts, striae, and bulbar and limbal hyperemia have been reduced among patients who wear silicone hydrogel contact lenses (Stapleton et al, 2006).
Coincident with advances in polymer chemistry have been breakthroughs in contact lens designs, resulting in improved optical clarity and crisper vision. Silicone hydrogel contact lenses are now available in a wide array of designs and parameters to correct spherical, astigmatic, and presbyopic refractive errors. Advanced toric soft contact lens designs have improved comfort for patients and have made these lenses more predictable to fit. Furthermore, newer manufacturing methods and technologies have eliminated many of the optical distortions and defects that used to plague cast-molded soft contact lenses.
These significant improvements have propelled silicone hydrogel contact lenses into a position of prominence (Nichols, 2012; Morgan et al, 2012), and they are now generally considered the state of the art in soft contact lens technology. In our opinion, silicone hydrogel-based contact lens materials will soon replace HEMA-based materials, similar to how the more oxygen-permeable GP materials supplanted PMMA materials.
|Silicone Hydrogel Contact Lenses|
||OXYGEN PERMEABILITY (DK)
||OXYGEN TRANSMISSIBILITY (DK/T) @–3.00D
|Bausch + Lomb
||Air Optix Night & Day Aqua
||Air Optix Aqua
||filcon II 3
||filcon II 3
||Acuvue TruEye (United States only)
Despite impressive advances, certain stumbling blocks remain on the road to contact lens perfection. These include:
Lens Comfort Fear of discomfort is the main reason why people are reluctant to try contact lenses, and lens comfort is a key issue for the estimated 125 million contact lens wearers around the globe (Richdale et al, 2007). According to a 2010 survey, dropout rates were 15.9 percent in the United States, 17.0 percent in the Americas (including the United States), 31 percent in Asia and the Pacific Rim, and 30.4 percent in Europe, the Middle East, and Africa (Rumpakis, 2010). Survey respondents cited discomfort as the main reason why they stopped wearing their lenses.
Lens design, lens fit, and deposit formation can affect comfort, but the most common symptom experienced by contact lens wearers is ocular dryness (Riley et al, 2006). In this regard, the most important attribute of a soft contact lens is the lens surface and its interaction with the tear film. Regardless of the reasons for complaints of discomfort, however, the prevalence of these complaints and the resulting discontinuations indicate that soft contact lens wear is still not optimal.
Tear Film Stability Successful, comfortable contact lens wear requires a stable tear film. The Dry Eye WorkShop (DEWS) implicated tear film instability as a core mechanism in the evaporative dry eye process, which includes contact lens-related dry eye (DEWS Report, 2007). To achieve tear film stability, the pre-corneal and pre-lens tear films must be continuously supported to maintain their integrity, which will enhance wettability, prevent dehydration, and create a smooth optical surface.
Research suggests that disturbances in the lipid layer play a dominant role in tear film instability, resulting in uneven distribution of the tear lipids, leading to evaporation of the aqueous layer and, in turn, causing dryness symptoms such as ocular discomfort (McCulley and Shine, 2004). Contact lenses can destabilize the tear film by creating a thinner lipid layer, which can cause corneal staining and reduced tear breakup times (Korb et al, 1996). Contact lens wear can also increase tear film evaporation, which prompts patients’ complaints of dryness (Guillon and Maissa, 2008).
Manufacturers have taken steps to address these issues by incorporating wetting and moisturizing agents in lens materials. In some contact lenses, the wetting agent remains in the lens matrix to maintain lubricity. In others, the wetting agent is released from the contact lens into the tears. Lenses that have blink-activated moisturizing agents that are gradually released during the day have demonstrated good tear film stability (Wolffsohn et al, 2010). These innovations have improved comfort, but, thus far, none of the silicone hydrogel contact lens materials offer the much-needed support to the critical lipid layer of the tear film.
Surface Properties At odds with the goals of comfort and ocular health, the surface properties of some silicone hydrogel contact lenses make them more hydrophilic. Thus, they attract lipids that accumulate on the lenses, causing discomfort and reduced visual acuity (Carney et al, 2008). To be compatible with the cornea and the tear film, the contact lens surface must mimic the cornea, attracting the aqueous layer but resisting deposits. Considering the high water content of the cornea and the tear film, a uniquely designed water gradient within a contact lens could mediate between the high oxygen transmissibility of silicone hydrogel and the natural composition at the point of contact. In the absence of such a panacea, contact lens wettability remains the more important objective, and daily disposable contact lens wear is perhaps the best resolution to this problem.
Care Solution-Induced Staining Corneal staining has been associated with care solution incompatibilities with silicone hydrogel lenses (Carnt et al, 2007). One study has shown a lower frequency of solution-induced corneal staining when hydrogen peroxide disinfecting systems were used with silicone hydrogel lenses (Holden, 2005).
Adverse Events Although the use of silicone hydrogel materials for contact lenses has reduced most hypoxia-related complications, other adverse events, such as infectious or inflammatory responses, may occur (Sweeney et al, 2004).
The Quest Continues
Despite advances in comfort, vision, health, and convenience, we must strive to close important gaps in unmet needs and expectations.
Comfort is a key need, and the role of tear film stability in contact lens comfort is well established. As Holden and Fonn (2005) concluded: “Today, we have the best lenses ever — well-designed, with high oxygen transmissibility and good surfaces. What we need on top of that is a tear film that behaves as though the surface on the lens is like the eye’s own surface… Our research indicates that the fundamental comfort barrier is creating a lubricious, wettable, long-lasting surface on the new generation of contact lenses.”
Fonn (2002) also noted: “Discomfort/dryness continues to remain an enigma and the Holy Grail of contact lenses must surely be a contact lens surface that can support a stable tear film as does the cornea and conjunctiva.” And according to Donald Korb (Korb et al, 1996), “Also, new contact lens materials that approximate the ocular surface more closely appear to be required in order to allow for optimum wettability.”
For eyecare professionals, ocular health will continue to be a primary need for future contact lens products. As we know, regardless of how well developed, designed, and manufactured, no contact lens can prevent all adverse health responses. Because contact lenses are not used in a vacuum, they are subject to factors that cannot be controlled. Contact lens wearers will make mistakes, either intentionally or unintentionally, and many eyecare professionals will default to modalities proven to have higher compliance rates, such as daily disposable and monthly replacement contact lenses (Dumbleton et al, 2010).
Much remains to be discovered, learned, and understood as advances in contact lens technology continue. Researchers are studying some interesting new applications, including contact lenses with embedded circuitry, as drug delivery devices, and as corneal inlays and onlays, proving that we are limited only by our own imaginations as we think outside the blister pack. CLS
The authors would like to acknowledge Anne Austin Thompson, MS, OD, for her help with this article.
A similar article printed in the Oct. 7, 2011 issue of Optician.
To obtain references for this article, please visit http://www.clspectrum.com/references.asp and click on document #204.
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