Solving the Rubik’s Cube of Daily Disposables
Many key factors must align perfectly to solve the puzzle of successful lens wear.
|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.|
|Dr. Hudson is head of Professional Affairs, Alcon Vision Care for UK & Ireland.|
By Inma Pérez-Gómez, PhD, MCOptom, FAAO, FBCLA; Tim Giles, OD, MBA, FBCLA, FIACLE; & Cameron Hudson, BSc (Hons), PhD, MCOptom
Contact lens wearers want convenience, comfort, clear vision, and value. Eyecare professionals want good ocular health, clear vision, and satisfied wearers. Creating the ideal contact lens is similar to solving Rubik’s Cube. Several key components must align in a precise and predictable way to solve the puzzle. In the case of daily disposable contact lenses, this means simultaneously achieving good ocular health, vision, comfort, and convenience. To reach this goal, we must optimize the desired features of a lens material while minimizing any inherent negative factors, striking a balance between the pros and the cons. In this article, we discuss several important factors that contribute to the success of a daily disposable contact lens, including oxygen transmissibility, lens-cornea interactions, and biocompatibility.
Oxygen for a Healthy Cornea
The driving force behind the development of silicone hydrogel contact lens materials was to satisfy the cornea’s need for oxygen (See “Seeking Normoxia” on opposite page). After a decade of clinical experience and numerous studies, we know that silicone hydrogel lenses are a healthy option for our patients. During a three-year clinical study, researchers found that signs and symptoms associated with corneal hypoxia were significantly reduced in patients wearing silicone hydrogel lenses compared with those wearing low-Dk HEMA lenses (Bergenske et al, 2007). In addition to a significant improvement in limbal and conjunctival hyperemia and corneal neovascularization, they found significantly fewer symptoms of lens awareness, dryness, photophobia, and blurred vision in patients wearing silicone hydrogel lenses. Highly oxygen-permeable silicone hydrogel materials have significantly reduced the number of hypoxia-related findings reported in daily wear, with microcysts, striae, and bulbar and limbal hyperemia virtually eliminated (Stapleton, 2006).
One of the most visually dramatic benefits of switching patients from hydrogel to silicone hydrogel lenses is the marked improvement in limbal and conjunctival hyperemia. Limbal hyperemia results from oxygen deprivation at the peripheral cornea and causes limbal stem cell damage. This damage makes the cornea vulnerable to deficient epithelialization, which can lead to recurrent corneal erosions, chronic keratitis, and vascularization (Stapleton, 2006).
The bottom line: Corneas need oxygen, and some need more than others. What’s more, the same cornea may have different oxygen needs depending on conditions. Although we don’t know the optimal Dk/t for each individual, we do know that increased oxygen transmissibility is good for the cornea. The quest for higher Dk/t materials is justified. The higher the Dk/t, the greater our confidence that the critical minimum oxygen requirements are being met for all contact lens wearers across the entire corneal surface and for lens powers other than –3.00D. Therefore, it makes sense to prescribe contact lenses that have the highest oxygen transmissibility available. Currently, three silicone hydrogel daily disposable lenses on the market have oxygen transmissibility values (Dk/t at –3.00D) ranging from 65 to 118. All other daily disposable contact lenses—traditional hydrogel (HEMA) lenses or polyvinyl alcohol (PVA) lenses—have Dk/t values ranging from 18 to 37 (Reeder, 2010).
|The oxygen transmissibility required for “normoxia,” the level of oxygen that allows for normal corneal physiology, has been an elusive figure in contact lenses. Researchers continue to explore the minimum Dk/t needed for a lens to provide for normal corneal oxygen demands (Fonn et al, 2010). Adding to the complexity of this issue is the fact that individuals may have varying corneal oxygen demands, and corneal swelling responses are multifactorial and not dependent on lens type alone (Moezzi et al, 2011). This would suggest that the minimum Dk/t requirement differs for each individual, and published data show that some corneas exhibit more swelling than others do under the same conditions (Moezzi et al, 2011; Mueller et al, 2001). A genetic predisposition to dystrophies, a history of trauma or toxicity, age, systemic disease, lens thickness profiles, and even altitude are variables that may affect normoxia (Fonn et al, 2005).|
Why Comfort Is So Important
Decreased comfort during contact lens wear continues to be the No. 1 reason why people stop wearing their lenses (Young et al, 2002; Rumpakis, 2010). 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). In this survey, 41.9 percent to 52.9 percent of dropouts cited discomfort as the main reason why they stopped wearing their lenses.
Ocular signs and symptoms of contact lens-related dryness are major causes of discomfort and subsequent lens intolerance (Chalmers et al, 2002). In fact, contact lens wearers are 12 times more likely than emmetropes are and five times more likely than spectacle wearers are to report dry eye symptoms (Nichols et al, 2005).
Most practitioners agree that improving comfort, particularly end-of-day comfort, could help reduce contact lens dropouts. Let’s look at how water content, lens dehydration, lens interactions, surface properties, and biocompatibility influence comfort.
One factor often implicated with contact lens-related dry eye symptoms is lens dehydration. A study has shown that traditional (poly-HEMA) lenses with higher water content dehydrate more compared to those with lower water content (Jones et al, 2002). On the other hand, silicone hydrogel lenses with lower water content may improve contact lens-related dryness symptoms (Chalmers et al, 2007), especially when symptoms are associated with age (Chalmers et al, 2009).
Factors other than water content may also contribute to lens dehydration. These include lens power and the resultant center thickness (Helton and Watson, 1991), the environment (Jones et al, 2002), and water-binding properties of the lens material (Larson et al, 1990).
A recent study examined the bulk dehydration rates of several daily disposable and frequent replacement lenses (silicone hydrogel and poly-HEMA) and concluded that bulk dehydration is probably not directly related to comfort (Jones and Jones, 2010). Rather, the researchers concluded that other factors, including surface hydration and wettability, are probably more important. Wettability describes how a fluid spreads across a surface. The more wettable the contact lens, the lower the wetting angle, which contributes to comfort.
The surface of a contact lens in vivo interacts with the precorneal tear film, the eyelids, and the corneal epithelium. The comfort of a lens depends upon its biocompatibility with these ocular environments (Morris, 2008), although other factors, such as edge geometry and thickness profile, also contribute to comfort.
Researchers have described the surface properties of the corneal epithelium as follows: Tiny microvilli extending from the epithelial cells of the corneal surface act as foci for the attachment of mucin from the tear film, creating a hydrophilic glycocalyx that promotes wettability and enhances the spread and continuity of the tear film (Nichols et al, 1983). Ideally, the contact lens surface should support this interface.
The tear film consists of three layers or phases: a thin, mucin-rich layer adjacent to the corneal epithelium, a thicker middle aqueous layer, and a lipid layer that interfaces with the air. Research suggests that disturbances in the lipid layer play a predominant role in tear film instability, leading to increased evaporation and osmolarity, which, in turn, causes a decrease in conjunctival goblet cells and corneal epithelial glycogen levels (Murube, 2006). These changes may lead to ocular discomfort and dry eye symptoms. With contact lens wear, the tear physiology can be adversely affected by an increased evaporation rate and reduced tear thinning time, demonstrated by reduced tear film breakup time (Thai et al, 2004). To address wettability and dehydration, lens manufacturers have incorporated wetting and moisturizing agents into silicone hydrogel polymers.
A stable tear film is necessary for successful and comfortable lens wear. Tear film stability requires that the precorneal and prelens tear films be supported in a way that maintains their natural integrity. During each blink, the eyelids sweep across the cornea or contact lens, clearing debris and replenishing the tear film. Intolerance to contact lens wear has been strongly correlated to reduced tear film stability (noninvasive tear film breakup time) and tear volume (tear meniscus area) (Glasson, 2003).
The interaction between the lens surface, the tear film, and the eyelids has an impact on contact lens success. Developing contact lens surfaces that have the same properties as the surface of the cornea itself will enhance comfort and vision for wearers.
Biocompatibility Supports Lens Lubricity
Contact lens materials and designs should be innately biocompatible or rendered that way to provide ongoing comfort and ocular health. Ideally, the lens surface mimics or supports the essential features of the cornea and the tear film, allowing the lens to exist in a mutually beneficial relationship with the eye. Biocompatibility may be achieved by:
• Creating a hydrophilic aqueous tear layer like the glycocalyx of the corneal epithelium.
• Providing a lubricious and protective surface for the cornea and eyelids.
• Supporting the tear film.
• Providing adequate oxygenation for a range of corneal oxygen demands.
Current silicone hydrogel lens materials contain a mix of hydrophilic and hydrophobic polymers and polymer segments. These polymers can reorient during lens wear so that the hydrophobic segments migrate to the lens surface. In the presence of lipids in the tear film or the air (if the tear film is unstable), the lens surface will become increasingly hydrophobic during wear (Epstein and Stone, 2010). Thus, silicone hydrogel materials, with their predominance of hydrophobic silicone elements, present a significant challenge to producing lens surfaces that are highly wettable. From a technological perspective, the next important development step for manufacturers is to produce silicone hydrogel lenses that exhibit good lubricity and high surface wettability that is maintained during wear.
According to Holden and Fonn (2005): “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.”
Health and comfort continue to be the primary prerequisites for contact lens success. To achieve these goals requires not only adequate corneal oxygenation, but also biocompatible lens surfaces with sustained wettability and good lubricity.
Contact lens wearers expect their lenses to be comfortable. Ideally they want to feel as if they are wearing no lenses at all. Of course, they also expect clear vision and convenience, but these are of little value if they must discontinue lens wear because of discomfort. Dissatisfaction with contact lenses, primarily owing to avoidable issues, such as discomfort, drives many consumers to choose alternative forms of refractive correction, including surgery.
With this in mind, researchers and manufacturers continue to explore new approaches for improving contact lens materials, designs, and surfaces. Further advances in polymer and surface chemistry will provide higher oxygen permeability, enhanced surfaces that are wettable and lubricious, and moisturizing ingredients that are released or that migrate to the lens surface or into the tear film during lens wear.
The frontiers of silicone hydrogel daily disposable lens technologies continue to expand. New technologies, chemistries and clinical perspectives portend a bright future for the contact lens industry. The newest frontier is the combination of a daily disposable contact lens with the high oxygen transmissibility of a silicone hydrogel material to leverage the advantages of both technologies. As eyecare professionals, we must stay informed about these advances so that we can meet the expectations of today’s contact lens consumers. CLS
A similar article appeared in the November 4, 2011 issue of Optician.
For references, please visit www.clspectrum.com/references.asp and click on document #207.