THE TEAR FILM AND LENS WEAR
The Tear Film and Contact Lens Wear
Part of the challenge of achieving comfortable contact lens wear is to mimic the functions of the tear film.
By John Buch, OD, MS, FAAO; Kristy Canavan, OD, FAAO; Zohra Fadli, PhD; & Charles Scales, PhD
The tear film is nature’s solution to creating a protective yet optically clear interface between the cornea and the environment. This solution is hardly simple: the 3- to 4-micron-thick tear film (Wang et al, 2003) contains more than 18 mucins (Mantelli and Argüeso, 2008), 491 proteins (de Souza et al, 2006), and at least 153 types of lipids (Rantamäki et al, 2011). Together, these hydrophilic, amphiphilic, and lipophilic components form a structured environment that mediates successfully between the hydrophilic environment of the corneal epithelium and the hydrophobic environment of air—while also performing a variety of vital optical and physiologic functions.
In the healthy eye, the tear film components work in harmony to continuously lubricate, moisturize, smooth, oxygenate, clean, and protect the ocular surface during and between blinks. The presence of a contact lens can dramatically alter the tear film structure, and the interaction between the contact lens and the ocular surface environment is a critical determinant of the quality of a patient’s contact lens wearing experience.
The Many Components of the Tear Film
The tear film is composed of a number of different components that enable it to carry out its many functions.
Mucin Matters Like aqueous, protein, and lipid components, mucins are a necessary class of constituents of a healthy tear film. In addition to their lubricating function, mucins are fundamental to solving the problem of how to prevent the aqueous layer of the tear film from simply sliding off of the hydrophobic cell membranes of the corneal epithelium. The ability of the tear film to adhere to the ocular surface is a prerequisite for all of its other functions, and this adhesion is made possible by mucins.
The base layer of the tear film is comprised of membrane-bound mucins, which have sites on their surfaces that interact with other tear film components. Forming a scaffold-like structure called the glycocalyx, these high-molecular-weight mucins are rooted within epithelial cells and extend through the hydrophobic cell membrane to the outside of the cell (Figure 1). The glycocalyx creates a hydrophilic bed to which the aqueous components can adhere. Soluble mucins (including pieces that break away from the glycocalyx) decrease surface tension so that the aqueous layer spreads evenly over the ocular surface (Mantelli and Argüeso, 2008; Abelson et al, 2011).
Figure 1. High-molecular-weight mucins are bound to the cornea on one end but have hydrophilic tails that extend into and hold the aqueous to the cell surface. Because cell membranes are largely hydrophobic, without these membrane-bound mucins, tears would run off the corneal surface.
More than Just Water Despite its name, the aqueous component is much more than water. It contains chemical entities of various sizes, including hundreds of proteins that protect the eye from various forms of insult, in addition to the physical protection afforded by the action of trapping and washing away particles (Rantamäki et al, 2011). The aqueous helps clean, protect, and transport nutrients and oxygen to the cornea (Abelson et al, 2011).
Learning About Lipids Above the aqueous is a complex, less well-understood lipid layer containing polar lipids at the aqueous/lipid interface and a thicker layer of nonpolar lipids facing the air (Green-Church et al, 2011). The lipid layer aids in lubrication, prevents loss of aqueous, and helps maintain a smooth refracting surface at the interface with the air. But without the amphiphilic polar-lipid interface, nonpolar lipids would spread poorly over the aqueous, leading to more rapid tear film breakup.
Mucins also help facilitate the spread of lipids; Green-Church et al (2011) reported that mucins are even in the lipid layer itself.
Impact of a Contact Lens
When the tear film is stressed—by movement through different external environments and/or by intense visual work—the delicate balance of components on the ocular surface can break down, leading to symptoms of dryness, discomfort, and visual disturbances as well as to signs such as corneal staining and conjunctival hyperemia.
The presence of a contact lens can alter mucin production, aqueous flow rate, and the concentration of certain tear proteins (Rohit et al, 2013). A contact lens on the eye compartmentalizes the tear film, isolating the mucin layer behind the lens, thinning the pre-lens tear film, and disrupting the lipid layer (Nichols et al, 2013). The less-robust pre-lens tear film layer breaks up and evaporates more readily compared to the normal (no lens) tear film, shortening tear film breakup time, which can impact vision and comfort (Nichols and Sinnott, 2006).
Simulating the Tear Environment
An ideal contact lens would create a condition on the lens surface similar to that of the surfaces of the eye, which could help mitigate the negative effects of lens wear on the tear film and the surrounding ocular tissues. One strategy for recreating conditions that mimic the normal ocular surface-tear film interactions is to create a lens that can interact with and support not just the water in the tear film, but tear film constituents from all classes, including mucins, water-soluble components, and lipids.
For example, if the contact lens is to function in a manner that mimics the corneal surface, it must effectively re-create the mucin layer at the lens surface, which, among other things, anchors and stabilizes the tear film. Mucins also protect the ocular surface from abrasion, lubricate cell surfaces so the corneal epithelium doesn’t stick to the tarsal conjunctiva, and reduce friction or drag during blinking. These functions must continue at high levels with a lens in place.
The Importance of Lubrication Mucins and lipids both have functions that lubricate and, with approximately 14,000 blink cycles per day, lubrication is essential. Without lubrication, frictional forces from the lid wiping across the cornea would soon produce signs of damage—we know this because even moderate loss of lubrication in dry eye disease, for example, produces corneal staining.
Lubrication reduces friction, and while we don’t often think about it, there is some degree of friction between the eyelid and the corneal surface during every blink. While the energy lost to friction in a single blink is extremely small, the total energy required for up to 14,000 blinks every day may be considerable.
To use the language of physics, blinking involves “work” in the form of frictional energy, which is the product of force × distance. In this case, the distance is the ~20mm that the upper lid travels during the blink cycle, and the force is the effort required to overcome frictional resistance to the blink.
Our eyes “work” during the blink process because energy is required to overcome the frictional forces resisting eyelid movement. Fortunately, the tear film lubricates the lid and ocular surface. But what happens if friction between the lids and the ocular surface increases as the day wears on—for example, with a contact lens whose surface becomes less lubricous over the course of a day?
In this situation, the repeated movement of the eyelid over an increasingly resistive surface thousands of times throughout the day can create a significant amount of additional physical work for the eye. This correlates with the common observation that many patients are comfortable in their contact lenses under certain conditions, yet experience symptoms of discomfort or eye fatigue with different environments, activities, or wear times (Nichols et al, 2013). Every practitioner has heard patients complain of the feeling of “tired eyes.” Could it be this reduction in lubrication function that is to blame?
Preserving Structure and Function of Tear Components In addition to maintaining the physical integrity of the tear film and its components, a contact lens should also maintain the functional elements of the tear film in their natural state.
Examples include protecting proteins (such as lysozyme) from denaturation, which can be caused by heat, drying, and exposure to air or chemicals; and shielding lipids from oxidation and degradation resulting from exposure to oxidizing agents such as ultraviolet radiation (Table 1). Not only is this necessary for these components to perform their intended functions, but allowing naturally occurring proteins and lipids to degrade has also been shown in vitro to result in the release of pro-inflammatory substances that can be irritating to the eye (Green-Church et al, 2011).
|Tear-friendly contact lenses need to:|
|•||Maintain the physical integrity of the tear film and its components.|
|•||Maintain the functional elements of the tear film in their natural state.|
|•||Protect proteins (such as lysozyme) from denaturation, which can be caused by heat, drying, and exposure to chemicals.|
|•||Shield lipids from oxidation resulting from exposure to ultraviolet radiation.|
Clinically, the causes of discomfort are very complex, and no single factor has been definitively identified as the culprit: all factors are potential contributors to the issue.
Comfort Matters to Patients More than 50% of contact lens wearers experience lens-related discomfort and dryness (Richdale et al, 2007). Unfortunately, many patients who experience contact lens discomfort and dryness will simply drop out of lens wear altogether. An international survey of 372 eyecare practitioners found mean dropout rates of 15.9% in the United States, 17.0% in the Americas (including the United States), 31.0% in Asia/Pacific Rim, and 30.4% in Europe/Middle East/Africa. These rates were all statistically significantly higher compared to the historically reported rate of 10% (P<0.05). The primary reason for contact lens intolerance and discontinuation was discomfort, often characterized as a feeling of “dryness” (Rumpakis, 2010; Nichols and Sinnott, 2006).
The frequency and negative consequences of contact lens discomfort make it vital for researchers to design lenses that stay comfortable and lubricous throughout the day. Fortunately, we have lenses that are helping us meet these criteria, and no doubt more will be developed in the future. But science is only part of the equation.
Innovation and the Patient Encounter
All of the great contact lens technology that is available today will be of no value unless patients decide to adopt it. As eyecare professionals, we are the gatekeepers, the interface through which patients access new technology—for both those seeking a better contact lens wearing experience or for first-time wearers.
When technology is developed based on patient needs and insights that can be expected to enhance contact lens wear, it is incumbent upon us to offer it to both new and current patients, and to do so in a way that makes it relevant and personalized. That way, patients are more likely to value the services we provide and can make a truly informed decision about whether they want the technology for themselves.
How, then, can we tailor our patient conversation to inform and to invite patients to try something new? While features and benefits of the new technology are important, it is even more important to relate those to each patient’s specific needs and ocular status. By having a basic understanding of how various contact lens materials interact with the eye and the tear film, we can make better selections to help meet the specific needs of our patients. Perhaps more importantly, bringing together the art of understanding our patient and the science of understanding the lens-eye interactions creates a tremendous opportunity for a richer dialogue with our patients.
It’s important to remember that “we don’t know what we don’t know.” For example, a new patient who has never tried contact lenses may have preconceptions of what contact lenses are like and probably won’t have many “problems” for you to solve. In this case, their concerns, lifestyle, and environment will help guide you to the best product and conversation. For a current wearer, you may have more clinical findings or symptoms to discuss; but don’t forget to re-evaluate their personal needs as well.
And remember, neither type of patient has ever seen the tear film and ocular surface close up! Help them understand the beauty and complexity of what we evaluate before we decide on a contact lens material and wearing and replacement schedule.
In both cases, the conversation is similar: “I listened…I looked…I selected…because….”
Start with what you know about their lifestyle and/or needs, and tell them what you have learned through evaluating their eyes and how these two things helped you come to the decision of the lens material and modality. Explain why you think that the contact lens you chose is the best contact lens for their particular situation.
An effective conversation can be quick, informative, and non-coercive. Whether or not our patients ultimately decide to wear advanced technology lenses, we will have fulfilled our obligation to offer them the best possible wearing experience to suit their particular eye physiology and lifestyle needs. CLS
For references, please visit www.clspectrum.com/references and click on document #243.
Dr. Buch is a principal research optometrist at Johnson & Johnson Vision Care, Inc. (JJVCI) and a graduate of the Ohio State and Indiana Universities.
Dr. Canavan is a principal research optometrist at JJVCI. Clinical research interests include interferometry, aberrometry, and tear film dynamics.
Dr. Fadli is the lead for the Reusable Spherical Platform at JJVCI. Her research interests include the biochemistry of tears, molecular biology, and biocompatibility.
Dr. Scales is a principal scientist leading the Scientific Evidence Generation team at JJVCI.