CPD Modules Available

Print this page

Contact Lenses and Tear Film Stability

2M CPD in Australia | TBA in New Zealand | 1 April 2018


By Mark Koszek

Technological advances are reducing the incidence of end of day contact lens discomfort. Although these sophisticated contact lenses may cost your patients more than they’ve previously been prepared to pay, it’s well worth taking the time out to explain the benefits that a premium lens will deliver… purchases are most often driven by price until the buyer understands the true value of the product.

LEARNING OBJECTIVES

1. Identify the effect that contact lenses have on tear film stability.
2. Identify the role that water gradient technology plays in improving dynamic tear film properties such as tear film break up time.
3. Understand that improved wettability is important for enhanced lens comfort.
4. Understand that improved wettability is associated with more consistent vision.


It’s been said that the only certainties in life are death and taxes. I started practicing in 1995 and I quickly realised there was a third certainty; anyone who wore contact lenses experienced end of day dryness and discomfort. Patients often reported the need to take their lenses out when they got home from work to give their eyes a break, which was a polite way of saying ‘my contact lenses are killing me’. Yet slowly this third certainty is changing with technological advancement, owing to a better understanding of the biochemistry of the tear film and material sciences.

Our tear film is a beautifully complex structure; for the lucky ones amongst us we can live our entire lives without giving a thought to the fact that our tear film is protecting our ocular surface from pathogens, keeping our eyes lubricated and our vision clear. Yet many of our patients do not have this luxury and one group that has suffered more than most has been our contact lens wearers. In 1995, anywhere between 10 to 30 per cent of patients would drop out of contact lens wear.1,2,3 Unfortunately, over the years this figure has changed little; Dumbelton et al’s 2013 study4 demonstrated that each year, 23 per cent of patients discontinued contact lens wear permanently and the major reasons were dryness and discomfort. If a quarter of our contact lens database drops out each year we’ve got to fit an equal amount of new wearers just to earn the same income. Obviously this is not great economics! It’s therefore imperative that we tether our practice to the right product, and to find the right product we need to understand the reasons behind contact lens discomfort. To begin this journey, let’s start with the truly amazing tear film.


THE TEAR FILM

As undergraduates we were taught the classic three-layer tear film model, which was described much like a ‘cake’, being composed of the mucin, aqueous and lipid layers. The mucin layer anchors the tears to the ocular surface; the aqueous layer provides the bulk of the eye’s moisture requirements, while the lipid layer prevents the tear film from evaporating. Not surprisingly the ‘cake’ model is too simplistic, in fact the tear film is more like a ‘cocktail’; a mixture of tear components in a relative gradient. For example, mucins are not just located in the mucin layer but can be found in the aqueous and even the lipid layer.5 There are believed to be 18 different mucins,6 491 proteins,7 and 153 types of lipids in the tear film.8

A contact lens that is 70-100 microns thick is a great disruption to the stability of the tear film which is only three to four microns thick,9 and causes it to split into two separate non-communicating layers; the pre-lens tear film (PrLTF) and the post lens tear film (PoLTF). The PrLTF is located on the surface of the lens and is approximately one to three microns thick, whereas the PoLTF is two to three microns thick10 and is wedged between the contact lens and cornea. Lu et al11 demonstrated that the PrLTF consisted of three interfaces, the air/lipid layer interface, a lipid layer/Aqueous-Mucin (A-M) interface, and an Aqueous-Mucin (A-M)/contact lens interface. There are also two post-contact lens interfaces, namely the contact lens/A-M layer and the A-M layer/ cornea interface. Unfortunately, the lipid layer on the PrLTF is thinner than in a noncontact lens wearer;12,13 the mean thickness of the lipid layer on the PrLTF was found to be 41-67nm, whereas the lipid layer in a normal eye is up to 170nm thick.14 A thinner PrLTF lipid layer adversely affects tear film stability by increasing evaporation,15 which is vital to the wettability of contact lenses.

The lipid layer is believed to have a bilayer structure where polar lipids overlay the aqueous layer and non-polar lipids face theair.16,17 Phospholipids are thought to reside in the polar layer i.e. the interface between the hydrophobic (water hating) lipid layer and the hydrophilic (water loving) aqueous layer. Phospholipids can be thought of as the peacekeepers of the tear film, bringing together opposing hydrophobic and hydrophilic moieties. Phospholipids achieve this by having a foot in each camp; their polar or charged heads orient towards the aqueous layer whereas their non-polar tails interact with the lipid layer. Phospholipids are believed to have an important function in reducing evaporation of the tear film. A deficiency in phospholipids means the oily lipid layer cannot spread evenly over the aqueous layer, leading to tear film disruption.  Butovich and Millar18 demonstrated that animals such as koalas and rabbits with high levels of phospholipids in their tear film had longer Inter-Blink Intervals (IBI), meaning they don’t have to blink as often because the tear film evaporates at a slower rate. There are believed to be two phospholipids predominant in tears, phosphatidylcholine and phosphatidylethanolamine, which account for 60 per cent of the phospholipid profile of the meibomian secretions.19 Contact lenses adsorb some phospholipids,20-23 which will decrease their availability in the tear film. Evaporation of the Pre-Lens Tear Film (PrLTF) therefore represents a great challenge to contact lens manufacturers. But first how do we measure tear film stability?

TEAR FILM STABILITY

Tear film break up time (TFBUT) is one way of evaluating tear film kinetics. As the pre-lens tear film evaporates, the tear film becomes thinner, exposing the contact lens surface to air, which is hydrophobic. Silicone hydrogels pose a particular problem since by nature they are hydrophobic and when exposed to air, the surface becomes even more hydrophobic due to conformational changes in the molecular structure of the silicone hydrogel backbone. Silicone moieties rotate towards the hydrophobic air, which causes progressive drying of the contact lens surface, which will then negatively affect comfort. Guillon24 et al demonstrated that tear film break up time (TFBUT) is highly related to patients reporting negative symptoms. Using the Ocular Surface Disease Index (OSDI) and a video tearscope, they were able to demonstrate that symptomatic contact lens wearers were more likely to have a shorter TFBUT than non-symptomatic wearers (4.7 seconds vs. 6.0 seconds). In contrast, non-contact lens wearers’ TFBUT will normally be between 12 to 15 seconds.25 The difference of 1.3 seconds between a symptomatic vs. non-symptomatic wearer may not sound like a lot but we need to consider this time frame in context of the Inter-Blink Interval (IBI) i.e. the time between blinks.

The average person blinks approximately 11 times per minute and thus has an IBI of approximately 5.7 seconds.26 Yet when we read, our blink rate can decreases to as low as 4.5 times per minute,27 meaning the IBI will be as long as 13 seconds, which is why many of our patients who use computers will report dry, burning eyes. If the TFBUT of a symptomatic CL wearer is 4.7 seconds and the IBI is a normal six seconds, then the contact lens surface is exposed to air for a critical 1.3 seconds before a new blink spreads the tear film over the contact lens surface. This is worse still if the patient is reading; the contact lens surface may be exposed to air for as long as eight seconds. Guillon et al also demonstrated that just before a new blink, a significant area of the contact lens surface has a greater degree of exposure in symptomatic wearers versus non-symptomatic wearers (9.4 per cent vs. 3.9 per cent). So, in terms of the kinetics of tear film spreading, 1.3 seconds is significant.

EVAPORATION OF THE TEAR FILM

The next question we need to ask ourselves is why is contact lens comfort compromised when there is greater evaporation of the tear film? The answer is friction. When a lens dehydrates its surface becomes drier and rougher. This has been the bugbear of contact lens manufacturers since day dot, and as a result they’ve played around with all sorts of features, such as water content, hydrophilic side chains and lubricants in the blister packets.

Ideally, we would like to have a lens that has the oxygen permeability of a silicone hydrogel, and the wettability of a traditional hydrogel. Alcon set about designing a lens to do just that and after a ten year development period, released Dailies Total1 contact lenses. Dailies Total1 are made
of a revolutionary material, consisting of a silicone hydrogel core and a surface gel layer, which approaches 100 per cent water content (Figure 1) at the lens surface.28,29

Figure 1: A schematic representation of the delefilcon A lens material including the surface gel layer that is integrated into and onto a silicone hydrogel core material. The entire graded gel layer is estimated to be on the order of 6 μm in thickness based on AFM mapping and fluorescence microscopy. The water content of the surface gel layer is greater than 80%, while core material is only 33%.

The surface gel layer has a thickness of six microns and a surface modulus of 0.025mPa,30 which is nearly as soft as a corneal epithelial cell.31 The integration between the silicone hydrogel core and the hydrophilic surface gel is a seamless transition, as demonstrated by atomic force microscopy (Figure 2).32

Figure 2: AFM topographic images of delefilcon A lens cross section near the lens surface in hydrated state. a) Topographic image (height profile) of a 20 μm wide section of the lens, and b) that a 10 μm wide section of the lens, near the lens surface.

 

CAPILLARY ACTION

The hydrophilic polymers of the surface gel have a fibril-like brush structure, which helps retain water on the lens surface by capillary action. What is capillary action I hear you ask? Well, it’s a combination of cohesion, adhesion and surface tension. Water is relatively sticky thanks to its cohesion properties; that is, water molecules like to stay close together. Water is also attracted and adherent to other substances. Think of spilling water on a table; due to its surface tension, it pools in a puddle rather than spreading evenly across the table. When you wipe it with a paper towel the liquid adheres itself to the paper fibers and the liquid moves into the spaces between and inside of the fibers. The surface gel of Dailies Total1 is not too dissimilar; the combination of cohesion, adhesion and surface tension causes water to bind to the brush like polymer fibrils. The tear film therefore closely associates with the surface gel. The question therefore becomes, where does the contact lens start and the tear film stop? The surface gel is uniform over the entire lens, which has been demonstrated using scanning laser surface fluorophotometry, where the hydrophilic polymers are tagged with fluorescent dyes to enable them to emit a green dye when excited (Figure 3).

Figure 3: Laser confocal image of the fluorescent tagged delefilcon A lens cross section showing complete and uniform coverage of the lens surface with the water gradient hydrophilic surface structure. The green colour is produced by the fluorescent tagged hydrophilic copolymer.

The hydrophilic surface gel, coupled with the capillary action of water, increases the overall lens wettability and delays surface dehydration.

Guillion et al33 assessed the Tear Film Kinetics (TFK) of Dailies Total1 lenses by using a cohort of tests rather than simply relying on Non-invasive Tear Film Break up Time alone. The other measures used included:

  • Dehydration Speed (DS): the speed of increased exposure between the first break and the spontaneous blink in mm2 per second.
  • Minimum Protection Area (MPA): the front contact lens surface area covered by the tear film at the time of the spontaneous blink (expressed as a percentage).

After three hours of wear, the Non-Invasive Break Up Time (NIBUT) was 7.1 seconds, the MPA was 93.4 per cent and the DS was 0.28mm2 per second. Remember, the average person has an Inter-Blink Interval (IBI) of six seconds, so we now have a lens with a longer break up time than the average IBI, which means the lens surface remains lubricated between blinks. The MPA is also impressive, which indicates that even when the lens is most dehydrated i.e. just before the blink, 93 per cent of the lens area is still covered by the tear film.

HOW DO WE MEASURE THESE THINGS IN PRACTICE?

Many of the latest topography systems have tear film analysis software, which calculates Non-Invasive Tear Break Up Time (NITBUT). Our practice has a Medmont E300 topographer, which I use to show patients the differences between their lenses and Dailies Total1. Below are the topography plots of Margaret, a 74-year old woman wearing her habitual daily disposable lens (Figure 4) and Dailies Total1 (Figure 5) on the same eye.

Figure 4

Figure 6

The Medmont E300 corneal topography system noninvasively examines changes in tear film stability by analysing the structure of the reflected placido disc image. The colours on the tear film maps represent tear film break up, not corneal curvature, with greater tear film instability being represented by warmer colours such as yellow and red. The software calculates the Tear Film Surface Quality (TFSQ) index. A TFSQ of 0.3 or greater corresponds to visible distortion in the placido ring pattern and from this, the TFSQ Area is calculated, which is the percentage of the tear film area with a TFSQ index greater than 0.3. Figures 4 and 5 were taken fifteen seconds after the patient was instructed not to blink. The difference between the lenses
is amazing, 88 per cent of the lens surface of the patient’s habitual lens was dry after 15 seconds. In contrast, only 5 per cent of the Dailies Total1 lens had significant tear film break-up. Remember, this is a 74-year old woman who, by virtue of her age and gender, is more likely to suffer from dry eyes. We really live in a wonderful age. Margaret, like many of my older patients, now finds it more comfortable to wear a contact lens than without.

One of the little tricks I get my patients to do after they’re finished with their Dailies Total1 lenses is to leave them on a dry surface and then wait to see how long the lenses take to shrivel up as they dehydrate. Most lenses will have significantly dehydrated within minutes, yet I’ve left my Dailies Total1 lenses on the kitchen bench for five days without any loss of shape. The following image is the shape of three commonly prescribed daily disposable lenses, Dailies Total1 being on the left. Each lens was removed from its blister packet and placed on a piece of paper. The photo was taken one hour after each lens was removed. The image speaks for itself!

Dailies Total1 also contains phosphatidylcholine, a polar lipid. Remember as discussed above, phospholipids help prevent tear film evaporation. In the manufacturing process of Dailies Total1 the phospholipids are loaded into the lens and over the course of the day are eluded from its matrix, which helps compensate for contact lens wearers’ thinner lipid layer in the PrLTF.



END OF DAY COMFORT

A wettable contact lens will provide a smooth, lubricious surface, resulting in reduced friction between the eyelid and the contact lens surface. Tribology is the science of interacting surfaces in relative motion, including the principals of friction and lubrication. We know that as wettability decreases, friction will increase. Coles and Brennan34 demonstrated that the principal property associated with end of day comfort was the coefficient of friction. Dailies Total1 has really changed the game with comfort. The continuous adhesion of water to the hydrophilic polymers in the surface gel allows the lens to maintain its wettability and lubricity (slipperiness) at the end of the day.35 Here arises my third certainty in life, end of day dryness and discomfort. A 2015 study by Michaud and Forcier36 demonstrated that symptomatic contact lens wearers wore their lenses on average for 7.6 hours per day. So if they put their lenses in at 7am, by 2:36 pm (to be precise) the discomfort would become so bad they would have to remove them. It hardly sounds worth it. When these same patients were switched to Dailies Total1, their wear time increased a further three hours to over 10 hours. This is impressive when considering they are symptomatic wearers.

It’s amazing how tolerant contact lens wearers become. One of the first questions I’ll ask any patient at their aftercare appointment is do you find your habitual lenses comfortable? The patient will invariably say yes, which may stem from a fear that you’ll take them out of contact lens wear. Yet I keep digging and as the consult proceeds they’ll tell you how they have to take their lenses out as soon as they get home from work due to dryness. Patient’s can’t know what they don’t know. I try to fit every eligible wearer with Dailies Total1 so they can appreciate what comfortable contact lens should be like.

END OF DAY VISION

Comfort is not the only aspect of the lens wearing experience that is compromised when the tear film evaporates. Vision will also be adversely affected. As the tear film dries and breaks up, the irregular surface will cause light scatter and a reduction in image quality,37,38 which is especially critical for multifocal lens wearers. Think about all the things that have to go right with a multifocal design; for starters when we’re explaining the optical design to patients, we find ourselves using the ‘C’ word; ‘compromise’. Additionally, the lens has to be well centered so that the patient is looking through the correct optical zone, and pupil size is also important. On top of this, if the lens dehydrates, it has two devastating effects; firstly the dry surface
will cause light scatter and reduce image quality. Secondly, the lens is also more likely to move around on the eye due to increased friction, thus decentering the lens optics. The quality and consistency of vision I’m finding with my Dailies Total1 multifocal lenses has been nothing short of extraordinary. Remember that multifocal contact lens wearers are also older and are often women, so to have a lens that can give them consistent clarity and better end of day comfort has been incredibly rewarding.

The benefits of Dailies Total1 speak for themselves but communicating this to patients is critical to the success of your contact lens business. I’m constantly reinforcing to patients the benefits of the lenses, “Did you know that a contact lens has been invented that has its own source of moisture so that the lenses are less likely to dry out at the end of the day?” There’s a big difference between value and price. Any purchase is driven by price until the consumer is educated about the benefits of the product. Contact lenses are no different, patients want the reassurance that they’re wearing the best product for the health of their eyes. In my opinion Dailies Total1 tick all the boxes, offering superior tear film stability, wettability and lubricity, which
has greatly reduced end of day dryness, discomfort and patient dropout. In fact, it’s been the greatest game changer in my contact lens career, so much so that the third certainty in life isn’t so certain after all. Let’s hope death and taxes can go the same way.

 

      Mark Koszek B.Optom, M.Optom, Grad Cert Oc.Ther, completed his Bachelor of Optometry from the University NSW in 1995, his Master of Optometry in 2002 and Ocular Therapeutics in 2012. He is a founding partner of EyeQ Optometrists and has a large full scope practice in the southern Sydney suburb of Ramsgate. He is a former councillor of the Optometry Association (NSW division) and is a Board Member of the Cornea and Contact Lens Society (CCLSA). Mark has an interest in orthokeratology and miniscleral lenses and has lectured extensively throughout Australia and internationally on contact lenses and dry eye management. 

 

References
1. Schlanger J. A study of contact lens failure. J Am Optom Assoc. 1993;64(3):220-224.
2. Weed K, Fonn D, Potvin R. Discontinuation of contact lens wear. Optom Vis Sci. 1993;70(12s):140.
3. Pritchard N, Fonn D, Brazeau D. Discontinuation of contact lens wear: A survey. ICLC. 1999;26:157-162.
4. Dumbleton K, Woods CA, Jones LW, Fonn D. The impact of contemporary contact lenses on contact lens discontinuation. Eye Contact Lens. Jan 2013;39(1):92-98.
5. Green-Church KB, Butovich I, Willcox M, Brochman D, Paulsen F, Barabino S, Glasgow BJ. The International Workshop on Meibomian Gland Dysfunction: Report of the Subcommittee on Tear Film Lipids and Lipid–Protein Interactions in Health and Disease. Invest Ophthalmol Vis
Sci. 2011 Mar 30;52:1979-1993.
6. Mantelli F, Argüeso P. Functions of ocular surface mucins in health and disease. Curr Opin Allergy Clin Immunol. 2008 Oct;8:477-483.
7. de Souza GA, Godoy LM, Mann M. Identification of 491 proteins in the tear fluid proteome reveals a large number of proteases and protease inhibitors. Genome Biol. 2006;7(8):R72.
8. Rantamäki AH, Seppänen-Laakso T, Oresic M, Juahiainen M, Holopainen JM. Human tear fluid lipidome: from composition to function. PLoS One. 2011 May 5;6:e19553.
9. Wang J, Fonn D, Simpson TL, Jones L. Precorneal and pre- and postlens tear film thickness measured indirectly with optical coherence tomography. Invest Ophthalmol Vis Sci. 2003 Jun;44:2524-2528.
10. Craig JP et al. The TFOS international workshop on contact lens discomfort: report of the contact lens interactions with the tear film subcommittee. Invest Ophthalmol Vis Sci 2013;54:TFOS123-56.
11. Lu H, Wang M, Wang J, Shen M. Tear film measurement by optical reflectometry technique. J Biomed Opt; 2014 Feb; 19(2): 027001.
12. Hamano H The change of precorneal tear film by the application of contact lenses. Contact Intraocul Lens Med J. 1981 Jul-Sep; 7(3):205-9.
13. Tomlinson A. Contact lens-induced dry eye. In:Tomlinson A., editor. Complications of Contact Lens Wear. Mosby; 1992. pp. 195–218.
14. Guillon J-P. Tear film structure and contact lenses In: Holly FJ (ed). The Preocular Tear Film. Dry Eye Institute: Lubbock, TX, 1986, pp 914–939.
15. Nichols J, Sinnott LT. Tear film, contact lens, and patient-related factors associated with contact lens-related dry eye. Invest Ophthal Vis Sci. 2006;47:1319-1328.
16. Yamada M, Mochizuki H, Kawai M, et al. Decreased tear lipocalin concentration in patients with meibomium gland dysfunction. Br J Ophthalmol. 2005 Jul;89(7):803-5.
17. Millar TJ, Mudgil P, Butovich IA, Palaniappan CK. Adsorption of lipocalin to human meibomian lipid films. Invest Ophthalmol Vis Sci. 2009 Jan;50(1):140-51.
18. Butovich IA, Millar TJ. In search of a better animal model of human tear film: comparative lipiomic analysis of human and animal meibum. Invest Ophthalmol Vis Sci. 2009;50: e-abstract 2545.
19. Greiner JV, Glonek T, Korb DR, et al. Phospholipids in gland secretion. Ophthalmic Res. 1996;28(1):44-9.
20. LorentzH, JonesL. Lipid deposition on hydrogel contact lenses: how history can help us today. Optom Vis Sci 2007;84:286–95.
21. Carney FP, Nash WL, Sentell KB. The adsorption of major tear film lipids in vitro to various silicone hydrogels over time. Invest Ophthal- mol Vis Sci 2008;49:120–4.
22. Mochizuki H, Yamada M, Hatou S, Kawashima M, Hata S. Deposition of lipid, protein, and secretory phospholipase A2 on hydro- philic contact lenses. Eye Contact Lens 2008;34:46–9.
23. Prager MD, Quintana RP. Radiochemical studies on contact lens soilation. II. Lens uptake of cholesteryl oleate and dioleoyl phosphati-dylcholine. J Biomed Mater Res 1997;37:207–11.
24. Guillon M1, Dumbleton KA, Theodoratos P, Wong S, Patel K, Banks G, Patel T. Association Between Contact Lens Discomfort and Pre-lens Tear Film Kinetics. Optom Vis Sci. 2016 Aug;93(8):881-91. doi: 10.1097/OPX.0000000000000866.
25. Jin Hak Lee, Chang Won Kee. The significance of tear film break up time in the diagnosis of dry eye syndrome. Kor J Ophthalmol; vol 2; 69-71 1988.
26. Johnston P, Rodriguez J, Lane K, Ousler G, Abelson M. The interblink interval in normal and dry eye subjects. Clin Ophthal 2013; 7: 253–259.
27. Bentivoglio A, Bressman S, Cassetta E, Carretta D, Tonali P, Albanese A. Analysis of blink rate patterns in normal subjects. Mov Disorders 1997 Nov;12(6):1028-34.
28. Angelini TE, Nixon RM, Dunn AC, et al. Viscoelasticity and mesh-size at the surface of hydrogels characterized with microrheology. Invest Oph & Vision Sci. 2013;54, E-Abstract 500.
29. Thekveli S, Qiu Y, Kapoor Y, Kumi A, Liang W, Pruitt J. Structure-property relationship of delefilcon A lenses. Cont Lens Anterior Eye. 2012;35(suppl 1):e14.
30. Dunn A, Urueña J, Huo Y, Perry S, Angelini T, Sawyer WG. Lubricity of Surface Hydrogel Layers. Tribology Letters. February 2013. 2013; 49(2): 371-378.
31. Kuznetsova T, Starodubtseva M, Yegorenkov N, Chizhik S, Zhdanov R. Atomic force microscopy probing of cell elasticity. Micron 38 (8), 824-833, 2007.
32. Sibichen Thekveli, Yongxing Qiu, and Augustine Kumi; Gradient Properties of Dailies Total 1 Surface; Alcon Internal Technical Report; March 2012.
33. Guillon M, Patel K, Gupta R, Patel T, Maissa C. Precontact lens tear film kinetics measurement repeatability under normal and adverse environmental conditions. ARVO 2017 Poster
34. Coles CML, Brennan NA. Coefficient of friction and soft contact lens comfort. American Academy of Optometry. 2012;E-abstract 125603.
35. Cecile Maissa, Anna Martin, David Kramer, Jared Nelson, Teresa DeCenzo-Verbeten. Evaluation of the Lubricity of DAILIES TOTAL1 Contact Lenses After Wear. American Academy of Optometry. Program 145195.
36. Michaud L, Forcier P. Comparing two different daily disposable lenses for improving discomfort related to contact lens wear. Contact Lens and Anterior Eye 39 (2016) 203–209.
37. Timberlake, GT, Doane, MG, Bertera, JH. (1992) Shortterm, low-contrast visual acuity reduction associated with in vivo contact lens drying Optom Vis Sci 69,755-760.
38. Tutt, R, Bradley, A, Begley, C, Thibos, LN. (2000) Optical and visual impact of tear break-up in human eyes Invest Ophthalmol Vis Sci 41,4117-4123.

' As the tear film dries and breaks up, the irregular surface will cause light scatter and a reduction in image quality... '