Influences on Tear Film Thickness
Read how interferometry is helping researchers learn how contact lenses interact with the tear film.
By Jason J. Nichols, OD, MPH, PhD, FAAO
As contact lenses sit on the eye, they partition the precorneal tear film into a prelens portion, which has a lipid surface over an aqueous layer, and a postlens portion, which comprises the base mucous-aqueous layer. Unlike the prelens tear film, which is responsible for image quality, the postlens tear film changes very little during contact lens wear.1 However, disruption of the lipid layer of the prelens tear film can lead to evaporation of the aqueous layer, ultimately causing contact lens dehydration, ocular dryness and discomfort.2
My colleagues and I have been using interferometry
to evaluate the prelens tear film, the contact lens and the postlens tear film and
how they're affected by wearing conditions, lens materials and lens care solutions.
In this article, I discuss how these factors affect tear film stability in contact
MEASURING THE TEAR FILM
Tear film thinning is caused by evaporation from the outer tear surface, either by fluid loss through the epithelium or contact lens or through tangential flow of the tear film on a surface. tangential flow may be influenced by gradients of surface tension in the tear film or by the curvature of the tear film surface. The concave tear meniscus near the edge of a contact lens generates negative pressure gradients that draw fluid from the nearby tear film.
In a recent study, my colleagues and
I used interferometry to evaluate tear film thinning in contact lens patients. Interferometry
is an accurate, noninvasive technique that calculates the thickness of the human
tear film by comparing light waves reflected from the various surfaces of the eye
including, the front and back of the contact lens, and the front of the cornea.3 We use data from these four target surfaces to quantify the thickness of the prelens tear film, the postlens tear film, the contact lens and the cornea either separately or as composite layers, such as the prelens tear film and the contact lens. For more information about interferometric methods, see "How Interferometry Works."
During our study, we measured the precorneal tear thickness of 20 patients before contact lens wear and prelens tear film thinning after wearing hydrogel contact lenses for 1 hour.4 The average initial thickness of the prelens tear film was 1.4 μm less than the precorneal tear film. The prelens tear film became thinner 1.8 times faster than the precorneal tear film on average. We concluded that the lower initial thickness and the more rapid thinning rate of the prelens tear film contributed to the faster thinning time, possibly due to evaporation or the tendency of hydrogel contact lenses to dewet when on the eye.
|Interferometry image showing a stable prelens tear film on a hydrogel contact lens. This layer coats the anterior surface of the contact lens uniformly, providing a smooth optical surface for good image quality.||Interferometry
image showing unstable prelens tear film on a hydrogel contact lens. Disruption
of the lipid layer creates a hydrophobic
surface, leading to lens dewetting and significant tear film breakup.
In additional studies, we used interferometry to evaluate other factors that influence tear film thickness and contact lens comfort. These variables include:
We tested prelens tear thickness
each minute after contact lens application for 30 minutes. Initial thickness was
4.5 μm, declining to 2.5 μm at the end of the study. The time constant
for the prelens tear film as it reached steady-state thickness was 7.1 minutes and
the final prelens thickness was 2.5 μm. The apparent center thickness of the
contact lens declined linearly at an
average rate of 0.051 μm/minute, and the postlens thickness remained constant at about 2.5 μm.
Based on this study, we concluded that tear film thinning is a complex process that involves evaporation, dewetting, pressure-gradient flow and surface tension gradients. Through interferometry, we were able to identify the distribution of tear film thinning and help identify the role of different mechanisms in tear film thinning and breakup.
ETAFILCON A CONTACT LENSES5
We measured the prelens
and postlens tear film in
patients wearing etafilcon A contact lenses. The refractive index of the tears was assumed to be 1.337 at 589 nm and the refractive index of the etafilcon A lens was set at 1.405, as reported by the manufacturer.
Mean pre- and postlens tear film thicknesses
were similar, 2.31 ± 0.82 μm and 2.33 ± 0.52 μm, respectively.
However, they were not correlated within patients,
indicating they are distinct layers.
EYE CLOSURE WITH SILICONE HYDROGEL LENSES6
We looked at the effects of eye closure on silicone hydrogel lens wear. The average postlens tear film thickness decreased to 1.58 μm after 5 minutes and to 1.20 μm after 15 minutes of eye closure. The postlens tear film thickness returned to normal levels about 15 minutes after the eyes were open normally.
MULTIPURPOSE SOLUTIONS AND THE TEAR FILM
Next, my colleagues and I conducted a study7 to compare tear film thickness in patients using Complete MoisturePlus (CMP) and Opti-Free Express (OFX) multipurpose solutions. CMP contains hydroxypropyl methylcellulose (HPMC) and propylene glycol (PG), which are thought to create a thicker prelens tear film layer than solutions that don't contain these demulcents.8 We used interferometry to measure tear film thickness during the study.
At the beginning of the study, all patients
(n = 31) were wearing Acuvue 2 contact lenses. They used one of the study solutions
according to manufacturer's guidelines for 7 days and returned for evaluation. After
abstaining for 24 hours, they resumed lens wear with a new pair of Acuvue 2 contact
lenses. The patients used the second study solution for 7 days. The investigators
were masked to which solutions the patients used
during the first and second arms of the study.
We used the interferometer to measure tear film thickness after both wearing periods. The overall average prelens tear film thickness with the HPMC solution was 3.02 μm. The tear film was thinner with the non-HPMC solution, measuring 2.72 μm.
After the second arm of the study, subjects were asked which solution they preferred and why, focusing on criteria such as comfort, convenience, contact lens handling, ocular health and vision. About two-thirds of the participants in the study preferred the HPMC solution (CMP) to the non-HPMC solution (OFX).
Our quantitative findings support the qualitative findings of the previous study8 of HPMC-containing multipurpose solutions, suggesting that multipurpose solutions may play a role in maintaining a stable prelens tear film and in improving contact lens comfort. In addition, these findings may have implications for patients who wear daily disposable contact lenses and do not use multipurpose solutions.
Interferometry is a quantitative and accurate means for measuring the components of the tear film, with or without contact lenses. We used this technology to identify variables that affect tear film thickness in contact lens wearers, such as lens materials and thickness-enhancing agents in multipurpose solutions. We conclude that both factors may contribute to maintaining a robust and stable prelens tear film.
In future studies, we'll address differences among lens material types, and further explore the mechanism of rapid prelens tear film thinning during contact lens wear. CLS
How Interferometry Works
Interferometers split light beams into two or more parts, which travel to and bounce off different targets. As the light waves return to the source, they're subject to the principle of interference, which states that differently phased wavefronts generate interference patterns when they return to the same time and place. Two wavefronts that coincide in the same phase (such as at their peak or trough) will amplify each other. If the crests of the two waves coincide, they will form a single wave with a higher crest and deeper trough than the single wave. Two wavefronts that come together in opposite phases (one peak, one trough) will cancel each other out. Waves that are out of phase with each other will come together in a systematic formation, or interference pattern. The phase difference of the waves in the interference pattern is used to calculate the distance between targets.