Contact lens wear may induce symptoms that are similar to those reported by patients suffering from Dry Eye conditions. These symptoms affect about 20 million people in the United States alone,1 and >70% of lens wearers report dryness and/or discomfort especially toward the end of day.2,3 Dryness and discomfort are often reported as the primary reasons for cessation of contact lens wear.4,5 While recognizing the magnitude of this problem, the International Dry Eye Work-Shop suggested that contact lens related dry eye may be considered a sub-classification of the dry eye syndrome.6
The success of a contact lens wear is determined by the tear-film stability and good pre- and postlens tear exchange characteristics.7 Hydrogel contact lenses exhibit a gradual reduction in surface hydrophilicity,8 with a resultant decline in comfort over time9,10 and lenses that are frequently replaced (e.g., disposable lenses) provide enhanced subjective performance as compared with traditional materials replaced less frequently.11,12 Although the exact reason for enhanced subjective performance is not clear, factors such as deposition of tear-film derived proteins or lipids,13,14 which can build up over time, may serve to reduce the wetting characteristics and/or optical quality of a lens by producing areas of hydrophobicity.15,16
Clinically, one of the means of assessing the performance of a contact lens in-eye is by estimating the combined relationship of soft contact lens surface dewetting and tear-film spread. This is often termed as “contact lens surface wettability” (CLW).17–19 However, the term wettability has been interpreted differently by other researchers.20 This is assessed by various methods such as examination with a slit-lamp,21 measurement of the non-invasive tear breakup time,22 determination of tear film thickness and stability clinically,12,23 using interferometry,24 on-eye contact angle and liquid spreading,17 interference fringes,25 aberrometry,26 and optical coherence tomography.27 Some of the clinical scales used to measure CLW, include the Centre for Contact Lens Research Unit Grading scale,28 Wettability Grading scales on a scale of 0 to 4,29,30 the Contact Lens Evaluation of Wettability grading scale,31 and the Centre for Contact Lens Research subjective dynamic in vivo wettability grading system, which includes the assessment of the optical quality of the prelens tear film and its dynamic change during and between blinks.18 In all these methods, in vivo CLW is assessed from a combination of various factors such as precorneal tear lipid layer interference patterns, appearance of dark spots in the specular reflection of the prelens tear film, tear breakup, and tear-film stability determined by the re-establishment of tear film following a blink.32 It is evident that the present methods are complex and the dynamic nature and temporal instability of the tear film makes the subjective methods for assessing the tear film dynamics and stability challenging.
A common slit lamp observation following a blink is the upward drift of the tear film driven by the surface tension gradient as a result of the upstroke movement of the upper lid.25,33,34 This has been observed and modeled both in the precorneal tear film of the non-lens wearing eye as well as the prelens tear film in a contact lens wearer.35,36 During the upward movement of the tear film, a free upward flow of tiny naturally occurring particles that are presumed to be reflective dust particles34,37 can be clearly observed. How well the particles move would depend on the ease of spread of the tear film across the ocular surface or the contact lens surface and interactions between the different layers of the tear film. If the spreading front of the surface of the contact lens biomaterial is more wettable (allows greater spontaneous spread), in theory the particles would move with a greater velocity as compared with movement on a less wettable surface.
The objective of this study is to propose a method of measuring tear-film particle velocity as an assessment of tear hydrodynamics by tracking the movement of reflective particles on the tear film. This may provide a more precise and objective method of clinically assessing the spread and stability of tears as compared with the present clinical methods that are highly subjective.
Ten subjects who were adapted contact lens wearers participated in the study. None of them had any ocular or systemic disease or were using any systemic or topical medications that affected ocular health. All subjects signed an Informed Consent Letter to a protocol approved through the Office of Research Ethics, University of Waterloo.
The study was conducted over 1 day, and data were collected in the morning and 8 h later. All the subjects wore the same type of silicone hydrogel lens that was inserted straight out of the blister packaging. After 10 min of lens settling, digital video images of the central region of the ocular surface were obtained using a CCD color video camera (Sony DXC-C33, Sony Corporation) mounted on a Zeiss slit-lamp. Images were recorded at a magnification setting of 8× and a capture rate of 30 frames per second. During the recording, the intensity of illumination was maintained at approximately 170 lux, as this was the minimal intensity that enabled a high particle contrast for easy visualization. The images were recorded when naturally seen particles were clearly visible in the tear-film following a blink. The illumination beam from the slit-lamp was focused on the anterior tear film surface at an angle of 30° and the width of the illumination was 15 mm. Subjects blinked normally during the 10 s interval of video recording. This procedure was repeated after 8 h of lens wear.
Using the software Fiji (ImageJ),38 the particle tracking plugin39 was customized to track the position of highly reflective tear-film particles through successive frames during one inter-blink interval. The plugin works with AVI movies of grayscale format and outputs trajectory information that includes the x and y coordinates of the particles in each frame. Images from 20 videos, without blinks and without an observable change in fixation were analyzed without any further postprocessing of the images. A trained investigator defined the XY coordinates of a particle to eliminate variability, improve repeatability, and measure the flow rate as determined by the distance traversed by the particle between two successive frames and recorded these in a table. As the particles traversed in the upward direction, they were often seen as streaks. The maximum intensity of the “particle streak ” was used to determine the particle position by the plugin.39 The velocity of particle movement between two successive video frame images was calculated by the formula, “Velocity = distance between points (X,Y location) in successive frames * mm/pixels * frames per second.” Comparisons were made between the measures at lens insertion and at end of day measurements.
Data were analyzed using the program R 2.1340 and ProFit 6.1 (QuantumSoft, Uetikon am See, Switzerland).
Calibration of the Macro
An image with a calibration scale was captured at the same resolution as the video images of the eye, and the known distances were measured using the Fiji program to determine the pixels per mm. Following this, a stack of images with objects set to give a predicted flow-rate value of 30.0 mm/s were measured using the custom macro. The mean measured value ±95% confidence interval was 30.78 ± 0.35 mm/s.
Results of Reflective Particle Movement
The average size of the particles (mean ± standard deviation) tracked was approximately 0.007 ± 0.0007 sq mm. There was no difference in measurements of particle size in images recorded after lens insertion and following 8 h of lens wear (p = 0.09); Fig. 1 shows typical velocity vs. time data in a single subject (S1). The plot demonstrates a number of characteristics of the dynamics of the tear film reflective particle movement. First, the upward particle velocity (UPA; y axis) was highest immediately after blinks, and declines with time (x axis). Second, in most subjects, the motion after the blink was higher in the morning than after 8 h of lens wear, as seen in Fig. 2 (non-selected example of a single subject's data).
In 9 of 10 subjects, the data were in the same pattern as seen in Fig. 1, with the declines in UPV after blink and a separation initially between the morning and 8 h lens wear data. In one subject, this was reversed, with higher UPV after 8 h of lens wear, compared with the earlier measures. There was a statistical difference between the morning and afternoon maximum UPV measures (using a paired difference t test, p = 0.001).
Fig. 3 shows the exponential Velocity = A × exp (−(Time/t0) + constant), fitted using least squares non-linear regression. There was a statistically significant difference between the exponentials' time constants that were (t0 ± SE) 346.02 ± 29.0 and 1413.13 ± 419.6, respectively, for the lens insertion at morning and afternoon measures after 8 h of lens wear.
This study introduces a novel objective in vivo imaging method to evaluate, tear-film particle dynamics over the surface of a contact lens following a blink. The method is advantageous, as it is non-invasive; there is a high signal-to-noise ratio that permits easy detection of the reflective tear particles as these are reflected against the darker background of the anterior chamber and the pupil. This method also enables the assessment of a relatively large area (∼15 to 17%) of the contact lens surface that covers the visible cornea, when compared with some of the other grading systems such as evaluation with the OCT or the CCLR grading system, both of which evaluate relatively smaller regions of the lens surface. The video images were captured at a relatively low magnification (8×) and any eye movement was much less than the particle movement. Also, the stack of frames was not aligned as previous studies indicated this was not necessary.34,41
The UPV could be measured in an objective way through image processing, and the results indicated that the UPV in the morning was different compared with measurements after 8 h. This method therefore enables the separation of groups, and this determines the construct validity of the measurement method. The UPV measurements recorded in the morning are similar to measurements of particle velocity that have been reported without contact lenses.34 The decrease in UPV after 8 h of lens wear may be due to both patient and lens-related factors,42–44 and these should be evaluated in a controlled environment to understand the effect of lens wearing time on UPV. The upper lid velocity is an important oculomotoric factor affecting tear-film thickness.35 Although diurnal variations in tear-film characteristics have been reported,45–48 it does not appear that systematic examination of diurnal variation in upper-lid velocity has been conducted. King-Smith et al. reported that the upward movement of the lipid layer of the tear film extends for a period >6 s.49 However the tear film particle dynamics measured in this study over a larger distance stabilized after about 1 s. Although any direct comparisons are not possible due to differences in the methodology and the tear components being measured, the differences in time may suggest that tear film particles measured in this study were not always on the surface of the lipid layer.
The spread of the tear film over the contact lens surface is determined by factors such as the contact lens/tear film interface, the surface tension gradients,50 tear film quality and stability, presence of lens deposits, gravity, and airflow, as these effect the spread of thin films. The contact lens/tear film interface and surface tension gradients play a more significant role. A hydrophobic contact lens front surface may cause spontaneous dewetting and the breakup of the tear film, whereas a hydrophilic contact lens front surface would result in a stable tear film. According to the hydrodynamic coating model,35 the thinning of the tear film is a function of the velocity of the upper lid's movement. Following a blink as the upper lid rises up, a fluid surface layer is created along the rising meniscus of the upper lid region. The rise is determined by the velocity of the upper lid movement and the radius of curvature of the tear film at the lid margin. The fluid rise is followed by the slower rise of the thicker lipid layer, a reforming of the tear film, and any disturbance in the thickness of the tear film is evened out by a curvature driven leveling with intermolecular forces acting on the tear film.51 The tear film spread may therefore be considered to be governed by the viscosity of the tears42 and the upward drag of the rising tear film. When the lid stops moving, the drag force diminishes while the tear film starts to thin, and eventually either drainage due to gravity or a rising film height reaching the effective range of the dewetting force may be responsible for breakup of the tear film. Other models of tear film kinetics suggest that the two-step response may be a simplification of tear spread following a blink.52 Through the procedures of interferometry and fluorophotometry, it has been shown that following the upstroke movement of the lid, horizontal lines of the lipid layer to move from the lower to the upper cornea.53,54 Additionally, the upward lid movement is associated with a superior movement of particles in the lipid layer34,49,55,56 due to surface tension gradients causing an upward Marangoni drift that usually lasts for about 1 s following a blink.34,57 As the upward spread of the tear film can be observed, it is possible that the present imaging method is indicative of the movement of the reflective particles upon the tear film. A limitation in the present procedure is that rapid initial fluid rise was not measured. However, the stability of the spreading tear film is better predicted by slower rise of the thicker lipid layer. The significant differences between the morning and evening measures indicate that imaging and tracking of upward particle movement is an in vivo and simple method of examining the ease of spread of the tear film upon the surface of the contact lens and also the stability of the tear film following its reformation.
The method can be easily extended to measure the UPV of very small particles seen during the initial rapid fluid rise, the subsequent UPV during the slow rise of the thicker lipid layer, and the eventual slower rise caused during the thinning of the tear film. An automation of the upward particle tracking would be ideal and a completely objective way of determining the ease of spread of the tear film.
In the case of the one subject whose UPV was lower in the morning as compared with end of day, it is possible that either lens surface-related factors or subject-related factors might have contributed to such presentation of the data.
In summary, this study reports a novel method of determining in vivo tear film particle dynamics over the surface of a contact lens using a simple but very effective imaging technique. Additional work is required to determine whether this method is able to differentiate in vivo, the wetting characteristics of different soft contact lenses following a blink. This may improve the understanding of contact lens-related dryness and discomfort with respect of tear film spread and stability. Regardless, this preliminary work establishes proof of concept and that tear-film particle dynamics over the contact lens surface may be different in the morning as compared with after 8 h of lens wear.
Centre for Contact Lens Research, School of Optometry
University of Waterloo
200 University Avenue West
Waterloo, ON N2L 3G1, Canada
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contact lens wettability; non-invasive tear-film stability; tear hydrodynamics; imaging; upward particle velocity