Optometry & Vision Science:
Estimating Tear Film Spread and Stability Through Tear Hydrodynamics
Varikooty, Jalaiah*; Keir, Nancy†; Simpson, Trefford‡
†OD, PhD, FAAO
Centre for Contact Lens Research, (JV, NK), and School of Optometry, University of Waterloo, Waterloo, Ontario, Canada.
Received October 1, 2011; accepted May 2, 2012.
Jalaiah Varikooty Centre for Contact Lens Research, School of Optometry University of Waterloo 200 University Avenue West Waterloo, ON N2L 3G1, Canada e-mail: email@example.com
Purpose. The stability and ease of spread of the tear-film over the contact lens surface may be an indicator of contact lens surface dewetting. The present in vivo methods of determining lens dewetting are complex. This study introduces a novel and objective way of determining the upward spread and stability of the tear-film through measurement of tear-film particle dynamics.
Methods. Ten adapted contact lens wearers wore the same type of contact lens. Using a video camera mounted to a slit-lamp, the tear-film spread over the lens surface was recorded after a blink, at 8× magnification and capture rate of 30 frames per second, at morning after lens insertion, and after 8 h of lens wear. Images from 20 videos, without blinks and without an observable change in fixation were analyzed without any further postprocessing of the images. Using a customized calibrated ImageJ macro for particle tracking, the velocity of naturally occurring reflective particles was determined. The results were analyzed using the R program and ProFit.
Results. The results established that the upward particle velocity was highest immediately after a blink and declined with time. The spread of the tear film measured through upward particle velocity was different on lens insertion than after 8 h of lens wear (p = 0.001). The exponential time constants ± SE were 346.02 ± 29.0 for lens insertion at morning and 1413.13 ± 419.6 after 8 h of lens wear.
Conclusions. A novel and non-invasive way to measure in vivo spread and stability of the prelens tear-film has been developed. Additional studies are needed to understand whether this simple measure is able to differentiate the performance of different soft contact lenses and how this method may help in the understanding other aspects of lens performance such as non-invasive tear breakup time, surface deposition, and lens comfort.
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
1. Gayton JL. Etiology, prevalence, and treatment of dry eye disease. Clin Ophthalmol 2009;3:405–12.
2. Begley CG, Chalmers RL, Mitchell GL, Nichols KK, Caffery B, Simpson T, DuToit R, Portello J, Davis L. Characterization of ocular surface symptoms from optometric practices in North America. Cornea 2001;20:610–8.
3. Dumbleton KA, Woods CA, Jones LW, Fonn D. Comfort and adaptation to silicone hydrogel lenses for daily wear. Eye Contact Lens 2008;34:215–23.
4. Pritchard N, Fonn D, Brazeau D. Discontinuation of contact lens wear: a survey. Int Contact Lens Clin 1999;26:15707–62.
5. Richdale K, Sinnott LT, Skadahl E, Nichols JJ. Frequency of and factors associated with contact lens dissatisfaction and discontinuation. Cornea 2007;26:168–74.
6. Lemp MA, Baudouin C, Baum J, Dogru M, Foulks GN, Kinoshita S, Laibson P, McCulley J, Murube J, Pflugfelder SC, Rolando M, Toda I. The definition and classification of dry eye disease: report of the Definition and Classification Subcommittee of the International Dry Eye WorkShop (2007). Ocul Surf 2007;5:75–92.
7. Stapleton F, Stretton S, Papas E, Skotnitsky C, Sweeney DF. Silicone hydrogel contact lenses and the ocular surface. Ocul Surf 2006;4:24–43.
8. Shirafkan A, Woodward EG, Port MJ, Hull CC. Surface wettability and hydrophilicity of soft contact lens materials, before and after wear. Ophthalmic Physiol Opt 1995;15:529–32.
9. Pritchard N, Fonn D, Weed K. Ocular and subjective responses to frequent replacement of daily wear soft contact lenses. CLAO J 1996;22:53–9.
10. Chalmers RL, Begley CG. Dryness symptoms among an unselected clinical population with and without contact lens wear. Cont Lens Anterior Eye 2006;29:25–30.
11. Poggio EC, Abelson M. Complications and symptoms in disposable extended wear lenses compared with conventional soft daily wear and soft extended wear lenses. CLAO J 1993;19:31–9.
12. Guillon M, Maissa C. Use of silicone hydrogel material for daily wear. Cont Lens Anterior Eye 2007;30:5–10.
13. Jones L, Senchyna M, Glasier MA, Schickler J, Forbes I, Louie D, May C. Lysozyme and lipid deposition on silicone hydrogel contact lens materials. Eye Contact Lens 2003;29:S75–9.
14. Jones L, Franklin V, Evans K, Sariri R, Tighe B. Spoilation and clinical performance of monthly vs. three monthly Group II disposable contact lenses. Optom Vis Sci 1996;73:16–21.
15. Gellatly KW, Brennan NA, Efron N. Visual decrement with deposit accumulation of HEMA contact lenses. Am J Optom Physiol Opt 1988;65:937–41.
16. Brennan NA, Coles ML. Deposits and symptomatology with soft contact lens wear. Int Contact Lens Clin 2000;27:75–100.
17. Haddad M, Morgan PB, Kelly JM, Maldonado-Codina C. A novel on-eye wettability analyzer for soft contact lenses. Optom Vis Sci 2011;88:1188–95.
18. Woods CA, Keir N, Fonn D. The development of a video based grading scale for in vivo front surface contact lens wettability. Cont Lens Anterio Eye 2011;34:258.
19. Guillon M, McGrogan L, Guillon JP, Styles E, Maissa C. Effect of material ionicity on the performance of daily disposable contact lenses. Cont Lens Anterior Eye 1997;20:3–8.
20. Radke CJ. To the editor: a novel on-eye wettability analyzer for soft contact lenses. Optom Vis Sci 2011;88:1529; author reply 30.
21. Elliott M, Fandrich H, Simpson T, Fonn D. Analysis of the repeatability of tear break-up time measurement techniques on asymptomatic subjects before, during and after contact lens wear. Cont Lens Anterior Eye 1998;21:98–103.
22. Guillon JP. Non-invasive tearscope plus routine for contact lens fitting. Cont Lens Anterior Eye 1998;21(suppl 1):S31–40.
23. Shiobara M, Schnider CM, Back A, Holden BA. Guide to the clinical assessment of on-eye wettability of rigid gas permeable lenses. Optom Vis Sci 1989;66:202–6.
24. Doane MG. An instrument for in vivo tear film interferometry. Optom Vis Sci 1989;66:383–8.
25. Nichols JJ, Mitchell GL, King-Smith PE. Thinning rate of the precorneal and prelens tear films. Invest Ophthalmol Vis Sci 2005;46:2353–61.
26. Liu H, Thibos L, Begley CG, Bradley A. Measurement of the time course of optical quality and visual deterioration during tear break-up. Invest Ophthalmol Vis Sci 2010;51:3318–26.
27. Chen Q, Wang J, Tao A, Shen M, Jiao S, Lu F. Ultrahigh-resolution measurement by optical coherence tomography of dynamic tear film changes on contact lenses. Invest Ophthalmol Vis Sci 2010;51:1988–93.
28. Stern J, Wong R, Naduvilath TJ, Stretton S, Holden BA, Sweeney DF. Comparison of the performance of 6- or 30-night extended wear schedules with silicone hydrogel lenses over 3 years. Optom Vis Sci 2004;81:398–406.
29. Long B, Robirds S, Grant T. Six months of in-practice experience with a high Dk lotrafilcon a soft contact lens. Cont Lens Anterior Eye 2000;23:112–8.
30. Morgan PB, Efron N. Comparative clinical performance of two silicone hydrogel contact lenses for continuous wear. Clin Exp Optom 2002;85:183–92.
31. Zhang J, Greenlee JD, Begley CG, Liu H, Simpson T, Chalmers RL, Wu Z, Himebaugh NL, Jansen ME. A novel contact lens wettability grading scale related to changes in visual function. Invest Ophthalmol Vis Sci 2011;52. E-Abstract 6529.
32. Cornea and Contact Lens Research Unit. The CCLRU grading scales. In: Philips AJ, Speedwell L, eds. Contact Lenses, 4th ed. Boston, MA: Butterworth-Heinemann; 1997:863–7.
33. King-Smith PE, Fink BA, Hill RM, Koelling KW, Tiffany JM. The thickness of the tear film. Curr Eye Res 2004;29:357–68.
34. Owens H, Phillips J. Spreading of the tears after a blink: velocity and stabilization time in healthy eyes. Cornea 2001;20:484–7.
35. Wong H, Fatt II, Radke CJ. Deposition and thinning of the human tear film. J Colloid Interface Sci 1996;184:44–51.
36. Chauhan A, Radke CJ. Settling and deformation of a thin elastic shell on a thin fluid layer lying on a solid surface. J Colloid Interface Sci 2002;245:187–97.
37. Khanal S, Millar TJ. Nanoscale phase dynamics of the normal tear film. Nanomedicine 2010;6:707–13.
38. Abramoff MD, Magalhaes PJ, Ram SJ. Image processing with ImageJ. Biophotonics Int 2004;11:36–42.
39. Sbalzarini IF, Koumoutsakos P. Feature point tracking and trajectory analysis for video imaging in cell biology. J Struct Biol 2005;151:182–95.
40. R Development Core Team. R: A Language and Environment for Statistical Computing. Vienna, Austria: R Foundation for Statistical Computing; 2011.
41. Doane MG. Interactions of eyelids and tears in corneal wetting and the dynamics of the normal human eyeblink. Am J Ophthalmol 1980;89:507–16.
42. McDonald JE. Surface phenomena of the tear film. Am J Ophthalmol 1969;67:5607–64.
43. Nichols JJ, Sinnott LT. Tear film, contact lens, and patient-related factors associated with contact lens-related dry eye. Invest Ophthalmol Vis Sci 2006;47:1319–28.
44. King-Smith PE, Nichols JJ, Nichols KK, Fink BA, Braun RJ. Contributions of evaporation and other mechanisms to tear film thinning and break-up. Optom Vis Sci 2008;85:623–30.
45. Patel S, Bevan R, Farrell JC. Diurnal variation in precorneal tear film stability. Am J Optom Physiol Opt 1988;65:151–4.
46. Lira M, Oliveira ME, Franco S. Comparison of the tear film clinical parameters at two different times of the day. Clin Exp Optom 2011;94:557–62.
47. Webber WR, Jones DP, Wright P. Fluorophotometric measurements of tear turnover rate in normal healthy persons: evidence for a circadian rhythm. Eye (Lond) 1987;1(pt 5):615–20.
48. Glasson MJ, Stapleton F, Keay L, Sweeney D, Willcox MD. Differences in clinical parameters and tear film of tolerant and intolerant contact lens wearers. Invest Ophthalmol Vis Sci 2003;44:5116–24.
49. King-Smith PE, Fink BA, Nichols JJ, Nichols KK, Braun RJ, McFadden GB. The contribution of lipid layer movement to tear film thinning and breakup. Invest Ophthalmol Vis Sci 2009;50:2747–56.
50. Nikolov AD, Wasa DT, Chengara A, Koczo K, Policello GA, Kolossvary I. Superspreading driven by Marangoni flow. Adv Colloid Interface Sci 2002;96:325–38.
51. Brown SI, Dervichian DG. Hydrodynamics of blinking. In vitro study of the interaction of the superficial oily layer and the tears. Arch Ophthalmol 1969;82:54107–7.
52. Szczesna DH, Iskander DR. Lateral shearing interferometry for analysis of tear film surface kinetics. Optom Vis Sci 2010;87:513–7.
53. Di Pascuale MA, Goto E, Tseng SC. Sequential changes of lipid tear film after the instillation of a single drop of a new emulsion eye drop in dry eye patients. Ophthalmology 2004;111:783–91.
54. Jones MB, McElwain DL, Fulford GR, Collins MJ, Roberts AP. The effect of the lipid layer on tear film behaviour. Bull Math Biol 2006;68:1355–81.
55. Wolff E. The muco-cutaneous junction of the lid margin and distribution of the tear fluid. Trans Ophthalmol Soc UK 1946;66:291–308.
56. Berger RE, Corrsin S. A surface tension gradient mechanism for driving the pre-corneal tear film after a blink. J Biomech 1974;7:22507–38.
57. Goto E, Tseng SC. Differentiation of lipid tear deficiency dry eye by kinetic analysis of tear interference images. Arch Ophthalmol 2003;121:173–80.
contact lens wettability; non-invasive tear-film stability; tear hydrodynamics; imaging; upward particle velocity
© 2012 American Academy of Optometry
Highlight selected keywords in the article text.