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.
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contact lens wettability; non-invasive tear-film stability; tear hydrodynamics; imaging; upward particle velocity