Estimating tear film spread and stability through tear hydrodynamics

Impact of the physical properties of contact lens materials on the discomfort: role of the coefficient of friction

Contact Lens Discomfort (CLD) is the main cause in contact lens (CLs) discontinuation, referred in literature as drop-out phenomenon. Despite such evidence was reported in several clinical studies, a relationship between physico-chemical properties of CLs and CLD is not still totally understood. In this regard, the friction of CLs surfaces seems to be related to discomfort feeling events, probably due to an alteration of the lubricate function of the tear film after the CL placement inside the ocular environment. In the last years, many studies have been finalized to the friction measurements of CLs surface, finding conflicting data due to a lack in standardized protocol. The aim of this review is primarily to show evident relationships between CLs surface properties (i.e. wettability, tear evaporation, tear film quality, etc.) and the coefficient of friction (CoF), resulting therefore the most relevant physical quantity in the CLs characterization. In addition, we reported the most recent studies in CLs tribology, which highlight that the introduction of a standard protocol in CoF measurements is necessary to obtain reproducible results, considering the aim to evaluate in a more precise way the relationship between this material surface property and comfort in CLs users.

Comparing Tear Film Viscosity between Sjögren and Non-Sjögren Dry Eye Disease

This study aimed to compare tear film viscosity (TFV) in Sjögren and non-Sjögren dry eye diseases (DEDs). This was a cross-sectional observational study. A total of 68 DED patients were enrolled, including 32 patients with Sjögren syndrome (SS) and 36 without SS. TFV was assessed by a tear film analyzer and determined by the momentary moving speed (MMS; MMS (t) = α × t-β, t = time (s)) with its power-law fitting-derived parameters (α and β). Among the four indices of TFV (MMS (0.1 s), MMS (2.0 s), α, and β), the SS-DED patients had significantly lower MMS (0.1 s) (p = 2.01 × 10-5), α (p = 0.0375), and β (p = 0.0076). The SS-DED group also had significantly higher OSDI, lower central and nasal tear meniscus height (TMH), and higher OSS. MMS (0.1 s) was significantly correlated with nasal TMH and OSS (ρ = 0.2520, p = 0.0381 in nasal TMH; ρ = -0.3487, p = 0.0036 in OSS). Index β was not correlated with any non-TFV tests. In conclusion, MMS (0.1 s), α, and β are promising TFV indices in distinguishing SS-DED from non-SS-DED patients early. Among these TFV indices, lower MMS is the best alternative clue for detecting SS-DED.

Assessment of condition-induced changes on the ocular surface using novel methods to assess the tear film dynamics and the lipid layer

OBJECTIVES

To assess the effects of different condition-induced changes on current and novel ocular surface metrics.

METHODS

Eighty-four healthy volunteers (22.4 ± 2.6 years) participated in this study. The ocular surface and tear film response to (1) computer use, (2) contact lens insertion and (3) tear film instillation during computer use with contact lenses were assessed. Current metrics included the ocular surface disease (OSDI) questionnaire, 5-item dry eye questionnaire (DEQ-5), bulbar redness, tear meniscus height (TMH) and non-invasive keratograph break-up time (NIKBUT). Novel metrics included the lipid layer thickness obtained from the intensity of the reflected Placido disk and the speed of tear film particles post-blink.

RESULTS

Higher dry eye symptoms, TMH and bulbar redness, and lower values in metrics related to the intensity of the Placido disk pattern and to particle speed were found after the computer reading task (p < 0.036). When a contact lens was fitted, lower TMH, NIKBUT and particle speed metrics were obtained (p < 0.044). Mixed ANOVA revealed that artificial tears significantly ameliorated the effect of computer reading on OSDI, DEQ-5, NIKBUT, metrics related to the intensity of the Placido disk pattern and metrics related to particle speed (p < 0.033).

CONCLUSIONS

Computer use and contact lens wear worsened dry eye signs and symptoms, but artificial tears ameliorated this effect. Newly developed methods can serve as a tool to detect changes in the tear film triggered by different ocular surface-disturbing conditions.

Viscoelastic properties of the human tear film

When considering eye blinking from a tribological perspective, tear viscosity is expected to play a fundamental role. The application of rheological techniques to describe the tear film dates back to the late 1980s, but there has been a continuous need of reappraisal due to new findings in tear film biochemistry and to the development of new methods of physico-chemical characterization. This review provides an overview on tear rheological behavior by analyzing the peer-reviewed literature on this topic. Specifically, examples of in-vitro and in-vivo viscosity measurements are detailed, highlighting experimental criticalities and the need of a standard convention for rheological techniques to compare data across different studies, of analyses on tears of single individuals even within the limits of the low volume available, and of a further development of in-vivo techniques. Then, the controversial role of specific tear components on viscosity is discussed, together with the alterations associated to dry eye disease and contact lens wear. Finally, an updated focus is reported on the viscosity of artificial tears formulations.

Grading reliability of the tear film viscosity examination

PURPOSE

To assess the reliability of a tear film (TF) viscosity video grading system.

METHODS

Thirty-four dynamic TF viscosity videos were obtained by a clinically available TF analyzer and objectively sorted according to the movement speed of three arbitrary reflective light particles. A 4-grade system was constructed on a specially designed window for simultaneous comparison with the three standard videos. Two masked graders were invited to grade these videos under a randomized procedure. Observer reliabilities were determined by Spearman's correlation analysis and Bland-Altman agreement analysis.

RESULTS

For this four-grade system, the intra-observer correlation was very strong in the two graders (ρ = 0.96 and 0.82; both P < 0.0001). However, the inter-observer correlation showed moderate strength in normal playback speed (ρ = 0.53, P = 0.002 and ρ = 0.52, P = 0.003 for 1st and 2nd gradings, respectively). In slower playback videos, the inter-observer correlation of the two graders was higher (ρ = 0.70 and P < 0.0001) when reduced to 0.8-times playback speed. Moreover, the 0.8-times mode was also significantly better than normal playback mode (P = 0.0204) in terms of inter-observer agreement.

CONCLUSIONS

The dynamic 4-grade system has an excellent intra-observer reliability and a good inter-observer reliability under 0.8-times playback speed. The grading system established in this study provides a promising solution for rapidly determining the level of TF viscosity.

An Emerging Method to Assess Tear Film Spread and Dynamics as Possible Tear Film Homeostasis Markers

Purpose: This study aims to assess the performance of an analysis method to measure in vivo the movement speed of tear film particles post-blink as a measure of tear film spreading.Materials and methods: Ocular surface parameters and the recording of tear film particles' spreading post-blink were assessed in eighty-one healthy volunteers (43.7 ± 27.0 years) using Keratograph 5 M. The developed software automatically decomposed the video into frames to manually track particles' position for 1.75 seconds after a blink. The following tear film-dynamic metrics were automatically calculated: mean, median, maximum, and minimum particles' speed at different times after blinking and time for particle speed to decrease to <1.20 mm/second. Repeatability of each tear film-dynamic metric and its correlations with ocular surface signs and symptoms were analyzed. Binomial logistic regression was performed to assess the predictability of new metrics to ocular parameters.Results: Repeatability tended to be lower just after blinking (variability of 12.24%), whereas the metrics from 0.5 s onwards had acceptable repeatability (variability below 10%). Tear film-dynamic metrics correlated positively with Non-Invasive Break-Up Time (NIKBUT) while negatively with meibomian gland drop-out. Binomial logistic regression analysis revealed that tear film-dynamic metrics were able to predict NIKBUT. Nevertheless, no statistically significant association was found with gland drop-out. This means that higher particle speed is related to larger NIKBUT. The metric "time for particle speed to decrease to <1.20 mm/second" can be considered the best metric to assess the quality of the tear film, since it was more strongly correlated with NIKBUT (r = 0.42, p = .004), it was more strongly associated in the binomial logistic regression analysis with NIKBUT and showed good repeatability (variability = 5.49%).Conclusions: Tear film-dynamic metrics are emerging homeostasis parameters for assessing indirectly the tear film quality in natural conditions with acceptable repeatability.

Impact of Improper Approach to Identify Lid Wiper Epitheliopathy (LWE)

Purpose

Variability in the use of ophthalmic dyes to diagnose lid wiper epitheliopathy (LWE) has led to division in the literature and clinical practice. The principal aim of this study was to evaluate whether the use of a non-optimal methodology to identify LWE had a potential for false negatives; in which LWE was overlooked.

Patients and Methods

A total of 20 participants were initially categorized to not have LWE and were enrolled in this study. The protocol examined whether or not LWE would later be revealed through the use of optimized methodology. Semi-automated analysis was performed of images taken after two different drop instillations with varying post-dye viewing times for both lissamine green (LG) and sodium fluorescein (NaFl).

Results

There was a significant increase in area of staining revealed when an optimal methodology for LWE identification was used. Comparisons for every non-optimal condition were statistically significantly different against the optimal condition (all p<0.01). The use of a non-optimal methodology resulted in a 70% false-negative rate when using LG and a 95% false-negative rate when using NaFl.

Conclusion

The study demonstrated that using a double instillation of dye was statistically different from a single-dose, even with extended wait time for clinical observation. A single instillation did not offer adequate volume of dye for adequate lid margin uptake. A careful adherence to volume as well as a repeat administration is key to revealing the full area of LWE. A non-optimal approach to diagnose LWE can lead to false negatives.

Predictive Potential of Eyelids and Tear Film in Determining Symptoms in Contact Lens Wearers

SIGNIFICANCE

Early diagnosis of clinical markers of contact lens discomfort can help clinicians set realistic expectations and monitor and provide prophylactic management for contact lens wearers.

PURPOSE

The purpose of this study was to evaluate the potential of eyelid- and tear film-related clinical markers to be used as predictive factors for diagnosing discomfort in contact lens wearers.

METHODS

A cross-sectional study was performed on 30 contact lens wearers (6 male, 24 female) with median age of 23 years (range, 18 to 41 years). Eyelid signs and tear film characteristics were evaluated during a single visit, and subjects completed the Contact Lens Dry Eye Questionnaire to evaluate ocular discomfort. Area under the curve (AUC) statistics and sensitivity and specificity values from receiver operating characteristic curves were analyzed to evaluate the predictive potential of clinical signs in discriminating symptoms of contact lens discomfort.

RESULTS

The presence of foam at meibomian gland orifices (AUC, 0.944; P < .05; sensitivity >0.7), meibomian gland secretion volume (AUC, 0.935; P < .05; sensitivity >0.7), quality (AUC, 0.969; P < .05; sensitivity >0.7), and expressibility (AUC, 0.933; P < .05; sensitivity >0.7) were significant and strong predictors of discomfort in lens wear. Tear evaporation rates with (AUC, 0.779; P < .05; sensitivity >0.7) or without contact lenses (AUC, 0.788; P < .05; sensitivity >0.7), palpebral conjunctival roughness (AUC, 0.859; P < .05; sensitivity >0.7), palpebral conjunctival staining (AUC, 0.817; P < .05; sensitivity >0.7), palpebral conjunctival hyperemia (AUC, 0.746; P < .05; sensitivity >0.7), meibomian gland orifice capping (AUC, 0.873; P < .05; sensitivity >0.7), pouting (AUC, 0.891; P < .05; sensitivity >0.7), and lid-parallel conjunctival folds (AUC, 0.770; P < .05; sensitivity >0.7) were other acceptable discriminators of symptoms of discomfort during contact lens wear. An equation was developed to identify symptomatic from asymptomatic lens wearers based on the significant predictors: Symptom discriminant function score = 3.378 (meibomian gland secretion grade) + 0.224 (meibomian gland morphology grade) + 0.61 (tear evaporation rate without contact lenses) + 0.439 (lid-parallel conjunctival folds grade) - 0.346 (palpebral conjunctival health grade) - 4.625.

CONCLUSIONS

This study demonstrated that clinical signs related to meibomian gland secretions and morphology, tear evaporation, lid-parallel conjunctival folds, and palpebral conjunctival health may successfully predict symptoms of discomfort in contact lens wearers.

Tracking the Reflective Light Particles Spreading on the Cornea: An Emerging Assessment for Tear Film Homeostasis

PURPOSE

To implement an emerging noninvasive approach for assessing the dynamic tear film (TF) homeostasis.

METHODS

The video records of dynamic TF from 12 healthy orthokeratology lens wearers were obtained by a clinically available TF analyzer and decomposed as image sequences. The trajectories of TF particles were analyzed by two tracking models, the full-span model (FSM) and the fixed-duration model (FDM). FSM tracked a particle for a complete opening blink cycle, while FDM tracked 1 second of the same cycle. A power-law fitting operation [Formula: see text] was used to extract homeostasis markers based on the tracking model for each subject.

RESULTS

Comparing two tracking models (N = 6), only one subject had statistical difference in averaged momentary moving speed (MMS; P = 0.0488), while none had significant difference in averaged momentary moving direction (MMD). However, both models showed good correlations in average MMS (ρ = 0.94, P = 0.0048) and MMD (ρ = 1.00, P < 0.0001) and all extracted homeostasis markers [α, β, MMS(0.1), and MMS(2.0)]. Assessing interblink reliability in these markers under FDM tracking (N = 12), only one subject in the MMS (0.1) and another subject in the MMS (2.0) were outside 95% limits of agreement, respectively.

CONCLUSIONS

FDM is a good alternative to FSM and has tracking properties of higher efficiency and easier implementation. The homeostasis markers under FDM tracking showed a good interblink consistence; therefore this approach will be a promising method for analyzing dynamic TF homeostasis in future practice.

TRANSLATIONAL RELEVANCE

FDM analytical architecture can practice the past experimental platform on a TF analyzer to obtain homeostasis markers of TF.

Supporting Dry Eye Diagnosis with a New Method for Noninvasive Tear Film Quality Assessment

SIGNIFICANCE

Noninvasive high-speed videokeratoscopy equipped with specific software has shown potential for assessing the homeostasis of tear film, providing clinicians with a fast and consistent tool for supporting dry eye diagnosis and management.

PURPOSE

The purpose of this study was to evaluate the efficacy of a recently proposed method for characterizing tear film dynamics using noninvasive high-speed videokeratoscopy in assessing the loss of homeostasis of tear film.

METHODS

Thirty subjects from a retrospective study, of which 11 were classified as dry eye and 19 as healthy, were included. High-speed videokeratoscopy measurements were performed using E300 videokeratoscope (Medmont Pty., Ltd., Melbourne, Australia). Raw data were analyzed using a recently proposed method to estimate the dynamics of the tear film based on a fractal dimension approach. This method provides three time-varying indicators related to the regularity of the reflected rings: tear film surface quality indicator, breaks feature indicator, and distortions feature indicator. From each indicator, five parameters were extracted and analyzed, including noninvasive breakup time, mean value of the indicator in the stability phase, mean value of the indicator in the whole interblink interval, mean value of the indicator in the leveling phase, and the general trend of the time series. Receiver operating characteristic curves were used to determine the sensitivity and specificity of each parameter in dry eye detection.

RESULTS

The best discrimination performance between dry eye and healthy subjects was achieved with the breaks feature indicator noninvasive breakup time parameter, with an area under the curve of 0.85. For a cutoff value of 10 seconds, the sensitivity was 100% and the specificity was 84%.

CONCLUSIONS

The analyzed method improves the assessment of tear film homeostasis in comparison with previous high-speed videokeratoscopy methods showing higher potential in assisting dry eye diagnosis.

Tear film evaluation and management in soft contact lens wear: a systematic approach

The human tear film is a highly ordered structure consisting of a thin layer of lipid on the surface and a thicker aqueous-mucin phase, which increases in mucin concentration toward the corneal epithelial cell layer. The health of the tear film and ocular surface influences the likelihood of being able to achieve successful contact lens wear. Contact lens discomfort and dryness are the most frequent reasons why contact lens wearers experience reduced wearing times, which can eventually lead to contact lens discontinuation. Comprehensive clinical assessment of tear film integrity and ocular surface health is therefore essential prior to commencing contact lens wear, to enable the ocular surface environment to be optimised to support lens wear. These parameters should also be evaluated over the course of contact lens wear, in order to identify any aspects requiring clinical management and ensure maintenance of optimal lens-wearing conditions. This review summarises current knowledge relating to the effects of soft contact lens wear on the tear film and ocular surface. It also provides a systematic approach to evaluating tear film and ocular surface integrity, in order to guide the clinical management of tear film anomalies with respect to contact lens wear.

Tear film dynamics on soft contact lenses

PURPOSE

To analyze and model the dynamics of tear film surface quality (TFSQ) in a group of subjects with healthy eyes, before and during contact lens (CL) wear, and in a group of subjects with dry eyes (DEs).

METHODS

Four sets of TFSQ measurements with lateral shearing interferometry were retrospectively analyzed on two groups of subjects. The first group included 13 CL wearers for which TFSQ measurements corresponding to baseline, Best CL, and Worst CL were selected. The second group included 13 DE subjects. The acquired TFSQ time series were fit with a power-exponential model. Tear film surface quality stability time, best TFSQ, and integrated poststability time characterizing the tear film deterioration process were derived.

RESULTS

The proposed power-exponential model was better suited (higher correlation values) for the TFSQ dynamics on CL rather than for those of baseline and DE measurements. The average baseline TFSQ Stability Time was significantly longer (p < 0.001) than those in the DE group and with both the best and worse CL. The average Best TFSQ achieved at baseline was statistically significantly better (p = 0.03) than that for the DE group. The average best TFSQ was significantly better (p < 0.01) for the Best CL than for that of the Worst CL. Deterioration of TFSQ on both best and Worst CL was substantially faster than that achieved for the DE group.

CONCLUSIONS

The observed distinct change in the sign of the TFSQ velocity on contact lenses suggests a two-phase dynamics in which the postblink stability phase is followed by a phase of dewetting. Lens material properties influence the first phase but play little role after the dewetting process occurs.

Tear exchange and contact lenses: a review

Tear exchange beneath a contact lens facilitates ongoing fluid replenishment between the ocular surface and the lens. This exchange is considerably lower during the wear of soft lenses compared with rigid lenses. As a result, the accumulation of tear film debris and metabolic by-products between the cornea and a soft contact lens increases, potentially leading to complications. Lens design innovations have been proposed, but no substantial improvement in soft lens tear exchange has been reported. Researchers have determined post-lens tear exchange using several methods, notably fluorophotometry. However, due to technological limitations, little remains known about tear hydrodynamics around the lens and, to-date, true tear exchange with contact lenses has not been shown. Further knowledge regarding tear exchange could be vital in aiding better contact lens design, with the prospect of alleviating certain adverse ocular responses. This article reviews the literature to-date on the significance, implications and measurement of tear exchange with contact lenses.

Dry eye disease: A review of diagnostic approaches and treatments

Dry eye (DE) is a common ocular disease that results in eye discomfort, visual disturbance and substantially affects the quality of life. It has a multifactorial etiology involving tear film instability, increased osmolarity of the tear film and inflammation of the ocular surface with potential damage to the ocular surface. This review discusses the classification, diagnostic approaches and treatments of DE.

Tear film stability: a review

Tear film stability can be assessed via a number of tools designed for clinical as well as research purposes. These techniques can give us insights into the tear film, and allow assessment of conditions that can lead to dry eye symptoms, and in severe cases, to significant ocular surface damage and deterioration of vision. Understanding what drives tear film instability and its assessment is also crucial for evaluating existing and new therapies. This review examines various techniques that are used to assess tear film instability: evaluation of tear break-up time and non-invasive break-time; topographic and interferometric techniques; confocal microscopic methods; aberrometry; and visual function tests. It also describes possible contributions of different tear film components; namely meibomian lipids, ocular mucins and proteins, and factors such as age, contact lens wear, ocular surgery and environmental stimuli, that may influence tear film instability.