Tear Exchange Research Articles

Tear Exchange, Intraocular Pressure, and Wear Characteristics of Quadrant-specific Versus Spherical Haptic Scleral Lenses.

To compare scleral lenses (SLs) with a quadrant-specific (study lens) or a spherical (habitual lens) landing zone in a crossover study. Seven participants (eight eyes) wore each of two lenses for 2 weeks before measurements. We measured visual acuity, contrast sensitivity, intraocular pressure (IOP), fluid reservoir clearance, corneal thickness, tear exchange, and lens experience. Variables were compared between lenses and before and after 2 hr of wear. The visual acuity was not different between the study lens, 0.12 logarithm of the minimum angle of resolution (logMAR), and habitual lens, 0.18 logMAR (median, P = 1.0). Contrast sensitivity was 1.3% under the study lens and 1.6% under the habitual lens ( P = 0.94). IOP did not change after 2 hr of wear for either lens (study lens, P = 0.33 and habitual lens, P = 0.74), and corneal thickness did not change during wear of either lens ( P = 0.44). The fluorescein concentration under the study lens did not change after 2 hr (99% of initial concentration; P = 0.84) but decreased to 46% of initial concentration under the habitual lens ( P = 0.008). Lens comfort was slightly better with the study lens (5.0 vs. 4.0, respectively; P = 0.05). SLs with spherical or quadrant-specific landing zones provide good vision and do not affect IOP or corneal thickness. However, tear exchange is greater under spherical lenses than under quadrant-specific lenses. The quadrant-specific lens provides greater patient comfort.

Limbal-Rigid Contact Lens Wear for the Treatment of Ocular Surface Disorders: A Review.

:Recently, the prescription of large-diameter rigid gas-permeable contact lenses (CLs), also known as “scleral lenses,” “corneoscleral lenses,” and “limbal-rigid CLs,” is on the rise for the treatment of both moderate and severe ocular surface disorders (OSDs). Compared with scleral lenses, the diameter of limbal-rigid CLs is generally smaller, that is, a diameter ranging from 13.0 to 14.0 mm, and they are designed so that the peripheral edge bears on the limbus. The Suncon Kyoto-CS (Sun Contact Lens Co., Ltd.) is a novel limbal-rigid CL design with multistep curves on the peripheral edge for easy tear exchange during blinking that removes debris and prevents lens clouding or fogging, thus allowing patients to enjoy a longer daily duration of CL wear. In severe OSD cases, limbal-rigid CL wear after surgery is a noninvasive therapeutic approach that can neutralize corneal irregularities, decrease dry eye symptoms, prevent the progression or recurrence of symblepharon, and improve the patient's visual acuity and overall quality of life. Thus, surgeries such as amniotic membrane transplantation and cultivated oral mucosal epithelial transplantation, as well as limbal-rigid CL wear, which is noninvasive, are valuable and effective treatment strategies that can now be applied for the management of patients afflicted with severe OSDs.

Rigid Gas-Permeable Semi-Scleral Contact Lenses after Radial Keratotomy: Apical Space, Lens Diameter, Limbal Clearance, Peripheral O-Rings, and Tear Exchange as Contact-Lens-Fitting Success Factors

The purpose of this case report is to verify if the adaptation of a rigid gas-permeable contact lens can improve VA and comfort in a patient with complications derived from radial keratotomy (RK) surgery. A semi-scleral contact lens was fitted in a 46-year-old female patient who had undergone bilateral RK before 30 years. The uncorrected distance visual acuity in the right eye (RE) and left eye (LE) was 0.5 and 0.6 logMAR (minimum angle resolution), respectively. The RE and LE manifest refractions were +2.00 − 4.25 × 155 diopters (D) and +2.00 − 3.00 × 15 D, respectively. A semi-scleral rigid gas-permeable Rose K2 XL™ lens was fitted in both eyes. The central clearance was 400–450 µm in the RE and 300–350 µm in the LE. The semi-scleral corrected distance visual acuity in the RE and LE was 0.1 and 0.16 logMAR, respectively. Semi-scleral contact lenses are comfortable and a beneficial choice for patients after RK when associated with visual problems and intolerance to other therapeutic options. Semi-scleral lenses permit prolonged duration of use without discomfort, adverse alterations, and improve the feeling of glare in scotopic vision. In the present case, there were multiple factors that influenced the resolution, such as the amount of apical space, lens diameter, limbal clearance, peripheral O-rings, and tear exchange.

Microengineered poly(HEMA) hydrogels for wearable contact lens biosensing.

Microchannels in hydrogels play an essential role in enabling a smart contact lens. However, microchannels have rarely been created in commercial hydrogel contact lenses due to their sensitivity to conventional microfabrication techniques. Here, we report the fabrication of microchannels in poly(2-hydroxyethyl methacrylate) (poly(HEMA)) hydrogels that are used in commercial contact lenses with a three-dimensional (3D) printed mold. We investigated the corresponding capillary flow behaviors in these microchannels. We observed different capillary flow regimes in these microchannels, depending on their hydration level. In particular, we found that a peristaltic pressure could reinstate flow in a dehydrated channel, indicating that the motion of eye-blinking may help tears flow in a microchannel-containing contact lens. Colorimetric pH and electrochemical Na+ sensing capabilities were demonstrated in these microchannels. This work paves the way for the development of microengineered poly(HEMA) hydrogels for various biomedical applications such as eye-care and wearable biosensing.

Changes in Corneal Thickness Associated With Variation of Central Corneal Clearance of Scleral Lenses

Purpose: Modern scleral lenses (ScCLs) promote little tear exchange, limiting oxygen delivery to the cornea, possibly resulting in corneal edema. Recent studies have investigated the hypoxic effects of varying central corneal clearance (CCC) in short-term ScCL wear, and theoretical models have been proposed. The purpose of this study was to observe if there were hypothesized changes in corneal thickness associated with low levels of corneal hypoxia in healthy participants wearing ScCLs of different (CCC) over a longer term of wear.

Fabrication of Hydrophilic Surface on Rigid Gas Permeable Contact Lenses to Enhance the Wettability Using Ultraviolet Laser System

The widely used rigid gas permeable (RGP) contact lenses provide higher oxygen permeability and tear exchange rate than do soft contact lenses. However, their wettability warrants improvement to enhance the wearing comfort. This study used UV laser (wavelength = 355 nm) to modify the surface properties of RGP contact lenses with materials of Boston XO® (Bausch & Lomb Incorporated). Briefly, the mesh pattern was fabricated on the RGP contact lens surface by using the laser and smoothed by using oxygen plasma; the enhanced hydrophilic efficiency was analyzed using contact angle measurement. The experiment results indicated that the contact angle of the lens material decreased by approximately 10°–20° when the pitch of mesh pattern was <50 μm under a 500-mm/s scanning speed. The oxygen plasma enhanced surface wettability with a decreased contact angle (40°). The hydrophilic characteristic of the UV laser and oxygen plasma–treated surface was twice that of oxygen plasma–treated and untreated surfaces. In the future, RGP contact lens edges could be treated with UV laser and oxygen plasma to enhance the tear wettability and wearing comfort.

Tear dynamics under scleral lenses.

To evaluate post-lens tear dynamics at two different time points during scleral lens wear in two cohorts with 10 neophytes each. All subjects wore bilaterally scleral lenses for 5 h on 3 separate visits. Post-lens tear exchange was measured using Out-in method, which required 5 μL of 2% FITC-Dextran instilled on the bulbar conjunctiva during lens wear. Time taken to observe the first sign of fluorescence in post-lens tear reservoir was recorded with a stopwatch. Out-in measurements were collected at 5-hour post-lens insertion in Group 1 and compared with those obtained at 20 min of lens wear in Group 2. Tear dynamics under the lens was observed in Group 2 with fluorogram using a modified slit-lamp technique (Tan et al., 2018) to monitor post-lens fluorescence intensity and with high-resolution spectral domain optical coherence tomography (ENVISU 2300; Bioptigen Inc.) to measure post-lens tear thickness (PoLTT) over 5 h of lens wear. 60% of subjects in Group 1 achieved Out-in times less than 5 min at 5-hour post-lens insertion, compared with 67% of subjects at 20-min lens wear (Tan et al., 2018). Using qualitative analysis on 60 series of data in Group 2 to compare the changes in fluorescence intensity and PoLTT with respect to lens-wearing time, 27% was due to lens settling, 13% was due to tear exchange and mixing while 60% indicated tear dynamics under scleral lenses was due to a combination of tear exchange, mixing, and lens settling. Tear flow into tear reservoir under a scleral lens on subjects with healthy cornea occurred at 20 min and 5 h after lens insertion. After 5 h of lens wear, roughly one third of the subjects had no tear flow into post-lens reservoir, as the observed decline in post-lens tear fluorescence was predominately due to lens settling.

A measure of tear inflow in habitual scleral lens wearers with and without midday fogging.

The purpose of this pilot study was to evaluate tear inflow in a scleral lens system using fluorophotometry, and indirectly assess the exchange of the tear reservoir in habitual scleral lens wearers with the presence or absence of midday fogging (MDF). Habitual scleral lens wearers (n=23) and normal scleral lens neophytes (n=10) were recruited. Of the 23 habitual wearers, 11 of them experienced MDF and 12 did not have a diagnosis of MDF. Contact lens-fitting characteristics were evaluated using ocular coherence tomograpy (OCT) and biomicroscopy. High molecular weight fluorescein (FITC) Dextran was instilled into the tear reservoir beneath the scleral lens, and the tear fluid fluorescein concentration was measured using the Fluorotron fluorophotometer. Calculated fluorescein concentrations were plotted over time to measure the fluorescein decay rate of the tear fluid beneath the scleral lens, which was used to calculate the tear exchange rate. There was no significant difference in tear inflow between the MDF group (mean: 0.111%) and the non-MDF group (mean: 0.417%), and there was a high amount of variability seen in the rates (p = 0.26). In addition, there was no significance between the tear reservoir thickness in the MDF (283um) and non-MDF (326um) groups (p = 0.53). The relationship between the amount of tear exchange during scleral lens wear and the incidence of MDF was not significant. Additional studies are needed to further examine the role of tear exchange in MDF and address the causes of variability to improve measurement techniques with fluorophotometry in the scleral lens system.

Tear exchange beneath a vaulted hybrid contact lens

Tear exchange beneath a vaulted hybrid contact lens

Silicone Hydrogel and Rigid Gas-Permeable Scleral Lens Tear Exchange.

To quantify tear elimination rate (ER) underneath silicone hydrogel (Si-Hy) and scleral gas permeable (GP) contact lenses (CLs). Subjects successfully using either well-fitting soft Si-Hy CLs or scleral GP CLs were recruited. Most scleral GP CL wearers had irregular corneas (e.g., keratoconus). An objective fluorometer measured decay of fluorescein isothiocyanate dextran dye signal (70 kD MW) from which the tear ER in %/min was calculated. For GP scleral lenses, the ER was determined for both the initial settling period and the 30- to 60-min period, and without lenses. All ERs were calculated from 5 to 30 min to avoid reflex tearing effects. Fourteen soft Si-Hy CL and 12 scleral GP CL wearers completed the study. The ER for the scleral GP CL wearers averaged 0.57 (±0.6) %/min for the 0- to 30-min and 0.42 (±0.5) %/min for the 30- to 60-min period (P=0.515). Non-CL wear tear ER in these same subjects averaged 34.17 (±15.9) %/min and was significantly different versus both scleral GP wear periods (both P values <0.001). The ER for the soft Si-Hy CL wearers, 5 to 30 min, averaged 6.09 (±2.8) %/min. Our data demonstrate significantly less ER in well-fit scleral GP CL wearers compared with soft Si-Hy CL wearers for both the settling and longer wear periods (both P values <0.001). Moreover, slightly greater tear exchange was observed during the scleral GP CL settling period than later, which may reflect a change over time in tear vault thickness.

Modeling Corneal Oxygen with Scleral Gas Permeable Lens Wear.

The main goal of this current work is to use an updated calculation paradigm, and updated boundary conditions, to provide theoretical guidelines to assist the clinician whose goal is to improve his or her scleral gas permeable (GP) contact lens wearing patients' anterior corneal oxygen supply. Our model uses a variable value of corneal oxygen consumption developed through Monod equations that disallows negative oxygen tensions within the stroma to predict oxygen tension at the anterior corneal surface of scleral GP contact lens wearing eyes, and to describe oxygen tension and flux profiles, for various boundary conditions, through the lens, tears, and cornea. We use several updated tissue and boundary parameters in our model. Tear exchange with GP scleral lenses is considered nonexistent in this model. The majority of current scleral GP contact lenses should produce some levels of corneal hypoxia under open eye conditions. Only lenses producing the thinnest of tear vaults should result in anterior corneal surface oxygen tensions greater than a presumed critical oxygen tension of 100 mmHg. We also find that corneal oxygen tension and flux are each more sensitive to modification in tear vault than to changes in lens oxygen permeability, within the ranges of current clinical manipulation. Our study suggests that clinicians would be prudent to prescribe scleral GP lenses manufactured from higher oxygen permeability materials and especially to fit without excessive corneal clearance.

Pseudomonas aeruginosa Survival at Posterior Contact Lens Surfaces after Daily Wear.

Pseudomonas aeruginosa keratitis is a sight-threatening complication of contact lens wear, yet mechanisms by which lenses predispose to infection remain unclear. Here, we tested the hypothesis that tear fluid at the posterior contact lens surface can lose antimicrobial activity over time during lens wear. Daily disposable lenses were worn for 1, 2, 4, 6, or 8 hours immediately after removal from their packaging or after presoaking in sterile saline for 2 days to remove packaging solution. Unworn lenses were also tested, some coated in tears "aged" in vitro for 1 or 8 hours. Lenses were placed anterior surface down into tryptic soy agar cradles containing gentamicin (100 μg/mL) to kill bacteria already on the lens and posterior surfaces inoculated with gentamicin-resistant P. aeruginosa for 3 hours. Surviving bacteria were enumerated by viable counts of lens homogenates. Posterior surfaces of lenses worn by patients for 8 hours supported more P. aeruginosa growth than lenses worn for only 1 hour, if lenses were presoaked before wear (∼ 2.4-fold, p = 0.01). This increase was offset if lenses were not presoaked to remove packaging solution (p = 0.04 at 2 and 4 hours). Irrespective of presoaking, lenses worn for 8 hours showed more growth on their posterior surface than unworn lenses coated with tear fluid that was aged for 8 hours in vitro (∼ 8.6-fold, presoaked, p = 0.003; ∼ 5.4-fold from packaging solution, p = 0.004). Indeed, in vitro incubation did not impact tear antimicrobial activity. This study shows that postlens tear fluid can lose antimicrobial activity over time during contact lens wear, supporting the idea that efficient tear exchange under a lens is critical for homeostasis. Additional studies are needed to determine applicability to other lens types, wearing modalities, and relevance to contact lens-related infections.

Calculated in situ tear oxygen tension under hybrid contact lenses.

We calculated corneal surface oxygen tension under hybrid contact lenses (CLs) by extending existing models of CL oxygen diffusion to the circumstances of hybrid CL designs with tear vaults. Theoretical oxygen tensions at the corneal surface, if tear mixing and exchange are excluded, are calculated for hybrid CL (modern, high oxygen-permeable rigid center) designs with a single chamber corneal model using a computer software spreadsheet. Several specific in vivo instances of hybrid CL tear vaults are measured by both slitlamp biomicroscopy and optical coherence tomography (OCT). No significant difference was found between central thickness of hybrid CL rigid portions measured by Vigor gauge and OCT. The amount of central tear vault thickness was found to be significantly greater, however, when interpolated from biomicroscope images (500-1,554 μm, simulating a poorly fit lens) than from either OCT images (0-91 μm) or direct measurement through OCT software (0-96 μm). Using all measurements, excluding any potential contributions from tear exchange or mixing, we predict corneal surface tear pO2 values under several hybrid CLs to range from zero under a lens with an excessive vault to 112 mm Hg under a lens with a minimal vault. Tear vaults beneath the gas permeable portion of the hybrid CLs of 100 μm or less resulted in acceptable corneal surface oxygen values. Where tear vaults are greater than about 100 μm, predicted anterior corneal surface pO2 values are less likely to be adequate for optimal corneal physiology. Any significant tear exchange, however, should increase these values. Although measuring tear thickness, we noted that the photographic biomicroscope and the OCT methods provided inconsistent results, and thus limited the reliability of our primary result.

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.

Contact lens material characteristics associated with hydrogel lens dehydration

To determine the association between material dehydration and hydrogel contact lens material characteristics, including water content and ionicity. Water content and refractive index data were derived from automated refractometry measurements of worn hydrogel contact lenses of 318 participants in the Contact Lens and Dry Eye Study (CLADES). Dehydration was determined in two ways; as the difference between nominal and measured (1) water content and (2) refractive index. Multiple regression models were used to examine the relation between dehydration and material characteristics, controlling for tear osmolality. The overall measured and nominal water content values were 52.58 +/- 7.49% and 56.88 +/- 7.81% respectively, while the measured and nominal refractive indices were 1.429 +/- 0.015 and 1.410 +/- 0.017. High water content and ionic hydrogel lens materials were associated with greater dehydration (p < 0.0001 for both) than low water content and non-ionic materials. When dehydration was assessed as the difference in refractive index, only high water content was associated with dehydration (p < 0.0001). High water content and ionic characteristics of hydrogel lens materials are associated with hydrogel lens dehydration, with the former being more strongly associated. Such dehydration changes could in turn lead to important clinical ramifications such as reduced oxygen transmissibility, greater lens adherence and reduced tear exchange.

Scalloped Channels Enhance Tear Mixing Under Hydrogel Contact Lenses

Tear exchange under a soft contact lens is directly related to the amount of lateral and transverse lens motion. Hydrodynamic modeling suggests that channels placed on the back surface of a soft lens will reduce fluid resistance and increase transverse lens movement. This study measured the effect of posterior lens surface scalloped channels on tear exchange. Tear exchange in the postlens tear film (PoLTF) was estimated using a fluorometer to measure the exponential depletion of high-MW fluorescein under the lens expressed as the time to deplete 95% of dye (T95). A total of 32 subjects wore two pairs of identical lenses except that the experimental lens had 12 scalloped channels placed radially in the midperiphery of the posterior lens surface, whereas lenses without channels served as controls. The mean +/- standard error T95 values for the channel lenses was 28 +/- 2 minutes compared with 32 +/- 2 minutes for the control lenses (p = 0.107). There was a marginally significant difference in T95 between two lens groups in Asian eyes (p = 0.054). Placing scallop-shaped channels on high-H2O content soft lenses improved the postlens tear pumping in Asian eyes.

Advances in Spectacle Free Refractive Corrections

Advances in Spectacle Free Refractive Corrections

Tear clearance implications for ocular surface health

Tear clearance/turnover provides a global assessment of the function of the lacrimal functional unit and of tear exchange on the ocular surface. It is an indirect measure of dry eye induced inflammation on the ocular surface. It shows better correlation with the severity of ocular irritation symptoms and corneal epithelial disease in dry eye than the Schirmer 1 test. Delayed tear clearance may prove to be the best measure for identifying patients with tear film disorders who may respond to anti-inflammatory therapy.

Tear Exchange and Oxygen Reservoir Effects in Silicone Hydrogel Systems

The effects of oxygen reservoir and tear exchange are known for conventional hydrogel contact lenses. This study attempted to (1) confirm their presence in lenses of a silicone hydrogel (SH) material and (2) evaluate their individual and combined contributions to hypoxic relief of the cornea. Corneal oxygen uptake rates were measured polarographically for 10 OD corneas immediately after 300-sec periods (equivalent to 60+ blink cycles) of: (1) nonblink wear of a 0 Dk/L polymethylmethacrylate (PMMA) cap lens; (2) nonblink wear of that cap lens with a SH lens inserted between it and the cornea; (3) regular blinked wear of that same lens combination; (4) nonblinked wear of the SH lens alone; and (5) normal open-eye, non-lens wear. The rates for each eye had a ratio with its own baseline (no lens) rate, and the total sample mean was calculated for each condition from those individual eye means. (1) Under nonblink conditions, a 28% reduction in corneal oxygen demand was observed when a SH lens was inserted under the PMMA cap lens versus without. (2) When regular blinking was added to that lens combination, corneal oxygen demand decreased another 8%, for a total of 36%. The combination of SH insertion and blinking did achieve a statistically significant difference (P<0.05 by the Dunnett test) from the nonblink, 0 Dk/L, maximum deprivation condition. The statistically significant hypoxia reduction observed with the SH lens insertion and blinking indicates the additive presence of two factors: (1) a lens reservoir effect caused by the SH lens and (2) a bulk-flow tear exchange effect caused by blinking. Their respective contributions to the reduction of corneal oxygen demand over the period studied were found to be in the ratio of 3.4:1.

Fenestrations enhance tear mixing under silicone-hydrogel contact lenses.

Extended wear of conventional hydrogel soft contact lenses (SCLs) is associated with adverse ocular responses. Some of these ocular events are thought to result from inadequate tear mixing in the postlens tear film (PoLTF). Therefore, strategies to increase tear mixing may improve the safety of extended-wear SCLs. Recently, it has been suggested that placing fenestrations in soft lenses increases tear mixing. In the present study, hydrodynamic modeling and tear-mixing measurements were used to explore the effects of fenestrations on tear exchange under an SCL. Tear mixing, expressed as the time to deplete 95% of fluorescent dye from the PoTLF (T(95)), was measured in 20 subjects fitted with two pairs of silicone hydrogel SCLs. The lenses were identical except that one pair was fenestrated (F) with 40 holes each 100 microm in diameter. The effect of fenestrations on improving tear mixing is explained and enhancement of tear mixing is predicted, with the use of hydrodynamic modeling. T(95) estimates were significantly lower (P < 0.001) in the F (mean T(95) = 18.3 minutes) lenses compared with the standard unfenestrated (S-uF) (mean T(95) = 22.6 minutes) lenses (DeltaT(95) = 4.3 minutes, 95% CI = 2.5-6.2) and were in general agreement with the proposed mixing model. Optimization of tear mixing can be achieved by selectively placing fenestrations in the pooling regions before the thinnest regions. The model predictions were sensitive to the distribution of tear-film thickness under the lens. Fenestrations improve tear-mixing efficiency. Accurate prediction of the effects of lens parameters on tear mixing, however, demands quantitative measurement of the postlens distribution of tear-film thickness.