A unifying parameter to describe the clinical mechanics of hydrogel contact lenses

All soft contact lenses are not created equal

Soft contact lenses that have been prescribed by eye care practitioners are sometimes substituted for alternative lenses by unqualified, unregulated and sometimes even fully regulated lens suppliers, in the mistaken belief that there is essentially no difference between different soft lens types. This review considers the implications of inappropriately substituting soft contact lens types in terms of (a) lens properties: surface treatment, internal wetting agents, material, total diameter, back optic zone radius, thickness, edge profile, back surface design, optical design, power, colour (tint) and ultraviolet protection; and (b) lens usage: wearing modality (daily versus overnight wear) and replacement frequency. Potential aspects of patient dissatisfaction and adverse events when prescribed soft lenses are substituted for lenses with different properties or intended usage are considered. Substitution of 15 of the 16 lens properties considered (i.e. except for back surface design) was found to be related to at least one - and as many as six - potential sources of patient dissatisfaction and adverse ocular events. Contact lens are medical devices which are prescribed and fitted; they should never be substituted for another lens type in the absence of a new prescription further to a full finalised fitting, for the simple reason that all soft contact lenses are not created equal. A substituted lens may have properties that results in undesirable consequences in respect of vision, ocular health, comfort and cosmetic appearance, and may be incompatible with the lifestyle of the patient.

Characterization of Soft Contact Lens Edge Fitting during Daily Wear Using Ultrahigh-Resolution Optical Coherence Tomography

Purpose

To determine conjunctival overlap over the edge of soft contact lens and to visualize the peripheral postlens tear film (PoLTF) underneath soft contact lenses using ultrahigh-resolution optical coherence tomography (UHR-OCT).

Methods

Twenty participants (4 males and 16 females, 23.0 ± 3.7 years) were fitted with two different types of soft contact lenses randomly. The limbus with lens was imaged with the UHR-OCT at the horizontal meridian every two hours up to 6 hours during lens wear. The conjunctival overlap was ranked as the percentage of the edge covered by the conjunctiva. The frequency of occurrence for visualized peripheral PoLTF was determined.

Results

The average conjunctival overlaps at insertion were 49% and 73% for galyfilcon A and balafilcon A lenses and increased significantly to 84% and 90% by 6 hours of lens wear (P < 0.001). Lenses with rounded edges had more conjunctival overlap than the lenses with angled edges (P=0.014). There were significant decreases for PoLTF on the conjunctiva (P=0.014) and peripheral cornea (P=0.004) over the study period compared to insertion. The percentage of subjects with PoLTF on the conjunctiva (32.5%) and peripheral cornea (36%) were greater in subjects wearing balafilcon A lenses (P=0.017).

Conclusions

Increased conjunctival overlap over the lens edges and reduced PoLTF underneath the peripheral region of soft contact lenses were shown during lens daily wear. The lens edge configuration may play a role in conjunctival response and peripheral PoLTF.

Impact of Contact Lens Material, Design, and Fitting on Discomfort

OBJECTIVE

To review the effect of contact lens (CL) material, design, and fitting characteristics on CL discomfort.

METHODS

A PubMed search identified publications describing subjective comfort and CL material, fitting, and design parameters. The review included clinical signs associated with discomfort that may be a consequence of these parameters.

RESULTS

Reduced lens movement or more CL tightness were associated with improved comfort. Increased lens-induced paralimbal conjunctival staining and indentation, considered as quasi-indicators of CL fitting or edge design, were also associated with better comfort. No recent studies have evaluated varying CL design parameters and subjective comfort. Silicone hydrogel CLs are no different in comfort compared with hydrogel CLs. Lower equilibrium water content is associated with improved comfort in hydrogel CL wear. Coefficient of friction shows promise as a material factor potentially associated with comfort. Lid wiper epitheliopathy and lid-parallel conjunctival folds have been linked with comfort in established wearers.

CONCLUSIONS

Recent studies have confirmed the association between more mobile CLs and more discomfort, whereas closer conformity of the CL to the bulbar conjunctiva improved subjective comfort. There is no evidence to support the perceived comfort difference between silicone hydrogel and hydrogel CL. There has been limited progress in understanding the impact of varying specific CL design parameters. Although specific clinical signs may be predictive of discomfort, their role in the natural history of discomfort remains unclear. A better understanding of the relationship between coefficient of friction and comfort and strategies to improve lubricity may hold promise for limiting CL discomfort.

Corneal swelling caused by conventional and new-design low-Dk soft contact lenses following a 10-day daily wear trial regime

PURPOSE

To investigate the efficacy and safety of a fenestrated and channelled soft contact lens (F-SCL) compared to a standard and non-fenestrated soft contact lens (S-SCL) in experienced soft contact lens (SCL) wearers.

METHODS

This was a randomised, crossover, single-blinded (subject), and multicentre clinical trial. Sixteen experienced SCL wearers were randomly divided into two groups (FS and SF). The FS group first wore F-SCLs followed by S-SCLs, each for 10 days, separated by a 1-week washout period, whereas the SF group wore the S-SCLs first and crossed over to F-SCLs in the same manner. The F-SCLs were designed with three equally spaced, symmetrical fenestrations and a partial-thickness, connecting, circumferential channel on the back surface of the mid-periphery of the lens. Measurement of central corneal thickness using ultrasonic pachymetry was performed on the day of screening, after the 1-week washout period, and after 10 days of wearing each kind of lens, based on which central corneal swelling was calculated and compared. One eye in each subject was chosen at random for analysis.

RESULTS

Central corneal swelling was 1.92±1.73% vs. 5.26±2.14% in F-SCLs vs. S-SCLs wearers, which was statistically significant (P<0.001). There was no significant difference between the groups in terms of SCL-corrected visual acuity or SCL-related adverse events.

CONCLUSION

The use of F-SCLs led to reduced corneal swelling compared to S-SCLs. The newly incorporated features appear to improve tear mixing and thereby the oxygen supply to the cornea, which results in reduced corneal oedema.

Micrometer-scale contact lens movements imaged by ultrahigh-resolution optical coherence tomography

PURPOSE

To dynamically evaluate contact lens movement and ocular surface shape using ultrahigh-resolution and ultralong-scan-depth optical coherence tomography (OCT).

DESIGN

Clinical research study of a laboratory technique.

METHODS

Four different types of soft contact lenses were tested on the left eye of 10 subjects (6 male and 4 female). Lens edges at primary gaze and temporal and nasal gazes were imaged by ultrahigh-resolution OCT. Excursion lag was obtained as the distance between the lens edge at primary gaze and immediately after the eye was quickly turned either nasally or temporally. The inferior lens edges were imaged continuously to track vertical movements during blinking. Ultralong-scan-depth OCT provided quantifiable images of the ocular surface, and the contour was acquired using custom software.

RESULTS

Excursion lag at the horizontal meridian was 366 ± 134 μm at temporal gaze and 320 ± 137 μm at nasal gaze (P > .05). The lens uplift at the vertical meridian was 342 ± 155 μm after blinking. There were significant differences in horizontal lags and vertical movements among different lenses (P < .05). Horizontal lags were correlated with radii of curvatures and sagittal heights at 6-mm and 14-mm horizontal meridian (P < .05). The blink-induced lens uplift first lowered by 104 ± 8 μm, and then lifted 342 ± 155 μm after the blink.

CONCLUSIONS

Ultrahigh-resolution and ultralong-scan-depth OCT can assess micrometer-scale lens movements and ocular surface contours. Both lens design and ocular surface shape affected lens movements.

Characterization of soft contact lens edge fitting using ultra-high resolution and ultra-long scan depth optical coherence tomography

PURPOSE

To characterize the edge fitting of soft contact lenses using ultra-high resolution optical coherence tomography (UHR-OCT) and ultra-long scan depth optical coherence tomography (UL-OCT).

METHODS

A total of 20 participants (11 men, 9 women; mean age, 32.3 years) were recruited. Four different types of soft contact lenses were randomly fitted to both eyes of each subject on two separate visits. After 30 minutes, the horizontal meridians of the corneal center, midperiphery, and limbus were imaged by UHR-OCT. UL-OCT imaged each lens in vitro and the ocular surface of a physical model eye.

RESULTS

Angle-edged lenses had significantly less conjunctival buildup than did round-edged lenses (P = 0.008). Limbal post-lens tear film gaps were present in 42% of the eyes, with the round-edged lenses having the most at 68%. Similarly, post-lens tear film gaps at the corneal mid-periphery were present in 47% of all eyes, with the round-edged lens having the most at 75%. Mismatches between the lens and the ocular surface were simulated based on UL-OCT images of the in vitro lenses and the model eye. The existence of tear film gaps and touching points were predicted in the simulation.

CONCLUSIONS

The soft contact lens edge fitting was characterized by the conjunctival buildup and tear film gaps. Different types of contact lenses presented different levels of conjunctival buildup as well as different frequencies of tear film gaps. The findings by UHR-OCT were predicted in the simulation by UL-OCT. The application of these new technologies may open new ways of designing lenses and evaluating their fit.

In-eye performance of soft contact lenses made from different materials

The in-eye performance of soft contact lenses may be affected by the material from which they are fabricated and may alter during wear. This study describes clinical and laboratory experiments that were conducted in order to examine the in-eye performance of eight soft contact lenses manufactured from different materials; these materials (and nominal water contents) were: HEMA/VP 40%, HEMA/VP 55%, HEMA/VP 70%, VP/MMA 55%, VP/MMA 70%, HEMA 40%, HEMA/MAA 55% and HEMA/MAA 70% (HEMA: 2-hydroxy-ethyl methacrylate, VP: vinyl pyrrolidone, MMA: methyl methacrylate, MAA: methacrylic acid). Two lenses from each of the eight soft contact lens groups were used in experiments concerning their parameter stability. Six subjects were fitted with lenses for 1 day. Verification of back optic zone radius, total diameter, back vertex power, centre thickness and water content was undertaken at 20 degrees C. In vitro measurements of total diameter were taken at 35 degrees C before lens fitting and after 6 h of lens wear. In vivo measurements of lens centration, up-gaze lag, post-blink movement, total diameter and subjective assessment of comfort were taken (a) immediately after lens insertion (1 min), (b) 20 min later and (c) 6 h later (same day). The majority of differences of the in vivo parameters between lens types (with respect to lens centration, lag, movement on blink, lens total diameter and comfort) were not found to be statistically significant throughout the 6 h wearing period. It was also found that lenses increase in diameter when first placed on the eye and decrease in diameter when they were removed from the eye. The information generated in study concerning the in-eye performance of soft contact lens materials may assist the contact lens industry and contact lens practitioners in developing and prescribing soft lenses with optimal performance characteristics.

Clinical performance of microchannel contact lenses

PURPOSE

It has been suggested that microchannels placed on the posterior surface of hydrogel contact lenses increase transverse lens movement and therefore increase tear mixing. Lens designs, which can improve tear exchange without adversely affecting lens performance, may play a significant role in improving the safety of contact lens wear. Our aim was to investigate the effect of posterior lens surface microchannels on contact lens performance.

METHODS

A randomized, single masked pilot study was conducted. Fifteen experienced hydrogel lens wearers were randomly assigned to wear a microchannel lens (MC) in one eye and a standard hydrogel lens design (STD) in the fellow eye over a period of 6 hours. A range of objective and subjective clinical performance measures were assessed, including visual acuity, lens movement, comfort and patient preference.

RESULTS

There were no significant differences between the MC and STD lenses with respect to vision measures, corneal staining, and bulbar and limbal redness (p > 0.05). The MC lenses produced significantly less conjunctival staining than the STD lenses and showed greater lens movement in primary gaze (p < 0.05). Lens distortion with blinking was evaluated through topographic assessment of the contact lenses on eye, and no significant differences were observed in central distortion levels between the two lens designs (p > 0.05). Subjective evaluation of the lenses with respect to comfort, vision, and overall satisfaction did not vary significantly between the two lens types (p > 0.05).

CONCLUSIONS

The results of this study suggest that posterior lens surface microchannels do not adversely affect contact lens performance or subjective lens preference over a short-term wearing period. The MC lenses appear to produce less physiological impact on the conjunctival surface than STD lenses with less conjunctival staining observed with the MC lenses. Clinical investigation of the performance of this type of lens design in a larger group of subjects, over longer wearing periods, is warranted.

Clinical evaluation of toric soft lens reproducibility

PURPOSE

The manufacturing variability of frequent replacement toric soft lenses is such that some resulting variation in clinical performance can be expected. The purpose of this study was to evaluate the effect of manufacturing variability on the clinical performance of three frequent replacement toric soft contact lenses.

METHODS

This was a three-part study in which 12 to 14 subjects were enrolled for each part. In each part, subjects were preadapted to the lens type 1 month prior to the assessment visits. Subjects wore a total of 12 test lenses in one eye and, in each case, the clinical performance was compared with a control lens of the same type worn in the other eye that was removed between assessments but not replaced. The study lenses were Acuvue Toric (Vistakon), Focus Toric (Ciba Vision) and FreshLook Toric (Wesley lessen).

RESULTS

Two lens types (Focus Toric and FreshLook Toric) showed significantly greater variability with the test lenses compared with the nonreplaced control lenses with several variables: lens fit, visual acuity, and subjective vision quality. One lens (Focus Toric) also showed greater variability of orientation position, and another (Acuvue Toric) showed greater variability in subjective vision quality.

CONCLUSIONS

The study demonstrated variability in aspects of clinical performance, which appear to be linked to variability of manufacturing output. With toric soft lenses used on a frequent replacement basis, better reproducibility is needed if consistent clinical performance is to be maintained.

Clinical evaluation of factors influencing toric soft contact lens fit

PURPOSE

Some knowledge of the orientation position of toric soft lenses is required as part of the fitting process. Although it is known that factors such as lid anatomy, lens design, and fit are important, there is little detailed understanding of how these relate to toric lens orientation. The purpose of this study was to evaluate various patient and lens factors that might influence toric soft lens fit and to determine the predictor variables for lens orientation.

METHODS

This was a clinical study in which 45 subjects were fitted with a prism-stabilized, mid-water, cast-molded, back surface toric soft lens. The lens was representative of commonly used designs for frequent replacement toric soft lenses. Patient and lens fit variables were assessed, including a range of lid topography features, which were measured using a digital photographic technique.

RESULTS

The main patient factors associated with lens orientation and stability were degree of myopia, palpebral aperture, and several angles relating to lid anatomy. Greater upward incline of the temporal lids was associated with increasing inferior-temporal lens orientation; lower myopia and smaller palpebral aperture size were associated with more stable lens orientation. The principal lens fit characteristics affecting lens orientation were postblink movement and lens tightness. Less movement was associated with more stable lens orientation, and increased tightness was associated with slower reorientation speed. Weaker but significant correlations were also shown between a number of other patient and lens variables.

CONCLUSIONS

A number of patient factors and lens fit characteristics influencing toric soft lens orientation were identified. The findings fall short of allowing practitioners to accurately predict toric lens orientation; however, they do provide some factors to consider when fitting toric soft lenses.

Comparative performance of disposable soft contact lenses

A short-term clinical study was undertaken to compare and evaluate the fit and clinical behaviour of eight disposable hydrogel contact lenses. Thirty subjects were enrolled in a 1-day, randomised, double-masked study. The lenses compared were Vistakon Acuvue (8.80), Vistakon Surevue (8.80), Aspect Frequency-55, Bausch & Lomb Medalist (SeeQuence 2), Ciba Focus (8.90), Lunelle Rythmic, Pilkington Barnes Hind Precision-Wand Wesley Jessen Freshlook. Three of the eight designs, Acuvue (86.7%), Precision-UV (86.7%) and Frequency-55 (90%), achieved relatively high levels of fitting success. The Rythmic lenses achieved the lowest success rate (70%) showing significantly greater downward decentration than the other designs. Although all the lenses were designed to fit a wide variety of eyes, they exhibited widely varying fitting characteristics. Some were more successful than others. The study shows that it is necessary for practitioners to utilise a range of lens designs in order to fit satisfactorily the typical contact lens population.

Soft lens movement: effect of blink rate on lens settling

Little is understood about the mechanism by which soft lenses settle on the eye, although it has been suggested that lens base curve, steepening with in-eye lens dehydration or tear film changes may influence lens movement in the initial period of wear. In this study, we investigated the role of postlens tear fluid expulsion by assessing lens movement in 20 subjects wearing 38% water content hydrogel lenses for 10 min under three different conditions: 10 blinks/min, 30 blinks/min and eye closure. Over the 10 min wearing period, the total decrease in median lens movement for the 10 blinks/min condition was 0.07 mm, which was not a significant change (Friedman ANOVA, p = 0.13), while significant decreases occurred with conditions of 30 blinks/min (0.19 mm, p = 0.004) and eye closure (0.43 mm, p = 0.0001). As expected, lens movement under the three conditions was the same at insertion, but was significantly higher thereafter for the slower blink rate condition compared to the faster blink rate or eye closure conditions (Wilcoxon test, Z = 2.8 and -3.0, p = 0.006 and 0.003, respectively). Based on these findings, we postulate the model that the extent of lens settling and the degree of postinsertion lens movement are determined by the timeaverage pressure for postlens tear film expulsion exerted on the lens by the eyelids.

Water transport in dehydrating hydrogel contact lenses: implications for corneal desiccation

This article provides direct evidence that the rate of evaporation from the anterior surface of a hydrogen contact lens is related to the bulk water transport within the contact lens, analogous to diffusion-controlled evaporation. The evaporative flux of water from the anterior surface of a contact lens, with free access to water at its posterior surface, was greater for contact lenses of greater overall water content. Blockage of the water entry to the posterior surface of the contact lens reduced the evaporative loss from the anterior surface by 44%. These results suggested that a decrease in the rate of lens bulk water transport during wear would lead to a reduced evaporation rate from the anterior lens surface, for example, if the contact lens water content was reduced due to lipid and mucus adsorption. In addition, a rise in the external temperature of 1.3 degrees was required to produce a significant increase (2 SD) in the evaporative flux from the contact lenses. This suggests that the steady-state water content in vivo may not be significantly influenced by temperature cycling during blinking.