Compressive behaviour of soft contact lenses and its effect on refractive power on the eye and handling off the eye.

PURPOSE: To investigate the stress-strain behaviour of 9 soft contact lens materials, that are commonly used in the market, under uniaxial compression loading. METHODS: Seven types of hydrogel and two types of silicone-hydrogel soft contact lens materials were hydrated in phosphate-buffered saline (PBS) solution then subjected to uniaxial compression loads. The load rate was set to 16.0 N/min starting with two consecutive initial 5.0 N loading cycles followed by three relaxation periods of 4.0 min within which there were two more 5.0 N loading cycles and eventually, a full loading cycle that stopped at a load of 49.0 N. The load and contraction data obtained experimentally were analysed to derive the stress-strain behaviour. Finite Element (FE) analysis was then utilised to evaluate the performance of soft contact lenses on the human eye and handling lenses off the eye. RESULTS: Unlike tensile tests, all tested materials showed nonlinear behaviour when tested under compression. When fitted to first-order Ogden hyperelastic model, parameter µ was found to be varying in the range 0.12 to 0.74 MPa and material parameter α was found to be varying in the range 8.2 to 20.326 among the nine tested materials. Compression modulus of elasticity was 2.2 times higher than the tensile modulus of elasticity on average. FE simulation with nonlinear Ogden constitutive model showed a limited change (8%~12%) in the optical performance when compared to other material models, however, it predicted higher stress when the lens was simulated under bending during off-eye handling. CONCLUSIONS: Compression tests revealed slightly nonlinear behaviour when materials were strained under compression stress down to 15% ~ 30% of their nominal heights. Considering the physiological compression loading range of 8 mmHg, secant moduli of elasticity were 1.5% to 6.9% higher than the tension moduli of elasticity depending on the material. Tensile-based moduli of elasticity could be used in FE analysis as a step towards simulating the optical performance of soft contact lenses on-eye. However, nonlinear compression-based material models are recommended for FE analysis of soft contact lenses when lens-handling is investigated off-eye.

Power profiles of centre-distance multifocal soft contact lenses.

PURPOSE: Centre-distance multifocal contact lenses (MFCLs) for myopia control are thought to slow myopia progression by providing both clear foveal vision and myopic defocus. Characterising the power profile of lenses is important to understanding their possible effects on retinal defocus when worn. The power profiles of three commercially available MFCLs were determined. METHODS: Three centre-distance MFCL designs were studied: Biofinity Multifocal D +2.50 add (comfilcon A), Proclear Multifocal D +2.50 add (omafilcon A), and NaturalVue Multifocal (etafilcon A). Two lenses each in power from -1.00D to -6.00D in 1D steps were stored in ISO 18369-3:2017 standard phosphate buffered saline for 24 h. Optical power profiles were measured in a wet cell with the SHSOphthalmic profiler accounting for centre thickness and manufacturer-reported material refractive index. Sagittal power maps from the SHSOphthalmic were exported, and custom MATLAB code was used to generate power profiles by averaging along the vertical and horizontal meridians. One-way anova with Tukey's HSD post-hoc t-tests were used to analyse maximum add power by lens design. RESULTS: Plus power increased out from the lens centre for all three MFCLs. Power profiles of Biofinity D and Proclear D MFCLs show three distinct areas within the optic zone; the distance zone (from lens centre to about 1.6 mm radius), intermediate zone (about 1.6 mm radius to 2.1 mm) and near zone (about 2 mm radius to 4 mm). For NaturalVue MFCLs, plus power starts increasing almost immediately from the lens centre, reaching maximum measured mean plus power at a radius of 2.7 mm. From 2.7 mm to 3.0 mm, there was a decrease in plus power, which was then generally maintained out to the optic zone edge. Across all lens powers, maximum add power was highest with the NaturalVue MFCL (+3.32 ± 0.44D), then Proclear D (+1.84 ± 0.28D) and Biofinity D (+1.47 ± 0.34D) MFCLs (all p < 0.04). Add power peaked at different locations for different lens powers and designs. CONCLUSIONS: Power profiles of MFCLs vary based on lens design and power. These power profiles are consistent with reported myopic and hyperopic changes in peripheral refraction with MFCLs and provide some explanation for reported differences in peripheral refraction with these MFCLs. Further work is needed to determine whether these power profile differences influence myopia progression.

Technical Report: Guidelines for Handling of Multipatient Contact Lenses in the Clinical Setting.

SIGNIFICANCE: Standardized guidelines that are clinically practical are needed to assist the prescriber in minimizing the risk of conveying infection through multiuse diagnostic contact lens use and reuse.Contact lens prescribers face the specter of transferring potential pathogens from one patient to another when reusing diagnostic (trial) contact lenses on multiple patients because infectious organisms have been recovered from worn contact lenses, although there is no evidence of transmission through this mechanism. These pathogens can be introduced into the system from one patient to another, or they may be introduced by clinician lens handling, storage, or both. These pathogens can cause acute or chronic systemic or ocular infection that can lead to significant morbidity (temporary or permanent) that includes vision loss.

Practitioner Learning Curve in Fitting Scleral Lenses in Irregular and Regular Corneas Using a Fitting Trial.

PURPOSE: To assess the learning curve of a novel practitioner with minor previous experience with scleral lenses (SL) fitting in the initial 156 consecutive fittings in irregular and regular corneas using a fitting trial. METHODS: Prospective dispensing case series involving a total of 85 subjects (156 eyes), 122 eyes with irregular corneas (IC Group) and 34 eyes with regular corneas (RC Group). All lenses were fitted by the same practitioner with minimal previous knowledge and practice on SL fitting. The first 156 consecutive fits were studied to estimate the number of trial lenses required to achieve the optimal fit and the number of reorders required. The results were divided into 8 chronological groups of 20 fittings (eyes) each. RESULTS: There was a decrease in the number of trial lenses required to achieve the optimal fit from 2.35±0.18 lenses in the first 20 fittings to 1.56±0.13 in the last fittings (p<0.05, Wilcoxon). There were no statistically significant differences between IC and RC groups. Regarding the number of reorders, there was also a decrease from 0.95±0.17 in the first fittings to 0.25±0.11 in the last fittings (p<0.05, Wilcoxon). Thought not statistically significant, there was an increase in the use of toric designs with increasing experience. CONCLUSIONS: Practitioner fitting experience reduced both the number of trial lenses required to achieve the best fit and the number of reorders with time. After the first 60 cases, there was a significant reduction in the trial lenses and reorders necessity.

Comfort, Ocular Dryness, and Equilibrium Water Content Changes of Daily Disposable Contact Lenses.

OBJECTIVES: The objective of this investigation was to evaluate the level of comfort and ocular dryness during wear with six daily disposable contact lenses (DDCL) and also determine the changes in contact lens equilibrium water content (EWC) resulting from their wear. METHODS: In this contralateral open trial, 27 subjects were randomly fitted with six DDCL (stenfilcon A, delefilcon A, nelfilcon A, narafilcon A, nesofilcon A, and omafilcon A). The evaluation of comfort and ocular dryness sensation was recorded by the participants at two moments of the day (11 AM and 5 PM) over a period of 10 days of contact lens wear. The assessment was made with the aid of visual analogue scales (0-10). The refractive index of 54 contact lenses was accessed by a single operator using a digital automated refractometer (CLR 12-70; Index Instruments). The EWC of the lenses was estimated based on its refractive index values. RESULTS: Comfort ratings were slightly higher for delefilcon A (9.56±0.67, P=0.01) and narafilcon A (9.40±0.93, P=0.01) and these lenses wearers also reported less ocular dryness. The results revealed a pronounced water content reduction for omafilcon A (P=0.002), narafilcon A (P=0.008), and nesofilcon A (P=0.003). CONCLUSION: Although changes in subjective responses and EWC were distinct among the materials analyzed, all the contact lenses performed well during the 10 days of wear.

Visual Performance and Optical Quality of Standardized Asymmetric Soft Contact Lenses in Patients With Keratoconus.

Purpose: To evaluate the visual performance and optical quality of a standardized asymmetric soft contact lens (SCL) used for correction of higher-order aberrations (HOAs) in eyes with keratoconus. Methods: We included 30 eyes (26 patients) with keratoconus (average K: 45.7 ± 2.3 diopters [D]). The patients were subjected to corneal tomography, aberrometry, measurements of manifest refraction and visual acuity (VA), and visual analog scale (VAS) assessments. The study lenses were made using a molding method and consisted of six standardized types, in which an asymmetric power distribution of approximately 2 to 12 D (2-D step) was used to correct HOAs. The lens type suitable for each eye was selected based on the corneal tomography and aberrometry data. The on-eye performance of the lens was evaluated using aberrometry (4-mm pupil), over refraction, VA, and VAS. Results: The standardized asymmetric SCL improved the best spectacle-corrected VA from -0.07 ± 0.09 to -0.11 ± 0.08 logMAR (P < 0.05) and the mean VAS score from 66.2 ± 21.8 to 75.4 ± 20.5 (P < 0.05). Vertical coma decreased significantly (-0.50 ± 0.36 µm without SCL; -0.36 ± 0.34 µm with SCL; P < 0.01). In subgroup analysis, subjects in the high VAS group (score ≥ 75) accounted for 70% of all subjects, and this was the group in which the vertical coma decreased significantly from the level without the lens. Conclusions: A standardized asymmetric SCL can reduce HOAs and improve vision quality when compared with spectacles in patients with keratoconus who wear rigid gas-permeable lenses.

Theoretical fitting characteristics of typical soft contact lens designs.

PUROSE: To calculate theoretical fitting success rates (SR) for a range of typical soft contact lens (SCL) designs using a mathematical model. METHOD: A spreadsheet mathematical model was used to calculate fitting SR for various SCL designs. Designs were evaluated using ocular topography data from 163 subjects. The model calculated SR based on acceptable edge strain (within range 0-6%) and horizontal diameter overlap (range 0.2-1.2mm). Where lenses had multiple base curves (BCs), eyes unsuccessful with the steeper BC were tested with the flatter BC and aggregate SR calculated. Calculations were based on typical, current, hydrogel and silicone hydrogel SCLs and allowed for appropriate on-eye shrinkage (1.0-2.3%). Theoretical results were compared with those from actual clinical trials. RESULTS: Theoretical success rates for one-BC lenses ranged from 60.7% (95% CI 7.2%) to 90.2% (95% CI 3.7%). With two-BC designs, most combinations showed a SR increase with a second BC (84.0%-90.2%). However, one of the two-BC combinations showed only negligible increase with a second BC (72.4%-73.0%). For designs with lower SR, the greatest contributor to failure was inadequate lens diameter. For a given design, differences in shrinkage (i.e. on-eye bulk dehydration) had a significant effect on success rate. In comparison with historical clinical data, there was a positive correlation between small lens fitting prevalence and discomfort reports (r=+0.95, P=<0.001) with a poor correlation between theoretical and actual tight/loose fittings. CONCLUSIONS: Mathematical modelling is a useful method for testing SCL design combinations. The results suggest that judicious choice of additional fittings can expand the range of fitting success.

Predicting Short-term Performance of Multifocal Contact Lenses.

OBJECTIVE: To investigate if initial multifocal contact lens (MFCL) performance predicts short-term dispensing performance. METHODS: A retrospective analysis of 55 participants (Px) in a masked, crossover, clinical trial, using ACUVUE OASYS for Presbyopia and AIR OPTIX AQUA Multifocal. Subjective questionnaires were administered at the following instances: initial fitting, two take home questionnaires (THQ) completed between days 2 and 4 and at assessment, ≥5 days after fitting. Questionnaires included vision clarity and lack of ghosting at distance, intermediate and near at day/night time points rated on a 1 to 10 (1-step, 10 most favorable) rating scale. Vision stability, vision while driving, overall vision satisfaction, willingness to purchase and comfort, as well as acuity-based measures were also collected. RESULTS: There were no statistical differences in comfort and vision at all distances, in vision stability or driving at either time points between THQ and assessment (P>0.05). However, there was a statistical decline in subjective overall vision satisfaction and comfort between fitting and assessment visits (P<0.001). Willingness to purchase remained the same at fitting and assessment in 68% of Px, whereas only 4% of Px converted to a positive willingness to purchase at assessment. The majority of acuity-based measures remained constant between fitting and assessment visits. CONCLUSION: Initial performance at fitting was not able to predict short-term performance of MFCL. Subjective measures peaked at fitting and declined thereafter whereas acuity-based measures remained constant. Utility of subjective rating tools may aid practitioners to gauge success of MFCL.

The Effect of Temperature on Soft Contact Lens Diameter.

PURPOSE: The diameters of soft contact lenses are conventionally measured at room temperature (RT), however, all soft lenses shrink when raised to eye temperature (ET). The purpose of this study was to evaluate RT to ET shrinkage in a representative sample of contemporary spherical soft contact lenses. METHODS: Twenty-four reusable and daily disposable lens types (10 hydrogel and 14 silicone hydrogel) in three powers were measured for total diameter at RT (20°C±1°C) and ET (34°C±1°C). Lens diameter measurements were undertaken after equilibration in ISO saline in a temperature controlled lens analyzer (Optimec Ltd, Malvern, UK). Theoretical changes in base curve radii were also calculated. RESULTS: All lens types reduced in diameter when raised to ET. The largest mean changes with silicone hydrogel and hydrogel lenses were with Avaria (Δ0.33 mm) and SofLens Daily Disposable (Δ0.69 mm), respectively. The smallest mean changes for silicone hydrogel and hydrogel lenses were with 1-DAY ACUVUE TruEye (Δ0.04 mm) and SofLens 38 (Δ0.11 mm), respectively. There was, however, a wide range in change between the different lenses tested. CONCLUSIONS: This study has identified the levels of shrinkage in current soft contact lenses. In many cases, these shrinkage levels can be expected to have significant effects on clinical performance.

Objective analysis of contact lens fit.

PURPOSE: To assess the validity and repeatability of objective compared to subjective contact lens fit analysis. METHODS: Thirty-five subjects (aged 22.0±3.0 years) wore two different soft contact lens designs. Four lens fit variables: centration, horizontal lag, post-blink movement in up-gaze and push-up recovery speed were assessed subjectively (four observers) and objectively from slit-lamp biomicroscopy captured images and video. The analysis was repeated a week later. RESULTS: The average of the four experienced observers was compared to objective measures, but centration, movement on blink, lag and push-up recovery speed all varied significantly between them (p<0.001). Horizontal lens centration was on average close to central as assessed both objectively and subjectively (p>0.05). The 95% confidence interval of subjective repeatability was better than objective assessment (±0.128 mm versus ±0.168 mm, p=0.417), but utilised only 78% of the objective range. Vertical centration assessed objectively showed a slight inferior decentration (0.371±0.381 mm) with good inter- and intrasession repeatability (p>0.05). Movement-on-blink was lower estimated subjectively than measured objectively (0.269±0.179 mm versus 0.352±0.355 mm; p=0.035), but had better repeatability (±0.124 mm versus ±0.314 mm 95% confidence interval) unless correcting for the smaller range (47%). Horizontal lag was lower estimated subjectively (0.562±0.259 mm) than measured objectively (0.708±0.374 mm, p<0.001), had poorer repeatability (±0.132 mm versus ±0.089 mm 95% confidence interval) and had a smaller range (63%). Subjective categorisation of push-up speed of recovery showed reasonable differentiation relative to objective measurement (p<0.001). CONCLUSIONS: The objective image analysis allows an accurate, reliable and repeatable assessment of soft contact lens fit characteristics, being a useful tool for research and optimisation of lens fit in clinical practice.

Prospective study of lotrafilcon B lenses comparing 2 versus 4 weeks of wear for objective and subjective measures of health, comfort, and vision.

OBJECTIVE: To examine potential changes in comfort, ocular health, and vision between 2 weeks and 1 month for wearers of lotrafilcon B lenses. METHODS: Up to 120 current wearers of lotrafilcon B lenses were recruited to wear a new pair of lenses for 1 month, using their habitual lens care system. Subjective ratings, biomicroscopy scores, visual acuity, and lens assessments were recorded at 2 weeks and at 1 month. In addition, participants responded to 4 Likert-type statements regarding comfort and vision after the full month of wear. RESULTS: No significant differences were noted in biomicroscopy scores between 2 weeks and 1 month. Noninferiority from 2 weeks to 1 month was established for subjective ratings of visual clarity (P=0.003; n=112) and ocular redness (P<0.001; n=112); the other 7 subjective ratings (for comfort, dryness, etc.) decreased less than 1 unit on 10-point scales. Significant majorities of respondents (all P<0.001 vs. 50%) agreed or strongly agreed that the lenses provided excellent comfort (83.5% of respondents) and clear vision (83.5%) throughout the month and that comfort (71.5%) and vision (77.9%) were the same at the end of the month as at the beginning of the month. Two adverse events occurred during the trial. Both were asymptomatic and considered clinically insignificant. CONCLUSIONS: Significant majorities of patients agreed that the lenses provided consistent comfort and vision throughout the month. Most wearers of lotrafilcon B lenses did not perceive substantial deteriorations in comfort or vision over the recommended wearing interval. Outcomes support monthly replacement of these lenses.

Applicability of contact angle techniques used in the analysis of contact lenses, part 1: comparative methodologies.

OBJECTIVES AND METHODS: Contact angle, as a representative measure of surface wettability, is often employed to interpret contact lens surface properties. The literature is often contradictory and can lead to confusion. This literature review is part of a series regarding the analysis of hydrogel contact lenses using contact angle techniques. Here we present an overview of contact angle terminology, methodology, and analysis. Having discussed this background material, subsequent parts of the series will discuss the analysis of contact lens contact angles and evaluate differences in published laboratory results. RESULTS: The concepts of contact angle, wettability and wetting are presented as an introduction. Contact angle hysteresis is outlined and highlights the advantages in using dynamic analytical techniques over static methods. The surface free energy of a material illustrates how contact angle analysis is capable of providing supplementary surface characterization. Although single values are able to distinguish individual material differences, surface free energy and dynamic methods provide an improved understanding of material behavior. The frequently used sessile drop, captive bubble, and Wilhelmy plate techniques are discussed. Their use as both dynamic and static methods, along with the advantages and disadvantages of each technique, is explained. CONCLUSIONS: No single contact angle technique fully characterizes the wettability of a material surface, and the application of complimenting methods allows increased characterization. At present, there is not an ISO standard method designed for soft materials. It is important that each contact angle technique has a standard protocol, as small protocol differences between laboratories often contribute to a variety of published data that are not easily comparable.

On-eye optical quality of daily disposable contact lenses for different wearing times.

PURPOSE: To quantify the optical quality of various daily disposable contact lenses in vivo and to ascertain its variation in terms of wearing time by means of objective non-invasive determination of wavefront patterns. METHODS: The crx1 adaptive-optics system was used to measure the wavefront aberrations in 15 myopic eyes before and at 2-h intervals after contact lens fitting, over a 12-h wearing period. Seven types of contact lenses having different material, water content and lens design were evaluated in this study: Dailies Total1, Dailies AquaComfort Plus, Proclear 1 Day, 1-Day Acuvue TruEye, 1-Day Acuvue moist, SofLens daily disposable and Clariti 1-Day. The aberration data were analysed by fitting Zernike polynomials up to the 5th-order for 3 and 5-mm pupils. The optical quality under each condition and at each point in time was described by means of the Root-Mean-Square (RMS) value of wavefront aberration, Modulation Transfer Function (MTF), Point Spread Function and cut-off spatial frequency. RESULTS: A RMS increase was observed after contact lens fitting as well as over time, both for a 3-mm and a 5-mm pupil. Each type of lens induced a different amount of wavefront aberrations, which vary over time also in a different manner. Dailies Total1 showed the lowest RMS values both at baseline and at the end of the day. In addition, Dailies Total1 provided the best MTF out of all the contact lenses that were assessed. These observations were reflected in higher cut-off spatial frequencies and visual resolution both at baseline and after 12 h of wearing time. CONCLUSIONS: Aberrometry makes it possible to analyse accurately and in vivo the optical quality of contact lenses and to assess how lenses having different characteristics - such as material or water content - behave for different wearing times. These variations across contact lenses may result in differences in visual performance.

Wettability and silicone hydrogel lenses: a review.

One of the major breakthroughs in the development of silicone hydrogel contact lenses has related to the ability of manufacturers to overcome the surface hydrophobicity that occurred with silicone elastomer lenses. However, the wettability of silicone hydrogel lenses continues to be of interest as a potential link between in vivo lens performance and contact lens-related comfort. This article will review some of the knowledge we have gained in the area of contact lens wettability over the past decade and will discuss some of the challenges related to its measurement.

A decade of silicone hydrogel development: surface properties, mechanical properties, and ocular compatibility.

Since the initial launch of silicone hydrogel lenses, there has been a considerable broadening in the range of available commercial material properties. The very mobile silicon-oxygen bonds convey distinctive surface and mechanical properties on silicone hydrogels, in which advantages of enhanced oxygen permeability, reduced protein deposition, and modest frictional interaction are balanced by increased lipid and elastic response. There are now some 15 silicone hydrogel material variants available to practitioners; arguably, the changes that have taken place have been strongly influenced by feedback based on clinical experience. Water content is one of the most influential properties, and the decade has seen a progressive rise from lotrafilcon-A (24%) to efrofilcon-A (74%). Moduli have decreased over the same period from 1.4 to 0.3 MPa, but not solely as a result of changes in water content. Surface properties do not correlate directly with water content, and ingenious approaches have been used to achieve desirable improvements (e.g., greater lubricity and lower contact angle hysteresis). This is demonstrated by comparing the hysteresis value of the earliest (lotrafilcon-A, >40°) and most recent (delefilcon-A, <10°) coated silicone hydrogels. Although wettability is important, it is not of itself a good predictor of ocular response because this involves a much wider range of physicochemical and biochemical factors. The interference of the lens with ocular dynamics is complex leading separately to tissue-material interactions involving anterior and posterior lens surfaces. The biochemical consequences of these interactions may hold the key to a greater understanding of ocular incompatibility and end of day discomfort.

Biocompatibility in the development of silicone-hydrogel lenses.

In response to patient demands for extended contact lens wearing times, the contact lens industry has developed novel silicone-hydrogel (SiHy) lens materials that combine the enhanced oxygen permeability of silicone polymers with the water-based comfort of conventional hydrogels. In the past 12 years since the successful launch of these SiHy lenses, much has been learned about their ability to provide the biocompatibility necessary to sustain ocular health and comfort. A review of the unique physiologic requirements for a successful extended wear lens and how the first and current SiHy lenses address them is provided.

Daily disposable lenses: the better alternative.

As the literature has described the increasing penetration of DDSL into the contact lens market worldwide, this review aimed to describe the development of this modality as a solution to the problems experienced with other types of contact lenses. The advantages of DDSL in comparison to reusable lenses are discussed. The important role of the practitioner in prescribing for and managing patients using this lens type are also examined, in particular with respect to non-compliance. Problems with the use of this lens type and possible solutions are explored and new applications and future development in this field are described.

A comparison of in-air and in-saline focimeter measurement of the back vertex power of spherical soft contact lenses.

PURPOSE: To compare the repeatability and validity of measurements of the back vertex power (BVP) of spherical soft contact lenses made in-air using the method specified in the International Standard (ISO 18369-3:2006) with the corresponding values when BVP was calculated from measurements made using a wet cell and focimeter method that is not approved by this Standard. METHODS: The BVP of 20 hydrogel and 20 silicone hydrogel lenses, with labelled powers ranging from +8.00 to -12.00 D, were measured with a focimeter by two operators on two occasions in-air in accordance with the relevant International Standard. Equivalent sets of measurements were made with the lenses immersed in-saline within a wet cell and their BVPs in-air were calculated. The validity of each method was assessed by comparing their results with an instrument that used the Hartmann method. RESULTS: The reliability results were generally a little better for the in-saline measurements than for the in-air measurements, although all reliability data demonstrated absolute values of mean errors (inter-operator and inter-session) that were <0.05 D for hydrogel lenses and <0.07 D for silicone hydrogel lenses. The in-air 95% confidence intervals were <0.45 D and <0.40 D for hydrogel and silicone hydrogel lenses, respectively and in-saline <0.39 and <0.31 D for hydrogel and silicone hydrogel lenses, respectively. The validity data revealed a relationship between measurement error and BVP for the in-air data (the focimeter overestimates the power of high plus and high minus lenses compared with the Hartmann instrument) and possibly a more complex relationship for the in-saline data. The 95% limits of agreement indicate better agreement for the in-saline validity data (-0.55 to +0.48 D for hydrogel lenses and -0.42 to +0.54 D for silicone hydrogel lenses) than those obtained in-air (-0.64 to +0.68 D for hydrogel lenses and -0.57 to +0.44 D for silicone hydrogel lenses). CONCLUSIONS: Using equipment readily available in a clinical setting, the wet cell method of measurement of the BVP of spherical soft contact lenses has been shown to provide results for reliability and validity that were at least as good as those obtained with the in-air method approved by the International Standard.

Measuring the modulus of silicone hydrogel contact lenses.

PURPOSE: The purpose of this study is to demonstrate a novel method for measuring the modulus of contact lenses in their as-received, variable-thickness form and to determine whether modulus varies with location within commercial lenses and whether it is dependent on lens geometry and temperature. METHODS: The thickness profiles of lenses having powers from -8 diopters (D) to +4 D were measured using a Rehder electronic thickness gauge. Strip-shaped specimens having a width of 5.5 mm were then cut from the lenses. Graphite particles were sprinkled on the specimen surface so that its motions could be tracked using digital image-correlation techniques. The specimens were mounted in a BioTester test system using BioRakes (rather than clamps) and stretched uniaxially until all parts of the lens between the attachment points had elongated by at least 10%. This procedure allowed local modulus values to be determined at 110 locations over the surface of each lens and any property variations within the lenses to be characterized. Tests were performed at 5, 23, and 37°C. RESULTS: Material modulus was found to be essentially constant within any given lens and was independent of the optical power of the lens. Young's Modulus values ranged from 0.3 to 1.9 MPa, depending on the lens manufacturer and product, and some lens materials showed a decrease in modulus with temperature. For the materials tested, those with lower water content had a tendency to exhibit higher moduli. CONCLUSIONS: Testing of the kind reported here is important for assessing the efficacy of current and proposed contact lens materials and designs, especially if such designs make use of variable properties to enhance function or fit.