Refractive index of rigid contact lens materials

A simple hand-held refractometer was used to measure the refractive index of 27 rigid gas permeable contact lens materials. As a general rule, lenses with refractive indices lower than 1.458 are made from fluorosilicone acrylates; lenses with refractive indices in the range of 1.458 to 1.469 are made from either fluorosilicone acrylates or silicone acrylates; and lenses with refractive indices greater than 1.469 are made from silicone acrylates. It is demonstrated how refractometry can be used by contact lens practitioners for the identification and verification of rigid contact lenses

Water properties of soft contact lens materials

The properties of water in soft contact lenses such as the water content, free-to-bound water ratio, and the extent to which soft lenses dehydrate during wear, are key determinants of their in eye performance and oxygen transmissibility characteristics. This study describes clinical and laboratory experiments that were conducted in order to examine the state of water in eight soft contact lenses manufactured from different materials. Specifically, lenses made from the following eight materials (and nominal water contents) were used: 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]. Differential scanning calorimetry (DSC) was used for measuring the free water content in the materials listed above. Some noticeable differences in water properties among soft contact lens materials having approximately the same water contents were revealed. Low water content materials exhibited a simple endotherm and all water melted around 0 degrees C. On the other hand, medium and high water content materials exhibited multiple melting endotherms, representing a broad range of interactions between water and the polymer. Low water content soft contact lenses have approximately the same amount of bound water as those with much higher water contents. Six subjects were then fitted with the same lenses for one day. In vitro measurements of water content and oxygen transmissibility were taken at 35 degrees C, both before lens fitting and after 6 h of lens wear. Water content and oxygen transmissibility were correlated with the water properties of the soft contact lens materials. The relative change in lens water content (%deltaWC) and relative change in lens oxygen transmissibility (%deltaDk/t) were calculated and correlated with the water properties of the eight soft contact lens materials. It was concluded that (a) oxygen transmissibility, free water content and free-to-bound water ratio are increased when the water content of a contact lens is increased and (b) water content, free water content and free-to-bound water ratio cannot be used for the prediction of soft contact lens dehydration in vivo

Scratch resistance of rigid contact lens materials

Practitioners have long recognised the importance of determining the extent of scratching on the surface of rigid contact lenses when deciding on lens replacement. Despite this, little research has been undertaken to define this problem. The extent of scratching of rigid contact lens materials was evaluated and this property was related to material oxygen permeability and refractive index. One hundred and forty lenses made from 28 different rigid materials were evaluated in a masked and randomised manner. Scratches were created on the front surface of the lenses using an apparatus that was specifically designed and constructed for this experiment. The extent of scratching was quantitatively evaluated using a computer-based scanning and image analysis system. The oxygen permeability of all materials was measured using the polarographic method. An Atago N3000 hand-held refractometer was used to measure the refractive index of these materials. Three significant correlations were revealed: an inverse relationship (r = -0.813, P < 0.0001) between oxygen permeability and refractive index; a positive relationship (r = 0.511, P < 0.008) between oxygen permeability and the extent of scratching; and an inverse relationship (r = -0.539, P < 0.0058) between refractive index and the extent of scratching. The information generated in this study concerning scratch resistance of rigid contact-lens materials may assist the contact lens industry and contact lens practitioners in developing and prescribing rigid lenses with optimal performance characteristics

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

Oxygen permeability of rigid contact lens materials

The oxygen permeability of 28 currently available rigid contact lens materials was measured using the polarographic method, and the data were corrected for boundary and edge effects. Comparison of the measured oxygen permeability values with data supplied by manufacturers (nominal data) reveals a large discrepancy, whereby the measured values are less than the quoted values. This may be attributed to either (a) the manufacturers citing values derived from techniques other than the polarographic method (such as the coulometric technique), or (b) the manufacturers citing values derived earlier that have not been corrected for the edge or boundary effects. Practitioners should continue to exercise caution when interpreting oxygen transmissibility data

Parameter stability of soft contact lenses made from different materials

The parameters of soft contact lenses may alter during wear. A series of clinical and laboratory experiments were conducted in order to examine the parameter stability of eight soft contact lenses manufactured from different materials. The following materials (and nominal water contents) were used: 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 the 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 water content, oxygen transmissibility, total diameter and back optic zone radius were taken at 35 degrees C before lens fitting and after 6h of lens wear. Distortion, discolouration and lens quality were assessed before and after lens wear. When the temperature was raised from 20 to 35 degrees C, a significant reduction in lens water content for all the lens types was observed, as well as a significant reduction in total lens diameter for the majority of the lens types. Water content, oxygen transmissibility, total diameter and back optic zone radius of all the lens types reduced, following a 6h open eye wearing period. For the majority of the lens types, these changes were found to be statistically significant. Distortion, discolouration and quality of the lenses remained unchanged throughout the study with the exception of the HEMA/MAA 70% lens. Correlating a number of parameters generated in this study, gave the following conclusions. High water content materials exhibit a low relative change in oxygen transmissibility following a 6h wear period. Soft contact lens dehydration leads to a decrease in oxygen transmissibility and total diameter, following a 6h wear period. These results will assist practitioners in predicting the alterations that occur in soft contact lens parameters and oxygen performance as a result of lens wear

International contact lens prescribing in 2015

We have been collecting data on worldwide contact lens prescribing habits for almost 20 years. Over this time period, we have amassed prospective information about 315,000 contact lens fits in 59 countries. This article marks our 15th report in Contact Lens Spectrum and features a breakdown of more than 23,000 contact lens fits in 34 markets. As in previous years, our international network of coordinators distributed survey forms to eyecare practitioners in their market who then recorded generic information about the first 10 patients fit with contact lenses after receipt. Information is gathered about patient age and gender; whether the contact lenses are prescribed as a new fit or a refit; contact lens material, design, and replacement frequency; number of intended days per week of use; wearing modality; and care system. Contact lens fits are weighted to reflect the number of fits undertaken by each eyecare practitioner. The study data were entered and processed at the University of Manchester and at the University of Waterloo