Rigid gas permeable lens identification using refractometry

A plentitude of rigid gas permeable lenses have become available to the contact lens practitioner. The materials vary in monomer compounds that produce specific optical polymers, refractive indices, specific gravities, and oxygen permeabilities as well as unique wetting characteristics. An in-office method of material identification and differentiation is desirable. In many instances, the clinical contact lens practitioner needs to know the type of contact lens material that a patient is wearing. This paper investigates the use of the Atago refractometer to test contact lens material identification. The refractive index of 20 unidentified rigid contact lens materials was measured by four investigators using the Atago N3000 refractometer. The contact lenses were fabricated and coded so that the material was unknown to the investigators before the study began. The study showed that the measured values for the refractive index were consistent for individual lens materials by each investigator. Specific lens identification was possible as well as a simple differentiation between a lens with and without fluorine. A refractive index value less than 1.460 indicated a fluropolymer, whereas a value of 1.460 or greater identified a silicone-acrylate polymer. A value of 1.49 indicated poly(methyl methacrylate).     

Effect of contact lens materials on tear physiology.

PURPOSE: This study measured evaporation rate, thinning characteristics, and lipid layer changes in the prelens tear film (PLTF) associated with wearing of different soft contact lens materials, in an attempt to determine the biocompatibility of the material with the PLTF. METHODS: Twenty habituated contact lens wearers wore five different soft materials in a random order on the left eye at visits separated by at least 24 h. The soft contact lens materials were polymacon (Optima 38), omafilcon A (Proclear Compatibles), phemfilcon A (DuraSoft 2), balafilcon A (PureVision), and etafilcon A (Acuvue). Tear film evaporation rate was measured by a modified Servo Med Evaporimeter and tear thinning time by HirCal grid. Tear film structure, elimination rate, and lens wetting ability were recorded dynamically with a Doane tear film video interferometer and graded according to a new system developed for the study. Baseline measurements were taken of the precorneal tear film before lens insertion, and PLTF was determined 30 min after commencing lens wear. RESULTS: No statistically significant differences were found for any of the baseline (precorneal tear film) data. There was also no significant difference in evaporation rate change (analysis of variance) and in tear thinning time (Friedmann) between the five contact lenses. In the PLTF structure grading, omafilcon A had significantly more stable grades than phemfilcon A (Friedmann, p = 0.0033) and polymacon (p = 0.004). In PLTF observation of tear thinning and elimination rate, there was a significantly slower rate of elimination observed for omafilcon A than phemfilcon A (Friedmann, p = 0.0023) and polymacon (p = 0.0023). There was no significant difference in the overall PLTF wetting ability grading between any of the lenses worn. CONCLUSION: Generally, all soft contact lens materials significantly and adversely affected tear physiology by increasing the evaporation rate and decreasing tear thinning time. The surface wetting ability of all contact lens materials exhibited no significantly difference irrespective of the special surface treatments. Only in PLTF structure and in PLTF elimination rate were differences found from the conventional low water content materials; omafilcon A was better in PLTF structure and in PLTF elimination.     

Comparison of two contact lens storage solution preservatives with respect to in-eye wetting of rigid oxygen-permeable lens surfaces and related factors

Wettability of contact lenses made of four rigid oxygen-permeable materials was assessed over a 4-month period when the lenses were worn in a daily-wear manner by 16 previously successful rigid contact lens wearers. The lenses were maintained with care regimens using two storage solution preservatives, 0.004% benzalkonium chloride (BAK) and 0.006% chlorhexidine digluconate (CHX), in isotonic-buffered saline. Wettability was evaluated by use of the in vivo contact angle, lens-surface break-up time, graded levels of surface deposition and discomfort, and by frequency of occurrence for verified presence and absence of functional wettability. Statistically significant effects of patient, lens material, care regimen (preservative), and their interactions were found, which led to a complicated statistical analysis of the results. Overall, low oxygen-permeability lens materials fared better with respect to wettability, as did the BAK-containing care regimen, but the results were patient-specific with regards to favor of any single lens material/care regimen combination. Results also varied according to the wettability assessment method used, although the methods have been previously shown to be loosely correlated. The contact lens practitioner is at a disadvantage for selecting the optimum lens material/care regimen combination for the individual patient, because the wettability assessment techniques at his/her disposal give conflicting results, because those results are highly specific with respect to the combination of patient, lens material, and care regimen and because he/she has a practical lack of information and limited diagnostic repertoire from which to make clinical predictions.     

Wetting angles of rigid contact lens plastics: the effect of contact lens wear

Several adapted rigid contact lens wearers were given duplicates of their present lenses made in different plastics. They were required to wear each material on each eye for a period of four hours. Both the average wetting angle and its range of values for each material was compiled. All lenses tended to give similar average wetting angles but ranges of angles for the PMMA/siloxanyl materials were greater than that for PMMA and GP IITM. Both values should be utilized when evaluating the wettability of a specific material.     

An instrument for in vivo tear film interferometry

An instrument has been designed and constructed that facilitates the measurement of in vivo tear film topology, thickness, and wetting, using the principles of thin film interferometry. The use of this tear film interferometer (TFI) allows objective evaluation of the breakup characteristics of the tear film on contact lens surfaces and provides a means for examining the dynamic changes in thickness, thickness distribution, and wetting properties during sequential interblink periods. The effects of contact lens material, wetting solutions, and cleaning regimens can be compared objectively. The instrument is easy to use, no additives to the natural tear fluid are required.     

人工晶状体材料的研究进展

随着白内障超声乳化吸除联合人工晶状体植入术的广泛应用，许多白内障患者都恢复了有效视力。术后人工晶状体混浊是评价生物相容性的重要指标，它会影响患者的视觉质量，不同材料人工晶状体混浊的表现及危险因素是不同的。然而，获得较好的视觉质量不是由单一因素决定的，人工晶状体的材料、患者对人工晶状体的反应等都是影响术后视觉质量的因素。随着科技的不断进步，越来越多的新材料在人工晶状体领域得到应用，充分了解不同人工晶状体材料的特性，为患者选择合适的人工晶状体，减少材料导致的并发症，将为患者带来福音。本文就不同人工晶状体材料的特性与人工晶状体植入术后发生混浊的表现及危险因素进行探讨。     

Blue-light-absorbing lenses and their effect on scotopic vision

PURPOSE: To correct assumptions about material transmission and provide a more realistic assessment of the optical properties of intraocular lenses (IOLs) made from AcrySof Natural material (Alcon). SETTING: University of Arizona Department of Ophthalmology, Tucson, Arizona, USA. METHODS: The transmission characteristics of conventional and AcrySof Natural IOL materials immersed in a balanced salt solution were examined to determine the differences in scotopic vision. The removal of the crystalline lens during cataract surgery and its effect on scotopic vision were also analyzed. RESULTS: When only considering the IOL properties, the AcrySof Natural lens decreased light entering the eye under scotopic by 14.6%, much lower than previously reported. Furthermore, when removal of the crystalline lens was taken into consideration, the AcrySof Natural material actually increased the amount of light entering the eye by 52% under these same conditions. CONCLUSIONS: Previous discrepancies in the transmission characteristics of IOL materials have led to an underestimation of the performance of the AcrySof Natural material under scotopic conditions. Accounting for both transmission and the crystalline lens demonstrates an increase in light levels entering the eye with the AcrySof Natural material when compared to young phakic subjects.     

Comparison of two care systems when used with low Dk and high Dk silicone/acrylate materials

The Boston care system (a chlorhexdine preserved conditioning solution and cleaning solution) was compared to the Sherman care system (a benzyl alcohol preserved wetting/soaking solution and a cleaning/disinfecting solution) on two different silicone/acrylate lens materials. Two groups of 12 patients each were used, one group wearing a low Dk (12) material and the other a higher Dk (55) material. The patients used one care system on one lens and the other care system on the opposite lens. The study was double masked: The solutions were coded so the patients did not know the brand of the solutions, and the examiner did not know which care system was being used on which lens. Tear film break-up time on the front of the lens was measured, surface debris and peripheral corneal staining was graded at each visit over the 24 weeks. The patients completed a questionnaire at each visit. There was no significant difference between the care systems with the low Dk material, however, the Sherman system tended to perform better on the higher Dk material. Break-up time on the higher Dk material was shorter, and debris was slightly greater on the lower Dk material.     

High gas permeability contact lens effects on the cornea: A comparative study

The aim of this paper is to compare the physiological responses, degree of comfort, vision quality, and acceptance of two types of high gas permeable contact lenses over a period of 3 months usage. Both contact lens materials were initially unknown to the authors; once the study was concluded, the manufacturing laboratories were consulted to determine the chemical composition of both materials. The lens types were fluoropolymer (Equalens) (lens A) and siloxane acrylate (lens B).Ocular responses of 30 patients to these lenses were studied. Fifteen of these patients were veteran contact lens wearers; the rest of the patients were new contact lens wearers. Each patient was fitted with lens A on one eye, and lens B, on the other eye. The degree of comfort experienced with both lens types as well as any changes in the corneal thickness, anterior corneal curvature, ocular refraction, and ocular physiology were noted.Lens A produced more discomfort than did lens B, due possibly to its wetting angle and edge design. No significant change in corneal curvature was found for either type of lens. Although lens B produced a greater increase in corneal thickness than did lens A, it is important to point out that, from the first week of contact lens usage through the 3 months of usage, corneal edema caused by both lenses was within the physiological limits of adaptation.     

Weight of edged spectacle lenses

Uncut spectacle lenses of various powers and materials were edged to three different eye shapes and three box eye sizes. Their weights were measured. This demonstrated that the eye shape has very little effect on the weight of edged lenses and that there is very little difference in weight for neighbouring eye sizes. The density of the lens material is of major importance, but other characteristics also influence the edged lens weight.     

Identification of intraocular lens materials using confocal Raman spectroscopy

PURPOSE: To develop and test a noninvasive method to identify intraocular lens (IOL) materials in vitro. SETTING: Center for Biomedical Engineering and the Department of Ophthalmology, University of Texas Medical Branch, Galveston, Texas, USA. METHODS: A laser confocal Raman spectroscopy system (Conforam) was used for the noninvasive assessment of Raman spectra in the lower and the higher spectral regions (299.1 to 1833.7 cm-1 and 2633.8 to 3819.6 cm-1, respectively) of 4 IOL materials: silicone, poly(methyl methacrylate) (PMMA), acrylic, and hydrogel. RESULTS: Each lens material showed a distinctive spectrum in both the higher and the lower spectral regions. Most materials had unique peaks and a distinct profile using 1 mW of laser power and a 1 second exposure time. All materials still had a unique spectrum in both the higher and the lower region that allowed 1 material to be distinguished from the others. CONCLUSIONS: A Conforam differentiated silicone, PMMA, acrylic, and hydrogel lenses in vitro. Raman spectroscopy using the Conforam may provide a fast, safe, and reliable noninvasive method to gain information about the material of an implanted IOL and the stability of lens materials and their coatings.     

UV-A absorbing characteristics of commercial sunglasses intended for recreational and general use.

Ultraviolet radiation (UVR) is known to cause photokeratitis with acute symptoms. It also has been demonstrated that UVR has the potential to accelerate human cataractogenesis. For certain recreational pursuits, protection from specific UVR wavebands is important to the prescriber, the dispenser, and the patient or consumer of sunglasses. By knowing the ultraviolet absorbance characteristics of lens materials, practitioners would be able to advise their patients. In this study, the UV-A spectra of pairs of sunglasses purchased anonymously were measured. The results indicate that manufacturers' claims of UVR protection, or absorbance, are valid with respect to the UV-A waveband. No association between UV-A absorbance and lens material or cost was established.     

Ultraviolet transmittance of monthly replacement lenses on the Scandinavian market

Ultraviolet (UV) radiation is reported to be harmful to the human retina even at relatively low levels of daily exposure. Advertising for UV-blocking contact lenses in the media suggests that these materials offer good protection of the eye from UV radiation. The protective effect of such lenses will be dependent on the absorption characteristics and the stability of the UV filter incorporated in the contact lens matrix. The transmittance characteristics of a selection of UV-protecting lenses on the market were evaluated by spectrophotometry. It was demonstrated that the transmittance characteristics were strongly dependent on the lens material. An increase in UV transmittance was observed for some of the lens materials after exposure to light corresponding to approximately 1 hour of sunlight per day through the period of use. None of the lenses offer complete protection from exposure to UV radiation.     

PHYSIOLOGY,PHARMACOLOGY AND CONTACT LENSES*

ABSTRACT: The materials likely to be present as contamination on the surface of a contact lens, from the patient's environment, and from the glandular secretion discharging into the precorneal tear film are mentioned. Cationic, anionic and nonionic surface active agents used in cleaning lenses, their mode of action and the concentration considered necessary in the maintenance of clean lenses, as well as antimicrobial substances used as preservatives, chelating agents and wetting agents included in contact lens wetting and soaking solutions, are described and commented upon.     

Surface deposits on low water content hydrogel contact lenses: Comparison of SEM and protein assay techniques

Comparative evaluations were made of the deposits on three low-water content hydrogel contact lens polymers, tetrafilcon A (Aquaflex), crofilcon (CSI), and tefilcon (Cibasoft) by scanning electron microscopy (SEM) and modified Lowry protein assay techniques. Ten patients wore each of the three lenses for a period of 1 week on a daily wear basis. Surfactant cleaning and chemical disinfection of lenses was carried out but no enzymatic cleaning was performed. No significant difference was found, by either technique, in the amount of deposit on the three lens materials. No direct correlation was found between the measurements by each technique. It is concluded that, quantitatively, the crofilcon (CSI) material is not more deposit resistant than are other similar low-water content, nonionic hydrogel lens polymers. This finding supports the hypothesis that water content and/or surface chemistry of the lens materials determines their deposit characteristics.     

Rigid contact lens base curve constancy between wet and dry lens storage conditions

The purpose of this study was to investigate dry storage of rigid gas-permeable contact lenses as an alternative to wet storage. This study evaluated the constancy of lens base curve between the two storage methods for lens materials that spanned the spectrum of oxygen permeability values. The differences found in lens base curve between wet and dry storage for any lens material evaluated did not reach a level considered to be clinically significant, although differences in average base curve constancy did exist among the lens materials. Base curve change did not increase with increasing lens material oxygen permeability value (Dk).     

Wettability of silicone-hydrogel contact lenses in the presence of tear-film components

INTRODUCTION: Modern application of soft contact lenses demands safe and comfortable wear over extended time periods up to one month. Lenses that exhibit and sustain complete water wetting allow thick tear-film deposition, minimize film rupture, and permit smooth tear recovery upon lid closure. Water contact angles determined using an air bubble captive on a lens best gauge the in-vivo wetting state. To achieve highly water wetting lenses demands that contact-angle hysteresis be eliminated and that the advancing and receding angles both approach zero. Since lens wear exposes the anterior surface to tear proteins, lens wettability should be measured in the presence of tear-film components. METHODS: A captive-bubble technique is applied to measure the advancing and receding contact angles of two commercial silicone-hydrogel lenses: PureVision (PV) and Focus Night & Day (CF) and a standard HEMA (hydroxethyl-methacrylate) hydrogel lens: Acuvue (AV). In the captive-bubble method, an air bubble immersed in aqueous solution is brought into contact with the contact lens. The contact angle through water during bubble expansion yields the receding angle. Bubble contraction gives the water advancing angle. Contact-angle hysteresis is the difference between the advancing and receding angles. RESULTS: In isotonic solution, all three lenses display considerable contact-angle hysteresis with advancing angles of almost 90 degrees. When lysozyme and/or mucin were added to the aqueous solution, hysteresis was eliminated, and equivalent and high water wetting was achieved for the three lenses. Only the advancing angle in isotonic solution provided discriminating evidence for differences in surface chemistry. Covalent attachment of polyethyleneglygol (PEG) to the PV lens surface achieved complete water wetting independent of the presence of tear protein in the solution. CONCLUSIONS: The captive-bubble technique provides contact angles that are relevant to on-eye lens wear. Both advancing and receding contact angles are important to lens wettability performance. When lysozyme and/or mucin are present in the solution, PV, CF, and AV lenses display low advancing and receding contact angles indicative of equivalent wettability performance. This result is due to molecular adsorption of the proteins onto the lens external surface. Covalently attached PEG on the PV lens not only provides complete water wetting but also minimizes or even eliminates protein adsorption.     

Eighteen-month clinical experience with extended wear silicone contact lenses on 400 patients

Four hundred patients wearing silicone elastomer contact lenses for extended wear were monitored over an 18-month period. Safety and efficacy were evaluated as related to corneal physiology and lens performance. These lenses proved to be reasonably durable and the lens material and design characteristics did not interfere with ocular physiology but permitted good visual acuity and comfort. The authors' clinical experiences, when compared with other reports of extended-wear lens designs and materials, indicate that this silicone elastomer lens provides superior performance characteristics.     

Contact lens material characteristics associated with hydrogel lens dehydration

Purpose: To determine the association between material dehydration and hydrogel contact lens material characteristics, including water content and ionicity. Methods: 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. Results: 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). Conclusions: 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.     

New design concepts for permeable rigid contact lenses.

Gas permeable rigid lens materials offer the opportunity to reevaluate contact lens design. This paper presents the rationale and procedures followed in the development of a design concept for the Polycon lens material.