Objective analysis of properties and material degradation in contact lens polymers using different techniques

Tese de Doutoramento em Ciências.Biocompatibility of contact lens polymers is the ability of the material to be worn in direct contact with the ocular surface without an adverse response of the host. In the contact lens field, it depends strongly on the ability of the material to respect the physiological needs of the ocular surface, and avoid or minimize other different forms of interaction. With moderns lenses, many aspects that caused problems in the past (i.e. oxygen transmissibility), have been solved, however, the impact of contact lenses on the ocular surface due to both topographic and mechanical characteristics and the dehydration process followed after insertion are still a matter of concern. The present Thesis address four main issues: 1) The evaluation of the pattern of contact lens fitting in our country and the symptoms more commonly associated with contact lens wear are covered in chapters 1 and 2; 2) Literature review of the main properties that characterize the contact lens materials and how they can interact with the ocular tissue as a consequence of wear and/or material spoliation covered in chapters 3 and 4; 3) Evaluation of different contact lens materials using different techniques to analyze some properties at the contact lens surface and the bulk of the material which are covered in chapters 5 to 10; 4) Analyze how some of those properties can change as a consequence of contact lens wear which is covered in chapters 11 to 13. Chapter 14 addresses the overall discussion of the results, some conclusions and proposals for future work to be developed in this filed with the body of knowledge acquired during the realization of this work. In the 2 introductory chapters we have observed that soft contact lenses are the most widely fitted in Portugal, with silicone hydrogel materials experiencing a significantly increase and already account for more than 20% of the new fits and refits despite the reduced proportion of brands within this field so far. Contact lens wearers usually report symptoms related with contact lens discomfort most frequently than non-contact lens wearers, and most of them could be related to dryness as the end of day discomfort, scratchiness and eye redness. The first experimental part of the Thesis has evidenced a specific and different behavior of silicone hydrogel materials regarding the relationship of equilibrium water content (EWC) and refractive index of the material if we compare them with the classical relationships followed by conventional hydrogels. These findings are of particular relevance when we need to characterize the dehydration of materials as a result of wear. Regarding microscopy, the microscopic technique that allows us to evaluate contact lenses in a less invasive way and in the natural hydrated state was atomic force microscopy (AFM). Again silicone hydrogels materials show a remarkable different pattern of surface topography, particularly those of the first generation including surface treatment to improve wettability. This technique also allows us to obtain information about the mechanical behavior of the material, with a nanometric precision. Complimentary, the oxygen permeability (Dk) and transmissibility (Dk/t) of some silicone hydrogel contact lens materials have been evaluated. These studies concluded that silicone hydrogel materials within a range of high oxygen permeability, are not expected to induce significant differences in their oxygen performance in physiological terms (i.e. evaluating the actual amount of oxygen reaching the contact lens-cornea interface) even with significant changes in their Dk/t values. The in vitro dehydration process of silicone hydrogel and conventional hydrogel materials is characteristic of each material, depending essentially on their EWC and several quantitative parameters have been obtained with this new approach. The second experimental part has been focused on the evaluation of the effects of wear on some characteristics of contact lens materials, particularly the topographic and mechanical parameters at the lens surface, their EWC and in vitro dehydration process. In the corresponding chapters, it has been observed that materials become less elastic and harder with wear, which is reflected as an increase in their elastic modulus of worn lenses when compared against unworn samples of the same characteristics under the same experimental conditions. Topographic information shows an overall increase of surface roughness of polymer surface, except in some samples where the high irregular surface of the unworn samples results in a partial uniformization of the surface elevation pattern because of the deposits that form films over the contact lens surface. The in vitro dehydration process shows a remarkable change in qualitative and quantitative terms with significantly higher initial dehydration rates. Samples worn for periods of one month demonstrated changes in their EWC with a trend towards decreasing EWC after several days in saline. Overall, the present work demonstrates objectively that some contact lens materials become more irregular and more rigid in their surfaces, decrease the EWC and increase their initial dehydration rates under in vitro conditions. These changes could cause a significant increase in the negative interactions between contact lenses and the most superficial tissues in the ocular surface.A biocompatibilidade dos materiais para lentes de contacto representa a capacidade dos mesmos para serem utilizados em contacto directo com a superfície ocular sem causar respostas adversas no olho. No âmbito das lentes de contacto esta biocompatibilidade depende da capacidade do material para respeitar as necessidades fisiológicas da superfície ocular e evitar ou minimizar outros tipos de interacções. Nas lentes actuais, muitos destes aspectos foram já melhorados ou até ultrapassados (p.e. a transmissibilidade ao oxigénio), no entanto, o impacto destas lentes na superfície ocular pelas suas características de topografia superficial e propriedades mecânicas ou a resistência dos materiais à desidratação e a adesão de depósitos ainda são aspectos a resolver. Esta Tese trata de abordar os seguintes aspectos: 1) A avaliação dos padrões actuais de prescrição e adaptação de lentes de contacto em Portugal e quais os sintomas que referem os usuários com maior frequência e que são abrangidos pelos capítulos 1 e 2; 2) Revisão da literatura sobre as principais propriedades que caracterizam os materiais para lentes de contacto, como se relacionam com a superfície ocular e como podem mudar estas relações pelo processo natural de deterioração dos materiais com o seu uso. Estes aspectos são abrangidos pelos capítulos 3 e 4; 3) Avaliação de distintos materiais de lentes de contacto utilizando diferentes técnicas para analisar algumas das propriedades da superfície e do interior das lentes de contacto, aspectos que são abrangidos pelos capítulos 5 a 10; 4) Avaliar em que medida sofrem alterações algumas das propriedades dos materiais como consequência do uso, o que é abordado nos capítulos 11 a 13. O capítulo 14 proporciona uma discussão geral dos resultados da Tese e as suas conclusões mais importantes, proporcionando ainda linhas de trabalho futuro utilizando o conhecimento adquirido durante a preparação deste trabalho de Tese. Nos dois capítulos introdutórios, observou-se que as lentes de contacto mais utilizadas em Portugal são as lentes hidrofílicas, e dentro destas destacam as lentes de silicone hidrogel, que já representam mais de um 20% das novas adaptações e readaptações, apesar de do ainda limitado número de marcas disponíveis neste segmento. Os usuários de lentes de contacto, geralmente apresentam mais queixas de desconforto que os não usuários e muitas destas queixas podem ser associadas à secura ocular. A primeira parte experimental da Tese evidenciou que os materiais de silicone hidrogel apresentam uma relação entre o seu teor de água e o índice de refracção do material hidratado que é diferente do que até agora se admitia para os materiais de hidrogel convencionais. Isto é importante para se poder avaliar o grau de desidratação de lentes de contacto de silicone hidrogel mediante técnicas de refractometria. Em relação com a microscopia, a técnica que permite uma melhor visualização das lentes de contacto hidrofílicas no seu estado hidratado é a microscopia de força atómica (AFM). Neste aspecto as lentes de silicone hidrogel apresentam também um padrão topográfico característico, principalmente as lentes de primeira geração que utilizam tratamentos de superfície para melhorar a humectabilidade. Esta técnica permite obter também informação acerca do comportamento mecânico dos materiais com elevada resolução. De um modo complementar também se estudo a permeabilidade ao oxigénio (Dk) e a transmissibilidade (Dk/t) de distintos materiais de silicone hidrogel. Estes estudos concluíram que com as lentes de hidrogel de silicone de alta permeabilidade não devem induzir alterações significativas na superfície ocular quanto à quantidade de oxigénio que atinge a superfície corneal mesmo que se produzissem mudanças significativas no valor da transmissibilidade. Estudou-se também o processo de desidratação in vitro de distintas lentes de hidrogel e de silicone hidrogel, demonstrando que este processo é característico de cada material e depende essencialmente do seu teor de água. Foram obtidos diferentes parâmetros quantitativos que caracterizam este processo utilizando este novo método de análise. A segunda parte experimental está focada na avaliação dos efeitos do uso em algumas propriedades dos materiais das lentes de contacto, em particular nos parâmetros topográficos e mecânicos da superfície das lentes, no teor de água e no processo de desidratação. Nos capítulos correspondentes, observou-se que com o uso, os materiais se tornam menos elásticos e mais duros. A informação da topografia da superfície mostra que no geral, aumenta a rugosidade da superfície da lente, com algumas excepções de materiais inicialmente muito rugosos nos que os depósito de materiais como consequência do uso resultam até numa regularização parcial da superfície. O processo de desidratação in vitro mostra também alterações importantes em termos qualitativos e quantitativos, com um aumento nos parÂmetros de deshidratação inicial. As amostras usadas também demonstrataram uma menor capacidade do polímero para recuperar a sua hidratação original mesmo após vários dias em solução salina. Globalmente, este trabalho mostra que alguns materiais de lentes de contacto se tornam mais irregulares e rígidos nas suas superfícies, diminuindo também, embora ligeiramente, a sua capacidade de hidratação e principalmente, a sua capacidade de retenção da hidratação alcançada, medida mediante técnicas de desidratação in vitro. Estas alterações poderão ter consequências negativas para o relacionamento entre a superfície dos materiais de lentes de contacto e a superfície ocular.European Social Funding.Foundation for Science and Technology (FCT

Influence of fogging lenses and cycloplegia on peripheral refraction

Purpose: To compare objective peripheral refraction measured with an open-field autorefractor without cycloplegia with the values obtained with fogging lenses or with cycloplegia to inhibit accommodation. Methods: For one hundred and sixty young adults aged 18 to 28 (mean 21.5 ± 2.3 years) their refraction was measured with the Grand Seiko (GS) autorefractor at the center and at four peripheral locations in the nasal and temporal directions under three different conditions: 1) without cycloplegia (GS); 2) without cycloplegia, but using a +2.00D fogging lens (GS_2D) and 3) with cycloplegia (GS_cycl). Results: Mean spherical equivalent refraction (M) was significantly more negative with the GS method in the hyperopic group for central and peripheral refraction, and only at the center and at 10º nasal eccentricity for the emmetropic group (P<0.05, Kruskal- Wallis). Paired comparison showed that differences of M values across techniques were larger for the GS-vs.-GS_2D comparison in myopes and emmetropes, and for the GS-vs.-GS_cycl one in hyperopes (P<0.001, Wilcoxon Signed Ranks Test). The gap between M values for all paired comparisons remained almost constant across all eccentric positions under analysis. Conclusions: Fogging lenses used with open-field autorefraction up to 20º in the nasal and temporal fields seem to provide similar accommodative relaxation to that provided by a cycloplegic. This is particularly important when refracting emmetropes and hyperopes. Moreover, this behavior seems to be independent of the eccentricity at which measurements are taken.Objetivo: Comparar las medidas objetivas de refracción periférica realizadas sin cicloplégico con los valores obtenidos con “lentes de miopización” o con cicloplegia, ambas técnicas utilizadas para inhibir la acomodación. Métodos: Se midió la refracción a 160 adultos jóvenes, con edades comprendidas entre 18 y 28 años (media=21,5± 2,3 años), con un autorrefractómetro Grand Seiko (GS), tanto en el centro del campo visual como en 4 regiones de la periferia situadas nasal y temporalmente, y todo ello en 3 condiciones diferentes: 1) sin cicloplegia (GS); 2) sin cicloplegia , pero utilizando una lente translúcida de +2.00 D (GS_2D) y 3) con cicloplegia (GS_cycl). Resultados: La media del equivalente esférico de la refracción (M) resultó ser significativamente más negativa en la condición GS en el grupo de los hipermétropes en lo que respecta a refracción central y periférica, mientras que en el grupo de los emétropes sólo ocurrió esto en el centro y a una excentricidad de 10º nasal (P<0,05, Kruskal-Wallis). La comparación por pares de muestras relacionadas reveló que la mayor diferencia de M entre distintas condiciones se obtuvo al comparar GS y GS_2D en el grupo de los miopes y en el de los emétropes, y al comparar GS y GS_cycl en el de los hipermétropes (P<0,001, contraste de Wilcoxon de rangos con signo). La discrepancia entre valores de M para las distintas comparaciones por pares se mantiene prácticamente constante para todas las excentricidades analizadas. Conclusiones: Las lentes de miopización, utilizadas con un autorrefractómetro con ventana de observación para medir excentricidades de hasta 20º en las direcciones nasal y temporal parece que logran una relajación acomodativa similar a la que proporciona el agente cicloplégico. Esto resulta especialmente relevante cuando se evalúa la refracción en emétropes e hipermétropes. Además, este comportamiento parece ser independiente de la excentricidad en la que se realiza la medida.(undefined

Prevalence of refractive error within a in Portuguese sample of optometric records

Purpose: Considering the burden of refractive error, clinical-based research methods are often used as epidemiological tools. This study aimed to generate evidence on the prevalence and distribution of refractive error in Portugal. Methods: A cross-sectional retrospective study was designed to review optometric records from Portuguese practices during July 2021. Results: 348 optometric records were analysed. Subjects had a mean age of 44.2 ± 19.2 years (range 6–81) and 58.4% were female. The mean spherical equivalent was myopic, −0.65 ± 2.38 Diopters (D), varying from a minimum of −13.63 to a maximum of 6.25 D. According to sex, the mean spherical equivalent was −0.76 ± 2.29 D for female and −0.49 ± 2.49 D for male, with no significant difference between them (p = .307). The distribution of the spherical equivalent mean across the age groups, linearly varies from a myopic −1.62 ± 1.74 D in the age group of [6 – 29]; −1.58 ± 2.80 D in [30 – 44]; −0.09 ± 2.40 in [45 – 59] to a hyperopic 0.67 ± 1.61 D in the group of [60 – 81]. High myopia had a prevalence of 2.7% in the sample. Myopia was the most prevalent refractive error in the sample representing 41.3%. In the age group [6–29], myopia had a prevalence of 69.3%. Hyperopia had a prevalence in the sample of 29.7%. Conclusions: Myopia represents the most prevalent refractive error within the sample and is the prevalence is higher in the younger age groups, demonstrating a shift towards an increase of myopia in the next years.(undefined

Changes in diadenosine polyphosphates during alignment-fit and orthokeratology rigid gas permeable lens wear

The authors thank Paragon Vision Science (Mesa, AZ), Lenticon SA (Madrid, Spain), and Interlenco SA (Madrid, Spain) for donating the lenses, and Bausch & Lomb (Rochester, NY) for donating the care solutions used in the study.PURPOSE: To evaluate the levels of dinucleotides diadenosine tetraphosphate (Ap(4)A) and diadenosine pentaphosphate (Ap(5)A) in tears of patients wearing rigid gas permeable (RGP) contact lenses on a daily wear basis and of patients wearing reverse-geometry RGP lenses overnight for orthokeratology treatment. METHODS: Twenty-two young volunteers (10 females, 12 males; 23.47 ± 4.49 years) were fitted with an alignment-fit RGP lens (paflufocon B) for a month, and after a 15-day washout period they were fitted with reverse-geometry RGP lenses for corneal reshaping (paflufocon D) for another month. During each period, tears were collected at baseline day 1, 7, 15, and 28. Ap(4)A and Ap(5)A were measured by high-pressure liquid chromatography (HPLC). Additionally, corneal staining, break-up time (BUT), Schirmer test, and dryness symptoms were evaluated. RESULTS: Ap(4)A concentrations increased significantly from baseline during the whole period of daily wear of RGP lenses (P < 0.001); concentration was also significantly higher than in the orthokeratology group, which remained at baseline levels during the study period except at day 1 (P < 0.001) and day 28 (P = 0.041). While BUT and Schirmer remained unchanged in both groups, discomfort and dryness were significantly increased during alignment-fit RGP daily wear but not during the orthokeratology period. CONCLUSIONS: Daily wear of RGP lenses increased the levels of Ap(4)A due to mechanical stimulation by blinking of the corneal epithelium, and this is associated with discomfort. Also, orthokeratology did not produce symptoms or signs of ocular dryness, which could be a potential advantage over soft contact lenses in terms of contact lens-induced dryness.Supported by Ministerio de Ciencia e Innovacion Grant SAF2010-16024, RETIC Red de Patologia ocular del envejecimiento, calidad visual, y calidad de vida Grant RD07/0062/0004, and BSCHUCM Grant GR58/08

What is the best alternative to the use of cycloplegic in the objective refraction

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Determination of oxygen permeability in soft contact lenses using a polarographic method : estimation of relevant physiological parameters

This work reports the apparent oxygen transmissibility Dk/t)app of four silicone hydrogel (Si-Hy) contact lenses (CLs). A method is described that allows the estimation of the oxygen tension at the lens-cornea interface for closed- and open-eyelids situations combining the instrument oxygen transmissibility (IOT) and corneal parameters such as corneal thickness, corneal permeability, and oxygen flux across the cornea. From these results, the biological oxygen apparent transmissibility (BOAT), equivalent oxygen percentage (EOP), partial pressure, (ptc), of oxygen at the cornea-CL interface and oxygen flux, (jc), were also obtained. This method allows the evaluation of the physiological environment under the lens using the formulations described in previous studies. The oxygen performance of four Si-Hy materials was evaluated using a polarographic cell coupled to a permeometer. Measurements of the apparent transmissibility, (Dk/t)app, and permeability, Dk, were performed in stacks containing from 1 to n repeated lenses and, from the values obtained, the error involved in the measurements in single samples was estimated. It was found that the values of (Dk/t)app and Dk obtained following the two different procedures (stack method and measurement of single sample) were significantly different from the nominal values given by the manufacturer, particularly for some samples. However, the impact of these differences on the values of the other physiologically relevant parameters (BOAT, EOP, ptc, and jc) was not significant. Furthermore, these parameters were similarfor the four lenses in spite of the different (Dk/t)app measured. The relationships of (Dk/t)app with theremaining physiological parameters were calculated and graphically represented for open and closed-eyelid conditions.Dirección General de Investigación Científica y Técnica (DGICYT) - bolsa MAT-2005-05648-C02-01Instituto de la Pequeña y Mediana Industria Valenciana (IMPIVA) - bolsa IMCOVA-2006/20.European Social Funding (ESF) - Contrato #8281/2002 para o bolseiro J.M.G.-M.Fundação para a Ciência e Tecnologia (FCT

Analysis of wavefront data obtained with a pyramidal sensor in pseudophakic eyes implanted with diffractive intraocular lenses

Purpose: To investigate the clinical validity of using wavefront measurements obtained with a recently available pyramidal aberrometer to assess the optical quality of eyes implanted with diffractive intraocular lenses (IOLs). Methods: Individual biometric data were used to create models of pseudophakic eyes implanted with two diffractive IOLs. Their synthetic wavefronts were calculated by ray-tracing with near infrared wavelength (0.85 μm). Comparisons of the through-focus visual acuity of 12 pseudophakic eyes were obtained with three different methods: clinical defocus curves; simulated defocus curves calculated from ray-tracing in the customized model eyes; and through-focus simulated defocus curves calculated from the wavefront data measured with a pyramidal aberrometer. Results: Image quality calculated from wavefront data obtained by ray-tracing with 0.85 μm wavelength, without scaling the phase to 0.55 μm, resulted in a significantly different through-focus curve compared to the reference values. Even so, after scaling of the wavefront data to 0.55 μm, the defocus curves calculated from the wavefronts measured with the pyramidal aberrometer did not match the shape and the depth of field of the clinical defocus curves or the theoretical expected values. Conclusions: Correcting for the longitudinal chromatic aberration of the eye when measuring the wavefront of eyes implanted with diffractive IOLs under near infrared light only accounts for the best focus shift due to the longitudinal chromatic aberration, but not for the wavelength dependence of the diffractive element. The pyramidal sensor does not seem to properly sample the slopes of a wavefront measured from a pseudophakic eye implanted with a presbyopia-correcting diffractive IOL to a clinically acceptable level.Acknowledgments: The authors thank CSO for providing the Osiris-T during the study and Pete Kollbaum and Matt Jaskulski from Indiana University for sharing the Matlab code used to parse the Osiris .wfm files