Biomedical subjects

José B Almeida

Publications and source records attributed to José B Almeida.

10 recordsLinked to original sources

PubMed2007

Equivalences between refractive index and equilibrium water content of conventional and silicone hydrogel soft contact lenses from automated and manual refractometry.

PURPOSE: The purpose of the present study was to develop mathematical relationships that allow obtaining equilibrium water content and refractive index of conventional and silicone hydrogel soft contact lenses from refractive index measures obtained with automated refractometry or equilibrium water content measures derived from manual refractometry, respectively. METHODS: Twelve HEMA-based hydrogels of different hydration and four siloxane-based polymers were assayed. A manual refractometer and a digital refractometer were used. Polynomial models obtained from the sucrose curves of equilibrium water content against refractive index and vice-versa were used either considering the whole range of sucrose concentrations (16-100% equilibrium water content) or a range confined to the equilibrium water content of current soft contact lenses (approximately 20-80% equilibrium water content). RESULTS: Values of equilibrium water content measured with the Atago N-2E and those derived from the refractive index measurement with CLR 12-70 by the applications of sucrose-based models displayed a strong linear correlation (r2 = 0.978). The same correlations were obtained when the models are applied to obtain refractive index values from the Atago N-2E and compared with those (values) given by the CLR 12-70 (r2 = 0.978). No significantly different results are obtained between models derived from the whole range of the sucrose solution or the model limited to the normal range of soft contact lens hydration. CONCLUSIONS: Present results will have implications for future experimental and clinical research regarding normal hydration and dehydration experiments with hydrogel polymers, and particularly in the field of contact lenses.

Biocompatible Materials↗

PubMed2006

Microscopic observations of superficial ultrastructure of unworn siloxane-hydrogel contact lenses by cryo-scanning electron microscopy.

The purpose of this study was to analyze three commercial siloxane-hydrogel contact lens materials, lotrafilcon A, balafilcon A, and galyfilcon A, by cryogenic scanning electron microscopy (cryoSEM). The fully hydrated lenses were frozen in slush liquid nitrogen and qualitatively observed in a cryogenic scanning electron microscope. The superficial ultrastructure of the siloxane-hydrogels was observed at the areas where the lens fractured during sample cryogenic preparation. There are qualitative differences among the three examined materials in the complex polymer network structure existing between the outer layer and the underlying polymer. CryoSEM, although destructive, is a useful tool to investigate the structure of polymers used in contact lenses. This technique allows the observation of the inner structure of polymers in the hydrated state. The ultrastructure, the polymer network underlying the outer surface of siloxane-hydrogels by cryoSEM microscopy, have never been reported before.

Contact Lenses, Hydrophilic↗

PubMed2006

Microscopic observation of unworn siloxane-hydrogel soft contact lenses by atomic force microscopy.

In the present study, samples of lotrafilcon A, balafilcon A, and galyfilcon A contact lenses were observed by atomic force microscopy (AFM) in tapping mode at areas ranging from 0.25 to 400 microm2. Mean roughness (Ra), root-mean-square roughness (Rms) and maximum roughness (Rmax) in nanometers were obtained for the three lens materials at different magnifications. The three contact lenses showed significantly different surface topography. However, roughness values were dependent of the surface area to be analyzed. For a 1 microm2 area, statistics revealed a significantly more irregular surface of balafilcon A (Ra = 6.44 nm; Rms = 8.30 nm; Rmax = 96.82 nm) compared with lotrafilcon A (Ra = 2.40 nm; Rms = 3.19 nm; Rmax = 40.89 nm) and galyfilcon A (Ra = 1.40 nm; Rms = 1.79 nm; Rmax = 15.33 nm). Ra and Rms were the most consistent parameters, with Rmax presenting more variability for larger surface areas. The higher roughness of balafilcon A is attributed to the plasma oxidation treatment used to improve wettability. Conversely, galyfilcon A displays a smoother surface. Present observations could have implications in clinical aspects of siloxane-hydrogel contact lens wear such as lens spoliation, resistance to bacterial adhesion, or mechanical interaction with the ocular surface.

Contact Lenses, Hydrophilic↗

PubMed2006

Correlations between central and peripheral changes in anterior corneal topography after myopic LASIK and their implications in postsurgical contact lens fitting.

PURPOSE: To investigate the changes in central and peripheral anterior corneal curvatures after myopic laser in situ keratomileusis (LASIK) and to correlate them with the manifest refractive change to discuss how such results could affect post-LASIK corneal topography profiles and midterm stability and their implications in postsurgical contact lens fitting. METHODS: Topographic and refractive data from 18 eyes of 11 patients that had undergone myopic LASIK were collected for 6 months after surgery. Short-term and midterm topographic responses were investigated and correlated with spherical equivalent manifest refractive changes. RESULTS: There was a strong correlation between eccentricity changes and manifest refractive change 15 days after surgery (r = 0.753, P < 0.001), with no significant changes thereafter for the following 6 months (r = 0.148, P = 0.114). A strong linear relationship was found between baseline manifest refraction and changes in corneal curvature at the center (r = 0.810, P < 0.001), 4-mm chord (r = 0.895, P < 0.001), and 6-mm chord (r = 0.696, P < 0.001). Statistically significant changes were also found after the first 15 days (P < 0.005) and showed a regression effect that affects the three zones. In this case, a weaker relationship was found between curvature regression and the final refractive change for the central location (r = 0.412, P = 0.004), 4-mm chord area (r = 0.430, P = 0.003), and 6-mm chord area (r = 0.283, P = 0.023). CONCLUSIONS.: After myopic LASIK, the anterior corneal dioptric power is expected to change, on average, approximately 77% of the attempted spherical equivalent correction at the center; 60% at the 4-mm chord region, where the stronger correlation between topographic and refractive change is found; and 30% at the 6-mm chord area. The paracentral area 4 mm from the center seems to be more likely to predict baseline corneal curvature from manifest refractive change. Some degree of regression in the midterm period is expected to occur after myopic LASIK, which shows a significant correlation with the manifest refractive change. Again, this effect is more evident and more accurately predicted at the 4-mm chord area. The results of the current study are of interest for those fitting contact lenses after myopic LASIK.

Adult↗

PubMed2006

Oxygen transmissibility of piggyback systems with conventional soft and silicone hydrogel contact lenses.

PURPOSE: To investigate the apparent oxygen transmissibility of various piggyback systems using conventional and silicone hydrogel soft contact lenses of different water content and permeability, rigid poly(methyl methacrylate), and rigid gas-permeable lenses of medium, high, and ultrahigh oxygen permeability. The aim of the study was to establish which material (rigid or hydrogel) is more representative of the resulting oxygen performance of piggyback systems. METHODS: The apparent oxygen transmissibility of 66 piggyback systems was measured with an electrochemical method. Eighteen of these combinations involved the use of silicone hydrogel contact lenses currently available. One hyperpermeable rigid gas-permeable contact lens (tisilfocon A) was also included in the study. RESULTS: Measured apparent transmissibility correlates with rigid lens permeability (r = 0.403; SE = +/-3.03 barrer/cm; P < 0.001) and hydrogel lens permeability (r = 0.334; SE = +/-3.2 barrer/cm; P < 0.001). As expected, a linear model comprising permeability values from both rigid and soft materials gave a more precise estimation of the piggyback transmissibility (r = 0.736; SE = +/-2.02 barrer/cm; P < 0.001). The highest values of apparent oxygen transmissibility were found for the combination of tisilfocon A rigid material with any of the 3 silicone hydrogel lenses. Tisilfocon A material significantly improved the transmissibility of all piggyback systems even when conventional hydrogels are involved. CONCLUSION: The combination of hypertransmissible rigid gas permeable lenses with silicone hydrogel soft materials should result in normal corneal function under daily wear conditions. When fitting piggyback systems, clinicians must be aware of material selection to optimize oxygen performance. This is of particular importance in already compromised corneas.

Contact Lenses, Hydrophilic↗

PubMed2006

Refractive index and equilibrium water content of conventional and silicone hydrogel contact lenses.

PURPOSE: The purpose of the present study was to measure equilibrium water content (EWC) and refractive index of conventional and silicone hydrogel soft contact lenses (SCL) using a hand refractometer and an automated refractometer. METHODS: Sixteen SCL were used in this study including 12 conventional SCL not containing siloxane moieties (equilibrium water content (EWC) range: 38.6-74%) and the four silicone hydrogel based contact lenses currently available (WC range: 24-47%). Two experienced observers performed the measurements in a randomised order being masked by a third party during the three sessions at which the measurements were collected. The Atago N-2E hand refractometer and the CLR 12-70 digital refractometer were used. Data were analysed separately for conventional and silicone hydrogel materials. RESULTS: Measured EWC and refractive index correlate better when measured with the instruments used in this study (r(2) = 0.979, p < 0.001) than the nominal parameters (r(2) = 0.666, p < 0.001). The linear relationship that correlates nominal and measured EWC shows higher spread of data when all lenses are analysed together (r(2) = 0.840) than when conventional hydrogel (r(2) = 0.953) and silicone hydrogel contact lenses (r(2) = 0.967) are analysed separately. Regarding refractive index, the relationship between nominal and measured values when all the lenses are considered together (r(2) = 0.794) becomes weaker when conventional hydrogel are considered separately (r(2) = 0.688), while a stronger relationship is observed for silicone hydrogel lenses (r(2) = 0.939). Hence, hand refractometry overestimates the EWC of silicone hydrogels, while automated refractive index measurements are more accurate in silicone hydrogels than in conventional hydrogels. CONCLUSIONS: New relationships are presented that correlate nominal and measured values of EWC and refractive index for the silicone containing hydrogels. The linear relationships derived fit well to the data. Hand refractometry overestimates the EWC of silicone hydrogel materials and this bias is related to the proportion of siloxane moieties in the material. Conversely, refractive index can be obtained more accurately with automated refractometry for silicone hydrogels than for conventional hydrogels. Present results are of interest in planning future clinical studies involving the measurement of EWC of current hydrogels.

Contact Lenses, Hydrophilic↗

PubMed2005

Theoretical calculation of a contact lens thickness designed to correct the eye's monochromatic aberrations.

PURPOSE: The aim of this work is the theoretical study of high order aberration compensation in the human eye. METHODS: This compensation should be achieved with uneven thickness contact lenses (rigid gas permeable or soft), capable of inducing a localized delay on the aberrated wavefront, so that light beams originated on a point object would reach the retina at a common point, irrespective of their point of passage through the pupil. RESULTS: The algorithm for optimal local contact lens thickness determination is explained. For this purpose the aberrated wavefront measured on a patient by the double-pass technique was expressed in terms of Zernike polynomials. The knowledge of the wave aberration as a function of the point of passage through the pupil allows determination of the delay to be introduced in order to produce a wavefront as would result from non-aberrated eye, where all rays would reach the same point on the retina. CONCLUSIONS: The explanation of how it would be possible to correct wavefront deviations, by varying an RGP or soft contact lens thickness, was made, and a particular eye exemplify.

Contact Lenses↗

PubMed2005

Retinoscopy/autorefraction: which is the best starting point for a noncycloplegic refraction?

PURPOSE: The aim of this study was to estimate the agreement between an autorefractor (Nidek ARK 700A, Gamagori, Japan) and retinoscopy with subjective refraction. METHODS: Measurements of autorefraction obtained with the ARK700A and retinoscopy were performed on 192 right eyes from 192 healthy young adults and compared with subjective refraction. These measurements were performed without cycloplegia. The age range was 18 to 34 years, with a mean value of 21.6 years and an SD of 2.66 years. RESULTS: A comparison of the autorefractor and subjective refraction results shows that (1) for the mean spherical equivalent (M), the autorefractor yields more negative values (-0.44 +/- 0.54 D; p = 0.000); (2) for the Jackson cross-cylinder at axis 0 degrees (J0), the autorefractor yields more positive values than the subjective ones (0.05 +/- 0.13 D; p = 0.000); and (3) for the Jackson cross-cylinder at axis 45 degrees (J45), the autorefractor results are more negative (-0.02 +/- 0.09 D; p = 0.019). The differences found for each component M, J0, and J45 are statistically significant. By comparing retinoscopy with the subjective examination, there are no statistically significant differences found for the M component (-0.02 +/- 0.33 D; p = 0.304). For the J0 and J45 components, the differences are statistically significant (0.07 +/- 0.10 D, p = 0.000; -0.01 +/- 0.08 D, p = 0.008). CONCLUSIONS: The present results confirm that when performed by an experienced clinician, retinoscopy is more accurate than automatic refraction, giving a better starting point to noncycloplegic refraction.

Adult↗

PubMedDate not supplied

Measuring corneal thickness with a rotary scanning system.

PURPOSE: Recently the authors presented a technique that allows measurement of the corneal thickness along a vertical meridian from optical sections obtained using a slit-lamp microscope. We present a new system that allows rotary scanning of the cornea and consequent measurement of the corneal thickness along any meridian. METHOD: The corneal thickness along two perpendicular meridians was measured from optical sections obtained with a new rotary scanning system. To obtain the optical sections, the cornea is illuminated with a light beam that is previously expanded in a fan by a small cylindrical lens rotated on a plane-containing axis to explore the cornea. The light diffused by both corneal surfaces is collected by two video cameras placed at an angle with the light beam. The axes of the two cameras define, with the visual axis, two planes normal to each other. After image acquisition, a processing algorithm is applied in order to compute the corneal thickness along the two meridians. RESULTS: Some preliminary results for the vertical and the horizontal meridians of one eye are shown, as well as all the intermediate computations. CONCLUSIONS: With this system, we expect to be able to measure corneal topography and thickness along an arbitrary meridian. The rotary scanning of the cornea is mechanically simple and will eventually allow automated scanning.

Cornea↗

PubMedDate not supplied

Corneal thickness and elevation maps computed from optical rotary scans.

PURPOSE: Recently, the authors presented a technique that allows corneal thickness measurements along any meridian from optical sections obtained using a rotary scanning system. This paper presents three-dimensional mapping of the corneal thickness and topography of both corneal surfaces, obtained with the rotary system. METHOD: Corneal thickness and topography are computed from optical sections obtained by illumination with a collimated beam expanded in a fan by a small cylindrical lens. This lens is provided with motor driven rotation to perform automated rotary scanning of the whole cornea. Two cameras are used to capture the images of the optical sections. RESULTS: With this system, it is possible to obtain measurements of corneal thickness, as well as corneal topography. Corneal thickness and elevation maps are shown. CONCLUSIONS: Although still under development, this new optical system allows measurement of the thickness of the whole cornea as well as topographical mapping of both corneal surfaces.

Corneal Topography↗