Single function crystalline lens capable of mimicking ciliary body accommodation.

The lens is a complex optical component of the human eye because of its physiological structure: the surface is aspherical and the structural entities create a gradient refractive index (GRIN). Most existent models of the lens deal with its external shape independently of the refractive index and, subsequently, through optimization processes, adjust the imaging properties. In this paper, we propose a physiologically realistic GRIN model of the lens based on a single function for the whole lens that accurately describes different accommodative states simultaneously providing the corresponding refractive index distribution and the external shape of the lens by changing a single parameter that we associate with the function of the ciliary body. This simple, but highly accurate model, is incorporated into a schematic eye constructed with reported experimental biometric data and accommodation is simulated over a range of 0 to 6 diopters to select the optimum levels of image quality. Changes with accommodation in equatorial and total axial lens thicknesses, as well as aberrations, are found to lie within reported biometric data ranges.

Big Data and diabetes: the applications of Big Data for diabetes care now and in the future.

We review current applications of Big Data in diabetes care and consider the future potential by carrying out a scoping study of the academic literature on Big Data and diabetes care. Healthcare data are being produced at ever-increasing rates, and this information has the potential to transform the provision of diabetes care. Big Data is beginning to have an impact on diabetes care through data research. The use of Big Data for routine clinical care is still a future application. Vast amounts of healthcare data are already being produced, and the key is harnessing these to produce actionable insights. Considerable development work is required to achieve these goals.

Composite modified Luneburg model of human eye lens.

A new lens model based on the gradient-index Luneburg lens and composed of two oblate half spheroids of different curvatures is presented. The spherically symmetric Luneburg lens is modified to create continuous isoindicial contours and to incorporate curvatures that are similar to those found in a human lens. The imaging capabilities of the model and the changes in the gradient index profile are tested for five object distances, for a fixed geometry and for a fixed image distance. The central refractive index decreases with decreasing object distance. This indicates that in order to focus at the same image distance as is required in the eye, a decrease in refractive power is needed for rays from closer objects that meet the lens surface at steeper angles compared to rays from more distant objects. This ensures a highly focused image with no spherical aberration.

Primary sequence contribution to the optical function of the eye lens.

The crystallins have relatively high refractive increments compared to other proteins. The Greek key motif in ßγ-crystallins was compared with that in other proteins, using predictive analysis from a protein database, to see whether this may be related to the refractive increment. Crystallins with Greek keys motifs have significantly higher refractive increments and more salt bridges than other proteins with Greek key domains. Specific amino acid substitutions: lysine and glutamic acid residues are replaced by arginine and aspartic acid, respectively as refractive increment increases. These trends are also seen in S-crystallins suggesting that the primary sequence of crystallins may be specifically enriched with amino acids with appropriate values of refractive increment to meet optical requirements. Comparison of crystallins from five species: two aquatic and three terrestrial shows that the lysine/arginine correlation with refractive increment occurs in all species investigated. This may be linked with formation and maintenance of salt bridges.

Age variations in intraocular pressure in a cohort of healthy Austrian school children.

PURPOSE: Limited data exist detailing the normal range of intraocular pressure (IOP) for healthy school age children. This study aims to describe the mean and normal range of IOP measurements that may be expected using the Icare rebound tonometer and to examine associations between visual function measures and IOP. METHODS: Six measurements of IOP from each eye were obtained from 211 normal children aged 6-15 years (79 females and 132 males) using the Icare tonometer. Other measures of visual function obtained included: visual acuity, non-cycloplegic retinoscopy, amplitude of accommodation, accommodative facility, and accommodative response. RESULTS: Statistical analysis (Mann-Whitney U test) demonstrated that the male subjects were more likely to have higher IOP measurements than the female subjects (mean IOP males 15.02 mm Hg (SD 2.19), mean IOP females 14.44 (SD 2.01) P=0.041). Analysis (Spearman's rho) showed a statistically significant association between age and IOP (right eye) in males (P<0.001) but no association for females (P=0.459). Using Spearman's rank analysis, statistically significant associations were found between IOP and amplitude of accommodation in males, for the whole data set (P=0.09) and for data up to age 11 (P<0.001). For females no statistically significant association was found for the whole data set (P=0.253) or for data up to age 11 (P=0.08). IOP was not significantly associated with visual acuity, refractive error, accommodative facility, or accommodative response. CONCLUSION: This study provides useful normative IOP data using the Icare tonometer for a European cohort of school age children.

Material properties of the cornea and sclera: a modelling approach to test experimental analysis.

Material properties of cornea and sclera are important for maintaining the shape of the eye and the requisite surface curvatures for optics. They also need to withstand the forces of external and internal musculature and fluctuations in intraocular pressure (IOP). These properties are difficult to measure and variable results have been reported. A previously published experimental procedure, from which the material properties of the eyeball coats were obtained, has been modelled in this paper using Finite Element Analysis, in order to test the accuracy of the experiment. Material parameters were calculated from the model and the resulting relationships between stress and strain for the cornea and sclera compared to their experimentally obtained counterparts. The comparison between model and experiment was close for the sclera but more varied for the cornea. The pressure vessel model can be applied for measuring the material properties of the sclera but is less accurate for the cornea.

Referrals to ophthalmology: optometric and general practice comparison.

PURPOSE: To assess referrals from primary care practitioners to an ophthalmological practice in order to determine whether any differences exist between the two primary care groups in the types of conditions referred, and in concurrence between reasons for referral and ophthalmological diagnosis. METHODS: This study investigated patient referrals from general practitioners and optometrists to an ophthalmologist's practice in Belfast, over a period of 3 months from January 2007 to the end of March 2007. Anonymised data were divided into two groups depending on the referring practitioner. Within the two groups, patient numbers were sub-divided into 15 categories covering the range of diagnoses made. Preliminary diagnoses or reasons for referral were compared with the definitive diagnoses made by the ophthalmologist. RESULTS: Optometrists contributed the greatest number of referrals (323) with 243 patients referred by general practitioners. Cataract was the most common condition referred by optometrists, and the second most common condition referred by general practitioners: agreement with ophthalmological diagnosis was high for both groups of primary care practitioners. Concurrence of referral reason with ophthalmological diagnosis for glaucoma was lower for general practitioners (56%) than for optometrists (76%), but optometrists referred more false positive patients for glaucoma than did general practitioners. The opposite was true for lid/tear duct/conjunctival conditions, the most common basis for referral by general practitioners. CONCLUSIONS: The differences between the two primary care practitioner groups may reflect variations in training, skills and practice. Further investigation of the particular strengths of both groups, and how they can be optimised to promote effective shared care, is required.

The elasticity and rigidity of the outer coats of the eye.

AIMS: The aim of this study was to measure the elastic moduli of the cornea and sclera and the rigidity of the eyeball. METHODS: Elastic moduli of the cornea and sclera were measured on 22 fresh intact porcine eyes using the relationship between elasticity and changes in pressure and volume applicable to pressure vessel models. Rigidity was determined from the relationship between changes in volume and intraocular pressure (IOP). Induced volume changes were up to 300 mul and within the physiological range of IOP (12-25 mm Hg). IOP readings taken with a transducer were used in calculations. Tonometric values were taken for comparison. RESULTS: For cornea and sclera, the relationship between circumferential stress and volumetric strain (which defines the elastic modulus) was closely approximated to linear (R(2) = 0.96), but better fitted with a second order polynomial (R(2) = 1.00). Elastic moduli ranged from 0.05 to 0.24 MPa and 0.15 to 0.83 MPa for the cornea and sclera, respectively. The mean value of ocular rigidity was found to be 0.0024. The effect of increasing IOP on the average corneal radius of curvature was not statistically significant, but it altered the scleral radius of curvature by 0.07 mm/mmHg. CONCLUSION: The elasticity of the porcine cornea and sclera could be approximated to constant over the physiological range, but showed a tendency to non-linearity. The modulus of elasticity for the sclera is 3 to 3.5 times that of the cornea. The cornea is more resistant than the sclera to IOP changes within the physiological range.

Patterns of crystallin distribution in porcine eye lenses.

PURPOSE: To measure the protein distribution patterns in single young porcine lenses. METHODS: Twenty fresh porcine lenses from 5 to 6 months old animals were fractionated into 8-10 concentric fractions by controlled dissolution in phosphate buffer. Proportions of soluble and insoluble protein were determined by Bradford assay. Water-soluble proteins in all layers were separated into HMW, MMW, and LMW fractions by size-exclusion HPLC and constituents of each class further characterized by SDS gel electrophoresis, as were the water-insoluble proteins. Size-exclusion fractions were further separated by reverse-phase HPLC and the molecular masses of each peak determined by MALDI-TOF mass spectrometry. RESULTS: The major soluble proteins in the porcine lens are beta-crystallins. They comprise around 45% of the total protein in the outer lens decreasing gradually to 35% in the central region. Soluble alpha-crystallins vary from 35% to 22% from outer to inner lens. The proportion of soluble gamma-crystallin levels, substantially lower than that of the other protein classes, increases gradually with progression into the lens center. Insoluble protein levels also increase from outer to inner lens layers. CONCLUSIONS: In the young porcine lens, there is relative constancy in the levels of all three crystallin classes in the outer lens with alpha- and beta-crystallins representing the predominant protein classes. The increase in gamma-crystallin in the inner lens may contribute to the refractive index gradient.

Children, Gillick competency and consent for involvement in research.

This paper looks at the issue of consent from children and whether the test of Gillick competency, applied in medical and healthcare practice, ought to extend to participation in research. It is argued that the relatively broad usage of the test of Gillick competency in the medical context should not be considered applicable for use in research. The question of who would and could determine Gillick competency in research raises further concerns relating to the training of the researcher to make such a decision as well as to the obvious issue of the researcher's personal interest in the project and possibility of benefiting from the outcome. These could affect the judgment of Gillick competency if the researcher is charged with making this decision. The above notwithstanding, there are two exceptional research situations in which Gillick competency might be legitimately applied: (1) when the research is likely to generate significant advantages for the participants while exposing them to relatively minor risks, and (2) when it is likely to generate great societal benefit, pose minimal risks for the participants and yet raise parental objection. In both cases, to ensure that autonomy is genuinely respected and to protect against personal interest, Gillick competency should be assessed by an individual who has no interest or involvement in the research.

Modeling internal stress distributions in the human lens: can opponent theories coexist?

The effects of material properties and equatorial stretching forces on the stress distribution and shape profile of human lenses were investigated to see whether support could be found for either or both current theories of accommodation. Finite element analysis was used to create models using shape parameters and material properties from published data. Models were constructed for two lenses of different ages. Material properties were varied to show differences between models with a single elastic modulus and those with different moduli for the cortex and the nucleus. Two levels of stretching forces were applied at the equator. Comparisons between experimental and model profiles were made, and stress distribution patterns were constructed. In all models, stretching produces a flattening in the peripheral curvature of the lens. In the younger lens, model and experimental results show that central curvature at some points is steeper for stretched than for unstretched profiles. In the older lens, gradients are flatter at all central points for stretched model and experimental profiles compared to the unstretched profile. In all models, there is a region of higher stress distribution within the lens that corresponds with the position of an inflection point that appears on the anterior surface and, in the older lens, also on the posterior surface. The results show that equatorial stretching forces can produce shape changes in support of both current theories of accommodation depending on the lens age, shape, and applied force.

Fluctuations in intraocular pressure and the potential effect on aberrations of the eye.

OBJECTIVE: To investigate the fluctuations in intraocular pressure during the day and to see if these are associated with changes in corneal shape and in the patterns of ocular aberrations. METHODS: Intraocular pressure, corneal curvature, refractive error, spherical equivalent and aberrations (defocus (sphere); cylinder (astigmatism); coma, trefoil and third order spherical aberration) were measured in 17 healthy subjects three times during the day. The first measurement was made between 9:00 and 9:30, the second at midday (12:30-13:00) and the third in the afternoon (17:00-17:30). Aberrations, corneal shape, refractive error and pupil size (for which correction was made) were measured with an Irx3 Dynamic Wavefront Aberrometer. Intraocular pressures were measured using a non-contact tonometer (Cambridge Instruments Inc.) and calibrated with the Goldmann applanation tonometer. RESULTS: Variations in intraocular pressures were unrelated to age or refractive error. Statistically significant differences in intraocular pressure between morning and midday as well as between midday and afternoon were found. Intraocular pressure variations between midday and afternoon were associated with changes in spherical equivalent, corneal radius of curvature and aberrations (defocus, cylinder, coma, trefoil and spherical aberration) over the same time period. Aberration patterns varied between individuals, and no association was found between two eyes of the same subject. CONCLUSIONS: Changes in intraocular pressure have no noticeable effect on image quality. This could be because the eye has a compensating mechanism to correct for any effect of ocular dynamics on corneal shape and refractive status. Such a mechanism may also affect the pattern of aberrations or it may be that aberrations alter in a way that offsets any potentially detrimental effects of intraocular pressure change on the retinal image. Variations in patterns of aberrations and how they may be related to ocular dynamics need to be investigated further before attempts at correction are made.

Modelling the elastic properties of the anterior eye and their contribution to maintenance of image quality: the role of the limbus.

PURPOSE: The elastic moduli of the cornea, sclera, and limbus for different corneal eccentricities (e) and varying levels of intraocular pressure (IOP) were modelled in order to determine how the rheological properties, especially those of the limbus, need to alter to maintain optical image quality when the eye is subjected to small variations in IOP. METHODS: Finite element analysis (FEA) was used to construct eyeball models with four different corneal eccentricities (e=0, 0.33, 0.5, 0.65). Three values for Young's modulus of the cornea were tested in all models (0.2 megapascal (MPa), 1.2 and 10 MPa). For each corneal modulus, scleral moduli of 3, 4, 5, 7, and 10 times that of the corneal modulus were selected. The limbal modulus was varied to optimise image quality of the eye model subjected to IOP variations of +/-0.8 mmHg for three different levels of IOP (8, 16, and 32 mmHg). RESULTS: The elastic modulus of the limbal ring increases with an increase in corneal modulus and rises to a peak when the ratio of scleral to corneal moduli is between 5 and 7 depending on corneal eccentricity. Different levels of IOP produce only slight differences in the relative moduli required to maintain optical image quality. CONCLUSIONS: The significance of a peak in the value of Young's modulus of the limbus is not clear but suggests that there may be an optimal limbal modulus that must be balanced with the moduli of cornea and sclera for preservation of image quality.

The effect of changing intraocular pressure on the corneal and scleral curvatures in the fresh porcine eye.

AIM: To measure corneal and scleral radii of curvature in response to intraocular pressure (IOP). METHODS: Using digital photographic profile images of 16 fresh porcine eyes, the curvatures of the cornea and sclera were determined in response to five consecutive incremental 100 mul saline intravitreal injections. IOP was measured and ocular rigidity calculated. Elastic moduli of the cornea and sclera were estimated. RESULTS: Intraocular pressure and the radius of curvature of the sclera increased linearly with increasing volume. There was no statistical change in corneal curvature. The elasticity of the cornea and sclera was constant during the 15-50 mm Hg increase in IOP. The estimated range of the elastic moduli of the cornea and sclera were, respectively 0.07-0.29 MPa and 0.2 MPa to 0.5 MPa. The scleral rigidity ranged from 0.0017 to 0.0022. CONCLUSIONS: The elastic moduli of the cornea and sclera are independent of IOP. The modulus of elasticity of the sclera is higher than that of the cornea. Elevation of IOP changes the curvature of the sclera but not that of the cornea. Porcine scleral rigidity is similar to human scleral rigidity. Scleral curvature could be a novel method for measuring IOP.

Refractive index distribution in the porcine eye lens for 532 nm and 633 nm light.

AIMS: To measure the refractive index distribution in porcine eye lenses for two wavelengths from the visible spectrum: 532 and 633 nm, in order to determine whether there are any discernible wavelength dependent differences in the shape of the profile and in the magnitude of refractive index. METHODS: Rays were traced through 17 porcine lenses of the same age group and of similar size. Ray trace parameters were used to calculate the refractive index distributions for 633 nm light in all 17 lenses and for 532 nm light in 10 lenses. The effect of the refractive index at the edge of the lens, on the rest of the profile, was considered because the mismatch between refractive index at the lens edge and the refractive index of the surrounding gel necessitated a further step in calculations. RESULTS: The shape of the refractive index distributions is parabolic. There is a small wavelength dependent difference in the magnitude of the refractive index across the profile and this increases very slightly into the centre of the lens. The value of the refractive index at the edge of the lens does not appreciably affect the index profile. CONCLUSIONS: The wavelength dependent differences in refractive index between light of 633 and 532 nm are small but discernible.

Corneal shape change during accommodation.

PURPOSE: To investigate whether accommodation induces any changes in central corneal curvature. METHODS: Shape changes were measured on 14 subjects using a keratometer modified to enable a change in focus to occur without a change in vergence. All subjects were emmetropic and their ages ranged from approximately 20 to 28 years. RESULTS: In 11 of the 14 subjects a difference in central corneal curvature, of around 0.4 D in at least one principal meridian, was found when focus was changed between distant and near targets. In 9 subjects the curvature was greater for near focus in at least one meridian. In 5 subjects the change in one meridian was opposite in effect to what would be expected, i.e. the curvature was greater at distance than at near. CONCLUSION: The study suggests that accommodation may have some effect on corneal shape.

A logistics evaluation of visual acuity as applied to the Bailey-Lovie chart.

AIMS: To discover whether as a result of the increasing use of the Bailey-Lovie chart some classes of patients may not be affected by the crowding of the smaller test characters, whose spacing is proportional to their size; and to determine acuities with a logistic function so that all of a patient's responses may be utilized. METHODS: 112 patients were tested both with the original chart and one in which the horizontal distance is kept constant, i.e., the letters are arranged in vertical columns. All of a patient's responses were recorded so that the constants of the logistic function might be determined. RESULTS: No difference was found for very high and very low acuity scores, but, for intermediate ones, the vertical columns yielded acuity ratings increased by some 13%. CONCLUSION: The use of the logistics function was successful in that the correlation between stimulus and response was between 0.9 and 1 for some 80% of those examined. A constant horizontal spacing may be of advantage to some patients with a conventionally measured visual acuity of approximately 0.9.

Age-related cataract and place of birth.

Cataract patient data from 13 Australian practitioners were analysed in order to compare the distribution of seven principal types of cataract in relation to gender, age and place of birth. Relative odds ratios were calculated for age-matched pairs of three identifiable ethnic groups and each type of cataract. The number of statistically significant relative odds ratios far exceeds the expectation due to chance. The results suggest a relatively greater risk for cortical cataract amongst younger when compared with older groups. In contrast, the relative odds ratios for nuclear cataract, independently of age, were consistently greater than unity when Australians or Northern Europeans were compared with Southern Europeans. There was no gender-based difference in cataract type distribution.

Investigation of the polarization optics of the living human cornea and lens with purkinje images.

We present what to our knowledge is a new method for assessing the polarization optics of the cornea and lens, by examining the state of polarization of the first, second, and fourth Purkinje images. When linearly polarized light is incident on the cornea at 70 degrees to the line of gaze along the horizontal meridian, and then traverses the cornea, or the cornea and the lens, the emergent light is elliptically polarized. The degree of ellipticity varies widely between subjects. The results indicate that both the cornea and the lens may be optically active and to our knowledge are the first to suggest that the cornea may exhibit circular birefringence.