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Biomedical subjects

B K Pierscionek

Publications and source records attributed to B K Pierscionek.

At least 19 recordsLinked to original sources
PubMed2006

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.

Animals↗
PubMed2006

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.

Biomechanical Phenomena↗
PubMed2005

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.

Animals↗
PubMed1999

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.

Adolescent↗
PubMed1998

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.

Age Distribution↗
PubMed1997

Refractive index contours in the human lens.

The refractive index values along the equatorial and sagittal planes of lenses of varying ages were measured using a reflectometric fibre optic sensor. In younger lenses (from the third decade) and in one older lens, the index profiles from the two planes did not concur when plotted on a normalized scale refuting, in these lenses, the assumption of concentric, isoindicial contours which follow the shape of the lens. Agreement between the normalized profiles did occur with all other lenses investigated (aged 47 and older).

Adult↗
PubMed1997

Is there a link between corneal structure and the 'corneal cross'?

The 'corneal cross', observed when polarised light is reflected from the cornea and viewed through a crossed analyser, has been attributed to the fine anisotropic structure of the cornea causing birefringence or, alternatively, multiple reflections. But when plane polarised light is similarly reflected from isotropic curved surfaces and viewed through a crossed analyser, isogyres are also seen. Moreover, they vanish with a gonioscopic lens neutralising corneal curvature. This suggests that the corneal cross is not a specific attribute of corneal birefringence.

Chromium↗
PubMed1996

Light intensity distributions in refracting structures placed between crossed polarizers.

A number of refracting structures each with different optical properties, were placed between crossed polarizers, illuminated with diffuse light and photographed. The images of the characteristic cross pattern (isogyres) seen in such structures, viewed under these conditions, were analysed to determine the relative distributions of light intensity within the patterns. The experimental findings show that intensity distributions are related to the optical properties of a refracting structure. Comparison of dimensional differences shows that in a larger sample the profile of light intensity distribution (in the quadrants between the isogyres) is more symmetrical and the peak is closer to the centre of the quadrant. Sharper peaks are seen in the intensity distribution profiles of gradient-index structures than in profiles of homogenous-index bodies of comparable dimensions. Some agreement was found between an experimental observation and a previously derived mathematical model for homogenous index, curved bodies.

Animals↗
PubMed1996

Risk factors and ocular senescence.

The first part of this paper is devoted to a brief survey of novel aspects of ocular senescence which are shown to fit into the general, apparently genetic, non-pathological framework of senescence. This is followed by an analysis of recent work on presbyopia, the sight of the elderly, which is easily remedied by the provision of reading glasses and, incidentally, seems to be an example of growth rather than senescence. A consideration of graver consequences of senescence in the eye lens is presented next in a detailed analysis of cataracts. Age-related afflictions of the retina conclude the survey.

Aging↗
PubMed1996

Odds ratios for different types of age-related cataract: ethnicity and environment.

Ethnic and environmental influences on cataract type are examined using patient data collected from an Australian region. Three ethnic groups are identified: native Australian Caucasians, southern and other Europeans. The Australians are further subdivided into 2 age groups. Apart from effects due to age, the following differences were found: the younger Australian group shows a greater relative risk for cortical cataracts; there is a gender-based difference among the southern European ethnic group; the Australian total has a lower relative risk for cortical cataracts than southern Europeans and a lower relative risk for cortical and mixed cataract than the European group. A comparison made with data published earlier for comparable groups in Britain shows that all Australian groups have a lower relative risk for nuclear but a higher one for cortical cataract and its combinations.

Aged↗
PubMed1995

The optics of the eye-lens and lenticular senescence. A review.

Although the lens of the eye is structurally a biological tissue, it functions as an optical element providing one third of the refracting power of the human eye, and a variable focus in younger years. Throughout a life-time the optical properties of the eye-lens alter, resulting in changes in function: there is a gradual depletion of the focussing amplitude from infancy to middle age, and a loss of transmittance in the later decades of life. The optical properties of the lens depend on its power, which in turn is determined by its physical dimensions (curvatures and thickness) and its refractive index as well as transmissivity and the organization of its internal components. The power of the functional lens is, however, modifiable by virtue of the lens being attached via the zonule to the ciliary muscle. The contraction and relaxation of the latter respectively increases and decreases lens power in accordance with innervations determined by the physical distance of external objects to be imaged on the retina. This review will consider many of these features and how alterations in any of them may lead to changes in lenticular function. However, as we have recently devoted a detailed study to presbyopia [1] its mechanism will not be considered here.

Accommodation, Ocular↗
PubMed1995

Variations in refractive index and absorbance of 670-nm light with age and cataract formation in human lenses.

A newly-developed fibre-optic sensor (wavelength of 670 nm) was used to measure the index gradient in 16 human lenses ranging in age from 38 to 78 years. Two of these lenses had nuclear cataract. The refractive index, calculated from the proportion of reflected light at the sensor/sample interface, appeared to increase with age and further with cataract formation. This was not consistent with previous studies. The increased losses of reflected light, which gave apparent increased values of refractive index, were reconsidered to be resulting from light absorption and an estimate of relative absorption was made by deducting excess light loss from a base level expected to yield the maximum index value. The characteristics of the relative absorption are similar to those of a red fluorophor reported in other studies.

Adult↗
PubMed1995

Age-related response of human lenses to stretching forces.

Five human lenses of varying ages were subjected to radial stretching forces which mimic the action of the ciliary muscle in vivo. Although the number of lenses was small, it was found that the human lens becomes more resistant, with age, to radial stretching forces and that lenses over the age of 50 years showed very little response to stretch. In prepresbyopic lenses, stretching has a greater effect on the shape of the anterior surface than it does on the shape of the posterior surface. The five lenses studied showed individual variations in lens size and shape and a trend to increased curvature with age, particularly marked for the anterior surface.

Accommodation, Ocular↗
PubMed1995

The refractive index along the optic axis of the bovine lens.

This study aimed to measure the refractive index values along the optic axes of adult bovine lenses. An optic fibre reflectometric sensor was used for this purpose. Seventeen lenses were examined and over the weight range studied there were no apparent age-related trends in the index gradient. The peak index values varied from 1.432 to 1.442. There was a greater variation in the edge index values, which may have reflected differences in hydration on lens surfaces or may be indicative of a slight decrease in surface index with age.

Animals↗
PubMed1995

Polarising light biomicroscopy and the relation between visual acuity and cataract.

The use of polarised light in biomicroscopy allows discrimination of lenticular features which are not discernible using standard biomicroscopic techniques. However, just as is true of most brightness-dependent photographic methods testing the effect of filters, past uses of polarised light did not involve any control of illumination. The present study involved the use of polarised light in the photo-biomicroscopy of the anterior segment in cataract patients, the consequential loss in illumination being mimicked with different neutral density filters. This served to distinguish between polarisation and attentuation. The patients were refracted and their corrected visual acuity was recorded. A qualitative scale was drawn up ranking weights of lenticular opacities. Spearman's rank correlation was applied to the photographs obtained with conventional and polarised-light biomicroscopy in turn. Brightness control served to confirm that biomicroscopy with polarised light offers some advantages over conventional methods, and correlates with visual acuity better than is true of conventional biomicroscopy. The Spearman ranks derived from the results obtained with polarised light correlate significantly with the patient's visual acuity. It is concluded that polarised light biomicroscopy, in conjunction with the standard examination, can improve the diagnosis of cataract and assist in relating it to a patient's visual acuity.

Aged↗
PubMed1995

The refractive index and protein distribution in the blue eye trevally lens.

BACKGROUND: The relationship between structure (crystallin distribution) and function (refractive index) in the lens is not understood and can be studied by comparing biochemical and optical properties. Such a comparison has been made using a blue eyed trevally lens. METHODS: The optical parameter of refractive index distribution was determined using a nondestructive ray tracing technique. The distributions of the various classes of proteins in the lens were determined by dissolving lenses in concentric layers and using biochemical protein assay. HPLC and SDS-PAGE electrophoresis were used to investigate the proportion of proteins in each layer. RESULTS: The refractive index distribution, from center to edge, follows a second order polynomial. The proteins do not vary in their proportions over most of the lens; only in the inner-most regions is there a rapid increase in insoluble protein and a concomitant decrease in the soluble protein classes. The smallest proteins (gamma crystallins) become insoluble later than the alpha- and beta-crystallins. CONCLUSIONS: There are no similarities in the distributions of any of the protein classes to that of the refractive index in the fish lens. This result indicates that a quantitative relationship cannot be derived by comparing protein to refractive index distributions. However, the findings are consistent with those made in other species: a high content of gamma-crystallins is always found in lenses which have steep refractive index gradients and high index magnitudes.

Animals↗