In vivo ultraosound elastographic evaluation of the age-related change of human lens nuclear stiffness.

BACKGROUND: To evaluate the age-related changes in the stiffness of the human lens nucleus in vivo. METHODS: A total of 78 volunteers with best-corrected visual acuity of 20/20with a mean ± standard deviation intraocular pressure (IOP) of 16 ± 2.5 mmHg were divided into 3 groups of 26. The mean ages of Groups A, B and C were 81 ± 5.5, 44 ± 3.2 and 21 ± 2.5 years, with mean axial lengths of 23.8 ± 0.5 mm, 23.8 ± 0.4 mm and 23.9 ± 0.3 mm, respectively. Using an elastographer, the ultrasound echolucency and elastic strain rate of the lens nucleus of one eye, selected randomly, of each subject were measured three times. The strain rate of the lens cortex could not be assessed. The qualitative differences in the strain rates across the groups were assessed, and differences in the strain rate ratios of the lens nuclei across groups were analysed by one-way ANOVA. RESULTS: The strain rates of the lens nuclei of Group A were much lower than those in Groups B and C, as assessed qualitatively; the elastograph images of the lens nuclei of the older group showed a blue colour.The strain rate ratios of the lens nuclei of Groups A, B and C were 0.02 ± 0.08, 0.69 ± 0.12 and 1.95 ± 0.85, respectively. The differences in the lens nucleus strain rate ratios across the groups were statistically significant, with p-values < 0.05. CONCLUSIONS: Ultrasound elastography demonstrated in vivo that an older age is associated with a statistically significantly lower lens nucleus strain rate ratio and therefore a markedly higher lens nuclear stiffness.

Shear modulus measurements on isolated human lens nuclei.

The use of a spinning lens test to determine ex vivo the shear modulus of 22 isolated human lens nuclei with ages ranging from 34 to 63 years is described. In this test procedure, the lens nucleus is spun about its polar axis. Images of the nucleus viewed from directions perpendicular to the polar axis are collected; these are used to quantify the deformations induced in the nucleus by the rotational motion. Data on these deformations are used to infer, by applying finite element inverse analysis, values for the shear modulus of the nucleus. The data on shear modulus obtained from this test program indicate that the nucleus stiffens very rapidly with age. These data are shown to compare well with the results of a related study (Wilde et al., 2012) in which the shear modulus of the nucleus is determined by similar spinning lens tests conducted on the entire lens substance.

Microindentation of the young porcine ocular lens.

Debate regarding the mechanisms of how the eye changes focus (accommodation) and why this ability is lost with age (presbyopia) has recently been rejoined due to the advent of surgical procedures for the correction of presbyopia. Due to inherent confounding factors in both in vivo and in vitro measurement techniques, mechanical modeling of the behavior of the ocular lens in accommodation has been attempted to settle the debate. However, a paucity of reliable mechanical property measurements has proven problematic in the development of a successful mechanical model of accommodation. Instrumented microindentation was utilized to directly measure the local elastic modulus and dynamic response at various locations in the lens. The young porcine lens exhibits a large modulus gradient with the highest modulus appearing at the center of the nucleus and exponentially decreasing with distance. The loss tangent was significantly higher in the decapsulated lens and the force waveform amplitude decreased significantly upon removal of the lens capsule. The findings indicate that localized measurements of the lens' mechanical properties are necessary to achieve accurate quantitative parameters suitable for mechanical modeling efforts. The results also indicate that the lens behaves as a crosslinked gel rather than as a collection of individual arched fiber cells.

Age-related compaction of lens fibers affects the structure and optical properties of rabbit lenses.

BACKGROUND: The goal of this investigation was to correlate particular age-related structural changes (compaction) to the amount of scatter in rabbit lenses and to determine if significant fiber compaction occurred in the nuclear and inner cortical regions. METHODS: New Zealand White rabbits at 16-20 months old (adult; n = 10) and at 3.5-4 years old (aged; n = 10) were utilized for this study. Immediately after euthanising, scatter was assessed in fresh lenses by low power helium-neon laser scan analysis. Scatter data was analyzed both for whole lenses and regionally, to facilitate correlation with morphometric data. After functional analysis, lenses were fixed and processed for scanning electron microcopy (SEM; right eyes) and light microscopy (LM; left eyes). Morphometric analysis of SEM images was utilized to evaluate compaction of nuclear fibers. Similarly, measurements from LM images were used to assess compaction of inner cortical fibers. RESULTS: Scatter was significantly greater in aged lenses as compared to adult lenses in all regions analyzed, however the difference in the mean was slightly more pronounced in the inner cortical region. The anterior and posterior elliptical angles at 1 mm (inner fetal nucleus) were significantly decreased in aged vs. adult lenses (anterior, p = 0.040; posterior, p = 0.036). However, the average elliptical angles at 2.5 mm (outer fetal nucleus) were not significantly different in adult and aged lenses since all lenses examined had comparable angles to inner fetal fibers of aged lenses, i.e. they were all compacted. In cortical fibers, measures of average cross-sectional fiber area were significantly different at diameters of both 6 and 7 mm as a function of age (p = 0.011 and p = 0.005, respectively). Accordingly, the estimated fiber volume was significantly decreased in aged as compared to adult lenses at both 6 mm diameter (p = 0.016) and 7 mm diameter (p = 0.010). CONCLUSION: Morphometric data indicates that inner cortical fibers undergo a greater degree of age-related compaction than nuclear fibers. Increased scatter appears to be only tentatively correlated with regions of fiber compaction, suggesting that it is simply one of an array of factors that contribute to the overall decreased transparency in aged rabbit lenses.

The shape of the human lens nucleus with accommodation.

Knowledge about geometric properties such as shape and volume and Poisson's ratio of the nucleus can be used in the mechanical and optical modeling of the accommodation process. Therefore, Scheimpflug imaging was used to determine the shape of the human lens nucleus during accommodation in five subjects. To describe the shape of the nucleus, we fitted a parametric model of the cross-sectional geometry to the gradient of the Scheimpflug images using the Hough transform. The geometric model made it possible to estimate the anterior and the posterior central radius, central thickness, equatorial diameter, and cross-sectional area of the nucleus. Assuming that the nucleus is rotationally symmetric, the volume of the nucleus can be estimated by integrating around the circumference. For all five subjects, the results show that during accommodation the nucleus became more convex and that the central thickness increased whereas the equatorial diameter decreased. This decrease in equatorial diameter of the nucleus with accommodation is in accordance with the Helmholtz accommodation theory. Finally, the volume of the nucleus (on average 35 mm(3)) showed no significant change during accommodation in any of the subjects, presumably due to the fact that the human nucleus consists of incompressible material with a Poisson's ratio that is near .5.

Dietary linolenic acid intake is positively associated with five-year change in eye lens nuclear density.

OBJECTIVE: Dietary fat may affect lens cell membrane composition and function, which are related to age-related cataract. The present study was designed to examine the associations between dietary fat and the change in nuclear lens opacification over five years. METHODS: Women aged 52 to 73 years without previously diagnosed cancer, diabetes and cataracts from the Boston, Massachusetts area were selected from the Nurses' Health Study cohort. Four hundred forty women participated in a baseline (1993-95) and a follow-up (1998-2000) eye examination. Intakes of total fat and selected fatty acids were calculated as the average intake from five food frequency questionnaires that were collected between 1980 and baseline. Change in the degree of nuclear density (opacification) was characterized by the difference between baseline and follow-up in pixel density at the central clear zone in the Scheimpflug slit image of the lens. RESULTS: Intake of alpha-linolenic acid (ALA) was positively associated with change in nuclear density. The geometric mean nuclear density change was 16% greater in the highest quartile category of ALA intake than in the lowest quartile category (P for trend = 0.05). For women in the high tertile category of baseline nuclear lens opacification, the geometric mean change in the highest quartile category of ALA acid intake was 70% higher than the change in the lowest quartile category (P for trend = 0.01). There were no significant associations between other dietary fats and change in nuclear density. CONCLUSION: Higher ALA intake was associated with a greater age-related change in lens nuclear density.

Insights into the age-related decline in the amplitude of accommodation of the human lens using a non-linear finite-element model.

AIM: To understand the effect of the geometric and material properties of the lens on the age-related decline in accommodative amplitude. METHODS: Using a non-linear finite-element model, a parametric assessment was carried out to determine the effect of stiffness of the cortex, nucleus, capsule and zonules, and that of thickness of the capsule and lens, on the change in central optical power (COP) associated with zonular traction. Convergence was required for all solutions. RESULTS: Increasing either capsular stiffness or capsular thickness was associated with an increase in the change in COP for any specific amount of zonular traction. Weakening the attachment between the capsule and its underlying cortex increased the magnitude of the change in COP. When the hardness of the total lens stroma, cortex or nucleus was increased, there was a reduction in the amount of change in COP associated with a fixed amount of zonular traction. CONCLUSIONS: Increasing lens hardness reduces accommodative amplitude; however, as hardness of the lens does not occur until after the fourth decade of life, the age-related decline in accommodative amplitude must be due to another mechanism. One explanation is a progressive decline in the magnitude of the maximum force exerted by the zonules with ageing.

Can reliable values of Young's modulus be deduced from Fisher's (1971) spinning lens measurements?

The current textbook view of the causes of presbyopia rests very largely on a series of experiments reported by R.F. Fisher some three decades ago, and in particular on the values of lens Young's modulus inferred from the deformation caused by spinning excised lenses about their optical axis (Fisher 1971) We studied the extent to which inferred values of Young's modulus are influenced by assumptions inherent in the mathematical procedures used by Fisher to interpret the test and we investigated several alternative interpretation methods. The results suggest that modelling assumptions inherent in Fisher's original method may have led to systematic errors in the determination of the Young's modulus of the cortex and nucleus. Fisher's conclusion that the cortex is stiffer than the nucleus, particularly in middle age, may be an artefact associated with these systematic errors. Moreover, none of the models we explored are able to account for Fisher's claim that the removal of the capsule has only a modest effect on the deformations induced in the spinning lens.

Massive increase in the stiffness of the human lens nucleus with age: the basis for presbyopia?

PURPOSE: To determine the stiffness of different regions of human lenses as a function of age, and to correlate the biophysical measurements in the lens center with nuclear water content. METHODS: A custom made probe fitted to a dynamic mechanical analyzer was employed to measure stiffness values at 1 mm increments across equatorial sections of individual human lenses. Thermogravimetric analysis was used to determine the percentage water content in the nuclei of human lenses. RESULTS: There was a pronounced increase in lens stiffness over the age range from 14 to 78. In the nucleus, stiffness values varied almost 1,000 fold over this age range, with the largest change observed in lenses between the ages of 20 to 60. Nuclear stiffness values increased on average by a factor of 450. By contrast, in the cortex the average increase in stiffness was approximately 20 fold over this same time period. In lenses younger than age 30, the nucleus was found to be softer than the cortex. This was true for all six lenses examined. In contrast all lenses older than 30 were characterized by having nuclear values higher than those of the cortex. In lenses over the age of 50, the lens nucleus was typically an order of magnitude more rigid than that of the cortex. The crossover age, when the cortical and nuclear stiffness values were similar, was in the 30s. There was no significant change in the water content of the human lens nucleus from age 13 to age 82. CONCLUSIONS: There is a marked increase in the stiffness of the human lens with age. This is most pronounced in the nucleus. Since in vivo data indicate that the nucleus must change shape significantly during accommodation, it is highly likely that these measured changes in physical properties will markedly diminish the ability of the lens to accommodate, and thus may be a major contributing factor to presbyopia. Since there was no measurable difference in the water contents of the nuclear regions of the lenses, this marked increase in stiffness is not due to compaction of the lens nucleus.

Changes in the internal structure of the human crystalline lens with age and accommodation.

Scheimpflug images were made of the unaccommodated and accommodated right eye of 102 subjects ranging in age between 16 and 65 years. In contrast with earlier Scheimpflug studies, the images were corrected for distortion due to the geometry of the Scheimpflug camera and the refraction of the cornea and the lens itself. The different nuclear and cortical layers of the human crystalline lens were determined using densitometry and it was investigated how the thickness of these layers change with age and accommodation. The results show that, with age, the increase in thickness of the cortex is approximately 7 times greater than that of the nucleus. The increase in thickness of the anterior cortex was found to be 1.5 times greater than that of the posterior cortex. It was also found that specific parts of the cortex, known as C1 and C3, showed no significant change in thickness with age, and that the thickening of the cortex is entirely due to the increase in thickness of the C2 zone. With age, the distance between the sulcus (centre of the nucleus) and the cornea does not change. With accommodation, the nucleus becomes thicker, but the thickness of the cortex remains constant.

Changes in the refractive index of lens fibre membranes during maturation--impact on lens transparency.

PURPOSE: Local variations in refractive index are the physical cause of light scattering in a material or tissue and also induce phase changes of propagating light waves. The goal of this study was to analyse local differences in refractive index by phase contrast microscopy of sections of human lenses. METHODS: Refractive index was estimated by immersion refractometry. Cryo-sections of quick-frozen human donor lenses were embedded in a graded series of bovine serum albumin solutions, and in immersion oil, ranging in refractive index from 1.34 to 1.52. RESULTS: Fibre membranes in the lens cortex prove to have a refractive index considerably above that of fibre cytoplasm at the same location. Fibre membranes in the lens nucleus have a refractive index approximately the same as that of fibre cytoplasm at the same location. CONCLUSION: In the lens cortex, transparency is obtained by a high spatial order of the lens fibre lattice to compensate for the light scattering caused by differences in refractive index between fibre membranes and cytoplasm. In the lens nucleus, high spatial order is less important, because the minor differences in the refractive index between fibre membranes and fibre cytoplasm lead only to minimal scattering.

Numerical modelling of the accommodating lens.

Data on geometric and material properties of the human lens derived from various published sources are used to construct axisymmetric, large displacement, finite element models of the accommodating lens of subjects aged 11, 29 and 45 years. The nucleus, cortex, capsule and zonule are modelled as linearly elastic materials. The numerical model of the 45-year lens is found to be significantly less effective in accommodating than the 29-year lens, suggesting that the modelling procedure is capable of capturing at least some of the features of presbyopia. The model of the 11-year lens shows some anomalous behaviour, and reasons for this are explored.

Aging of the human lens: changes in lens shape at zero-diopter accommodation.

Scheimpflug photographs of the zero-diopter-accommodated anterior segments of 100 human subjects, aged 18 to 70 yr and evenly spaced over this range, were digitized and analyzed to characterize lens and lens nucleus shape as a function of age by the Hough transform and other image analysis methods. Anterior and posterior lens surface curves exhibit a decrease in radius of curvature with increasing age, in qualitative but not quantitative agreement with the earlier observations of Brown [Exp. Eye Res. 19, 175 (1974)]. In contrast, the shape of the lens nuclear boundaries changes little with age. Overall lens volume at zero diopters increases with age, but the volume of the lens nucleus remains unchanged. The lens center of mass moves anteriorly with increasing age, as does the central clear region of the lens. Although these data sets were found to be more variable than those of Brown, the complementary variability of other factors, such as anterior chamber depth, for each subject leads to a very high statistical correlation between lens shape and lens location relative to the cornea. This supports the finding of previous work that image formation on the retina for a given individual results from the multifactorial balancing of related factors.

Estimation of anterior nucleus of lens by Scheimpflug image before and after pupil dilatation.

PURPOSE: To compare the accuracy of lens transparency evaluations by Scheimpflug image in the anterior nucleus of the lens before and after pupil dilatation. METHODS: Scheimpflug lens images were recorded in 70 eyes of 38 subjects (age: 28-75 years) before and after pupil dilatation, and light scattering intensity measurements before and after dilatation were compared. RESULTS: There was a significant positive correlation between the light scattering intensity before and after dilatation at the anterior cortex, anterior nucleus, and central clear zone of the lens (r > 0.9, P <.0001). CONCLUSIONS: It is possible to estimate the transparency in the anterior nucleus of the lens from the Scheimpflug image without pupil dilatation. If nuclear type cataracts are regarded as a structural marker of aging in epidemiological studies, measuring the light scattering intensity in the anterior nucleus of the lens without dilatation seems to be a safe, useful, and quantitative method.

[A study of coloring in human lens nucleus--association of four inorganic elements and dielectric behavior with nuclear color].

We classified senile cataractous lenses before surgery into five grades on the basis of their nuclear color and analyzed the extracted nuclei on wet weight, water content (weight %), inorganic element content (sodium, potassium, calcium and magnesium), and dielectric behavior. With increase in the grade, water content (p < 0.01) and potassium content (p < 0.05) significantly decreased, and wet weight (p < 0.01) and sodium (p < 0.05), calcium (p < 0.01) and magnesium (p < 0.001) content significantly increased. In dielectric behavior, limiting conductivity at low frequencies significantly decreased with increase in the grade (p < 0.05), and limiting permittivity at high frequencies showed a trend to decrease, but it was not significant (p = 0.052). These results indicate that changes in water content and in inorganic element content are associated with advancement in color grade, suggesting a close relation between the color grade and the membrane functions of lens fibers. In the nuclei with high color grade, we concluded that part of the active transport systems of the fiber membranes was damaged, although the electrical barrier of the membranes was retained.

Electrical impedance tomography of the eye: in vitro measurements of the cornea and the lens.

This study was designed to perform multifrequency impedance measurements on the pig's eye. On one hand, impedance of the ocular tissues was measured from 10 kHz to 10 MHz. The aqueous and vitreous humours and the cornea showed no relaxation in this range of frequencies, whereas the lens and its parts (the cortex and the nucleus) did. On the other hand, multifrequency EIT and dynamic imaging were performed on the lens and on the whole eye. Data and images obtained after thermal and chemical injuries are presented and discussed.

Mechanical testing of isolated senile human eye lens nuclei.

Phacoemulsification is a cataract surgery technique during which the eye lens nucleus is carefully dissected by an oscillating hollow needle simultaneously serving as a suction line for lens fragments. As the success of this procedure depends greatly on the mechanical properties of the nucleus, the basic mechanical properties of extracted senile human eye lens nuclei were studied. For this purpose a novel measuring system had to be developed enabling the study of the biomaterial's responses to any chosen excitation function. Either force- or displacement-controlled testing signals can be selected serving as a one-dimensional input. The resultant quantity can be recorded in the time or frequency domain. An almost linear viscoelastic behavior of lens nuclei is observed. Typical phenomena such as hysteresis, creep, relaxation, and a frequency-dependent compliance are evaluated. The ultimate strength of the specimen is studied by penetration tests. Finally the possible correlation between mechanical and optical parameters of lens nuclei is discussed. If correlated, a surgeon could use this information when deciding whether an extracapsular extraction or a phacoemulsification should be the preferred approach.

In vivo light scattering intensity in the lens versus in vitro spectral transmission in the nuclear region.

In order to understand the correlation between nuclear opacity and spectral light transmission factor, the in vivo light scattering intensity of Scheimpflug lens images and the light transmission through slices of surgically removed nucleus were examined densitometrically as well as spectrophotometrically respectively, using both human and Brown Norway rat lenses. The experiments using rat lenses showed that the increase of the light scattering intensity in the nuclear regions is correlated with a decrease of the spectral transmission factor. The transparent anterior cortical region did not influence the light transmission (T%) through the nucleus. No significant influence was seen from the application of freezing and thawing procedures to measure light transmission. The examinations of the human subjects which were based on the rat study showed that the light transmission of nuclear cataracts objectively evaluated as grade I was almost the same as that observed in noncataractous nuclei. The light transmission of nuclear cataracts graded II or above, however, showed around 50% of the spectral light transmission of noncataractous nuclei.

[A real-time analysis of 133 normal lens images by a computer].

According to the theory that the light scattering of the lens is correlated with the lens density, we designed a computerized system for the real-time analysis of the lens image. By measurement of the gray scale value of the lens optical section, the lens density is measured objectively and numerically. With the help of the system, 133 normal lens images were investigated. From the analysis of various age groups, we found that the grey scale values of the anterior cortex, posterior cortex and nucleus increase with the increase of age (P < 0.01), the grey scale values of the anterior and posterior cortex are significantly higher than the value of the nucleus (P < 0.05), but there is no significant difference between the values of the anterior and posterior cortex (P > 0.05), and there is also no significant difference in the comparisons of the grey scale values of the respective corresponding lens areas between the male and female.

Spatial distribution of back scattering in the nuclear area of the non-cataractous human lens.

The spatial distribution of back scattering from a vertical cross-section of the nuclear area in non-cataractous human lens was investigated. The cross-section was centred on the pupil. There is no significant difference in average nuclear back scattering between sides. The distribution of back scattering along an axis perpendicular to the anatomical axis can be modelled as a second-order polynomial. It is believed that the increase of back scattering in the central region corresponds to the peak protein concentration in the centre of the nucleus. This information was obtained by imaging back scattering with Scheimpflug photography. The back scattering was measured photometrically in a 3 x 10 matrix of measuring areas within the lens nucleus. Each measuring area was 0.20 x 0.20 mm in size. It is anticipated that the established model will allow detection of nuclear cataract with high sensitivity, especially in early stages.