Lens Equatorial Diameter Research Articles

Rotational Stability, Decentration, and Tilt of a New Hydrophobic Acrylic Intraocular Lens Platform

To evaluate rotational stability, decentration and tilt of the monofocal intraocular lens (IOL) Nanex (NC1-SP; HOYA Surgical Optics). Prospective interventional case series. The study was performed at the Department of Ophthalmology, Medical University of Vienna. The study population comprised 130 eyes of 68 patients with age-related cataract who underwent cataract surgery with implantation of a Nanex IOL. Baseline image for rotational stability evaluation was obtained at the end of surgery (EoS) and compared to retroillumination images taken at 1 hour, 1 week, 1 month, and 6 months after surgery. Axis alignment was assessed using nonmovable landmarks on the sclera and the optic-haptic junctions of the IOL. Anterior segment OCT images were performed to evaluate decentration and tilt. The main outcome measure was absolute rotation from EoS to 6 months postoperatively. Median IOL rotation of all eyes from EoS to 6 months was 1.9° (interquartile range 0.1°-37.5°). Ten eyes (9.71%) rotated more than 5° and 2 eyes (1.94%), more than 10°. IOL rotation did not correlate with axial eye length (Spearman r=-0.042, P=.46), crystalline lens thickness (Spearman r=0.134, P=.19), and crystalline lens equatorial diameter (Spearman r=0.101, P=.325). IOL rotation positively correlated with anterior fibrosis severity (Spearman r=0.321, P=.002). Preoperative decentration (0.2 ± 0.12 mm) and tilt (5.7 ± 1.6°) did not change significantly after surgery (0.22 ± 0.12 mm and 5.62 ± 1.49°, respectively). The investigated IOL presented good rotational stability and low decentration and tilt values. Nevertheless, anterior capsule fibrosis development led to a higher tendency of IOL rotation after 1 week.

Intraoperative Measurement of Crystalline Lens Diameter in Living Humans.

There are no reports on accurate measurement of lens equatorial diameter of the living human eye. This study aimed to measure lens equatorial diameter with a special measurement device during cataract surgery and examine the relationships with preoperative parameters. From April 7 to December 1, 2019, the equatorial diameters of 24 eyes from 24 patients who underwent cataract surgery at Kushimoto Arita Hospital were measured with a loop shaped measurement device during cataract surgery. Correlations between the value of the diameters and various preoperative parameters measured by CASIA2® were evaluated. The average value of the measured equatorial diameter using the device was 10.5 ± 0.4 mm and the value estimated by circular approximation using the CASIA2®️ was 10.1 ± 0.7 mm. A significant difference was observed between these two groups (P = 0.016), and only a weak correlation was observed (γ = 0.31). A positive correlation was observed between equatorial diameter and anterior chamber depth (ACD) or anterior chamber width (ACW) (γ = 0.57 and 0.47, respectively). No significant correlation was found between other parameters and the value measured by the device. Our method is a completely new approach to measuring the living human lens equatorial region of the eye. No complications were observed in any of the cases. One new finding was the values of the lens equatorial diameters are actually longer than those reported previously. The results suggest that the values of the equatorial diameter measured by the loop device and those estimated by CASIA2® measurement were closer than those reported previously by other methods, although both were slightly different. We conclude that it is still difficult to estimate the equatorial diameter of the living human lens using preoperative examination parameters. This research will greatly contribute to the development of accommodative intraocular lenses in the future.

Effect of age and cycloplegia on the morphology of the human crystalline lens: swept-source OCT study.

To evaluate the effect of age and cycloplegia on the morphology of the crystalline lens using a swept-source optical coherence tomography (SS-OCT) system. Hospital. Prospective cross-sectional study. The parameters including anterior chamber depth (ACD), the radii of curvature of the anterior and posterior surface of the crystalline lens (ALR and PLR), lens thickness (LT), lens equatorial diameter (LED), and lens vault (LV) were quantified by the SS-OCT before and after cycloplegia. The paired t test was used to compare the parameters before and after cycloplegia. A multivariate linear regression model was built to analyze the association between the parameters/cycloplegia-induced changes and age, while adjusting for the effect of axial length, refractive status, and sex. 76 individuals (age range, 18 to 86 years) were recruited. The ALR and ACD were negatively correlated with age (P ≤ .002), and the LT, LV, and LED were positively correlated with age (P ≤ .004). In participants younger than 60 years, the ALR and ACD significantly increased, whereas the LV and LT significantly decreased after cycloplegia (all P < .001). With aging, cycloplegia-induced differences of ALR (P = .001) and ACD (P = .014) significantly decreased, and of LT (P < .001), LT (P < .001), and LV (P = .001) significantly increased. The crystalline lens morphology measured by the SS-OCT revealed steepening anterior surface and increasing equatorial diameter with age. Cycloplegia caused a significant change of anterior surface morphology in participants younger than 60 years, and this effect diminished with age.

A numerical investigation of changes in lens shape during accommodation

The purpose of this study was to investigate how the mechanical properties and geometry of the lens influence the changes in lens shape during accommodation. To do so, ex vivo stretching tests of the isolated lens were simulated via finite element analysis. In these tests, the lens is stretched from the accommodated state to the non-accommodated state. Several key characteristics of the lens were studied: the stiffness gradient of the lens material, the distribution of the capsule thickness, the mechanical properties of the capsule and the material comprising the lens, nucleus and cortex, and the influence of two different age-related lens geometries (17 and 29 y/o subjects). To determine the effects on the changes in lens shape during accommodation, changes in the anterior and posterior radius, the lens and nucleus thicknesses and the equatorial lens diameter were analysed. The results suggest that multiple factors exert statistically significant influences on how the lens changes its shape, but two factors predominate over the rest: the stiffness ratio between the nucleus and cortex and the stiffness of the capsule, specifically the posterior surface.

Isolated human crystalline lens three-dimensional shape: A comparison between Indian and European populations

There have been many studies on lens properties in specific populations (e.g. in China, Europe, Singapore, etc.) some of which suggest there may be differences between populations. Differences could be caused by ethnic or environmental influences or experimental procedures. The purpose of this study is to evaluate if any differences exist between Indian and European populations in the central geometric and full shape properties of human lenses. Two custom-developed spectral domain optical coherence tomography systems were used to acquire the crystalline lens geometry: one in India (69 lenses from 59 donors) and the other in Spain (24 lenses from 19 donors). The steps for obtaining accurate 3-D models from optical coherence tomography raw images comprised of image segmentation, fan and optical distortion correction, tilt removal and registration. The outcome variables were lens equatorial diameter, lens thickness, anterior and posterior lens thicknesses and their ratio, central radius of curvature of the anterior and posterior lens surfaces, lens volume and lens surface area. A mixed effects model by maximum likelihood estimation was used to evaluate the effect of age, population and their interaction (age*population) on lens parameters. After adjusting for age, there were no population differences observed in anterior and posterior radii of curvature, equatorial diameter, lens thickness, anterior and posterior lens thicknesses and their ratio, volume and surface area (all p ≥ 0.08). There was also no effect of the interaction term on anterior and posterior radii of curvature, equatorial diameter, lens thickness, anterior and posterior lens thicknesses and their ratio, volume and surface area (all p ≥ 0.06). All central geometric and full shape parameters appeared to be comparable between the European and Indian populations. This is the first study to compare geometric and full shape lens parameters between different populations in vitro.

Deformations and Ruptures in Human Lenses With Cortical Cataract Subjected to Ex Vivo Simulated Accommodation.

PurposeHuman cortical opacities are most commonly accompanied by changes in lens fiber structure in the equatorial region at the lens nucleus–cortex interface. Cortex and nucleus have different elastic properties, which change with age. We therefore subjected ex vivo lenses to simulated accommodation and studied the internal deformations to better understand the mechanism of cortical cataract formation.MethodsNine human donor lenses (33–88 years old) were tested using a bespoke radial stretching device for anterior eye segments. Seven of the lenses exhibited cortical cataracts. The other two lenses, without cataract, were used as controls. Frontal and cross-sectional images of the lens obtained during stretching facilitated measurements on equatorial lens diameter and central lens thickness in the stretched and unstretched states.ResultsStretching caused the lens equatorial diameter to increase in all cases. Conversely, the lens central thickness showed no systematic variation during stretching. For four of the lenses with cortical cataract, ruptures were observed during stretching at the nucleus–cortex boundary adjacent to the cortical cataracts. Ruptures were not observed in the control lenses or in the three other lenses with cortical cataract.ConclusionsInternal ruptures can occur in aged ex vivo lenses subjected to simulated disaccommodation. These ruptures occur at the nucleus–cortex interface; at this location, a significant stiffness discontinuity is expected to develop with age. It is hypothesized that ruptures occur in in vivo lenses during accommodation—or attempted accommodation.

Panoramic Observation of Crystalline Lenses with 25 MHz Ultrasonography.

Purpose To visualize and assess in vivo the age-related changes in crystalline lens size and contour. Methods Seventy-nine healthy volunteers, 39 females and 40 males, with a mean age of 41.53 + 11.32 years (range: 21 to 60 years) were enrolled in this study. The axial lens thickness (ALT), equatorial lens diameter (ELD), and anterior (Ra) and posterior (Rp) lens surface radii of curvatures of the subjects' left eyes were measured with a 25 MHz ultrasound probe. Results The mean ALT and ELD were 4.178 mm + 0.288 mm and 9.209 mm + 0.214 mm, respectively. There was a statistically significant increase in both ALT (slope = 11 μm/year, r = 0.88, p < 0.01) and ELD (slope = 6 μm/year, r = 0.60, p < 0.01) with age. Ra negatively correlated, and Rp did not change with age. Conclusion There were no statistically significant relationships between any studied values and gender. Independent of gender, the lens grows equatorially and axially with age while its central anterior lens surface steepens and its posterior central surface curvature does not change.

Morphological changes of human crystalline lens in myopia.

Ocular biometric parameters, including full shape crystalline lens, were assessed in myopes and emmetropes using 3-D optical coherence tomography. The anterior chamber depth, the radius of the curvature of the anterior cornea, anterior lens, and posterior lens, lens thickness, lens equatorial diameter, surface area, equatorial position, volume, and power, were evaluated as functions of refractive errors and axial lengths while controlling for age effects. The crystalline lens appears to change with myopia consistent with lens thinning, equatorial, and capsular stretching while keeping constant volume. Axial elongation appears counteracted by a crystalline lens power reduction, while corneal power remains unaffected.

Change in human lens dimensions, lens refractive index distribution and ciliary body ring diameter with accommodation.

We investigated changes in ciliary body ring diameter, lens dimensions and lens refractive index distributions with accommodation in young adults. A 3T clinical magnetic resonance imaging scanner imaged right eyes of 38 18-29 year old participants using a multiple spin echo sequence to determine accommodation-induced changes along lens axial and equatorial directions. Accommodation stimuli were approximately 1 D and 5 D. With accommodation, ciliary body ring diameter, and equatorial lens diameter decreased (-0.43 ± 0.31 mm and -0.30 ± 0.23 mm, respectively), and axial lens thickness increased ( + 0.34 ± 0.16 mm). Lens shape changes cause redistribution of the lens internal structure, leading to change in refractive index distribution profiles. With accommodation, in the axial direction refractive index profiles became flatter in the center and steeper near the periphery of the lens, while in the equatorial direction they became steeper in the center and flatter in the periphery. The results suggest that the anatomical accuracy of lens optical models can be improved by accounting for changes in the refractive index profile during accommodation.

The effect of varying glucose levels on the ex vivo crystalline lens: implications for hyperglycaemia-induced refractive changes.

Refractive changes in diabetic eyes have long been reported but with equivocal results. The lens has been a more recent focus as the source of any change but it is possible that multiple sources of variation have made it difficult to demonstrate a systematic change clinically. The aim of this study was therefore to use a bovine lens model to investigate the optical changes in hyperglycaemia and when lenses are returned to normal glucose levels as would occur following commencement of treatment. Bovine eyes were obtained and their lenses excised under sterile conditions before placing them in culture medium within an incubator using standard tissue culture techniques. In the first experiment, lenses were transferred into culture medium containing 5mm (n=12), 15mm (n=12) and 30mm (n=12) glucose. Measurements were made of the change in back vertex focusing distance with equatorial lens diameter using the ScanTox(™) measurement system. From these measurements, the back vertex focal length and primary longitudinal spherical aberration were derived. In a second experiment, lenses maintained at 30mm glucose (n=7) were stepped down to 5mm glucose to simulate starting diabetic therapy and measured in the same way. Changes over time were assessed with a linear regression model. A trend towards myopia was observed with increasing hyperglycaemia, this was not statistically significant. When lenses were stepped-down from hyperglycaemia to normal physiological levels of glucose, a hyperopic shift was observed in line with published clinical studies that again failed to reach statistical significance. High variability in the measurement on longitudinal spherical aberration prevented any significant trends being measured. Our results suggest that there are no consistent crystalline lens-induced refractive changes following exposure to hyperglycaemia for time-periods up to 5days used in the current study. It is possible that bovine lenses are able to offset the raised osmotic pressure from high glucose levels in the short-term by a process of osmoregulation and that repeated osmotic stress or longer term exposure may be required to induce the changes in refraction that are seen clinically.

Comparison between carbachol iontophoresis and intravenous pilocarpine stimulated accommodation in anesthetized rhesus monkeys

Rhesus monkeys are an animal model for human accommodation and presbyopia and consistent and repeatable methods are needed to stimulate and measure accommodation in anesthetized rhesus monkeys. Accommodation has typically been pharmacologically stimulated with topical pilocarpine or carbachol iontophoresis. Intravenous (i.v.) pilocarpine has recently been shown to produce more natural, rapid and reproducible accommodative responses compared to topical pilocarpine. Here, i.v. pilocarpine was compared to carbachol iontophoresis stimulated accommodation. Experiments were performed under anaesthesia on five previously iridectomized monkeys aged 10–16 years. In three monkeys, accommodation was stimulated with carbachol iontophoresis in five successive experiments and refraction measured with a Hartinger coincidence refractometer. In separate experiments, accommodation was stimulated using a 5 mg/kg bolus of i.v. pilocarpine given over 30 s followed by a continuous infusion of 20 mg/kg/hr for 5.5 min in three successive experiments with the same monkeys as well as in single experiments with two additional monkeys. Refraction was measured continuously using photorefraction with baseline and accommodated refraction also measured with the Hartinger. In subsequent i.v. pilocarpine experiments with each monkey, accommodative changes in lens equatorial diameter were measured in real-time with video-image analysis. Maximum accommodation of three monkeys with carbachol iontophoresis (five repeats) was (mean ± SD; range) 14.0 ± 3.5; 9.9–20.3 D and with i.v. pilocarpine stimulation (three repeats) was 11.1 ± 1.1; 9.9–13.0 D. The average of the standard deviations of maximum accommodation from each monkey was 0.8 ± 0.3 D from carbachol iontophoresis and 0.3 ± 0.2 from i.v. pilocarpine. The average latency to the start of the response after carbachol iontophoresis was 2.5 ± 3.9; 0.0–12.0 min with a time constant of 12.7 ± 9.5; 2.3–29.2 min. The average latency after i.v. pilocarpine was 0.31 ± 0.03; 0.25–0.34 min with a time constant of 0.19 ± 0.07; 0.11–0.31 s. During i.v. pilocarpine stimulated accommodation in five monkeys, lens diameters decreased by 0.54 ± 0.09; 0.42–0.64 mm with a rate of change of 0.052 ± 0.002; 0.050–0.055 mm/D. Accommodative responses with i.v. pilocarpine were more rapid, consistent and stable than those with carbachol iontophoresis. The accommodative decrease in lens diameter with i.v. pilocarpine as a function of age was consistent with previous results using constant topical pilocarpine. Intravenous pilocarpine stimulated accommodation is safe, more consistent and more rapid than carbachol iontophoresis and it requires no contact with or obstruction of the eye thus allowing continuous and uninterrupted refraction and ocular biometry measurements.

Quantification of Age-Related and per Diopter Accommodative Changes of the Lens and Ciliary Muscle in the Emmetropic Human Eye

To calculate age-related and per diopter (D) accommodative changes in crystalline lens and ciliary muscle dimensions in vivo in a single cohort of emmetropic human adults ages 30 to 50 years. The right eyes of 26 emmetropic adults were examined using ultrasonography, phakometry, anterior segment optical coherence tomography, and high resolution magnetic resonance imaging. Accommodation was measured both subjectively and objectively. In agreement with previous research, older age was linearly correlated with a thicker lens, steeper anterior lens curvature, shallower anterior chamber, and lower lens equivalent refractive index (all P < 0.01). Age was not related to ciliary muscle ring diameter (CMRD) or lens equatorial diameter (LED). With accommodation, lens thickness increased (+0.064 mm/D, P < 0.001), LED decreased (-0.075 mm/D, P < 0.001), CMRD decreased (-0.105 mm/D, P < 0.001), and the ciliary muscle thickened anteriorly (+0.013 to +0.026 mm/D, P < 0.001) and thinned posteriorly (-0.011 to -0.015, P < 0.01). The changes per diopter of accommodation in LED, CMRD, and ciliary muscle thickness were not related to subject age. The per diopter ciliary muscle contraction is age independent, even as total accommodative amplitude declines. Quantifying normal biometric dimensions of the accommodative structures and changes with age and accommodative effort will further the development of new IOLs designed to harness ciliary muscle forces.

Three-Dimensional Magnetic Resonance Imaging of the Phakic Crystalline Lens during Accommodation

To quantify changes in crystalline lens curvature, thickness, equatorial diameter, surface area, and volume during accommodation using a novel two-dimensional magnetic resonance imaging (MRI) paradigm to generate a complete three-dimensional crystalline lens surface model. Nineteen volunteers, aged 19 to 30 years, were recruited. T(2)-weighted MRIs, optimized to show fluid-filled chambers of the eye, were acquired using an eight-channel radio frequency head coil. Twenty-four oblique-axial slices of 0.8 mm thickness, with no interslice gaps, were acquired to visualize the crystalline lens. Three Maltese cross-type accommodative stimuli (at 0.17, 4.0, and 8.0 D) were presented randomly to the subjects in the MRI to examine lenticular changes with accommodation. MRIs were analyzed to generate a three-dimensional surface model. During accommodation, mean crystalline lens thickness increased (F = 33.39, P < 0.001), whereas lens equatorial diameter (F = 24.00, P < 0.001) and surface radii both decreased (anterior surface, F = 21.78, P < 0.001; posterior surface, F = 13.81, P < 0.001). Over the same stimulus range, mean crystalline lens surface area decreased (F = 7.04, P < 0.005) with a corresponding increase in lens volume (F = 6.06, P = 0.005). These biometric changes represent a 1.82% decrease and 2.30% increase in crystalline lens surface area and volume, respectively. CONCLUSIONS; The results indicate that the capsular bag undergoes elastic deformation during accommodation, causing reduced surface area, and the observed volumetric changes oppose the theory that the lens is incompressible.

Age-dependence of the optomechanical responses of ex vivo human lenses from India and the USA, and the force required to produce these in a lens stretcher: The similarity to in vivo disaccommodation

The purpose of this study was to study the age-dependence of the optomechanical properties of human lenses during simulated disaccommodation in a mechanical lens stretcher, designed to determine accommodative forces as a function of stretch distance, to compare the results with in vivo disaccommodation and to examine whether differences exist between eyes harvested in the USA and India. Postmortem human eyes obtained in the USA ( n = 46, age = 6–83 years) and India ( n = 91, age = 1 day–85 years) were mounted in an optomechanical lens stretching system and dissected to expose the lens complete with its accommodating framework, including zonules, ciliary body, anterior vitreous and a segmented rim of sclera. Disaccommodation was simulated through radial stretching of the sectioned globe by 2 mm in increments of 0.25 mm. The load, inner ciliary ring diameter, lens equatorial diameter, central thickness and power were measured at each step. Changes in these parameters were examined as a function of age, as were the dimension/load and power/load responses. Unstretched lens diameter and thickness increased over the whole age range examined and were indistinguishable from those of in vivo lenses as well as those of in vitro lenses freed from zonular attachments. Stretching increased the diameter and decreased the thickness in all lenses examined but the amount of change decreased with age. Unstretched lens power decreased with age and the accommodative amplitude decreased to zero by age 45–50. The load required to produce maximum stretch was independent of age (median 80 mN) whereas the change in lens diameter and power per unit load decreased significantly with age. The age related changes in the properties of human lenses, as observed in the lens stretching device, are similar to those observed in vivo and are consistent with the classical Helmholtz theory of accommodation. The response of lens diameter and power to disaccommodative (stretching) forces decreases with age, consistent with lens nuclear stiffening.

MRI study of the changes in crystalline lens shape with accommodation and aging in humans

Magnetic Resonance Imaging was used to study changes in the crystalline lens and ciliary body with accommodation and aging. Monocular images were obtained in 15 young (19-29 years) and 15 older (60-70 years) emmetropes when viewing at far (6 m) and at individual near points (14.5 to 20.9 cm) in the younger group. With accommodation, lens thickness increased (mean ± 95% CI: 0.33 ± 0.06 mm) by a similar magnitude to the decrease in anterior chamber depth (0.31 ± 0.07 mm) and equatorial diameter (0.32 ± 0.04 mm) with a decrease in the radius of curvature of the posterior lens surface (0.58 ± 0.30 mm). Anterior lens surface shape could not be determined due to the overlapping region with the iris. Ciliary ring diameter decreased (0.44 ± 0.17 mm) with no decrease in circumlental space or forward ciliary body movement. With aging, lens thickness increased (mean ± 95% CI: 0.97 ± 0.24 mm) similar in magnitude to the sum of the decrease in anterior chamber depth (0.45 ± 0.21 mm) and increase in anterior segment depth (0.52 ± 0.23 mm). Equatorial lens diameter increased (0.28 ± 0.23 mm) with no change in the posterior lens surface radius of curvature. Ciliary ring diameter decreased (0.57 ± 0.41 mm) with reduced circumlental space (0.43 ± 0.15 mm) and no forward ciliary body movement. Accommodative changes support the Helmholtz theory of accommodation including an increase in posterior lens surface curvature. Certain aspects of aging changes mimic accommodation.

Lentotomie mittels fs-Laserpulsen: Behandlung der Presbyopie durch Erzeugen von Gleitebenen in der Linse

Based on the Helmholtz theory for accommodation, increasing sclerosis of the lens nucleus and cortex is the main cause for the development of presbyopia. Existing therapies, however, do not reverse the stiffness of the crystalline lens and thus do not regain real accommodation ability. A new approach to restore the flexibility of the lens has been realised by utilising the non-linear interaction of ultrafast laser pulses with transparent tissue, the so-called photodisruption. This process has been used to create micro-incisions which act as gliding planes inside the crystalline lens without opening the eye globe. This treatment method, known as fs-lentotomy, enables regeneration of real dynamic accommodation. For the first time, 3D structures for gliding planes were successfully generated in experiments with human donor lenses of different ages. An average increase in anterior-posterior lens thickness of 100 mum accompanied by a decrease of equatorial lens diameter was observed as a direct consequence of fs-lentotomy. This is attributed to the increased flexibility, as the force of the capsule bag moulds the lens tissue more spherically. Moreover, in vivo experiments on rabbit eye lenses did not induce an increasing opacification (cataract) over a six-month follow-up period. However, the incisions were still detectable using Scheimpflug imaging and histopathological techniques, although the visibility of the incisions was declining. Furthermore, no side effects were observed during the wound healing process and during a six-months follow-up period. Based on these findings fs-lentotomy might have the potential to become a procedure for the reversal of presbyopia.

Effect of Anterior Zonule Transection on the Change in Lens Diameter and Power in Cynomolgus Monkeys during Simulated Accommodation

To quantify the role of anterior zonular tension on the optomechanical lens response during simulation of accommodation in primates. Postmortem cynomolgus monkey eyes (n = 14; age range, 3.0-11.5 years) were dissected leaving intact the lens, zonules, ciliary body, hyaloid membrane, anterior vitreous, and a scleral rim. The lens was mounted in a lens-stretching system and stretched radially in step-wise fashion. The load, and the lens diameter and power were measured at each step and the diameter- and power-load relationships were quantified. The anterior zonular fibers were then transected, and the experiment was repeated. The equatorial lens diameter and lens optical power before and after zonular transection were compared. Stretching increased the lens diameter by 0.25 +/- 0.09 mm (median +/- interquartile range) before and 0.25 +/- 0.19 mm after zonular transection. Stretching decreased the lens power by 13.0 +/- 6.5 D before and 10.6 +/- 8.0 D after zonular transection. The load required to change the diameter of the lens by 1 mm decreased from 18.8 +/- 10.7 g before to 15.0 +/- 7.8 g after zonular transection. The absolute change in power per gram of loading decreased from 2.5 +/- 1.1 before to 2.0 +/- 1.2 D after zonular transection. The cynomolgus monkey lens retains a significant fraction of its accommodative ability after transection of the anterior zonules in simulated accommodation experiments.

Accommodative changes in lens diameter in rhesus monkeys.

Some debate surrounds the accommodative mechanism in primates, particularly whether the lens equatorial diameter increases or decreases during accommodation. This study has been undertaken to measure the relationship between changes in lens diameter and refraction during accommodation in rhesus monkeys. Photorefraction was used to measure accommodation, and goniovideography was used to measure accommodative changes in lens diameter in the iridectomized eyes of two rhesus monkeys. Accommodation was stimulated through the full amplitude available to each eye by stimulation of the Edinger-Westphal nucleus of the brain. Dynamic measurement of refractive changes followed by dynamic measurements of changes in lens diameter for the same stimulus current amplitudes allow the relationship between refraction and lens diameter to be determined. Lens diameter decreased relatively linearly during accommodation by 0.055 mm/diopter (D), resulting in an overall decrease in lens diameter of approximately 7% of the unaccommodated lens diameter for approximately 12 D of accommodation. The rhesus monkey lens diameter decreases systematically with the refractive change during accommodation in accordance with the Helmholtz accommodative mechanism and in contrast to the accommodative mechanism originally proposed by Tscherning.

Spatially variant changes in lens power during ocular accommodation in a rhesus monkey eye.

This study investigated the changes in ocular aberrations that occur over the entire lens equatorial diameter during accommodation in iridectomized rhesus monkey eyes to understand the nature of accommodative lenticular deformation. Accommodation was centrally stimulated to a range of different response amplitudes (0 D to approximately 11 D), and ocular aberrations were measured with a Shack-Hartmann wavefront sensor in both eyes of one previously iridectomized 10-year-old rhesus monkey. At the highest amplitude in the two eyes, aberrations were analyzed over entrance pupil diameters ranging from 3 to 8 mm in steps of 1 mm. Root mean square error of the total measured aberrations, excluding defocus, increased systematically with increasing accommodation from about 1 to 3.5 microns. Spherical aberration became systematically more negative, and vertical coma increased significantly in magnitude with accommodation. There was a strong accommodative change in power near the center of the lens and little change in power at the periphery. At the highest accommodative state, decreasing the analyzed entrance pupil diameter from 8 to 3 mm considerably reduced the wavefront error. The greater increase in optical power near the central region of the lens, combined with an accommodative pupillary miosis, would serve to maximize accommodative refractive change while maintaining acceptable image quality.

In vivo increase of the human lens equatorial diameter during accommodation.

The movement of the human lens equator during accommodation was examined in vivo. High-resolution ultrasound images of the lens equator were obtained from young human subjects whose amplitude of accommodation was controlled with 1% tropicamide and 2% pilocarpine. To avoid errors that otherwise arise from eye rotation or other movement, the cornea and sclera were used as positional references in comparative studies of the video images obtained from the unaccommodated and accommodated states. During accommodation, the movement at the lens equator involved small displacement; i.e., < 100 microns, and the equator did not move anteriorly or posteriorly but peripherally toward the sclera. These results indicate that the lens equator is under increased zonular tension during accommodation, in contradiction to Helmholtz's widely accepted theory of accommodation.