The IOL horizon: accommodative intraocular lenses.

BACKGROUND: For more than half a century, intraocular lenses have been used effectively to correct vision after cataract extraction. New developments in intraocular lens design have led to the creation of lenses that may have a significant role in the restoration of accommodation as well. PURPOSE: Accommodative lens technology is constantly evolving. Many different lens designs are currently under development. It is important for optometrists to be familiar with this technology and the various lenses being developed, as accommodative intraocular lenses stand to become the mainstay for cataract surgery, as well as for the surgical treatment of presbyopia. METHOD: The literature is reviewed in order to summarize the developments in accommodative lens technology.     

Accommodative intraocular lenses: current status

PURPOSE OF REVIEW: The possibility of using a monofocal IOL with accommodative ability allows refractive cataract surgery with a clearly decreased potential of photic phenomena. Three IOLs of different designs and materials have demonstrated accommodative ability, but the degree of accommodative amplitude has been reported to different extents and variabilities. The plate-haptic CrystaLens has a hinged design that might permit forward movement of the optic as a result of pressure changes in the vitreous cavity. The 1CU has modified haptics that bend in the bag as the lens capsule contracts, which are supposed to cause anterior displacement of the lens optic. With the dual-optic one-piece Synchrony, springlike haptics separate a high-plus anterior lens from a posterior minus lens. With accommodative effort, the capsular bag expands and the springs express kinetic energy, which might allow the optics to separate as the anterior lens moves forward. RECENT FINDINGS: This article seeks to clarify and distinguish the concepts of true accommodation and pseudo accommodation. Current designs of accommodative IOLs are supposed to work by the focus-shift principle to allow true pseudophakic accommodation. Studies that biometrically assessed optic shift found no or only low amplitudes of forward movement. The amount of forward movement, if present, was highly variable between patients. To date, most studies present psychophysical data for the proof of concept, which alone seems insufficient. Capsule bag performance and posterior capsule opacification with accommodative IOLs seem worse than those with standard intraocular lenses. SUMMARY: The potential clinical benefits of accommodative IOL technology for both cataract patients and refractive patients may place accommodative IOLs in a competitive position with multifocal IOL technology.     

Simultaneous measurements of refraction and A-scan biometry during accommodation in humans.

PURPOSE: Accommodation is a dioptric change in power of the crystalline lens resulting from ciliary muscle contraction that leads to an increase in lens surface curvatures and thickness and changes in the position of lens surfaces. Previous studies have used A-scan ultrasound to measure changes in the position of lens surfaces with voluntary accommodation, but have not simultaneously measured the change in refraction. The goal of this study is to simultaneously measure and correlate refractive and biometric changes in the lens during voluntary accommodation in humans. METHODS: Refraction was measured off-axis in the right eye and biometry on-axis in the left eye simultaneously during voluntary accommodation in 22 human subjects between the ages of 21 and 30 years (mean +/- standard deviation: 25.8 +/- 2.3 years). Subjects viewed a distant target and four near targets spanning the full accommodative range available to evaluate refraction and lens surface position at each accommodative state. RESULTS: Maximum objectively measured accommodative amplitude of all subjects was 5.64 +/- 0.21 D (mean +/- standard error of mean). Biometric and refractive changes during accommodation were linearly correlated. The mean +/- standard error of mean decrease in anterior chamber depth was 0.051 +/- 0.008 mm/D, increase in lens thickness was 0.067 +/- 0.008 mm/D, and increase in anterior segment length was 0.017 +/- 0.005 mm/D during accommodation. There was a net anterior movement of the lens center of 0.017 +/- 0.005 mm/D. CONCLUSION: Anterior chamber depth, lens thickness, and anterior segment length change linearly with refraction during accommodation. Per-diopter changes in the lens were greater in the current study compared with previous studies in which only accommodative demand was measured, which overestimates the accommodative response.     

Accommodative changes in lens diameter in rhesus monkeys

PURPOSE: 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. METHODS: 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. RESULTS: 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. CONCLUSIONS: 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.     

Soft bifocal contact lenses for patients with nearpoint asthenopia

Bifocal contact lenses have generally been considered solely for the presbyopic population. However, pre-presbyopic individuals with accommodative or vergence disorders who respond favorably to plus lenses at near may be good candidates for bifocal contact lenses. A group of ten non-presbyopic patients who manifest accommodative dysfunctions or near point esophoria were fit with the Ciba BI-SOFT contact lenses. Nearpoint accommodative and vergence findings were evaluated through both near point plus spectacle lenses and the bifocal soft lenses. Results indicate that the BI-SOFT bifocal contact lens was effective in changing the near point visual posture. However, poor distance vision acuity due to ghost images reduced the subjective success rate of the BI-SOFT lens on this non-presbyopic population.     

Accommodation in the northern rock bass (Ambloplites rupestris rupestris) in response to natural stimuli

A television-videotape system was used to monitor and record the accommodative lens movements of a fish, the northern rock bass (Ambloplites rupestris rupestris). The results demonstrate that large accommodative changes in the rostral-caudal direction occur naturally in response to behaviorally meaningful visual stimuli, such as a feeding stimulus. Such responses have been previously induced artificially with pharmocological agents.     

Lens Refilling

Cataract surgery has developed into a very safe and highly reproducible procedure but the ultimate goal to also restore physiological accommodation has not yet been achieved. A variety of accommodative intraocular lenses (IOLs) and surgical techniques have been suggested to cure presbyopia but all showed only poor accommodative effects by objective measurements. Complete lens refilling with flexible polymers might be an interesting alternative. Recent investigations on accommodation and presbyopia have given support to the lens refilling method. After development of suitable surgical techniques and filling materials only secondary cataract formation and the unsolved intraoperative control of refraction restrict the clinical use of this technique.     

Tangent Streak RGP bifocal contact lenses in the treatment of accommodative esotropia with high AC/A ratio.

Patients with the diagnosis of high accommodative convergence/accommodation (AC/A) ratio, accommodative esotropia traditionally have been fit with bifocal spectacles. For cosmetic reasons, however, many patients prefer non-spectacle correction. Orthoptics, miotics, contact lenses, and surgery have been used as alternatives. We describe the treatment of three patients with the Tangent Streak bifocal rigid gas permeable (RGP) contact lens in which functional control of the patient's deviation was attained. Tangent Streak RGP bifocal contact lenses offer an acceptable alternative to bifocal spectacles, surgery, miotics, or orthoptics in the treatment of accommodative esotropia with high AC/A ratio.     

Accommodating intraocular lenses

PURPOSE OF REVIEW: With the advent of interest in accommodative lenses as a solution for presbyopia and the growing baby-boomer demographic, ophthalmic surgeons will have the opportunity to provide this technology to facilitate near, intermediate, and distance vision for their patients. RECENT FINDINGS: At present, six corporate entities and lens designs are attempting to commercialize accommodative intraocular lens devices. One Food and Drug Administration (FDA) clinical trial has been completed and the first FDA-approved accommodating lens is available. SUMMARY: For the first time, ophthalmic surgeons will be able to provide a full range of visual focus in each eye of a patient to maintain binocular function while also avoiding the unwanted mesopic and scotopic visual disturbances that are experienced with monovision and multifocal lens technologies. Accommodative intraocular lenses could revolutionize not only cataract visual rehabilitation but also the surgical approach to presbyopia.     

Pseudophakic accommodation with translation lenses – dual optic vs mono optic

PURPOSE: To investigate the pseudophakic accommodation effect in dual and mono optic translation accommodative intraocular lenses (AIOL) using linear matrix methods in the paraxial space. METHODS: Dual (anterior optic of power +32 D linked to a compensatory posterior optic of negative power) and mono lens power was determined in the non-accommodated state using linear geometric optics based on the Gullstrand model eye. The position of the AIOL was calculated from a regression formula. Pseudophakic accommodation was assessed with three systems: (1) forward shift of the mono optic lens, (2) anterior translation of the anterior optic in the dual optic lens system with an unchanged position of the posterior minus lens and (3) symmetrical anterior and posterior translation of the anterior and posterior lens. The Gullstrand model eye was modified by changing the axial length (and proportionally changing the phakic anterior chamber depth) to investigate the accommodative effect in myopic and hyperopic eyes. RESULTS: The dual optic lens system (2) yields a nearly constant accommodation amplitude of 2.4-2.5 D mm(-1) movement over the total range of axial lengths. The mono optic lens (1) provides a higher accommodative effect only in extremely short eyes (high refractive power of the lens), whereas for normal eyes (1.4-1.5 D mm(-1) movement) and for long (myopic) eyes the accommodative effect is much less than the dual optic lens. The dual optic lens system under condition (3) yields less accommodation amplitude compared with the dual optic system under condition (2) over the total range of axial length but provides higher accommodation amplitude compared with the mono optic lens system (1) with axial lengths greater than 22.3 mm (lens power 25.5 D). In the accommodated state, with lens translation of 1 mm, the absolute value of the lateral magnification increases with the refractive power of the mono optic lens (1) and decreases in both dual optic lens systems (under conditions 2 and 3). CONCLUSIONS: A mathematical strategy is presented for calculation of the accommodative effect of mono-optic and dual optic AIOL. The dual optic lens yielded a nearly constant accommodation amplitude of about 2.4-2.5 D mm(-1) translation, whereas the mono optic lens yielded an accommodative response of <2 D mm(-1) translation in long myopic or normal eyes. Only in extremely short eyes is the accommodative amplitude of the mono-optic lens higher than the dual optic lens.     

On the relationship between lens stiffness and accommodative amplitude

The purpose of this study was to investigate the relationship between the stiffness of the material comprising the lens and the loss of accommodative amplitude with age. We used a validated mechanical model to determine the changes in the shape of the lens during accommodation and disaccommodation. The relative contribution of lens stiffness to loss of accommodative amplitude with age was determined by varying lens stiffness in the model. The changes in lens stiffness with age were based on the results of two recently published studies. In the first study we showed that lens stiffness increases exponentially with age, and in the second study we showed that there is a considerable stiffness gradient within the lens that changes with age. The results of both studies were incorporated in the mechanical model. The model showed that it is not the increasing stiffness of the lens with age, but rather the changing stiffness gradient that influences accommodative amplitude. The results show that the changing stiffness gradient in the lens may be responsible for almost the entire loss of accommodation with age.     

The importance of the test parameters in the clinical assessment of accommodative facility

Normative data for accommodative facility have recently become available. This study investigates the effect of varying the test parameters of lens power, target size, test distance, and test duration on binocular accommodative facility. Binocular accommodative facility decreased as a function of increasing lens power and decreasing target size. Differences in response times to the plus and minus phases of the flip test were not evident under any of the conditions. Interesting effects of proximity occurred at the nearer test distance. The results show the importance of strict control of testing procedures to achieve meaningful data.     

Fatigue reduces tonic accommodation

Ocular accommodation adopts a mean baseline response level of approximately 1.0 D in the absence of blur feedback (open-loop state). This baseline or tonic accommodation (TA) can be elevated following a sustained monocular accommodative response to a dioptric stimulus (lens adaptation) that exceeds the baseline open-loop level of TA. The accommodative response to the lens persists in the open-loop state (accommodative hysteresis), and eventually decays to a stable end-point. Interestingly, if the baseline TA is high, the monocularly adapted accommodative state can decay to an end-point that is below the initial pre-adapted baseline level of the TA (counter-adaptive response) (McBrien, N.A. and Millodot, M., (1988). Differences in adaptation of TA with refractive state. Invest. Ophthalmol. Vis. Sci., 29, 460-469). We have investigated the possible contribution of accommodation fatigue to the counter-adaptive change in baseline TA following sustained accommodation. Two fatigue procedures were used while viewing a target at 66 or 33 cm. In a monocular condition, accommodation was stimulated for 3 min with lens values alternating from -1.5 to +1.5 D at a rate of 0.25 Hz. In the binocular condition, convergence was stimulated with alternating prism values from 9 prism diopters (PD) base-out to 9 PD base-in. Both monocular and binocular tasks resulted in a significant reduction of TA. These results suggest that previously reported reductions of baseline TA following sustained monocular accommodation or binocular convergence could have resulted from fatigue of the accommodative system. Accommodative fatigue could be responsible for the lower values of TA observed in counter-adaptive responses to sustained accommodative or convergence effort.     

Magnetic resonance imaging study of the effects of age and accommodation on the human lens cross-sectional area

PURPOSE: To evaluate the effect of age and accommodation on lens cross-sectional area (CSA). METHODS: High-resolution magnetic resonance images of the eye were acquired from 25 subjects ranging in age from 22 to 50 years during accommodation and with accommodation at rest. The images were analyzed to obtain the total lens CSA and the CSAs of the anterior and posterior portions of the lens. RESULTS: The total lens CSA and the CSA of the anterior portion increased with age in both accommodative states. With accommodation, the CSA was larger in these portions of the lens; however, this difference decreased with age. Conversely, the CSA of the posterior portion of the lens remained statistically independent of both age and accommodative state. CONCLUSIONS: This preliminary study documents, in vivo, that the lens grows with age. This growth appears to be confined to the anterior portion. A quite unexpected finding is that both the total lens CSA and the CSA of the anterior portion are greater during accommodation when zonular tension is minimized. This accommodative change in CSA, which decreases with age, may be due to compression of the lens material during relaxed accommodation when zonular tension is greatest. That both age and accommodative changes in CSA appear to be limited to the anterior portion of the lens may be related to properties of the anterior capsule and lens material, the position of the zonular attachments, and the location of the fetal nucleus.     

Lens diameter and thickness as a function of age and pharmacologically stimulated accommodation in rhesus monkeys

Uncertainty exists regarding accommodative and age changes in lens diameter and thickness in humans and monkeys. In this study, unaccommodated and accommodated refraction, lens diameter, and lens thickness were measured in rhesus monkeys across a range of ages. Iridectomized eyes were studied in 33 anesthetized monkeys aged 4-23 years. Refraction was measured using a Hartinger coincidence refractometer and lens thickness was measured with A-scan ultrasound. Lens diameters were measured with image analysis from slit-lamp images captured via a video camera while a saline filled, plano perfusion lens was placed on the cornea. Accommodation was pharmacologically stimulated with 2% pilocarpine via the perfusion lens in 21 of the monkeys and lens diameters were measured until a stable minimum was achieved. Refraction and lens thickness were measured again after the eye was accommodated. Unaccommodated lens thickness increased linearly with age by 0.029 mm/year while unaccommodated lens diameter showed no systematic change with age. Accommodative amplitude decreased by 0.462 D/year in response to pilocarpine. The accommodative increase in lens thickness decreased with age by 0.022 mm/year. The accommodative decrease in lens diameter declined linearly with age by 0.021 mm/year. Rhesus monkeys undergo the expected presbyopic changes including increasing lens thickness and a decreasing ability of the lens to undergo changes in thickness and diameter with accommodation, however without an age-related change in unaccommodated lens diameter. As in humans, the age-related decrease in accommodative amplitude in rhesus monkeys cannot be attributed to an age-related increase in lens diameter.     

Deterioration of amplitude of the accommodation with age and its possible restoration in the intraocular lens implanted eye]

This study aimed to determine whether pseudophakia which have deteriorated amplitude of accommodation with age, i.e., presbyopia and which received implantation of refractively changeable intraocular lenses after cataract extraction are able to regain accommodative function. According surveys on aging in the literature, functional deterioration of the ciliary muscle and the zonular fiber is not so striking compared to the changes of crystalline lens material and capsule. Consequently the artificially pseudophakic eye can be expected to regain accommodative function when the crystalline lens material could be replaced by an appropriately visco-elastic material before 80 years of age when the crystalline lens capsule still retain its elasticity.     

Accommodative intraocular lenses: considerations on use, function and design

PURPOSE OF REVIEW: To review the development, design and clinical implications of the first US Food and Drugs Administration approved accommodative lens. To inform the reader of forthcoming accommodative intraocular lens technologies that are being clinically tested and other technologies that are under design. RECENT FINDINGS: We present clinical data that are currently available or pending release on accommodative intraocular lenses. As much as the results of the technology can be appreciated, the approach toward the refractive mindset of the presbyopic intraocular lens patient must be understood and mastered. SUMMARY: Accommodative intraocular lens design and development are at a feverish pace. The lure is the perceived holy grail for the solution of presbyopia.     

低度近视性屈光参差不同矫正方式的调节功能研究

目的:研究分别采用单焦框架眼镜与角膜塑形镜矫正低度近视性屈光参差的儿童单眼及双眼调节的差异。方法:回顾性研究。收集2019-11/2020-04于我院眼科门诊就诊的低度近视性屈光参差(1.0D≤双眼等效球镜差<2.5D)儿童47例94眼,其中配戴框架眼镜者27例54眼作为A组,配戴角膜塑形镜者20例40眼作为B组。分别记录并比较两组患儿矫正1mo后的矫正视力及调节参数[调节反应(AR)、调节灵活度(AF)、调节幅度(AMP)]的差异。结果:两组患儿AR测量值均为正值,表现为调节滞后,A组患儿屈光高度眼调节滞后量显著高于屈光低度眼(0.63±0.21D vs 0.25±0.34D,P<0.001),但B组患儿双眼调节滞后量无差异(P=0.104),且两组患儿屈光高度眼与屈光低度眼单眼矫正视力和单眼AMP均无差异(P>0.05)。A组患儿双眼AR差值高于B组(0.38±0.36D vs 0.10±0.26D,P=0.005),双眼AF低于B组(8.22±1.15c/min vs 9.95±0.89c/min,P<0.001),但两组患儿双眼AMP无明显差异(P=0.280)。结论:低度近视性屈光参差儿童配戴角膜塑形镜比单焦框架眼镜矫正可获得更高的双眼调节灵活度,降低双眼调节反应差值,从而维持更协调的双眼调节功能。     

A model for accommodation in the young human eye: The effects of lens elastic anisotropy on the mechanism

An explanation of the mechanism of visual accommodation depends heavily on understanding the mechanical properties of the lens, as well as the way in which its shape is altered in small accommodative changes. An initial attempt to relate these properties to a mechanism has already been performed (Koretz and Handelman, 1982) for the young (age 11 yr) human lens, using certain simplifying assumptions (spherical curvature on the anterior lens surface and elastic isotropy). However, since it has been shown that the lens behaves as an anisotropic body, the previous treatment has been extended to include the variation of lens elastic properties in the polar and radial directions. With this modified representation, it is found that only one combination of elastic constants is consistent with the generally accepted qualitative theory of accommodation and with clinical data on the accommodative range of the emmetropic age 11 human eye. For this unique solution of the equations, however, the general mechanism already suggested by us, which includes support by the vitreous and alteration of the magnitude and angle of application of zonular force with accommodation, remains little changed.