A dual optic accommodating foldable intraocular lens

Polymeric Materials for Eye Surface and Intraocular Applications

Ocular applications of polymeric materials have been widely investigated for medical diagnostics, treatment, and vision improvement. The human eye is a vital organ that connects us to the outside world so when the eye is injured, infected, or impaired, it needs immediate medical treatment to maintain clear vision and quality of life. Moreover, several essential parts of the eye lose their functions upon aging, causing diminished vision. Modern polymer science and polymeric materials offer various alternatives, such as corneal and scleral implants, artificial ocular lenses, and vitreous substitutes, to replace the damaged parts of the eye. In addition to the use of polymers for medical treatment, polymeric contact lenses can provide not only vision correction, but they can also be used as wearable electronics. In this Review, we highlight the evolution of polymeric materials for specific ocular applications such as intraocular lenses and current state-of-the-art polymeric systems with unique properties for contact lens, corneal, scleral, and vitreous body applications. We organize this Review paper by following the path of light as it travels through the eye. Starting from the outside of the eye (contact lenses), we move onto the eye's surface (cornea and sclera) and conclude with intraocular applications (intraocular lens and vitreous body) of mostly synthetic polymers and several biopolymers. Initially, we briefly describe the anatomy and physiology of the eye as a reminder of the eye parts and their functions. The rest of the Review provides an overview of recent advancements in next-generation contact lenses and contact lens sensors, corneal and scleral implants, solid and injectable intraocular lenses, and artificial vitreous body. Current limitations for future improvements are also briefly discussed.

Optimized multielement accommodative intraocular lens with a four-freeform-surface Alvarez lens and a separate aspheric lens

This paper proposes an accommodative intraocular lens (IOL), which consists of a two-element Alvarez lens and an aspheric lens for changing focal power and refractive power, respectively. The four-freeform-surface Alvarez lens is optimized for a multiple field of view; further, the aspheric lens also corrects the aberrations induced by the corneal asphericity of the human eye over the whole range of accommodation. A simulation using optical design software demonstrates its excellent performance in that the values of the modulation transfer function at 100 cycles/mm all reach ∼0.4 with a ±5° field of view for 3 and 5 mm pupils.

Clinical application of accommodating intraocular lens

The present review describes recent advances in application of accommodating intraocular lenses (AIOLs). Standard monofocal intraocular lenses (MIOLs) only correct distance vision, while AIOLs are designed to allow both good distance vision and near vision, which is achieved through the contraction and relaxation of ciliary muscles by providing transformation of the axial movement or curvature of the lens. Thus, AIOLs may be a better choice for those patients who demand a higher level of visual performance. Since techniques to analyze the performance of AIOLs have not been standardized, and there is a variety of both subjective and objective methods, it is hard to measure the performance of these intraocular lenses. By evaluating advantages and disadvantages of various AIOLs, and introducing techniques for measurement the performance postoperative, this paper can provide some relative information on choosing the type of AIOLs in the clinic.

Axial movement of the dual-optic accommodating intraocular lens for the correction of the presbyopia: optical performance and clinical outcomes

Presbyopia occurs in the aging eye due to changes in the ciliary muscle, zonular fibers, crystalline lens, and an increased lens sclerosis. As a consequence, the capacity of accommodation decreases, which hampers to focus near objects. With the aim of restoring near vision, different devices that produce multiple focuses have been developed and introduced. However, these devices are still unable to restore accommodation. In order to achieve that goal, dual-optic accommodating Intraocular Lenses have been designed, whose anterior optic displaces axially to increase ocular power, and focus near objects. Although dual-optic accommodating IOLs are relatively new, their outcomes are promising, as they provide large amplitudes of accommodation and a greater IOL displacement than single-optic accommodating IOLs. The outcomes show comfortable near vision, higher patients' satisfaction rates, and minimal postoperative complications like Posterior Capsular Opacification and Anterior Capsular Opacification, due to their design and material.

Refractive lens exchange in modern practice: when and when not to do it?

Cataract surgery due to advances in small incision surgery evolved from a procedure concerned with the primary focus on the safe removal of cataractous lens to a procedure focused on the best possible postoperative refractive result. As the outcomes of cataract surgery became better, the use of lens surgery as a refractive modality in patients without cataracts has increased in interest and in popularity. Removal of the crystalline lens for refractive purposes or refractive lens exchange (RLE) presents several advantages over corneal refractive surgery. Patients with high degrees of myopia, hyperopia and astigmatism are still not good candidates for laser surgery. Moreover, presbyopia can currently only be corrected with monovision or reading spectacles.

RLE supplemented with multifocal or accommodating intraocular lenses (IOLs) in combination with corneal astigmatic procedures might address all refractive errors including presbyopia, and eliminate the future need for cataract surgery.

Clinical performance of a new aspheric dual-optic accommodating intraocular lens

Purpose

To describe the clinical performance of a new dual-optic intraocular lens (IOL) with an enhanced optic profile designed to mimic natural accommodation.

Patients and methods

Prospective multicenter clinical study with the new dual-optic aspheric accommodating IOL (Synchrony Vu) in 74 patients (148 eyes) undergoing cataract surgery. Refractive target was emmetropia. Examinations at 1 month and 6 months included subjective refractions; visual acuities at near, intermediate, and far; mesopic contrast sensitivity with and without glare; safety data; and subjective survey on dysphotopsia (halos and glare).

Results

Clinical data at 6 months showed 89% of the eyes within ±1.0 D spherical equivalent refraction. Mean binocular uncorrected and distance-corrected visual acuity was 20/20 at far (0.00±0.11 logMAR and −0.06±0.08 logMAR, respectively), 20/20 at intermediate (0.01±0.13 logMAR and −0.01±0.10 logMAR, respectively), and 20/25 at near (0.10±0.14 logMAR and 0.14±0.15 logMAR, respectively). Mesopic contrast sensitivity was within normal limits. Seventy-eight percent of the patients had no spectacles and 70% had no dysphotopsia. One eye had IOL repositioning within 1 month of surgery.

Conclusion

The new aspheric Synchrony Vu accommodating IOL provided good visual performance at a range of distances without affecting quality of vision and with minimal safety considerations.

Developments in the correction of presbyopia II: surgical approaches

PURPOSE

To discuss the various static and dynamic surgical approaches which attempt to give presbyopes good vision at far, intermediate and near viewing distances.

CONTENT

Static methods broadly adopt the same optical techniques as those used in presbyopic contact lens correction and aim to satisfy the needs of the presbyope by increasing binocular depth-of-focus, often using monovision as well as simultaneous-imagery. Dynamic methods generally attempt to make use of at least some of the still-active elements of the accommodation system. They include procedures which are supposed to modify the relative geometry of the ciliary muscle and lens, or which reduce the stiffness of the presbyopic lens either by replacing it with other natural or man-made material or by subjecting it to femtosecond laser treatment. Alternatively the natural lens may be replaced by some form of intraocular lens which changes power as a result of forces derived from the still-active ciliary muscle, zonule and capsule, or other sources.

CONCLUSIONS

At present, multifocal intraocular lenses appear to offer the most consistent and reliable surgical approach to surgical presbyopic correction. They have obvious advantages in convenience and stability over optically-similar, simultaneous-image presbyopic contact lenses but this must be balanced against their relative inflexibility in cases of patient dissatisfaction. Dynamic methods remain largely experimental. Although some approaches show promise, as yet no method has demonstrated a reliable, long-term ability to correct distance refractive error and to appropriately change ocular power in response to changes in viewing distance over the normal range of interest.

Intraocular lens based on double-liquid variable-focus lens

In this work, the crystalline lens in the Gullstrand-Le Grand human eye model is replaced by a double-liquid variable-focus lens, the structure data of which are based on theoretical analysis and experimental results. When the pseudoaphakic eye is built in Zemax, aspherical surfaces are introduced to the double-liquid variable-focus lens to reduce the axial spherical aberration existent in the system. After optimization, the zoom range of the pseudoaphakic eye greatly exceeds that of normal human eyes, and the spot size on an image plane basically reaches the normal human eye's limit of resolution.

Magnifications of single and dual element accommodative intraocular lenses: paraxial optics analysis

PURPOSE

Using an analytical approach of paraxial optics, we evaluated the magnification of a model eye implanted with single-element (1E) and dual-element (2E) translating-optics accommodative intraocular lenses (AIOL) with an objective of understanding key control parameters relevant to their design. Potential clinical implications of the results arising from pseudophakic accommodation were also considered.

METHODS

Lateral and angular magnifications in a pseudophakic model eye were analyzed using the matrix method of paraxial optics. The effects of key control parameters such as direction (forward or backward) and distance (0 to 2 mm) of translation, power combinations of the 2E-AIOL elements (front element power range +20.0 D to +40.0 D), and amplitudes of accommodation (0 to 4 D) were tested. Relative magnification, defined as the ratio of the retinal image size of the accommodated eye to that of unaccommodated phakic (rLM(1)) or pseudophakic (rLM(2)) model eyes, was computed to determine how retinal image size changes with pseudophakic accommodation.

RESULTS

Both lateral and angular magnifications increased with increased power of the front element in 2E-AIOL and amplitude of accommodation. For a 2E-AIOL with front element power of +35 D, rLM(1) and rLM(2) increased by 17.0% and 16.3%, respectively, per millimetre of forward translation of the element, compared to the magnification at distance focus (unaccommodated). These changes correspond to a change of 9.4% and 6.5% per dioptre of accommodation, respectively. Angular magnification also increased with pseudophakic accommodation. 1E-AIOLs produced consistently less magnification than 2E-AIOLs. Relative retinal image size decreased at a rate of 0.25% with each dioptre of accommodation in the phakic model eye. The position of the image space nodal point shifted away from the retina (towards the cornea) with both phakic and pseudophakic accommodation.

CONCLUSION

Power of the mobile element, and amount and direction of the translation (or the achieved accommodative amplitude) are important parameters in determining the magnifications of the AIOLs. The results highlight the need for caution in the prescribing of AIOL. Aniso-accommodation or inter-ocular differences in AIOL designs (or relative to the natural lens of the contralateral eye) may introduce dynamic aniseikonia and consequent impaired binocular vision. Nevertheless, some designs, offering greater increases in magnification on accommodation, may provide enhanced near vision depending on patient needs.

Evaluation of the performance of accommodating IOLs using a paraxial optics analysis

PURPOSE

We employed an analytical approach to evaluate the key parameters for the potential design optimisation of accommodating intra-ocular lenses (AIOL) and to use these parameters to predict their accommodative performance.

METHODS

Paraxial thin-lens equations to predict the accommodative performances of single-element (1E) and two-element (2E) AIOLs were developed. 2E-AIOLs with either mobile front or back lens elements were analysed as well as 1E-AIOL for their accommodative performance. A paraxial model including key ocular components (corneal surfaces, pupil and retina) as well as AIOL was used to evaluate the key control parameters and optimal design configurations. A range of variants of the model, representing varying powers of front and back optical elements and with either front or back optical element mobile was tested.

RESULTS

Optimal accommodative performance of 2E-AIOL is governed by the power combinations of its optical elements; design variants with higher positive front element power produced greater accommodative efficacy, while mobility of the front element contributed more to the accommodative performance than the back element. The performance of 1E-AIOL is primarily governed by the power of the AIOL; the higher the AIOL power, the better the accommodative performance.

CONCLUSIONS

From an accommodative performance standpoint, the optimal design of 2E-AIOL should comprise a high plus power front element. Considering the maximum potential amounts of element translation available clinically, 2E-AIOLs are predicted to offer higher accommodative performance compared to 1E-AIOL.

Design and fabrication of accommodating fluidic intraocular lens

Intraocular lens (IOL) is a replacement lens for patients with crystalline lens problems. One key problem for today's IOL is its limited ability to retain the original accommodation capability inherent to human eyes. Unlike conventional optical lenses, a bio-inspired fluidic lens changes the curvature of the lens under an external force, resulting in a lens tuning power about 5 times as large as the current devices. By mimicking this desirable feature, a fluidic intraocular lens can achieve a large accommodation range as well. The device is designed that the lens can fully utilize the ciliary muscle force and the deformation of capsular bag during accommodation. Experimental results on fluidic IOL demonstrated a record high 12 Diopter (D) tuning range with a movement around its equator as small as 0.3 mm, achieved under a modest amount of force of 6 grams or 0.06 Newtons. The tuning range of the bio-inspired fluidic lens is comparable to a young, healthy eye, but the force and the movement required for ciliary muscle in order to achieve the wide accommodation range is comparable to an aged eye.

Bio-inspired accommodating fluidic intraocular lens

The invention of intraocular lens (IOL), a substitute for crystalline lens, represents a major advancement in cataract surgery. After about sixty years of IOL development, one key remaining problem is its limited accommodation range compared with natural eyes. To overcome this performance limit, we explore bio-inspired fluidic IOL. By mimicking the working principle of natural eyes, a fluidic intraocular lens can achieve an exceedingly large accommodation range. An experiment on fluidic IOL demonstrated a very high tuning range of 12 D. This accommodation range was achieved with a modest amount of force (0.06 N) and equatorial radius change (0.286 mm), in conditions matching well with the characteristics of aged eyes.

Restoration of accommodation: surgical options for correction of presbyopia

Accommodation is a dioptric change in the power of the eye to see clearly at near. Ciliary muscle contraction causes a release in zonular tension at the lens equator, which permits the elastic capsule to mould the young lens into an accommodated form. Presbyopia, the gradual age-related loss of accommodation, occurs primarily through a gradual age-related stiffening of the lens. While there are many possible options for relieving the symptoms of presbyopia, only relatively recently has consideration been given to surgical restoration of accommodation to the presbyopic eye. To understand how this might be achieved, it is necessary to understand the accommodative anatomy, the mechanism of accommodation and the causes of presbyopia. A variety of different kinds of surgical procedures has been considered for restoring accommodation to the presbyopic eye, including surgical expansion of the sclera, using femtosecond lasers to treat the lens or with so-called accommodative intraocular lenses (IOLs). Evidence suggests that scleral expansion cannot and does not restore accommodation. Laser treatments of the lens are in their early infancy. Development and testing of accommodative IOLs are proliferating. They are designed to produce a myopic refractive change in the eye in response to ciliary muscle contraction either through a movement of an optic or through a change in surface curvature. Three general design principles are being considered. These are single optic IOLs that rely on a forward shift of the optic, dual optic IOLs that rely on an increased separation between the two optics, or IOLs that permit a change in surface curvature to produce an increase in optical power in response to ciliary muscle contraction. Several of these different IOLs are available and being used clinically, while many are still in research and development.

Biomechanical analysis of the accommodative apparatus in primates

BACKGROUND

The restoration of natural accommodation in the presbyopic and cataract affected eye is a subject of intense research effort. A new instrument has been developed to test the viability and efficacy of procedures and methods to restore accommodation ex vivo in animal or human eyes.

METHODS

A section of the globe containing the crystalline lens, zonules, ciliary muscle and sclera is bonded into eight curved shoes. After dissecting the sclera between the shoes, even radial load is applied to stretch the zonules and capsular bag to simulate the natural accommodative process. The associated change in optical power is measured using a modified Scheiner's disk method. Changes in the diameter of the lens and ciliary processes are recorded, as well as zonular load.

RESULTS

No effective change in power was observed for the three presbyopic human eyes under four millimetres diameter stretching; the diameter of the ciliary aperture increased by between 1.8 mm and 2.3 mm, while the maximum increase in lens diameter was 0.19 mm. For the three younger monkey eyes, the diameter of the ciliary aperture increased by 1.4 mm with a corresponding increase in the lens diameter of between 0.50 mm and 0.65 mm. Stretching forces were generally higher for the human than for the monkey eyes, reaching maxima of 35 mN and 52 mN, respectively. The monkey eyes changed power by between 9.1 and 10.1 dioptres. An almost identical, progressive increase for lens diameter, power and stretching force versus stretch distance was found for all three monkey eyes.

CONCLUSION

The better understanding of the mechanisms and forces involved in the primate accommodative apparatus will assist with the development of accommodating IOLs and other methods to restore accommodation.

Optomechanical response of human and monkey lenses in a lens stretcher

PURPOSE

To quantify the forces necessary to change the shape and optical power of human and monkey lenses.

METHODS

Cynomolgus monkey (n = 48; age: 3.8-11 years), rhesus monkey (n = 35; age: 0.7-17 years) and human (n = 20, age 8-70 years) eyes obtained postmortem, including the lens, capsule, zonules, ciliary body, and sclera were mounted in an optomechanical lens-stretching system. Starting at zero load, the lenses were symmetrically stretched in a stepwise fashion in 0.25- or 0.5-mm steps. The load, lens diameter, inner ciliary body diameter, and lens power were measured at each step and the diameter- and power-load responses were quantified.

RESULTS

The diameter- and power-load responses were found to be linear in the physiologically relevant range of stretching. The average change in cynomolgus, rhesus, and human lens diameter, respectively, was 0.094, 0.109, and 0.069 mm/g in young lenses, and 0.069, 0.067, and 0.036, mm/g in older lenses. For the same lenses, the average change in lens power was -3.73, -2.83, and -1.22 D/g in young lenses and -2.46, -2.16, and -0.49 D/g in older lenses.

CONCLUSIONS

The force necessary to change the lens diameter and lens power increases with age in human and monkey lenses. The results agree with the Helmholtz theory of accommodation and with presbyopia theories that predict that the force required to disaccommodate the lens increases with age.

Study of visible and mid-infrared laser ablation mechanism of PMMA and intraocular lenses: experimental and theoretical results

Laser-polymer interactions have attracted extensive attention both for understanding the inherent basic ablation mechanism and for development of tissue simulators in several biomedical laser applications such as in human ophthalmology. Ablation experiments were performed on polymethylmethacrylate used as cornea tissue simulator and PMMA intraocular lenses. The polymer-ablation mechanism was examined with two different wavelengths and pulse durations. The experiments were conducted with Nd:YAG and Er:YAG solid-state lasers, and the ablation rates were simulated by a mathematical model in each case. Furthermore, to investigate the role of tissue hydration during laser ablation, we performed a set of experiments in which Er:YAG laser ablation of hydrophilic acrylic intraocular lenses, with different H(2)O and D(2)O concentrations, was studied. The hydrophilic acrylic lenses with the higher concentration of H(2)O gave the most satisfactory results regarding both the ablation efficiency and the quality of the ablated craters.

[Triple-optic approach to the Artificial Accommodation System]

BACKGROUND

Use of an implantable mechatronic microsystem (artificial accommodation system) is a novel approach to restoration of the accommodative capacity of the human eye. One such system, based on the triple-optic approach, has been selected for investigation of its suitability.

METHODS

Optical modelling of the dioptric apparatus is carried out by means of an optical simulation tool. Postoperative refractive compensation plus at least 3 dpt of accommodation are required.

RESULTS

An axial shift of 300 microm or less is achieved for the entire range of biometric data ascertained before surgery. In addition, the defined requirements made of optical imaging quality are met.

CONCLUSIONS

The triple-optic approach is basically suitable for application in the restoration of accommodative capacity. It seems that flexure hinges could be used in association with conducting polymers as actuator material. One question that is still open concerns the deformability of such implants as this relates to minimally invasive corneal incisions.

Status of accommodative intraocular lenses

PURPOSE OF REVIEW

This review examines the recent peer-reviewed literature of the past 18 months, with the goal of providing an overview of the pseudophakic lenses designed to provide accommodative action. Accommodative lenses take advantage of axial movement, refractive change and bag filling to provide their effect. Single and dual optic design, as well as unique technologies, were reviewed.

RECENT FINDINGS

An overview is presented of the various accommodative lenses available for investigation and use. There is positive evidence that current accommodative intraocular lenses provide superior potential for near vision compared with standard intraocular lenses.

SUMMARY

The available data suggest that accommodative amplitude and pseudoaccommodation are both important factors in the functional ability of accommodative intraocular lenses to provide a range of distance, intermediate and near vision.

Synchrony dual-optic accommodating intraocular lens

PURPOSE

To describe a dual-optic accommodating intraocular lens (IOL) based on theoretical considerations.

SETTING

University and independent research group.

METHODS

Ray-tracing analysis using optical modeling software (ZEMAXTM, Focus Software Inc., Tucson, Ariz) in a theoretical model eye was used to analyze lens configurations to optimize the accommodative and magnification effects of axial lens displacement. Finite-element modelling using a commercially available PC-based software package (COSMOS DesignSTAR) was applied to design the biomechanical parameters of the inter-optic articulations and optics.

RESULTS

Ray-tracing analysis indicated that a dual-optic design with a high plus-powered front optic coupled to a minus posterior optic produced greater change in conjugation power of the eye compared to a single-optic intraocular lens and that magnification effects were unlikely to account for improved near vision. Finite-element modelling indicated that the 2 optics can be linked by spring-loaded haptics that allow anterior and posterior axial displacement of the front optic in response to changes in ciliary body tone and capsular tension.

CONCLUSION

A dual-optic design linked by spring haptics increases the accommodative effect of axial optic displacement with minimal magnification effect and has promise for improving the performance of accommodative intraocular lenses.

Synchrony dual-optic accommodating intraocular lens

PURPOSE

To evaluate the clinical outcomes of an accommodating dual-optic intraocular lens (IOL).

SETTING

Private practice and university centers.

METHODS

A prospective noncomparative case series with retrospective control comprised 21 patients (26 eyes) scheduled for small-incision extracapsular cataract extraction by phacoemulsification with implantation of the Synchrony dual-optic accommodating IOL (Visiogen) (accommodating IOL group) and 10 patients who had small-incision extracapsular phacoemulsification with implantation of a monofocal, single-optic IOL at least 6 months previously (control group). Patients were examined 1, 3, 6, and 12 months after surgery. Defocus curves in the accommodating IOL group were compared with those in the control group. The main outcome measures were postoperative distance uncorrected and best corrected visual acuity; near uncorrected, distance corrected, and near corrected visual acuity; and accommodative range based on defocus curves.

RESULTS

Twenty-four eyes were available at the 6-month follow-up visit. All eyes had best corrected distance visual acuity of 20/40 or better, and 19 eyes (79%) had an uncorrected distance visual acuity of 20/40 or better. Uncorrected near visual acuity was 20/40 or better in all eyes. With distance correction, 23 eyes (96%) had an acuity of 20/40 or better at near. Defocus curve analysis suggested a mean accommodative range of 3.22 diopters (D) +/- 0.88 (SD) (range 1.00 to 5.00 D) in the accommodating IOL group and 1.65 +/- 0.58 D in the control group (range 1.00 to 2.50 D) (P<.05).

CONCLUSION

The Synchrony dual-optic IOL shows promise as an option to provide accommodative function in pseudophakic patients.

Accommodating intraocular lenses: a critical review of present and future concepts

BACKGROUND

Significant efforts have been made to develop lens implants or refilling procedures that restore accommodation. Even with monofocal implants, apparent or pseudoaccommodation may provide the patient with substantial though varying spectacle independence. True pseudophakic accommodation with a change of overall refractive power of the eye may be induced either by an anterior shift or a change in curvature of the lens optic.

MATERIALS AND METHODS

Passive-shift lenses were designed to move forward under ciliary muscle contraction. This is the only accommodative lens type currently marketed (43E/S by Morcher; 1CU by HumanOptics; AT-45 by Eyeonics). The working principle relies on various hypothetical assumptions regarding the mechanism of natural accommodation. Dual-optic lenses were designed to increase the dioptric impact of optic shift. They consist of a mobile front optic and a stationary rear optic which are interconnected with spring-type haptics. With active-shift lens systems the driving force is provided by repulsing mini-magnets. Lens refilling procedures replace the lens content by an elastic material and provide accommodation by an increase of surface curvature.

RESULTS

Findings with passive-shift lenses have been contradictory. While uncorrected reading vision results were initially reported to be favorable with the 1CU, and excellent with the AT-45 lens, distant-corrected near vision did not exceed that with standard monofocal lenses in later studies. Mean axial shift from laser interferometric measurements under stimulation with pilocarpine showed a moderate anterior shift with the 1CU, while the AT-45 paradoxically exhibited a small posterior shift. With the 1CU, the shift-induced accommodative effect was calculated to be less than +0.5 D in most cases, while +1 D was achieved in a single case only. Ranges and standard deviations were very large in relation to the mean values. Under physiological near-point stimulation, however, no shift was seen at all. Prevention of capsule fibrosis by extensive capsule polishing did not enhance the functional performance. Dual optic lenses are under clinical investigation and are reported to provide a significant amount of accommodation. However, possible long-term formation of interlenticular opacifications remains to be excluded. Regarding magnet-driven active-shift lens systems, initial clinical experience has been promising. Prevention of fibrotic capsular contraction is crucial, and it has been effectively counteracted with a special capsular tension ring, or lens fixation technique, together with capsule polishing. Lens refilling has been extensively studied in the laboratory and in primates. Though it offers great potential for fully restoring accommodation, a variety of problems must be solved, such as achieving emmetropia in the relaxed state, adequate response to ciliary muscle contraction, satisfying image quality over the entire range of accommodation and sustained functioning. The key problem, however, is again after-cataract prevention.

CONCLUSIONS

As opposed to psychophysical evaluation techniques, laser interferometry measures what shift lenses are designed to provide: axial shift on accommodative effort. While under pilocarpine some movement was recorded, no movement at all was found under near-point stimulation with any of the lenses currently marketed. In contrast, magnetic-driven active-shift lens systems carry the potential of sufficiently topping up apparent accommodation to provide for clinically useful accommodation while using conventional lens designs with proven after-cataract performance. Dual optic implants significantly increase the impact of axial optic shift. The main potential problem, however, is delayed formation of interlenticular regenerates. Lens refilling procedures offer the potential of fully restoring accommodation due to the great impact of increase in surface curvature on refractive lens power. However, various problems remain to be solved before clinical use can be envisaged, above all, again, after-cataract prevention. The concept of passive single-optic shift lenses has failed. Concomitant poor capsular bag performance makes these lenses an unacceptable trade-off. Magnet-assisted systems potentially combine clinically useful accommodation with satisfactory after-cataract performance. Dual optic lenses theoretically offer substantial accommodative potential but may allow for interlenticular after-cataract formation. Lens refilling procedures have the greatest potential for fully restoring natural accommodation, but will again require years of extensive laboratory and animal investigations before they may function in the human eye.

Dynamic performance of accommodating intraocular lenses in a negative feedback control system: a simulation-based study

A dynamic model of ocular accommodation is used to simulate the stability and dynamic performance of accommodating intraocular lenses (A-IOLs) that replace the hardened natural ocular lens that is unable to change focus. Accommodation simulations of an older eye with A-IOL materials having biomechanical properties of a younger eye illustrate overshoots and oscillations resulting from decreased visco-elasticity of the A-IOL. Stable dynamics of an A-IOL are restored by adaptation of phasic and tonic neural-control properties of accommodation. Simulations indicate that neural control must be recalibrated to avoid unstable dynamic accommodation with A-IOLs. An interactive web-model of A-IOL illustrating these properties is available at http://schorlab.berkeley.edu.

Predicting the performance of accommodating intraocular lenses using ray tracing

PURPOSE

To predict and compare the amount of accommodation achievable by pseudophakic accommodating intraocular lenses (IOLs) using optical ray-tracing analysis.

SETTING

Computational laboratory.

METHODS

Two-element IOLs (2E-IOL, with mobile front or back optical elements) were compared with single-element IOLs (1E-IOL). Modeling using computer-assisted ray tracing of both IOL types assumed lens elements were equiconvex/equiconcave. The 4 possible combinations of configurations representing a wide range of varying positive and negative power (up to +40 diopters [D]) of front and back optical elements were evaluated.

RESULTS

The 1E-IOLs offered limited amplitude of accommodation with axial shift (approximately 1.2 D/mm). For 2E-IOLs, configurations with high positive-power front elements returned the best amplitude of accommodation (up to approximately 3.0 D/mm when the front element power was +40 D).

CONCLUSIONS

Considering the maximum potential amounts of axial shifts available, 1E-IOLs were predicted to provide 1.0 D of accommodation or less and 2E-IOLs were predicted to provide up to 3.0 D to 4.0 D depending on design configuration and amount of axial shift achievable. Potential issues relating to accommodative aniseikonia and spherical aberration have been identified.

Interlenticular opacification: Dual-optic versus piggyback intraocular lenses

PURPOSE

To evaluate and compare the incidence of capsular bag opacification, particularly interlenticular opacification (ILO), in rabbit eyes implanted with a dual-optic silicone intraocular lens (IOL) or piggyback lenses.

SETTING

John A. Moran Eye Center, University of Utah, Salt Lake City, Utah, USA.

METHODS

Ten dual-optic study IOLs (Synchrony), 10 control pairs of piggyback silicone-plate lenses, and 10 control pairs of piggyback single-piece hydrophobic acrylic lenses were implanted in the capsular bag of 30 rabbit eyes following phacoemulsification. After a 6-week follow-up, the rabbits were killed and their eyes enucleated. Anterior capsule opacification and posterior capsule opacification were graded on a 0 to 4 scale from a posterior or Miyake-Apple view. Interlenticular opacification was noted in relation to the center of the interlenticular space (periphery, paracentral, and central area) and to the number of quadrants involved. The eyes were then evaluated histopathologically.

RESULTS

Postoperative inflammatory reaction was similar in all groups. Interlenticular opacification formation was statistically different among the 3 groups of lenses (ILO extension, P = .0013, and ILO extension x ILO quadrants, P = .0023; Kruskal-Wallis test). Pairwise post comparisons of ILO formation showed that the differences between the study IOL group and the silicone-plate lens group were not significant. Interlenticular opacification post comparisons between the hydrophobic acrylic lenses and the study lens or the silicone-plate lenses were significant (P = .002 and P = .001, respectively). Histopathologic examination showed extension of the proliferating cortical material from the peripheral Soemmering's ring into the interlenticular space, causing ILO, especially with the pairs of hydrophobic acrylic lenses.

CONCLUSIONS

In this rabbit model, ILO was significantly associated with pairs of hydrophobic acrylic lenses implanted in the bag. This study appears to confirm clinical observations that implantation of 2 silicone-plate lenses in the bag is not associated with ILO. There was also a relative lack of ILO with the dual-optic silicone lens.

Optical analysis of an accommodating intraocular lens

PURPOSE

To model the feasibility of an accommodating intraocular lens (IOL) that allows near vision by means of an anterior translation within the capsular bag.

SETTING

University of Waterloo, Waterloo, Ontario, Canada.

METHODS

Model eyes were constructed and analyzed based on experimental data using Code V, a computerized optical design tool. The potential near vision of IOLs of different powers was calculated as they were moved anteriorly within the capsular bag. The conditions under which a spherical lens performs well and when an aspheric design should be considered were determined.

RESULTS

Accommodation (the dioptric change from the far to the near point) varies linearly with lens movement and is sensitive to corneal and IOL powers. Simple equations were derived and accurately predicted induced accommodation. Retinal image quality varies significantly with pupil size and IOL power. However, image quality is minimally affected by the amount of induced accommodation.

CONCLUSIONS

From an optical standpoint accommodation with adequate image quality can be achieved by anterior movement of a spherical IOL within the capsular bag.

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.

Optical principles, biomechanics, and initial clinical performance of a dual-optic accommodating intraocular lens (an American Ophthalmological Society thesis)

PURPOSE

To design and develop an accommodating intraocular lens (IOL) for endocapsular fixation with extended accommodative range that can be adapted to current standard extracapsular phacoemulsification technique.

METHODS

Ray tracing analysis and lens design; finite element modeling of biomechanical properties; cadaver eye implantation; initial clinical evaluation.

RESULTS

Ray tracing analysis indicated that a dual-optic design with a high plus-power front optic coupled to an optically compensatory minus posterior optic produced greater change in conjugation power of the eye compared to that produced by axial movement of a single-optic IOL, and that magnification effects were unlikely to account for improved near vision. Finite element modeling indicated that the two optics can be linked by spring-loaded haptics that allow anterior and posterior axial displacement of the front optic in response to changes in ciliary body tone and capsular tension. A dual-optic single-piece foldable silicone lens was constructed based on these principles. Subsequent initial clinical evaluation in 24 human eyes after phacoemulsification for cataract indicated mean 3.22 diopters of accommodation (range, 1 to 5 D) based on defocus curve measurement. Accommodative amplitude evaluation at 1- and 6-month follow-up in all eyes indicated that the accommodative range was maintained and that the lens was well tolerated.

CONCLUSIONS

A dual-optic design increases the accommodative effect of axial optic displacement, with minimal magnification effect. Initial clinical trials suggest that IOLs designed on this principle might provide true pseudophakic accommodation following cataract extraction and lens implantation.

New intraocular lens technology

PURPOSE

To review the current status of phakic intraocular lenses (IOLs), intraocular treatment of presbyopia, and IOLs that filter some blue light.

DESIGN

Review of current information on the subject from numerous sources.

METHODS

Medline search and Internet search engines on the topics of phakic IOLs, presbyopia or multifocal IOLs, and blue light and macular degeneration.

RESULTS

An iris fixated phakic IOLs is now approved in the United States (US). There are concerns for corneal endothelial stability and late dislocation. Other approaches include anterior chamber fixation with concerns of corneal endothelial stability and pupil elongation, and posterior chamber fixation with concerns of cataract formation, IOL dislocation, and pigment dispersion. Intraocular treatment of presbyopia includes monovision, multifocal, and accommodative IOLs. Which approach is superior today is still not clear. There are IOLs designed to block some blue light to potentially lessen the risk of age-related macular degeneration (ARMD). While there is presumptive evidence of this, no definitive study shows such a correlation. Color perception issues are unlikely to be a problem. While decreased scotopic vision has been proposed, there is no study that proves this is an issue of clinical significance.

CONCLUSIONS

The IOL field is dynamic with many new choices. Phakic IOLs and treatment of presbyopia will be an increasingly important part of ophthalmology; however, there are important unresolved issues. With better evidence that blue light is an important variable in ARMD, such an approach could rapidly become the standard.

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.

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.

Capsular bag opacification after experimental implantation of a new accommodating intraocular lens in rabbit eyes

PURPOSE

To evaluate the development of capsular bag opacification in rabbit eyes after implantation of an intraocular lens (IOL) designed to minimize contact between the anterior capsule and the IOL and ensure expansion of the capsular bag.

SETTING

David J. Apple, MD Laboratories for Ophthalmic Devices Research, John A. Moran Eye Center, University of Utah, Salt Lake City, Utah, USA.

METHODS

Ten New Zealand white rabbits had a study IOL (new accommodating silicone IOL [Synchrony, Visiogen, Inc.]) implanted in 1 eye and a control IOL (1-piece plate silicone IOL with large fixation holes) implanted in the other eye. Intraocular lens position, anterior capsule opacification (ACO), and posterior capsule opacification (PCO) were qualitatively assessed using slitlamp retroillumination photographs of the dilated eyes. Anterior capsule opacification and PCO were graded on a 0 to 4 scale after the eyes were enucleated (Miyake-Apple posterior and anterior views after excision of the cornea and iris). The eyes were also evaluated histopathologically.

RESULTS

The rate of ACO and PCO was significantly higher in the control group. Fibrosis and ACO were almost absent in the study group; the control group exhibited extensive capsulorhexis contraction, including capsulorhexis occlusion. Postoperative IOL dislocation into the anterior chamber and pupillary block syndrome were observed in some eyes in the study group.

CONCLUSIONS

The special design features associated with the study IOL appeared to help prevent PCO. Complications in the study group were probably caused by the increased posterior vitreous pressure in rabbit eyes compared to human eyes and the relatively large size of the study IOL relative to the anterior segment of rabbit eyes.

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.