Accommodation is a change in the focus of the eye from distant to near objects. This is defined as a dioptric change in the eye’s optical power. Presbyopia, defined as the physiologic decrease in accommodative amplitude during the aging process, may be due in part to a combination of factors, such as loss of elasticity of the crystalline lens, increase of the equatorial diameter of the lens, alterations in the elastic part of the Bruch membrane, or changes in the ciliary muscle.
The issue of restoring accommodation following cataract surgery or through refractive lens exchange is becoming an increasingly important topic in ophthalmology. Several different approaches can be taken to address this problem. Current approaches for treating presbyopia include monovision (this being the most often chosen method by ophthalmologists for treating their own eyes), presbyLASIK, corneal inlays, and multifocal intraocular lenses (IOLs). Great interest has been generated in the field of presbyopia correction after the approval of the accommodating IOL in the United States in 2001. Accommodating IOLs may avoid problems associated with multifocal IOLs, such as decreased contrast sensitivity and glare/halos, and have the potential to provide near, intermediate, and distance vision without correction. Smart accommodative intraocular lenses equipped with advanced electronics to facilitate accommodation may be the future of presbyopia correction.
One important issue with the new accommodative IOL technology is the measurement of the performance of the IOL. To clarify whether accommodation is restored by an IOL, it is necessary to demonstrate objectively that the eye undergoes an increase in power with accommodation. Although no standardized methods currently exist for measuring phakic or pseudophakic accommodation, a variety of techniques are available and can be routinely used in clinical practice. It is also important to differentiate the so-called pseudoaccommodation from true accommodation, which can result from multifocality, monovision, a small pupil that increases the depth of focus, and ocular aberrations. The aim in the implantation of the accommodating IOL is to maintain binocularity at all distances. The aim of pseudoaccommodation is to provide the functional value of accommodation by generating two separate focal points along the optical axis in order to provide good near and distance vision, and functional intermediate vision.
The ideal accommodating IOL would be one that mimics the function and properties of the juvenile lens, a lens that changes in shape and dioptric power when the ciliary muscle contracts. Several obstacles will need to be overcome to achieve feasible fluid-/gel-filled capsular bag lens implants, including surgical technique, correct volume and shape of the refilled lens, and capsular opacification. Although different accommodating IOL technologies have been developed and have shown acceptable results, accommodation remains one characteristic of the human lens that no conventional intraocular device can perfectly match. In order to achieve success with true accommodating IOLs, several important steps should be taken, such as the quantification of accommodation by subjective and objective tests; identification of pseudoaccommodation in the outcomes; testing of outcomes by homologated charts for near (40-cm) and intermediate (70-cm) vision; and the confirmation of results by multicentric, longitudinal studies.The approaches for accommodative IOL are based on the mechanisms of change in axial position, change in the refractive index of the cornea, or change in the shape of the cornea.
Smart Intraocular Lenses
The idea behind smart IOLs is based on the need for an actuator and controller that guides the change in the power of the lens and a sensor that detects the information to facilitate accommodation. The mechanism for accommodation can be anything from a variable refractive index lens, deforming liquid optics, to movable multioptic lens and custom lens based on pupil dynamic and wavefront characteristics. Other smart IOLs have been focused on glucose sensing; having clearer vision in low light conditions; having magnified vision; treating glaucoma based on IOP measurements; and on collecting biometric data, such as body temperature and blood alcohol content, and sensing external conditions, such as allergens.
Among the smart accommodating IOLs focused on treating presbyopia are FluidVision (PowerVision), Dynacurve (NuLens), and FlexOptix (FlexOptix GmbH).
Single-Optic, Flexible Haptic Support
Single-power IOLs with flexible haptics allow for the optic of the lens system to displace anteriorly when the ciliary muscles contract.
The Crystalens is made of silicone and has an overall diameter of 11.5 mm. It has a 4.5-mm optic and grooved plate haptics with ends of polymide ( Fig. 41.1 ). Cumming et al. postulated that the ciliary muscle, while bulking backwards when it constricts, increases the pressure of the vitreous on the intraocular lens optic. The latter is thus shifted forward, also by the action of the forward shift of the zonular plane. The optic then reverts back to its initial position upon relaxation of the accommodative muscles in the eye.
The Akkommodative 1CU lens is made of hydrophylic acrylic material. It has an overall diameter of 9.5 mm, a 5.5-mm optic and four haptics ( Fig. 41.2 ). During relaxation of the capsular bag induced by ciliary muscle contraction, the IOL is displaced forward at the hinges of the four haptics.
A disadvantage in these types of lens design is that the generated accommodative power induced by a 1-mm anterior displacement is proportional to the power of the IOL; lower-power IOLs will generate less accommodation than higher-power IOLs. Using high-precision, high-resolution, dual-beam partial coherence interferometry, Findl and associates determined the movement of accommodating IOLs compared to that of conventional monofocal IOLs. Their results showed that the anterior displacement of the accommodative IOLs was equivalent to approximately 0.5 diopters (D) in the majority of cases ( n = 62), which is within the realm of pseudoaccommodation. Neither polishing of the capsule bag nor a posterior capsulorhexis could enhance the accommodative ability.
Results of a clinical study by Mastropasqua et al. showed that the accommodating IOL 1CU provided better useful spectacle correction–free near visual acuity versus a monofocal IOL. The mean amplitude in the accommodating IOL group was 1.14 ± 0.44 D (range, 0.75–2.00 D) at 7 days, 2.36 ± 0.28 D (range, 2.00–2.75 D) at 30 and 90 days, and 1.90 ± 0.77 D (range, 0.75–2.75 D) at 6 months as compared to the monofocal IOL group, which showed no accommodative amplitude.
In clinical studies by Cumming et al. on the AT-45 Crystalens accommodating IOL, uncorrected distance acuity of better than or equal to 20/40 and near acuity of better than or equal to 20/30 were obtained in 97% of cases. Kuchle et al. and Kanellopoulos reported similar results with the 1CU IOL. Mastropasqua et al. reported excellent uncorrected distance and near visual acuity as compared to a conventional monofocal IOL. However, they also reported a decrease in near vision acuity at 6 months after the surgery in the accommodating IOL, which approximated that of the monofocal IOL group. They noted these results in patients who developed anterior capsular opacity (ACO) and posterior capsular opacity (PCO), and in patients who developed both ACO and PCO. These observations lead to the hypothesis that fibrosis may make the capsular bags more rigid, thereby interfering with the flexion of the haptics responsible for the forward displacement of the optic, resulting in increased accommodative power of the IOL. The study suggested that the resultant fibrosis may be avoided with the improvement of IOL edge design and the material used in its manufacture.
ACO development has been shown to be related to hydrophilic acrylic IOLs. The 1CU IOL is made of this material and may be the cause for the development of ACO. The anterior displacement of the 1CU optic during ciliary muscle contraction may allow for the migration of lens epithelial cells (LECs) to the posterior capsule, leading to PCO development. The material and design of static power, axially displacing IOLs may be limited in efficacy owing to the development of capsular fibrosis and opacities that hinder its mechanism for inducing pseudophakic accommodation. In the long term, contraction of the capsule becomes inevitable, and the 1CU IOL loses its accommodative property. A study reported higher incidence of ACO and PCO in 100% of their patients (14 eyes) within postoperative year 1 and a complete loss of the accommodative property of the 1CU IOL. Research in IOL materials and design may improve and maintain the accommodative properties of this simple but effective concept. Physicians must be very judicious and forthcoming in suggesting the use of this type of accommodating lens; although ingenious and effective, valid concerns have been expressed as to their limitations.
Sadoughi et al. reported significant improvements in uncorrected and distance corrected near visual acuities with the implantation of Crystalens HD. Good visual outcomes have been reported using Crystalens for distance vision; however, some studies reported unfavorable visual outcomes regarding intermediate and near visual acuities in comparison to the monofocal IOLs. Objective measurements of accommodative response, such as laser ray tracing aberrometry, were reported to be lower by 0.4 D in Crystalens compared to the monofocal IOL. Alió et al. reported poor defocus curves in Crystalens compared to both multifocal refractive IOLs and dual-optic accommodating IOLs. The authors also reported greater incidence of higher-order aberrations and PCO postoperatively compared to the dual-optic accommodating Synchrony IOL.
Crystalens Surgical Technique
The protocol is based on our routine protocol for cataract surgery, with some modifications. Pupil dilation is performed using 4 drops of tropicamide 0.5% and phenylephrine 10% at 15-minute intervals, 1 hour prior to surgery. Peribulbar anesthesia with ropivacaine 0.75% is then applied, followed by one drop of 0.5% proparacaine HCl. Before surgery, the following equipment and medications are prepared alongside the routine cataract sets:
10-0 vicryl suture
atropine 1% drops
cyclopentolate 1% drops
Crystalens IOL and spare monofocal IOL.
Other Single-Optic Systems
The Kellen Tetraflex (Lenstec, Inc.) IOL is intended for active lifestyles that require good near vision and crisp intermediate and distance vision. It is a 11.5-mm-long accommodating lens made of hydrophilic acrylic material (medical grade hydroxyethylmethacrylate [HEMA], with 26% water content), with an optic of 5.5 mm and an anterior vault of 5 degrees that is designed to promote movement during ciliary contraction. The Tetraflex IOL can be used for both refractive and cataract lensectomy surgery. Currently, it is available for use only outside the United States and is undergoing US Food and Drug Administration (FDA) review for premarket approval.
From the studies done on the mechanism of action of Tetraflex, its positions appear to be relatively fixed in the eye and, although the ocular aberrations of the eye were reported to change with the increasing accommodative demands, the changes did not appear to be consistent among all individuals. Some of the reported benefits of the Tetraflex IOL in terms of near visual acuity therefore appear to be due to optical aberration changes from the IOL flexure rather than due to the forward movement of the lens within the capsular bag. Owing to high flexibility of the hydrophilic acrylic material of the IOL, there is greater susceptibility of the capsular bag contraction, with subsequent flexing of the haptic part of the lens, making the accommodating IOL exchange necessary.