Presbyopia / SPO - 3.2.2.2. The evolution of accommodative lenses

3.2.2.2. The evolution of accommodative lenses

Fernando Silva

Hospital Privado de Braga, Portugal

INTRODUCTION

The correction of presbyopia is the last major challenge in the field of refractive surgery. The physiological degradation of the accommodation has multifactorial causes, which makes this treatment more difficult.

With age, there is a progressive thickening and loss of elasticity of the lens and its capsule, as well as an anterior displacement of the ciliary muscle, an increase in the volume of the ciliary body and an increase in its connective tissue1,2.

On the other hand, the fact that presbyopia is universal with the aging of the eye, with millions of people in need of treatment3, makes the investigation and development of accommodating intraocular lenses a topic of increasing interest.

The development of a lens capable of providing a change in optical power in a smooth and progressive manner would have a high potential. To this end, various types of accommodative lenses are currently under study.

After removal of the crystalline lens there is a deepening of the anterior chamber and a posterior displacement of the ciliary body, with several studies demonstrating that the ciliary body maintains the capacity to react to accommodative stimuli4, which may alter the position or index of refraction of an intraocular lens. The initial hypothesis about the mechanism of action of these lenses was centered on the anterior translational movement of the lens. Several studies have shown different results regarding the degree and direction of this movement. It is known today that there is a combination of this movement with the tilting and flexion of the lens optics itself5.

Because accommodative lenses use a monofocal optic, they have no problem of diminishing visual quality and contrast sensitivity, as with multifocal lenses, where light is divided into 2 or 3 outbreaks, with the formation of conflicting images on the retina. There are also fewer dysphotopic phenomena such as halos and glare. Visual quality is also less affected by possible small residual astigmatism.

They may be indicated when there are contraindications to the use of multifocal lenses, or when their performance is suboptimal, such as patients with macular problems (DMI, retinopathy, epiretinal membrane), advanced glaucoma and irregular astigmatism.

The distant and intermediate visual acuity is high, but for near vision a large percentage of patients require the use of glasses. One of the strategies to be used, in order to minimize the need for glasses, is to have a refractive target in the dominant eye of 0/-0.25 D and in the non-dominant eye of -0.75/-1.00 D if patients value independence of glasses highly.

ACCOMMODATION AND PSEUDO-ACCOMMODATION

Accommodation is the process by which a change in the power of the lens, associated with the contraction of the ciliary body, occurs to form a clear image on the retina, at different distances. According to Helmholtz's theory, with contraction of the ciliary body there is a reduction of tension on the zonular fibers and consequently an increase in the anterior and posterior curvature of the lens, as well as an increase in its thickness and anterior displacement6.

Increased crystalline hardness (up to 1000 times during life)7 is mainly responsible for the loss of accommodation, maintaining the contractility of the ciliary body almost unchanged8. Therefore, changes in the position and shape of an intraocular lens may occur under the contractile effect of the ciliary body. The challenge to overcome is to create a lens that is capable of achieving it in an effective, predictable and long-lasting way.

Pseudo-accommodation consists of any other mechanism that alters the power of the eye's optics or the way it functions, so that there is good near vision and therefore not a true restoration of accommodation. These are static and non-dynamic mechanisms, such as those occurring in accommodation.

The factors that may help with pseudo-accommodation are residual myopia, pinhole effect of a miotic pupil (2 D of pseudo-accommodation in a 2.5 mm pupil), mild myopic astigmatism against the rule, spherical corneal aberration, corneal multifocality, corneal coma, neural processing, recognition of visual patterns and the personal motivation of each patient5.

The pseudo-accommodation can go up to 2 D, is not dependent on the presence of an intraocular lens, and therefore a true pseudophakic accommodation has to demonstrate a lens movement and/or a change in its optical power, correlated with accommodative effort.

TYPES OF ACCOMMODATIVE LENSES

In the 1980s, Cummings observed that some patients with silicone intraocular lenses were able to have good vision at close range, showing an optical movement of 0.7 mm, and these data were later confirmed by Thornton9.

These results have created a great deal of interest in the investigation of accommodative lenses, without close addition or multifocality, in order to reduce the phenomena of halos and glare associated with multifocal lenses.

Several models with accommodative capacity have been proposed, in order to maintain some near vision capacity.

There are several optics and haptics designs designed to take advantage of the movement of the ciliary body and the vitreous in order to alter the position and/or shape of the intraocular lens. However, this movement will have different accommodative effects, depending on lens power, corneal curvature and axial length.

The accommodation obtained by the anterior motion of 1 mm of an intraocular lens may range from 0.8 D in a long eye to 2.3 D in a short eye10. In addition, the objective measures performed vary widely with the methods used in different studies, but these are always small movements, and cannot justify even 1 D of accommodation. There are studies that demonstrate even posterior movements in about half of the lenses11.

It should also be noted that there is progressive capsular bag fibrosis and lens blockage, with decreased movement and effectiveness.

Implantation in sulcus may eventually overcome these problems because it uses the forces generated by the ciliary body, regardless of the integrity of the posterior capsule.

We can classify these lenses according to their mechanism of action, namely how they change in response to ciliary body contraction:

Change in position (single (A) or double (B) optics)
Change in shape

3- Filling of the Capsular Bag with a Gel or Polymer

4- Modulation of the Refractive Index

1. POSITION CHANGE

These lenses were developed to improve near visual acuity through anterior displacement of the lens optics.

A- SINGLE OPTICS

CRYSTALENS

Biconvex lens made of a 3rd generation silicone elastomer (biosil), with T-shaped haptics, for attachment to the capsular bag. It has bevels between the optic and the haptics, in order to allow changes in the axial position and the curvature of the lens (Figure 1). The Crystalens AT-45 was the first compliant lens approved by the FDA in 2003. Currently there are the AO and HD versions. The visual acuities for distant and intermediate are high with this lens, but for near the results are contradictory and may be due mainly to phenomena of pseudo-accommodation12.

Figure 1 - Implanted Crystalens, 1st day after surgery

Recent studies have demonstrated an objective accommodation of 0.4 D, with improvements in visual acuity for near in relation to monofocal lenses, probably due to geometric changes and lens alignment, which occur with the accommodative effort, provoking an increase in the depth of focus by pseudo mechanisms (astigmatism, spherical aberration, trefoil and coma)13.

Dysphotopic phenomena are smaller and the contrast sensitivity is higher in this lens than in multifocal lenses.

1CU

Hydrophilic acrylic lens with biconvex optics and 4 flexible haptics, which fold with contraction of the ciliary muscle, allowing anterior movement of the lens. The lens depends greatly on the flexibility of the capsule, losing much effectiveness with the capsular contraction that occurs over time. One study showed an average accommodation of 1.90 D at 6 months and 0.30 D at 12 months14. Other studies measured a range of objective accommodation of about 0.68 ± 0.49 D at 4 years15.

TETRAFLEX

Very flexible hydroxyethyl methacrylate (HEMA) lens with angled haptics, allowing movement of the lens, along with the capsular bag, during accommodation. Studies have shown some improvement in near visual acuity in relation to monofocal lenses, but the lens remained relatively fixed, this benefit probably being due to the induction of high-order aberrations16.

B- DOUBLE OPTICS

SYNCHRONY

Lens consisting of two silicon plate optics, one anterior with a very high positive power (about +32 D) and a posterior one with negative power to achieve ametropia. These 2 lenses are connected by 4 haptics that work as springs. When there is accommodation the capsular tension is reduced and there is expansion of the capsular bag, there being an anterior axial movement of the anterior optic with a dynamic increase of the dioptric power of the intraocular lens. The results of a study of 74 patients with bilateral Synchrony lens implantation showed visual acuity for near of 20/27 and 70% of patients did not require glasses17.

LUMINA

This lens has two optical elements of a hydrophobic acrylic polymer. These two components are implanted in the ciliary sulcus and are capable of moving on top of each other. The posterior element corrects for far vision and the anterior element is designed to provide 5 D of correction. Each component has an aspheric surface, linearly increasing its power with lens movement as the ciliary muscle contracts, focusing the light for near vision.

In a 61-eyes study the near-corrected visual acuity, corrected for distance, was 0.11 LogRAD and an accommodation of 1.27 D was objectively manifested18. Placing the lens in sulcus may prevent the effects of fibrosis and capsular contraction, which may disrupt the performance of the lens placed in the capsular bag.

2. SHAPE CHANGE

Small changes in the curvature of a lens can produce large changes in its diopter power. Several lenses work on this principle.

NULENS

Lens with PMMA haptics that are placed in the ciliary sulcus, with an anterior lens that corrects vision for distance, a small chamber with a silicone gel and a posterior piston with a central opening. When there is contraction of the ciliary muscle the piston induces a bulging of the silicone gel, thus increasing the optical power of the lens. In a study with 10 patients a slight gain in visual acuity was achieved for near, but a large loss of endothelial cells and 60% of posterior capsular opacification occurred19.

WIOL-CF

Lens with a polyfocal optics that changes its shape during accommodation, trying to mimic the structure and shape of a young lens. It is a full-optic, haptic-free hydrogel lens that fills the entire capsular bag. Theoretically the lens works by its polyfocality and hyperbolic design, with increased depth of focus and deformation due to ciliary muscle contraction, with increased thickness and change in curvature of the lens itself. In a prospective study with 50 eyes, uncorrected near visual acuity of 20/25 or higher was achieved in 72% of cases20. In another, more recent study, about half of the patients did not need glasses for near, and near-corrected far vision was 0.26 LogMAR at 6 months21.

FLUIDVISION

Lens with a 6-mm hydrophobic acrylic optic, filled with silicone oil, bonded to hollow haptics also filled with silicone. When accommodation occurs, there is an oil passage, with a change in the curvature of the optics of the lens. The fifth generation of this lens has recently started clinical trials. In a multicenter study, with the fourth generation of the lens, which involved 117 patients, visual acuities of 0.3 LogMAR were achieved for near, with objective measures of accommodation between 1.81 and 2.17 D.

JUVENE

Lens with a modular system, consisting of a fixed lens, with an aspherical optical base surrounded by a haptic in 360º, filling the entire capsular bag and by a second lens that is fixed in the anterior one, being able to change its curvature in direct response to the capsular changes due to accommodation. The first clinical trials were planned for the year 2018.

3. FILLING OF THE CAPSULAR BAG

During many years the possibility of filling the capsular bag with a transparent and flexible gel or polymer was investigated, allowing changes in shape with the accommodative effort. However, several difficulties have been encountered related to the high incidence of capsular opacification, polymer leakage during or after surgery, volume control of injected material and production of viscoelastic characteristics that allow controlled deformation and at the same time formation of a retinal image of quality. Recently, Nishi et al22 carried out preliminary studies in pre-presbyopic monkeys, performing phacoemulsification with anterior and posterior capsulorhexis, inserting a silicone lens on the posterior capsule and injecting another collapsible, accommodating lens on the former. Between the two lenses was injected a silicone polymer. In this study 2.5 D accommodation amplitudes were achieved23.

4. MODULATION OF THE REFRACTIVE INDEX

LIQUILENS

This is a lens with an optic that contains two non-miscible solutions with different refractive indices. When the patient looks ahead, he sees through the lower fluid and when looking down he sees through both, with a composite refractive index, allowing a better near vision.

ELECTRO-ADAPTIVE LENSES

Diffractive liquid crystal lens, with electric control of the refractive index in response to accommodation (Elenza Inc.). It has a central hydrophobic aspherical component, and a diffractive liquid crystal is activated for near vision in response to micro-sensors that detect physiological changes in light, caused by the effort of accommodation24.

Other proposed lenses alter the refractive index of their optics by regulating the electric field in response to the movement of the ciliary body. This motion is detected by pressure or electric field transducers25.

CONCLUSIONS

In order to restore accommodation, several attempts have been made, with different lens designs and various mechanisms of action. These lenses are in different stages of development and commercialization. The ideal accommodative lens does not yet exist, but significant steps have been taken in this direction. There are already some lenses that provide good distant and intermediate visual acuities, with few dysphotopic phenomena, but the visual acuities for near, besides not ideal, worsen significantly with time.

We need to develop a truly accommodative lens that does not depend on the effects of pseudo-accommodation, with predictable results, maintaining long-term efficacy and less dependent on capsular fibrosis.

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