Restoring accommodation: a dream or an approaching reality?

Presbyopia: Effectiveness of correction strategies

Presbyopia is a global problem affecting over a billion people worldwide. The prevalence of unmanaged presbyopia is as high as 50% of those over 50 years of age in developing world populations, due to a lack of awareness and accessibility to affordable treatment, and is even as high as 34% in developed countries. Definitions of presbyopia are inconsistent and varied, so we propose a redefinition that states "presbyopia occurs when the physiologically normal age-related reduction in the eye's focusing range reaches a point, when optimally corrected for distance vision, that the clarity of vision at near is insufficient to satisfy an individual's requirements". Strategies for correcting presbyopia include separate optical devices located in front of the visual system (reading glasses) or a change in the direction of gaze to view through optical zones of different optical powers (bifocal, trifocal or progressive addition spectacle lenses), monovision (with contact lenses, intraocular lenses, laser refractive surgery and corneal collagen shrinkage), simultaneous images (with contact lenses, intraocular lenses and corneal inlays), pinhole depth of focus expansion (with intraocular lenses, corneal inlays and pharmaceuticals), crystalline lens softening (with lasers or pharmaceuticals) or restored dynamics (with 'accommodating' intraocular lenses, scleral expansion techniques and ciliary muscle electrostimulation); these strategies may be applied differently to the two eyes to optimise the range of clear focus for an individual's task requirements and minimise adverse visual effects. However, none fully overcome presbyopia in all patients. While the restoration of natural accommodation or an equivalent remains elusive, guidance is given on presbyopic correction evaluation techniques.

Accommodative intraocular lenses: where are we and where we are going

Presbyopia still remains the last frontier of refractive surgery. Its surgical management is under constant evolution due to the limitations that exist today with respect to its management, which is probably in relation with the multifactorial basis in which presbyopia is clinically developed in the human. Until currently, virtually all surgical techniques that have been proposed for its correction are based on the induction of pseudoaccommodation in the presbyopic eye, including multifocality. However, the real restoration of accommodation is more complex, and it has been tried by the use of different, so called, “accommodative” pseudophakic intraocular lenses (AIOL). Overall, the reported results with these lenses by independent authors have been modest in relation with the restoration of the accommodative power of the eye and these modest benefits are usually lost with time due to the long term changes in the capsular bag. This fact made these lenses to be almost abandoned in the last few years, but there are currently other AIOL models being used with innovative mechanisms of action and different anatomical support outside the capsular bag that offer encouraging preliminary results that could bring a new potential of application to these types of lenses.

In this article, we will update the modern refractive surgeon about the fundamentals and provide updated information about the outcomes of AIOLs by reviewing the concept of accommodation, the different attempts that have been accomplished in the past, their demonstrated published results in human clinical trials, and the future alternatives that may arrive in the near future.

Graphene electrodes for adaptive liquid crystal contact lenses

The superlatives of graphene cover a whole range of properties: electrical, chemical, mechanical, thermal and others. These special properties earn graphene a place in current or future applications. Here we demonstrate one such application - adaptive contact lenses based on liquid crystals, where simultaneously the high electrical conductivity, transparency, flexibility and elasticity of graphene are being utilised. In our devices graphene is used as a transparent conductive coating on curved PMMA substrates. The adaptive lenses provide a + 0.7 D change in optical power with an applied voltage of 7.1 V_rms - perfect to correct presbyopia, the age-related condition that limits the near focus ability of the eye.

Novel switching mode in a vertically aligned liquid crystal contact lens

Liquid crystal (LC) contact lenses are emerging as an exciting technology for vision correction. A homeotropically (vertical) aligned LC lens is reported that offers improved optical quality and simplified construction techniques over previously reported LC contact lens designs. The lens has no polarization dependence in the off state and produces a continuous change in optical power of up to 2.00 ± 0.25 D with a voltage applied. The variation in optical power results from the voltage-induced change in refractive index of the nematic LC layer, from 1.52 to a maximum of 1.72. One device substrate is treated with an alignment layer that is a mixture of planar and homeotropic polyimides, rubbed to induce a preferred director orientation in the switched state. Defects that could occur during switching are thus avoided and the lens exhibits excellent optical quality with a continuous variation in focal power.

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.

Functionalised polysiloxanes as injectable, in situ curable accommodating intraocular lenses

The aged eye's ability to change focus (accommodation) may be restored by replacing the hardened natural lens with a soft gel. Functionalised polysiloxane macromonomers, designed for application as an injectable, in situ curable accommodating intraocular lens (A-IOL), were prepared via a two-step synthesis. Prepolymers were synthesised via ring opening polymerisation (ROP) of octamethylcyclotetrasiloxane (D(4)) and 2,4,6,8-tetramethylcyclotetrasiloxane (D(4)(H)) in toluene using trifluoromethanesulfonic acid (TfOH) as catalyst. Hexaethyldisiloxane (HEDS) was used as the end group to control the molecular weight of the prepolymers, which were then converted to macromonomers by hydrosilylation of the SiH groups with allyl methacrylate (AM) to introduce polymerisable groups. The resulting macromonomers had an injectable consistency and thus, were able to be injected into and refill the empty lens capsular bag. The macromonomers also contained a low ratio of polymerisable groups so that they may be cured on demand, in situ, under irradiation of blue light, in the presence of a photo-initiator, to form a soft polysiloxane gel (an intraocular lens) in the eye. The pre-cure viscosity and post-cure modulus of the polysiloxanes, which are crucial factors for an injectable, in situ curable A-IOL application, were controlled by adjusting the end group and D(4)(H) concentrations, respectively, in the ROP. The macromonomers were fully cured within 5 min under light irradiation, as shown by the rapid change in modulus monitored by photo-rheology. Ex vivo primate lens stretching experiments on an Ex Vivo Accommodation Simulator (EVAS) showed that the polysiloxane gel refilled lenses achieved over 60% of the accommodation amplitude of the natural lens. An in vivo biocompatibility study in rabbits using the lens refilling (Phaco-Ersatz) procedure demonstrated that the soft gels had good biocompatibility with the ocular tissue. The polysiloxane macromonomers meet the targeted optical and mechanical properties of a young natural crystalline lens and show promise as candidate materials for use as injectable, in situ curable A-IOLs for lens refilling procedures.

Far and near visual acuity with multifocal intraocular lenses in an optomechanical eye model with imaging capability

PURPOSE

To compare the quantitative and qualitative visual performances of different multifocal intraocular lenses (IOLs) in an experimental model of the human eye.

SETTING

University Hospital San Raffaele, Milan, Italy.

METHODS

Five multifocal IOLs and 1 monofocal IOL were implanted in an optomechanical eye model with imaging capability. The comparative optical characterization of the imaging performance included aberrometry, simulated visual acuity testing at variable contrast for far and near distance, glare tests, and image records of optotype charts.

RESULTS

The maximum recorded far visual acuity for the monofocal IOL was between 20/12.5 and 20/16; the multifocal IOLs decreased visual acuity by 1 to 2 lines. The difference tended to increase at reduced contrast. Full-contrast near visual acuity with multifocal IOLs ranged between 20/63 and 20/25; the near distance performance of the monofocal IOL without an additional correcting lens was worse by 1 to 3 lines of acuity with large pupils but was comparable with small pupils. Multifocal IOLs of different designs showed marked differences as a function of contrast, which tended to balance between far and near behaviors.

CONCLUSIONS

Multifocal IOLs of different optical designs were well characterized and distinguished by simulated contrast acuity testing in an experimental eye model, allowing quantitative comparison. Their overall visual performance, averaged over contrast and distance, was not superior to the performance of a monofocal IOL without an additional correcting lens.

[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.