Understanding the accommodating intraocular lens

Injectable nanocomposite hydrogels as an emerging platform for biomedical applications: A review

Hydrogels have attracted much attention for biomedical and pharmaceutical applications due to the similarity of their biomimetic structure to the extracellular matrix of natural living tissues, tunable soft porous microarchitecture, superb biomechanical properties, proper biocompatibility, etc. Injectable hydrogels are an exciting type of hydrogels that can be easily injected into the target sites using needles or catheters in a minimally invasive manner. The more comfortable use, less pain, faster recovery period, lower costs, and fewer side effects make injectable hydrogels more attractive to both patients and clinicians in comparison to non-injectable hydrogels. However, it is difficult to achieve an ideal injectable hydrogel using just a single material (i.e., polymer). This challenge can be overcome by incorporating nanofillers into the polymeric matrix to engineer injectable nanocomposite hydrogels with combined or synergistic properties gained from the constituents. This work aims to critically review injectable nanocomposite hydrogels, their preparation methods, properties, functionalities, and versatile biomedical and pharmaceutical applications such as tissue engineering, drug delivery, and cancer labeling and therapy. The most common natural and synthetic polymers as matrices together with the most popular nanomaterials as reinforcements, including nanoceramics, carbon-based nanostructures, metallic nanomaterials, and various nanosized polymeric materials, are highlighted in this review.

Analysis and applications of accommodative lenses for vision corrections

Analysis and applications of vision correction via accommodating intraocular lens (AIOL) are presented. By Gaussian optics, analytic formulas for the accommodation rate function (M) for two-optics and three-optics systems are derived and compared with the exact numerical results. In a single-optics AIOL, typical value of M is (0.5-1.5) D∕mm, for an IOL power of (10-20) diopter. For a given IOL power, higher M is achieved in positive-IOL than negative-IOL. In the dual-optics AIOL, maximum accommodation is predicted when the front positive-optics moves toward the corneal plan and the back negative-optics moves backward. Our analytic formulas predict that greater accommodative rate may be achieved by using a positive-powered front optics, a general feature when either front or back optics is mobile. The M function is used to find the piggy-back IOL power for customized design based on the individual ocular parameters. Many of the new features demonstrated in this study can be easily realized by our analytic formulas, but not by raytracing method.

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.

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.

Feasibility and development of a high-power real accommodating intraocular lens

PURPOSE

To develop an accommodating intraocular lens (IOL) that changes power with ciliary body action and generates a minimum of 8.0 diopters (D) by manipulation of a flexible material between a sulcus-fixated rigid plate and a ciliary muscle-operated capsular diaphragm.

SETTING

Vissum-Instituto Oftalmológico de Alicante and Miguel Hernández University, Alicante, Spain.

METHODS

The feasibility of the concept was tested by a laboratory lens model. An implantable measuring device was constructed to simulate the lens action. The device was implanted in monkey eyes to measure the various parameters involved with such IOLs. Based on these measurements, an accommodating IOL prototype was built and implanted in monkeys' eyes. Pharmacologic agents were used to achieve ciliary relaxation and spasm. Ultrasound biomicroscopy (UBM) imaging was used to document the active changes of the IOL flexible lens curvature as related to the ciliary muscles status.

RESULTS

The laboratory model produced more than 50.0 D of accommodation. The UBM demonstrated changes in lens curvature between cyclospasm to cycloplegia of calculated 9.0 to 53.0 D for flexible material with a refractive index of 1.41.

CONCLUSIONS

Flexible material is capable of being manipulated when placed between a scleral-fixated rigid plane and the ciliary muscles-operated capsular diaphragm. Such manipulation as used by the IOL concept presented here has generated an active change of more than 40.0 D in the monkey eye.

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.

Surgical treatment of presbyopia: scleral, corneal, and lenticular

PURPOSE OF REVIEW

Having solved most of the problems concerning myopia, hyperopia, and astigmatism, it is perfectly understandable that the surgical treatment of presbyopia should be next on the agenda. It is a new challenge where every possible aspect is explored.

RECENT FINDINGS

The past five years have been troubled by the debate over von Helmholtz theory on accommodation. Numerous investigations have been carried out on the primate and humans using various procedures. The more we learn about this mechanism, the nearer we will be to finding a solution to presbyopia. It appears essential to refer to recent works confirming von Helmholtz theory. Therefore, understanding presbyopia requires a great deal of optical ingenuity such as monovision, scleral modifications, which still remain controversial, or clear lens exchange or refilling.

SUMMARY

In 2003, with all the different techniques available, the surgeon has a wide choice to offer patients that are satisfactory from a practical, theoretical, and ethical point of view. However, these techniques must only be proposed once the patients have been carefully informed.