|Year : 2013 | Volume
| Issue : 2 | Page : 113-117
Newer intraocular lens materials and design
Head of R & D, Care Group India, India
|Date of Submission||23-Oct-2012|
|Date of Acceptance||11-Feb-2013|
|Date of Web Publication||20-May-2013|
Block No. 310, Dabhasa, Ta. Padra, Vadodara - 391 440, Gujarat
Source of Support: None, Conflict of Interest: None
The continued development of new intraocular lens (IOL) material and design has provided cataract surgeons more lens-based options than ever before. Surgeons must carefully evaluate, which IOLs may be the best for their patients and their practices. The roles of refractive index, water content, optic coloration (blue- or violet-light-blocking), and design of acrylic IOLs are widely debatable among surgeons. Ease of use, availability, cost, and surgeon preference are also important factors that influence surgeons' IOL selection.
Keywords: Diffractive, glistening, intraocular lens, light adjustable, multifocal, tackiness, trifocal
|How to cite this article:|
Argal S. Newer intraocular lens materials and design. J Clin Ophthalmol Res 2013;1:113-7
The IOLs have undergone a sea change since the time that Harold Ridley first implanted a polymer lens in the eye to replace the natural crystalline lens of the eye. One of the earlier year's ophthalmologists was overheard saying at a conference, "You have taken the simple cataract surgery of our times and have transformed it into a complexity!" The challenges have changed, the expectations have changed, and technological advances in the surgical technique and lens materials and manufacturing methods have made return to near normal vision and beyond a reality today. Earlier aphakia was the challenge and then it was the best corrected visual acuity and today we are talking about the quality of vision, accommodation and providing the best corrected visual acuity at all possible points of focus.
The changes in cataract surgery can be said to have taken place on three fronts:
- Advances in cataract surgery and techniques
- Advances in IOL materials
- Advances in IOL designs.
Advances in IOL materials
Polymethyl metha acrylate (PMMA) ruled the roost as the material of choice for a long time before it was eventually replaced by silicone lenses and then by the acrylic lenses both hydrophobic and hydrophilic.
They are biocompatible with good optical clarity. Certain features of the silicone material put them at a disadvantage. They tend to turn opalescent or have a slight tinge after a period of time. They open with a snap while unfolding hence one has to be careful while implantation.
An interesting feature of silicone material is its malleability and elasticity, which makes them a good choice for the accommodating lenses since these lenses would have to deform millions of time over the life time of the patient. In an analysis conducted by Arthur P. Little, it was estimated that the average frequency of accommodation is probably on the order of 20-120 per hour (1 cycle every 3 min to one every 30 s). Assuming that an average person is awake 16 h per day, the range would be from 20 cycles per hour (120,000 per year) to 120 cycles per hour (720,000 per year). With a fatigue life-time exceeding 20 million cycles, it has been roughly estimated that the crystalens should last a minimum of 30 years and probably much longer.
Hydrophobic and hydrophilic acrylic lenses
The acrylic lenses took over from the silicone lenses and have been the front runners in the IOL market with the highest percentage share of lens manufacturing and implantation. Their excellent biocompatibility and optical clarity with ease of manufacture and handling as well as ease of implantation have made them the material of choice for most of the IOLs today. Even as they are bunched as acrylic lenses significant differences exist between different lenses from different manufacturers with each making necessary changes to drive home the advantages and the debate about hydrophobic and hydrophilic rages on. The materials comparisons at various attributes can be summarized as follows:
- Effect on consistency of refractive outcome
Hydrophilic, as the name suggests, absorbs and retains water. Water acts as a plasticizer for the polymer chains and polymer folds. It is the consistency in the ability of the material to take up and retain the water, which makes IOL, caused refractive power and elasticity deviations tending to zero. Hydrophilic IOL, which has undergone the controlled and seamless production procedure, is more likely to have consistent orientation of the matrix and hence consistent water content.
Hydrophobic material absorbs water to minimal amount. The molecular orientation of hydrophobic material, even if varies from batch to batch, may not lead to significant change in its applicable characteristics as those characteristics are not dependent on the molecular orientation but on molecules itself. It is this reason the reproducibility, accuracy, and sensitivity of the refractive outcome will be high in case of hydrophobic IOLs.
- Effect on tackiness
Hydrophilic material does not possess tackiness. The reason is simple-water is not tacky. It makes hydrophilic IOL freely movable and easily injectable. However, since it doesn't easily adhere to the capsule it has a higher rate of posterior capsular opacification (PCO).
Hydrophobic material is tacky. Such lens will not move easily in bag once implanted. It will also adhere well to the capsule reducing the rate of PCO with these lenses.
- structure and its effect on mechanical stability inside the capsular bag
Material structure decides the refractive index (RI), which in turn decides the center thickness. Hence, for a given design mechanical stability of the hydrophilic material is more than hydrophobic material.
- Material structure and glistening
Glistening are the air vacuoles. Generally, hydrophilic material will not have glistening. However, not so perfect hydrophobic IOL material may have them. As there is nothing to fill inside the matrix, glistening is more prevalent in the hydrophobic material. ,
- Material structure and cats eye effect
Lenses with higher refractive indices have a greater tendency to reflect light. Hence, the hydrophobic lenses with higher RI would give a shiny reflex from the pupil at night.
- Yellow IOL material-to block or not to block
The hypothesis that filtering blue-light might increase visual performance was first suggested in the 1970s.  Protagonists of these lenses argued that such an IOL would increase visual quality by reducing longitudinal chromatic aberration, which is three times higher with clear ultraviolet (UV)-blocking IOLs compared with the crystalline lens.  Opponents of blue-light-filter IOLs argue that these lenses might have a negative influence on the scotopic and mesopic contrast sensitivity due to the Purkinje shift, since blue-light is much more important for scotopic than for photopic vision.
The scotopic luminous efficiency peak, mainly contributed to by rods, is at 507 nm, whereas, photopic luminous efficiency peak is at 555 nm, mainly contributed to by cones.  Blocking blue-light up to 500 nm
- should theoretically result in a decrease in mesopic vision. Brockmann et al. have recently shown that commercially available blue-light-filter IOLs have a different transmission spectrum, especially, the orange IOL,  and UV transmission spectrum depending on the IOL material used. There is a significant difference between hydrophilic and hydrophobic acrylic materials.  Mainster actually propagates implantation of orange IOLs to filter violet instead of blue-light. This would protect the retina from the phototoxic short wavelengths between 400 nm and 440 nm and transmit blue-light of more than 440 nm for better scotopic vision. ,, However, the orange IOL has been shown to have a transmission spectrum of less than 60% at 500 nm, whereas, the yellow IOLs have a transmission spectrum of 80-90% at 500 nm. The crystalline lens of a 53-year-old person shows a transmission spectrum of 70% at 500 nm and so, in theory, scotopic vision should actually be better with yellow IOLs in pseudophakic patients than in the phakic eye of a 53-year-old patient. ,
Advances in the design
Most IOL designs are rotationally symmetric, meaning that they can only correct for spherical refractive error in the eye. Toric IOLs have different powers along different meridians and can therefore, correct for cylinder error. Two potential difficulties arise with toric IOLs. First, accurate alignment and fixation of the lens is required. The axis of the IOL must be aligned with the axis of the cylinder error or a reduced correction will be seen. Furthermore, the rotational alignment must remain fixed following surgery, so any post-surgical rotation of the lens would degrade correction and can even introduce additional cylinder error if the rotation was large enough and molded hydrophobic lenses which are inherently tacky in nature are most preferred platform for toric lenses. Acriol EC Toric molded hydrophobic lens introduced by Care Group is fast emerging as preferred toric IOL.
Multifocal IOLs are used in an attempt to simultaneously provide good distance and near vision in a pseudophakic patient. Various designs have been tried to achieve this multifocality, however, the designs fall into two categories : r0 efractive multifocal and diffractive multifocal.
Refractive multifocal IOLs
Refractive multifocal IOLs typically have a series of concentric zones with different optical powers. Implantation of these types of lenses produces two simultaneous images to be formed on the retina of the patient. One image has distant objects in focus and the second image has near objects in focus. It is up to the patient to accept the in-focus portions of both images and ignore the out-of-focus portions. Typically, these patients show losses in contrast sensitivity for distance vision, in exchange for improved near vision. Modulation of the number and dimensions of the near and distance zones in refractive IOLs has been performed by manufacturers in an attempt to optimize the performance of these lenses. These lenses offer a good range of foci from far to near, however, the quality of vision is not good and it is totally a pupil dependent lens. The Rezoom (Abbot Laboratories Inc., Illinios, USA) and Preziol (Care Group, Vadodara, Gujarat, India) Lenses are perfect examples of refractive multifocal lenses.
Diffractive multifocal IOLs
Diffractive IOLs take advantage of diffraction caused by small, closely spaced, annular grooves cut in to the lens surface. The diffraction causes an infinite number of foci for the lens, however, most of the power goes in to the first two foci. By adjusting the spacing and shape of the grooves, the optical properties of the diffractive IOL can be adjusted to be suitable for a multifocal lens. The diffractive IOL gives two very distinct foci one for distance and one for near. At both these distances the clarity of vision is very good. The intermediate vision in these lenses however, has been typically compromised. Another problem is the problem of glare and halos caused by the randomly diffracted rays of light. Furthermore, the contrast and scotopic vision are compromised because at any given focal point only a portion of total light intensity is utilized for creating an image. Most of the lenses split light in the ration of 60:40 (distance: near - light distribution) Many innovative changes have attempted to correct these problems.
The height and spacing of the diffractive rings are reduced from the center to the periphery. This reduces the scatter from the periphery hence reducing glare. The rings are also made with smooth rounded edges in AcriLisa (Zeiss Germany) lens to reduce glare. Restor (Alcon Labs Inc. Forthworth, Texas) has partial optic diffractive lens where the diffractive rings only occupy central 3 mm of the lens the peripheral lens is clear giving a better night time driving vision. Tecnis from AMO is a full optic diffractive lens, which provides near and far vision.
iDIFF Plus and AcriDIFF (Trifocal)
Another interesting design modification has been adopted by the Care Group iDIFF Plus and AcriDIFF (Trifocal) lenses, [Figure 1]. Along with the above mentioned changes in the diffractive rings from the center to the periphery the diffractive rings are also progressively sloped greater from the center to the periphery. This changes the angle of diffraction of light affording good intermediate vision. The iDIFF Plus lens is a hydrophilic lens while the AcriDIFF (Trifocal) lens is hydrophobic lens.
The new IOLs are designed with haptics that will cause the lens to move back and forth in the eye as the ciliary muscle puts pressure on the haptics [Figure 2]. These IOLs provide good distance, intermediate, and near vision by allowing the lens to shift position in the eye to adjust focus. This technology is based on the assumption that the ciliary muscle in older adults still functions properly, however, the crystalline lens has become too stiff or large to accommodate properly.
The crystalens [Figure 3], is a modified plate-haptic lens manufactured from a high (RI = 1.430), third-generation non-reflective silicone material (Biosil), which contains an UV filter. The lens is hinged adjacent to the optic and has small looped polyimide haptic, which have been shown to fixate firmly in the capsular bag. The grooves across the plates adjacent to the optic make the junction of the optic with the plate haptic the most flexible part of the haptic/optic design.
Accurate the medennium smart IOL
In the quest to provide pseudophakic accommodation, one idea has always been to refill the capsular bag with a compressible, clear material. The goal would be to utilize the eye's natural accommodative mechanism according to the Helmholtz theory.
The concept behind the medennium smart IOL, [Figure 4] (Medennium, Inc., Irvine, CA) may overcome many of the aforementioned obstacles. The lens is composed of a hydrophobic acrylic material with unique thermoplastic properties that permit a temperature-induced change in its shape. Chemically, bonding wax to the acrylic polymer creates a "smart" material, which remains in a solid state at room temperature. Because the wax component melts at body temperature, adjusting the percentage of wax content produces a semisoft gelatinous polymer once the lens is in the eye.
The concept of implanting the smart IOL is first to create an optic that fills the bag with an appropriate shape and dioptric power. The lens is then heated and compressed so that a solid, thin, 50 mm long rod results upon cooling. Next, the rod can be implanted through a small incision (about 3.5 mm in widths), through a standard capsulor rhexis, and into the capsular bag. As the rod warms to body temperature, it changes back to a pliable lens measuring 10 mm in diameter and 3.5 mm in thickness. The lens fills the capsular bag as it recovers the pre-determined shape and dioptric power. The smart IOL is still a long way from becoming a clinical option. However, it represents an ingenious approach to reviving the natural mechanism of accommodation.
Kellan tetraflex IOL
The tetraflex IOL features a square-edge design with a 5.75-mm optic, which may be inserted through a 2-mm incision. The lens is composed of PolyHEMA (hydroxyethylmetha acrylate), a highly biocompatible material consisting of 26% water. The structural configuration of the tetraflex is entirely different from that of crystalens, which is a hinged accommodative lens designed to vault posteriorly against the capsular. This movement is dependent on positive vitreous pressure to shift the lens forward. The tetraflex has no hinges and it is angulated forward (i.e., away from the capsular bag) and therefore, has a unique mechanism of accommodation independent of positive vitreous pressure. The haptic configuration of the tetraflex allows the lens to move with the entire capsular bag [Figure 5]. Unlike with the crystalens, no atropinisation is necessary with the tetraflex IOL.
Visiogen synchrony IOL
The visiogen synchrony IOL  (Visiogen, Inc., Irvine, CA) is the first dual-optic accommodating IOL to undergo clinical trials. For any IOL design that relies upon a moving optic to produce near focus, the amount of accommodative shift is proportional to the dioptric power of the optic. Consequently, hyperopes who are implanted with high plus power IOLs should enjoy much greater accommodative amplitude than myopes. The principal goal behind the dual-optic system is to afford every patient a moving +34.00D optic. This aim is accomplished by pairing it with a variable minus power optic in order to provide each individual with the necessary net IOL power for emmetropia. The two silicone optics is connected by a system of spring-like struts that pushes the optics apart.
The synchrony IOL is designed to utilize the natural mechanism of accommodation according to the Helmholtz theory. Made of the latest generation of silicone, the single-piece design is sized so as to distend and fill the capsular bag. With a relaxed ciliary muscle, the zonules become tense, and the taut capsular bag compresses two optics together. As the ciliary muscle contracts, the zonules and capsular bag relax. This relaxation permits the +34.00D anterior optic to move forward. A small-diameter, 4.5-mm capsulor rhexis is needed to confine the moving anterior optic (5 mm diameter) to the capsular bag. An injector system has been developed to deliver the lens through a 3.5-mm incision. 
Another emerging technology is light-adjustable IOLs. These lenses are made with a material that can change shape if exposed to UV light. The concept behind these lenses is to implant a lens that is close to the ideal power, and then adjust the lens power with UV light following the cataract procedure and appropriate healing. Once the lens has been adjusted to the proper power, it is "fixed" so that additional UV exposure cannot change the shape of the lens. This technology may also be able to correct for astigmatic errors and even aberrations in the eye.
Calhoun vision's light adjustable lens (LAL)
The LAL [Figure 5] (LAL; Calhoun Vision, Inc., Pasadena, CA) enables surgeons to adjust the lens' power in situ to correct for refractive errors occurring after the LAL's implantation. The three-piece IOL has modified C-loop, blue PMMA haptics and a 6-mm, square-edge optic. The lens is composed of a photosensitive silicone material that undergoes a controlled change in shape when exposed to a specific UV-beam intensity from the light-delivery device. The altered shape of the lens produces a corresponding power adjustment.
| Conclusion|| |
By neutralizing spherical aberration, one of the leading causes of patients' dissatisfaction with conventional IOLs, multifocal (Trifocal) IOL promises to be a successful means of reviving accommodation to the post cataract patients and the second issues is PCO, which can be minimized with cast molded hydrophobic lenses.
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[Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5]
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