Continuing Education

Author: Dr Leon Davies, Dr James Wolffsohn and Dr Shehzad Naroo

In the second part of our short CET series on intraocular lenses, Dr Leon Davies, Dr James Wolffsohn and Dr Shehzad Naroo describe advances in IOL technology and what they may mean for future cataract surgery. C4941, one standard CET point

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In the centenary year of Sir Harold Ridley's birth, referral for cataract extraction and subsequent intraocular lens (IOL) implantation is commonplace for optometrists and ophthalmologists alike.

Indeed, the last 50 years have seen the implantation of single-piece IOLs transform from an experimental, contentious, infrequent procedure to one that is routinely performed by ophthalmologists around the world.1

Moreover, rather than the traditional replacement of a crystalline lens following the development of a cataract, today some surgeons are electing to perform 'clear lens extractions' led by advancements in IOL technology, refraction and biometry techniques. In such cases, the technique becomes an elected refractive surgery procedure, rather than a treatment for a sight-threatening condition.

The advent of new surgical procedures, improved materials and more advanced measurement techniques has also seen an exponential growth in the raft of IOLs available to today's ophthalmic surgeons. As demand for and supply of such lenses continues to grow, an even greater number of lenses will become available to meet the needs of an ever-ageing global population. From aspheric2 to multifocal,3 from one-piece4 to small-incision,5 and from UV-blocking6 to blue-blocking,7 IOL types continue to expand to meet the needs of individual patients.

The early development of IOLs, their nature and the techniques used to implant them have been recently discussed.8 This article outlines current and future advances in IOL technology and is not an exhaustive list, but aims to furnish readers with the latest developments.

ELIMINATING POSTERIOR CAPSULAR OPACIFICATION

Posterior capsule opacification (PCO) is undeniably the most common post-operative complication of otherwise uncomplicated cataract surgery.9 Essentially, the principles of preventing PCO can be divided into two categories. First, the surgeon must try to minimise the amount of retained or regenerated cells and cortex that is, Soemmering's ring formation following cortical clean-up. Second, if there are remaining cells, the surgeon must create a physical barrier with the edge of the IOL optic to prevent cell growth and migration from the equator to centrally over the visual axis.10

With rates of PCO approaching 50 per cent for some IOL designs and surgical techniques, many ophthalmologists think of YAG laser capsulotomy as an inevitable procedure.11 Evidence, however, has now begun to suggest it may be possible to reduce significantly the need for YAG capsulotomy.12,13 While surgical technique and skill is unquestionably indispensable in this area, certain features of IOL design are identified as having displayed benefits in reducing the likelihood of YAG capsulotomy. Specifically, the square IOL edge configuration has demonstrated a substantial decrease in YAG capsulotomy rates.12

The concept of a square-edged IOL, preventing PCO by inhibiting the migration of lens epithelial cells (LEC), was first proposed by Okihiro Nishi and Kayo Nishi.12

Their study compared two different IOL materials with square-edge designs in rabbit eyes. As expected, prominent Soemmering's ring formation developed in both sets of eyes. Importantly, however, both posterior capsules of both IOL types appeared completely clear a finding that identified the edge and not the lens material as the significant factor.

Nishi observed that a distinct change in the posterior capsule's configuration occurred against the square edge of the IOL, which he termed the 'capsular bend'. He proposed this sharp angle of contact effectively inhibited the migration of LEC onto the posterior capsule and thereby kept the space optically clear (Figure 1). A second complementary theory, proposed by David Spalton and his team, suggested that the reduction of PCO with a square-edge design was in fact due to the greater distance traversed by the cells migrating around the square edge,13 and the greater pressure exerted by the square edge on the posterior capsule.14

The square-edge design, however, can be compromised. Two factors appear to have a profoundly deleterious effect on a square optic edge's ability to prevent PCO. The first involves using hydrophilic acrylic material for the optic as increased cellular proliferation onto the surface of this material has been well documented.15 Second, the inhibition of capsular fusion, and thus capsular bend formation for example, where IOL haptics prevent capsular fusion due to their bulk and position - may also negate the efficacy of the square-edge design.16 Work by David Apple and colleagues at Rayner, however, has attempted to overcome this by adding an additional lip at the edge of the optic to prevent LECs migrating along the unprotected haptic to the optic.

UV AND BLUE-BLOCKING IOLs

It is well established that ultraviolet (UV) and visible radiation have the potential to damage the retina and pigment epithelium. Indeed, UV radiation is responsible for 67 per cent of acute phototoxicity in the part of the electromagnetic spectrum that reaches the retina.17 The human retina, however, is protected from harmful short-wavelength radiation by the cornea, which absorbs below 295nm, and the lens, which absorbs below 400nm.18 Consequently, the human retina is only exposed to the visible component of the electromagnetic spectrum, 400-760nm, and some short-wavelength infrared (IR) light.

Scientific evidence suggests light may damage the retina through various mechanisms. Short, intense exposures, between 0.1 to 10 seconds may result in thermal damage (due to a rise in temperature) when incident light is absorbed by a tissue. Conversely, longer exposures of 10 seconds or more to much less intense light sources may cause retinal damage by a photochemical mechanism. It is this mechanism that is most likely to be of relevance to the development of age-related macular degeneration (AMD).

Given this, in an attempt to limit retinal phototoxicity, the majority of currently available IOLs are now equipped with UV-blocking chromophores. In recent years, however, several studies have suggested that IOLs should also block blue visible light, as this may have a role in the pathogenesis of AMD, as illustrated in the animal model.19 The AcrySof SN60AT (Alcon, Fort Worth, Texas, US), Hoya AF-1 (Hoya Medical Europe, Frankfurt, Germany) and MicroSil Yellow (HumanOptics, Erlangen, Germany) are such lenses. The introduction of these lenses has, however, produced some controversy in the literature, most notably by the ophthalmologist-physicist Martin Mainster. He was instrumental in the development of UV-blocking IOLs,18 but hypothesises that the use of blue-blocking filters in IOLs impair the natural circadian (day-night) rhythms of the human body, in turn affecting an individual's well-being and cognitive ability. In addition, Mainster suggests blue-blocking IOLs reduce scotopic sensitivity. He argues that, in bright environments, pseudophakes should instead wear sunglasses.20 Consequently, the future of blue-blocking IOLs is unclear. To move forward, further studies are needed on the trade-offs between photoprotection and photoreception induced by blue-blocking IOLs.

IOLs FOR THE PRESBYOPIC PATIENT

Refractive surgery for presbyopia has been steadily increasing over the last 20 years.21 As in the field of contact lenses, monovision was the first attempt to correct the condition. Where successful, patients are usually happy, however, there are concerns over the reduction of binocular function with increasing levels of anisometropia.22 The practice is also less popular in the UK than in the US.

The next evolution was the introduction of multifocal IOL designs, similar to those designs employed in contact lenses. The first such commercial lens was the AMO Array (AMO, Santa Ana, California, USA). The Array is a refractive type lens where 50 per cent of light is focused for distance, 35 per cent at near and approximately 15 per cent for intermediate vision. The inherent perception of haloes produced by the refractive rings at night, particularly in unilateral implantations, however, led to an increase in patient IOL rejection. This lens has now been superseded by AMO's second-generation refractive IOL the ReZoom (Figure 2).

An alternative multifocal mechanism is the diffractive optic IOL. Here, the light is split for vision at distance and near, with less effective results found for intermediate vision. In addition, due to the process of diffraction, up to 17 per cent of light entering the eye can be lost.21 The AcrySof ReSTOR (Alcon, Fort Worth, Texas, US), is a diffractive, single-piece IOL made of hydrophobic acrylic. It employs diffractive optics in the central 3.5mm of the optic with each successive diffractive ring decreasing in height in an attempt to minimise glare at night. Moreover, as the pupil expands in low light conditions, light increasingly passes through the distance portion of the optic again decreasing the problems of haloes at night. Studies have shown that the ReSTOR provides predictably good uncorrected distance and uncorrected near acuities and, compared to monofocal IOLs, spectacle independence is significantly higher, which outweighs the photic symptoms it causes.3,23 Multizone aspheric lenses, such as the M-flex by Rayner, and many other multifocal designs are also being released onto the market.

An exciting development in the battle against presbyopia is the advent of accommodating intraocular lenses (AIOLs). At present, AIOLs are showing variable results.24-27 However, they provide much promise. The CrystaLens AT-45 (Eyeonics, Aliso Viejo, California, US) AIOL is a biconvex lens with a 4.5mm optic and flexible hinged-plate haptics that allow forward movement of the optic during accommodative effort to provide near and intermediate vision in pseudophakic patients. The lens design incorporates grooves, or hinges, across the plates adjacent to the lens optic that allows forward and backward movement of plate-haptic lenses against the anterior vitreous face. The proposed mechanism of the CrystaLens AT-45 IOL is that with accommodative effort, there is a redistribution of the ciliary muscle mass that causes increased vitreous pressure and forward movement of the IOL.28

Other AIOLs currently available include the 1CU (HumanOptics, Erlangen, Germany) (Figure 3) and the TetraFlex KH3500 (Lenstec, St Petersburg, Florida, US). Essentially, these lenses work on the principle of forward movement of the optic, within the capsular bag to increase the effective power of the IOL. However, this means that the maximum accommodative response will be limited by the power of the IOL. For example, in order to achieve emmetropia, a myopic patient is fitted with a low powered IOL. As such, this AIOL needs to move more in the eye to achieve the same accommodative response as a hyperopic eye fitted with a high powered AIOL. Furthermore, physiological optics suggests that, for a typical IOL power of 20.0D, the IOL optic needs to move by 1.5mm to produce 1.0D of accommodation. Clearly then, in order to achieve approximately 3.0D of accommodation, these lenses would need to move anteriorly by, on average, 4.5mm. As the average depth of the anterior chamber is only approximately 3.0mm, this movement is a physical impossibility.

NEXT GENERATION IOLs

In an attempt to overcome the limitations of current AIOLs, the dual-optic design has been developed. On June 23, 2006, Visiogen announced that it had received approval in Europe for its dual-optic AIOL.

The CE Mark designation for the Visiogen Synchrony AIOL signifies the device conforms to the essential requirements of the Medical Devices Directive (MDD). The Synchrony dual-optic AIOL (Visiogen, Irvine, California, US) has the potential to allow greater accommodation versus single-optic AIOLs. The lens is silicone, and has a 5.5mm high-powered anterior optic connected to a 6.0mm negative-powered optic by haptics that act like springs. When the lens is at rest, the optics are separated by a spring action that links the optics together. When implanted, however, the capsular bag tension compresses the optics together, thus reducing the inter-optic distance. On contraction of the ciliary body, the zonules relax and the tension on the capsular bag is released, allowing the optics to move apart, thus allowing the effective power of the eye to alter. The benefit of the lens is that it is influenced less by the initial refraction of the patient, and as the anterior optic is highly powered, it requires less anterior displacement to produce the accommodative response required by the patient. Furthermore, Bausch & Lomb also has a dual-optic accommodating IOL design which is currently in clinical trials.

The Smart IOL (Medennium, Irvine California, US) is a another intriguing design. In the quest to provide pseudophakic accommodation, one concept has also been to refill the capsular bag with a compressible clear liquid. This, however, has inherent problems, such as leakage through the anterior capsule capsularhexis, leakage through the posterior capsule following YAG laser capsulotomy, bubbles in the gelatinous material and varying power distribution across the lens surface. The Smart IOL appears to overcome these obstacles. The lens is composed of a hydrophobic acrylic material with unique thermoplastic properties that permit temperature-induced changes in its shape. The concept requires the lens to be moulded to the correct shape and size for the patient. The material is then heated and compressed so that a solid, thin 50mm long rod results upon cooling. The surgeon then inserts the rod through a small corneal incision. Once in the capsule, the physiological warmth of the eye alters the consistency of the material so that it assumes its original, predetermined shape.

Finally, a recent study has evaluated a new hydrophobic acrylic IOL with photochromic properties in vitro and in vivo in the rabbit eye.29 Given the current interest in the effect of UV/blue light on the development and progression of AMD it will be interesting to see how well this concept works when tested in the human eye.

SUMMARY

Clearly then, the design of the IOL has changed dramatically since the concept was first introduced by Ridley. Techniques have also advanced dramatically, allowing IOLs to be used as an option for the refractive surgery patient. Ridley's initial corneal incision of over 12mm has been replaced with self-sealing wounds of less than 3mm that require no sutures.

Modern surgery also uses sophisticated visco-elastics that allow the anterior chamber to be reformed and protect the endothelium during surgery, whereas Ridley's surgery was conducted without modern pharmaceutical intervention and without the use of an operating microscope.

Finally, the 'final frontier' in vision correction, presbyopia, is being tackled head on too by IOL designers, ophthalmologists and researchers alike. Consequently, in the next decade, we should see more functional IOLs with more useable accommodation for our patients.

References

1 Charman WN. Restoring accommodation to the presbyopic eye: how do we measure success? J Cataract Refract Surg, 200329:2251-2254.

2 Muñoz G, Albarrán-Diego C, Montés-Micó R, Rodríguez-Galietero A, Alió JL. Spherical aberration and contrast sensitivity after cataract surgery with the Tecnis Z9000 intraocular lens. J Cataract Refract Surg, 200632: 1320-1327.

3 Chiam PJT, Chan JH, Aggarwal RK, Kasaby S. ReSTOR intraocular lens implantation in cataract surgery: Quality of vision. J Cataract Refract Surg, 200632:1459-1463.

4 Kleinmann G, Apple DJ, Chef J, Stevens S, Hunter B, Larson S, Mamalis N, Olson RJ. Hydrophilic acrylic intraocular lens as a drug-delivery system: Pilot study. J Cataract Refract Surg, 200632:652-654.

5 Condon GP. Simplified small-incision peripheral iris fixation of an AcrySof intraocular lens in the absence of capsule support. J Cataract Refract Surg, 200329:1663-1667.

6 Cionni RJ, Tsai JH. Colour perception with AcrySof Natural and AcrySof single-piece intraocular lenses under photopic and mesopic conditions. J Cataract Refract Surg, 200632:236-242.

7 Yanagi Y, Inoue Y, Iriyama A, Jang WD. Effects of yellow intraocular lenses on light-induced upregulation of vascular endothelial growth factor. J Cataract Refract Surg, 200632:1540-1544.

8 Davies LN, Wolffsohn J S, Naroo SA. The intraocular lens: A brief history of surgical procedures and IOL technology. Optician, 2006232:26-28.

9 Dewey SH. Six keys for clearer posterior capsules: significant advances in the struggle to eliminate PCO. Cataract and Refractive Surgery Today, March 2002.

10 Apple DJ, Peng Q, Visessook N et al. Eradication of posterior capsule opacification. Documentation of a marked decrease in Nd:YAG laser posterior capsulotomy rates noted in an analysis of 5,416 pseudophakic human eyes obtained postmortem Ophthalmology, 108:505-518, 2001.

11 Aslam TM, Devlin H, Dhillon B. Use of Nd:YAG laser capsulotomy. Surv Ophthalmol, 200348:594-612.

12 Nishi O, Nishi K. Preventing posterior capsule opacification by creating a discontinuous sharp bend in the capsule. J Cataract Refract Surg, 199925:521-526.

13 Bhermi G S, Spalton DJ, El-Osta AA, Marshall J. Failure of a discontinuous bend to prevent lens epithelial cell migration in vitro. J Cataract Refract Surg, 200228:1256-1261.

14 Boyce JF, Bhermi G S, Spalton DJ, El-Osta A R. Mathematical modeling of the forces between an intraocular lens and the capsule. J Cataract Refract Surg, 200228:1853-1859.

15 Heatley C. Comparison of PCO with Hydrophilic and Hydrophobic Single-Piece Acrylic IOL. ASCRS/ASOA Symposium on Cataract, IOL and Refractive Surgery, June 2003 San Francisco, CA.

16 Nishi O, Nishi K. Effect of the optic size of a single-piece acrylic intraocular lens on a posterior capsule opacification. J Cataract Refract Surg, 200228:1236-1240.

17 Mainster MA. Spectral transmittance of intraocular lenses and retinal damage from intense light sourses. Am J Ophthalmol, 197885:167-170.

18 Margrain T H, Boulton M, Marshall J, Sliney D H. Do blue light filters confer protection against age-related macular degeneration? Prog Retin Eye Res, 200423:523-531.

19 Rapp LM, Smith SC. Morphologic comparisons between rhodopsin-mediated and short-wavelength classes of retinal light damage. Invest Ophthalmol Vis Sci, 199233:3367-3377.

20 Mainster MA. Violet and blue light blocking intraocular lenses: photoprotection versus photo reception, Br J Ophthalmol. 200690:784-792.

21 Olson RJ, Werner L, Mamalis N, Cionni R. New intraocular lens technology. Am J Ophthalmol, 2005140:709-716.

22 Brooks SE, Johnson D, Fischer N. Anisometropia and binocularity. Ophthalmology, 1996103:2007-2012.

23 Blaylock JF, Si Z, Vickers C. Visual and refractive status at different focal distances after implantation of the ReSTOR multifocal intraocular lens. J Cataract Refract Surg, 200632:1464-73.

24 Davies LN, Hunt OA, Wolffsohn JS, Naroo S, Hurcomb PG, Shah S, Cunliffe IA, Benson MT. Performance of the KH3500 accommodative intraocular lens. Optom Vis Sci, 200582:E-abstract 050437.

25 Dick HB. Accommodative intraocular lenses: current status. Current Opinion in Ophthalmology, 200516:8-26.

26 Wolffsohn JS, Hunt OA, Naroo S, Gilmartin B, Shah S, Cunliffe I A, Benson MT, Mantry S. Objective Accommodative Amplitude and Dynamics with the 1CU Accommodative Intraocular Lens. Invest Ophthalmol Vis Sci, 2006a47:1230-1235.

27 Wolffsohn JS, Naroo SA, Motwani NK, Shah S, Hunt OA, Mantry S, Sira M, Cunliffe IA, Benson MT. Subjective and objective performance of the Lenstec KH-3500 'accommodative' intraocular lens. Br J Ophthalmol, 2006b90:693-696.

28 Cumming JS, Colvard DM, Dell SJ, Doane J, Fine IH, Hoffman RS, Packer M, Slade SG. Clinical evaluation of the Crystalens AT-45 accommodating intraocular lens: results of the US Food and Drug Administration clinical trial. J Cataract Refract Surg, 200632:812-25.

29 Werner L, Mamalis N, Romaniv N, Haymore J, Haugen B, Hunter B, Stevens S. New photochromic foldable intraocular lens: Preliminary study of feasibility and biocompatibility. J Cataract Refract Surg, 200632:1214-1221.

 Dr Leon Davies is a lecturer in the School of Life and Health Sciences, Aston University. Dr James Wolffsohn is a reader in the School of Life and Health Sciences, Aston University, and immediate past president of the British Contact Lens Association. Dr Shehzad Naroo is a lecturer in the School of Life and Health Sciences, Aston University, past treasurer of the British Society for Refractive Surgery, and editor-in-chief of the British Contact Lens Association's journal Contact Lens and Anterior Eye

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