Intraocular lens implantation - Optometry Today

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Sponsored by Module 3 Part 11 tissues in their physical properties and the smooth, hydrophilic nature minimises mechanical friction with ocular tissues and contributes to superior biocompatibility 10 . This is reported to produce less damage to the corneal endothelium after inadvertent touch on implantation than PMMA 11 . The hydrophilic nature of the lens surface causes low interfacial tension (wettability) of the hydrogel in aqueous solution and reduces the tendency of proteins to denature on the surface of the polymer, thus reducing biological rejection mechanisms. This possibly also prevents adhesion between the lens and the capsular bag. As the dimension of the hydrogel changes in direct proportion to the degree of water saturation, it enables implantation of a semi-hydrated lens through a small incision, for it to later expand in the eye as it becomes fully hydrated. Hydrophilic acrylics have the advantage of undergoing less damage during YAG laser capsulotomy. When there is direct impact of the YAG laser beam on the lens, mild to moderate localised pitting occurs, without the radial fracturing seen on a PMMA implant. This is due to the resilience of the material and its ability to act as a shock absorber rather than cracking under stress. Although high water-content polymers are usually mechanically weak, the hydrogels can be very strong owing to certain changes of the polymer produced during the process of polymerisation. Examples of hydrophilic acrylic IOLs are the Hydroview lens, EasAcryl, Inject-A, Centerflex and the Memory Lens. Studies on the Hydroview IOL (Bausch & Lomb) have shown that it is associated with fewer surface inflammatory cells than PMMA and a second-generation silicone IOL. However, a significant lens epithelial cell (LECs) reaction on the anterior IOL surface was reported in addition to a higher incidence of PCO 12,13 . AcrySof (Alcon) is an example of a hydrophobic acrylic IOL, which has become the most commonly inserted IOL in the USA. The AcrySof polymer was developed from the same backbone used in PMMA. It has a higher refractive index (1.55) than PMMA or silicone. This means that lens implants made from AcrySof are thinner, thereby facilitating folding and insertion through a smaller incision 14 . Acrylic polymers change their mechanical properties with temperature, being hard and glassy at low temperature, and soft and fluid at high temperature. This means that an IOL inserted at room temperature unfolds slowly and in a controlled manner 14 . This avoids the rapid, explosive opening which can be seen with three-piece silicone IOLs which may cause iatrogenic damage to the capsule or other anterior segment structures. The AcrySof IOL is, however, found by many surgeons to be more difficult to fold than silicone lenses, as the lens is more rigid if cool. In addition, due to its tacky nature, the AcrySof IOL has a tendency to stick to forceps or between two parts of the IOL on insertion 15 . This characteristic may mean that the IOL sticks to the capsular bag (which is discussed later and may have advantages), however, the adhesion means that the IOLs are more difficult to explant in cases of anisometropia or incorrect power calculation 16 . The IOLs are not slippery when wet in contrast to silicone IOLs. The AcrySof IOL has been found to be associated with dramatically reduced rates of PCO 17 . This is thought to be due to both mechanical and material features. This was the first lens implant to be manufactured with a square optic edge, and it is thought that this edge acts as a barrier to the migration of LECs onto the posterior capsule, reducing PCO. The tacky nature of the implant leads to increased adhesion of the IOL to the capsule which also probably limits the migration of LECs onto the posterior capsule. An IOL may have excellent cytological biocompatibility (a limited/nil inflammatory reaction), but poor capsular biocompatibility (the effect on anterior capsule and PCO), for example, the Hydroview IOL. Therefore, the goal is to find an IOL which has both cytological and capsular biocompatibility. Design features, such as a sharp optic edge, and material composition appear to be equally important. To date, the second-generation silicone IOLs and AcrySof IOLs appear to have the best biocompatibility. Table 4 Advantages and disadvantages of the IOL types IOL Type Advantages Disadvantages PMMA Long term experience Rigid so need large incision Good biocompatibility Pits with YAG laser Cheap High incidence of PCO The prevalence of lens materials in the 1997 survey of the American Society of Cataract and Refractive Surgery was 38% acrylic, 20% silicone, and 40% PMMA (Kohnen, 1998). The implantation of foldable IOLs in Australia rose from 1% in 1991 to 67% in 1997 18 . Multifocal IOLs Standard IOLs are monofocal and so the loss of accommodation which increases with age becomes absolute with surgery, and the need to correct the resultant presbyopia is apparent. Multifocal and bifocal IOLs have been designed in an attempt to provide both distance and near vision without additional spectacle correction, as they form separate images of near and distance objects. The goal of multifocal implants has been to enable patients to be less dependent on spectacles following surgery. Multifocal IOLs are based on the simultaneous vision principle – if the power difference between two optical systems is 3.00DS or more, then the images are dissimilar enough for the brain to interpret them as separate. An example is the ARRAY multifocal IOL by Allergan which has concentric rings of varying optical power around a central power for distance. The rings of power are for near distances, while the central distance power is dominant. Fifty per cent of the lens is dedicated to distance vision, 36% to near Silicone Foldable - small incision Low refractive index - thicker IOLs (first generation silicone) Fairly low incidence of PCO High refractive index - thinner IOLs (second generation silicone) (particularly second Pits with YAG laser generation silicone) Rapid unfolding in the eye Dislocation after YAG More decentration More anterior capsule contraction Slippery when wet Cannot use with silicone oil AcrySoft Foldable - small incision Short experience High refractive index - thin IOLs Tacky surface - sticks to forceps Very low incidence of PCO More difficult to fold LEC regression ?Glistenings Biocompatible ?Glare Fewer pits with YAG laser Slow uncontrolled folding Hydroview Foldable - small incision LECs on anterior IOL surface Good biocompatibility High incidence of PCO - low inflammatory cell reaction Fewer pits with YAG laser Controlled unfolding Less endothelial cell damage with cornea touch www.optometry.co.uk 31
ot vision and 14% to intermediate distances. One study compared 100 patients implanted with the ARRAY lens in both eyes after bilateral cataract surgery with 100 patients with a standard monofocal IOL implanted bilaterally 19 . It was reported that 41% of patients with the ARRAY lens did not wear spectacles compared to 12% of patients with monofocal IOLs. For near tasks, 38% of multifocal patients did not wear spectacles compared to 10% of monofocal patients. In addition, 85% of those with multifocal IOLs did not wear spectacles for distance compared to 52% with monofocal IOLs. The downside was that patients with multifocal lenses were more bothered with glare and haloes from oncoming headlights at night and found night driving more difficult than the patients with monofocal IOLs. This “halo-effect” can also occur with other bright/dark contrast situations. An additional disadvantage of multifocal lenses is that they reduce contrast sensitivity. This results in loss of sharpness of vision particularly under poor visibility conditions such as low light or fog. Therefore, patients who spend their lives in a twilight environment and night-time work conditions may be disturbed by this loss of contrast sensitivity. Patient selection is a crucial step when considering whether a multifocal or monofocal lens should be implanted. Suitable candidates are those whose myopic astigmatism does not exceed 1.00D, spend most of their lives in normally lit environments and have no other ocular diseases. Bilateral implantation results in the most satisfying outcome, and leaving the patient emmetropic or slightly hypermetropic minimises glare and visual disturbances. Accommodative IOLs Certain research groups are studying ways of refilling the capsular bag with flexible IOLs capable of accommodation. One study removed lenses from a group of rabbits and primates, and refilled the capsular bag with an inflatable endocapsular balloon 20 . Accommodation could be demonstrated following this technique in primates 21 , however, the incidence of PCO was reported to be very high (94%). The post-operative amplitude of accommodation was small and decreased with time, which was likely to be due to the increase in PCO with capsular fibrosis leading to a loss of capsular pliability. The use of a soft injectable liquid to fill the bag has been attempted in rabbit eyes 22 and primates 23 : after making a minicapsulorhexis, phacoemulsification was performed with a tiny tip. Silicone material was then injected into the capsular bag, and the bag then closed with a silicone plug. Some accommodation appeared to be present postoperatively. However, all eyes suffered thick PCO soon after surgery. YAG laser capsulotomy would obviously not be appropriate as this could annul the attained accommodation. However, research has continued in this field and two prototypes will be marketed for use in humans next year (see previous CPD article, Outcome in Cataract Surgery, 05/10/01). Conclusion IOL design has developed through the years and has now become extremely successful with very few complications. The complications of corneal decompensation, glaucoma, hyphaema and uveitis have largely been resolved with the use of posterior chamber IOLs placed within the capsular bag. Similarly, lens decentration and dislocation is much rarer with the in-the-bag positioning of the lens implant with capsulorhexis. However, one complication remains, despite improvements in surgical technique – PCO. Nevertheless, developments in intraocular lens design are having a dramatic effect on reducing the incidence of PCO. About the author Emma Hollick works at Moorfields Eye Hospital. She recently carried out research at St Thomas’ Hospital looking at intraocular lens implant biocompatiblity and posterior capsular opacification. References 1. Marcher A. Memoirs of Giacomo Casanova de Seingalt. 8, 45-50, R&R Clarke for Limited Editions Club 1940. 2. Spalton DJ. Harold Ridley’s first patient. J Cataract Refract Surg 1999; 25: 156. 3. Letocha CE, Pavlin CJ. Follow-up of three patients with Ridley intraocular lens implantation. J Cataract Refract Surg 1999; 25: 587-591. 4. Ridley H. Intra-ocular acrylic lens- past, present and future. Trans Ophthalmol Soc UK 1964; 84: 5-14. 5. Smith RT, Campbell CJ, Koester CJ et al. The barrier function in extracapsular cataract surgery. Ophthalmology 1990; 97: 90-95. 6. Newland TJ, Auffarth GU, Wesendale TA, Apple DJ. Neodymium: YAG laser damage on silicone intraocular lenses. A comparison of lesions on explanted lenses and experimentally produced lesions. J Cataract Refract Surg 1994; 20: 527-533. 7. Newland TJ, McDermott ML, Eliott D, Hazlett LD, Apple DJ, Lambert RJ, Barett RP. Experimental neodymium:YAG laser damage to acrylic, poly(methylmethacrylate) and silicone intraocular lens materials. J Cataract Refract Surg 1999; 25: 72-76. 8. Amon M, Menapace R. In vivo documentation of cellular reactions on lens surfaces for assessing the biocompatibility of different intraocular lens implants. Eye 1994: 8: 649-656. 9. Dahlauser KF, Wrolewski KJ, Mader TH. Anterior capsule contraction with foldable silicone intraocular lenses. J Cataract Refract Surg 1998; 24: 1216-1219. 10. Blumenthal M. The use of high water content hydrogels as intraocular lenses. In: Soft implant lenses in cataract surgery. Mazzocco TR, Rajacich GM, Epstein E (eds), New Jersey, Slack Inc, 1986; pp107-117. 11. Blumenthal M, Yalon M. Interaction of soft and hard intraocular lenses with cat cornea endothelium. Cornea 1982; 1: 129-132. 12. Hollick EJ, Spalton DJ, and Ursell PG. Surface cytology on intraocular lenses: Can increased biocompatibility have disadvantages? Archives Ophthalmol 1999a; 117: 872-878. 13. Hollick EJ, Spalton DJ, Ursell PG, Meacock WR, Barman SA, Boyce JF. Posterior capsular opacification with Hydogel, PMMA and Silicone intraocular lenses: two-year results of a prospective randomised clinical trial. Am J Ophthalmol 2000; 129: 577-584. 14. Anderson C, Koch DD, Green G, et al. Alcon AcrySof acrylic intraocular lens. In: Martin RG, Gills JP, Sanders DR, eds, Foldable Intraocular Lenses. Thorofare, NJ, Slack, 1993; 161-177. 15. Komatsu M. Soft acrylic lens. In: Tobari I, ed. Textbook of Ophthalmic Surgery. Vol 2, Tokyo, Kanehara, 1995; 346-348. 16. Neuhann TH. Intraocular folding of an acrylic lens for explantation through a small incision cataract wound. J Cataract Refract Surg 1996; 22: 1383-6. 17. Hollick EJ, Spalton DJ, Ursell PG, Pande MV, Barman SA, Boyce JF, Tilling K. The effect of PMMA, silicone and polyacrylic intraocular lenses on posterior capsular opacification three years after cataract surgery. Ophthalmology 1999b; 106: 49-54. 18. Loughnan M. Intraocular lens materials and styles. Aust NZ J Ophthalmol 1997; 25: 251. 19. Javitt MD et al. Outcomes of cataract surgery with multifocal intraocular lens implantation: Functional status and quality of life. Ophthalmology 1997; 104: 589-599. 20. Nishi O, Nakai Y, Mizumoto Y, Yamada Y. Capsule opacification after refilling the capsule with an inflatable endocapsular balloon. J Cataract Refract Surg 1997a, 23: 1548-1555. 21. Nishi O, Nakai Y, Yamada Y, Mizumoto Y. Amplitudes of accommodation of primate lenses refilled with two types of inflatable endocapsular balloon. Arch Ophthalmol 1993; 111: 1677-1684. 22. Nishi O, Nishi K, Mano C, Ichihara M, Honda T. Lens refilling with injectable silicone in rabbit eyes. J Cataract Refract Surg 1998a, 24: 975-982. 23. Nishi O, Nishi K, Mano C, Ichihara M, Honda T. Accommodation amplitude after lens refilling with injectable silicone by sealing the capsule with a plug in primate eyes. Arch Ophthalmol 1998b, 116: 1358- 1361. 32 November 2, 2001 OT www.optometry.co.uk
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