Soft intraocular lenses

articles Soft intraocular lenses Liaquat Allarakhia, M.D. Randall L. Knoll, Ph .D. Minneapolis, Minnesota St. Paul, Minnesota Richard L. Lindstrom, M.D. Minneapolis, Minnesota ABSTRACT An in-depth review of the current status of soft intraocular lenses (IOLs) is presented. We have outlined the historical aspects of IOL development from polymethylmethacrylate material to newer soft polymers such as silicones and hydrogels. Chemical, physical , and biomaterial properties as well as the advantages and disadvantages of these materials and lenses are discussed. Clinical results from international investigators are presented, along with some thoughts about future trends in small incision cataract surgery and IOL substitutes. Key Words: advantages, disadvantages, clinical expe rience, hydrogel, intraocular l en , ilicone, s 'mall incision cataract surge ry The concept of intraocular lens (IOL) implantation following cataract extraction dates back to 1765 when Tadini, an Italian oculist practicing in Warsaw, conceived the idea of implanting crystal glass balls to correct aphakia. 1 However, contemporaries in the medical and scientific communities ridiculed and forced him out of Warsaw without allowing him to carry out this interesting surgical concept. In this paper we will discuss the evolution from this early attempt, through hard implant materials to current research with soft implant materials. The basic polymer science and currently reported clinical results with soft implants will be reviewed. Long after Tadini, Ridley revived this idea and performed the first hard IOL implantation in 1949. 2 Following World War II, many veteran fighter pilots were observed to have Perspex (polymethylmethacrylate) fragments lodged in their corneas, anterior cham-

bers, or posterior segments with almost no deleterious effects.3 This prompted Ridley to use a hard acrylic, polymethylmethacrylate (PMMA), as a possible material for intraocular implantation. Unsatisfactory results caused by a combination of factors including inadequate surgical techniques, poor lens design, heavy lens weight, and lack of manufacturing technology prevented Ridley from pursuing this goal successfully. However, Ridley's name remains in ophthalmic history as the first man to implant an IOL to correct aphakia. The idea was kept alive by surgeons such as Epstein,4,5 Strampelli,6 and Dannheim 7 and more recently by Choyce, 8 Binkhorst,9 Fyodorov,lO and others. Lenses were implanted in the posterior chamber, the anterior chamber, or supported by the iris. A multitude of lens designs were tried over the years, but one material has consistently performed well-

From the Department of Ophthalmology, University of Minnesota, Minneapolis, and Vision Care/3M, St. Paul , Minnesota. Ann Chiginsky and Lisa Neaton assisted with the preparation of this manuscript. Supported in part by an unrestricted grant from Research to Prevent Blindness, Inc., and 3M Corporation . Reprint requests to Richard L. Lindstrom, M.D ., University of Minnesota, Department of Ophthalmology, Box 493 Mayo Memorial Building, 516 Delaware Street SE, Minneapolis, Minnesota 55455.

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PMMA. A variety of materials such as nylon, polypropylene, polyimide, and metal have been used for haptic manufacture. However, biodegradation of some materials, tissue trauma, and inflammation are prompting many ophthalmologists to return to the use of all-PMMA lenses. The stimulus-response relationship between science, technology, and medicine has fueled the search for different and better materials for lens design. Considerable advances in biomedical technology for ophthalmology and other medical fields has led to continuous improvement in lens design. Advances in the contact lens industry have also affected IOL design. This has led over the last several years to a considerable interest in the design and use of soft or foldable IOLs. Dreifus, Wichterle, and Lim l l conceived the idea of using soft materials for IOL manufacture in the late 1950s. They performed the first animal experiments in 1960 by implanting hydrogel anterior chamber lenses in rabbits. In the mid 1970s, Epstein 12 performed experimental work in monkeys, placing the first soft implant in a human patient in 1976. Since then considerable clinical research, both animal and human, has been conducted by Epstein, 12 Mazzocco, 13,14 Mehta,15,16 Blumenthal,17,18 Yalon,19 Fyodorov,20 Siepser,21 Zhou,22 Chen,23 Zheng,24 and others with very promising results. Many IOL manufacturing companies are currently investigating a variety of soft materials and the immediate future will see the release of several interesting and, we hope, useful soft IOLs. One impetus for the interest in soft lenses is that cataract surgery can be performed by phacoemulsification through a small incision and a soft lens can be inserted through this small incision. Utilizing the physical properties of a class of polymers called hydrogels, which are water swollen, cross-linked structures of hydrophilic homopolymers or copolymers, a lens design that is much smaller when dry than a conventional PMMA lens can be produced. The dehydrated hydrogel lens can be inserted through a small incision and, on subsequent hydration within the eye, will become larger and flexible, taking up its ideal position within the capsular bag. Cataract surgery through a small incision has the advantage of minimizing postoperative astigmatism and reducing dependence on wound healing, allowing early rehabilitation. With the advances in microsurgery and particularly phacoemulsification, small incision cataract surgery is now within the reach of any trained microsurgeon. In addition, some soft materials such as poly-HEMA (hydrogel) are less traumatic to the corneal endothelium and other ocular tissues. 25-28 Sterilization is simplified because silicones and hydrogels can be autoclaved without any damage or degradation. In addition, the lenses are soft and relate well to the 0

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elasticity and plasticity of the normal tissues of the eye, reducing undue mechanical trauma to the implanted eye. MATERIALS

o·

Medical Background Scales,29 as far back as the early 1950s, suggested that an ideal implant material should be (1) chemically inert, (2) stable and not physically modified by contact with tissues, (3) acceptable to the body, with no inflammation, foreign body reaction, or tissue chafe, (4) noncarcinogenic, (5) nonallergenic, (6) capable of being fabricated into the desired form, (7) able to resist mechanical strains, and (8) readily sterilizable. In addition, for ophthalmic applications, the material (9) should be optically transparent and remain so for long periods; (lO) it should have a high resolving power, (ll) should be capable of blocking ultraviolet radiation in the 300 nm to 400 nm wavelength, and (12) should be imphmtable through the smallest incision current cataract removal technology is capable of creating. Polymethylmethacrylate has been used successfully with excellent tissue tolerance since 1940. 30 It is currently being used in orthopedics, dentistry, plastic and reconstructive surgery, and ophthalmology. It has been used in the eye for almost 40 years and has fulfilled many of the criteria mentioned. However, it is a hard and rigid material and, thus, has been shown in some cases to produce mechanical irritation of sensitive uveal tissues, resulting in chronic low grade inflammation. 31 Also, PMMAS hydrophobic nature may cause endothelial cells to adhere to its surface on contact with the corneal endothelium. 32 Because PMMA is not flexible, a larger incision is required to insert the lens and this may delay postoperative visual and physical rehabilitation. Thus, it was a natural step to investigate softer materials or polymers to overcome some of the limitations of these rigid lenses. The soft polymers that have been most widely investigated include silicone and polyhydroxyethylmethacrylate (poly-HEMA). Of these, silicone has been the most widely used in medicine (over 30 years),33 principally for prosthetic heart valves, hydrocephalic shunts, metacarpophalyngeal joints, and breast augmentation. In ophthalmology, silicone has been used for contact lenses, scleral buckles, keratoprosthesis, glaucoma shunts, and nasolacrimal intubation tubes. Silicone in clinical use has been shown to be extremely inert, stable at high temperatures, flexible and elastic at a wide range of temperatures, and nonadhesive to tissues . While its long-term effects in the eye are not known, it is expected to be a very favorable and biocompatible material. Hydrogels including poly-HEMA have also been extensively studied. The main application has been soft

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contact lenses; however, hydrogels have also been successfully used in other fields of medicine. 33-35 These polymers are known to resemble living tissue in their physical properties more than any other class of synthetic polymers for biomedical application . Wichterle 36-38 developed poly-HEMA in 1952 and it was first applied to human use in 1954 as an orbital implant. Since then it has been placed subcutaneously, intraperitoneally, intravenously, and intracardially with very satisfactory results. 39 ,40 It has also been used for ureter prosthetic devices and as coatings for various other biomedical devices. Recently, considerable knowledge has been gained from its use as a corneal inlay41,42 for the correction of high refractive errors. Promising initial results with these soft materials have warranted further long-term research into biocompatibility that is underway at various institutions. Properties The use of polymers as implant materials has several advantages. They can be made with a wide variety of mechanical, physical, and chemical properties, can be readily formed into the desired shape and size, are usually inert, and are economical. Researchers have overcome the lack of tissue compatibility (toxic leachants to tissues) and the lack of resistance to their biological environments (degradation) by improved purification methods and by altering their properties and manufacturing processes. Polymers derive their wide range of properties from their chemical composition and their structure. Each polymer is composed of long chains of repeating units (monomers) that can reach molecular weights of hundreds of thousands of, dalto~s. Vinyl monomers of the general formulation C = C undergo "addition polymerization"; for exa~ple, the double bond between the two carbon atoms (C) open up, facilitating the formation of bonds to other monomers at either end . Depending on the formulation used, these chains may be linear, possess branches of varying length, or may be cross-linked to adjacent side chains. It is the combination of various structural features with the chemical composition that produces polymers that are hard, brittle, soft, or elastic. Elastomers are polymers of entangled long-chain molecules capable of large and reversible deformations. The cross-linked structure serves as a limit to elongation and encourages the polymer to return to its original configuration. Crosslinked polymers (thermosets) are infusable and insoluble, whereas non-cross-linked polymers (thermoplastics) are generally soluble and processable by solvent casting or injection molding and extrusion. The properties can be further extended by copolymerization of two or more polymers. Various copolymer and terpolymer systems formed from a combination of monomeric components are becoming

available for specific applications, which may require a balance of several distinct properties such as hydrophilic surfaces, physical strength, and transmissibility of gases and fluids. Modifications can also be made by adding low molecular weight materials to plasticize hard, rigid materials. The plasticizers act by separating the relatively rigid polymer chain through solvent action. However, care in the choice of plasticizers is warranted since leaching of plasticizers into body fluids or tissues can be harmful. 43 The following paragraphs summarize the features of several currently used IOL polymers. Acrylic Most currently used IOLs are made from PMMA with either PMMA or polypropylene haptics. The chemical structure of PMMA is shown in Figure 1.

Fig. 1.

(Allarakhia) Chemical structure of polymethylmethacrylate (PMMA).

Polymethylmethacrylate is a hard, rigid, and strong thermoplastic material with excellent optical properties. It has a low water diffusion constant and is highly resistant to the effects of light, oxygen, and hydrolysis. The pure polymer has a high degree of ultraviolet light transmissibility, but can be made to absorb these harmful wavelengths by special compounding. Table 1 summarizes some of the physical properties of PMMA.44,45 Silicone The name silicone denotes a synthetic polymer having a generalized structure of a polyorganosiloxane as shown in Figure 2'. . A silicone contains a repeating silicon-oxygen backbone and organic groups R, attached to a significant number of the silicon atoms by silicon-carbon bonds. Industrial silicones have R groups that are mostly methyl; however, longer alkyl, fluoroalkyl, and phenyl

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Table 1. Properties of PMMA.

Table 2. Properties of silicone elastomer.

Property

Property

Value

Value

Specific gravity

1.18 glcc

Specific gravity

1.14 glcc

Refractive index

1.49

Refractive index

1.43

Tensile strength

8,000-10,000 psi

Tensile strength

850-1,200 psi

Ultimate elongation

2.5-5.4%

Rigidity modulus

14,500 psi 65-70°

Equilibrium water contact angle

Rigidity modules

1,160 psi 97°

From Boretos 44 and Stone and Philips 45

Silicones, as with most other polymers, cannot be fully characterized once formulated and processed. Therefore, knowledge of polymer formulation processing is required when reviewing the bioassessment for safety and efficacy. The term "medical grade" refers to silicone elastomers that have been specifically designed and qualified to provide a high level of safety and efficacy in medically related uses. 47

o n (Allarakhia) Chemical structure of polyorganosiloxane .

groups may be substituted for specific purposes. The molecular structure of silicones can vary considerably and includes linear, branched, and cross-linked configurations. Different forms can be made including a variety of elastomers, gels, greases, fluids, foams, and antifoams. At present, approximately 60,000 siliconecontaining compounds are known. The silicones used in biomedical applications are primarily elastomers prepared from polydimethylsiloxane. 46

Silicone Elastomer Silicone elastomers are an important class of silicone compounds of appropriate molecular weight (polysiloxane 750,000 daltons) cross-linked to provide elastomeric properties. Fillers, such as large surface area fumed silica, are used to increase the tensile strength. The typical catalysts for this cross-linking reaction are trace quantities of rare metals such as platinum or a peroxide such as 2,4-dichlorobenzyl peroxide. A variety of catalysts, cross-linkings, or additives can be used depending on the property desired. However, most medical grade elastomers contain no organic additives, ultraviolet chromophores, accelerators, dyes, or plasticizers because of their poorly known biocompatibility. Typical physical properties of medical grade silicone elastomers are outlined in Table 2.44,45 610

350-600%

Equilibrium water contact angle

From Boretos 44 and Stone and Philips 45

Fig. 2.

Ultimate elongation

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Hydrogel The term hydrogel refers to a broad class of polymeric materials that swell extensively (greater than 20%) on contact with water, but do not dissolve in water. Included in this definition is a wide variety of materials of both natural (gelatin, polysaccharides) and synthetic polymeric origin. Of particular interest in recent years have been those hydrogels formulated from polymers and copolymers of methacrylate esters containing at least one hydroxyl group in the side chain. Three basic classes of vinyl-type monomers are used: neutral, anionic, and cationic. For example, the principle monomer 2-hydroxyethylmethacrylate (HEMA) can be copolymerized with other hydrophilic polymers such as n-vinyl pyrrolidine (NVP) and crosslinked with a diester, mostly ethylene dimethacrylate derivatives. 48 The structural formula of HEMA is outlined in Figure 3. Wichterle and Lim 36-38 described and synthesized acrylic cross-linked polymers in which the monomer

Fig . 3.

(Allarakhia) Chemical structure of polyhydroxyethyl methacrylate (poly-HEMA).

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was polymerized in an aqueous solution, resulting in a polymer with a soft and rubbery consistency. Their objective was to produce a polymeric network composed primarily of stable carbon-carbon bonds, with side groups providing good hydrophilic properties. The hydrogels are rigid in the dehydrated state and become soft and rubber-like on hydration. The degree of swelling, elastic modulus, gas permeability, and optical properties can be varied over a large range by altering the polymerization. The typical physical properties of HEMA-based hydrogels are outlined in Table 3. 44 ,45 Table 3. Properties of HEMA hydrogels. Value

Property Specific gravity

1.16 glcc

Refractive index

1.43

Tensile strength

70 psi

Rigidity modulus

725 psi

20°

Equilibrium water contact angle Water content

38%

From Boretos 44 and Stone and Philips 45

Because of the expanded nature of their network, hydro gels facilitate the extraction of polymerization initiators, solvents, and other extraneous molecules prior to clinical use . This property is extremely useful in reducing harmful toxic effects to ocular or other tissues following implantation. ADVANTAGES AND DISADVANTAGES OF SOFT MATERIALS As with most materials and devices used in medicine, both advantages and disadvantages of soft materials exist. All clinicians and biomedical engineers wish to minimize the disadvantages as much as possible. As more research is undertaken and as more data become available, we hope that some of the disadvantages listed will be reduced in importance. The inherent properties and designs of soft IOLs confer many important advantages . It is important to elaborate the major advantages and disadvantages so that surgeons can determine the most appropriate IOL for their patients.

Advantages Size of Incision. Most studies have involved extracapsular cataract extraction and posterior chamber lens implantation. Use of the phacoemulsifier allows surgeons to perform extracapsular extraction through a 3 mm incision. Experienced phacoemulsification surgeons have shown that the complication rate with this technique is no greater than with other methods. Silicone IOLs can easily be inserted through a 3.5 mm J CATARACT

to 4.5 mm incision, facilitated by various inserting devices. In addition, hydrogels in the dehydrated state are smaller, allowing small incision insertion. A 4.3 mm hydrogel optic can, on hydration, reach 6.0 mm, while a 6.0 mm dry optic can reach 8.4 mm upon hydration. Also, because of the elasticity and plasticity of these IOLs, they can be squeezed or compressed through an incision much smaller than the lens size. In addition through the use of special suturing techniques and th~ availability of intraoperative keratometry, postoperative astigmatism can almost be eliminated, thus allowing earlier visual and physical rehabilitation. This is a great advantage since a large number of patients receiving implants are young and active, and still employed. Endothelium. Several studies have reported reduced trauma to the corneal endothelium in animal experiments. Kassar and Varnell's27 in vitro experiments on rabbit corneas suggested that silicone IOLs produced less endothelial damage than PMMA lenses following brief physical contact. Polymethylmethacrylate materials caused considerably more damage than silicone lenses as evidenced by marked staining and stripping of endothelial cells. Kassar and Varnell also noted that surface modified silicone lenses (electrical molecular rearrangement) produced less damage than unmodified lenses. A study by Herzog and Peiffer28 showed that silicone lenses produced less endothelial damage than PMMA lenses following a 10 gram weight contact with rabbit corneas for 30 seconds. Following staining with alizarin red and trypan blue, linear damage that was attributed to mechanical trauma was noted with both lenses. However, PMMA lenses produced areas of endothelial cell membrane stripping (due to its adhesive properties) that was not observed with silicone lenses. Yalon et al. 25 have recently reported that 45 second contact between a soft hydrogel lens and the corneal endothelium of cats resulted in no damage to the cell membranes. Barrett and Constable 26 have also shown less cell loss after hydrogel contact than after PMMA contact in rabbit corneas. Uveal Tissues. Because of their soft cushioning nature, these materials are potentially more gentle to uveal tissues. Complications such as iris chafing and atrophy, pigment dispersion, hemorrhage, fibrosis, and inflammation as occasionally seen with PMMA lenses may be reduced. However, this may be compromised in some cases due to higher adherence forces related to surface charges on the lens, in spite of the lens being soft and compliant. No comparative studies of the effects of soft IOLs on uveal tissue have been performed; however, several clinical reports are available. Mehta et ai.I 5,16 have noted that the eyes in their series have been quiet postoperatively and no iris

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atrophy was noted up to 30 months following hydrogel IOL implantation. In addition, no pigment deposition on the lens surface or evidence of pigment dispersion was noted. Barrett et al. 49 also found in their series of hydrogel implantations that the incidence of pigment dispersion syndrome was similar to that following PMMA lens implantation. However, Fogle et al. 50 in a study of cynomolgus monkeys noted early pigment dispersion in the silicone implanted cases which was not present in the PMMA cases. Also, Watt ("Pigment Dispersion Syndrome Associated with Silicone IOLs," Ocular Surgery News, July 1,1987) reported two cases of pigment dispersion syndrome with elevated intraocular pressure in humans. This was attributed to poor lens edges in one case and possibly mechanical abrasion of the iris from ciliary sulcus fixation in the other. However, no iris chafing or pigment dispersion was noted in the bag-fixation cases. Weight. Soft lenses can be made from polymers with low specific gravities; therefore, their weight can be considerably less than present PMMA lenses. The lens weight may be compromised, however, by a lower refractive index in some polymers. The bulk or thickness of the optic has to be increased to attain the necessary optical power because of these lower refractive indices. The average weight of a posterior chamber PMMA IOL is between 2.0 mg and 3.5 mg in aqueous and many of the hydrogel IOLs are in this range on full hydration. Some of the proprietary silicone elastomers have lower specific gravities, enabling the lenses to be considerably lighter. With some IOLs this advantage could lessen the risk of compromising the zonular fibers and lens subluxation or dislocation. This is of particular interest in patients who have weak zonules, such as very high myopes and patients with Marfan's syndrome, homocystinuria, a history of trauma or pseudoexfoliation. Optical Quality. Silicone lenses can be manufactured by cast or injection molding and, therefore, the optical qualities are reproducible. During this process, no finishing or polishing is required so residual chemicals harmful to the eye are not present. However, a potential problem is molding Hash, which may be present along the lens edges. This occurs as the polymer escapes along the parting line where the two halves of the mold meet. Recently this has been reported with an explanted silicone lens. 51.52 This may have potentially deleterious effects on tissues and may lead to complications such as iris chafing, pigment dispersion, chronic inHammation, and uveitis-glaucoma-hyphema syndrome. 51 .52 To date, molding Hash has not been noted with hydrogels because these are manufactured in the hard state and are easier to finish than silicones which are soft and difficult to finish . PolyHEMA is a hard material in the dehydrated state and, thus, can be lathe cut and subsequently polished. 612

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Medical grade silicone used in IOL manufacture is noted to have a transmission of greater than 95% in the 400 nm to 800 nm range. Some elastomers are also capable of filtering harmful ultraviolet rays without further additives. Both silicones and hydrogels have excellent surface properties that reduce reHections and glare. Chemical . Hydrogels and silicones are known to be resistant to hydrolytic and oxidative degradation. During the manufacturing process of hydrogel lenses, it is possible to extract harmful residual monomers completely. In addition, there is a low interfacial free energy and work of adhesion between the hydrogel surface and the aqueous media, which has been postulated to prevent or reduce protein adsorption onto the lens surface. This has been noted clinically as a reduced prevalence of lens surface precipitates, both pigmentary and inHammatory. Sterilization . Both silicones and hydrogels can be autoclaved safely, avoiding the use of any toxic gases or chemicals for ·sterilization that may occasionally be harmful to the eye. If the lens is contaminated at the time of surgery, it can easily be cleaned and resterilized within minutes for reuse. However, the haptics on some soft lenses are made of materials that are not au toclavable. Explantation. There is a considerable amount of fibrosis around an implanted PMMA lens which varies particularly with the type of haptic used, whether rigid or semirigid, and whether anchored in the anterior chamber angle, ciliary sulcus, or capsular bag. It is not uncommon for surgeons to explant IOLs for a variety of reasons and considerable difficulty may be encountered. There is practically no adherence between the soft lens haptics or optic and the surrounding tissues. Thus, these lenses are potentially easier to remove without significant damage to tissues. No comparative studies evaluating the ease or difficulty of explantation in relation to materials used and mode of fixation have been performed . However, several reports of posterior chamber IOL explantations are available. (No reports of anterior chamber IOL explantations are yet available .) Our experience with a proprietary hydrogel posterior chamber IOL in a primate model has shown much easier explantation because of the lack of adhesions. Silicone lenses were readily removed from the ciliary sulcus in a primate model,50 and Newman et al. 51 reported a case of a silicone posterior chamber lens that was explanted without difficulty from the ciliary sulcus approximately three months after initial implantation. Mehta et al.I 5,16 and Barrett et al. 49 have stated that these lenses can be easily removed using a cryoprobe, as one would for an intracapsular cataract extraction. Laser Effects . Bath et al.5 3 ,54 have shown that the amount of damage to silicone lenses is no greater than

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to injection molded PMMA lenses. However, both these materials are more vulnerable to laser damage than lathe cut or cast molded PMMA lenses. Scanning electron microscopy studies on various PMMA and silicone lenses show the characteristic splash crater pattern with raised irregular melted edges in silicone lenses. Figure 4 shows laboratory-induced YAG laser pits to the IOGEL (Alcon) lens using 15 m] bursts; Figure 5 shows pits on an injection molded PMMA lens using identical settings. Toxicity studies in our laboratory have shown that lase ring various soft lenses at high energy laser settings does not cause the release of harmful toxic products. In one study, 55 IOLs immersed in serum-free cell culture medium were purposely hit with a range of exaggerated doses of laser energy to cause extensive damage to Alcon hydrogel and control PMMA lenses. The resultant solutions were assayed for cytotoxicity in a

bioassay system using monolayers of fourth passage human corneal endothelial cells. Up to 50 bursts of 15 m] energy exposure were inflicted on each lens. No cytotoxicity was noted with either lens up to an incubation period of 72 hours. In this study we also noted that the damage inflicted on these hydrogel lenses was much less than on PMMA lenses. Allergan Medical Optics ("Preclinical Evaluation of an Elastomeric Silicone Material for Use as a Small Incision Intraocular Lens, " Technical Report Series #16) has shown that its silicone lens (SLM-I/UV) is similar to Perspex CQ lenses following YAG laser damage and the pit edges in its silicone lenses are generally smoother and do not show radial cracking. Clinically, Barrett et al. 49 have noted that YAG lasers cause less marking on poly-HEMA lenses than on PMMA lenses, although this has not been objectively measured.

Disadvantages

Fig. 4.

(Allarakhia) YAG laser damage to Alcon IOGEL hydrogel lens using 15 m] burst. Magnification x 150 (laboratory experiment courtesy D. Skelnik).

Fig . 5.

(Allarakhia) YAG laser damage to IOLAB PMMA injection molded lens using 15 m] bursts. Magnification x 150 (laboratory experiment courtesy D. Skelnik).

Biocompatibility. Although silicones have been shown to be biocompatible in various areas of the body, their biocompatibility in an intraocular setting is not fully known. The bathing fluids within the eye are somewhat different and, therefore, long-term data are required before silicones can be determined to be safe. Similarly, the data on other soft materials are short term and require further study. Yalon et al. 19 have shown that hydrogel implants in cat eyes lead to microvilli formation in endothelial cells, which may be due to impurities in the polymers. These authors noted that an aminopolyamide formulation demonstrated a monolayer of fibroblasts on the lens surface after implantation . They speculated that this formulation may have provided a favorable surface for cell growth . This response indicates the importance of recognizing differences in polymer molecular structure, hydration , ionic charge , surface topography, and purity in order to predict the polymer's implant behavior. Mechanical Strength. Once the lens is implanted, a high tensile strength of the soft lens is less important; however, the lens should be able to withstand mechanical forces during surgical manipulation. Soft polymers such as silicone have tensile strengths of approximately 1,000 psi compared to 8,000 psi for PMMA. Tensile strength of poly-HEMA lenses is even lower (approximately 100 psi) and therefore care is required during surgery. Data on the long-term changes that may occur with folding or rolling of silicone lenses are also inadequate. Sizing and Fixation. If the soft lens is to be positioned in the capsular bag, the sizing is less critical than if it is to be positioned in the ciliary sulcus or the angle. However, placement in any of these areas requires careful designing and more accurate sizing than for PMMA lenses. Several reports that elucidate

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problems from improper and inadequate fixation of fixed-size silicone lenses are available. The major complications of capsular bag fixation with earlier designs have been decentration 23 and lens tilting because of bending at the optic-haptic junction that leads to an increase in myopic astigmatism. 56 This usually occurs four to six weeks after cataract and implantation surgery because of localized fibrous cicatrization and capsular bag shrinkage . Complications have also arisen with ciliary sulcus fixation. Chen 23 noted six cases of decentration in a series of 205 cases. This was presumably due to small lenses leading to inadequate fixation in the sulcus. Newman et al. 51 ,52 reported a case in which a silicone lens was explanted because of the windshield wiper syndrome with chronic inflammation and secondary glaucoma. The lens was placed in the ciliary sulcus, but the fixed haptic configuration and reduced lens size prevented firm fixation. This led to lens propellering within the posterior chamber. A pathology report from Crawford and Faulkner57 showed adequate fixation of a silicone lens in the ciliary sulcus with the pigment epithelium in the sulcus partially atrophic but with no erosion through the epithelial basement membrane. Epstein 58 reported a case in which an 11 .0 mm by 5.0 mm hydrogel poly-HEMA lens in the dehydrated state was implanted in the ciliary sulcus. On hydration, it enlarged to 12.8 mm. This was subsequently found to be arched anteriorly, causing a myopia of 4 diopters, and thus had to be exchanged for a smaller lens. From these reports it is obvious that fixation and sizing is very critical; however, newer modifications and designs, including the availability of flexible looped haptics, may overcome some of these initial problems. Implantation Damage. Because of the soft and compliant nature of these lenses, there is a potential of damaging the optics and haptics with possible visual problems postoperatively. Hydrogels are particularly vulnerable because of their low mechanical strength but silicones are also prone to implantation damage despite greater mechanical strength. Permanent crease or fold marks from folding or inserting with specialized instruments may be visible on the lens optic. In addition, undue pressure by forceps on the lens during implantation may lead to grooves and scratch marks which may potentially lead to glare and reduced vision. Newman et al. 51 in their pathological report of an explanted silicone lens have demonstrated the presence of several grooves and indentations on the surface , parallel to the long axis of the lens. These marks would correlate with the forceps used during implantation. It is important for the surgeon to use utmost care in handling these lenses. The handling that he or she may use with PMMA lenses may cause damage with soft lenses. Discoloration. The original STAAR silicone polymer 614

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(RMX-l) has a slight opalescence, resembling a crystalline lens in an elderly patient. Mazzocco (Ocular Surgery News, March 15,1985, pp 26-30,32,34) states that this does not harm the eye and, in fact, may be beneficial in that it may absorb between 20% and 50% of the harmful visible and invisible rays. However, this opalescence has made YAG laser capsulotomy more difficult for some investigators. 59 This polymer has been superseded by a new STAAR polymer (RMX-3) which does not have this opalescence. Mehta et aP6 have observed discoloration of hydrogel IOLs following the use of fluorescein for corneal staining. They recommend that a minimal amount of stain be used with these IOLs. This may also be true with intravenous fluorescein use in angiographies, although Blumenthal et alP and Epstein 58 have not noted any discoloration in their series of cases. The long-term effects of topical and systemic medications to these lenses are not known. Hydrogel soft contact lenses do imbibe various topically applied drugs; however, their concentrations in the aqueous may be too minimal to cause any deleterious effects. Research to determine the absorption and subsequent wash-out of various drugs is needed. Capsular Opacification . No comparative studies of differences in the incidence of posterior capsule fibrosis following implantation of lenses of various materials have been reported . Several conflicting clinical reports of opacification following silicone lens implantation are available. Neumann59 has reported an unacceptably high incidence of posterior capsule fibrosis early in the postoperative period compared to his control PMMA cases. In a series of over 200 patients, Chen23 noted central capsule opacification and Elschnig pearls less frequently; however, there were no control cases in this study. No controlled studies of capsule opacification following hydrogel posterior chamber lenses are available; however, the clinical impression of several surgeons I5 ,16,49,58 indicates a reduced incidence. Further controlled studies are warranted to determine the incidence of this complication.

Types of Lenses Soft lenses have been used as iris fixated lenses or implanted in the anterior or posterior chamber. Posterior chamber implantation demonstrates the most promise. In the posterior chamber, lenses have been implanted in the ciliary sulcus and the capsular bag. Thus, depending on the planned position of the lens, the configuration of the optic and particularly the haptic varies. A number of shapes and sizes have been tried with a variety of haptic configurations. The STAAR surgical silicone lens (Figure 6) is available as a one piece lens with a webbed haptic and reinforced haptic edges or a silicone optic with polyimide open

SURG- VOL 13, NOVEMBER 1987

R = 17mm

12mm

Fig. 6.

(Allarakhia) Silicone posterior chamber lenses produced by STAAR Surgical Company (courtesy STAAR Surgical Company).

-12.012.5,mm

t

I

loop haptics in a Sinskey-style configuration for implantation in the capsular bag or the ciliary sulcus. This lens has FDA approval for insertion by folding techniques. The AMO lens has a silicone optic with polypropylene hap tics in a Sinskey-style configuration (Figure 7). This lens can also be folded and is suitable for ciliary sulcus or capsular bag fixation. There are a variety of hydrogel poly-HEMA lenses with water content in the range of 38%. Most of these are one piece lenses, oblong in shape, with various degrees of tapering at either end. These lenses can be

Fig. 7.

(Allarakhia) Silicone posterior chamber lens produced by Allergan Medical Optics (courtesy Allergan Medical Optics).

Fig. 8.

(Allarakhia) Poly-HEMA hydrogel posterior chamber lens produced by Alcon (courtesy Alcon Laboratories).

flat and uniplanar or saucer-shaped (Alcon IOGEL, Figure 8). Most of these lenses are biconvex and are available with or without positioning holes, depending on the surgeon's preference. Mehta et aP5,16 have used a one piece iris fixated poly-HEMA lens with a posteriorly placed lenticule optic and an anterior supporting flange. The hydrogels can be implanted in the dehydrated state (Epstein, Siepser and Severin), with slight moistening of the surfaces prior to insertion. This facilitates easier insertion with minimal trauma to the endothelium and iris. The hydrogels can also be inserted in the fully hydrated state without folding or rolling, as done by Mehta et aI., 15,16 Barrett et aI.,49 and Blumenthal et aP7

Insertion Devices Because of their flexible nature, insertion of soft lenses can be somewhat different and difficult for surgeons who routinely implant rigid PMMA lenses. As interest in small incision cataract surgery has increased, devices to facilitate insertion through smaller incisions have been developed. Mazzocc0 14 has suggested that silicone soft lenses be inserted without folding through a 4.5 mm to 5.0 mm incision. Since the lenses are pliable, they can be squeezed through such an incision without causing any substantial deformation. For this technique, the lens is held in its longitudinal axis with a Kelman-McPherson-type forceps. However, insertion through an even smaller incision requires the lenses to be folded. The first generation folding devices, known as "bar folders," required incision sizes to be slightly larger than a

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Fig. 9.

(Allarakhia) Faulkner lens folding forceps (courtesy STAAR Surgical Company).

phacoemulsification incision. These forceps created pressure marks and grooves on the lens surface and, in addition, required that caution be used during manipulation in the anterior chamber to avoid trauma to the endothelium . A newer lens folding forceps (Faulkner design) that overcomes some of these problems is shown in Figure 9. The second generation devices enabled insertion through a 3.5 mm to 4.0 mm incision. These are "tube folders" which are basically tubes into which the lens is placed in a rolled fashion. The Bartel tube folder 14 has a plunger that pushes the lens into the desired position (Figure 10); the Frenchik-Mazzocco device 14 has a split half tube construction that allows retraction of one half of the tube, thus leaving the lens in the desired position. The hydrogel lenses are, as yet, not recommended to be folded or rolled and, therefore, no special devices are required. A variety of normally used lens introducing forceps can be used with blunt probes 'or hooks to position the superior haptic. RESULTS The Hydrogel Experience (Tables 4 and 6) Epstein (South Africa). Epstein58 has the largest series with the longest follow-up. The reported series

Fig. 10.

(Allarakhia) Bartel tube folder showing extrusion of silicone lens from tip of tube (courtesy STAAR Surgical Company).

consists of 350 cases of poly-HEMA hydrogel lenses implanted from 1976 to 1983. Manual planned extracapsular cataract extractions were performed in all cases and lenses were implanted in the posterior chamber. In this series, no dislocations have been reported, including three cases that received severe blows to the eye. There was no pupil capture, posterior synechias, or lens deposits, and only one case of cystoid macular edema was reported four months after surgery in a patient who had a primary capsulotomy. Epstein performed fluorescein angiography in five cases and did not note any IOL discoloration or other changes. The incidence of postoperative iritis was less than in the patients with PMMA lenses, including the black patients who usually have a higher incidence . Late pigment dispersion was not noted and any pigment on the lens surface following surgery was minimal and transient. The best corrected visual acuity was 20/40 or better in 85% of cases. If the other 15% of cases, having prior retinal disease, were excluded, the best case visual acuity of 20/40 or better was 100%. Epstein has also implanted silicone posterior chamber lenses (STAAR Surgical) since 1983 and reported satisfactory results in 98 cases (European Intraocular Implant Society Meeting, Cannes, October 1985). Mehta (India). Mehta et aP5,16 reported a series of

Table 4. Chronology of cases and visual results with the hydrogel IOL. Author

Lens Type

First Implant

Number of Cases

20/40 (Total)

20/40 (Best Case)

Epstein

PCL-one piece

1976

Mehta

IFL-one piece

1978

350

85%

100%

135

80% (20/30)

Blumenthal

PCL-one piece

NA

1983

52

87%

NA

Barrett

PCL-one piece

1983

108

92%

98%

PCL = posterior chamber lens IFL = iris fixated lens NA = not available

616

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Table 5. Chronology of cases and visual results with the silicone IOL. Lens Type

Author

First Implant

20/40 (Total)

Number of Cases

20/40 (Best Case)

Zhou

ACLlIFL-one piece

1978

50

NA

NA

Zheng

ACLlIFL-one piece

1978

45

NA

NA

Fyodorov

PCL-one piece

1979

350

91%

NA

Schlegel

ACUPCL-one piece

1981

500+

NA

NA

Fyodorov

ACL-one piece

1982

103

96%

NA

Epstein

PCL-one piece

1983

98

NA

NA

PCL-one and two piece

1984

109

80%

NA

Chen

PCL-one piece

1984

205

88%

94%

Hunkeler

PCL-two piece

1986

23

91%

100%

Mazzocco

ACL IFL PCL NA

= = = =

anterior chamber lens iris fixated lens posterior chamber lens not available

135 cases of iris fixated poly-HEMA hydrogel lenses implanted since 1978. In this series, 97 cases had manual planned extracapsular cataract extraction and the remaining 38 had intracapsular extractions. Eighty-five percent of patients were over 60 years of age and the remaining were between 40 and 60 years. Results of a follow-up period of 30 months indicated corrected visual acuities of 20/30 or better in 101 cases (80%) (explanted cases excluded). Seventy-eight percent of the extracapsular group had 20/30 or better corrected acuity; 86% of the intracapsular group had 20/30 or better corrected acuity. The authors attributed this difference to capsular distortion of the lens optic in the extracapsular group. The major complications noted were severe iritis in 22 cases (17%); six were very severe and the lenses had to be explanted to control the iritis. Polishing compound residues were felt to be the causative factor in the earlier cases. Striate keratopathy, probably due to poor surgical technique, was noted in 23% of the cases.

Blumenthal (Israel). Blumenthal (European Intraocular Implant Society Meeting, Cannes, October 1985) has implanted poly-HEMA hydrogel posterior chamber lenses in 52 patients. The mean age was 73 years with a range of 47 to 89 years. The surgical technique consisted of mechanical planned extracapsular cataract extraction; no iridectomies were performed. Complications consisted of decent ration in three cases (6%), transient uyeitis in two cases (4%), and lens precipitates in one case (2%). The mean endothelial cell loss was 8.9%. The best corrected visual acuity was 20/40 or better in 87.2% 'of c~ses. BlumenthaP7 has also reported his observation that glare was subjectively less in patients implanted with hydrogels than in patients with PMMA lenses. This is presumably due to a lower reflection factor of the hydrogel lens surface. Barrett (Australia). Barrett et al. 49 have reported a series of 108 single piece poly-HEMA hydrogel posterior chamber lens (Alcon, Figure 8) implantations. The

Table 6. Reported complications of soft IOLs. Subluxation

IOL Removal

Uveitis

CME

0

2 (0.6%)

18 (5.0%)

1 (0.3%)

0

0

22 (17.0%)

4 (3.0%)

Corneal Edema

Secondary Glaucoma

Author

Lens Type

Total Cases

Epstein

Hydrogel

350

Mehta

Hydrogel

135

Blumenthal

Hydrogel

52

3 (6.0%)

3 (6.0%)

3 (6.0%)

0

0

0

Barrett

Hydrogel

108

6 (6.0%)

6 (6 .0%)

6 (6.0%)

0

0

0

0 29 (21.0%)

Zhou

Silicone

50

0

6 (12 .0%)

0

0

6 (12.0%)

Fyodorov

Silicone

350

0

0

4 (1.0%)

0

0

Chen

Silicone

205

Hunkeler

Silicone

23

1 (0.3%) (pupil block) 4 (3.0%) (pupil block)

NA 0

8 (4.0%)

0

0

1 (0.5%)

0

1 (0.5%)

0

0

0

0

0

0

NA = not available

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lenses were implanted from August 1983 to June 1985 with a follow-up of two years (mean 12 months). After full hydration, the lenses were placed in the capsular bag or the ciliary sulcus, without folding or inserting through a small incision. There was significant decentration in six cases because of improper bag fixation; however, these were easily recentered at a secondary procedure. Two cases of endophthalmitis were attributed to coincidental bacterial infections and six cases of significant iritis with hypopyon formation in three cases were attributed to inadequate polishing and finishing techniques that have been resolved. No cases of cystoid macular edema were observed. Sixty-eight percent of all patients had a visual acuity of 20/20 or better; 86% had 20/30 or better and 92% had 20/40 or better corrected acuity. The best case analysis, excluding patients with preoperative ocular pathology, showed that 98% of the patients achieved 20/40 or better acuity at latest followup. The authors also noted less glare in eyes with hydrogel lenses than in fellow eyes with PMMA lenses. Yalon and Siepser (USA). To date, no human implantations have been performed by Yalon 18 .19 or Siepser21 and most of the data conc~rn animal implantation. Considerable in vivo experimentation to determine biocompatibility of hydrogel polymers has been done. They have determined that poly-HEMA and other hydrogels have excellent properties and potentially are satisfactory materials for intraocular implantation. Based on the data and experience from his animal experiments, Siepser has developed various designs of single piece hydrogel posterior chamber lenses. The Silicone Experience (Tables 5 and 6) Zhou (China). Zhou 22 has reported using suture fixated anterior chamber silicone lenses in 50 cases between August 1978 and July 1979. Forty two of these were implanted as primary implantations and the rest as' secondary procedures. He also reported the use of iris fixated and posterior chamber lenses which were, however, not included in this series. During a followup period of five to 16 months, the only major complication encountered was significant corneal edema in six cases (12%) attributed to thicker, first generation designs. Fyodorov (USSR). Fyodorov (United Kingdom Intraocular Implant Meeting, London , December 1985) has implanted both anterior and posterior chamber silicone lenses. Results of his series consisting of 350 posterior chamber and 103 anterior chamber lenses over a follow-up period of three years revealed posterior capsule opacification in 5.4% of the cases and iritis in 1.1%. Visual acuity in the 20/20 to 20/40 range was obtained in 91.1% of cases. In the anterior chamber lens series, the visual acuity was 20/40 or better in 96% of cases and the endothelial cell loss was 8.8%. 618

Schlegel (West Germany). Schlegel has been associated with CooperVision/Cilco and has performed anterior and posterior chamber silicone implantation in over 500 humans. The FDA core study in the U.S. includes a series of 50 cases with a six month follow-up. Posterior capsule opacification was noted in 14 eyes (28%). Other complications included persistent iritis up to six months in one case (2%) and secondary glaucoma in one case (2%). Visual acuity of 20/40 or better was found in 40 cases (80%) and 20/40 to 20/80 in a further seven (14%). Three patients (6%) had a visual acuity of 20/80 to 20/200. Mazzocco (USA). Mazzocco 14 has implanted silicone posterior chamber lenses (STAAR Surgical) in 109 eyes during the 20 months preceding January 1986. These consisted of single piece lenses with 6 mm optics and webbed flange haptics or two piece lenses consisting of silicone optics and polyimide flexible open looped haptics. Phacoemulsification was performed in all cases; the lenses were placed in the bag in 33 cases and in the sulcus in 76. Visual results of 101 eyes (eight lost to follow-up) showed a best corrected acuity of20/40 or better in 80 eyes (80%). The remaining had acuities ranging from 20/40 to 20/200 or worse because of preexisting macular degeneration or other unrelated ocular pathology. Chen (Taiwan). Chen 23 has reported a series of 205 silicone posterior chamber lens implants (STAAR Surgical) inserted in the Ciliary sulcus with a six to 23 month follow-up (mean 12.3 months). Best corrected visual acuity was 20/40 or better in 88.3% of cases; when cases with pre-existing pathology were excluded, 94.1 % achieved an acuity of 20/40 or better. Complications included eight cases (4%) of decentrations, two cases (1 %) of capsular opacification, and one case each of cystoid macular edema and secondary glaucoma. Hunkeler(USA). Allergan Medical Optics has developed a two piece posterior chamber lens consisting of a 6 mm silicone optic and modified J-polypropylene loops (SI -18B, Figure 7). It is the first soft lens reported to contain a bonded UV chromophore. Hunkeler (Eighth Royal Hawaiian Eye Meeting, Kona, February 1987) reported a series of 23 cases with a mean age of 75 years and visual acuity results of 20/40 or better in 91 % of cases at three months. If two cases having macular degeneration are excluded, the best case analysis is 100% with 20/40 or better. Many of these reports are small and the follow-up period short. In addition, several authors have furnished incomplete data, particularly regarding postoperative complications and visual results, which makes interpretation difficult. In total, over 5,000 lenses have been implanted worldwide in humans; the longest follow-up is 10 years. More detailed follow-up with accurate and controlled studies and a larger database are necessary before any definitive conclu-

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sions can be made. However, preliminary results from all over the world are very encouraging and soft IOLs are likely to play an increasingly greater role.

THE FUTURE Injectable lens substitutes 60 may be available for use by surgeons near the end of this decade. Liquid polymer gels can be injected into the capsular bag following a small incision cataract extraction. The liquid gel solidifies over several hours ,or can be polymerized at the time of surgery using special techniques. During the process of solidification from the gel state, no toxic components are released, Studies so far show the gel to remain clear after injection; in addition, the dioptric power can be altered by controlling the quantity injected into the capsular bag. Also, preliminary animal data indicate the presence of some accommodative power which is an added advantage. Considerable animal studies are underway to improve surgical techniques to facilitate preservation of the anterior and posterior capsules. Further research of instrumentation to facilitate endocapsular cataract extraction and anti-capsular opacification agents is warranted. This field of cataract surgery is very exciting and it is hoped that this new technology will be available to surgeons in the near future. ' In conclusion, the use of soft materials for · IOL implantation is an exciting new frontier with much promise. Fortunately, the outstanding results currently obtainable with PMMA implants allow us to develop this new technology in a careful and controlled fashion. . REFERENCES 1. Fechner PU, Fechner MU, Reis H: Tadini, the man who invente d the artificial lens. Bull Soc Beige Ophthalmol 183:9-23, 1979 2. Ridley H : Intra-ocular acrylic lenses. Trans Ophthalmol Soc UK 71:617-621, 1951 3. Ridley H : The origin and objectives of intraocular lenticular implants. Trans Am Acad Ophthalmol Otolaryngol 81:65-66, 1976 4. Epstein E: The Ridley lens implant . Br J Ophthalmol 41:368-376, 1957 5. Epstein E: Modified Ridley lenses. Br J Ophthalmol43:29-33, 1959 6. Strampelli B: Anterior chamber lenses; present technique. Arch Ophthalmol66:12-17, 1961 7. Dannheim H: Vorderkammerlinse mit elastischen Halteschlingen. Dtsch Ophthalmol Ges 60:267-268, 1956 8. Choyce DP: All-acrylic anterior-chamber implants in ophthalmic surgery. Lancet 2:165-171, 1961 9. Binkhorst CD: Iris-supported artificial pseudophakia. A new development in intra-ocular artificial lens surgery (iris clip lens). Trans Ophthalmol Soc UK 79:569-584, 1959 10. Fyodorov SN: Long-term results of2,000 operations of implantation of Fyodorov intraocular lenses performed in the Soviet Union . Am Intra-Ocular Implant Soc J 3(2):101, 1977 11 . Dreifus M, Wichterle 0, Lim D: Intrakameralni cocky z hydrokoloidnich akrylatu. CS Optalmologic 16:154-159, 1960

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12. Epstein E: History of intraocular lens implant surgery. In:Mazzocco TR, Rajacich GM, Epstein E, eds, Soft Implant LRnses in Cataract Surgery, New Jersey, Slack, Inc, 1986, pp 1-10 13. Mazzocco TR: Progress report: Silicone 10Ls . Cataract 1(4):18-19, 1984 14. Mazzocco TR, Davidson BM : Insertion technique and clinical experience with silicone lenses. In: Mazzocco TR, Rajacich GM, Epstein E, eds, Soft Implant LRnses in Cataract Surgery, New Jersey, Slack, Inc, 1986, pp 97-106 15. Mehta KR, Sathe SN, Karyekar SD: The new soft intraocular lens implant . Am Intra~Ocular Implant Soc J 4:200-204, 1978 16. Mehta KR, Sathe SN, Karyekar SD: The soft intraocular implant. In:. Trevor-Roper PD, ed, Vlth Congress of the European Society of Ophthalmology; the Cornea in Health and Disease, London , Royal Society of Medicine , 1981 , pp 859-863 17. Blumenthal M, Moisseiev J, Bartov E : The present trend of hydrogel materials as intraocular lenses. In: Materials in Intraocular LRnses . Transactions of Eleventh Annual Meeting of the SOCiety for Biomaterials, 1985, p 207 18. Blumenthal M, Yalon M: Interaction of soft and hard intraocular lenses with cat cornea endothelium. Cornea 1:129-132 , 1982 19. Yalon M, Blumenthal M, Goldberg EP: Preliminary study of hydrophilic hydrogel intraocular lens implants in cats. Am Intra-Ocular Implant Soc J 10:315-317, 1984 20. Fyodorov SN: Initial clinical testing of a silicone intraocular lens (IOL). Proceedings of the Interzonal Scientific and Practical Conference of Ophthalmologists of Western and Eastern Siberia and the Far East, 1983, pp 22-24 21. Siepser SB: Expansile hydrogel intraocular lenses. In: Mazzocco TR, Rajacich GM, Epstein E, eds, Soft Implant LRnses in Cataract Surgery, New Jersey, Slack , Inc, 1986, pp 119-142. 22. Zhou KY: Silicon intraocular lenses in 50 cataract cases. Chin Med J 96(3):175-176, 1983 23. Chen TT: Clinical experience with soft intraocular lens implantation. J Cataract Refract Surg 13:50-53, 1987 24. Zheng YR: Clinical report of transparent silicone intraocular lens implantation . Chung Hua Yen Ko Tsa Chih 17(1):17-20 , 1981 25 . Yalon M, Sheets JW Jr, Reich S, Goldberg EP: Quantitative aspects of endothelium damage due to intraocular lens contacts: Effect of hydrophilic polymer graft coatings. Int Congr Ophthalmol 24:273-276, 1983 26. Barrett G, Constable IJ : Corneal endothelial loss with new intraocular lenses. Am J Ophthalmol 98:157-165, 1984 27. Kassar BS , Varnell ED: Effect of PMMA and silicone lens materials on normal rabbit corneal endothelium: An in vitro study. Am Intra-Ocular Implant Soc J 6:344-346, 1980 28. Herzog WR, Peiffer RL Jr: Comparison of the effect of polymethylmethacrylate and silicone intraocular lenses on rabbit corneal endothelium in vitro . J Cataract R efract Surg 13:397-400, 1987 29. Scales JT: Discussion on metals and synthetic materials in relation to tissues; tissue reactions to synthetic materials. Proc R Soc Med 46:647-652, 1953 30. Dennis C: Prolonged dependent drainage with "Lucite" drains in the treatment of chronic osteomyelitis. Surgery 13:900-910, 1943 31. Apple DJ, Mamalis N, Loftfield K, Googe JM, et al: Complications of intraocular lenses, A historical and histopathological review. Surv Ophthalmol29:1-54, 1984 32. Kaufman HE, Katz J, Valenti J, Sheets JW, e t al: Corneal endothelium damage with intraocular lenses: Contact adhesion between surgical materials and tissue . Science 198: 525-527, 1977

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33. Refojo MF: Current status of biomaterials in ophthalmology. Surv Ophthalmo126:257-265 , i982 34. Andrade JD, ed: Hydrogels for Medical and Related·Applications. American Chemical Society Symposium Series #31 , American Chemical Society; Washington DC, 1976 35. Peddley DG, Skelley pJ, Tighe BJ: HydrogeJs.in biomedical applications. Br Polyin J 12:99-110, 1980 36. Wichterle 0, Lim D:' Hydrophilic gels for biological use , Nature 185:H7-118, 1960 37. Wichterle 0: Hydrogels. In: Mark HF, Gaylord NG; Bikales N, eds, En'qjclopedia 'of Polymer SCience and Technology, New York, Interscience; 1971, pp 273-290 38. Wichterle 0: The beginning of the soft lens. In: Ruben M, ed, 'Soft Contact Lenses: Clinical and Applied Technology, New York, John Wiley and Sons, 1978, pp 3-5 39. $impson BJ: Hydron: A hydrophilic polymer. Biomed Eng 4:65-68, 1969 . 40. Singh MP: Hydron in the right atrium. Biomed Eng 4:68-69, 1969 41. Binder PS, Deg JK, Zavala EY, Grossman KR: Hydrogel keratophakia in non-human primates. Curr Eye Res 1:535-542, 1981 42. Werblin TP, Blaydes JE, Fryczkowski AW, Peiffer R: Stability of hydrogel intracorneal implants in non-human primates. CLAO J 9:157-161 , 1983 43. Leininger RI: Polymers as surgical implants. CRC Crit Rev Bioeng 1:333-381 , 1972 44 . Boretos JW: Concise Guide to Biomedical Polymers. Springfield, IL, CharIesC Th0lilas; 1973 45. Stone j, PhilipsAJ; eds: Contact Lenses; a Textbook for Practitioner and Student. London, Butterworths, 1981 46. Hardman .BB, Torkelson A: Silicon compounds (silicones). In: Encyclopedia 'of Chemical Technology, vol 20, Wiley-International , 1982, pp 922-962 47. Frisch EE·: Technology of silicones in biomedical applications. In: Rubin LR, ed, Biomaterials in Reconstructive Surgery, St Louis, CV Mosby Co, 1983, pp 78-79 48. Ratner BD, Hoffman AS: Synthetic hydrogels for biomedical applications. In: Andrade JD, ed, Hydrogels for Medical and Related Applications, American Chemical SOCiety, Washington DC, 1976, pp 1-35

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49 . Barrett GD, Constable IJ, Stewart AD: Clinical results of hydrogel lens implantation . J Cataract Refract Surg 12:623-631, 1986 50. Fogle JA, Blaydes JE , Fritz KJ, Blaydes SH , et al: Clinicopathologic observations of a silicone posterior chamber lens in a primate model. J Cataract Refract Surg 12:281-284, 19$6 51. Newman DA, McIntyre DJ, Apple DJ, Popham JK, et al: Pathologic findings of an explanted silicone intraocular lens. J Cataract Refract Surg 12:292-297, 1986 52. Popham JK, Apple DJ, Newman DA, Isenberg RA, et al: Advantages and limitations of soft intraocular lenses : Ascientific perspective. In : Mazzocco TR, Rajacich GM, Epstein E, eds, Soft Implant Lenses in Catar:a ct Surgery, New Jersey, Slack, Inc, 1986, pp 11-30 53. Bath PE, Romberger AB, Brown P: A comparison of Nd:YAG laser damage thresholds for PMMA and silicone intraocular lenses. Invest Ophthalmol Vis Sci 27:795-798, 1986 54. Bath PE, Boerner CF, Dang Y: Pathology and physics ofYAGlaser intraocular lens damage. J Cataract Refract Surg 13:47-49, 1987 55. Skelnik DL, Lindstrom RL, Allarakhia L, Tamulinas C, et al: Neodymium: YAG laser interaction with Alcon 10GEL intraocular lenses: An in vitro toxicity assay. J Cataract Refract Surg 13:662-668, 1987 56. Faulkner Go.: Early experience with STAAR™ silicone elastic lens implants. J Cataract Refract Surg 12:36-39, 1986 57. Crawford JB, Faulkner GD: Pathology report on the foldable silicone posterior chamber lens. J Cataract Refract Surg 12:297-300, 1986 58. Epstein E : Insertion techniques and clinical experience with HEMA lenses. In : Mazzocco TR, Rajacich GM, Epstein E, eds, Soft Implant Lenses in Cataract Surgery, New Jersey, Slack, Inc, 1986, pp 143-150 59. Neumann AC , McCarty GR, Osher RH : Complications associated with STAAR silicone implants. J Cataract Refract Surg 13:653-656, 1987 60. Banker D, Sodero EC, Mazzocco TR, Rajacich GM, et al: Lenses of the future: Gel substitute , compressible silicone, expansile hydrogel. In : Abrahamson lA, ed, Cataract Surgery, New York, McGraw-Hili, 1986, pp 266-280

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