Polymethylmethacrylate Intraocular Lenses - Cataract Surgery, 3rd Edition

Cataract Surgery, 3rd Edition

PART VI – Intraocular Lenses

Chapter 36 – Polymethylmethacrylate Intraocular Lenses

Richard L. Lindstrom, MD

Contents

	•	Optic Materials
	•	Loop and Haptic Materials
	•	Manufacturing Techniques
	•	Sterilization
	•	General Design Characteristics, and Optic Size and Shape
	•	Loop Size, Shape, and Configuration
	•	Surface Modification of Polymethylmethacrylate Lenses
	•	Conclusions

CHAPTER HIGHLIGHTS

	≫	Original material and design considerations
	≫	Factors in biocompatibility of intraocular lens
	≫	Lessons learned in manufacturing

This chapter reviews the historical evolution and current status of intraocular lenses (IOLs) with an optic manufactured of polymethylmethacrylate (PMMA). Although a comprehensive review of the literature has been performed and selected references provided, the perspectives presented are those of the author's. In the following chapters, the historical evolution and current status of IOLs with an optic manufactured from foldable materials and of those with a multifocal optic also are reviewed.

The earliest reference to lens implantation is credited to Tadini, an eighteenth-century oculist.^[1–3] According to his memoirs, Casanova met him in 1766 in Warsaw, where Tadini showed him a box with small spheres that were well polished and suggested that such globes might be placed under the cornea in the place of the crystalline lens. No confirmation is available that Tadini ever actually did perform such an implant operation.

Approximately 30 years later, in 1795, a Dresden ophthalmologist, Casaamata, performed a cataract operation and implanted an artificial lens.^[1–3] Apparently, Casaamata performed the procedure by inserting the glass lens through a wound in the cornea. He immediately realized the procedure would not be successful as the glass lens fell deeply into the vitreous. Thus the first implantation of an IOL and the first severe complication, total lens dislocation into the vitreous, appear to belong to Casaamata.

The modern era of lens implantation begins with Harold Ridley of London.^[4–6] At the end of a cataract operation in the fall of 1949, Ridley reported he was asked by a medical student why he did not replace the cataractous lens he was removing with a new one. Apparently this gave Ridley the impetus to explore the possibility of lens implantation. During World War II, many ophthalmologists had noted that perforating eye injuries from airplane canopies made from acrylic Perspex plastic often resulted in minimal intraocular irritation secondary to the material itself. It, therefore, became accepted that acrylic was relatively inert in the eye. This, and the fact that acrylic has a relatively high refractive index of 1.49 and a low specific gravity of 1.19, prompted Harold Ridley to select this material for his initial investigations into lens implantation.

Ridley originally designed his lens to imitate the natural lens. Its diameter was 8.35mm, and its weight was 112mg in air and 70.4mg in water, as compared with a modern IOL, which weighs less than 4mg in water. On November 29, 1949, at St. Thomas Hospital in London, Harold Ridley implanted the first posterior-chamber lens into the capsular bag after an extracapsular cataract extraction. It is amazing that his original choice of material, method of cataract extraction, and selection of in-the-bag implantation have been affirmed after more than 40 years of trial-and-error investigation in this field.

The second lens was implanted almost 1 year later, on August 23, 1950. Unfortunately, the initial two patients’ postoperative refractive results were significantly myopic, one refracting at −20.0 and one at −15diopters (D). Ridley then recalculated the basic optics for the lens and began a series of about 750 implants, which extended to approximately 1959. These early lens-implant patients had a significant rate of complications, including severe postoperative inflammation and lens dislocation. Lens dislocation occurred in approximately 13% of the cases, usually into the vitreous. Many patients also developed late secondary glaucoma. Nonetheless, many of these implants performed well for many years (Figure 36-1).

Ridley's work stimulated several other surgeons to become interested in the idea of lens implantation. In an attempt to reduce the high incidence of dislocation, most of these surgeons abandoned the posterior chamber and began to design anterior-chamber, angle-fixated lenses. The first published reports of such lenses came from Strampelli.^[7] His lens can be considered the precursor of the rigid, one-piece, anterior-chamber lenses. Of particular interest is the original work by Dahnheim, whose closed-loop, anterior-chamber lens most closely resembles the closed-loop, anterior-chamber lenses that gained great popularity in the United States in the 1980s before being withdrawn from the market for an unexpectedly high incidence of uveitis, glaucoma, hyphema, cystoid macular edema, and corneal decompensation.^[8] Of equal interest is the open-loop, anterior-chamber lens of Barraquer, which was later modified successfully as a posterior-chamber lens by Shearing.^[9,][10] It remains today one of the most popular IOL designs in the world.

Unfortunately, the early anterior-chamber, angle-fixated lenses resulted in a very high incidence of secondary corneal decompensation. Barraquer actually reported a 67% incidence of late corneal decompensation, and he had to remove 50% of his lens implants, many of which were implanted for the correction of myopia in phakic eyes.^[9]

These anterior-chamber angle-fixated lenses were the first lenses that suggested to ophthalmologists that long-term follow-up might be required to confirm the level and severity of complications. For example, Strampelli did not notice a high incidence of bullous keratopathy until almost 5 years postoperatively, with most of his patients doing well in the initial postoperative period.

In 1964, Peter Choyce designed a lens, named the Mark VIII, based on his continuing work in anterior-chamber angle fixation (Figure 36-2).^[8] This was the first lens implant to perform in a reasonable fashion over an extended period in many surgeons’ hands. This lens was implanted without any major change in design between 1964 and 1978. One of the major factors in the improved success of the Mark VIII lens of Choyce was a significant improvement in the quality of manufacturing, which was quite crude in many of the early implant designs.

A continuing significant complication rate with the anterior-chamber angle-fixated lens caused many ophthalmologists to turn to the pupil or iris for fixation of a lens implant. Pioneers of iris-fixated lenses included Epstein of South Africa,^[11] Binkhorst^[12–14] and Worst^[15] in the Netherlands, and Fyodorov^[8] in the Soviet Union.

The original work of Edward Epstein led to a lens shaped like a Maltese cross, with four wings extending from a central optical part. Epstein abandoned this design because of secondary complications, especially inflammation. A similar lens under the name of Copeland was widely used in the United States in the 1970s (Figure 36-3). Unfortunately, a high incidence of chronic iris irritation, secondary cystoid macular edema, and bullous keratopathy caused this lens also to fall into disuse.

A major contributor to the generation of iris support and later iridocapsular lenses was Cornelius Binkhorst, who developed the concept of the iris clip implant. Modifications of this lens, especially the Binkhorst four-loop iris clip lens, gave results superior to those that had been obtained with the early posterior-chamber and anterior-chamber angle-fixated implants.

Jan Worst, also working in Holland, had the concept of improved fixation through the use of a suture or metal clip. This led to a series of lenses called the Medallion lens implants, which also presented improved results over the previous generation of implants.

At the same time, working in the Soviet Union, Syvataslav Fyodorov developed a group of lenses commonly called the Sputnik lenses, which also became popular. However, there continued to be a significant rate of dislocation, cystoid macular edema, and secondary corneal decompensation with these lens implants. Unsatisfied with the results himself, Binkhorst recognized the benefits of extracapsular cataract extraction. He went on to develop a series of iridocapsular lenses, which achieved their primary fixation from the capsular bag after extracapsular cataract extraction and a secondary fixation and centration through pupil support.^[13] It was soon discovered that the frequency and severity of cystoid macular edema, dislocation, and corneal decompensation were significantly lower with the capsular-fixated lenses.

The concept of iridocapsular fixation gained increasing support and led to a renewed interest in extracapsular cataract extraction. This set the stage for a return to the posterior chamber.

In 1977, John Pearce, working in England, reevaluated the concept of capsular fixation of a posterior chamber lens.^[16][,17] Beginning with a small, 4mm optic, tripod-shaped lens, which he sutured to the iris to obtain secondary fixation, he showed that posterior-chamber lenses could be safely and effectively implanted without a high complication rate. The work of Pearce stimulated the imagination of several other ophthalmologists, including Shearing^[10] and Simcoe,^[18] who modified the Barraquer flexible, open-loop anterior-chamber lens for use in the posterior chamber. Shearing developed the J-loop posterior-chamber lens, with semiflexible loops for implantation either into the ciliary sulcus or the capsular bag^[10] (Figure 36-4). This lens proved almost immediately to be successful and demonstrated a relative ease of implantation with a lower complication rate than had been noted with any other implant design. Many brilliant and innovative surgeons then dedicated themselves to improving on the open-loop posterior-chamber lens implant.^[19–50] The end result of the evolution is discussed later in this chapter.

At about the same time that one group of pioneers was returning to the posterior chamber with good success, there was a rebirth of interest in the closed-loop anterior-chamber lens (Figure 36-5). The attractiveness of this concept was clear in that most surgeons were accomplished at intracapsular cataract extraction and did not wish to learn the skill of extracapsular extraction required for posterior-chamber lens implantation. The popularity of the closed-loop anterior-chamber lenses peaked in approximately 1982, followed by a total withdrawal from the market by 1988 because of an intolerable incidence of secondary complications, including cystoid macular edema, secondary glaucoma, uveitis, hyphema, and corneal decompensation^[51–55] (Figure 36-6).

About the same time, Charles Kelman introduced his flexible, three-point and four-point fixation, one-piece PMMA open-loop anterior chamber lenses, which have continued to perform successfully to this day (Figure 36-7).



Figure 36-1 Ridley posterior-chamber lens implant in an eye, nearly 40 years after implantation.



Figure 36-2 Choyce–Tennant, rigid, anterior-chamber intraocular lens.



Figure 36-3 Copeland pupil-supported lens.



Figure 36-4 Original Shearing J-loop posterior-chamber lens used a 5mm optic and polypropylene loops.



Figure 36-5 The popularity of the Leiske closed-loop anterior-chamber lens peaked between 1982 and 1983.



Figure 36-6 Pupil-blocked glaucoma associated with uveitis and recurrent microhyphema in a patient who received a Leiske closed-loop anterior-chamber lens implant.



Figure 36-7 Kelman flexible, open-loop, one-piece, anterior-chamber lens.

Optic materials

The optic material for an IOL is required to meet several challenges. It must be able to be lathed or molded and polished to a high optical quality, it must be biocompatible and durable with minimal induction of inflammation, it must be nonantigenic and noncarcinogenic, and it must be sterilizable. To meet the requirement for a light weight, it requires a high index of refraction. It is remarkable that Harold Ridley, in his first work, selected a material that has continued to stand the test of time, PMMA.

PMMA is a polymer of methylmethacrylate monomer (Figure 36-8). PMMA is manufactured through the addition polymerization of methylacrylic acid methyl ester, which is itself derived from acrylic acid.^[56] Additional agents such as ultraviolet light absorbers may be added to the plastic to enhance its capabilities.^[57–59] Various forms of PMMA are available commercially. PMMA used in lathe-cut or compression-molded IOLs is a high-molecular-weight type, such as the Perspex CQ, manufactured by Imperial Chemical Industries. Another form of manufacturing, injection molding, uses a lower-molecular-weight PMMA, such as that manufactured by Rohm & Haas.



Figure 36-8 Chemical structure of polymethylmethacrylate.

PMMA is a light, durable material with a specific gravity of 1.19. It has a refractive index of 1.49. The average molecular weight of Perspex CQ is in the range of 2.5 to 3 million Daltons. At temperatures lower than 100° C it is hard, but the material can melt at temperatures of 140° C or higher.

Although the monomer is toxic, the polymer is inert and is well tolerated in the eye with minimal inflammatory reaction.^[60–62] Release of monomer from PMMA was not a concern until the development of the neodymium:yttrium-aluminum-garnet (Nd:YAG) laser for capsulotomy, which has the potential to damage the PMMA optic.^[63–67] It has been shown that substances toxic to cultures of various ocular cell lines can be released from PMMA IOLs that are directly hit with Nd:YAG laser bursts of approximately 5mJ. Fortunately, this has not resulted in significant problems, because much lower energy levels are used, and direct blows to the lens are rare. The higher-molecular-weight, lathe-cut, compression-molded, and cast-molded IOLs are more resistant to Nd:YAG laser damage than the injection molded lenses.

Although PMMA is relatively inert, it is not totally inert in the eye. It does not appear to activate complement or induce chemotaxis of leukocytes, but a cellular reaction does occur on its surface, even in clinically well-tolerated implants.^[68–70]

PMMA transmits a broad spectrum of light, including near-ultraviolet light, a possible source of retinal damage.^[71] Therefore, ultraviolet-absorbing materials have been added to PMMA optics to reduce this potential toxicity.^[57–59] The ultraviolet absorber may be added through covalent bonding or by simply mixing the chromophores with the PMMA. Although it has not been confirmed that ultraviolet absorption has a clinical value, most current PMMA lenses contain ultraviolet light absorbers, usually a benzophenone or benzotriazole.

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Loop and haptic materials

Most PMMA optic lenses are supported by one or more loops or haptic configurations. These may be broadly divided onto one-piece and multipiece IOLs. In the one-piece lenses, the entire lens is manufactured of the same PMMA. In the multipiece lenses, the loop or haptic is composed of a second material. These alternative materials are discussed in the following paragraphs.

An initial material selected for support loops was polyamide, a synthetic material consisting of long molecular chains that contain the amido group at regular intervals. The polyamides include materials such as Nylon 6, Supramid, and Perlon. In the American literature, nylon is basically synonymous with polyamide. These materials retain a high degree of breakage resistance and are quite flexible. They can be manufactured in various diameters, and the manufacturing technology to create various shapes is readily available. Unfortunately, nylon undergoes hydrolysis, resulting in fragmentation over time after implantation into tissue.^[15,][72] For this reason, polyamide materials were abandoned for use as loops to support optics.

Another suture material, polyethyleneglycolterephthalate, commonly known as Dacron or Mersilene, is also a potential material for IOL loops.^[73] Unlike nylon, it is hydrophobic rather than hydrophilic and does not appear to undergo biodegradation. However, it is less elastic and stiffer than the favored polypropylene. Although it is not currently popular for use in IOL loops, Mersilene suture is often used for wound closure and can be used for iris fixation or transscleral fixation of posterior chamber or iris-supported IOLs.

The most popular material for three-piece posterior chamber lenses other than PMMA has been polypropylene (Prolene).^[74] Polypropylene has a high tensile strength and, in contrast to nylon, has no hydrolysable binding sites. In a vascular tissue, it is biologically inert and relatively stable, although it is subject to oxidative biodegradation, especially when exposed to light.^[72] Elongation of polypropylene is significantly greater than that of Mersilene. It also appears to have a relatively short structural memory, making it possible to implant lenses with compressible polypropylene loops in a space smaller than the diameter of the lens without the loops causing continuous pressure on the tissues.

Polypropylene continues to be a widely employed material for loops of posterior chamber lenses. However, it has two potential problems.

In vitro, polypropylene materials have demonstrated increased levels of complement fragments, which attract circulating inflammatory cells.^[70] The clinical importance of this finding is not clear. To date, only one study by Tuberville and Wood^[75] has measured complement levels in pseudophakic eyes. This study found that complement levels in posterior chamber IOL pseudophakic eyes were not different than in phakic eyes. The type of haptic material in the patients studied was not differentiated, however.

One in-vitro assay indicated that more bacteria adhere to polypropylene than to PMMA haptics.^[76] In a retrospective case-control clinical comparison of endophthalmitis, Menikoff et al.^[77] reported that 87% of their cases involved polypropylene-looped lenses. Because of the low incidence of endophthalmitis and the time frame of this study, it is not clear if the more frequent involvement of polypropylene-looped lenses is a function of other confounding factors or represents a clinically significant phenomenon.

Nonetheless, polypropylene has stood the test of time as a support loop in posterior chamber lenses. Yet most surgeons have come to favor PMMA. Metal-looped, iris-supported lenses had a short period of popularity, particularly platinum-iridium and titanium. Today, all metal-looped lenses have been removed from the market because of a high rate of complications.^[78]

Another potential haptic material put to use more recently is polyimide. This is a synthetic material of variable construction that contains an imino (NH) group and benzoyl ring. This material is capable of withstanding high temperatures and high-energy radiation, and may be heat sterilized. At this time, polyimide is used as a support loop in some three-piece silicone IOLs, and it is performing in a satisfactory fashion.

In summary, the most popular materials to provide haptic support for a lens implant continue to be PMMA itself, polypropylene, and polyimide. All three appear to perform in a satisfactory fashion, especially if placed inside the capsular bag. The possible oxidative biodegradation of polypropylene and its ability to induce inflammation have reduced its popularity, especially when sulcus fixation or anterior-chamber angle fixation is selected. The dominant trend is toward one-piece, all-PMMA posterior-chamber lenses.

Manufacturing techniques

There are at least five different ways to manufacture IOLs today from PMMA material.^[78] Although the exact details of the manufacturing process are often proprietary, a brief review follows.

The most popular method is lathe cutting, in which lenses are cut out of a PMMA blank, usually of a high-molecular-weight acrylic, such as Perspex CQ. Initially, PMMA cast sheets are received as raw material. PMMA cast sheets are preformed and then, using a computer-aided microlathe, lens blanks are cut to the appropriate specific spherical radius for the optic power. The lens blanks are polished. In three-piece lenses, the loops are then formed and attached. In one-piece lenses, the lens optic is lathe cut with a diamond-tip tool to the radius of curvature, and the haptic shape is milled to size and shape. A modern lathe can be programmed to produce a variety of lens shapes and powers. After formation, most lenses are tumble polished, using a drum filled with small spheres that rotate slowly for hours to days. This results in very smooth edges. As an alternative, polishing pads may be used to buff the surface and edges.

In a second manufacturing technique, compression molding is added to the lathe cutting. After creating the lens with a lathe, the implant is placed into a mold, and heat and pressure are applied to shape the lens into its final form. The lens is then carefully polished. This manufacturing technique produces a lens that requires less polishing than a lens produced solely by lathe cutting.

A third method is compression polymerization, in which well-aged and dried base material is poured into a hard stainless-steel mold and slowly warmed under high pressure until polymerization occurs. High pressure is maintained as the material is cooled by blowing.

Fourth, the lenses may be cast molded. Cast molding requires the use of resin in a distilled and purified form. PMMA monomer is crystallized into a pregel or prepolymer residue. The pregel is vacuum processed, filtered, and poured into molds of the desired optical configuration. Lens blanks are cast in a curing cycle similar to that used to prepare cast sheet PMMA.

Finally, lenses may be injection molded. In injection molding, the plastic is heated and then injected into a steel mold. As it softens, it takes the shape of the PMMA mold. After the PMMA cools, the blank is removed from the mold and the edges are polished.

It is clear that manufacture of IOLs by any of these techniques is sophisticated and requires highly skilled technical capability. For an IOL to be suitable for implantation it must be within 0.25D of stated power; have the proper shape and configuration within 0.25mm; have a uniform, smooth surface and edges; be chemically pure without any residual monomer, ethylene oxide, or contaminants; be clean of surface debris; and be sterile. Although all manufacturers incorporate careful practices and all IOLs are inspected, it is the surgeon's responsibility to perform a final inspection as well. This may be done under high power with the operating microscope. If the lens shows any imperfection, it should be rejected for implantation and returned to the manufacturer.

Sterilization

PMMA lenses cannot be heat sterilized because the material will melt. Most PMMA lenses are sterilized by ethylene oxide, which is the only technique that is approved by the United States Food and Drug Administration. Ethylene oxide is a cyclic ether that can be toxic to tissue and adheres only to plastic. Although it is an effective sterilizer, all IOLs must be quarantined after sterilization until excess ethylene oxide evaporates and reaches a nontoxic level. Ethylene oxide residual continues to be a concern as a possible cause of postoperative inflammation after implantation of IOLs. The alternative technique of sodium hydroxide sterilization is not used in the United States, but is potentially effective.

General design characteristics, and optic size and shape

IOL optics range in size from 4.5 to 7.5mm. The potential advantage of a larger optic is a lens that is more forgiving of decentration and one that is less likely to produce unwanted optical aberration as a result of light deflecting off the edge of the optic.^[79,][80] In addition, there is a reduced likelihood of complications such as pupillary capture. An optic size of 6.5–7mm is appropriately popular with surgeons using planned extracapsular cataract extraction in which a large incision is made (Figure 36-9).



Figure 36-9 Modern one-piece, all-polymethylmethacrylate, open-loop, posterior-chamber lens with a 6.5mm optic. (Courtesy IOLAB Corporation, Claremont, Calif.)

As more surgeons convert to phacoemulsification and use of continuous-tear anterior capsulectomy, smaller optic lenses are becoming more popular.^[81–83] In particular, lenses with a round optic of 5–5.5mm are currently favored by many phacoemulsification surgeons (Figure 36-10). Because lens implants placed inside the bag after capsulorrhexis show minimal decentration, this optic size seems satisfactory for most older patients.^[79] However, these lenses have the potential for increased optical aberration as a result of edge light deflection. Several manufacturers are working on edge treatments to reduce the unwanted visual images created when a lens implant edge is exposed in the pupil.



Figure 36-10 A 7mm round optic, one-piece, all-polymethylmethacrylate, posterior-chamber lens side by side with a 5 × 6mm oval lens. (Courtesy Storz Corporation, St. Louis, Mo.)

Oval optics, especially those 5 × 6mm in diameter, are available and also allow implantation through a relatively small incision. Unfortunately, inside a continuous-tear circular anterior capsulectomy, some lenses decenter in the axis perpendicular to the loops, rendering the extra optical diameter in the opposite meridian less helpful. In addition, the oval lenses appear to induce a higher incidence of unwanted glare or reflection off the edge of the 5mm portion of the optic, which is usually somewhat thicker. Currently, oval lenses are rarely used as most surgeons favor round optic lenses.

In regard to shape, biconvex lenses appear to be the soundest design. In addition to simulating the natural lens and providing a good optical quality, the posterior convex portion of the optic in close apposition to the posterior capsule appears capable of retarding opacification from Elschnig pearls.^[84–86] A biconvex implant design also presents a relatively low profile, which enhances implant-to-iris clearance.

Other design shapes, including meniscus, planoconvex, and those with laser spacing ridges, are still preferred by some surgeons but are significantly less popular. Many lenses have one or more small holes placed in the optic or adjacent to the optic–haptic junction to aid the surgeon in positioning the lens within the eye. Although some surgeons find these useful, with modern flexible-loop capsular bag lenses, positioning holes are probably unnecessary and in some cases can produce unwanted optical aberrations.^[37] Use of the optic–haptic junction for manipulation as an alternative to positioning holes in the optic is currently favored by most surgeons because this configuration reduces the chance of postoperative unwanted visual aberrations.

Loop size, shape, and configuration

Loop materials currently used in most posterior chamber lenses include PMMA, polypropylene, and polyamide. One-piece and three-piece PMMA lenses, three-piece polypropylene lenses, and polyamide haptic lenses have excellent track records with no strong scientific evidence to support one over the other. However, there is an increasing trend toward use of one-piece, all PMMA lenses among experienced surgeons. Theoretically, the absence of a junction between two materials reduces the chance for a discontinuity at which inflammatory cells and debris could accumulate. Several studies have shown that the capsular bag stretches only from approximately 9.5 to approximately 11mm after cataract extraction and that the diameter of the ciliary sulcus is only 11.5–12.5mm.^[46] This has led to a trend toward shortening the overall tip-to-tip diameter of posterior chamber IOLs. These so-called capsular bag lenses are usually 11.5–12.5mm from tip to tip. These lenses provide a relative ease of implantation without excess capsular stretch and striae in the postoperative period. Nonetheless, the longer tip-to-tip diameter lenses have achieved an excellent track record over the past 10 years.

Multiple-loop configurations – beginning with the J-loop lens, followed by the Y-loop configurations popularized by Kratz and Sinskey, through the longer and more gradual C-loop, and on to the modified C loop, which is most popular today – are available without any strong scientific proof favoring one over the other. The choice of loop configuration is usually made by the preference of the individual surgeon and influenced by ease of implantation. The majority of surgeons use a modified or short C configuration. Loops are usually angled to a small degree, in the belief that this will reduce pupil capture and iris chafe and place the optic more directly in contact with the posterior capsule.^[87] An angulation of 3–10° is usually preferred. Some loops also contain holes or notches for specialized implantation forceps or for use in scleral fixation. Some loops are colored to provide easier visualization during and after implantation into the eye. Increased visibility of the loops can assist in ensuring that the implant is inside the capsular bag (Figure 36-11).



Figure 36-11 A colored-loop, one-piece, all-polymethylmethacrylate, posterior-chamber lens inside the capsular bag, as viewed by a Miyake eye model posterior photograph. (Courtesy IOLAB Corporation, Claremont, Calif.)

The most popular lenses of today appear to be those manufactured with biconvex optics, which are round and between 5 and 6.5mm in diameter. In most cases, these lenses are one-piece or three-piece tumble-polished biconvex optics with haptics of PMMA. The preferred overall diameter is 12–12.5mm, with a modified short C loop and an angulation of approximately 5°. Results with these lenses are superb, and surgeons can use them with confidence. Other alternative lens designs continue to attract small but loyal followings, including various closed-loop designs such as those advocated by Sheets and Anis for capsular-bag fixation.^[29,][88] In addition, disc or plate lenses have been investigated, especially in Europe.^[6]

Surface modification of polymethylmethacrylate lenses

With improvements in surgical technique and equipment, the incidence of postoperative inflammation following cataract surgery has reduced dramatically. Nevertheless, evidence has shown that a low-grade inflammatory response occurs to the IOL implant.^[89–91] Clinical and histologic studies have demonstrated that a foreign-body reaction likely occurs in all eyes following IOL implantation.^[92–94] This inflammatory reaction may result in synechiae, cellular and pigmented deposits on the IOL, and uveitis. Certain conditions, such as pre-existing uveitis, diabetes, glaucoma, or a young age may predispose a patient to this event.

Despite PMMA being a relatively inert material in the human eye, its biocompatibility is compromised in one respect, that of surface molecular structure. The ends of the long polymer chains of PMMA, when exposed to the surface of the IOL, can induce inflammatory responses. Knowledge of this interaction potential has stimulated much research into methods of modifying the surface of PMMA lenses such that the exposed polymer ends are either chemically modified or coated with another material.^[42,][43]

In particular, the use of a heparin coating on the IOL has been investigated and has been shown to reduce early postoperative inflammation in high-risk eyes.^[44,][45] This IOL is created by inducing electrostatic adsorption of heparin onto the surface of the PMMA IOL. In-vitro experiments have demonstrated a reduced activation of human granulocytes with heparin coating of PMMA. Furthermore, platelet adhesion and growth of human fibroblasts are reduced.^[95]

A coating of heparin has been shown to decrease the number and severity of lens deposits. In addition, the likelihood of formation of adhesions to the IOL is decreased.^[96–98] Some have found heparin-coated IOLs to be free of cellular deposits and to reduce the incidence of inflammatory complications, compared with unmodified PMMA lenses.^[99] In addition, fewer lens deposits lead to a clearer implant and better visual acuity.^[100] These benefits have been found not only for high-risk eyes but also for routine cases.^[101] Care must be taken in implanting heparin-coated IOLs because the heparin can be mechanically destroyed in the areas where the lens is grasped.^[102] The clinical consequences of this occurrence are not currently known.

Another modification of PMMA IOL surfaces has been introduced by Bausch & Lomb Surgical. This innovation, known as fluorine surface modification, is unique in that it produces a permanent, irreversible change in the molecular structure and composition of the IOL surface. Again, this change results in an enhanced biocompatibility with decreased cellular adsorption and adhesions.^[103] Fluorine-surface modified lenses have been used in the correction of pediatric aphakia, a procedure in which the postoperative inflammatory response can be significant. These lenses have demonstrated a reduced cicatricial response compared with unmodified PMMA lenses.^[104]

Conclusions

Although IOLs have historically been placed in the anterior-chamber angle, pupillary space, or posterior chamber, the posterior chamber has been confirmed to be the safest and most effective for primary implantation.

Surgeons have the opportunity to select from a large variety of IOL designs. Most of the differences are small modifications based on individual preference, but some design features have been shown to be advantageous through clinical experience and scientific study. PMMA is clearly a superior material for lens implant. Biconvex optics appear to be preferred over other shapes. In addition, a round optic appears to be superior to an oval optic. The trend is, therefore, toward one-piece, biconvex PMMA lenses, which may have a slight advantage over three-piece lenses.

The short modified C-loop haptic appears to be a good compromise between ease of implantation and solid fixation and centration. Reduction in the overall tip-to-tip loop diameter to 12–12.5mm is consistent with ocular anatomy, and loops that angle forward 3–10° reduce contact between the lens and the iris, and thereby the chance for pupillary capture. In addition, implantation within the capsular bag appears to be superior to sulcus fixation because the lens is sequestered from contact with vascular tissue.

Positioning holes and notches on the lens loops are unnecessary for most surgeons for routine implantation and may result in secondary complications, such as optical aberration or difficulty in IOL removal. Most lenses incorporate an ultraviolet light absorber in spite of the absence of strong evidence in its favor. Loop coloration may ease implantation for the surgeon beginning in this field, but is not required for most experienced surgeons. Surface modification may be useful to reduce the postoperative inflammatory response, particularly in high-risk cases.

The evolution to the current state of the art in posterior chamber lenses over a period of more than 40 years represents a marvel of collaboration between manufacturers, ophthalmic surgeons, and patients throughout the world.

References

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[11]. Epstein E.: Modified Ridley lenses. Br J Ophthalmol 1959; 43:29-33.

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