Refractive Eyecare — April 2011
CATARACT SURGERY: IOL Lens Materials: Choice and Clinical Implications
When deciding on the most appropriate iOL for a given situation and patient, surgeons should include lens materials among the factors that can influence clinical outcome.
In the early days of intraocular lens (IOL) implantation, all IOLs were made of rigid polymethylmethacrylate acrylic (PMMA). The early 1980s brought the introduction of the foldable lenses— manufactured from silicone or hydrophilic acrylics—and these, in turn, were followed by hydrophobic acrylic lenses.Today IOL materials fall into these two basic categories: acrylic (including PMMA, hydrophobic acrylic, and hydrophilic acrylic) and silicone.1 In addition, new IOL materials within these major categories are moving through various stages of clinical investigation, and some of these are already in use overseas.
From a surgeon’s perspective, there are advantages and disadvantages to each material category. However, the properties that are most desirable in an IOL can vary depending on a given surgical situation and patient. A review of these differences begins with basic material properties.
Refractive index is one of the most fundamental differences between acrylic and silicone lenses. In general, acrylic has a higher refractive index (1.46 or greater), which allows these lenses to be thinner than silicone lenses of the same refractive power. In addition, each type of foldable acrylic lens consists of a slightly different copolymer. Here, too, refractive index varies: from 1.55 for Alcon’s hydrophobic AcrySof® to 1.46 for Rayner’s hydrophilic C-flex.
The first silicone IOLs were made of polydimethylsiloxane, with a refractive index of 1.41. Newer generation silicone IOLs have a higher refractive index. The silicone used in the SI-40 and Clariflex® lenses (Abbott Medical Optics), for example, has a refractive index of 1.46.
Glass transition temperature is another differentiating property in IOL materials. This is the temperature above which a polymer transitions from rigid to flexible. For foldable acrylics, it occurs around room temperature, and when implanted, these lenses tend to unfold in a slow, controlled manner. By contrast, silicone’s considerably lower glass transition temperature produces an IOL that unfolds quickly. Surgeons differ in their preferences. But in general, more controlled unfolding is gentler on a compromised capsular bag.
Water content varies between different copolymers to produce different mechanical properties. Hydrophobic acrylic lenses generally have very low water content (< 1%), although Advanced Vision Science (AVS) currently manufacturers a hydrophobic acrylic material with water content of approximately 4%, which is the only hydrophobic acrylic lens packaged in solution (0.9% saline). By contrast, most hydrophilic acrylic lenses have water contents ranging from 18% to 38%. In general, hydrophilic materials enable the degree of IOL compressibility needed for micro-incision surgery.
INTRAOCULAR LENS MATERIALS
Foldable acrylics (hydrophobic and hydrophilic)
— Thinner than silicone for same refractive power
— Allows controlled unfolding
— Calcification risk with some hydrophilic acrylics
— Glistenings most associated with hydrophobic acrylics
— Interlenticular opacification associated with piggybacked in-the-bag hydrophobic IOLs
— Capsular dyes stain high water content hydrophilic
— More rapid unfolding
— Anterior capsule fibrosis/opacification more common with older silicone materials
— Calcification risk in eyes with asteroid hyalosis
— Interact with silicone oil (vitreoretinal surgery)
Lens materials under investigation
— Silicone light-adjustable
IOL Materials in the Pipeline
IOL materials with photochromic, or light-adjustable, properties are now available overseas, but still moving through or towards clinical investigation in the United States. Calhoun Vision (Pasadena, CA) is developing a silicone, light-adjustable material with photosensitive silicone macromers that move within the lens optic when exposed to low intensity, near-ultraviolet light, enabling the surgeon to fine tune refractive power noninvasively after implantation.2,3 Medennium (Irvine, CA) has developed a hydrophobic acrylic optic with a photochromic chromophore that changes from clear to yellow when exposed to strong UV light outdoors, transitioning back to colorless indoors.4
A number of clinical studies have compared the inflammatory reactions provoked by different IOL materials. In 2006, Abela-Formanek and colleagues reported no clinically relevant difference in postoperative flare values associated with different foldable IOL Materials in 142 cataract patients, including some with a history of uveitis.5 Postoperative recovery was similar.
By contrast, another study of 100 eyes suggested that hydrophobic acrylic IOLs have a greater tendency to provoke late foreign-body cell reactions, although none proved clinically significant.6 In 2000, clearer biocompatibility differences were found in a report of 460 cadaver eyes, with anterior capsule opacification and fibrosis being more common with silicone IOLs (vs acrylic); this was particularly the case with plate haptic designs that increased the area of contact with the anterior capsule.7 However, more recent studies have failed to demonstrate such an association with IOLs made from the latest generation of silicone materials.8
Most recently, Abela-Formanek’s group reported the results of their 7-year, post-implantation follow up on 136 uveitis eyes, with results suggesting that modern hydrophilic acrylic IOLs provided greater uveal biocompatibility than hydrophobic acrylic IOLs, at least during the early postoperative period.9
Some have speculated that differences in material may influence the risk of posterior capsule opacification. If so, such differences are dwarfed by the influence of lens geometry— a square posterior optic edge providing the best protection against this complication.10,11 As a group, currently available hydrophilic acrylic lenses have less square edges.
Postoperative calcification has been associated with some hydrophilic acrylic lenses, and further research is needed to tease out how this may relate to differences in manufacturing techniques and/or material subtypes.12 Silicone lenses, in turn, appear to predispose to calcification in eyes with asteroid hyalosis.13 To date, the literature describes 22 cases of calcification related to eight silicone lens designs, 86% of these occurring in eyes with confirmed asteroid hyalosis. As a result, asteroid hyalosis, when present, should be among the prominent factors that guide a surgeon’s choice of IOL.
Silicone oil-silicone IOL interactions are well-documented when standard silicone lenses are implanted in the context of vitreoretinal surgery.14 Irreversible adherence of silicone oil to the IOL optic can cause visual disturbances and visual loss and obstruct the vitreoretinal surgeon’s view into the eye. Therefore, silicone IOLs should be avoided in patients with vitreoretinal disease.
Glistenings, or fluid-filled microvacuoles, can form within any type of IOL optic.15 They are most associated with hydrophobic acrylic lenses, though to varying degrees that may reflect differences in material subtypes, manufacturing techniques, and even packaging. Predisposing patient factors may include glaucoma and other conditions that break down the blood-aqueous barrier. Certain ocular medications may likewise increase the risk of this complication.The effect on visual function remains unclear, and IOL explantation due to glistenings is rarely reported.
Interlenticular opacification involves the opposing surfaces of piggybacked IOLs and is primarily associated with hydrophobic acrylic lenses.16 To date, all cases analyzed in our laboratory have involved two posterior chamber hydrophobic acrylic IOLs implanted in the capsular bag through a small capsulorhexis with margins overlapping the optic edge of the anterior IOL for 360 degrees. This complication can Be prevented by implanting the anterior IOL in the sulcus and the posterior IOL in the bag with a small rhexis. However, the IOL to be placed in the sulcus should have appropriate design characteristics, such as round, smooth edges, and thin haptics.17
Finally, high water content hydrogel IOLs should be avoided immediately after using capsular dyes, as they tend to absorb even minimal amounts of dye.18
THE BOTTOM LINE
There is no one ideal material for the manufacture of IOLs. Rather, clinically relevant differences in material properties should be among the factors the surgeon considers when selecting the best IOL for a given patient. In general, newer IOL materials appear to be more biocompatible; silicone lenses should be avoided in eyes with asteroid hyalosis or vitreoretinal disease; hydrophobic acrylic IOLs should not be piggybacked within the capsule; and high water content hydrogels should not be used in the presence of capsular dye.
Liliana Werner, MD, PhD, is an associate professor at the John A. Moran Eye Center of the University of Utah, in Salt Lake City, and the co-director of its Intermountain Ocular Research Center. She states that she has received research funding from Alcon, AMO, Anew Optics, AVS, Bausch + Lomb, Calhoun, Medennium, and Rayner and has been a consultant with AMO/Visiogen and Powervision.Freelance writer Deborah Hoffman assisted in the preparation of this manuscript.
1. Werner L. Biocompatibility of intraocular lens materials. Curr Opin Ophthalmol. 2008;19:41-9.
2. Schwartz DM, Sandstedt CA, Chang SH, Kornfield JA, Grubbs RH. Light-adjustable lens: development of in vitro nomograms. Trans Am Ophthalmol Soc. 2004;102:67-72.
3. Werner L, Yeh O, Haymore J, Haugen B, Romaniv N, Mamalis N. Corneal endothelial safety with the irradiation system for light-adjustable intraocular lenses. J Cataract Refract Surg. 2007;33:873-8.
4. Werner L, Mamalis N, Romaniv N, et al. New photochromic foldable intraocular lens: Preliminary study on feasibility and biocompatibility. J Cataract Refract Surg. 2006;32:1214-21.
5. Abela-Formanek C, Amon M, Schild G, et al. Inflammation after implantation of hydrophilic acrylic, hydrophobic acrylic, or silicone intraocular lenses in eyes with cataract and uveitis: comparison to a control group. J Cataract Refract Surg. 2002;28:1153-9.
6. Mullner-Eidenbock A, Amon M, Schauersberger J, et al. Cellular reaction on the anterior surface of 4 types of intraocular lenses. J Cataract Refract Surg. 2001;27:734-40.
7. Werner L, Pandey SK, Escobar-Gomez M, Visessook N, Peng Q, Apple DJ. Anterior capsule opacification: A histopathological study comparing different IOL styles. Ophthalmology. 2000;107:463-71.
8. Sacu S, Menapace R, Findl O. Effect of optic material and haptic design on anterior capsule opacification and capsulorhexis contraction. Am J Ophthalmol. 2006; 141:488-93.
9. Abela-Formanek C, Amon M, Kahraman G, Schauersberger J, Dunavoelgyi R. Biocompatibility of hydrophilic acrylic, hydrophobic acrylic, and silicone intraocular lenses in eyes with uveitis having cataract surgery: Long-term follow-up. J Cataract Refract Surg. 2011;37:104-12.
10. Werner L, Müller M, Tetz M. Evaluating and defining the sharpness of intraocular lenses. Microedge structure of commercially available square-edged hydrophobic lenses. J Cataract Refract Surg. 2008; 34:310-7.
11. Werner L, Tetz M, Feldmann I, Bücker M. Evaluating and defining the sharpness of intraocular lenses: microedge structure of commercially available square-edged hydrophilic intraocular lenses. J Cataract Refract Surg. 2009;35:556-66.
12. Werner L. Causes of intraocular lens opacification or discoloration. J Cataract Refract Surg. 2007; 33:713-26.
13. Stringham J, Werner L, Monson B, Theodosis R, Mamalis N. Calcification of different designs of silicone intraocular lenses in eyes with asteroid hyalosis. Ophthalmology. 2010;117:1486-92.
14. Apple DJ, Federman JL, Krolicki TJ, et al. Irreversible silicone oil adhesion to silicone intraocular lenses: A clinicopathologic analysis. Ophthalmology. 1996; 103:1555-61.
15. Werner L. Glistenings and surface light scattering in intraocular lenses. J Cataract Refract Surg. 2010; 36(8):1398-1420.
16. Werner L, Apple DJ, Pandey SK, et al. Analysis of elements of interlenticular opacification. Am J Ophthalmol. 2002;133:320-6.
17. Chang WH, Werner L, Fry LL, et al. Pigmentary dispersion syndrome with a secondary piggyback 3-piece hydrophobic acrylic lens: Case report with clinicopathological correlation. J Cataract Refract Surg. 2007;33:1106-1109.
18. Werner L, Apple DJ, Crema AS, et al. Permanent blue discoloration of a hydrogel intraocular lens caused by intraoperative use of trypan blue. J Cataract Refract Surg. 2002;28:1279-86.