Scanning electron microscopic analysis of foldable acrylic and hydrogel intraocular lenses

Scanning electron microscopic analysis of foldable acrylic and hydrogel intraocular lenses Thomas Kohnen, MD, Gerd Magdowski, BS, Douglas D. Koch, MD

ABSTRACT Objective: To analyze the surface quality of foldable acrylic and hydrogel intraocular lenses (IOLs). Setting: Cullen Eye Institute, Baylor College of Medicine, Houston, Texas, and Institute of Anatomy and Cellular Biology, University of Giessen, Germany. Methods: We studied eight foldable IOL models with optics made of six different acrylate/methacrylate polymers: Acrylens ACR360 (Ioptex), AcrySof MA60BM (Alcon), MemoryLens U940A (Mentor), 92S and 92C (Morcher), Hydroview H60M (Storz), HydroSof SH30BC (Alcon), and ISH66 (Corneal). Four IOLs of each design were examined. Light and scanning electron microscopy were performed before and after IOL folding with forceps. Results: All IOL models had excellent optic and haptic surfaces. The haptic-optic junctions revealed minimal empty spaces or irregularities in three of the five three-piece IOLs and smooth surfaces in all one-piece IOLs. Minimal surface alterations and superficial defects caused by folding were detectable in the two acrylate (acrylic) IOLs (Ioptex ACR360, Alcon MA60BM) with low water content. Conclusion: Intraocular lenses of acrylate/methacrylate polymers had excellent surface quality. The acrylic IOLs were vulnerable to mild folding or forceps defects; however, these were less marked than those previously noted with poly(methyl methacrylate) IOLs. J Cataract Refract Surg 1996; 22:1342-1350

C

ataract surgery performed through a small incision has been effective in minimizing postoperative astigmatism and inflammation and in accelerating visual recovery and patients' rehabilitation. I - 3 In 1984, the

From the Cullen Eye Institute. Baylor College ofMedicine, Department ofOphthalmology, Houston, Texas (Kohnen, Koch), and the Imtitute of Anatomy and Cellular Biology, University of Giessen. Germany (Magdowski). Reprint requests to Thomas Kohnen, MD, Cullen Eye Imtitute, Baylor College ofMedicine, Department ofOphthalmology. 6501 Fannin, NC200, Houston, Texas 77030.

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first foldable intraocular lens (IOL) made of silicone was implanted in humans,4 and to date silicone elastomers have been the preferred materials for foldable IOLs. 5 •6 Other foldable materials that have been used for IOL optics are hydrogel acrylics (hydrogelsf-9 and nonhydrogel acrylics (acrylics).10·11 Both these materials are members of the same family of acrylate/methacrylate polymers as the rigid poly(methyl methacrylate) (PMMA) IOLs; these foldable acrylate/methacrylate polymers have been developed by altering the side groups of the standard methacrylate polymer backbone 12 (Table O.

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Table 1. Intraocular lens specifications as provided by manufacturers.

Type

Design

Acrylens ACR360 (Ioptex)t AcrySof MA60BM (Alcon) MemoryLens U940A (Mentor) 92S (Morcher)* 92C (Morcher) HOHEM Hydroview H60M (Storz) HydroSof SH30BC (Alcon) ISH66 (Corneal)

Three piece Three piece Three piece Three piece One piece, plate haptic Three piece One piece One-piece, plate haptic

Optic Material

Optic Total Water ARefractive Content* Diameter Diameter Haptic Material (mm) (mm) (%) constant Index

EAlEMA PEAlPEMA MMAlHEMAI EGDMA MMAlHEMA MMAlHEMA

Polypropylene PMMA Polypropylene

6.0 6.0 6.0

13.65 13.00 13.00

118.5 118.9 119.0

1.47 1.55 1.47

<1 <1 20

Polypropylene MMAlHEMA

6.0 6.0

13.00 10.50

118.1 118.1

1.46 1.46

28 28

HEMAlHOHEXMA HEMA HEMA

PMMA HEMA HEMA

6.0 5.5 6.0

12.50 12.00 11.00

118.3 118.4 119.0

1.47 1.44 1.44

18 38 38

EA = ethyl acrylate; EMA = ethyl methacrylate; PEA = 2-phenylethyl acrylate; PEMA = 2-phenylethyl methacrylate; MMA = methyl methacrylate; HEMA = 2-hydroxyethyl methacrylate; EDGMA = ethylene glycol dimethacrylate; HOHEXMA = 6-hydroxyhexyl methacrylate 'Approximate tCurrently manufactured by Allergan, Inc., as ClariFlex *Currently produced with PMMA C-Ioops as model 92L

Surface properties of IOLs have been shown to influence postoperative inflammation 13 and long-term ac14

ceptance of the IOL. We previously reported on the surface characteristics of silicone IOLs and found some surface irregularities that might affect lens performance 15

Materials and Methods Eight different acrylic or hydrogel IOLs (five threepiece IOLs with polypropylene or PMMA loops and three one-piece IOLs) were examined in this study

In this study, we evaluated the surface qual-

(Table 1). Four IOLs of each design were obtained from

ity of several currently available acrylic and hydrogel IOLs. Since we (Figure 1) and others l6 have clinically observed surface deposits or defects following acrylic

the hospital stock or received as implantation samples from the manufacturer. All IOLs were first inspected

IOL implantation, we also studied the surfaces of acrylic

processed for scanning electron microscopy (SEM) di-

and hydrogel IOLs after folding.

rectly before (two each) or after (two each) they had been

in the eye.

with a dissecting microscope (Stereolupe Wild) and then

Figure 1. (Kohnen) Slitiamp photographs of foldable acrylic IOLs after forceps implantation. Left: Six months after implantation, an Acrylens ACR360 IOL has two surface abnormalities produced by the folding forceps. Right: Fine marks or scratches on the surface of an AcrySof MA60BM 3 months after implantation.

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Results

Table 2. Folding devices used in the study. IOL Type Acrylens ACR360 (Ioptex) AcrySof MA60BM (Alcon) MemoryLens U940A (Mentor) 92S (Morcher) 92C (Morcher) Hydroview H60M (Storz) HydroSof SH30BC (Alcon) ISH 66 (Corneal)

Folding Device

All eight IOLs had a smooth and homogeneous optic surface (Figures 2 to 9). A distinctive surface pattern that was evident in one 10L at 150 times magnification was an artifact (Figure 2H).15.17 The edge finish of all optics showed no evidence of ridges or flash lines. Only four IOLs had some surface abnormality at specific areas (Table 3). Photographs of the haptic-optic junctions showed an irregular surface of the haptic of one IOL (Figure 4H) and some empty space between the haptic and optic in three of the five three-piece IOLs (Figures 3B, 4B, and 5B). The haptic-optic junction in the onepiece IOLs did not have any surface irregularities (figures 6B, 8B, and 9A). The loop ends of all IOLs had an excellent surface finish with one slightly irregular pattern of a polypropylene haptic (Figure 2e). After folding, no IOL had significant fissuring or cracking. At high magnification, afine pattern of superficial scratches on both three-piece loptex IOLs was detected in the area of the forceps' contact (Figure 2D), as was a fine tear line on an AcrySofIOL (Figure 3D). A tiny trench on the optic of a Morcher 91c one-piece plate-haptic IOL was detected only with extreme highpower magnification (600 times) (Figure 6D). However, the mark was not in the region of the fold or in the area of the forceps' contact with the IOL and was therefore believed unrelated to the folding process. No defects were detected in any other IOL after folding.

Folding forceps (Moria) Ernest-McDonald lens folding forceps K5-8228 (Katena) Ernest-McDonald lens folding forceps K5-8228 (Katena) Forceps for folding soft lenses G-31902 (Geuder) H.-A. Koch plate haptic folder G-31906 (Geuder) Hydro-Inserter implantation forceps SP7 -52607 A (Storz) HydroSof implantation forceps 1NS440 (Alcon) Ernest-McDonald lens folding forceps K5-8228 (Katen a)

folded for 1 minute with an implantation forceps at room temperature of 20°C (Table 2). The folding instruments were meticulously cleaned before IOL folding, and the nonhydrogel IOLs (ACR360, MA60BM, U940A, 91s, and 91C) were covered with sodium hyaluronate (Healon®) for folding. The three-piece IOLs were folded along the 6 to 12 0' clock axis, the two platehaptic IOLs along their longitudinal axis. All IOLs were sputter-coated with gold for 6 minutes at 5 rnA in a Balser SCD 040 instrument (Bal-Tec AG). Scanning electron microscopy was done using a PSEM 500 machine (Philips, Industrial Electronics GmbH) at high voltage (more than 1000 volt). Photographs were taken with a Steinheil M20 Oscillophot camera (Steinheil Lear Siegler AG) at 5 to 300 times magnification. Special attention was given to the optic surfaces and edges, haptic-optic junctions, and the haptic itself. Between the different steps of the experiment, the IOLs were examined using the dissecting microscope for any defects that could have been produced by the examIners.

Table 3. Surface abnormalities noted in IOL Type Acrylens ACR360 (Ioptex) AcrySof MA60BM (Alcon) MemoryLens U940A (Mentor) 92S (Morcher)

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Optic

Discussion The quality of modern PMMA IOLs has been perfected during the past two decades14.18-23 and sets a standard for other IOL materials.24 Several investigators have demonstrated the importance of surface properties for IOL safety, particularly for the lens' long-term biocompatibility. Rough and irregular surfaces and sharp

acrylate/methacrylat~olymer IOLs before and after folding .

Optic Margin

Haptlo-Optic Junction

Sharp edge

Gap Irregular surface, gap Gap

Haptic Irregular surface

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SURFACE ANALYSIS OF ACRYLIC AND HYDROGEL IOLs

Figure 2. (Kohnen) Scanning electron micrographs of a three-piece ethyl acrylate/ethyl methacrylate IOL with polypropylene haptics (Acrylens ACR360). A: Overview of the IOL before folding shows a smooth surface finish (original magnification x 10). B: The optic margin and optichaptic junction have an excellent finish. The folds of the IOL surface (arrow) are artifacts from the SEM procedure (original magnification x 150). C: The haptic finish has a smooth surface with occasional fine irregularities (arrow) (original magnification x 300). 0: Magnification of the optic surface shows fine forceps scratches after forceps folding (original magnification x 300).

Figure 3. (Kohnen) Scanning electron micrographs of a three-piece 2-phenylethyl acrylate/2-phenylethyl methacrylate IOL with PMMA haptics (AcrySof MA60BM). A: Overview of the IOL before folding shows a smooth surface (original magnification x 10). B: The optic margin shows a smooth surface but an abrupt transition at the optic edge. The optic-haptic junction has a tiny empty space and fine irregularities (curved arrow) (original magnification x 75). C: The haptic shows optimal surface conditions (original magnification x 300). 0: Magnification of the optic surface after folding reveals a fine tear line (arrow = dust particle as artifact) (original magnification x 300).

edges can damage delicate intraocular tissues 14 and lead to disastrous consequences. The primary analytical tool in our study of different IOLs was SEM,15,25-27 an important methodology for evaluating the surface properties of IOLs. 14 ,18-21,23,28,29 Goldberg and coauthors 30 advocated low-voltage SEM using minimal or no conducting surface coating to analyze IOL surfaces. In our studies, IOL sputtering was performed with low energy

over a prolonged time; we used SEM with high voltage (25 kV) but with low emission energy of approximately 2 J,LA. As our data and images indicate, SEM using these parameters provided detailed information concerning the surfaces of these foldable IOLs. Further studies are required to compare the value of the two methods for examining IOL surfaces. In a previous study, 15 we evaluated the surface properties of 11 silicone elastomer IOLs. We found generally

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Figure 4. (Kohnen) Scanning electron micrographs of a three-piece methyl methacrylate/2-hydroxyethyl methacrylate IOL with PMMA haptics (MemoryLens U940A). A: Overview of the IOL before folding shows a smooth surface (original magnification x 10). B: Optic-haptic junction shows empty space (curved arrow) and irregularities (straight large arrow) (small arrow = particles as artifacts) (original magnification x 300). C: The haptic has an excellent finish (original magnification x 150). 0: The optic margin shows an excellent round finish (original magnification x 150).

Figure 5. (Kohnen) Scanning electron micrographs of a three-piece methyl methacrylate/2-hydroxyethyl methacrylate IOL with polypropylene haptics (92S). A: Overview of the IOL before folding shows an excellent surface condition with a prominent optic margin (original magnification x 10). B: The haptic-optic junction shows a solid connection but reveals a small gap (arrow). The optic margin has a fine notch not caused by folding (original magnification x 150). C: The haptic shows excellent surface conditions (original magnification x 300). 0: The optic margin has a smooth surface. The parallel lines on the surface (arrows) indicate incomplete polishing (original magnification x 300).

acceptable surface properties (Figure lOA, C) that were better than those of previously studied silicone lenses. 29 ,31 However, most IOLs had regional surface irregularities of varying magnitude (Figure lOB,D). Omar et al. 24 also found an adequate lens finish in silicone IOLs but irregularities and molding flash in three of the four IOLs examined. In the current study, we used SEM to evaluate the surface properties of new acrylate/methacrylate polymer 1346

IOLs and uniformly found excellent surface quality with no surplus material or molding flash. All but one haptic had a smooth, even surface and, although empty spaces were noted in three of the five three-piece IOLs, the haptic and optic surfaces in these regions were smooth and regular. The second part of our study was designed to evaluate the surfaces of acrylate/methacrylate polymer IOLs after folding. The hydrogel IOLs showed no signs of

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Figure 6. (Kohnen) Scanning electron micrographs of a one-piece, plate-haptic methyl methacrylate/2-hydroxyethyl methacrylate IOL (92C). A: Overview of the IOL before folding shows an excellent surface (original magnification x 10). B: The optichaptic connection reveals a smooth surface (original magnification x 75). C: The haptic finish has no irregularities (original magnification x 75). 0: High magnification of the optic surface after folding reveals a tiny trench (arrow) but no evidence of tearing. The mark was not in the region of the fold or in the area of the forceps contact with the IOL. Dust particles as artifacts after folding can also be seen (original magnification x 600).

Figure 7. (Kohnen) Scanning electron micrographs of a three-piece 6-hydroxyhexyl methacrylate/2-hydroxyethyl methacrylate IOL with fused PMMA haptics (Hydroview H60M). A: Overview of the IOL before folding shows an excellent overall surface quality (original magnification x 10). B: The optichaptic connection reveals perfect fusion of the two materials: 6-hydroxyhexyl methacrylate/2-hydroxyethyl methacrylate and PMMA (original magnification x 75). C: The haptic shows excellent surface conditions (original magnification x 150). 0: Magnification of the optic margin reveals a smooth, round surface (few dust particles as artifacts after folding) (original magnification x 150).

surface alterations, perhaps because of their high water content (i.e., 20%, MemoryLens; 28% Morcher IOLs; 18%, Hydroview; 38%, HydroSof), which makes them extremely soft and flexible. On the other hand, the nonhydrogel acrylic 10Ls with a lower water content (Ioptex ACR.360, Alcon MA60BM) showed subtle scratching or tearing of the optic surface. These findings parallel the clinical observations of surface debris or marks that are occasionally seen on acrylic 10Ls after folding (Figure 1).

Recent SEM studies have shown that metal implantation forceps can injure the surface of PMMA25 and heparin-surface-modified27 10Ls. Significant surface defects were not detected with SEM in silicone 10Ls after folding with forceps or injectOfs. 15 ,32 It seems that acrylic 10Ls are vulnerable to damage during folding if not handled appropriately. We avoid this problem by meticulously cleaning the folding instruments and placing balanced salt solution or a viscoelastic on the IOL before folding. In addition, the 10L is folded slowly to

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Figure 8. (Kohnen) Scanning electron micrographs of a one-piece open-loop 2-hydroxyethyl methacrylate IOL (HydroSof SH30BC). A: Overview of the IOL before folding shows excellent overall surface quality (original magnification x 10). B: Surface quality is excellent at the optichaptic junction (original magnification x 75). C: The haptic has an excellent surface with a central trench characteristic of this IOL (original magnification x 150). 0: Magnification (x 300) of the optic margin reveals a smooth and round surface.

Figure 9. (Kohnen) Scanning electron micrographs of a one-piece, plate-haptic 2-hydroxyethyl methacrylate IOL (ISH66). A: Overview of the IOL shows an excellent overall surface quality (original magnification x 10). B: The IOL corner also reveals excellent surface conditions (original magnification x 75).

eliminate sudden stresses that might produce surface fissures or fractures. 33 ,34 The clinical impact of surface irregularities of the magnitude seen on foldable IOLs remains to be established, especially in "capsular surgery."35 However, these lenses are sometimes placed in the ciliary sulcus. It is conceivable that surplus material or surface defects might result in deposition of inflammatory cells, protein, or micro-organisms and might predispose to syn1348

echia formation. Considering the sometimes delayed manifestations ofIOL complications, long-term clinical studies may be required to verifY the current clinical impression that all these lenses are safe. The quality of foldable IOLs approaches and in many instances meets the gold standard set by PMMA IOLs. We believe that the safest option for our patients is implantation of IOLs with perfectly smooth, regular surfaces and that current studies reveal areas for poten-

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Figure 10. (Kohnen) Scanning electron micrographs of silicone IOls. A: One-piece, plate-haptic silicone IOl (Silens SP, Domilens); the positioning hole and optic margin have a smooth and round surface (original magnification x 75). B: One-piece, platehaptic silicone IOl (AA-4203, Staar); the positioning hole shows surplus silicone material (molding flash) (arrows) (original magnification x 75). C: Three-piece silicone IOl with PMMA haptics ryvS 127, lovision); the optic-haptiC junction has a "welded" appearance without gaps or ridges (original magnification x 150). This IOl is currently manufactured by Pharmacia-Upjohn as Cee ON 920. 0: Three-piece silicone IOl with PMMA haptics (Soflex Ll41U, IOLAS); the silicone protuberance into which the haptic is anchored shows molding flash (straight arrows), and there is a small gap at the haptic-optiC junction (curved arrow) (original magnification x 150). (Reprinted with permission from Klin Monatsbl Augenheilkd 1995; 207:253-263.)

tial improvement in foldable IOL design and manufacturing. We encourage IOL manufacturers to continue to develop and refine ophthalmic biomaterials. 36

8. 9.

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Presented in part at the Annual Meeting ofthe Association for Research in Vision and Ophthalmology, Ft. Lauderdale, Florida, May 1995; 8th Annual Meeting of German Ophthalmic Surgeons in Niirnberg, Germany, June 1995; and Symposium of the International Intra-Ocular Implant Club, Stockholm, Sweden, October 1996 Supported in part by grants from the Deutsche Forschungsgemeinschaft (Postdoctoral Research Grant DFG-Ko 159511-1 and 1-2), Bonn, Germany, and Research to Prevent Blindness, Inc., New York, New York. Dr. Koch is paid consultant to Storz, St. Louis, Missouri, and Alcon, Ft. Worth, Texas. Dr. Kohnen and Mr. Magdowski have no financial interest in any ofthe foldable IOLs. GrahamD. Barrett, FRACO, FRACS, Perth, WesternAustralia, andF. Richard Christ, MS MBA, Laguna Beach, California, helped classifY foldable intraocular lens materials, and Alexander S. Kogan and Gilma Miranda, Houston, Texas, prepared the figures.

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