Variations in manufacturing quality of diffractive multifocal lenses Martin R. Wenzel, M.D., Elke M. Imkamp, M.D., David

Several bifocal and multifocal intraocular lenses (IOLs) have been introduced in recent years. These IOLs are purported to have the potential to improve the quality of life of pseudophakic patients significantly because they provide lens power for both near and distance vision. A well-publicized design, the diffractive multifocal IOU- 4 operates on the principle of simultaneous vision. The lens possesses two powers simultaneously; it can form images from two different object conjugates. l To achieve this with diffraction optics, a stepped discontinuity at the zone boundaries of the lens posterior surface is used. This requires manufacturing precision to a fraction of wavelength of light. 2 The step height at the zone boundaries is less than 2 Ilm, and there are approximately 30 zones on the surface of a lens with a 6 mm diameter optic. l We have obtained several multi focal diffractive IOLs from different manufacturers and studied them by specular microscopy5 and scanning electron microscopy. 6 In this report we discuss the appearance of lenses obtained in 1989 from two

J.

Apple, M.D.

manufacturers (3M Corporation and Morcher Corporation) . MATERIALS AND METHODS In the Aachen Eye Clinic, patients are routinely examined with the specular microscope after IOL implantation. This technique analyzes such factors as the presence or absence of inflammation after implantation. 5 We have also found this to be a means to examine the optical quality of implanted multifocal IOLs and we have used the technique on the IOLs described in this study. Focusing on both the anterior and posterior IOL surfaces, one can distinguish two different reflexes. Because of the convexity of the anterior lens surface, the examined area appears round and disclike with bright central illumination. On a plano posterior surface the entire area is illuminated evenly. It is more difficult to find the specular area on planar surfaces. To examine the posterior surface, it is important to focus on the specular area.

From the Department o/Ophthalmology, Rheinisch-Westfalische Technische Hochschule, Aachen, Germany (Wenzel, Imkamp), and the Department o/Ophthalmology, Medical University o/South Carolina, Charleston, South Carolina (Apple). Supported in part by an unrestricted grant/rom Research to Prevent Blindness, Inc., New York, New York. Reprint requests to M. Wenzel, M.D., Augenklinik der RWTH, Pauwelsstrasse, D 5100 Aachen, Germany.

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Case Reports Case 1. A 75-year-old man had bilateral cataracts, with 20/20 visual acuity in the right eye and 20/50 in the left eye. There was no pre-existing macular pathology. Cataract surgery was performed under local anesthesia on the left eye. After manual expression of the nucleus, the cortex was aspirated with an irrigation/aspiration unit. A diffractive multifocal IOL (20.0 diopters [D] + 3.5; 53-15200NB, 3M Lot DP6625) was inserted into the capsular bag (Figure 1). Before implantation, the lens was examined with the operating microscope; no faults in lens quality were observed. Three months postoperatively, the patient had 20/20 visual acuity with a distance correction of -2.5 D sph -l.0 D cyl. With this distance correction he could also read Jl at near satisfactorily, but he preferred reading with an extra addition of +3.0 D (= +0.5 D sph - l.0 D cyl) or without any glasses. We examined his contrast sensitivity using Landoldt's rings with the Mesoptometer 2. He could recognize the Landoldt's ring (20/200) with

both eyes at a contrast of 1:23 and a brightness of 0.1 cd/m 2 but not at a brightness of 0.032 cd/m 2 • These data do not meet the German requirements for a license to drive at night. Case 2. A 74-year-old woman had bilateral cataracts with a visual acuity of 20/200 in the right eye and 20/100 in the left eye. She had mild age-related macular degeneration in both eyes. Cataract surgery on the right eye was performed with a technique similar to that described in Case 1. A diffractive multifocal IOL (19.0 D + 3.5; 5325290NB, 3M Lot DP7840) was inserted into the capsular bag (Figure 2). After surgery, the patient first complained of diplopia and fell on two occasions, but these problems subsided in the first postoperative weeks. Three months after the first operation, the left eye was operated on and a monofocal IOL was implanted. Both eyes were examined two months later. The patient did not achieve 20/ 20 acuity with either eye. With the multifocal IOL, she had 20/40 with a distance correction of + l.25 sph -2.5 D cyl. With the monofocal IOL, she had 20/30 with a distance correction of plano sph -2.0 D cyl. She was unable to read J 10 with either eye when using the distance correction. She was able, however, to read J 5 using a reading addition of +3 D sph with each eye (right, +4.25 D sph

Fig. 1.

Fig. 2.

Even slight decentration can make it impossible to recognize low-contrast phenomena. RESULTS

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(Wenzel) Specular microscopy of the posterior surface of a 3M diffractive multifocal IOL (5315200NN, Lot DP6625) in vivo (Case 1). Between the 2-llm boundaries (1), many smaller nonhomogeneous rims (2) which appear to be manufacturing artifacts can be distinguished (original magnification X 80).

(Wenzel) Specular microscopy of the posterior surface of a 3M diffractive multifocal IOL (5325290NB, Lot DP7840) in vivo (Case 2). A bright reflex on the anterior surface (1) is out of focus. Between the 2-llm boundaries (2), many smaller nonhomogeneous rims (3) can be distinguished. These are similar to those in Figure 1. Small gray or colored spots (4) are seen in areas of contact with the posterior capsule (original magnification X 80).

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-2.5 cyl; left, +3 .0 D sph -2.0 D cyl). We examined her contrast sensitivity with the Mesoptometer 2. She could not recognize the Landoldt's ring (20/200) with the multifocal IOL at contrast of 1:23 and a brightness of 0.1 cd/m 2 • She reported much better contrast vision with the monofocal IOL and recognized the Landoldt's ring at a brightness of 0.1 cd/m 2 but not at a brightness of 0.032 cd/m 2 . Specular Microscopy The IOLs were routinely examined with the specular microscope three months after surgery in both cases. Except for an inactive giant cell seen on the lens surface in the second patient, no significant cellular residues were found on either lens. The optical quality of the anterior surface of each multifocal IOL was good. While looking for posterior surface reflexes we studied the boundaries that are responsible for the multifocal quality of the lens. There were about 30 of them with approximately 200 Ilm between one and the next. The distance between two boundaries was larger in the central part of the lens than in the peripheral areas. The step height at the zone boundaries was less than 2 Ilm. Many smaller nonhomogeneous rims could be easily distinguished (Figures 1 and 2). Some showed a'change of intensity from rim to rim. There were up to 16 of these smaller rims between two diffractive boundaries (Figure 1). These findings prompted us to discontinue implanting this diffractive IOL model and to examine other IOLs in vitro using specular microscopy. Two lenses from the same manufacturer (3M Corporation) confirmed the presence of the same smaller rims (53-15190NB, Lot DZ1294 and 5315200NB, Lot DY3945). However, a lens from another company, Morcher Corporation (No. T032786, Lot 040389), showed no small rims between the zone boundaries (Figure 3). Finding such differences, we examined the lenses using other microscopic techniques, including phasecontrast microscopy and scanning electron microscopy (Figure 4). These differences were always reproducible and different examiners consistently achieved similar results. We concluded that the smaller rims were not optical artifacts but represented differences in the manufacturing quality of diffractive multifocal lenses.

DISCUSSION Implanting a monofocal IOL should theoretically not induce glare disability. 7 In all diffractive lenses, there is some loss of image contrast because of the presence of the defocused light that is directed to

Fig. 3.

(Wenzel) Specular microscopy of a Morcher diffractive multifocal lens (T032786, Lot 040389). Between the zone boundaries, there are no smaller rims (original magnification X 80).

Fig. 4. (Wenzel) Scanning electron microscopy of the surface of a 3M diffractive multifocal lens (Lot DZ1294) . The nonhomogeneous rims are visible when observing the lens at an angle of 30 degrees or less (original magnification X 80). Compare with Figures 1 and 2.

the second image. Ideally, the input light would be divided so that 50% of the energy is used for the near image and 50% for the distance image. In practice, nature does not allow a 1: 1 split for a nonabsorbing diffractive lens. The multifocal IOL actually sends about 41 % of the light to each image at the designated wavelength. The rest of the energy is divided between higher diffractive orders. This produces lens powers that are multiples of the add power, and they form very low intensity im-

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ages that are defocused on the retina. 1 The same reduction of contrast sensitivity occurs in fresnel prisms. 8 Besides these inherent problems of diffractive IOLs, in the selected lenses from a particular lot manufactured by 3M Corporation in the late 1980s there appeared to be manufacturing problems that we first noted with specular microscopy. Specular microscopy of the crystalline lens was introduced by Vogt. 9 He described this technique in detail in his classic atlas, but it went out of practice after 1943. Specular microscopy of implanted IOLs was first described by Ohara in 1985.10 Our specular microscopic and scanning electron microscopic studies show that diffractive IOLs may vary in quality. 4 The findings reported here reflect lenses manufactured from certain lots during a given time period, namely two years ago. Manufacturing quality may have improved since then. However, using specular microscopy we examined the latest biconvex model 825X from 3M Corporation and did find the same type of rims. There is one significant difference between the manufacturing techniques of the two companies studied; i.e., 3M does not polish its diffractive lenses. It believes that polishing may reduce the diffractive quality of the lens. Indeed, polishing may create other problems that are more severe than the rims of the 3M lenses. Reich et al. 11 for example noted findings different from those in this study. They found fine rims on the 3M type 815LE IOL, but demonstrated no significant surface deficiencies. However, they found deeper rims, irregularities, and deposits (possible polishing material) on the Morcher type 53D (serial number T065143 and T072684) IOL. Studies regarding the optical performance of multifocal IOLs are in progress. 12 Ellingson 13 has recently reported on explantation of 3M diffractive IOLs. He notes that the major issue is not whether the diffractive IOL is capable of producing bifocal vision, 3 but whether the IOL is capable of producing visual acuity comparable to that provided by a monofocallens. Ellingson noted that the tolerance and ability to adjust to bifocal vision varies from patient to patient, but he feels that the diffractive lenses can cause a significant lack of clarity or crispness in their vision. He feels that the presence of macular degeneration, as noted in Case 2, is a def-

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inite contraindication to multifocal IOLs because these patients record the best resolution capability. Since maximum optical clarity is mandatory with multifocal IOLs, detailed attention to manufacturing quality is warranted. Each surgeon implanting diffractive lenses should be able to evaluate the IOL quality. Patients can be examined using specular microscopy, but this technique can be difficult to learn and is not widely available. Furthermore, some models of specular microscopes are not designed to allow examination of IOLs. 3 A better strategy would be to examine IOLs from each manufacturer using a specular microscope before implantation. After such an examination the IOL can be implanted or returned to the manufacturer if the quality is inadequate. REFERENCES 1. Simpson MJ. The diffractive multifocal intraocular lens. Eur J Imp-lant Refract Surg 1989; 1:115-121 2. Percival P. Early experience with the diffractive bifocallens. Eur J Implant Refract Surg 1989; 1 :3-4 3. Duffey RJ, Zabel RW, Lindstrom RL. Multifocal intraocular lenses. J Cataract Refract Surg 1990; 16:423429 4. Percival P. Indications for the multi zone bifocal implant. J Cataract Refract Surg 1990; 16:193-197 5. Wenzel M, Reim M, Heinze M, Bocking A. Cellular invasion on the surface of intraocular lenses. In vivo cytological observations following lens implantation. Craefes Arch Clin Exp Ophthalmol 1988; 226:449454 6. Apple DJ, Kincaid MC, Mamalis N, Olson RJ. Intraocular Lenses. Evolution, Designs, Complications, and Pathology. Baltimore, Williams & Wilkins, 1989 7. Masket S. Reversal of glare disability after cataract surgery. J Cataract Refract Surg 1989; 15:165-168 8. Woo CC, Campbell FW, Ing B. Effect offresnel prism dispersion on contrast sensitivity function. Ophthalmic Physiol Opt 1986; 6:415-418 9. Vogt A. Lehrbuch und Atlas der Spaltlampenmikroskopie des lebenden Auges, Vol 1-3. Berlin, Springer, 1930-1942 10. Ohara K. Biomicroscopy of surface deposits resembling foreign-body giant cells on implanted intraocular lenses. Am J Ophthalmol1985; 99:304-311 11. Reich ME, Waltersdorfer R, Hanselmeyer H, et al. Surfaces of different diffractive IOLs. In: Schott K, Jacobi KW, Freyler H. 4. Kongress der Deutschen Cesellschaft fur Intraokularlinsen Implantation. Berlin, Heidelberg, New York, Springer 1991: 374-376 12. Holladay]T, van Dijk H, Lang A, et al. Optical performance of multifocal intraocular lenses. 1990; 16:413-422 13. Ellingson Fr. Explantation of 3M diffractive intraocular lenses. J Cataract Refract Surg 1990; 16:697-702

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Variations in manufacturing quality of diffractive multifocal lenses.

The new generation of multifocal intraocular lenses (IOLs) based on diffraction optics is characterized by a stepped discontinuity at zone boundaries ...
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