Piggyback intraocular lenses

LETTERS

groups and the Q-values of eyes without significant stigmatism analyzed separately. Then, the Q-values for myopic and hypermetropic eyes would be more reliable. YAþAR SAKARYA, MD SADY´ K KAVAKLY´ , MD SAMED SY´ TKY´ ERMIþ, MD ERTUO˘ RUL CU¨ NEYT IþY´ K, MD Istanbul, Turkey

References 1. Budak T, Khater TT, Friedman NJ, et al. Evaluation of relationships among refractive and topographic parameters. J Cataract Refract Surg 1999; 25:814 – 820 2. Holladay JT. Corneal topography using the Holladay Diagnostic Summary. J Cataract Refract Surg 1997; 23:209 –221

Reply: Although we appreciate the comments about our article, we do not agree with the assertion that the asphericity value we obtained (⫺0.03) was skewed by our inclusion of eyes with astigmatism, nor do we agree that the astigmatic cornea has both prolate and oblate meridians. In fact, all meridians are prolate in the typical unoperated cornea, and the degree of prolateness (or asphericity) tends to be equal in both astigmatic meridians, despite the differences in central curvature. In corneas in which the values for asphericity are equal, one sees a perfectly symmetric bow tie in refractive power maps. When the bow tie in a refractive map is compressed (dumbell) or elongated, the asphericities are still negative (i.e., prolate), even though they are not identical. Dr. Sakarya and colleagues also question the validity of our calculating mean asphericity in each eye by averaging the asphericity in all meridians. However, this is the method that has been used by every previous study of corneal asphericity. The only variation among studies is the size of the zone over which asphericity is measured. In previous studies, the size of this zone ranged from 3.0 to 8.0 mm, whereas our study used the 4.5 mm zone provided by the software of the EyeSys topographer. We think it is intriguing that eyes with a higher degree of myopia show an increasing Q-value (less prolate). One possible explanation is the “emmetropization” process of the human eye. In many eyes, an increase in myopia is due to an increase in axial length; this is especially true in eyes with myopia above ⫺6.0 D. As the axial length increases, the design of the peripheral optics of the cornea might change to reduce spherical aberrations, thereby resulting in the increasing Q-value in these eyes. Although intriguing, this is obviously speculation.—Douglas D. Koch, MD, Jack T. Holladay, MD, Koray Budak, MD 308

Piggyback Intraocular Lenses

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e would like to comment on case reports and an editorial that deal with piggyback intraocular lenses (IOLs). The editorial1 contains a short summary of our case report on the contact zone of piggyback acrylic IOLs and also discusses a potential multifocal gain that could be achieved with these IOLs. To avoid confusion, we would like to clarify something concerning the latter. In our case report, we suggest there is a second focus caused by the flatter contact zone between the IOLs. This second focus would, however, lie behind the retinal plane. Therefore, this “inverse bifocal IOL” could be beneficial to the patient under only 2 circumstances: The patient wears (⫹) lenses to correct the central contact zone for distance, or the patient ends up myopic after surgery because of inaccuracies in biometry, power calculation, or both. This would allow the patient to view distance through the central contact zone and near through the peripheral noncontact area of the IOLs. However, it is still unclear how large such a contact zone has to be to enable satisfactory visual acuity. The editorial includes a slitlamp photograph of 2 plate-haptic piggyback IOLs and describes the findings as “a pearl disrupting the optical continuity” of the IOLs. We interpret the findings as a contact zone surrounded by a fine “membrane-like” ring. The contact zone contains a slightly smaller zone of inhomogeneous appearance. We have also seen such changes with silicone piggyback IOLs. They can be found immediately after surgery and do not seem to change with time. We think this inhomogeneous zone is caused by debris (maybe partly traces of viscoelastic material) that becomes trapped between the IOLs, within the contact zone, during surgery. This would explain why it is not washed away by irrigation/aspiration at the end of surgery or by circulating aqueous during the first few postoperative days. The clear zone just outside this inhomogeneous zone is probably the result of a slight enlargement of the contact zone as a result of capsule shrinkage, forcing the IOLs together, during the first few postoperative months. Later, the membrane-like ring forms at the border of the contact zone, possibly a first sign of opacification. In the case report by Shugar and Schwartz,2 a late hyperopic shift in 6 eyes in 3 piggyback patients with

J CATARACT REFRACT SURG—VOL 26, MARCH 2000

LETTERS

interpseudophakos Elsching pearls in interpreted in 3 ways or possibly as a combination: proliferating pearls displacing the posterior IOL backward, a change in zonular tension displacing the entire IOL/capsule complex backward, or pearls proliferating under the capsulorhexis resulting in posterior displacement of both IOLs. The range of hyperopic shifts in the 6 eyes is 2.00 to 4.75 diopters (D). To achieve a hyperopic shift of more than 4.00 D, the IOLs would have to move backward significantly, in 1 of the cases by more than 1.50 mm, as the authors point out. We would like to add a fourth possible explanation. Since we have seen a contact zone with all piggyback IOLs that we have implanted, we infer that most, if not all, the 6 eyes also have central contact zones. If this is the case, the reduction in visual acuity peripheral to the contact zone because of the pearls results in the patient using the central contact zone for subjective refraction. Since this zone is flatter than the surrounding zone, the refraction will be more hyperopic. In 3 eyes with acrylic IOLs (AcrySof MA60BM), we found a refraction difference of 3.0 to 5.5 D between the central contact zone and the peripheral zone (unpublished data). This would provide a simple explanation of the strong hyperopic shifts the authors have seen. OLIVER FINDL, MD RUPERT MENAPACE, MD Vienna, Austria References 1. Rosen ES. Face to face with IOLs (editorial). J Cataract Refract Surg 1999; 25:729 2. Shugar JK, Schwartz T. Interpseudophakos Elschnig pearls associated with late hyperopic shift: a complication of piggyback posterior chamber intraocular lens implantation. J Cataract Refract Surg 1999; 25:863– 867

Reply: Drs. Findl and Menapace suggest a new putative etiology for late hyperopic shift in conjunction with peripheral ingrowth of interpseudophakos Elschnig pearls. They hypothesize that cellular ingrowth obscures the image produced through the area outside the central contact zone between the IOLs, following which the flattened central contact zone is preferred for refraction. This novel explanation may account for some of the late hyperopic shift observed in the 4 eyes in our series with piggyback acrylic IOLs but would not account for that observed in the 2 eyes with poly(methyl methacrylate) IOLs, in which no flattening of the central contact area could be expected. The average clear zone central to the interpseudophakos Elschnig pearls in the initial 4 eyes measured about 4.0 mm at

the slitlamp. Presumably, much of this area would represent the previously reported contact area between IOLs, which was not quantified in the initial report. Because of the deformability of the acrylic material, the amount of flattening and hyperopic shift in IOL surface power would be greatest at the center of the contact zone and would decrease radially outward until the edge of the contact zone. From this point outward to the periphery of the IOLs, the surface power would remain constant. Therefore, the central contact zone should prove polyfocal. Reoperation in a number of these eyes, by myself and by Johnny Gayton, MD, has proven necessary because of reduced vision from central interpseudophakos opacification (IPO). Multiple IOLs have been explanted and sent for pathophysiological study (Q. Peng, MD, “Pathological Examination of Two Pairs of Explanted Posterior Chamber Intraocular Lenses,” presented at the annual meeting of the American Academy of Ophthalmology [AAO], Orlando, Florida, USA, October 1999). In some cases, the central optics were so adherent that the IOLs could not be separated in vitro (J.L. Gayton, MD, “Long-Term Follow-up of Piggyback Implantation,” presented at the annual meeting of the AAO, Orlando, Florida, USA, October 1999). In 2 eyes, the IOLs could be separated and the peripheral pearls aspirated, following which the hyperopic shift resolved (J.K. Shugar, MD, “Interpseudophakos Opacification,” presented at the annual meeting of the AAO, Orlando, Florida, USA, October 1999). The theory proposed by Drs. Findl and Menapace would be consistent with this finding. Yet a fifth possible mechanism for the hyperopic shift observed with piggyback acrylic IOLs would be flexion of the IOL peripheries apart by ingrowth of Elschnig pearls while the centers remain stuck together. Such IOL bending would change the effective lens power. Further study should elucidate which putative mechanisms are at play. Even more important will be the outcome of various strategies to prevent IPO, as well as treatment for cases presenting with late IPO. I thank Drs. Findl and Menapace for their suggestion of this ingenious explanation and encourage them to continue their important work in this area.—Joel K. Shugar, MD, MSEE

Surgical Treatment of Pellucid Marginal Degeneration Associated with Cataract

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ellucid marginal degeneration (PMD) is an uncommon bilateral ectatic disorder of the cornea, orginally described by Schlaeppi in 1957.1 It is characterized by an area of noninflammatory crescent-shaped thinning in the lower periphery of the cornea. In PMD, the keratocytes reduce or alter the normal sulfatation pattern of the keratan sulfates.2

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