- Email: [email protected]
Verisyse IOL implantation in a child with anisometropic amblyopia
1057
LETTERS
lenses (IOLs) using the ISO eye model as the basis for comparison. The authors attempt to draw inferences relative to vision based on modulation transfer function (MTF) testing in the eye model. The ISO eye model is, however, not valid for such comparisons because it has an aberration-free cornea. The ISO eye model was conceived before the introduction of aspherical lenses. Because such lenses are designed to reduce or compensate for the spherical aberration of the human cornea, the ISO eye model is not suitable for assessing their performance in the human eyeda fact that has been overlooked in a number of studies, the one by Artigas et al. being the most recent. As the author of the 1999 ISO Optical Standard,2 I can make that statement with some authority and objectivity. I apologize to Artigas et al. and to other authors for not clearly communicating the shortcomings of the present ISO eye model until now. The ISO standard is under revision; for the next edition, an eye model3 that has a cornea with the same level of spherical aberration as the average human cornea is being considered. When the ISO standard was written, all IOLs were spherical and the eye model correctly graded lenses with spherical surfaces, although the measured MTF performance is far better than can be expected in the human eye. With aspherical IOLs, even the grading is incorrect. The first standard for IOLs was the 1984 ANSI Z80.7,4 in which the optical quality of IOLs was assessed by imaging the USAF 3-bar target through them. The measurement was taken in air with a 3.0 mm pupil and required a resolution of 100 line pairs/mm, or 60% resolution efficiency. The first generation of foldable IOLs could not meet this requirement because of the low refractive index of the silicone used in their manufacture. To obtain the same power with a lower refractive index requires steeper curvatures, and the steeper the curvature, the more spherical aberration there will be. This dilemma was circumvented by measuring the foldable IOLs immersed in water, but that test is so undemanding that almost any IOL would pass. One might think that measuring in water would be more relevant to the use than measuring in air. However, in the context of the eye, the IOL is working at a larger numerical aperture than it is in isolation in water. By chance, the numerical aperture in isolation in air is about the same as in the context of the eye.5 The ISO group validated testing in the eye model versus 3-bar target resolution in air.6 In both cases, a 3.0 mm pupil at the IOL was used. The 60% resolution efficiency in air was found to correspond to an MTF of 0.43 at 100 cycles/mm in the eye model. Both methods are acceptable under the ISO
standard; however, their primary purpose is to ensure manufacturing quality not in vivo performance. A separate standard exists for multifocal IOLs, in which measurement is done in the eye model with a small pupil (range 2.0 to 3.0 mm) and a large pupil (range 4.0 to 5.0 mm).7 The choice of the exact pupil size within the ranges is up to the manufacturer with regard to the design. This standard is also primarily intended to ensure manufacturing quality. Sverker Norrby, PhD Leek, The Netherlands REFERENCES 1. Artigas JM, Menezo JL, Peris C, Felipe A, Dı´az-Llopis M. Image quality with multifocal intraocular lenses and the effect of pupil size; comparison of refractive and hybrid refractive-diffractive designs. J Cataract Refract Surg 2007; 33:2111–2117 2. International organization for standardization. Ophthalmic implantsdIntraocular lensesdPart 2. Optical properties and test methods. Geneva, Switzerland, 1999 (ISO 11979–2) 3. Norrby S, Piers P, Campbell C, van der Mooren M. Model eyes for evaluation of intraocular lenses. Appl Optics 2007; 46:6595–6605 4. American National Standards Institute, Inc. American National Standard for OphthalmicsdIntraocular Lenses. ANSI Z80.7, 1984 5. Norrby NES. Standardized methods for assessing the imaging quality of intraocular lenses. Appl Optics 1995; 34:7327–7333 6. Norrby NES, Grossman LW, Geraghty EP, Kreiner CF, Mihori M, Patel AS, Portney V, Silberman DM. Determining the imaging quality of intraocular lenses. J Cataract Refract Surg 1998; 24:703–714 7. International organization for standardization. Ophthalmic implantsdIntraocular lensesdPart 9. Multifocal intraocular lenses. Geneva, Switzerland, 2006 (ISO 11979–9)
Verisyse IOL implantation in a child with anisometropic amblyopia I have a few questions about the report by Assil et al.1 of a 3-year-old boy who was implanted with a Verisyse iris-fixated anterior chamber intraocular lens (IOL) (Advanced Medical Optics) for unilateral high myopia. The authors state that one reason for contact lens failure was ‘‘socioeconomic status.’’ Can they explain how the family afforded the travel from Australia to Beverly Hills, California, USA? How long did they stay in the United States? Did they make the trip more than once? Who paid for the procedure? Where are Sturm and Chang from? Is one of them the Australian comanaging ophthalmologist? How much of the reported postoperative information was based on personal examination by the operating surgeon? If some of it was forwarded from Australia, was the Australian provider an ophthalmologist, optometrist, cataract specialist, or pediatric ophthalmologist? How cooperative was the child for
J CATARACT REFRACT SURG - VOL 34, JULY 2008
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LETTERS
a detailed, high magnification, bright-light slitlamp examination in the office (to check for low-grade chronic uveitis)? Were additional examinations under anesthesia performed? Was there pigment dispersion? Did iris transillumination defects develop in the areas of enclavation? What was the intraocular pressure? Was the pupil deformed (hour glass) under dim illumination? The 4-year follow-up period is an unusually good opportunity for the authors to contribute safety data regarding the use of iris-fixated IOLs in children. However, the only postoperative information given was refractive error, visual acuity, and endothelial cell count. Particularly since Assil is a paid consul-
tant to AMO, the authors must meticulously provide all the clinical information a prudent ophthalmologist would need to decide, independently, whether the procedure was in the child’s best long-term interest. Sandra M. Brown, MD Concord, North Carolina, USA
REFERENCE 1. Assil KK, Sturm JM, Chang SH. Verisyse intraocular lens implantation in a child with anisometropic amblyopia: four-year followup. J Cataract Refract Surg 2007; 33:1985–1986
ERRATUM In the paper entitled ‘‘New Intraocular Lens for Achromatizing the Human Eye’’ (J Cataract Refract Surg 2007; 33:1296–1302), there are 2 small errors that may affect the outcomes if one tries to perform some calculations. On page 1298, the equation in the last paragraph should not have parentheses; that is, nZn0 þ A=l þ B=l3:5 . On page 1299, the data for ‘‘Aqueous’’ and ‘‘Intraocular Lens’’ in Table 2 are incorrect. The correct table is as follows: Table 2. Values of no, A, and B used in the model to modulate the refraction index variation with wavelength. Value no A B
Cornea
Aqueous
Intraocular Lens
Vitreous
1.36468149 0.00478075 0.00049921
1.32628942 0.00441874 0.00055639
1.4732734 0.00918165 0.000447667
1.32468149 0.00478075 0.00049921
J CATARACT REFRACT SURG - VOL 34, JULY 2008
LETTERS
lenses (IOLs) using the ISO eye model as the basis for comparison. The authors attempt to draw inferences relative to vision based on modulation transfer function (MTF) testing in the eye model. The ISO eye model is, however, not valid for such comparisons because it has an aberration-free cornea. The ISO eye model was conceived before the introduction of aspherical lenses. Because such lenses are designed to reduce or compensate for the spherical aberration of the human cornea, the ISO eye model is not suitable for assessing their performance in the human eyeda fact that has been overlooked in a number of studies, the one by Artigas et al. being the most recent. As the author of the 1999 ISO Optical Standard,2 I can make that statement with some authority and objectivity. I apologize to Artigas et al. and to other authors for not clearly communicating the shortcomings of the present ISO eye model until now. The ISO standard is under revision; for the next edition, an eye model3 that has a cornea with the same level of spherical aberration as the average human cornea is being considered. When the ISO standard was written, all IOLs were spherical and the eye model correctly graded lenses with spherical surfaces, although the measured MTF performance is far better than can be expected in the human eye. With aspherical IOLs, even the grading is incorrect. The first standard for IOLs was the 1984 ANSI Z80.7,4 in which the optical quality of IOLs was assessed by imaging the USAF 3-bar target through them. The measurement was taken in air with a 3.0 mm pupil and required a resolution of 100 line pairs/mm, or 60% resolution efficiency. The first generation of foldable IOLs could not meet this requirement because of the low refractive index of the silicone used in their manufacture. To obtain the same power with a lower refractive index requires steeper curvatures, and the steeper the curvature, the more spherical aberration there will be. This dilemma was circumvented by measuring the foldable IOLs immersed in water, but that test is so undemanding that almost any IOL would pass. One might think that measuring in water would be more relevant to the use than measuring in air. However, in the context of the eye, the IOL is working at a larger numerical aperture than it is in isolation in water. By chance, the numerical aperture in isolation in air is about the same as in the context of the eye.5 The ISO group validated testing in the eye model versus 3-bar target resolution in air.6 In both cases, a 3.0 mm pupil at the IOL was used. The 60% resolution efficiency in air was found to correspond to an MTF of 0.43 at 100 cycles/mm in the eye model. Both methods are acceptable under the ISO
standard; however, their primary purpose is to ensure manufacturing quality not in vivo performance. A separate standard exists for multifocal IOLs, in which measurement is done in the eye model with a small pupil (range 2.0 to 3.0 mm) and a large pupil (range 4.0 to 5.0 mm).7 The choice of the exact pupil size within the ranges is up to the manufacturer with regard to the design. This standard is also primarily intended to ensure manufacturing quality. Sverker Norrby, PhD Leek, The Netherlands REFERENCES 1. Artigas JM, Menezo JL, Peris C, Felipe A, Dı´az-Llopis M. Image quality with multifocal intraocular lenses and the effect of pupil size; comparison of refractive and hybrid refractive-diffractive designs. J Cataract Refract Surg 2007; 33:2111–2117 2. International organization for standardization. Ophthalmic implantsdIntraocular lensesdPart 2. Optical properties and test methods. Geneva, Switzerland, 1999 (ISO 11979–2) 3. Norrby S, Piers P, Campbell C, van der Mooren M. Model eyes for evaluation of intraocular lenses. Appl Optics 2007; 46:6595–6605 4. American National Standards Institute, Inc. American National Standard for OphthalmicsdIntraocular Lenses. ANSI Z80.7, 1984 5. Norrby NES. Standardized methods for assessing the imaging quality of intraocular lenses. Appl Optics 1995; 34:7327–7333 6. Norrby NES, Grossman LW, Geraghty EP, Kreiner CF, Mihori M, Patel AS, Portney V, Silberman DM. Determining the imaging quality of intraocular lenses. J Cataract Refract Surg 1998; 24:703–714 7. International organization for standardization. Ophthalmic implantsdIntraocular lensesdPart 9. Multifocal intraocular lenses. Geneva, Switzerland, 2006 (ISO 11979–9)
Verisyse IOL implantation in a child with anisometropic amblyopia I have a few questions about the report by Assil et al.1 of a 3-year-old boy who was implanted with a Verisyse iris-fixated anterior chamber intraocular lens (IOL) (Advanced Medical Optics) for unilateral high myopia. The authors state that one reason for contact lens failure was ‘‘socioeconomic status.’’ Can they explain how the family afforded the travel from Australia to Beverly Hills, California, USA? How long did they stay in the United States? Did they make the trip more than once? Who paid for the procedure? Where are Sturm and Chang from? Is one of them the Australian comanaging ophthalmologist? How much of the reported postoperative information was based on personal examination by the operating surgeon? If some of it was forwarded from Australia, was the Australian provider an ophthalmologist, optometrist, cataract specialist, or pediatric ophthalmologist? How cooperative was the child for
J CATARACT REFRACT SURG - VOL 34, JULY 2008
1058
LETTERS
a detailed, high magnification, bright-light slitlamp examination in the office (to check for low-grade chronic uveitis)? Were additional examinations under anesthesia performed? Was there pigment dispersion? Did iris transillumination defects develop in the areas of enclavation? What was the intraocular pressure? Was the pupil deformed (hour glass) under dim illumination? The 4-year follow-up period is an unusually good opportunity for the authors to contribute safety data regarding the use of iris-fixated IOLs in children. However, the only postoperative information given was refractive error, visual acuity, and endothelial cell count. Particularly since Assil is a paid consul-
tant to AMO, the authors must meticulously provide all the clinical information a prudent ophthalmologist would need to decide, independently, whether the procedure was in the child’s best long-term interest. Sandra M. Brown, MD Concord, North Carolina, USA
REFERENCE 1. Assil KK, Sturm JM, Chang SH. Verisyse intraocular lens implantation in a child with anisometropic amblyopia: four-year followup. J Cataract Refract Surg 2007; 33:1985–1986
ERRATUM In the paper entitled ‘‘New Intraocular Lens for Achromatizing the Human Eye’’ (J Cataract Refract Surg 2007; 33:1296–1302), there are 2 small errors that may affect the outcomes if one tries to perform some calculations. On page 1298, the equation in the last paragraph should not have parentheses; that is, nZn0 þ A=l þ B=l3:5 . On page 1299, the data for ‘‘Aqueous’’ and ‘‘Intraocular Lens’’ in Table 2 are incorrect. The correct table is as follows: Table 2. Values of no, A, and B used in the model to modulate the refraction index variation with wavelength. Value no A B
Cornea
Aqueous
Intraocular Lens
Vitreous
1.36468149 0.00478075 0.00049921
1.32628942 0.00441874 0.00055639
1.4732734 0.00918165 0.000447667
1.32468149 0.00478075 0.00049921
J CATARACT REFRACT SURG - VOL 34, JULY 2008