Evaluating hydrophilic acrylic lenses: Reply:

LETTERS

Reply: It is not clear exactly what Dr. Memmen takes exception to regarding my conclusions. The purpose of the editorial was to bring attention to potential problems associated with this newer class of IOL materials, the hydrophilic acrylics. Nowhere in the editorial was it stated that all lenses made of this material were the same or that these IOLs were classified simply by water content. In fact, there are great differences among the various hydrogel materials used to manufacture IOLs. The limited spaced available in an editorial precludes a complete discussion of the differences in the materials used in the manufacture of IOLs. All hydrophilic acrylic IOLs are certainly not alike. There are problems that have been associated with various hydrophilic acrylic IOLs, such as calcification on the surface of some types of lenses as well as calcium within the polymer itself in others. Also, a different set of complications relating to toxic anterior segment syndrome was found to be unique to the MemoryLens.1 This was thought to be due to residual polishing compound on this particular hydrophilic acrylic IOL. Nowhere in the editorial was it stated that there are no problems associated with other foldable IOL lens materials such as hydrophobic acrylic IOLs. Dr. Memmen mentions the article by Kato et al. in which they were able to generate glistenings within a hydrophobic acrylic IOL by manipulating the temperature of the lens in the laboratory. I am well aware of the problems that can result from glistenings within this type of IOL as our center was the first to report patients with glistenings in lenses packaged in the AcryPak2 and my laboratory was the first to analyze these phenomena.3 I am also well aware of the potential for problems with glare and visual aberrations associated with certain hydrophobic acrylic IOLs, which has been well documented in an annual survey of explanted foldable IOLs.4 I could not agree more with Dr. Memmen’s statement, “We need to be eternally vigilant in regard to all the materials we implant.” However, I must restate my conclusions in the editorial that newer IOL materials such as hydrophilic acrylics may have potential problems. Continued analysis of these materials, as well as long-term follow-up of patients implanted with hydrophilic acrylic IOLs, is necessary to ensure excellent outcomes in patients having phacoemulsification with foldable IOL implantation.—Nick Mamalis, MD References 1. Jehan FS, Mamalis N, Spencer TS, et al. Postoperative sterile endophthalmitis (TASS) associated with the MemoryLens. J Cataract Refract Surg 2000; 26:1773–1777 2. Dhaliwal DK, Mamalis N, Olson RJ, et al. Visual significance of glistenings seen in the AcrySof intraocular lens. J Cataract Refract Surg 1996; 22:452– 457 3. Omar O, Pirayesh A, Mamalis N, Olson RJ. In vitro analysis of AcrySof intraocular lens glistenings in AcryPak and Wagon Wheel packaging. J Cataract Refract Surg 1998; 24:107–113 4. Mamalis N, Spencer TS. Complications of foldable intraocular lenses requiring explantation of secondary intervention—2000 survey update. J Cataract Refract Surg 2001; 27:1310 –1317 384

Intraocular Pressure Measurement After Refractive Surgery

W

e read with interest the article about changes in the measured intraocular pressure (IOP) after hyperopic photorefractive keratectomy (PRK).1 Since IOP was the focus of the study, the methods used to record it were of primary significance. We wish to raise certain concerns about the methodology used. Only 1 reading with the Goldmann tonometer was used throughout the study. Past studies have shown that single IOP measurements are subject to variability.2,3 Remedies suggested include the use of an average of 2 readings3 or a median of 3 consecutive readings.4 These have been shown to significantly decrease the variability of a single measurement. It was not specified whether all observations were made by the same observer, using the same instrument throughout, as these factors account for variability in the measurement of IOP with the Goldmann tonometer. The authors used the Goldmann tonometer and the BioRad Tono-Pen (XL) tonometer to measure the IOP. In the Discussion, it is not clear whether these 2 methods were used in exclusion of each other for 2 separate sets of patients or as alternatives. Although there is good correlation between the 2 methods in recording the IOP,5 it is known that a significant difference of 1.7 to 2.4 mm Hg exists between the methods.5,6 This assumes significance as the reported lowering of IOP after hyperopic PRK in this study was 1.0 to 1.8 mm Hg. Thus, using them as alternatives would have significantly affected the study results. We believe these factors have significantly compromised the results of this study and highlight the inherent limitations of a retrospective study. TUSHAR AGARWAL, MBBS ROBIT SAXENA, MD RASIK B. VAJPAYEE, MS New Delhi, India References 1. Munger R, Dohadwala AA, Hodge WG, et al. Changes in measured intraocular pressure after hyperopic photorefractive keratectomy. J Cataract Refract Surg 2001; 27:1254 –1262 2. Moses RA, Liu CH. Repeated applanation tonometry. Am J Ophthalmol 1968; 66(1):89 –91 3. Motolko MA, Feldman F, Hyde M, Hudy D. Sources of variability in the results of applanation tonometry. Can J Ophthalmol 1982; 17(3):93–95 4. Dielemans I, Vingerling JR, Hofman A, et al. Reliability of in-

J CATARACT REFRACT SURG—VOL 28, MARCH 2002