Keratectasia after LASIK

letters Capsule Stabilization for Phacoemulsification he article by Lee and Bloom1 describes a method in which the lens capsule and enclosed cataractous lens are stabilized by insertion of iris retractors during phacoemulsification. I first learned of this technique when it was presented by John Shepherd at a meeting of the Chicago Ophthalmological Society in May 1993. I used it sporadically but found that the flexible and relatively short retractors often slipped over the capsulorhexis during lens manipulation. This required frequent repositioning of the retractors or worse, as nucleus dislocation could sometimes occur when capsule stabilization was lost. In addition, the lack of equatorial zonular support left the peripheral capsule free to collapse inwardly as the nucleus was removed, risking damage to the capsule by aspiration. I therefore designed relatively long metallic retractors that extend into the capsular fornix and remain securely in place throughout the procedure. With these instruments in place (I insert them every 45 degrees to provide excellent support), the lens is held securely and the capsular fornix is well supported. Cortex removal is easily accomplished without attracting the equatorial lens capsule, and following this, an endocapsular tension ring (with or without scleral suturing) can be inserted prior to posterior chamber intraocular lens implantation. These are certainly not the quickest procedures to perform, but the technique is effective. The Cataract Support System is currently available from Duckworth & Kent. I do have a proprietary interest (patent pending).

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during the course of phacoemulsification. However, we have found that this is more common early in the learning curve of the technique. Later, experience dictates that a moderate amount of tension can be placed on the anterior capsule opening by the stabilizing microhooks. This minimizes the need to reposition hooks because of greater tension within the capsulorhexis. In this regard, the key element is a square rather than a round capsulorhexis after placement of the microhooks, indicating that the hooks are providing significant circumferential traction. We have not noted any induced capsule tears with this technique. Nor have we seen nucleus dislocation; using the technique described above, we would imagine that this is extremely rare. We agree that the lack of equatorial zonular support is sometimes a further problem in these complex cases. A solution we have used, although not specifically reported in our article, is to combine the microhook capsular stabilization technique with the use of an endocapsular tension ring. We have most commonly inserted the ring after phacoemulsification and cortical aspiration but before lens implantation. In cases of severe lens instability, we have also combined the microhook stabilization technique with the use of an endocapsular tension ring inserted after capsulorhexis and hydrodissection but before phacoemulsification. This method has the disadvantage of occasionally resulting in cortical soft lens matter becoming trapped between the ring and equatorial lens capsule, although this can usually be removed by gentle aspiration. We were interested in Dr. Mackool’s innovative approach to equatorial zonular support by extending the hooks into the recesses of the capsular bag. However, this method might also be subject to the disadvantage of equatorial soft lens matter becoming trapped between the elongated hook, extending behind the pupil, and the capsular bag. We look forward to trying Dr. Mackool’s capsular support system. Of course, the technique we described has the advantage of using equipment that is readily available in most operating suites and familiar to most surgeons.—Philip Bloom, FRCS, FRCOphth, Vincent Lee, FRACS

RICHARD J. MACKOOL, MD Astoria, New York, USA Reference 1. Lee V, Bloom P. Microhook stabilization for phacoemulsification in eyes with pseudoexfoliation-syndrome-induced lens instability. J Cataract Refract Surg 1999; 25:1567–1570

Reply: After reading Dr. Mackool’s discussion of capsular stabilization for phacoemulsification, we would agree that flexible iris retractors do sometimes slip over the capsulorhexis © 2000 ASCRS and ESCRS Published by Elsevier Science Inc.

Keratectasia After LASIK heo Seiler has published1,2 and lectured about the occurrence of keratectasia after laser in situ keratomileusis (LASIK).1,2 In a recent guest editorial,3 Seiler reminded us that this feared iatrogenic complication is potentially present in every case of myopic LASIK, es-

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LETTERS

pecially (but not limited to) those in the higher ranges of myopia. Considering the scarcity of reports about this complication, a question arises: Is Dr. Seiler the only refractive surgeon to whom this happens or “is the currently published number of fewer than 20 cases only the tip of an iceberg”? Yes, Dr. Seiler, they are the very tip of an iceberg. In this sense, I think Dr. Seiler should be recognized for his humility and intellectual honesty. In LASIK, 250 ␮m remains the gold standard when we consider the thickness of corneal tissue that must be left in place. This is the “security factor,” the paradigm. Nevertheless, biology rarely surrenders entirely to mathematics, and many paradigms of this type have fallen and will fall in the future. This is, after all, the essence of scientific progress. To cite just 1 example: In glaucoma, an intraocular pressure of 21.0 mm Hg was the security factor for many years. But then it became evident that many individuals were biologically sensitive to pressure far lower than this, and the concept of normal-tension glaucoma emerged. Similarly, 250 ␮m of residual corneal tissue after LASIK will probably suffice for most but not all patients. Without considering the numerous potential sources of error and/or predisposing factors at work in the reported cases of iatrogenic keratectasia (forme fruste keratoconus, inconsistencies in flap thickness, miscalculations, differences in ablation rates in dehydrated stromas, etc.), the undeniable fact is that the tensile strength of corneas will vary among individuals and reducing corneal thickness by 50% or more will weaken the mechanical properties in a way almost completely unknown to us. Finally, the words of Dr. Seiler about stopping and reflecting are a warning that seems absolutely logical and deserves the most judicious consideration. JORGE MURAVCHIK, MD Buenos Aires, Argentina

References 1. Seiler T, Koufala K, Richter G. Iatrogenic keratectasia after laser in situ keratomileusis. J Refract Surg 1998; 14:312–317 2. Seiler T, Quurke AW. Iatrogenic keratectasia after LASIK in a case of forme fruste keratoconus. J Cataract Refract Surg 1998; 24:1007–1009 3. Seiler T. Iatrogenic keratectasia: academic anxiety or serious risk? (editorial) J Cataract Refract Surg 1999; 25:1307–1308 630

Pneumotonometry Versus Goldmann Tonometry

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n their article comparing pneumotonometry and Goldmann tonometry after laser in situ keratomileusis (LASIK) for myopia, Zadok et al.1 suggest that pneumotonometric measurements are probably less affected than Goldman applanation tonometry by the flattening and thinning of the central cornea that occurs after LASIK. I think the following points should be clarified: 1. Drance reported that the range of circadian intraocular pressure (IOP) variation was 3.7 mm Hg ⫾ 1.8 (SD) in healthy eyes.2 Most authors generally report a maximal range of circadian IOP variation less than 7.0 mm Hg,3,4 but some exceptional cases range up to 12.0 mm Hg.5 For more accurate results, the authors should measure the IOP at the same time of day. However, they have provided no information on this point. 2. When 2 IOP measurements are taken with the Goldmann applanation tonometer within a short time, using 1 instrument and 1 examiner6 or 2 instruments and 2 examiners,7 at least 30% of 2 measurements will vary by 2.0 mm Hg and 3.0 mm Hg or more, respectively. There was no information about these variables in the manuscript. 3. As stated in the Discussion, there was relatively poor reproducibility of Goldmann applanation tonometry. For this reason, a control group was needed. The effects of reproducibility could then be eliminated, and the effects of LASIK on IOP measurements could be assessed more reliable. As a conclusion, the authors state that pneumotonometric measurements were probably less affected by LASIK than Goldmann applanation tonometry. Since there was no control group, the study results might also be interpreted as indicating that the interpersonal variability of the Goldmann applanation tonometer was higher than that of the pneumotonometer. 4. When the tonometer prisms are in their usual orientation, with the mires displaced horizontally, the IOP is underestimated by about 1.0 mm Hg for every 4.0 diopters (D) of corneal astigmatism for with-therule astigmatism and overestimated by about the same amount for against-the-rule astigmatism.8 To minimize

J CATARACT REFRACT SURG—VOL 26, MAY 2000