Obtaining lenticules for lamellar transplantation

LETTERS

of dye in deep lamellar keratoplasty (LK). We fully agree that this innovative idea will help corneal surgeons. However, we would like to highlight some important points. This is by no means a criticism of the excellent study but rather will further the simplified technique. Balestrazzi and coauthors have rightly highlighted the shortcomings of deep stromal dissection of the lamellar bed in the recipient cornea. They also mention the various methods to facilitate easy stromal separation that have been reported. We have compared these techniques and found hydrodelamination to be a better method.2 The authors may have overlooked this article. The following points are to be noted: 1. How much trypan blue dye was injected intrastromally while attempting the divide-and-conquer technique of separation? Was additional fluid (balanced salt solution [BSS威]) injected intrastromally before the trypan blue dye was injected? 2. From Figure 1, it is evident that the dye is injected during the open technique of dissection. In fact, it would be much easier with a closed technique.3 3. The authors mention that a limbal paracentesis was made to lower the intraocular pressure (IOP) to facilitate lamellar dissection without globe perforation. But lamellar dissection in a eye with low IOP is a tedious process. The limbal entry has been described for injecting air into the anterior chamber4; the presence of air enhances optical visualization while maintaining the IOP. 4. While performing deep LK up to Descemet’s membrane, the authors might have noted that the clear Descemet’s membrane appears dark because of the iris reflex. This can be prevented by injecting air into the anterior chamber. Our experience using trypan blue dye with fluid LK indicates that it provides a uniform appearance compared to viscolamellar and air lamellar keratoplasty, which provide a patchy appearance. 5. The statement about the indications for deep LK is not correct as a conventional anterior LK would satisfy the need for epithelial corneal dystrophies. Again, we wish to commend the authors for describing another use of trypan blue dye. MAYANK S. PANGTEY, MD ANITA PANDA, MD New Delhi, India

References 1. Balestrazzi E, Balestrazzi A, Mosca L, Balestrazzi A. Deep lamellar keratoplasty with trypan blue intrastromal staining. J Cataract Refract Surg 2002; 28:929 –931 2. Panda A, Sharma N, Singh R. Intralamellar dissection. Cornea 1999; 19:22–25 3. Panda A. Lamellolamellar sclerokeratoplasty: where do we stand? Eye 1999; 13:221–225 4. Melles RJG, Rietveld JRF, Beekhuis WH, Binder SP. A technique to visualize corneal incision and lamellar dissection depth during surgery. Cornea 1999; 18:80 –85

Reply: We appreciate and thank Drs. Pangtey and Panda for their interest in our deep lamellar technique. Regarding the questions about the technique, it is important to clarify some points. In our experience, it is not necessary to inject BSS before the trypan blue solution. We usually carry out stromal hydrodelamination with the dye solution. After manual removal of the superficial corneal layers, when we have to dissect the deep stromal layers, we inject the trypan blue dye again. The first dye injection is carried out with a closed technique. After the first injection, during removal of the deep stroma, we continue to wash the surgical field using an open technique. This step allows visualization and removal of residual stromal fibers. With the limbal paracentesis, we usually obtain only a minimal decrease in the IOP. Limbal paracentesis is not absolutely necessary, but we think it may prevent microperiorations during removal of the deep stromal layers. We have not performed air LK and in our experience, with viscodissection it is easy to have complications such as perforations. We are satisfied with the results obtained using trypan blue hydrodelamination. With this technique, we have the impression of a greater safety margin near Descemet’s membrane and we have seen a decrease in microperforations. —Emilio Balestrazzi, MD, Angelo Balestrazzi, MD, Luigi Mosca, MD, Alessandra Balestrazzi, MD

Obtaining Lenticules for Lamellar Transplantation he article by Li et al.1 describes a modification of the original technique for improving accuracy in obtaining lenticules for lamellar transplantation.2 While we agree that adequate intrachamber pressure is imperative for obtaining consistent lenticular diameter and thickness, in our experience, intrachamber pressure is best confirmed with Barraquer tonometry not TonoPen (Mentor) tonometry. TonoPen tonometry has been

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LETTERS

shown to be an accurate method of tonometry of whole globes3; however, its use in corneoscleral rims in an artificial anterior chamber system has not been extensively studied. We found confirmation of intrachamber pressure with the TonoPen to be time consuming, variable between TonoPens, and consistently over or under the readings obtained with Barraquer tonometry. Elevating the bottle of irrigant to the maximal height allowable is certainly a viable method of obtaining adequate intrachamber pressure. When we used this technique, we experienced corneal perforations with the 350 ␮m head as well as with the 250 ␮m head. We suspect this complication is due to subtle herniation of tissue from high intrachamber pressure and an inadequate corneoscleral rim diameter, not inadequate corneal thickness as suggested by Li et al. For this reason, we have modified our technique by titrating the bottle height to correspond to a reading of 65 mm Hg with Barraquer tonometry and request that corncoscleral rims used for lamellar transplantation have at least a 2.0 mm rim. Since implementing these modifications, we have not experienced corneal perforations. CLARK L. SPRINGS, MD LEE A. WILEY, MD Morgantown, West Virginia, USA

artificial anterior chamber and found no significant differences. However, we observed superior reproducibility using the thinnest head tested (180 ␮m) in terms of the lowest standard deviation.1 This may account for more standardized and widespread use of the microkeratome with this head thickness. It is likely that the use of thicker heads may require conditions that differ from those used with thinner “standard” cutting heads. Drs. Springs and Wiley titrated the bottle height to maintain 65 mm Hg and obtained no perforations. This observation was also seen in our original series using a lower intrachamber pressure (TonoPen ⬃53 mm Hg). Nonetheless, the thickness variability using the thick-cutting heads was higher and the accuracy was lower.1,2 Therefore, we agree with Drs. Springs and Wiley that the intrachamber pressure is an important factor to consider to avoid unexpected perforations. We also believe that their hypothesis of a subtle herniation of tissue using a higher pressure might play a role in corneal perforations. The cut diameter and thickness should be taken into account when selecting the intrachamber pressure to be used. The biomechanical properties of the cornea may change with deeper and larger cuts using an artificial anterior chamber. The intrachamber pressure should therefore be adapted to these variables to obtain accuracy in the expected disk dimensions. We are grateful to Drs. Springs and Wiley for the contribution of their important observations to the development of new insights for the use of this instrument. There are several confounding factors that must be addressed to understand corneal biomechanics under these ex vivo circumstances. —Ashley Behrens, MD, Li Li, MD, Roy S. Chuck, MD

References 1. Li L, Behrens A, Sweet PM, et al. Corneal lenticule harvest using a microkeratome and an artificial anterior chamber system at high intrachamber pressure. J Cataract Refract Surg 2002; 28:860 –865 2. Behrens A, Dolorico AMT, Kara DT, et al. Precision and accuracy of an artificial anterior chamber in obtaining corneal lenticules for lamellar keratoplasty. J Cataract Refract Surg 2001; 27:1680 –1687 3. Eisenberg DL, Sherman BG, McKeown CA, Schuman JA. Tonometry in adults and children: a manometric evaluation of pneumotonometry, applanation and TonoPen in vitro and in vivo. Ophthalmology 1998; 105:1173–1181

Reply: We thank Drs. Springs and Wiley for the interesting points raised in their letter. They postulate some differences between Barraquer and TonoPen tonometry that have not been documented using this system. The artificial anterior chamber certainly constitutes a different environment to confidently measure the intrachamber pressure with preexisting technologies. However, we believe the common instruments for tonometry may show a close approximation for comparative purposes. Although our data are not published, we compared pneumotonometry and TonoPen tonometry using the 6

References 1. Behrens A, Dolorico AMT, Kara DT, et al. Precision and accuracy of an artificial anterior chamber in obtaining corneal lenticules for lamellar keratoplasty. J Cataract Refract Surg 2001; 27:1680 –1687 2. Li L, Behrens A, Sweet PM, et al. Corneal lenticule harvest using a microkeratome and an artificial anterior chamber system at high intrachamber pressure. J Cataract Refract Surg 2002; 28:860 –865

Defining Mesopic and Scotopic

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n their paper comparing the pupil card and the pupillometer,1 Pop and coauthors make the following statement: “Colvard reports that it is difficult for clinicians to measure pupil size with the Rosenbaum card in scotopic luminance. Because of the examiner bias, the present study showed that measurements done in scotopic luminance with the Colvard pupillometer were

J CATARACT REFRACT SURG—VOL 29, JANUARY 2003