Microkeratome accuracy in LASIK
2001; Elsevier BV; Volume: 108; Issue: 11 Linguagem: Inglês
10.1016/s0161-6420(01)00741-2
ISSN1549-4713
Autores Tópico(s)Glaucoma and retinal disorders
ResumoThe recent article by Durairaj et al, concurs with earlier reports1Yildrim R. Aras C. Ozdamar A. et al.Reproducibility of corneal flap thickness in laser in situ keratomileusis using the Hansatome microkeratome.J Cataract Refract Surg. 2000; 26: 1729-1732Abstract Full Text Full Text PDF PubMed Scopus (113) Google Scholar, 2Behrens A. Langenbucher A. Kus M.M. et al.Experimental evaluation of two current-generation automated microkeratomes the Hansatome and the Supratome.Am J Ophthalmol. 2000; 129: 59-67Abstract Full Text Full Text PDF PubMed Scopus (67) Google Scholar, 3Perez-Santonja J.J. Bellot J. Claramonte P. et al.Laser in situ keratomileusis to correct high myopia.J Cataract Refract Surg. 1997; 23: 372-385Abstract Full Text PDF PubMed Scopus (300) Google Scholar that currently available microkeratomes create corneal flaps that are thinner than intended. These studies also indicate that variability in flap thickness is outside the safety limits for laser in situ keratomileusis (LASIK). An indirect method is used in these studies to estimate flap thickness—central corneal pachymetry is performed at the start of the procedure and is repeated in the stromal bed after cutting and lifting the corneal flap. The difference between these two values represents the thickness of the corneal flap. Durairaj et al stated in their discussion section (paragraph 2 and lines 13–14) that “several flaps measured little more than 50 μm.” Because this is the thickness of the corneal epithelium, the clinical appearance and behavior of these flaps is likely to be quite different from flaps of standard thickness. If such thin flaps are actually produced, intraoperative repositioning will be difficult, there may be flap wrinkling postoperatively, and the refractive outcomes are also likely to be unsatisfactory.4Yi W.M. Joo C.K. Corneal flap thickness in laser in situ keratomileusis using an SCMD manual microkratome.J Cataract Refract Surg. 1999; 25: 1087-1092Abstract Full Text Full Text PDF PubMed Scopus (76) Google Scholar We would be interested to know such clinical information, but that was not mentioned in the report. It is possible that this extreme thinness may be an artifact of the indirect measurement method. Changes in stromal bed thickness caused by altered corneal hydration could affect the accuracy of this method. Excessive drying of the stromal bed would result in corneal compaction and underestimation of its thickness, with resultant increase in the computed thickness of the corneal flap. Conversely, overhydration of the stroma could result in the prediction of a flap that is thinner than that actually created. Studies in vitro have also shown that intraocular pressure fluctuations during the vacuum phase and lamellar cut using the microkeratome could reach unphysiologic levels.5Pico J.F. Stamper R.L. McMenemy M. Intraocular pressure and corneal curvature changes on application of limbal-scleral suction fixation ring in rabbits.Cornea. 1993; 12: 25-28Crossref PubMed Scopus (7) Google Scholar Intraoperative measurements during the LASIK procedure could be affected because of these changes, resulting in inaccurate estimation of the flap thickness. These inaccuracies can be partially overcome by direct measurements of the gap width of the microkeratome head. The microkeratome gap width is the shortest distance from the tip of the blade to the footplate of the microkeratome. This is the most important controllable factor that determines the maximal corneal flap thickness. A larger microkeratome gap width will constantly produce a thicker flap under normal conditions. Similarly, a smaller gap width will constantly produce a thinner flap. We have successfully measured and compared the gap width of four SCMD Ltd. (Fountain Hills, AZ) microkeratomes using scanning electron microscopy. The manufacturer’s gap width specification in all four instruments was 150 μm. However, two of the four microkeratomes with the gap width measured had errors greater than 25% from the manufacturer’s specification. This is well beyond the standard of tolerance that can normally be accepted in LASIK surgery. Further studies are warranted to correlate the gap width of instruments with the maximal corneal thickness of the flaps created. Another method of determining the thickness of the created flap would be the use of noninvasive measurement techniques in the postoperative period. The use of optical coherence tomography for this purpose has been described,6Ustundag C. Bahcecioglu H. Ozdamar A. et al.Optical coherence tomography for evaluation of anatomical changes in the cornea after laser in situ keratomileusis.J Cataract Refract Surg. 2000; 26: 1458-1462Abstract Full Text Full Text PDF PubMed Scopus (62) Google Scholar and it is also possible that techniques like confocal microscopy7Vesaluoma M. Perez-Santonja J. Petroll W.M. et al.Corneal stromal changes induced by myopic LASIK.Invest Ophthalmol Vis Sci. 2000; 41 ([erratum in Invest Ophthalmol Vis Sci 2000;41:2027]): 369-376PubMed Google Scholar can be used. Thus, published reports and our study8Liu K.Y. Lam D.S.C. Direct measurement of microkeratome gap width by electron microscopy.J Cataract Refract Surg. 2001; 27: 924-927Abstract Full Text Full Text PDF PubMed Scopus (20) Google Scholar indicate that there is considerable variability in the microkeratomes supplied by manufacturers, and this along with intraoperative factors can lead to unacceptable variations in the thickness of the created flap. It is therefore important for surgeons to validate the instruments that they use and to use a standardized technique for LASIK, using blades specified by the manufacturer. In addition, it may be prudent to perform intraoperative pachymetry, particularly in eyes with high myopia or thin corneas, to ensure that the planned stromal thickness is retained. Microkeratome accuracy in LASIK: Author replyOphthalmologyVol. 108Issue 11Preview Full-Text PDF
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