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M.S. Muallem, S.H. Yoo, F.B. Marangon, J.C. Schiffman, W.W. Culbertson; Flap Thickness in LASIK Enhancement . Invest. Ophthalmol. Vis. Sci. 2003;44(13):2594.
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Purpose: to evaluate whether flap thickness changes after LASIK procedure, and to assess the accuracy of intraoperative pachymetry. Methods: interventional non-comparative case series. Fifty seven eyes of 42 patients who underwent LASIK enhancement between June 2001 and September 2002 were included. During enhancement, the original flap was lifted in all cases. Corneal thickness was measured intraopratively by ultrasound pachymetry. The patient age, eye, refraction, date of primary LASIK, central corneal thickness, central corneal bed thickness, depth of ablation, flap thickness (subtraction pachymetry), date of enhancement, central corneal thickness at enhancement, central corneal bed thickness at enhancement, and flap thickness at enhancement were recorded. We included only patients who underwent LASIK and enhancement by the same surgeon at our institute, and had intraoperative pachymetry readings for both procedures. Exclusion criteria included previous ocular surgery or complications during the first LASIK procedure. Results: the mean age for all patients was 51±13. Hyperopic patients were older than myopic ones (59±7 vs 44±12, respectively. P<0.001). The mean spherical equivalent was –1.9±4.3 D (-5.4±2.4 for myopic, and +2.2±1.2 for hyperopic eyes). The average central corneal bed ablation in myopic eyes was 69±32 µ. The average time between the LASIK procedure to enhancement was 218±115 days [248±136 for hyperopic, and 193±88 for myopic eyes (p=0.068)]. The flap tended to be thicker in enhancement than in primary LASIK procedures by 9.3± 25.7 µ for myopic (p=0.054) and 10.5±16.6 µ for hyperopic (p=0.004) eyes. There was a correlation between flap thickness at first and second procedures (r=0.6). No correlation was found between the difference in flap thickness and the time until enhancement (r=0.009 in myopic, and 0.01 in hyperopic eyes). In myopic eyes, the estimated central corneal bed thickness after the first procedure (central bed thickness – ablation depth) was about the same as the one measured directly at enhancement (p=0.54, paired t-test). Conclusions: to our best knowledge, no data has been published regarding flap thickness in LASIK enhancement. In our study, flap thickness tended to be thicker in enhancement than in primary LASIK. This might be due to the healing process that takes place in the flap after its cutting. Intraoperative pachymetry proved to be a precise tool, as shown by comparing the estimated central bed thickness after ablation in the first procedure to the one measured directly at enhancement. This information may be helpful in planning LASIK enhancement.
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