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R. Vasaiwala, J. de la Cruz, J. Hallak, D. Azar; Evaluation of Direct and Indirect Techniques of LASIK Flap Thickness Measurement Using IntraLase Femtosecond Laser. Invest. Ophthalmol. Vis. Sci. 2010;51(13):2871.
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© ARVO (1962-2015); The Authors (2016-present)
(1) To compare whether the ultrasound indirect method of Laser Assisted In Situ Keratomileusis (LASIK) flap thickness calculation is equivalent to the indirect method using Orbscan optical pachymetry, and (2) to compare direct and indirect methods of measuring flap thickness in LASIK.
Fifty eyes treated with LASIK refractive surgery using IntraLase femtosecond laser were included in this retrospective study. All procedures were performed with the IntraLase flap thickness parameter set to 115 microns. Flap thickness was calculated using two types of techniques: direct and indirect. The direct method involved measurement of the corneal flap thickness immediately after flap construction in the presence of cavitation bubbles. The indirect method entailed subtraction of the intraoperative post-lift corneal thickness measured via ultrasound pachymetry from the preoperative central corneal thickness (CCT) obtained via ultrasound or Orbscan pachymetry. Two analyses were performed: (1) CCT measured with ultrasound and Orbscan pachymetry were compared using the two-tailed paired t-test. The values obtained from the two indirect methods were also compared with the two-tailed paired t-test.; and (2) the mean flap thickness obtained from the direct method was compared to those from the indirect methods using the one-tailed paired t-test.
In the first analysis, average preoperative CCT measured with ultrasound and Orbscan pachymetry were 567.3±5.1 (mean ± SEM) microns and 567.9±5.1 microns, respectively. This difference was not statistically significant (p=0.8). The difference in flap thickness calculated by the two indirect techniques was similarly not statistically significant (p=0.8). In the second analysis, direct measurement of LASIK flap thickness averaged 116.7±1.1 microns. Indirect calculations using ultrasound and Orbscan pachymetry yielded mean values of 122.8±3.5 microns and 123.5±3.6 microns, respectively. The differences between the mean measurement via the direct method and the calculations using the two indirect methods were statistically significant (p≤0.05).
Our retrospective analysis suggests that ultrasound pachymetry is equivalent to Orbscan pachymetry for calculating LASIK flap thickness using the indirect method. The direct method of LASIK flap thickness measurement, obtained in the presence of stromal cavitation bubbles, yielded significantly lower values compared to the indirect calculation method.
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