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Direct and Indirect Flap Measurements in Femtosecond Laser–Assisted In Situ Keratomileusis

Crispim, Joao, MD*,†; Allemann, Norma, MD*,†; Hallak, Joelle A., PhD*; Azar, Dimitri T., MD*; De La Cruz, Jose, MD*

doi: 10.1097/ICO.0000000000001836
Clinical Science

Purpose: To compare direct and indirect LASIK flap thickness measurements using ultrasound and Scheimpflug technology.

Methods: Eighty-two eyes treated with laser-assisted in situ keratomileusis refractive surgery using a femtosecond laser (IntraLase FS150) were prospectively included in the study. Flap thickness was set to 115 μm. Corneal flap thickness was measured using the direct method—ie, ultrasound pachymetry immediately after flap construction in the presence of cavitation bubbles—and indirect methods, with subtraction of intraoperative post-lift corneal thickness measured using ultrasound pachymetry (intrastroma) from preoperative central corneal thickness using ultrasound (Indirect-US) or Scheimpflug thinnest pachymetry (Indirect-Scheimpflug).

Results: Mean flap thickness was overestimated using the indirect methods, Indirect-US and Indirect-Scheimpflug (122.6 ± 24.5 μm and 128.1 ± 26.1 μm, respectively; P < 0.0060 and P < 0.0001, respectively). There were no significant correlations between the direct and indirect methods. Indirect-Scheimpflug was significantly higher (P = 0.0122) than Indirect-US. The closest average flap thickness compared with the set parameter of 115 μm was that of the direct method (115.6 ± 8.6 μm; 95% confidence interval: −1.3 to 2.5; P = 0.5163). The direct method provided the lowest SD of all groups (SD: 8.64).

Conclusions: The direct method of flap thickness measurement was the most comparable to the set parameter compared with the indirect subtraction methods. Additional studies are needed to determine which method allows for the most accurate measurement of flap thickness.

*Department of Ophthalmology and Visual Sciences, Illinois Eye & Ear Infirmary, University of Illinois at Chicago, Chicago, IL; and

Department of Ophthalmology and Visual Sciences, Federal University of São Paulo, São Paulo, Brazil.

Correspondence: Jose De La Cruz, MD, Department of Ophthalmology and Visual Sciences, Illinois Eye & Ear Infirmary, University of Illinois at Chicago, 1855 W Taylor St, Chicago, IL 60612 (e-mail:

Supported by an unrestricted institutional grant from Research to Prevent Blindness (RPB) to the Illinois Eye & Ear Infirmary, University of Illinois at Chicago.

The authors have no conflicts of interest to disclose.

Received June 18, 2018

Accepted November 05, 2018

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