Analysis of Thin Femtosecond Flaps with Anterior Segment Optical Coherence Tomography

US Ophthalmic Review, 2007,2:37-9

Greater corneal biomechanical stability has been demonstrated following photorefractive keratectomy (PRK) compared with traditional microkeratome laser-assisted in situ keratomileusis (LASIK).1,2 Thin LASIK flaps exhibit biomechanical properties similar to PRK but with the advantage of no haze formation or pain.2 These findings suggest that thin flaps created with consistent thickness across the cornea are the ideal choice for optimizing refractive and biomechanical outcomes. This article is an overview of our published studies using anterior segment optical coherence tomography (OCT) to analyze thin flaps created with the IntraLase® femtosecond (FS) laser (IntraLase, Inc. Irvine, California).3

Methods
In this prospective study, 25 eyes with myopia with or without astigmatism had excimer laser ablation following thin flap creation by the 60kHz IntraLase FS. Standardized flap parameters were programmed for each procedure with a superior hinged flap. Desired flap thickness was 110μm. Previous experience with our 60kHz Intralase FS laser demonstrated that we need to program the laser to a thickness of 100μm to obtain a thickness of 110μm. The other laser parameters were: 8.5mm diameter, hinge angle of 50º, side-cut angle of 75º, raster pattern energy of 1.30μJ, pulse separation of 8x8μm, and side-cut energy of 2.00μJ with the pocket enabled.

The eyes were evaluated during a one-month post-operative examination using the Visante™ anterior segment OCT (Carl Zeiss Meditec, Inc.) to image each flap. A skilled technician performed all the high-resolution corneal scans with 512 A-scans per line sampling and 0.25 seconds per line acquisition time (2,048 scans per second). The Visante can display a cross-section of an image at any specified meridian. We chose to display the 45°, 90°, 135°, and 180° meridians for each flap. Visante OCT requires manual (semi-automated) measurement. The Visante flap tool is a computer-controlled cursor that is placed on the corneal image at the desired location and that automatically measures corneal thickness. Within this total corneal thickness measurement, the cursor is manually placed on the visualized flap interface. Using the software’s flap tool, the flap interface was visualized and marked by the examiner to measure flap thickness and residual stromal bed thickness. The flap thickness was measured at four points for each of these cross-sections by one examiner who was masked to the attempted flap depth. Two points were ±3–4mm from center, and two were ±1–2mm from center. Thus, each flap’s thickness was measured at a total of 16 points.

Statistical Analysis
The 16 flap thickness measurements were entered into 16 separate columns in an Excel spreadsheet. The data were then analyzed with three different statistical methods: by pooling all data, by looking at each column separately, and by multivariate statistical analysis on the 16 columns simultaneously.

Various statistics were calculated for each method, such as averages, standard deviations (SD), 95% confidence intervals (CI), and p values. The p values were from the one-sample T test, where the data are compared with a hypothesized average value. For the multivariate statistical analysis, the corresponding test is Hotelling’s one-sample T-squared test. A p value ≤0.05 was considered statistically significant.
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