Three-dimensional LASIK flap thickness variability: topographic central, paracentral and peripheral assessment, in flaps created by a mechanical microkeratome (M2) and two different femtosecond lasers (FS60 and FS200)

Abstract

Purpose: To evaluate programmed versus achieved laser-assisted in situ keratomileusis (LASIK) flap central thickness and investigate topographic flap thickness variability, as well as the effect of potential epithelial remodeling interference on flap thickness variability.

Patients and methods: Flap thickness was investigated in 110 eyes that had had bilateral myopic LASIK several years ago (average 4.5 ± 2.7 years; range 2-7 years). Three age-matched study groups were formed, based on the method of primary flap creation: Group A (flaps made by the Moria Surgical M2 microkeratome [Antony, France]), Group B (flaps made by the Abbott Medical Optics IntraLase™ FS60 femtosecond laser [Santa Ana, CA, USA]), and Group C (flaps made by the Alcon WaveLight(®) FS200 femtosecond laser [Fort Worth, TX, USA]). Whole-cornea topographic maps of flap and epithelial thickness were obtained by scanning high-frequency ultrasound biomicroscopy. On each eye, topographic flap and epithelial thickness variability was computed by the standard deviation of thickness corresponding to 21 equally spaced points over the entire corneal area imaged.

Results: The average central flap thickness for each group was 138.33 ± 12.38 μm (mean ± standard deviation) in Group A, 128.46 ± 5.72 μm in Group B, and 122.00 ± 5.64 μm in Group C. Topographic flap thickness variability was 9.73 ± 4.93 μm for Group A, 8.48 ± 4.23 μm for Group B, and 4.84 ± 1.88 μm for Group C. The smaller topographic flap thickness variability of Group C (FS200) was statistically significant compared with that of Group A (M2) (P = 0.004), indicating improved topographic flap thickness consistency - that is, improved precision - over the entire flap area affected.

Conclusions: The two femtosecond lasers produced a smaller flap thickness and reduced variability than the mechanical microkeratome. In addition, our study suggests that there may be a significant difference in topographic flap thickness variability between the results achieved by the two femtosecond lasers examined.

Keywords: 400 Hz excimer; IntraLase FS60; Moria M2; WaveLight® FS200, Allegretto Wave® Eye-Q; ultrasound biomicroscopy.

Figure 1

Standard corneal analysis report used in our investigation. Note: This specific flap has been created with the FS200 femtosecond laser. Abbreviation: LASIK, laser-assisted in situ keratomileusis.

Figure 2

Detail from the lower-left table of the corneal analysis report depicted in Figure 1, showing data recorded for mean epithelial thickness, mean flap depth (0–6 mm), central flap depth (0–3 mm), and peripheral flap depth (3–6 mm).

Figure 3

Three representative flap thickness maps (8 mm diameter) from flaps created with the modalities studied in this paper: (A) M2 microkeratome (Moria Surgical, Antony, France), (B) Intralase™ FS60 femtosecond laser (Abbott Medical Optics, Santa Ana, CA, USA), (C) WaveLight^® FS200 femtosecond laser (Alcon, Fort Worth, TX, USA). Note: The values over the 21 points are those used for the flap thickness mean and topographic flap thickness variability study.

Figure 4

Postoperative topographic flap thickness variability for the three groups examined. Notes: “FS60” refers to the IntraLase™ FS60 femtosecond laser manufactured by Abbott Medical Optics, Santa Ana, CA, USA; “FS200” refers to the WaveLight^® FS200 femtosecond laser manufactured by Alcon, Fort Worth, TX, USA; “M2” refers to the M2 microkeratome manufactured by Moria Surgical, Antony, France.

Figure 5

Postoperative epithelial thickness and topographic epithelial thickness variability for the three groups examined. Notes: “FS60” refers to the Intralase™ FS60 femtosecond laser manufactured by Abbott Medical Optics, Santa Ana, CA, USA; “FS200” refers to the WaveLight^® FS200 femtosecond laser manufactured by Alcon, Fort Worth, TX, USA; “M2” refers to the M2 microkeratome manufactured by Moria Surgical, Antony, France.

Figure 6

Schematic of the architectural differences between the (A) Intralase™ FS60 (Abbott Medical Optics, Santa Ana, CA, USA) and (B) WaveLight^® FS200 (Alcon, Fort Worth, TX, USA) femtosecond lasers. Notes: In the initial phase of flap creation with the FS60, a stromal “gas decompression” pocket is created, while, with the FS200, a channel through the hinge is created to help the gas escape.