This prospective study included 35 eyes of 18 patients (5 males, 13 females) with a mean age of 26.20 ± 6.30 years (standard deviation, SD) (range, 19.0 to 40.0 years). The mean preoperative spherical equivalent (SE) was -4.86 ± 2.34 diopters (D). The mean preoperative corneal keratometry was 43.31 ± 1.50 D, and the mean preoperative corneal pachymetry on ultrasound was 557.40 ± 31.05 mm. Table 1 shows the preoperative demographics of the patients.

Since the initial LASIK procedures were performed in 1990 [12], multiple mechanical microkeratomes have been used to create the corneal flap [13-21]. The current microkeratomes have a very precise and reliable performance and show standard deviations in flap thickness between 15 and 35 µm [22]. However, the accuracy of mechanical microkeratomes is due to their specifications, and their method of cutting is limited. Therefore, they still can cause some complications during the cutting process. Among these are incomplete cuts, free caps, off centered flaps, button holes, inconsistent flap thickness, epithelial defects, induction of higher-order corneal aberrations, etc. [23,24]. A different approach to cutting corneal flaps is the use of laser energy. Ultrashort laser pulses such as those emitted by pico- and femtosecond lasers, have been tested, and femtosecond laser technology has shown the most promising results in flap separation [25]. The femtosecond laser is a focusable infrared laser, which uses pulses in the femtosecond (10^-15 s) duration range. Contiguous pulses are placed at a precise depth within the cornea. The 1,053-nm wavelength of light used by the laser is transparent to the cornea, so it resects only targeted tissue while leaving the surrounding tissue unaltered. Because the energy and firing patterns are controlled by a computer, the laser is capable of cutting tissue at various depths and patterns, which produces minimal inflammation or collateral tissue damage [26-29]. Thermal damage to adjacent tissue in the cornea has been measured to be in a 1-mm radius [30]. The laser essentially vaporizes small volumes of tissue by photodisruption, producing plasma, a shock wave, cavitation, and gas (CO₂ and H₂O) bubbles. The femtosecond laser creates a resection plane for a lamellar cut using a spot size of 2.5 mm, a pulse-repetition rate of 15 to 60 kHz and a pulse width of 600 to 800 femtoseconds. The intensity of the laser energy can be varied at the operator's discretion. A side (trephination) cut can also be fashioned from the stroma to the surface of the cornea for anterior flaps in LASIK. The cut angle can be varied between 45° and 90°. The femtosecond laser is also capable of creating lamellar dissections ranging from 5 mm to 9.5 mm in diameter.

The femtosecond laser's range of achieved flap thickness is narrower than that created with conventional microkeratomes. This advantage translates into few thicker-than-expected flaps, which reduces the risk for ectasia, and very few thinner flaps, which reduces the risk for cap perforation and corneal striae. Mechanical microkeratomes tend to create meniscus-shaped corneal flaps that are thinner in the center and thicker in the periphery, whereas the femtosecond laser typically creates a more uniform planar flap. Alió and Piñero [8] found that the peripheral pachymetric values of the Moria M2 microkeratome flaps were always higher than those corresponding with the IntraLase flap.

The femtosecond laser is not perfect. Unexpectedly thinner or thicker flaps sometimes occur with the femtosecond laser and can lead to surgical complications [9]. Flap thickness has been an important measure of femtosecond-assisted LASIK safety. Pfaeffl et al. [31] found that the following factors had a low correlation with preoperative keratometry in predicting the variance of the achieved flap thickness, selection of the right suction ring, suction time, use of the pocket option, flap diameter, temperature, and humidity when using the femtosecond laser microkeratome. Binder [32] suggested that the femtosecond laser photodisrupts more deeply when set between 110 µm and 120 µm with an SD of 12 µm but dissects less deeply when set between 130 µm and 140 µm with an SD of 18.5 µm. Sutton and Hodge [33] reported no significant correlation in either group between keratometry and flap thickness, or between preoperative pachymetry and flap thickness. Additionally, 74.9% of patients treated with 15 kHz had a corneal flap within 20 µm of the intended location, which was increased to 97.9% with the 30-kHz laser [33].

In this study, the intended flap thickness was 110, 120 or 130 µm depending on the surgeon's preference. One week posteratively, the mean achieved flap thickness was 115.21 ± 4.98 µm (range, 104 to 128 µm) in the 110-µm group, 121.90 ± 5.79 µm (range, 109 to 130 µm) in the 120-µm group and 134.38 ± 5.04 µm (range, 123 to 147 µm) in the 130-µm group. There was no significant difference between the 110-µm and the 120-µm groups when compared with the 130-µm group. No significant correlations were found between the residual of the achieved flap thickness and age, preoperative keratometry, pachymetry, or SE. In our study, the periphery of the flap showed higher variations in thickness and greater deviations from the intended flap thickness. The mean achieved flap thickness in the central 1.5-mm radius area was thicker than that in the peripheral 3.0 to 4.0-mm radius area. The difference of the mean achieved-flap thickness between the central and peripheral areas ranged from 3.33 µm to 5.58 µm (Table 5). Li et al. [34] reported that the repeatability of OCT corneal-thickness measurements was 2 µm. Other studies found that the mean femtosecond laser flap thickness increased slightly toward the periphery [35].

The possible factor is the biomechanical characteristics of the cornea. Dawson et al. [36] reported a difference in tensile strength between the central and peripheral cornea, with increasing tensile strength moving from the center to the periphery. However, when boundaries between the corneal flap and the stroma become ambiguous, especially in the central portion of the cornea, corneal-flap thickness measurements using OCT are not absolutely precise.

In conclusion, the ultimate goal of LASIK surgery is to provide patients with the best possible visual and refractive outcomes with the lowest risk of complications. Therefore flap thickness is an important indicator of LASIK safety because of the importance of stromal preservation. Sub-Bowman keratomileusis (SBK) is becoming a prominent procedure because SBK is said to combine the advantages of LASIK (i.e., low risk for haze and almost no pain) and classic photo-refractive keratectomy in the hopes of maintaining better corneal strength. However, these newer excimer laser techniques still have disadvantages caused by an unstable corneal flap. Further randomized studies are required to evaluate factors affecting flap thickness.