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ETIOLOGY AND INCIDENCE OF THIN AND THICK FLAPS
A flap is considered thin when the keratome or laser cuts are within or above the 12-microns (µm)-thick Bowman’s layer. This is recognized by a shiny reflex on the stromal surface. The use of corneal pachymeter before and after lifting the flap may be helpful in recognizing this occurrence. A measurement below 60 µm is suspicious, as the thickness of the corneal epithelium is approximately 50 µm. The definition of thick flap is not clear in the literature, but usually involves flaps with a thickness resulting in a lesser-than-intended residual stromal bed (ie, residual stromal bed < 300 µm after excimer laser treatment). The incidence of thin flaps after LASIK has been reported to vary between 0.3% and 0.75%.1–3 With femtosecond LASIK, the rate is approximately 0.08%.1,2 Thick flap incidence is not reported in the literature.
Femtosecond LASIK
Cavitation bubbles from the femtosecond laser can dissect upwards toward the epithelium and may stay below the Bowman’s membrane to create a focal or diffuse thinning in the flap. Also, air bubbles may diffuse accidently deeper, creating a thick flap.
Microkeratome LASIK
Higher keratometric values offer increased resistance to cutting when applanated, leading to upward or downward movement of the blade, and may result in thin and thick flaps. Also, a lack of synchronization between translational flat keratome movement and oscillatory blade movement results in forward displacement of corneal tissue, leading to thin or thick flaps. Flat corneas may also result in a thin flap, as they could be below the adequate cutting plane in certain locations. Blade positioning in the microkeratome and the preset space for the blade in the microkeratome may affect flap thickness in the absence of an irregular flap shape.
FEMTOSECOND LASIK COMPLICATIONS AND IMMEDIATE SOLUTIONS
Complication #1: Thin Regular Flap
Video section: 0 minutes 5 seconds
Platform: IntraLase FS60 kilohertz (kHz) (Abbott Medical Optics)
Flap diameter: 9.3 mm
Flap target depth: 110 µm
The initial surgery resulted in a thin regular flap that disrupted during flap dissection (video 11; time: 0 minutes 5 seconds; Figures 11-1, 11-2, and 11-3).
Some practical measures are as follows:
- Assess the flap regularity.
- Apply excimer laser treatment with regular flap and an adequate-sized stromal bed.
- Try to reposition the flap and epithelial defect in the best anatomical configuration.
- Place a contact lens.
Figure 11-1. Initial surgery resulted in a thin flap with a tear during dissection.
Figure 11-2. Successful excimer laser treatment.
Figure 11-3. Flap was repositioned. At 3 months postoperatively, the flap was clear and well-centered with no signs of epithelial ingrowth.
Figure 11-4. Initial surgery resulted in an irregular flap cut pattern. Figure 11-5. Dissection resulted in a flap tear at 9 o’clock on an unusual thin flap. Complication #2: Localized Thin Regular Flap Video section: 1 minute 53 seconds Platform: WaveLight FS200 (Alcon Labs) Flap diameter: 9.3 mm Flap target depth: 100 µm The initial surgery resulted in a thin flap in the periphery with a tear during dissection (video 11; time: 1 minute 53 seconds; Figures 11-4, 11-5, 11-6, and 11-7). Figure 11-6. Further dissection resulted in an extension of the tear. Flap was repositioned, and the surgery was aborted. Figure 11-7. Flap was repositioned, and the surgery was aborted. A surface refractive procedure was performed 1 week later.
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