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Up Front | Oct 2008

Thin-Flap LASIK: How Thin Should We Go?

A 100-µm flap is safely within stromal tissue but preserves more underlying stroma than standard flaps.

In the early days of LASIK, thin flaps were considered the gold standard. However, after the appearance of complications including flap folds and irregularities, the trend moved toward thicker flaps in the hopes of reducing postoperative side effects.1 Microkeratome-related complications subsequently decreased; however, surgical complications continued.

Over the past 2 years, we have begun to see a resurgence of the thin flap, which has been driven by the growing need to preserve the stroma for both higher correction capabilities and safety reasons, such as avoiding keratectasia. The precision of modern laser keratomes, including the IntraLase femtosecond laser (Advanced Medical Optics, Inc., Irvine, California), has enabled us to consistently cut flaps as thin as 90 µm. This begs the question: How thin should we go to promote the safest results for our patients?

BIOMECHANICS
To adequately answer this question, we must first consider corneal biomechanics. According to Marshall et al,2 the anterior-most 160 µm of corneal stroma is much stronger than the deeper stroma. Additionally, the mid-periphery (8–12 mm) is stronger than the central region (6 mm) of the cornea. Therefore, we must ensure we are creating the flap cut as superficially as possible—especially in the mid-periphery—to maintain the integrity of the cornea.

Microkeratomes create a meniscus-shaped flap. Femtosecond lasers, on the other hand, generate planar flaps that are uniformly thick across the cornea.3 Generally speaking, planar flaps cut by the femtosecond laser minimize corneal weakening.

Tran et al4 confirmed that planar flaps have less biomechanical effect on the cornea compared with mechanical microkeratome-created flaps. Measurable spherical aberrations were noted in patients treated with the Hansatome microkeratome (Bausch & Lomb, Rochester, New York); however, no aberrations were seen in patients treated with the IntraLase.

SIDECUT DESIGN
Another question regarding post-LASIK corneal function is the role of sidecut design. The iFS Advanced Femtosecond Laser (Advanced Medical Optics, Inc.) can customize the sidecut for each patient. But does the sidecut angle matter clinically? According to the latest research, it does.

Marshall et al5 found that a steeper sidecut (70°) weakened the cornea more than an inverted sidecut (150°; Figure 1). One explanation for this is that the beveled edge of the flap is tucked under the lip of the peripheral stroma in an inverted sidecut (Figure 1), preserving the strongest part of the cornea. Marshall concluded that shallow sidecuts weaken the cornea because supportive superficial fibers are severed; however, inverted sidecuts at 150° inflict less biomechanical change. Marshall's findings have since been replicated (see How Can We Influence Flap Healing?).6

MAKING A THINNER FLAP
We have established: (1) the mid-periphery of the cornea must stay as intact as possible, (2) femtosecond lasers create planar flaps that promote corneal stability, and (3) inverted sidecuts maximize corneal strength. Now, we can discuss how thin to make a flap.

No studies have tested the efficacy of thin versus thick LASIK flaps; however, two retrospective studies speculate that 100-µm flaps provide visual quality as good as or better than thick flaps.7,8 Thin flaps also (1) weaken the cornea less, (2) make less biomechanical impact, and (3) leave more stroma for ablation. Thick flaps are more stable and have less tendency to form microfolds; however, they are also more harmful to corneal stability and limit higher corrections.

With the depth of the epithelium approximately 50 µm to 60 µm and Bowman's layer 10 µm, we could theoretically create a flap as thin as 70 to 80 µm; however, the thickness of the epithelium varies between individuals, and applanation cones have a certain variability, too. For this reason, using a flap thickness of 100 µm provides a margin of safety with which I am comfortable.

Rather than standard flap thickness (range, 130–160 µm), I recommend 100-µm flaps cuts made with the femtosecond laser. I believe a femtosecond laser is mandatory when performing thin-flap LASIK because it creates predictable, uniform, planar flaps. The inverted sidecut may also enhance thinner flaps. Lastly, to prevent flap folds that result from overstretching, laser energy should be set accordingly to achieve easy flap separation.

Michael C. Knorz, MD, is a Professor of Ophthalmology, University of Heidelberg, Medical Faculty Mannheim, and the Medical Director of the FreeVis LASIK Centre, Mannheim, Germany. He states that he is a consultant to Advanced Medical Optics, Inc. Dr. Knorz is a member of the CRST Europe Editorial Board. He may be reached at tel: +49 621 383 3410; e-mail: knorz@eyes.de.

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