Historically, thin irregular flaps have been characterized with increased rates of irregular astigmatism. Some surgeons find thin flaps undesirable due to past experiences with thin irregular flaps and postoperative irregular astigmatism.

A distinction needs to be drawn between regular flaps, thin flaps, and irregular flaps. Thin flaps are not new to lamellar surgery and many surgeons have knowingly and unknowingly been creating thin flaps with the Automated Corneal Shaper microkeratome (Bausch & Lomb, Claremont, CA) for years. In fact, some surgeons still use the Automated Corneal Shaper microkeratome when they need to create a thin flap of 100 µm or less. My experience with thin flaps came from the suggestion of Tory Prestera MD, PhD, of Carlsbad, CA, and out of the necessity of treating high myopia with average or less-than-average corneal thickness. Using the Nidek MK-2000 microkeratome (Fremont, CA), with a 130-µm head and correct blade lot, I have consistently been able to cut 80- to 100-µm corneal flaps. Each new blade lot is checked for flap thickness created with that lot number for my specific microkeratome.

Perry Binder MD, of San Diego, CA, has demonstrated thin flap creation with the SKBM microkeratome (Alcon Surgical, Fort Worth, TX) (personal communication; December 14, 2001, mean flap thickness, 110 µm). After observing the clinical benefits of thin flaps for high myopia in my practice (Table 1), I have transitioned to thin flaps for all patients. My results over the last year and 1,000 eyes indicate no greater levels of micro- or macrostriae, epithelial ingrowth, flap displacement, or buttonholes.

Thin flap creation has started to take on greater importance as more research has been performed in the area of corneal structure and biomechanics.1-6

There is a significant amount of basic science research that suggests that the anterior third of corneal collagen lamellae proportionally contributes a greater amount to corneal structural integrity. Currently, there is no definitive method of determining which patient may develop corneal ectasia. Surgeons may not want to leave only 250 or 300 µm of residual stroma, but focus instead on leaving as much tissue available as possible.

Currently in vogue, larger ablation zones may also require additional cornea tissue that would be consumed in the production of thicker (160 µm) flaps. Many surgeons believe thin flaps are more striae-prone, but based on my experience and observations, I have not found that to be the case for the reasons listed below:

1. Fewer epithelial defects. There is a gap between the microkeratome footplate and blade. As the flap is made thinner, less tissue passes through this gap and the shear stress applied to the epithelium is reduced beyond the threshold that normally creates an epithelial defect. With a reduction in epithelial macrotrauma (defects/slides) and microtrauma (decreased barrier function), the physiologic parameters that facilitate flap adherence are optimized, and the probability of epithelial defect-associated DLK is decreased. There have been case reports of DLK associated with epithelial defects at the time of surgery and postoperatively several months after surgery.

2. Decreased flap mass. As the surgeon decreases the flap thickness, the flap mass is reduced. Thin flaps demonstrate less prominent edge effect of the keratotomy, thereby optimizing sealing of the keratotomy. Flap-bed cohesion is maximized and nasal-hinged flaps will require less cohesive force to maintain optimal position against lid effects and gravity. Given current proposed mechanisms for flap coherence, a lower mass/thinner flap will require less attractive forces to maintain flap stability. When total flap cohesion is achieved, the possibility of microshifts within the flap that create focal or segmental microstriae is greatly minimized.

3. Increased stretchability. As flap thickness decreases, the flap is more easily stretched, thereby allowing the surgeon to minimize the base curve disparity between the flap and corneal bed. As the flap is stretched back into place and the keratotomy gutter is closed, all or most of the multidirectional and circumlinear microstriae seen in high myopic ablations can be eliminated. Many surgeons have stated that it is not possible to perform high myopic ablations without creating striae; my experience with thin flaps does not involve suturing, only simple stretching. The endpoint for this type of a stretching is visualization of the ablation impression pattern translated to the flap surface (broad-beam diaphragm pattern).

4. Fewer cases of dry eye. In my clinical experience, patients with thinner flaps appear to manifest fewer dry-eye clinical findings and complaints. The improved ocular surface decreases the shear forces between the lids and flap surface seen in post-LASIK dry eye patients who may also demonstrate concomitant micro- and macrostriae development.

In the past, microkeratome technology has been the main impediment to successful thin flap creation. In conjunction with improved microkeratome technology, better instruments and techniques are required to safely and consistently create and manipulate thin flaps. Thin flap creation may not be desirable for all surgeons, but mounting evidence regarding corneal stromal structural integrity suggests that progressively thinner flaps may decrease the risk of ectasia in appropriately selected patients.

Sam Omar, MD, is from Advanced Vision Institute in Orlando, Florida. He does not hold a financial interest in any of the materials mentioned herein. Dr. Omar may be reached at (407) 389-0800; Omar_eye@yahoo.com
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2. Smolek MK: Interlamellar cohesive strength in the vertical meridian of human eye bank corneas. Investigative Ophthalmology & Visual Science 34:2962-2969, 1993
3. Smolek MK, Klyce SD: Is keratoconus a true ectasia? An evaluation of corneal surface area. Arch Ophthalmol 118:1179-1186, 2000
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5. Jue BD, Maurice DM: The mechanical properties of the rabbit and human cornea. J Biomechanics 19:847-853, 1986
6. Mueller LJ, Pels E, Vrensen GF: The specfic architecture of the anterior stroma accounts for maintenance of corneal curvature. Br J Ophthalmol 85:437-443, 2001 (April)