Closing the Gap

It is a common standard today to leave at least 250 µm of residual stromal bed after LASIK to avoid further corneal ectasiae.1 Formerly, Professor José I. Barraquer, MD, suggested a minimum of 300 µm of residual stromal bed for myopic keratomileusis.2 In addition to this discrepancy, these limitations indicate that patients with thin corneas and/or high ametropia may not be suitable candidates for LASIK due to a low residual stromal bed after ablation. A thinner flap, however, may help expose more stromal thickness that can be ablated, thereby expanding the population of patients that might benefit from this procedure.

Accordingly, Moria, Inc. (Antony, France), has developed a special microkeratome head with a 100-µm depth plate, designed in our clinic by Enrique Suárez, MD, to be coupled with the commercially available LSK One turbine and Evolution II unit. This device features a shorter space between the cutting edge of the blade and the cut depth plate, to an extent of 100 µm. Previously, the thinnest cut available for this microkeratome was produced by a 130-µm head, which has commonly been considered the minimum desirable flap thickness for LASIK. In the experience of our clinic (more than 43,000 procedures since 1995), however, a thinner flap may be feasible if the appropriate techniques are followed.

OUR EXPERIENCE
We have performed LASIK in more than 350 eyes using the 100-µm microkeratome head, without experiencing any major intraoperative problems. The technique we use operates in a manner similar to a standard microkeratome procedure. Because the Moria LSK One microkeratome is a manually guided instrument, the surgeon should be careful with the speed with which the microkeratome passes along the dovetails of the suction ring. This will prevent paper-thin flaps that often occur with a quick pass of the microkeratome head. We observed a few incidences of this in some of the preliminary cases, but there were no significant consequences. The microkeratome advancement must be constant and at a stable speed.

Using the new head, no free caps were obtained. One of the strong points of this type of microkeratome is the intraoperative visibility it allows. In addition to the manually guided pass, the surgeon can visually control its extension under the microscope, and can consequently stop the advancement of the head at will if the flap hinge is too small. A free cap is more likely to occur if the hinge is short, due to the easier detachment of the head in the reverse movement with the weakened resistance of the small hinge. A stop ring is also available as an additional safety feature in this microkeratome unit to avoid free caps (Figure 1).

Regarding the cut surface quality, no irregularities other than those made with standard heads were observed under the operating microscope.3 The stromal bed surface was smooth, and we noted no macroscopic chatter. We noticed a similar quality in the flap cut surface counterpart (Figure 2). The flap manipulation is also similar to that of the standard procedure.4 The thinner flap did not add any problems for flap lifting or replacement. We regularly use the Suárez flap manipulator for flap lifting. After the laser ablation, we thoroughly wash the interface with BSS (Alcon Laboratories, Fort Worth, TX), using the Vidaurri double-armed LASIK (Katena Products, Inc., Denville, NJ) cannula to eliminate debris in that area. It is important to consider that a hypothetically higher incidence of flap striae and folds may be foreseen using a thin flap. In our preliminary series, striae have been observed in some cases, but we have not compared these data with a similar myopic-range group. Most of the patients included in the 100-µm microkeratome head group were highly myopic, therefore requiring a greater depth of ablation, which has been associated with a higher incidence of flap striae, regardless of the flap thickness used. Further studies are required to assess the possible association of thin flaps and striae.

FLAP STRETCHING
One of the practical pearls for avoiding flap folds and striae relates to surgical technique. A flap-stretching maneuver should be attempted routinely in all LASIK procedures after flap replacement, and a special effort should be made in cases using this microkeratome head. We have observed that a flap stretching maneuver avoids striae in a majority of eyes. This is performed using a moist microsurgical sponge (Merocel Scientific Products, Jacksonville, FL), first in a horizontal direction from the flap hinge to the opposite end of the flap, followed by several radial movements centered on the midportion of the hinge towards various directions to cover the total circumference of the flap. The use of different applanators and other instruments developed to avoid folds and striae may also be helpful.

We routinely use a microfiltered air source over the flap at the end of the procedure to promote adhesion and to detect striae that might affect the visual outcome. As the corneal surface dries under the effect of the air, striae may become visually evident through the microscope. This technique helps to avoid postoperative surprises with striae and allows an immediate correction of such occurrences before the patient leaves the surgical suite.

Contrary to what may be expected, we did not see buttonhole or irregular flaps in our series. The incidence of epithelial defects was also similar to that found in other series with flaps of standard thicknesses. The novel 100-µm microkeratome head is a new addition to the LASIK family, and it allows a wider range of patients to be considered for the surgical correction of ametropia.

Ashley Behrens, MD, is a cornea and refractive surgery specialist at the Centro Médico Docente La Trinidad, Caracas, Venezuela, and also serves as Clinical Assistant Professor of Ophthalmology at the Hospital F.A. Rísquez in Caracas, Venezuela. He does not hold a financial interest in any of the products mentioned herein. Dr. Behrens may be reached at +58 (212) 949-6235; behrens@doctor.com
Francia Torres, MD, is a cornea and refractive surgery specialist at the Centro Médico Docente La Trinidad, Caracas, Venezuela. She does not hold a financial interest in any of the products mentioned herein. Dr. Torres may be reached at +58 (212) 949-6235; cvelasik@telcel.net.ve
Luis A. Rodríguez, MD, is a cornea and refractive surgery specialist at the Centro Médico Docente La Trinidad, Caracas, Venezuela. He does not hold a financial interest in any of the products mentioned herein. Dr. Rodríguez may be reached at +58 (212) 949-6235; cvelasik@telcel.net.ve
Enrique Suárez, MD, is Chairman of the Department of Ophthalmology at the Centro Médico Docente La Trinidad. He does not hold a financial interest in any of the products mentioned herein. Dr. Suárez may be reached at +58 (212) 949-6235; cvelasik@telcel.net.ve
1. Seiler T, Koufala K, Richter G: Iatrogenic keratectasia after laser in situ keratomileusis. J Refract Surg 14:312-317, 1998
2. Barraquer JI: Cirugía de la córnea. Instituto Barraquer de América.Tomo I Bogotá, 1989
3. Behrens A, Seitz B, Langenbucher A, et al: Evaluation of corneal flap dimensions and cut quality using a manually guided microkeratome. J Refract Surg 15:118-23, 1999
4. Suárez E, Cárdenas J: Intraoperative complications of LASIK, in Buratto L, Brint S, (eds): LASIK Principles and Techniques. Thorofare, NJ, SLACK Inc.,1998, pp 371-379