Corneal laser surgery has developed continually since the introduction of PRK in 1990. In the mid-1990s, LASIK using a mechanical microkeratome to create a superficial stromal flap of the cornea and an excimer laser to perform intrastromal ablation was developed. A few years ago, the femtosecond laser was introduced as a nonmechanical technique to create the corneal flap for LASIK. Now, an all-in-one corneal laser procedure for refractive correction, refractive lenticule extraction (ReLEx; Carl Zeiss Meditec, Jena, Germany) is available. ReLEx is a new application that allows surgeons to perform complete laser vision correction procedures using only one laser platform, the VisuMax femtosecond laser (Carl Zeiss Meditec).
The typical set-up for modern corneal refractive laser surgery consists of two advanced laser systems, the excimer and the femtosecond. In some operating rooms, the patient is moved between two patient beds; in more integrated systems, the patient bed is rotated between the lasers or a smaller femtosecond laser is brought over to the eye receiving treatment. In either case, the logistics of the treatment are complex and time consuming.
Challenges associated with the use of two laser systems are resolved with the availability of the all-in-one femtosecond laser treatment of ReLEx. This procedure simplifies corneal refractive laser surgery because the corrective lenticule and overlying corneal flap are created in one step using one laser (Figure 1). This avoids movement of the patient between laser systems. After femtosecond-laser cutting of the cornea and lifting of the corneal flap—the same steps used in conventional LASIK7mdash;the femtosecond laser is used to remove the corrective lenticule, and the corneal flap is then repositioned. At this time, only the VisuMax femtosecond laser is capable of such controlled intrastromal lenticule formation in a procedure called femtosecond lenticule extraction (FLEX).
Suction pressure during laser cutting with FLEX, part of the ReLEx technology, is considerably lower than with microkeratome-based LASIK. The refractive lenticule is cut, and the corneal surface is almost untouched. Additionally, variable corneal hydration does not affect the procedure's refractive precision, as it can during excimer laser ablation in conventional LASIK. After a short learning curve with FLEX, surgeons can avoid the potential complications associated with microkeratome and femtosecond-laser–based LASIK.
We have performed FLEX in 50 eyes with moderate to high myopia (6.00 to 10.00 D). One day postoperative, BCVA is comparable to or better than that after conventional LASIK with the femtosecond laser; there is also an improvement in precision of the procedure. No surgical complications occurred, and no retreatments were necessary.
The current FLEX procedure is the first step in the transition to other corneal refractive lenticule extraction techniques. Modifications to the shape of the corrective lenticule may refine the procedure in terms of further reducing aberrations that inevitably occur after corneal refractive surgery for high myopia. Advanced in vivo confocal studies of treated corneas may also help to fine-tune the energy and spacing of applied laser pulses.
Small incision lenticule extraction (SMILE) is a modification of the FLEX procedure. Instead of a full corneal flap to remove the intrastromal lenticule, a corneal microincision is created. Only part of the anterior flap sidecut is completed to the surface (Figure 2), followed by removal of the lenticule through a 4- to 5-mm superior corneal tunnel. The theoretical advantages of SMILE are numerous: less postoperative irritation due to the small epithelial cut; the possibility for less loss of corneal sensibility; and less effect on tear production, because the small incision cuts fewer corneal nerves. Additionally, there is the potential for greater biomechanical stability and a lower risk for corneal ectasia due to the preservation of continuous anterior stromal lamellae across the cornea.
Our initial experience with FLEX in patients with moderate to high myopia produced higher precision compared with conventional LASIK. Although no patient required it in our FLEX series, retreatment may be performed when necessary using the excimer laser after manual lifting of the flap. After SMILE, surface ablation is more convenient if retreatment is necessary. Another thus far unexplored option is placing intrastromal relaxing incisions with the femtosecond laser to compensate for small refractive errors or to provide touch-up for small changes that occur with age in the other refractive components of the eye.
Jesper Hjortdal, MD, PhD, is the Director of Corneal and Refractive Surgery, Department of Opthalmology, Aarhus University Hospital, Denmark. Professor Hjortdal states that he has no financial interest in the products or companies mentioned. He may be reached at e-mail: jesper. firstname.lastname@example.org.