Murat V. Kalayoglu, M.D., Ph.D.
Although LASIK is generally safe and wildly popular, refractive eye surgeons still sweat when using microkeratomes to create corneal flaps. The initial lamellar resection used to create the flap remains the leading cause of intra- and post-operative complications. Flap complications such as flap slippage or loss may occur in 1-2% of patients undergoing LASIK. In order to further reduce flap complications, newer microkeratomes are developed every year; indeed, there are over 20 microkeratomes currently on the market differing in hinge position (superior vs. nasal), blade-oscillation speed, suction-ring size and thickness, travel mechanism (manual vs. automated), long-term use (disposable vs. reusable), blade angle, movement (rotational vs. translational), blade material, and, if automated, motor drives for blade oscillation and travel (single vs. independent). However, although eye surgeons have made staggering advances in corneal flap creation since Jose Barraquer created his first free corneal flap 50 years ago, microkeratome-assisted flap creation is far from being perfect.
A non-mechanical alternative is available to create the LASIK flap and may avoid many of the risks associated with using the microkeratome. The technology uses an ultrashort-pulse duration laser – measured in femtoseconds, or 1x10-15 seconds - to create a flap. The femtosecond laser is different than an excimer laser, which uses an ultraviolet beam (193 nm) to photoablate corneal tissue. In contrast, the femtosecond laser uses an infrared beam (1053 nm) to cause photodisruption via laser-induced optical breakdown. When performing femtosecond laser – assisted flap creation, the fluence (defined as energy / area) at the laser focus reaches a threshold, then transforms corneal tissue from its normal state into plasma. Since the pulse energy is quickly absorbed within plasma, pressure and temperature both increase rapidly, causing expansion of tissue. Tissue expansion leads to a micro-shock wave, thereby destroying the tissue and causing formation of a cavitation bubble.
Ophthalmic photodisruption is not a new concept: it has been performed since the 1980s and is the technology behind the Nd:YAG laser. However, the resulting large shock waves and cavitation bubbles traditionally have produced too much collateral tissue damage to permit contiguous pulse-to-pulse placement. Even the nanosecond Nd:YAG laser or the picosecond Nd:YLF laser cause big shock waves. Collateral damage may be minimized by either reducing the focal spot size or decreasing the laser’s pulse duration. Focal spot size is not easily reduced beyond a few microns due to optical aberrations; however, the duration of the pulse can be reduced several orders of magnitude to the femtosecond range. The ultra-short duration of the femtosecond laser minimizes the shock and collateral tissue damage, making it ideally suited to cut a lamellar flap by contiguous photodisruption.
The femtosecond laser allows the refractive surgeon to be more flexible when considering creation of the flap diameter, flap thickness, hinge location, and hinge width. Indeed, the flap can theoretically be of any shape, size and thickness with a femtosecond laser. The borders of the flap are created by stacking the cavitation bubbles vertically until the epithelium is disrupted; vertical positioning of the flap’s borders may decrease the risk of flap displacement and epithelial ingrowth compared with using the microkeratome. In addition, the femtosecond laser couples with the cornea via a disposable suction ring and requires little vacuum compared with the microkeratome. Remarkably, the intraocular pressure is raised to only 40 mm Hg, unlike intraocular pressures reaching >70-80 mm Hg when using a conventional microkeratome.
One caveat to using the femtolaser is that relatively little data are available comparing it to the newer microkeratomes. A recent study by Kezirian et al. (J Cataract Refract Surg. 2004 Apr 30:804-11) is a welcome addition to the literature. In this study, the authors compared flaps created by the IntraLase femtosecond laser to those created by two new, popular microkeratomes. Their results showed that flaps created by the femtosecond laser had more predictable flap thickness, maintained better astigmatic neutrality and were dramatically less likely to have epithelial injury. Similar studies with larger cohorts may bolster the use of the femtolaser in refractive surgery.
For many patients, the biggest fear for refractive surgery remains “the fear of the blade”. The femtolaser offers a blade-free treatment and minimizes mechanical contact with the eye. Further reducing pulse duration may lead to in-situ stromal photodestruction, obviating the need to create flaps for a separate excimer laser treatment. For now, however, the femtolaser remains an attractive – albeit expensive – alternative to the microkeratome.