Murat V. Kalayoglu, M.D., Ph.D.
Traditionally, excimer laser treatment of the superficial cornea with photorefractive keratectomy (PRK) used to be the preferred method of refractive surgery. However, with the advent of LASIK, surgeons moved away from surface ablation and towards stromal ablation, chiefly to reduce post-operative pain and shorten the recovery period. LASIK took hold and has become the preferred refractive technique in the United States. However, LASIK also may lead to some intra- and post-operative complications, including flap wrinkling, intrastromal deposits, buttonholes and flap loss. With LASIK, there is also a chance of corneal ectasia if the flap is too thick. Furthermore, because microkeratomes often cut long corneal nerves during flap creation, some surgeons have raised concerns about the procedure causing decreased corneal sensation and dry eyes. Some of these potential complications can be avoided by using a femtosecond laser to create the flap. Nevertheless, a growing number of surgeons are re-evaluating key differences between superficial versus stromal ablation, and considering ways to combine the advantages of each technique to help their patients.
An emerging area of interest is the creation of a superficial epithelial flap; if surgeons can reliably lift only the corneal epithelium, then they could treat solely the anterior stroma. This would obviate the need for a microkeratome-assisted (or femtosecond laser-assisted) stromal cut, and avoid many of the potential complications associated with stromal surgery. Indeed, a technique called laser-assisted subepithelial keratomileusis (LASEK) was developed to reduce risks associated with LASIK but also to reduce pain and hazy vision associated with PRK. The technique was first described by Dr. Massimo Camellin at the 1999 ASCRS meeting, and consisted of using 20% ethyl alcohol for 30 seconds to create an epithelial flap. In the original technique, a spatula was used to lift the flap, and the excimer laser was then used to ablate the anterior stroma. After the ablation therapy, chilled buffered saline solution was applied to the cornea and flap, and the flap was gently re-layered back. A soft contact lens was then placed over the eye to allow healing and prevent flap displacement.
Several modifications to the original LASEK procedure have been described. For example, a “butterfly LASEK” technique, named because it creates two separate flaps from a single paracentral cut, was developed in order to preserve epithelial viability through maintaining limbal epithelial adhesion. The “cruciform” technique used a rotating microbrush to cut into the epithelium, also to improve epithelial viability through maintaining the limbal epithelial connections. A gel-assisted technique using viscous hydroxypropyl cellulose (0.3%) instead of ethyl alcohol has been developed to gently create the epithelial flap and reduce dehydration of the cornea.
One of the main concerns that surgeons have when using LASEK is that although the technique tends to produce good results, the level of reproducibility is somewhat less than that obtained with LASIK. A chief reason for this may be that LASEK requires chemical treatment of the corneal epithelium rather than a mechanical cut. In addition, patients are somewhat less comfortable post-operatively with LASEK compared with LASIK, again likely due to ethyl alcohol treatment and epithelial cell loss. A new modification of LASEK avoids chemical treatment of the superficial cornea altogether, instead using a modified keratome to create the epithelial flap. This technique, termed epi-LASIK, separates the epithelium from the stroma through passing an automated microkeratome-like device under suction. The epithelium maintains a good degree of viability, and is reflected out of the way to allow the excimer access to the anterior stroma. Once the treatment is complete, the epithelial sheet is reposited back onto the cornea. Histological examination of epi-LASIK flaps show good separation of the epithelium from the stroma, and many surgeons believe that the flaps are easier to create than with LASEK’s chemical treatment methods.
Randomized clinical trials are currently underway to compare surgeon and patient preferences, ease of use, complication rates, visual recovery and end visual acuity using epi-LASIK, LASEK and LASIK. Early results suggest that LASEK may be equivalent or superior to LASIK vis-à-vis wavefront-based clinical outcomes. LASEK may be especially valuable for patients with relatively thin corneas, since a LASIK-based stromal flap may increase the chance of corneal ectasia in these patients. These types of studies will help determine if stromal surgery is necessarily better than superficial ablations in the coming years.