15 November 2011
Posted in
Pearls in Ophthalmology
Advanced Surface Ablation (ASA) is a term that describes advancements in photorefractive keratectomy encompassing different techniques for epithelial removal, wound healing response, and pain control. In this month’s Refractive Surgery column of Pearls in Ophthalmology, Jeremy Kieval, MD outlines and describes the history of ASA and the differences between surface ablation technique, highlighting the proposed advantages of each iteration.
-George O. Waring IV, MD
Senior Editor, Refractive Surgery Section
Pearls in Ophthalmology
Variations in Advanced Surface Ablation
Jeremy Z. Kieval, MD
Lexington Eye Associates, Lexington, MA
Introduction
The development of the excimer laser in 1983 established ablative procedures such as photorefractive keratectomy (PRK), and ultimately, laser in situ keratomileusis (LASIK) as the gold standard in refractive surgery. Though LASIK became the more popular refractive procedure,1 PRK maintains a high level of interest among patients and surgeons, as it obviates lamellar complications introduced with the creation of a stromal flap. PRK has since been transformed into to laser-assisted subepithelial keratectomy (LASEK), epipolis laser in situ keratomileusis (epi-LASIK), and several other reported techniques. Each involves the same basic principle in achieving laser vision correction with slight alterations in surgical procedure. These modifications are often classified as “advanced surface ablation” procedures, with all attempting to optimize visual outcomes while minimizing the postoperative complications that have hindered the success of PRK.
Surgical Techniques
Photorefractive Keratectomy
Photorefractive keratectomy (PRK) is a surface ablation technique involving the mechanical removal of the corneal epithelium with a surgical blade, rotating brush, or laser transepithelial ablation. Excimer laser ablation is then performed through the Bowman’s layer and corneal stroma. Although the refractive results were very good, problems with pain, delayed visual recovery, and corneal stromal opacification (haze) limited its success. In 1991, the advent of LASIK essentially limited PRK to select groups of patients seeking refractive surgery.
Laser-Assisted Subepithelial Keratectomy
The success of refractive surgery and the desire to improve surgical outcomes generated a surge in research on corneal wound healing. The understanding of the biochemical processes that occur after PRK led to the concept of an epithelial flap into the PRK technique. Similar to PRK, LASEK involved surface ablation under the epithelium. However, rather than mechanically removing the epithelium, a replaceable flap was created using a trephine filled with alcohol solution that breaks the epithelial bonds to the underlying Bowman’s layer. Similar to LASIK, the flap would then be reflected back on a hinge of tissue, eventually to be repositioned following excimer laser ablation. Initial studies suggested that patients experienced less pain, had a faster visual recovery, and developed less haze compared to standard PRK.2,3
Epipolis Laser in Situ Keratomileusis
In 2003, concerns about epithelial toxicity with alcohol during the LASEK procedure gave rise to the introduction of an epikeratome that would be used in a similar fashion as a microkeratome during LASIK. Epi-LASIK involves the creation of a precise subepithelial incision, producing a flap that did not require alcohol to disrupt the epithelial anchors. It was believed that the preservation of an epithelial flap would serve to protect the underlying corneal stroma from inflammatory mediators, albeit with fewer flap related complications than that of LASIK. Light and electron microscopy data also demonstrated that the epithelial basement membrane was preserved with an epikeratome possibly providing improved epithelial flap integrity when compared to cells exposed to ethanol.4
Other Techniques in Advanced Surface Ablation
Additional advanced surface ablation procedures have been described. Camellin and Wyler introduced a new technique they called epi-LASEK.5 The procedure combines features of both LASEK and epi-LASIK in which the epithelial flap is created using alcohol followed by an epikeratome. Lamellar epithelial debridement (LED) has also been described in the literature in which an epikeratome is used to create an epithelial flap; however, rather than repositioning the flap as done in epi-LASIK, the flap is removed. Though data has been intriguing with regard to these variations in technique, limited studies are available to allow for a thorough comparison of outcomes and complications when compared to other techniques of surface ablation.
Review of Outcomes
Though the data presented above can be found to support conflicting results, a complete review of the literature suggests that best corrected visual acuity, postoperative pain, corneal haze, and visual recovery have similar results in various techniques of surface ablation. Though there are proponents of each technique, many prospective comparative studies have not been able to demonstrate a clear advantage of any one procedure.6,7,8
Conclusion
There are inherent challenges in comparing a surgical procedure in evolution. First, is the fact that advances in technology may impact the results of one study versus another. As progress is made in surgeon experience, excimer laser technology, and surgical instrumentation, the ability to compare studies becomes increasingly difficult. Additionally, the abundance of minute variations in technique makes large, randomized, comparative studies difficult to perform or even interpret. Lastly, mention is warranted of the more recent widespread use of intraoperative mitomycin C that has improved outcomes and revitalized interest in all techniques of surface ablation.
In the end, we are left with several different surgical techniques that each accomplishes a similar outcome, with similar complication rates, and their own unique challenges. In the absence of conclusive and consistent evidence, individual surgeon experience and comfort will likely determine one’s method of choice.
References:
1 Duffey RJ, Leaming D. U.S. trends in refractive surgery: 2001 International Society of Refractive Surgery Survey. J Refract Surg 2002;18:185–188.
2 Shah S, Sebai Sarhan AR, Doyle SJ, et al. The epithelial flap for photorefractive keratectomy. Br J Ophthalmol 2001;85:393–396.
3 Lee JB, Seong GJ, Lee JH, et al. Comparison of laser epithelial keratomileusis and photorefractive keratectomy for low to moderate myopia. J Cataract Refract Surg 2001;27:565–570.
4 Pallikaris IG, Naoumidi II, Kalyvianaki MI, et al. Epi-LASIK: comparative histological evaluation of mechanical and alcohol-assisted epithelial separation. J Cataract Refract Surg 2003;29:1496–1501.
5 Camellin M, Wyler D. Epi-LASIK versus epi-LASEK. J Refract Surg 2008;24:S57–S63.
6 Litwak S, Zadok D, Garcia-de Quevedo V, et al. Laser-assisted subepithelial keratectomy versus photorefractive keratectomy for the correction of myopia. A prospective comparative study. J Cataract Refract Surg 2002;28:1330–1333.
7 Lee JB, Seong GJ, Lee JH, et al. Comparison of laser epithelial keratomileusis and photorefractive keratectomy for low to moderate myopia. J Cataract Refract Surg 2001;27:565–570.
8 O’Doherty M, Kirwan C, O’Keeffe M, O’Doherty J. Postoperative pain following Epi-LASIK, LASEK and PRK for myopia. J Refract Surg 2007;23:133– 8.
