In 1995, the U.S. Food and Drug Administration (FDA) approved the excimer laser for use in laser vision correction, and nearly all of the initial surgical corrections employed photorefractive keratectomy (PRK). Because of slow healing, pain, and the risk of the vision-threatening haze associated with PRK, the industry quickly adapted a thin, lamellar corneal-flap procedure which became popular with both surgeon and patient alike. Laser-assisted in situ keratomileusis (LASIK) addressed the shortcomings of PRK by promising rapid visual recovery, low pain, and a low risk of haze. And despite the femtosecond laser and newer mechanical microkeratomes that offer the surgeon new choices and improvements in corneal-flap procedures, the risks associated with LASIK, as well as smoother laser ablations, have prompted a resurgence of surface ablation techniques. This discussion explores the reasons behind the resurgence and describes the surgical techniques that maximize safety and optimize visual outcomes in refractive surgeries.
Many refractive surgeons have seen at least one case of post-LASIK corneal ectasia, and they wonder whether foregoing the flap could have prevented the condition. How does the patient benefit from a flap that compromises corneal strength (Schmack et al, 2005:433-445)? Is there truly a residual stromal bed depth at which the risk of ectasia is zero? As patients opt for custom-driven ablations that remove even more corneal tissue, a greater risk of ectasia arises.
Given the concerns over post-LASIK complications, the femtosecond laser and newer mechanical microkeratomes have gained popularity. These new devices promise increased safety and accuracy. Even so, investigators have shown that both the femtosecond laser and mechanical microkeratomes induce higher-order aberrations (HOAs), though fewer HOAs were produced with laser flaps (Tran et al, 2005:97-105). On the other hand, flaps produced by laser have reportedly led to significant stromal inflammation, which likely causes increased incidence of diffuse lamellar keratitis (DLK) and transient light sensitivity (TLS).
The chance of LASIK-flap complications favors surface ablations. Good candidates for surface treatment have been patients with a high risk of flap complications—such as higher myopes in whom residual corneal bed depths may be compromised—those patients with thin corneas, and those with non-orthogonal astigmatism. In general, patients reluctant to have any refractive surgery may also favor surface treatment as it offers no risk of flap complications or HOAs.
Types of Surface Ablations
Three techniques for surface treatment are currently available: PRK, Laser-assisted epithelial keratomileusis (LASEK), and Epi-LASIK. Each method employs some method or methods of removing the corneal epithelium prior to application of the excimer laser.
PRK utilizes scraping, with or without the use of a 20% solution of alcohol (ETOH), or laser to permanently remove the 50-µm layer of epithelial cells. This bare stromal bed is then covered with a bandage contact lens as the new cells repopulate.
LASEK uses 20% ETOH applied to the corneal surface to loosen the epithelium which is then mechanically removed using a Beaver blade or other device that sweeps away the sheet of loosened cells. Following the laser ablation, this sheet of cells is replaced on top of the stromal bed and the contact lens is added. It is unknown what role this sheet of cells has on healing and surgical outcomes. The sheet is thought to prevent pain and postoperative vision disturbances by acting as a physiologic contact lens, limiting the release of cytokines which may lead to the proliferation of myofibroblasts and result in corneal haze (Pallikaris et al, 2003:1496-1501).
Epi-LASIK is the most recent and fashionable of the three techniques. The procedure depends on a mechanical epikeratome to remove the surface epithelium from Bowman’s layer in a complete, hinged sheet. This sheet is then placed back on the stromal bed after the laser ablation, providing similar benefit as afforded with LASEK. A bandage contact lens is placed on the eye until new epithelium repopulates the cornea. Four epikeratomes are currently available: EpiBlade (Advanced Medical Optics), Epi-K (Moria), Centurion (Norwood Eye Care), and EpiLift (Advanced Refractive Technologies). These units are sold to the surgeon; additional costs for suction-tubing and blades are charged per procedure.
The Resurgence of Surface Ablation Procedures
Some converts to surface ablations feel that quality of vision is the driving force, and many recognize the growing place of custom and guided ablations in refractive surgery. Custom ablations depend on slight optical variances on the corneal surface, subtleties that can be muffled by the 160-µm flap produced in LASIK procedures. In contrast, surface ablations create only a 50-µm blanket (the thickness of the epithelial layer), ensuring that more of the optical corrections reach the surface of the cornea where they do their work.
To date, there are no studies that show a definitive advantage of any surface technique over another. Using chilled Balanced Salt Solution (BSS) for pain and mitomycin C (MCC) for healing, new post-op pharmacologic manipulation clouds the influence of various surgical techniques. While PRK lost favor in the mid-1990s as a result of pain and vision disturbances such as haze, these issues are now manageable. Pain can be limited with the use of intra-operative chilled BSS, topical nonsteroidal anti-inflammatory drugs (NSAIDs), better bandage contact lenses, and narcotics. Haze can be controlled through smoother ablations profiles delivered by newer lasers, as well as the use of adjunctive mitomycin C. And because of its low cost, PRK is likely favored by the surgeon and patient alike.
As surgeons, we constantly strive to deliver the best possible care to our patients, and that means providing the safest procedures that result in optimal quality of vision. The recent resurgence in surface treatment is justified, but this view is often hard to sell to patients and surgeons who have marveled at LASIK for so many years. Risks of keratectasia, flap complications, and induction of HOAs exist whether or not the flap is created with a laser or mechanical microkeratome. While PRK may involve the most time investment postoperatively from both surgeon and patient, it requires the least capital outlay, is likely to be safest, and provides the optimal platform for custom-driven ablations.
||Schmack I, Dawson DG, McCarey BE, Waring GO, Grossniklaus HE, Edelhauser HF. Cohesive tensile strength of human LASIK wounds with histologic, ultrastructural, and clinical correlations. J Refract Surg. 2005;21:433-445.|
||Pallikaris IG, Naoumidi II, Kalyvianaki MI, Katsanevaki VJ. Epi-LASIK: comparative histological evaluation of mechanical and alcohol-assisted epithelial separation.|
J Cataract Refract Surg. 2003;29:1496-1501.
||Tran DB, Sarayba MA, Bor Z, et al. Randomized prospective clinical study comparing induced aberrations with IntraLase and Hansatome flap creation in fellow eyes: potential impact on wavefront-guided laser in situ keratomileusis. J Cataract Refract Surg. 2005;31:97-105.|
The author states that he has no financial relationship with the manufacturer or provider of any product or service discussed in this article or with the manufacturer or provider of any competing product or service.