Refractive Laser Ablations on the Cornea: Understanding new Terminology in Refractive Surgery

By Renato Ambrósio Jr., MD, PhD^1,2 and Frederico Guerra, MD<sup>3

1</sup>Clinical Director of Cornea and Refractive Surgery of Instituto de Olhos Renato Ambrósio, Visare Personal Laser and Refracta-RIO

²Scientific Coordinator of the Rio de Janeiro Corneal Tomography and Biomechancis Study Group

³Anterior Segment Fellow of Instituto Benjamin Constant

Refractive Surgery has over one hundred years of history and is an exceedingly evolving field. Since the introduction of 'modern' radial keratotomy, in the early 1980s, there were major developments in this field. Corneal laser ablations, the preferred methods for refractive surgeons, represent the most common elective procedures performed in the world today. The fantastically fast expansion in concepts and technology related to such procedures directed to the development of different techniques and terminologies to describe these procedures, which may instigate confusion for the clinician. This brief review provides a critical update on the terminology used to describe the many options available for corneal laser reshaping. Typically, the 193-nm argon fluoride excimer laser is used in many available commercial platforms, but new solid state laser platform with 213-nm wavelength has been introduced in the market of corneal laser surgery.

Laser energy for reshaping the corneal stroma can be delivered on its anterior surface (surface ablation) or under a lamellar flap (lamellar ablation). Originally, only two major procedures were introduced. Photorefractive keratectomy (PRK) was the first technique performed, in which the epithelium is removed for surface ablation. The combination of Prof. J.I. Barraquer´s keratomileusis concept with laser stromal reshaping leaded to the development of laser in situ keratomileusis (LASIK). In LASIK, a flap with the epithelium and anterior stroma with 120 to 180 micron of thickness is created with the microkeratome for exposing the stroma for lamellar ablation. LASIK eclipsed PRK and still remains as the dominant procedure. The main advantage of LASIK over PRK is related to maintaining the central corneal epithelium intact, which increases comfort during the early postoperative period, allows for rapid visual recovery, and attenuates wound healing response. However, PRK remained an excellent option particularly for mild to moderate corrections. Along with the early phase of healing, haze formation associated with over 6D of myopic treatments were the major drawbacks and limitations of PRK. The advent of Mitomycin C conscious and efficacious use to prevent haze formation has made possible the treatment of higher refractive errors. Cases associated with thin corneas, recurrent erosions, or with a predisposition for eye trauma (martial arts, military, etc.) were among the most important indications.

Interestingly, due to multiple reasons including crescent reports of ectasia after LASIK (many with no known or recognized risk factors), there was a significant interest on revisiting surface ablation procedures, which have a lower impact on the biomechanical properties of the cornea. Modifications on the standard PRK technique have been proposed to improve surface ablation procedures, as the concept of Advanced Surface Ablation (ASA). These were based on advances in understanding corneal wound healing. The main concepts are that a smoother stromal surface with a healthy epithelium would enable a fast and more organized recovery with a normal basement membrane. Attempts to create an epithelial flap with viable basal cells that would be repositioned after the ablation have also been used. Such procedures were made possible because of major advances in technology and instrumentation. In LASIK, the concept to create a thinner flap, with just enough thickness to expose the cornea stroma lamellae using new microkeratomes and the femtosecond laser also fit this perception. Table 1 lists the terminology, definitions and critical considerations of these new surface ablation and lamellar procedures.

There are major advantages on doing a less invasively refractive ablation related to more biomechanical stability and more refractive predictability. Thin flap LASIK or SBK is possible with new generation microkeratomes, such as the Moria OUP (France) and the Carriazo-Pendular (Schwind, Germany). Femtosecond lasers from different companies are available and they promise the benefits of potentially being adjusted accordingly to the refractive ablation shape. Also, regarding the architecture of the flap, the femtosecond laser has a constant depth of focus, so that a planar flap with constant thickness is created as opposed to a meniscus flap as created by standard microkeratomes. Mechanical SBK with microkeratome have a semi-planar flap architecture.

Advanced surface ablation procedures are related to having a smooth corneal surface with healthy epithelial on the periphery so that the epithelial healing response is accelerated (Figure 1). In addition to the epithelial removal and ablation technique, the concept of ocular surface optimization with nutritional supplementation including omega 3 essential fatty acids (EFA) and vitamin C, along with treatment of underlying tear film dysfunction are critical requirements for success.

Figure 1. Image of both eyes of the same patient that had sequential bilateral ASA. In the right eye (image on the right), epithelial PTK removal, while the left eye had Epi-LASIK with epithelial flap off. Both eyes had epithelial healing and contact lens removal by day 5.

Correspondence:
Dr. Renato Ambrósio Jr.
Rua Visconde de Pirajá, 595/808 - Ipanema – Rio de Janeiro, RJ – Brazil
55-21-2274 5694 - renatoambrosiojr@wavefront.com..br