LASIK is a well-known procedure for the correction of myopia, hyperopia, and astigmatism.1-3 Since its introduction, LASIK's favorability among refractive procedures has continued to increase. Unlike radial keratotomy (RK), in which radial surface incisions change the shape of the cornea, LASIK precisely sculpts the cornea with a laser, making it far more predictable and stable.4 And unlike PRK and other corneal surface ablation techniques, LASIK ablates the stromal bed, neatly sidestepping the pain, corneal haze, regression, and slow visual rehabilitation that result from destroying the epithelium. Finally, unlike the side effects that plagued earlier mechanical keratomileusis procedures, modern LASIK uses a hinged flap rather than a free corneal cap, avoiding irregular astigmatism, significant residual refractive error, and corneal instability.
LASIK is currently the most popular refractive procedure worldwide, with more than 3 million treatments conducted annually throughout the world. In the 20 years that have elapsed since the first procedure at the University of Crete in Greece (Figure 1), a lot has changed. Technological advances in the field of refractive surgery have contributed to the evolution of LASIK.
The first LASIK clinical trials in the United States began in 1994; US Food and Drug Administration (FDA) approval was obtained in 1999. The early Barraquer microkeratomes (Figure 2) were manual, and therefore the procedures, which required exceptional skill, were fraught with complications. These drawbacks were slowly overcome, and even the most conservative surgeons gained confidence that LASIK was a safe refractive procedure. Today, it is a secure assumption that the birth of LASIK signaled a new era in ophthalmology, one that will continue to evolve as the treatment undergoes future refinements.
New microkeratome technology, thin flaps. With the evolution of LASIK, new microkeratome technology has aimed to create thinner flaps in an attempt to avoid post- LASIK ectasia. Until recently, the ideal flap thickness was thought to be 130 µm or greater; surgeons believed this to facilitate easier intraoperative manipulations, better flap-tobed fit, fewer striae, and fewer intraoperative complications such as buttonholes or irregular flaps.5-7 However, the possibility of post-LASIK corneal ectasia due to limited residual corneal bed thickness after flap creation and ablation,8 the trend toward larger ablation zones, the availability of supplementary topography- or wavefront-guided treatments,9 and the risk of flap-induced aberrations10 have combined to encourage a shift toward thinner flaps.
Seiler and colleagues11 proposed in 1998 that 250 μm of residual corneal tissue was a safe limit for long-term biomechanical stability of the cornea. More recent reports have shown the occurrence of corneal ectasia with even more than 300 µm of residual corneal tissue after LASIK. The need for higher attempted corrections with less risk of post-LASIK ectasia has led surgeons to prefer flaps thinner than 100 µm, even though these are difficult to manage and therefore may increase the risk of flap striae and irregular astigmatism.5-7 Most surgeons now follow a conservative approach and leave at least 300 µm of residual corneal tissue. Patients today can undergo thin-flap LASIK with the Schwind Carriazo Pendular microkeratome (Schwind eye-tech-solutions, Kleinostheim, Germany); the Moria M2 microkeratome 90- μm head (Moria, Antony, France);12-13 or the femtosecond laser, which is so popular right now that it deserves its own section for discussion (see Femtosecond laser era).
New visual supportive technology. New technology to map the anterior and posterior corneal surfaces, including videokeratography, Placido-disc–based topography, Scheimpflug photography, and optical coherence tomography (OCT), contribute to more detailed screening of patients' suitability for LASIK. Such technologies are used to detect forme fruste keratoconus and other corneal surface irregularities and to minimize the percentage of post-LASIK ectasia cases.
Customized LASIK treatments. The future of LASIK, and refractive surgery in general, is based on customized treatments, especially platforms employing wavefront-guided14 principles. These treatments can minimize induced higherorder aberrations and achieve optimal vision quality postoperatively. These kinds of ablation profiles have been shown to be safe and effective, without the incidence of any associated complications.
Femtosecond laser era. The femtosecond laser (Figure 3) has rapidly gained popularity; this technology creates corneal incisions by delivering thousands of microphotodisruptive pulses focused on a precise plane of the cornea. The result is a smooth cut and a stromal flap with parallel anterior and posterior surfaces. The femtosecond laser provides a new level of precision and safety due to its characteristics of predictability, reproducibility, and fast visual rehabilitation. Femtosecond laser technology has been refined to decrease the energy necessary to incise tissues and to minimize thermal damage to the surrounding structures.
The principal application for the femtosecond laser is flap creation during LASIK.15-16 As an alternative to mechanical microkeratomes, the device seems to be effective in terms of predictability and uniformity of flap thickness. When the femtosecond laser is used, flap thickness is not affected by preoperative corneal curvature, corneal thickness, translation speed, or intraocular pressure. The flaps are clear, with no debris, minimizing the risk of infection and diffuse lamellar keratitis. The flap shape is planar (compared with meniscus flaps created by mechanical microkeratomes), inducing minimal aberrations and corneal flattening. The flap architecture practically eliminates epithelial ingrowth and the occurrence of severe dry eye.17-18 Femtosecond laser flap creation is definitely a part of the future of LASIK.
LASIK has been evolving as a refractive procedure since the beginning of its entrance into clinical practice. The future of LASIK lies within customized treatments and the use of femtosecond lasers. Today's technology provides surgeons with the means to create custom ablation profiles that consider parameters such as monovision, binocular vision, dynamic accommodation, and pseudoaccommodation. Now we understand profoundly the importance and necessity of certain aberrations and the dynamics of vision in general.
As technology improves, the goal of LASIK, as with any kind of refractive surgery, is not to produce super-vision but to better understand vision dynamics to achieve natural vision that meets patients' current and future needs.
Dimitrios I. Bouzoukis, MD, practices in the Department of Ophthalmology and the Institute of Vision and Optics, University of Crete, Greece. Dr. Bouzoukis states that he has no financial interest in the products or companies mentioned. He may be reached at tel: +30 2810 371800; fax: +30 2810 394653; email: Dbouzoukis@hotmail.com.
Harilaos S. Ginis, PhD, practices in the Department of Ophthalmology and the Institute of Vision and Optics, University of Crete, Greece. Dr. Ginis states that he has no financial interest in the products or companies mentioned. He may be reached at e-mail: firstname.lastname@example.org.
George D. Kymionis, MD, PhD, is a Lecturer in Ophthalmology at the University of Crete, Heraklion, Greece. Dr. Kymionis states that he has no financial interest in the products or companies mentioned. He may be reached at e-mail: email@example.com.
Ioannis G. Pallikaris, MD, PhD, is a Professor of Ophthalmology at the University of Crete, and Director of the Institute of Vision and Optics, Heraklion, Greece. Professor Pallikaris states that he has no financial interest in the products or companies mentioned. He may be reached at tel: +302810371800; fax: +302810394653; e-mail: firstname.lastname@example.org.
Dimitra M. Portaliou, MD, is a Clinical and Research Fellow in Institute of Vision and Optics, Heraklion, Greece. Dr. Portaliou states that she has no financial interest in the products or companies mentioned. She may be reached at e-mail: email@example.com.