Classification of excimer laser profiles

More than 20 years have passed since the first excimer laser treatment for the correction of a refractive error was performed. The procedure was done in a myopic patient. Today, although demographic data show there are more hyperopes than myopes,1 most treatments are myopic or myopic-astigmatic corrections. The classic laser algorithms are based on Munnerlyn's formula for calculating a laser profile for spherocylindrical ablations.2 Lasers have changed from broad-beam to scanning-spot or flying-spot devices; over the years, advanced laser profiles based on different concepts have been developed. In this editorial, I would like to summarize the current laser algorithms, particularly those for myopic and myopic-astigmatic eyes. In an excellent summary of ablation profiles in corneal laser surgery, Mrochen et al.3 divide treatments into those based on the total optical system and those based on the cornea. Treatments based on the total optical system are the classic Munnerlyn's formula, wavefront-guided profiles, and wavefront-optimized profiles. Those based on the cornea are topography-guided profiles and aspheric/Q-factor-adjusted profiles. MUNNERLYN'S FORMULA The classic ablation profile for the correction of myopia and myopic astigmatism is based on the Munnerlyn formula, which removes a convex-concave lenticule of corneal tissue with spherocylindrical surfaces to remodel the corneal curvature. The laser profile is based on subjective and objective measurements of refraction. Wavefront-Guided Ablation Using a wavefront sensor (including Hartmann-Shack sensors, the Tscherning aberrometer, and scanning-slit imaging), the wavefront of the eye is measured and approximated in Zernike polynomials or Fourier analysis. The treatment goals are to correct the refractive error, minimize induction of higher-order aberrations (HOAs), and reduce preexisting aberrations. Wavefront-Optimized Laser Profile Because standard spherocylindrical excimer ablations induce positive spherical aberrations,4–6 a wavefront-optimized laser profile to preserve the eye's preexisting optical aberrations without introducing new aberrations has been developed. The natural physiological situation should be maintained after surgery; it should not change due to the refractive procedure. Topography-Guided Ablation Based on topography measurements, the elevation profile of the corneal surface is calculated. The desired corneal surface is determined with the goal of correcting the refractive error and corneal HOAs. The difference between the preexisting surface and the desired surface is used to calculate the ablation profile. Because of the wavefront component in this laser profile, the term corneal wavefront ablation is sometimes used. Aspheric or Q-Factor-Adjusted Laser Profile This treatment is designed to improve the eye's optical quality by optimizing the asphericity of the cornea. The Q-factor-based ablation profile is described by Koller et al. in this issue (pages 584–589). The Q-factor is a measure of the asphericity of the cornea. In the normal population, the mean Q-factor is –0.25, which indicates a slightly prolate shape. After determining the ideal asphericity of the cornea in a model, some researchers have suggested the optimum Q-factor should be around –0.4.7 Treatment is designed to provide each cornea with this Q-factor, independent of the overall spherical aberration of the eye. Based on 1 of the 2 concepts of current excimer ablation profiles—total optical or cornea—companies with different laser technology use various laser profiles for their treatment of refractive errors. Comparative studies of the optical quality and long-term safety of various profiles including the influence of cyclotorsional recognition, faster eye trackers, laser spot size, and the biomechanical effects of the cornea are obviously needed. Currently, there are no prospective clinical trials that investigate the possible advantages or disadvantages of the various profiles. Thus, we have not yet established the optimum laser profiles; ie, which profiles should be used in which eyes. With the worldwide effort to improve these technologies, we will see further advances in laser profiles for the correction of refractive errors and optimization of human visual acuity.