Optical Physics of Customized Laser Ablation Profiles

Abstract

Excimer laser is used to reshape the cornea by means of photoablation and is a safe and predictable method for correcting vision. Significant advancements such as scanning spot lasers, high repetition rates and high-speed eye tracking have refined this process. Coinciding with technical advancements have been increasingly customizable and sophisticated treatment planning abilities (ablation profile algorithms) that have significantly contributed to improved predictability of post-surgical results.

This chapter describes the currently used ablation profiles with their advantages and disadvantages and provides an outlook to future methods for the calculation of ablation profiles.

The first ablation formula was the Munnerlyn Formula that estimated the amount of corneal tissue to be excised to correct lower order aberrations (LOAs), or spherocylindrical refractive errors.

A wavefront-optimized profile was first introduced around the year 2000, intending to avoid the creation of HOAs by pre-compensating for post-surgical biomechanical and epithelial remodeling changes. This proved successful in improving post-surgical visual acuity, particularly in low-contrast environments but did not improve those with significant pre-existing HOAs.

Topography-guided ablations were introduced for the treatment of highly aberrated and irregular corneas. The major advantage over wavefront-guided corrections was the ability to reduce severe corneal irregularities that produced HOAs. The treatment of such corneal irregularities requires the selection of a target shape of the postoperative cornea including radius of curvature and corneal asphericity (Q-value).

Conventional ablation profiles rely on the sole subjective and cycloplegic refraction of the individual patient. The actual interaction between the excimer laser and the cornea is another factor contributing to the asphericity of the cornea. Due to the curvature of the cornea, the posteriorly located peripheral zone is inherently further away from the laser and thus receives less energy and less ablation than intended. Advanced generations of the excimer laser have implemented a function known as radial compensation which reduces this factor.

Hyperopic ablations pose a greater challenge than myopic ablations due to the increased difficulty in ablating an annular region rather than the central cornea. Epithelial remodeling is another complicating factor. Epithelial hyperplasia occurring in the treated periphery may lead to a possible reduction of the hyperopic ablation effect over time. Larger corrections requiring deeper ablation depth are correlated to further healing responses with marked glare impacting visual acuity. Like myopic correction, initial attempts were limited to small optical zones, and improved outcomes were eventually achieved with larger optical zones.

Many studies have documented the occurrence of HOAs following surgical intervention for refractive errors. Myopic ablations mainly induce positive spherical aberration and hyperopic treatments mainly induce negative spherical aberration. Wavefront-optimized (WFO) ablation profiles were created to pre-compensate for 4th-order spherical aberration and higher-order astigmatism induced by sphero-cylindrical corrections, thereby maintaining the natural physiological state of the eye to be operated.

Pre-compensation is achieved by removing more stromal tissue peripherally than the conventional profiles, to retain the cornea’s prolate shape. Each dioptre of myopic correction induces approximately 0.1 μm of spherical aberration.

WFO treatment offers the advantage of excluding the need for expensive aberrometry and complex, time-consuming interpretation of wavefront measurement. The major disadvantages are its limitation in treating only lower-order spherocylindrical aberrations and the need to remove more tissue peripherally than the classic profiles.

The Q factor represents the asphericity of the cornea as a measured variable. In the general population, this variable was found to be between −0.8 and 0.4 with a mean value of −0.2. This value corresponds to the cornea having a slightly prolate form, flattening slightly from the centre to the periphery (Q < 0). Myopic LASIK tends to create an oblate cornea.

Wave-front guided (WFG) ablation profiles allow for sphero-cylindrical corrections with additional correction of HOAs of the total eye. Since the first wavefront treatment by Seiler in 1999, the wavefront-guided ablation profile has increasingly become a ‘gold standard’ for initial treatment with some of the laser platforms. In this procedure, information obtained from a wavefront-sensing aberrometer is electronically transferred to the treatment laser to program the ablation. The difference between the desired and the actual wavefront is used to generate a 3-dimensional map of the planned ablation.

Combining all the above (refraction, topography, tomography, wavefront data and the axial length, anterior chamber depth and lens thickness by means of 820 nm wavelength optical biometry, ray tracing profiles can be generated using the patient’s own eye measurements to create a virtual eye model on which to calculate the ablation profile.

Finally, presbyopia profiles can also be created whereby the cornea becomes either hyper-prolate or alternatively, multifocal, depending on the laser being utilised.