Ideally, the perfect refractive laser integrates a femtosecond laser into the body of an excimer laser. The femtosecond laser delivery system is preferably handheld and attached to an easily maneuverable articulated arm, which would allow patients to remain stationary between flap creation and ablation procedure. The surgeon could perform and control the entire procedure from a single working position without having to move about or change microscopes.

The handheld laser could be guided manually and turned around for left and right eye treatment. The femtosecond laser should create uniformly thin corneal flaps with a smooth, evenly hydrated, symmetrical stromal bed. The edges of the flap must be beveled to facilitate realignment and flap positioning. Parameters (eg, hinge position and width, flap centration and diameter) must be easily adjustable; flap thickness must be independent of corneal thickness and reproducible with narrow standard deviations. Producing consistently thin flaps allows for corneal tissue preservation and an extended treatable diopter range, increasing the base of potential LASIK candidates.

Future Models
Future excimer laser models will perform treatments with fast pulse repetition (?500 Hz). This type of treatment minimizes the effects of corneal dehydration and external influences during surgery and also improves predictability. Ablation profile precision and smoothness of the ablated surface is improved. The laser spot size can be decreased — without extending the treatment times. In addition to the medical advantages, the higher repetition rate also improves patient comfort during the laser treatment.

The main drawback of high-frequency lasers (although it could change) is the frequent replacement of the cavity, which increases overall surgery costs. This is one of the reasons why there is a sustained interest in solid-state lasers. Whether solid-state lasers will supplant argon-fluoride excimer lasers is unclear. They may be cheaper and less service intensive, however, they compete with excimer lasers that have undergone several generations of refinement.

The laser spot size will further decrease (<0.7 mm), enabling laser systems to work with a very high resolution to create the finest curvatures on the cornea, as required for customized laser vision correction. The small spot creates the desired contour of the correction profile with utmost precision, thereby minimizing the total overall ablation depth. Combining a wavefront-optimized beam profile with small laser spots allows for the creation of a large effective optical zone and a very small transition area.

Eye-Tracker Frequency
The eye-tracker frequency of a perfect laser would be in harmony with the firing frequency of the laser (latency time between image capture and firing reduced to <6 milliseconds). The tracking would still be video based, but — in standard treatments — it would take into account not only the pupil center, but also anatomical data outside the limbus (eg, conjunctival blood vessels to cope with possible cyclotorsion). Even standard treatments will need to integrate some data from the corneal topography (kappa angle) or the wavefront analysis. Laser treatments would no longer be centered on the variable pupil center but on the visual axis, as the visual axis may not be identical to the pupil center.

This ideal laser guarantees real-time monitoring of its performance (eg, spectrometry of the stromal bed to detect the presence of fluid; intraoperative noncontact pachymetry measurement of the flap and corneal bed pre- and postablation; and monitoring the tissue removed per pulse). The surgeon could establish a safer strategy, maintaining precise flap and bed measurements. At the end of the procedure, checking the flap and searching the interface for debris may be done with a built-in slit lamp.

Improved ablation profiles with this laser would minimize induced spherical aberration, attempting to preserve the original corneal shape and integrating an important wavefront feature in standard laser vision correction. Laser surgery executed without a wavefront-optimized ablation profile may result in altering the corneal asphericity in the peripheral correction area. These induced spherical aberrations in the wavefront image may result in the patient experiencing problems with night vision or under mesopic conditions.

Preventing Aberrations
Utilizing a wavefront-optimized ablation profile is designed to prevent such spherical aberrations of the cornea, thereby minimizing the risk of night vision problems or halos. Even standard treatments will be centered on the visual axis and not on the pupil center, meaning integration of corneal topography data. Wavefront data will be integrated with corneal topography data, resulting in one customized treatment profile. Combined with postoperation simulation software, this will enhance the precision and predictability of the refractive results when retreating irregular corneas or treating irregular virgin corneas.

Adjustment of corneal asphericity — described by its Q-value — may influence patients' sight in mesopic and scotopic conditions. Some aspheric ablation profiles have multifocal characteristics and will probably be used in the presbyopic patient. Q-value adjustment allows the physician to customize the asphericity of the cornea. Patients will need to choose the type of correction that is best for them. Presbyopic patients may want to retain some higher-order aberrations for greater depth of field, while younger patients will choose for the sharpest distance vision with minimal optical aberrations.

JerÙme C. Vryghem is from the Brussels Eye Doctors in Brussels, Belgium. He may be reached at j.c.vryghem@vryghem.be or +32 2 741 69 99.