Like refractive surgery itself, excimer lasers did not evolve along a linear path. Their evolution, however, has helped create a viable marketplace for many laser models — none of which can truly be said to be perfect for every practice. In fact, refractive surgeons often disagree on what is the best laser. Join me in reading Cataract & Refractive Surgery Today Europe's refractive mini focus, where some of Europe's top refractive surgeons debate what constitutes the perfect laser. Perhaps this perfect laser is on the horizon in our practice.

The Past
IBM (White Plains, NY) developed the excimer laser in the 1970s to etch precise lines on tiny semiconductor chips. The laser combined argon and fluoride gases into a compound that, when molecularly excited, produced a UV beam of light. At wavelengths of 192 nm, this cold laser energy selectively removed several cell layers at a time to effectively reshape the cornea and spare underlying tissue.

In the late 1980s, ophthalmologists first used the excimer laser to alter the corneal curvature in PRK. During PRK, a small circular area of central corneal epithelium is removed manually before applying the excimer laser to the underlying cornea. PRK is effective for treating a broad range of refractive errors, but the procedure is not without problems relating to epithelial resection, healing and scarring.

LASIK combines the corneal epithelial flap of automated lamellar keratoplasty (ALK) and the stromal ablation of PRK to provide similar visual correction without adverse effects. Because the sensitive corneal top layer is not disturbed during LASIK, there is essentially no pain. Recovery is fast, and the visual improvement is immediate.

The Present
Today's excimer lasers (eg, the Ladar 6000; Alcon Laboratories, Fort Worth, Texas) (Figure 1) are wonderfully sophisticated and delicate instruments that help refractive surgeons achieve outstanding results. Excimer lasers have been called marvels of modern medicine. Although some finer points of laser physics interest only engineers, most refractive surgeons strive to understand how excimer lasers work. Why? They want to compare models and choose the laser that fits best into their practice.

Advances in laser technology have not only kept pace with surgical innovations, but at times have anticipated them. Among the more useful features of current lasers are integrated wavefront-based diagnostics and laser treatment; small beams with flying spot energy delivery; automated, ultrafast tracking to follow eye movements during treatment; and built-in registration to ensure exact overlay of the laser beam on the mapped area of the cornea in need of treatment.

Integrated laser diagnostics and treatment. These dual-function systems use an aberrometer to create a wavefront map of the eye. The map (transmitted to the laser) guides treatment. My experience is with the Ladarwave (Alcon) wavefront device, which can measure higher-order aberrations up to the eighth order. The refractive power is basically unlimited. The Ladarwave also distinguishes between the individual terms for easy clinical use. A software algorithm, modified by my own nomogram, translates the wavefront data into an ablation profile (Figure 2) for the Ladarvision laser.

The Ladarvision Customcornea system was the first LASIK procedure approved by the US Food and Drug Administration to provide this tailor-made seamless melding of corneal diagnostics and treatment addressing each eye's specific problems. In my practice, the Customcornea eyes have much less coma and spherical aberration than conventionally treated eyes, and >98% have 20/20 vision or better.

Small beam scanning lasers. More precise and accurate corneal reshaping is achieved with finer laser beams. For example, the Ladarvision 4000 has a
0.8-mm Gaussian beam that focuses the laser energy on a small area so that the target tissue is removed in tiny localized increments. Each pulse of the laser removes approximately 0.004 mm of corneal stroma. The surrounding tissues are unaffected. Excimer laser treatment does not injure, shrink or distort adjacent tissue, and combined with the precise accuracy of sculpting, this makes it an effective and accurate tool for vision correction.

The Ladarvision is a true flying spot laser. These lasers are considered superior to variable spot lasers, which in turn are superior to broad-beam lasers. Scanning lasers make for smoother ablation surfaces, fewer stromal irregularities, the capability to correct for higher-order aberrations and increased surgical precision.

Tracking. To perform wavefront-guided corrections, the laser must keep up with the eye's position at all times. The Ladarvision system uses a closed-loop tracker that samples the cellular field 4,000 times/second (Figure 3). The laser receives a still image of the eye, which is useful when the laser fires. Even eyes with perfect fixation move, and in patients who have numerous saccades, this may be problematic. Ladarvision tracking mirrors constantly monitor the position of the eye, adjust the laser beam, realign it and create a space-stabilized image before the laser fires — all at a speed of 4,000 Hz.

One of the critical elements of eye trackers is the latency period (the time it takes the laser to know where the eye is and make the adjustment). Automatic registration and fast tracking make for extremely accurate laser discharge.

Autoregistration. Registration refers to the ability of the system to superimpose the wavefront map on the treatment area. Precise registration is the key to successful treatment because if the laser spots do not land
on the exact same places where the aberrations were measured, the visual correction will be incomplete. The Ladarvision system uses limbal registration and crosshairs with sputniks to mark the 3:00 and 9:00 meridians to compensate for variable pupil dilation, cyclotorsion and eye drift when the patient is under the laser.

The Future
The Ladar 6000 improves upon the capture-match-treat technology of the Ladarvision system to correct a wide range of refractive errors. The Ladar 6000 incorporates several innovations that offer three important benefits: greater control, easier operation and significant time savings.

Increased automation. New autoregistration software aligns the wavefront profile on the eye with greater precision in a fraction of the time. Registration will be completed through an ingenious method of scleral vessel recognition and locking. Once the wavefront profile is obtained, patients may be taken directly to the laser or scheduled for surgery later; the unique scleral vessel-identification system ensures an exact match of the wavefront map to the eye during treatment. The registration process will be entirely automated, with surgeon prompts throughout for maximum efficiency.

The Ladar 6000 also offers higher illumination of the laser microscope (Figures 4,5) and a ring light array to simplify patient fixation. Off-axis lights improve the working distance for lamellar flap dissection.

Control through design. A console featuring more room for charts and files is one new feature of the Ladar 6000. The monitor is easier to orient during surgery, and a verification of alignment and proper registration is viewed as a graphic display of the wavefront image.

Faster ablation rate. The Ladar 6000 system operates at a 50% faster rate than its predecessor. The average treatment is completed in <1 minute. Faster ablation and shorter treatment times translate into improved patient flow. Additionally, a new gas cavity requires less maintenance and refills weekly rather than daily.

Refractive surgeons may disagree on what is the best laser for their practice. Until the perfect laser comes along, one that will treat all possible refractions under a wide array of conditions, the Ladar 6000 will do very well.

Joseph Colin, MD, is the chief of services, ophthalmology, at CHU Chirurgie Refractive in Bordeaux, France. Professor Colin is the cochief medical editor of Cataract & Refractive Surgery Today Europe. He did not provide financial disclosure informaton. He may be reached at joseph.colin@chu-bordeaux.fr or +33 05 56 79 56 08.