Many people believe “a laser is a laser is a laser.” It’s understandable, given that all LASIK systems use a laser beam to reshape corneas and help correct vision problems. But there are many differences in LASIK excimer laser equipment.
* The Allegretto Wave LASIK Laser uses a small-spot Gaussian beam and follows eye movement with a closed-loop eye tracker. It has a true optic treatment zone up to 8mm.
* The VisX LASIK Laser uses a broad beam and a video open-loop tracker. It has a true optic treatment zone up to 6.5mm.
The first, and perhaps most important, difference in lasers is the eye tracker. It tracks your eye movements so that even when your eye moves during surgery -and it will- the laser beam still makes precise contact with the cornea in exactly the right spot. You may never be aware of it, but everyone’s eyes move about 100 times a second. These involuntary movements, called saccadic movements, can interfere with the placement and accuracy of the LASIK laser beam. So a tracker has to be fast enough to register these movements, and it has to track them continuously. Tracker speeds range from 60 times per second to more than 4,000 times per second, depending on the type of tracker. If the tracker is at least as fast as the eye’s movement, the likelihood is greater that the laser beam will focus precisely on the area targeted for correction.
Trackers employ open-loop vs. closed-loop technologies. Open loop video systems like the VisX follow eye drift, but shut the laser off if the eye moves beyond limits set by the doctor. No video tracker is FDA approved to improve the accuracy of the laser during surgery. A closed-loop system like the Allegretto actually measures and compensates for eye movements. By locking on to the eye, and staying locked on for the duration of the surgery, the laser can accurately predict where the eye will be at the exact moment the laser beam makes contact.
The second important thing to consider is the kind of laser beam used. They’re really not all the same. Laser beams can range from 9mm to less than 1mm in width. It’s the width of the beam that defines it as either a broad-beam or small-spot beam laser. Like a wide paintbrush, broad-beam lasers can cover a large amount of surface quickly but may not be precise and gradual in the shaping of the cornea. A small-spot beam, on the other hand, is less than 1mm in width and lets the surgeon do corneal shaping in fine, gradual, precise increments.
The third important thing to look for is the true optic treatment zone of the LASIK laser. The optic zone should match or exceed your pupil size in dim illumination so that at night, when your pupils expand, you still see through a fully corrected cornea. This will minimize any chance of glare, halos or starbursts at night. Don’t be misled by “blend zones.” These are transition areas that gradually smooth out the edges of the laser treatment area to minimize night vision problems. But blend zones don’t enlarge the actual optic treatment zone. Make sure your pupil size is measured in low light and choose a laser with an optic zone that meets or exceeds that measurement.
The Allegretto Wave system employs a flying small-spot laser beam that is 0.9 mm wide. This is important because only a very small, narrow beam permits precise, gradual corneal shaping. The beam used, called a Gaussian beam, also has a special shape. The tip is U-shaped, with smooth round corners, unlike other laser beam tips which are flat. The Gaussian beam shape aids in producing a smooth surface on the cornea. The Allegretto laser operates at a pulse rate of 200 pulses per second, which is more than twice as fast as other “scanning spot” systems.
Because the Allegretto laser treatment is so fast, the challenge of eye movement during LASIK is minimized. Still, the Allegretto’s incredibly fast and accurate eye tracking system monitors eye movement 200 times per second. That’s almost 2.5 times faster than the VisX system primarily used in Southwest Florida. It’s more than fast enough to follow, and compensate for, eye movement.
Furthermore, this is a closed-loop tracker. That means the information about eye movement is continuously relayed to the system, and the system continuously compensates for the movements, ensuring accurate placement of the laser beam. The laser radar tracker tells the laser where the eye will be when the beam is sent. To the patient undergoing LASIK surgery, the Allegretto Wave tracker should add a great degree of confidence that any movement of your eyes during the procedure will not affect the beam’s aim.
The Allegretto can extend to a true 8.0mm treatment area, with a blend zone reaching to 9.0mm. That’s larger than any other excimer laser optic zone, making Allegretto Wave the choice for people with large pupils and those concerned with night vision.