THE EXCIMER LASER - Star S4 Laser

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The Visx Star S4 with Active Trak and Auto-centering is the most recent and state-of-the-art laser on the US market today, and can be used for both conventional and custom LASIK, PRK, and LASEK. We at Grendahl Eye associates have been using this since June 2002. The tracking device follows extremely small eye movements enabling the surgeons at Grendahl Eye associates to fine tune ablations done for PRK, LASIK and LASEK to achieve improved outcomes. The Visx Star S4 Active Trak uses a video-based tracker composed of two infrared light sources and two cameras that establish a 3-D coordinate system on the eye, allowing it to track an eye in the z-axis ( in and out) as well as in the x- axis (left or right) and in the y-axis (up and down). The cameras are connected to the computer in the laser. When the eye moves to the right, left, up, down or in and out the computer directs the laser beam to move to the right, left, up, down or in and out by the same amount that the eye moved.

The Visx Star S4 Active Trak provides direct pupil measurement in real time. The laser shines the infrared light on the eye, and the computer performs a centroid calculation to find the center of the pupil. Even if the pupil changes size during the procedure due to varying lighting conditions this calculated pupil center won't change. It has a major advantage over other laser tracking systems because it dose not require the pupil to be dilated. Because the position of the pupil is measured directly by the infrared camera, this system is much more accurate.

Just centering the laser treatment on the pupil will not ensure that the treatment zone is centered after the laser surgery. Beam shape and placement on the cornea both will affect the outcome. The shape of the laser beam that reaches the corneal plane can differ significantly from the shape of the laser beam that leaves the laser if the eye is looking to far off the patient central fixation light. The beams angle relative to the corneal and iris plane is a key factor in centration of the treatment. All trackers track the iris plane and not the corneal surface, in order to center the laser beam over the pupil. When the patient does not fixate properly on the central fixation light in the laser, the eye drifts or moves off to one side. Under this condition the cornea moves a greater distance than the iris does (parallax). At this point, pupil centration is no longer accurate and the laser beam falling onto the cornea is ovalized (oval shaped) because of the increased angle.

When this occurs the laser pauses and will not firer until the patient refixates on the central fixation light in the laser. In order to ensure proper laser beam placement and avoid tilted oval shapes, the tracker has preset boundaries to prevent the laser from firing if the eye moves too far. These types of passive limitations are designed into the Visx Star S4 Active Trak. When the laser firers, it hits perpendicular to the corneal plane. This allows a uniform ablation pattern on the cornea. As the eye moves within the pause limits set in the lasers computer, the beam is angled and moved to the side to compensate for the eye movement. If the eye motion is too great the laser pauses so that the beam profile is not compromised. The passive limits prevent the beam from extending outside an optimal boundary.

The Active Trak's passive limitations enable the tracking to stay active following small eye movement, but stop the laser from firing if the eye moves more than 1.5mm. from the staring central position. This enables the tracker to maintain a direct relationship between the tracking of the iris and the target ablation zone on the cornea.

In addition to the Active Trak technology, the VISX Star S4 has Auto-centering capabilities as well. The Auto-centering device in the Star S4 allows the treatment to be automatically centered over the pupil. This assures that misalignment will not occur, and is thus safer and more accurate during laser vision correction procedure as a result. Both the Active Trak and Auto-centering capabilities embedded into the Star S4 allow the patient to relax during their procedure, knowing that they are receiving the safest and most accurate treatment available.