The 400 Hz Allegretto Wave Eye-Q (Wavelight Technologie AG, Erlangen, Germany) (Figure 1) boasts a 400-Hz eye tracker, while the 200 Hz laser has a 250-Hz eye tracker. Both models make sense in that the eye tracker tracks at the rate that the laser fires, and each tracker is infrared and active.

Once the eye is tracked, a pulse is fired and repeated either 200 or 400 times per second, depending on the repetition rate of the laser. The laser only fires when the eye tracker has located the center of the pupil. The tracker has three illumination pods, and hence never finds a pupil that it cannot track. If the flap is reflected into the path of one camera, the two remaining cameras will capture the pupil. The response time (ie, the time from when the pupil is tracked to when the pulse is fired) is between 6 milliseconds and 8 milliseconds.

One important feature of the Eye-Q is that the center of the ablation can be manually decentered, depending on angle kappa and angle lambda. This decentration may also occur automatically for custom procedures (eg, wavefront-guided or topography-guided).

THEN VS NOW
I have been performing LASIK since 1993, and initially—even when the laser had an eye tracker—I would complete procedures without using the tracker. Eye tracker technology was so poor that the procedure was easier and results were better when the tracker was switched off. Since obtaining my first Allegretto Eye-Q in 1999, however, I have not done one case without the tracker. First, I would not want to (because the tracker works so well and improves the result), and second, I would not even know how to switch it off. I have had an 180º switch in attitude toward eye trackers, from not wanting to use them to not wanting to do a procedure without them.

Approximately 14,000 procedures later, the eye tracker has not once failed and, in all cases, has managed to capture the pupil. This tracker captures the pupil, irrespective of whether the (1) iris is dark brown or light blue or (2) pupil is small or large. When the pupil diameter is less than 1.8 mm, the tracker may have difficulty, however, it is easily overcome by placing a single mydriatic drop in the eye. When the pupil is more than 8.5 mm under illumination—a very rare occurrence—placing a miotic drop into the eye facilitates tracking, if turning up the ambient illumination does not work.

CORRECT PLACEMENT
The 400 Hz laser fires 400 pulses per second. The pulses are 0.95 mm in diameter, and it is essential that they be correctly placed to achieve high quality results. Placement is even more important when trying to correct specific errors with topography-guided or wavefront-guided ablations. If the ablation is delivered to the incorrect area, the potential exists to make the eye worse rather than better.

Using the tracker gives the surgeon ultimate control and confidence, because you know that the pulses are going to go where you want them. The patient has a similar confidence: They know that the ablation will be placed correctly, even if they fail to keep their eye still. A very common patient question is, "Will I be able to keep my eye still?" With this tracker, the answer is "It does not matter; the tracker will track your eye and ensure that the treatment is delivered to the correct area." Patient benefits, therefore, include more accurate corrections, higher quality corrections, and all around better vision. This results in fewer enhancements for both residual refractive errors as well as quality of vision issues.

EYE-Q PEARLS
There are a number of pearls when using the Eye-Q tracker. First, there is a catch 22 situation: Because the tracker is so good, surgeons may get lazy with regard to aligning the eye correctly. Why should you be concerned about alignment when you have such a good tracker? Well, the system tracks the pupil while the laser ablates the corneal stroma, some 300 µm to 500 µm anterior to the pupil. If the patient is incorrectly aligned, the pupil will still be perfectly tracked, however, the incorrect part of the cornea will be treated. This results in decentration and subsequent coma. Therefore, it is imperative to pay attention to detail and align the eye perfectly under the tracker.

At the start of the procedure, ask the patient to look at the green flashing light directly ahead. The surgeon should see the green light reflected in the center of the pupil when the alignment is approaching adequacy. Then, the patient should adjust their head so that the green flashing light is directly in the middle of the four flashing yellow lights that are in a rectangular formation around the green light. This ensures that the x- and y-axis alignment is correct. Z-axis alignment is achieved by adjusting the height of the bed and ensuring that the two red diode lights meet centrally on the cornea, in the middle of the pupil. If the eye is perfectly aligned, the brightest red dot is reflected on the center of the cornea, and two additional red dots appear that are almost always (ie, greater than or equal to 90% of the time) on the pupil margin on the iris. The converging diode light beams meet at the corneal apex, forming the bright red dot and then cross to fall on the pupil margin at 3 o'clock and 9 o'clock—if the eye is correctly aligned. These three red dots (ie, pupil margin 3 o'clock, central cornea, and pupil margin 9 o'clock) need to be in a straight line and bisect the pupil in two equal halves. Only then is the eye perfectly aligned to the x-, y-, and z-axes.

CYCLOTORSION
The next issue is that of cyclotorsion: The tracker has a cross-hair that is projected onto the eye and can be used to help monitor cyclotorsion. One simply marks the horizontal or vertical axis at the slit lamp and then positions the eye in such a way that the cross-hair projector falls onto these marks. If the eye is cyclotorted, the head can be tilted to improve the cyclotorsional orientation.

The four yellow lights that surround the green central light flash for the duration of the treatment. Their function is, which I call for lack of a better term, neurosensory tracking. If the patient looks at the green flashing light without the four yellow lights being activated, one can often see cyclotorsion occurring. The moment that the four yellow lights are activated, however, the cyclotorsion stops or reduces greatly. The effect is so marked that I always have the four yellow lights on, irrespective of what treatment I am performing.

Recently, my colleagues and I analyzed results in patients with large cylinders (ie, more than 2.50 D) that were particularly prone to cyclotorsional errors. If cyclotorsion did not occur to a significant degree, the results would be as near as possible to perfect. If cyclotorsion was a problem, the results would be much poorer due to ablation occurring in the incorrect axis or axes.¹

Michael Mrochen, PhD, analyzed the data and concluded that our results would be better in 20% of cases if we had a perfect cyclotorsional eye tracker for cases with high astigmatism. In our results, 20% of these eyes still had a residual astigmatism of more than 0.50 D, and Dr. Mrochen believes that this group is due to cyclotorsional issues. This implies that, in 80% of cases, cyclotorsion was not an issue. Dr. Mrochen also created an astigmatism calculator from the data that predicts the following:
• If a 2.00 D was treated and 5º of cyclotorsion took place, the residual astigmatism would be 0.35 D.
• If this cylinder was 4.00 D in magnitude, the residual error would be 0.70 D.
• A 2.00 D that was corrected at 10º off-axis would leave 0.69 D of residual error behind.
• At 15º off-axis, the residual error would increase to 1.04 D, and at 20º to 1.37 D.

There is no doubt that cyclotorsion is important, especially when the astigmatism starts becoming more significant. Cyclotorsion would have an equally significant effect on custom treatments such as wavefront- or topography-guided, if they were significantly off-axis.

Arthur Cummings, MB ChB, MMed(Ophth), FCS(SA), FRCS(Ed), is consultant ophthalmologist at the Wellington Eye Clinic, in Dublin, Ireland. He states that he has no financial interest in any of the products or companies mentioned. He is a member of the CRST Europe Editorial Board. Dr. Cummings may be reached at abc@wellingtoneyeclinic.com or +353 1 6608821.

1. Mrochen M., Laser assisted in situ Keratomileusis for high myopic astigmatism with the WaveLight Allegretto Wave. J Cataract Refract Surg. 2006;782.