Increased precision of eye tracking module vital for customised ablations of large corneal areas

Dermot McGrath in Rome

THE dynamic eye tracking module of the Bausch & Lomb 217z excimer laser permits treatment of large regions of the cornea with extreme precision and minimises the risk of complications, according to Erik Mertens MD, EBOD.

Speaking at the ESCRS Winter Refractive Surgery meeting, Dr Mertens said the increasing use of small scanning spot lasers placed greater emphasis on the need for each laser pulse to be accurately directed onto the cornea, especially during customised ablation procedures.

"With the increased use of flying spot lasers, there is an increased need for precision when treating hyperopia and higher order aberrations. For a good outcome during a customised laser procedure, each laser pulse has to hit the cornea in the exact spot where and when you need it," he said.

The dynamic eye tracker also helped to allay patients’ fears about the consequences of eye movement during LASIK surgery, he added.
Dr Mertens explained that eye movements during Zyoptix laser refractive surgery were measured using a video-based eye tracker at a speed of 120Hz. The tracker works by shining infrared light on the pupil, producing a very distinct, black pupil and a light-coloured iris.

Then an infrared video camera tracks the image of the pupil moving around. The speed of the eye over the time between two camera frames is the absolute positional change of the eye divided by 8.3ms.

Dr Mertens noted that all dynamic or active eye trackers are active only to a point; they stop tracking and will not allow the laser to fire if the patient moves too far out of the range. With the 217z system, if the speed of the eye movements is higher than 24mm/s, the software interrupts laser pulsing until the eye comes back within range.

"This is vital. Because eye movements can cause displacement of single laser pulses and therefore an error in the ablation pattern, this is something that we would prefer to avoid if possible. The patient’s eye is mainly stabilised by voluntary visual fixation, but you can have involuntary eye movements, head movements, and sometimes in cases involving higher refractive corrections, fixation problems of the patient," Dr Mertens said.

Reviewing the different kinds of eye movements that cause problems during laser refractive surgery,

Dr Martens said very fast saccadic movements up to 170mm/s were beyond the range of even the fastest eye tracker.
"That is why the dynamic eye tracking module is engaged and stops the laser from firing until the eye tracker catches up. The eye tracker acts as the police officer of the laser system.

"When the dynamic eye tracker checks the speed of the eye and computes that the eye is moving faster than 24mm/s, it stops the laser. Then when the eye movement slows again, it recommences in the exact ablation position from which it left," Dr Mertens said.

US specialist Jack Holladay MD, MSEE, FACS who chaired the session, stressed the importance of understanding the difference between active and passive trackers. He cited a performance test of the B&L dynamic tracker using a single laser shot projected on a moving artificial eye which found that 4.5% of all eye movements were faster than that figure of 24mm/s.

"Active trackers move the beam to shoot in the position that you want, and a passive tracker actually turns it off and doesn’t shoot when you have it out of range or you don’t want it for whatever reason. So about 4.5% of the time, there are these very fast saccades that no tracker can keep up with and it’s smarter to turn it off than to try to stick with the eye movements during that time and have 4.5% of your shots in the wrong place," Dr Holladay said.

Dr Mertens agreed that the 4.5% of inaccurate laser spots could affect the quality of the ablation, which was important in treating higher order aberrations using a customised treatment. But he noted the effects would probably be insignificant during conventional sphere and cylinder treatments.

"Pupil dilation is another key issue. Some people have said that the eye tracker of the B&L system doesn’t work well when a pupil is dilated. Well, when I do a Zyoptix treatment, a customised ablation, I deliberately dilate the pupil and the eye tracker has no problems following the eye movement," he commented.
Dr Mertens noted that fast saccadic eye movements and fixation problems of patients could affect the accuracy of the ablation pattern, so it is important to use an eye tracker that could take these factors into account.

"When treating higher order aberrations, you need the pulses there when you want them to be. Therefore the dynamic eye tracking module reduces the potential for an error in the ablation pattern and the results are optimised," he concluded.

Erik Mertens MD, EBOD
Antwerp Eye Centre, Antwerp, Belgium
Email: e.mertens@skynet.be

Jack T Holladay MD, MSEE, FACS
Houston, Texas, US
Email: holladay@docholladay.com

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