Keeping track of tracking and registration

The importance of both eye tracking and registration for optimal custom ablation was a recurring theme throughout the symposium. Dr. Francesco Carones provided a detailed analysis of just why tracking and registration are so important in the wavefront era.

"We now know that in order to perform effective custom ablations we need reliable and accurate measurements of both low and high order aberrations. We then need a laser capable of providing the intended ablation pattern, that is, a small flying spot laser system. Then, equally important, we need to match what the wavefront device measures with the actual treatment hence the importance of registration. Finally, we need to maintain this alignment properly with accurate eye tracking."

The first objective of the eye tracker is to align the treatment. Once the surgeon decides where to centre the ablation, one of the most important tasks of the eye tracker is to keep this position during the whole procedure. These are two different processes; eye movement detection and redirection of laser beam to follow the detected movement.

"We know the eyes are moving constantly, even in normal vision. Indeed, we know eye movement is essential for focusing. So for the patient to see the fixation light properly during the ablation, the eye needs to move. These are very small movements, the saccadic movements and high frequency tremors, but these are all necessary for the vision process."

The tracker has to follow all of these movements. But the tracker also has other purposes. One is to position each single laser spot exactly where it is intended. This becomes even more crucial when the goal is to correct higher order aberrations, because small eye movements can have large impacts on visual outcomes in such cases.

However, it is not the objective of the eye tracker to centre the procedure. It is still up to the surgeon to decide where to centre the procedure; the tracker just follows the surgeon's orders. There is still considerable debate on the appropriate point on which to centre the ablation. Different surgeons advocate either the centre of the natural pupil, the centre of the constricted pupil, or the centre of the dilated pupil.

The tracker issue rose to prominence with the debut of the flying spot laser. With earlier laser systems eye tracking was not mandatory because a large spot could fall even within a relatively small range of error without any significant change in visual outcome. But now with the use of flying spot systems, imprecise pulse positioning can lead to irregular ablation, lower accuracy, longer time to recovery and induction of higher order aberrations, Dr. Carones noted.

Not all eye trackers are created equal. The detection frequency of the LADARVision® 4000 eye tracking system, for example, is 4000 Hz, much faster than any other system. Moreover, the LADAR 4000 tracker is a closed-loop, laser-based system rather than infrared-based system. It measures the eye position four times per second, and offers automatic acquisition, real-time self-optimisation, and performance monitoring.
The slower eye tracking systems have a greater latency period. With open-loop systems with no independent tracking mirrors there are some movements that cannot be detected Such trackers are totally 'blind' at certain times and cannot compensate for the eye motion during the 'blind' moments. The closed-loop LADAR 4000 tracker has the smallest amount of delay and compensation, compared to other systems on the market, and eye movement is always followed, Dr. Carones explained.

Even very small tracking errors can produce misaligned ablations. Clinical experience indicates that even a 10% misalignment in axis can result in 30% residual cylinder aberration. This becomes more critical when the goal is to treat higher order aberrations.

The LADARVision® system optimises accuracy through three pre and intraoperative steps. First, the LADARWave™ system sensor takes a 'snapshot' of the eye with the natural pupil and defines two regions, the limbus and the pupil, to get an x-y reference position of the centre of the natural pupil related to the limbus. Second, the system determines the extent of the wavefront error in the dilated eye to get as much information as possible on a wide area than that of the undilated pupil. Immediately before getting that measurement, two Gentian dye marks are applied to the eye. This provides extremely important information for the registration process: a software line that cross the two Gentian dye marks compensates for angular misalignment errors. Third, immediately after wavefront measurement, the patient undergoes treatment. At the LADAR Vision ® 4000 monitor, the actual patient's limbus is matched with that defined at the LADARWave™ to register the measured wavefront, and the software line is oriented to cross the two dye marks for compensation of cyclotorsional errors. This allows for precise registration and alignment during the ablation.

"Effective tracking is mandatory not just for centreing the procedure but to follow all eye movements and guarantee a correct pulse positioning during the procedure. But this is not enough, particularly for correcting higher order aberrations. Registration is a fundamental step. It guarantees a precise matching of what we have measured with what we are going to treat with the laser. This option is currently only available on the LADARVision® 4000 platform," he told the symposium.