Early clinical experience is positive with the Visx ActiveTrak eye tracking system

The increasing demand for custom ablation requires excimer laser technology to continually evolve. Larger treatment zones, variable spot beams and the need for high precision and high patient expectations all underscore the need for highly accurate ablation alignment. As excimer laser technology advances, the ability to accurately compensate for these eye movements is becoming critical. Also, there has always been a clear need to compensate for eye motion in patients who have excess eye movement due to anxiety, obesity or other causes.

First-generation eye trackers provided some advantages but have fallen short of the ideal design. As a result, many ophthalmic surgeons hesitate to rely too heavily on automated eye trackers. First-generation eye tracking technology also required a dilated pupil diameter of at least 7 mm pre-imaging, and lacked 3-D capabilities.

In an effort to overcome these issues, several new trackers are now in development. Since July 2000, we have been evaluating the latest advancement in eye tracking technology, the Visx Star S3 ActiveTrak excimer laser system.

Tracker technology

---A scattergram of attempted vs. achieved correction at 1 month for untracked eyes.

The new tracker is an active, three-dimensional, infrared, obliquely angled dual camera system designed to track rapid eye movements regardless of iris color, pupil diameter or contrast. The infrared system identifies the pupillary margin by detecting the difference between the reflected infrared energy from the iris and pupil using high-speed processors. This information then controls the laser guidance system.

In the past, video tracker technology was limited by relatively low speed. The new infrared tracker operates at 60 Hz; studies show this speed is more than adequate for capturing more than 99% of all eye movements during LASIK.

Microsaccades, tremors, drifts and gross translation/rotation occur in three planes, and this infrared tracker monitors pupil location in all three axes - x, y and z - using triangulation. The z-axis tracking (toward and away from the laser) is designed to ensure that the corneal surface does not move outside the vertical focus of the beam.

Clinical experience

---A scattergram of attempted vs. achieved correction at 1 month for tracked eyes.

Although most refractive surgeons agree that ablations should be centered over the entrance pupil, some prefer to center the ablation over the visual axis. The infrared tracker will operate with surgeon-specified decentration up to 0.5 mm. Also, the tracker's cameras automatically change sensitivity depending on the room lighting, so it is important to turn the tracker on after the lights are properly adjusted and the flap is lifted. During treatment, the tracker actively compensates for eye movements within 1.5 mm of the ideal optical alignment by repositioning the beam. If the eye moves outside of this range, treatment automatically pauses until fixation is regained. Our early clinical results with the ActiveTrak have been as good as or better than those achieved with the Star S2.

Prospective evaluation

We performed a prospective evaluation of 213 myopic eyes undergoing LASIK with the Visx Star S3 ActiveTrak excimer laser. Of those eyes, 122 were treated with the tracker on and 91 were treated with the tracker off.

Surgical technique

---The new tracker is an active, 3-dimensional, infrared, obliquely angled dual camera system designed to track rapid eye movements regardless of iris color, pupil diameter or contrast.

All patients underwent the same basic technique and received the same postoperative regimen. Before surgery, the surgeon carefully focused the eyepieces. Laser calibration was performed every two patients. After patient preparation, which included topical proparacaine and lubrication with Refresh Tears, a microkeratome was used to create a superior-hinged, 180-µm-thick flap. After adjustment of hydration, the tracker was engaged and treatment proceeded according to our usual nomogram. After ablation, the flap and stromal bed were irrigated with balanced salt solution and the flap was floated back into position and smoothed over. The flap edges were dried with a Merocel sponge and the flap was allowed to dry in place. Postoperatively, patients received ofloxacin and loteprednol four times a day for 2 weeks, with Refresh Tears as needed.

Results

---During treatment, the tracker actively compensates for eye movements within 1.5 mm of the ideal optical alignment by repositioning the beam. If the eye moves outside of this range, treatment automatically pauses until fixation is regained.

One-month follow-up was available for 107 of the eyes that were tracked and 76 of the eyes that were not tracked. Table 1 shows the preoperative patient characteristics. Tables 2 and 3 show uncorrected visual acuity (UCVA) results for each group at 1 day and 1 month postop. Table 4 shows the 1 month UCVA results as a function of preoperative degree of myopia. Tables 5 and 6 show the deviation from emmetropia at 1 day and 1 month for each group. Figures 1 and 2 show a scattergram of the attempted-versus-achieved correction at 1 month for each group.

Complications

The complication rate was low. The incidence of epithelial defects was 3%, limited epithelial ingrowth 1%, microstriae 9.4%, minor interface debris 19% and stage 1 diffuse lamellar keratitis 6%. Three patients (1.4%) lost 2 lines of best corrected visual acuity (BCVA) at 1 month. All loss of BCVA was attributable to dry eye.

Conclusion and discussion

Based on this data, we feel that these early results of the Star S3 ActiveTrak laser are promising. To date, we have found the eye tracker easy to use, capable of keeping up with significant eye movement and able to lock on to the pupil virtually 100% of the time. We have not noted any decentrations on postop topography or related patient symptoms. We have also been able to track under the LASIK flap when using instruments to retract the flap, or when using suction devices to control bleeding.

In our experience, elimination of dilation has resulted in several clinical benefits, including prevention of mydriasis-related delays in the operating room, distortion or decentration related to mydriasis, an immediate return to normal vision and easier patient fixation. We believe this latter benefit is extremely important, because anything that diminishes the patient's ability to see the fixation target will reduce reproducibility of surgery.

All patients in this study had a very rapid recovery of UCVA and BCVA. There was no statistically significant difference in the results of this study with our small sample group. However, the tracked group had a statistically higher spherical equivalent preoperatively, which could account for an apparent lack of difference. Based on this study, we found no need for adjustments to our established nomogram. Further studies and longer-term follow-up are needed to determine the final refractive stability.

The true clinical benefit of accurate eye tracking is difficult to demonstrate statistically, given that it is case-dependent. However, when viewed in context of upcoming new technologies, such as custom ablations and variable spot scanning using the Star platform, we believe accurate eye tracking will become an invaluable tool that will help us realize better clinical outcomes in the years ahead.

For Your Information:

• Richard L. Lindstrom, MD, Elizabeth A. Davis, MD, and David R. Hardten, MD, FACS, can be reached at Minnesota Eye Consultants PA, 710 E. 24th St., Ste. 106, Minneapolis, MN 55420; (612) 813-3633; fax: (612) 813-3660. Drs. Lindstrom, Davis and Hardten do not have a direct financial interest in any of the products mentioned in this article. However, they are paid consultants for Visx Inc.

References:

• Hardten DR, Lindstrom RL, Chu YR. LASIK for myopic astigmatism: Results using the Visx Star and Star S2 lasers. Visx Inc. 1999.
• Wyatt HJ. The form of the human pupil. Vision Res. 1995;35(14):2021-2036.
• Wilson MA, Campbell MC, Simonet P. Change of pupil centration with change of illumination and pupil size. Optom Vis Sci. 1992;69(2):129-136.
• Walsh G. The effect of mydriasis on the papillary centration of the human eye. Ophthalmic Physil Opt. 1988;8(2):178-182.