Currently available eye trackers have made inaccurately placed excimer laser treatments avoidable circumstances in surgery. Most systems, however, continue to rely on preoperative scans to locate the ablation zone and iris, scleral, or limbal registration to safely place laser treatments. The main disadvantage to relying on preoperative scans and registration is that once laser vision correction begins, no more adjustments can be made during surgery.
The eye tracker on the Allegretto Wave Eye-Q (WaveLight AG, Erlangen, Germany) scans eye position intraoperatively and validates the placement of each laser spot at a rate of 200 to 500 Hz, matching the repetition rate of the laser. The delay between the scan and placement of the spot is less than 5 milliseconds.
PREVENTING CYCLOTORSION
There are six common causes of decentered ablations: (1) saccadic eye movements, (2) improper head alignment, (3) cyclotorsion, (4) pupil shift/centroid shift, (5) eye rolling, and (6) technical misalignment of the laser beam. Although all eye trackers compensate similarly for saccadic eye movements, there are three approaches to address cyclotorsion (ie, the rotation of the globe along the sagittal axis). The first approach is to readjust the treatment profile to the degree of cyclotorsion present; the second is to monitor the degree of cyclotorsion in the patient's eye; and the third is to prevent it.
The Allegretto Eye-Q has a unique approach to preventing cyclotorsion, which commonly occurs when the patient moves from a seated to supine position and may cause decentered ablations, over- or undercorrections, or a shift in the axis of the astigmatic treatment (Figure 1). Whereas most eye trackers reorient the ablation pattern by using the last preoperative scan to compensate for cyclotorsion, the Allegretto Eye-Q does not use a specific algorithm to adjust for cyclotorsion. Instead of allowing a preablation adjustment, the laser's NeuroTrack system (WaveLight AG) eliminates cyclotorsion at its source by controlling optokinesis, a neural response of the visual system to use spatial cues to stabilize retinal images (Figure 2). It automatically rotates the eye to compensate for head position. Therefore, decentered ablations and induced higher-order aberrations are reduced, and the retinal image quality is maintained.
The NeuroTrack system, consisting of four flashing orange-yellow lights placed around a green fixation light in a rectangular pattern (Figure 3), provides the brain with horizontal and vertical reference lines that are used to control the optokinetic reflex. The presence of this pattern either eliminates or reduces cyclotorsion to negligible levels because the patient's eye stops rotating and stabilizes in the proper position. The patient is correctly aligned when two helium-neon laser beams meet in the center of the pupil on the corneal surface and the green flashing light is superimposed onto the red corneal image. Monitoring is used to confirm proper placement, indicated when the green light is centered inside the rectangle.
PATIENT-FRIENDLY EYE TRACKER
The NeuroTrack system is easy to use. Once the patient is positioned and the machine is turned on, an operator can confirm that the patient is properly aligned by asking three questions:
- Do you see the green light?
- Do you see that four orange-yellow lights surround the green light, forming a rectangle?
- Is the green light directly in the center of the rectangle?
Once properly aligned, it is safe to proceed with the laser vision correction. The eye tracker is engaged by depressing the right-hand footpedal to activate the tracker. A red beam then appears on the cornea as a Purkinje image, indicating that the tracker is locked on the pupil.
When combined with fast treatment times (eg, 10 seconds for a -5.00 D correction), the Allegretto Eye-Q eliminates the opportunity for further misalignment due to residual reflexes. We know that this laser is reliable because of the refractive outcomes achieved during the US Food and Drug Administration (FDA) study, in which 76% of myopic patients who underwent wavefront-optimized treatment achieved a UCVA of 20/16 (Table 1). With wavefront-optimized treatment, the average amount of induced higher-order aberrations was 12%. According to WaveLight, it is the only company that has effectively demonstrated the ability to reduce higher-order aberrations.
CLINICAL OUTCOMES
Regardless of what technology is used, what truly matters is the clinical outcome. Recently, we examined our clinical results for mixed and high astigmatism to validate the effectiveness of the NeuroTrack system compared with those that use cyclotorsional eye trackers. In our study, we did not have any major over- or undercorrections. At two independent practices, the Wellington Eye Clinic in Dublin, Ireland, and Southline Laser Center in Huntsville, Australia, 20 cases of high astigmatism (more than 2.50 D) were treated without NeuroTrack and 141 cases with NeuroTrack. In 10% of eyes treated without NeuroTrack, at least 1.00 D of residual astigmatism was still present; however, the residual astigmatism in eyes treated with NeuroTrack was 1.00 D or less. Additionally, average BCVA increased by 0.6 in eyes treated with NeuroTrack and 0.37 in eyes treated without NeuroTrack. More lines were gained while using NeuroTrack.
Improved accuracy for the compensation of cyclotorsion is especially important in patients with astigmatism or irregular corneas. In the majority of my cases, I have found that custom wavefront treatments are unnecessary because patients boast good postoperative acuities after wavefront-optimized LASIK with the Allegretto Wave Eye-Q. Clinical outcomes with the Allegretto Wave Eye-Q exceed other lasers in terms of high astigmatic treatments and wavefront results. The NeuroTrack system enhances the accuracy of the Allegretto Wave Eye-Q laser, leading to better results and increased patient satisfaction with wavefront-optimized treatments.
Arthur Cummings, FRCS(Ed), is the Medical Director of the Wellington Eye Clinic, Dublin, Ireland. He states that he is an investigator and member of the beta-site group for WaveLight AG. Mr. Cummings may be reached at +353 1 2930470; abc@wellingtoneyeclinic.com.
