Analysis of the effects of Eye-Tracker performance on the pulse positioning errors during refractive surgery☆

Method to determine the centration of a lenticule of tissue extracted from a cornea

A simple and novel method to analyse the centration of a lenticule of tissue extracted from a cornea has been developed, in which the centre of "mass" of the individual differences between post and preoperative maps of several corneal metrics represents the lenticule centration and its spatial distance to a reference point (aimed centration) determines the decentration. Different parameters have been evaluated to weight the centre of "mass". The robustness of the methods has been evaluated using perturbation analysis (adding white-noise to the data) based on realistic uncertainties. A clipped analysis has been performed to prevent large, localised areas of lacking/missing data from affecting the centre of "mass". The method has been tested on a pilot cohort of clinical data showing 30% and 63% of the treatments within 200 µm of decentration for corneal thickness and refractive equivalent power, respectively. Except for anterior elevation with a total standard deviation of 17 µm, all other metrics show excellent precision of ∼5 µm. The method provides a reliable and objective way to determine the centration of a lenticule of tissue extracted from a cornea and it can be applied to any topo- or tomographic derived metric.

Symmetric offset versus asymmetric offset ablation with transepithelial refractive keratectomy

BACKGROUND

In eyes with hyperopia, astigmatism, and mixed astigmatism Transepithelial photorefractive keratectomy (TransPRK) is a modality of surface ablation surgery. We center on the corneal vertex for all our treatments (all have an offset to the center of the pupil) and wanted to compare the visual results of symmetrical profile treatments versus asymmetrical profile treatments (the center of the treatment on the vertex and the boundaries with the pupil center) using TransPRK as corneal refractive surgery.

METHODS

We retrospectively analyzed two consecutive groups of eyes treated with TransPRK in the Aurelios Augenlaserzentrum Recklinghausen: 47 eyes treated with symmetrical offset and 51 eyes treated with asymmetrical offset. The intergroup comparisons were assessed using unpaired Student's T-tests, whereas preoperative to postoperative changes were assessed using paired Student's T-tests.

RESULTS

Refractive outcomes were good for both groups. 83 and 88% of eyes were within the spherical equivalent of 0.5 D from the target in the symmetric and asymmetric offset groups, respectively. 85 and 84% of eyes had a postoperative astigmatism of 0.5 D or lower in the symmetric and asymmetric offset groups, respectively.

CONCLUSION

We have not found a significant difference in the refractive outcomes between the symmetric group and the asymmetric group of eyes treated both with TransPRK for preoperatively hyperopic or mixed astigmatism.

Eye tracking: empirical foundations for a minimal reporting guideline

In this paper, we present a review of how the various aspects of any study using an eye tracker (such as the instrument, methodology, environment, participant, etc.) affect the quality of the recorded eye-tracking data and the obtained eye-movement and gaze measures. We take this review to represent the empirical foundation for reporting guidelines of any study involving an eye tracker. We compare this empirical foundation to five existing reporting guidelines and to a database of 207 published eye-tracking studies. We find that reporting guidelines vary substantially and do not match with actual reporting practices. We end by deriving a minimal, flexible reporting guideline based on empirical research (Section "An empirically based minimal reporting guideline").

Non-contact direct selective laser trabeculoplasty: light propagation analysis

Selective laser trabeculoplasty (SLT), used to treat glaucoma and ocular hypertension, requires the use of a gonioscope placed on the cornea to visualize and irradiate the trabecular meshwork (TM). Alternatively, non-contact direct SLT (DSLT) irradiates the TM through the overlying tissues. Here we analyze this innovative procedure using analytical modeling and Monte Carlo simulations to quantify the laser energy reaching the TM through the overlying tissues. Compared with energy launched from the laser, DSLT energy transmission to the TM is 2.8 times less than SLT, which verifies the efficacy of non-contact DSLT given the lowest reported effective SLT energies.

Immediate and short term visual recovery after SmartSurfACE photorefractive keratectomy

PURPOSE

To investigate immediate and short-term visual recovery in a large cohort of 2093 myopic eyes (with or without astigmatism) treated with SmartSurf^ACE procedure, a combination of Transepithelial Photo Refractive Keratectomy (PRK) and Smart Pulse Technology (SPT, SCHWIND eye-tech-solutions GmbH, Kleinostheim, Germany).

METHODS

In this retrospective case series, post-operative outcomes were evaluated immediately after the surgery (Day 0), at day 1 and 3 months postoperatively, after myopic SmartSurf^ACE treatment with mean pre-operative spherical equivalent -4.65±2.53D and range from -16.13D to -0.13D. In all cases, pre and postoperative standard examinations were performed. The analysis evaluated preoperative Corrected and Uncorrected Distance Visual Acuity (CDVA and UDVA, respectively), and postoperative UDVA, monocularly and binocularly, immediately after the surgery, at day 1 and 3 month follow up.

RESULTS

Sixty-two percent eyes achieved monocular UDVA 20/40 or better immediately after the surgery, while 82% patients achieved binocular UDVA 20/32 or better immediately after the surgery. At 3-month postoperatively, monocular UDVA 20/25 or better was achieved in 94% eyes. Treated eyes achieved immediately after the surgery or by the next day mean UDVA 20/41±8. UDVA improved significantly from Day 1 to 3-months follow up (p<0.0001 for both OS and OD) to mean UDVA 20/21±5 (equal to preoperative CDVA 20/21±8).

CONCLUSION

Immediate and short-term visual recovery after SmartSurf^ACE PRK in our large cohort was rapid, providing functional binocular UDVA immediately after the surgery.

LASIK ablation centration: an objective digitized assessment and comparison between two generations of an excimer laser

PURPOSE

To objectively define the effective centration of myopic femtosecond laser-assisted LASIK ablation pattern, evaluate the difference between achieved versus planned excimer laser ablation centration, and compare these results from two different generations of an excimer laser system.

METHODS

The study retrospectively evaluated 280 eyes subjected to myopic LASIK. Digital image analysis was performed on Scheimpflug sagittal curvature maps (difference of preoperative to postoperative). Centration was assessed via proprietary software digital analysis of the coordinate displacement between the achieved ablation geometric center and the planned ablation center, which was the corneal vertex. Results from two different excimer laser generations (Eye-Q 400 [140 eyes] and EX500 [140 eyes]; Alcon/WaveLight, Fort Worth, TX) were compared.

RESULTS

Radial displacement was on average 360 ± 220 µm (range: 0 to 1,030 µm) in the Eye-Q 400 laser group and 120 ± 110 µm (range: 0 to 580 µm) in the EX500 laser group (P < .01). The percentage of eyes with displacement of greater than 300 µm was 52% in the Eye-Q 400 laser group and 4% in the EX500 laser group.

CONCLUSIONS

Displacement of ablation pattern may depend on the laser platform used. The improvement in the efficiency of centration indicates that newer generation excimer lasers with faster eye tracking and active centration control appear to achieve a significantly more accurate centration of myopic ablation patterns. The authors propose this novel, objective technique for laser refractive surgeon evaluation may point out significant outcome measures not currently used in standard metrics of refractive laser efficiency.

Eye-tracker-guided non-mechanical excimer laser assisted penetrating keratoplasty

PURPOSE

The purpose of the study was to implement a new eye tracking mask which could be used to guide the laser beam in automated non-mechanical excimer laser assisted penetrating keratoplasty.

MATERIALS AND METHODS

A new trephination mask design with an elevated surface geometry has been proposed with a step formation between conical and flat interfaces. Two recipient masks of 7.5/8.0 mm have been manufactured and tested. The masks have outer diameter of 12.5 mm, step formation at 10.5 mm, and slope of conical surfaces 15°. Its functionality has been tested in different lateral positions and tilts on a planar surface, and pig eye experiments. After successful validation on porcine eyes, new masks have been produced and tested on two patients.

RESULTS

The build-in eye tracking software of the MEL 70 was always able to capture the masks. It has been shown that the unwanted pigmentation/pattern induced by the laser pulses on the mask surface does not influence the eye-tracking efficiency. The masks could be tracked within the 18 × 14 mm lateral displacement and up to 12° tilt. Two patient cases are demonstrated. No complications were observed during the surgery, although it needs some attention for aligning the mask horizontally before trephination. Stability of eye tracking masks is emphasized by inducing on purpose movements of the patient head.

CONCLUSION

Eye-tracking-guided penetrating keratoplasty was successfully applied in clinical practice, which enables robust tracking criteria within an extended range. It facilitates the automated trephination procedure of excimer laser-assisted penetrating keratoplasty.