Comparison of pachymetry measurements between the Alcon Wavelight EX500 and Sonogage Corneo-Gage plus platforms

Intraoperative optical pachymetry in photorefractive keratectomy

PURPOSE

To study the effect of variables on the accuracy and reliability of the optical pachymeter built into the WaveLight EX500 excimer laser during photorefractive keratectomy (PRK).

SETTING

John A. Moran Eye Center, University of Utah, Salt Lake City, USA.

DESIGN

Retrospective case series.

METHODS

A chart review of 352 eyes (181 patients) that had excimer laser PRK was performed. Programmed excimer laser residual stromal bed (RSB) measurements, optical pachymeter measurements after ablation, and Scheimpflug pachymetry measurements (Pentacam) at the 1-year follow-up were compared. Variables included ablation time, preoperative spherical equivalent (SE), 1-year SE, mitomycin-C use, operating room temperature and humidity, and programmed monovision.

RESULTS

The mean programmed RSB was 27 μm greater than the optical pachymetry post-ablation measurement (P < .001). Of patients with a 1-year follow-up, the 1-year Scheimpflug pachymetry RSB was 24 μm greater than the optical pachymetry post-ablation RSB (P < .001). Comparison of the programmed RSB with the optical pachymetry post-ablation RSB showed that the preoperative SE and ablation time had a Pearson correlation coefficient of -0.36 and 0.30, respectively (P < .001). There was no correlation between operating room temperature, humidity, or programmed monovision with these differences.

CONCLUSIONS

The RSB post-ablation values measured by optical pachymetry during PRK were significantly lower than the programmed excimer laser RSB value and 1-year Scheimpflug pachymetry RSB value. Intraoperative pachymetry during PRK underpredicted the actual long-term RSB thickness. The greater temporary drying effect associated with increased ablation time in higher myopic corrections might have caused this error.

Accuracy of Alcon WaveLight® EX500 optical pachymetry during LASIK

Purpose

To study the accuracy and reliability of optical pachymetry using the Alcon WaveLight EX500 during laser-assisted in situ keratomileusis (LASIK).

Materials and methods

This was a retrospective chart review of 90 eyes from 45 patients who had undergone LASIK (mean age 35.2±8.2 years; 19 males, 26 females). The WaveLight FS200 femtosecond laser was programmed to cut LASIK flaps at a desired depth of 120 μm. Optical low-coherence reflectometry (WaveLight EX500) was used to measure central corneal thickness prior to lifting the flap, and the residual stromal bed immediately after excimer ablation. Flap thickness (FT) was calculated using simple subtraction. Optical coherence tomography (OCT) was used to measure central corneal thickness, flap thickness, and residual stromal bed in the postoperative period and the results compared to intraoperative measurements.

Results

Mean programmed FS200 FT was 119 μm. Mean FT using EX500 optical pachymetry was 109 μm. The difference between FS200- programmed and EX500-measured FT was 9 μm (P<0.001). There was also a significant difference between the EX500 and OCT FT (109 μm vs 119 μm, respectively; P<0.001).

Conclusion

FT values calculated using intraoperative EX500 optical pachymetry were significantly lower than programmed FS200 values or OCT measurements.