Trans-Epithelial Ptk Ablation Rate With The Mel 90 Excimer Laser Measured By Insight 100 Very High-Frequency Digital Ultrasound

To measure the ablation rate for trans-epithelial PTK (transPTK) treatments using the MEL 80 or MEL 90 excimer laser based on Insight 100 very high-frequency digital ultrasound (VHFDU) epithelial thickness maps.

London Vision Clinic, London, UK

Retrospective analysis of 64 eyes that underwent VHFDU-assisted transPTK to treat stromal surface irregularities masked by epithelial remodelling. VHFDU epithelial thickness maps were obtained and a series of maps was generated, showing the epithelial thickness profile expected at increasing 8-mm ablation depths. The first ablation depth was chosen to expose an area of stroma that would be identifiable when compared to the breakthrough map. The breakthrough pattern was compared to the predicted patterns to estimate the ablation depth. Epithelial ablation rate was calculated as the ratio between the laser and observed depths. The laser ablation rate was calibrated over time, and a conversion factor was used to calculate the laser depth.

The mean first target transPTK depth was 51±6 µm (range: 38 to 67 µm). The mean first transPTK laser depth value was 62±8 µm (range: 43 to 92 µm). The mean achieved epithelial removal depth was 53±7 µm (range: 38 to 75 µm). The mean ablation conversion factor used for the first ablation was 1.20±0.11 (range: 1.07 to 1.49). The mean actual ablation conversion factor based on observed breakthrough was 1.16±0.10 (range: 1.03 to 1.46).

The epithelial ablation rate for transPTK treatments with the MEL 80 or 90 excimer laser was found to be 1.16. This demonstrates the difference in ablation rate in epithelium compared to stromal tissue. Intraoperative assessment of the epithelial ablation rate enables further transPTK ablations to be calibrated individually for each eye to ensure accurate total epithelial removal.