Improved effectiveness of transepithelial PTK versus topography-guided ablation for stromal irregularities masked by epithelial compensation.

PURPOSE: To demonstrate improved effectiveness of transepithelial phototherapeutic keratectomy (PTK) where topography-guided ablation is degraded by epithelial compensation for localized irregularities.

METHODS: Artemis very high-frequency digital ultrasound (ArcScan Inc., Morrison, CO) scanning was performed on five eyes (four patients) after LASIK in which a truncated nasal flap was created but the ablation was still performed, resulting in irregular astigmatism and double vision. The stromal ablation pattern of a transepithelial PTK ablation was modeled and compared to topography-guided ablation simulations. Artemis-assisted transepithelial PTK procedures were performed in three eyes (two patients).

RESULTS: In all cases, Artemis very high-frequency digital ultrasound demonstrated a crevice on the stromal surface caused by ablation on the underside of the flap nasally, which had been compensated for by epithelial thickening, thus masking a significant proportion of the stromal surface irregularity from corneal surface topography. The transepithelial PTK ablation pattern appeared logically to reduce the ridge nasal to the crevice, whereas topography-guided ablation patterns demonstrated poor targeting of the stromal ridge. Artemis-assisted transepithelial PTK greatly reduced the stromal irregularity, resulting in smoother epithelium and topography. In one eye, the epithelium had compensated fully for the irregularity and the patient reported 95% subjective improvement. One patient (two eyes) reported little change in visual symptoms despite significant anatomical regularization due to the partial nature of epithelial compensation, demonstrating that multiple procedures may often be required.

CONCLUSION: Compensatory epithelial remodeling masks stromal irregularities from the corneal surface, meaning that topography-guided (or wavefront-guided) ablations may not provide an adequate treatment option and may potentially worsen the irregularity. The effectiveness of transepithelial PTK will depend on the proportion of epithelial masking due to remodeling, related to the rate of change of curvature of the stromal surface.