Intraoperative OCT-Based Stromal Topography in Phototherapeutic Keratectomy

Objectives: This study aims to investigate whether anterior corneal curvature can be reliably measured by combined Placido-optical coherence tomography (OCT) during phototherapeutic keratectomy (PTK) despite intraoperative stromal swelling, and to quantify the immediate smoothing effect of PTK on anterior stromal higher-order aberrations (HOAs). Methods: In this prospective study, 20 eyes with epithelial basement membrane dystrophy (EBMD) undergoing PTK were examined using the MS-39© (CSO©) at four timepoints: preoperatively (t1), after epithelial debridement (t2), after PTK ablation (t3), and at 3-month follow-up (t4). Anterior curvature, corneal and stromal thickness, and epithelial thickness were analyzed across 17 corneal sectors within an 8 mm zone using two-way repeated-measures analysis of variance (ANOVA) (timepoint × sector). Anterior Zernike HOAs were computed globally, and the smoothing effect of PTK ablation was assessed by paired comparison of t2 and t3, both obtained on de-epithelialized stroma. Results: Mean central sagittal anterior curvature remained stable across intraoperative timepoints (t1: 44.27 ± 2.32 D; t2: 44.83 ± 2.75 D; t3: 45.01 ± 2.48 D; p = 0.490), despite substantial stromal swelling: Central corneal thickness increased by 114.4 ± 53.0 μm from t2 to t3 (p < 0.001), exceeding preoperative values. At 3-month follow-up, a significant flattening of -1.13 ± 1.38 D was observed (t3→t4; p = 0.002). Stromal HOA analysis revealed a significant smoothing effect of PTK ablation: total HOA root mean square (RMS) decreased from 2.28 ± 2.14 to 0.99 ± 0.46 μm (t2→t3; p = 0.001), with coma RMS showing the most consistent reduction (Δ = -0.38 ± 0.33 μm, p < 0.001; 19/20 eyes). Epithelial thickness distribution narrowed at t4 (interquartile range (IQR) -52.2%), consistent with epithelial remodeling after stromal surface regularization. Conclusions: Anterior corneal curvature remains stable during PTK despite intraoperative stromal swelling, supporting the feasibility of OCT-based intraoperative topographic assessment. In contrast, pachymetric readings are rendered unreliable by stromal hydration. Anterior stromal HOA, particularly coma RMS, may serve as a quantitative intraoperative marker of the smoothing effect achieved by PTK.