Effect of acute biomechanical changes on corneal curvature after photokeratectomy

Unintended hyperopic shift is a common yet poorly understood complication of phototherapeutic keratectomy (PTK) that raises fundamental questions about the etiology of corneal curvature change in PRK and LASIK. We investigated the relative contributions of ablation profile and peripheral stromal thickening to intraoperative PTK-induced central flattening, and propose a biomechanical model of the acute corneal response to central ablation.

Fourteen de-epithelialized eye bank globes from seven donors underwent either broadbeam ablation (approximately 100-microm depth, no programmed dioptric change) or sham photoablation in paired-control fashion. Peripheral stromal thickness changes and the pattern of thickness loss across each ablation zone were evaluated by optical section image analysis as predictors of acute corneal flattening.

Relative to sham ablation, keratectomy caused significant anterior corneal flattening (-6.3+/-3.2 D, P = .002). Concomitant peripheral stromal thickening (+57+/-43 microm, P = .01) was a significant predictor of acute hyperopic shift (r = 0.68, P = .047). Ablation pattern bias did not consistently favor hyperopia and was a poor lone predictor of hyperopic shift.

Unintended keratectomy-induced hyperopic shift is replicable in a human donor model and is associated with significant thickening of the unablated peripheral stroma. This biomechanical response may have a considerable impact on early refractive outcomes in PTK, PRK, and LASIK.