Santhiago and associates are to be commended for their paper showing the association of the percent tissue altered (PTA) for keratectasia after laser in situ keratomileusis (LASIK) in eyes with normal topography. PTA is introduced as the percentage of flap thickness plus ablation depth from preoperative central thickness. PTA was higher than 40 in 29 of 30 ectasia eyes from 16 patients (97% sensitivity) and in 19 of 174 stable LASIK eyes with over 3 years follow-up (89% specificity). PTA ≥40 was found to be the most significant predictor of ectasia risk, with significantly higher odds ratio than residual stromal bed, age, and the ectasia risk score.
We agree with the relevance of the biomechanical impact from the refractive procedure on the cornea for ectasia progression. In fact, it was already proposed that, instead of the “classic” 250 μm rule for residual stromal bed (RSB), more than half of corneal thickness should be preserved for maintaining corneal stability. Interestingly, the 2 cases with unilateral ectasia had 281 and 274 μm of RSB and PTA values of 47 and 52 in the ectatic eyes, while PTA was 38 and 37 in their stable eyes.
We also agree that there are 2 possible mechanisms for keratectasia after refractive corneal surgery. First, a susceptible cornea with weak preoperative biomechanics, such as one with mild keratoconus, is at very high risk to undergo progressive “iatrogenic” ectasia if altered by keratorefractive surgery. Second, chronic biomechanical failure may also occur owing to significant weakening caused by the procedure itself. Thereby, preoperative susceptibility or predisposition for biomechanical failure of any given cornea is dynamically mingled with the degree of biomechanical impact from the procedure, so that any cornea may undergo ectasia depending on the combination of these factors.
However, we have some considerations:
Normal topography does not exclude having mild (subclinical) ectatic corneal disease, such as the eyes with normal curvature maps from patients with keratoconus in the fellow eye. A total of 90% of such asymmetric cases, referred to as forme fruste keratoconus, have tomographic abnormailies.
Central thickness may significantly overestimate the true thinnest point, so that tomographic thinnest point value should be considered when available.
The biomechanical impact from LASIK depends on the number of lamellae that are severed. While PTA calculates similar weights for flap cut and ablation, a higher biomechanical impact on the cornea from the cut is expected, depending on flap diameter and geometry.
In a series of previously published cases, we found 13 eyes among 23 cases of ectasia after LASIK with normal bilateral curvature maps preoperatively. Normal curvature was objectively defined as regular patterns or with less than 0.5 diopter of inferior steepening without skewed radial axis, considering the high variability of subjective classifications. From those, only 38.5% (5/13) had PTA ≥40. Interestingly, only 1 eye (0.34%) from 266 stable LASIK cases had PTA ≥40. Such discrepancies in accuracy determine the need for future studies. However, enhanced ectasia susceptibility screening should integrate the impact of the procedure and preoperative corneal properties (tomography and biomechanics), using validated artificial intelligence technioques.