Effect of Corneal Hydration on the Quality of the Femtosecond Laser Anterior Lamellar Cut

The goal of this study was to assess the effect of corneal hydration on the quality of the femtosecond laser (FSL) anterior lamellar cut. The Visumax FSL was used to dissect an 8-mm-diameter corneal flap in 22 eye bank corneas showing various levels of hydration. The intended ablation depth was 220 mm in all eyes, which corresponded to the maximal depth available with this laser. After the cut, the achieved ablation depth was measured using optical coherence tomography images, flap separability was assessed by measuring the mean force generated to detach the flap, and stromal bed roughness was assessed by measuring the Haralick contrast level on the 1000 x scanning electron microscopy images of the ablated surfaces. The preoperative central corneal thickness ranged from 547 to 1104 mu m (mean +/- SEM: 833 +/- 30 mu m). A negative correlation was found between the level of corneal hydration and the ablation depth measured in the midperipheral cornea (r = -0.626, p = 0.003), the ablation being more superficial in more edematous corneas. The Haralick contrast also tended to increase as a function of corneal hydration ( r = 0.416, p = 0.061), suggesting that laser ablation in edematous corneas results in rougher stromal surfaces. These results support the hypothesis that the quality of the FSL lamellar cut decreases as the level of corneal hydration increases. Although FSL is still considered in the field as the tool of the future for corneal dissection, a better understanding of the limits of this tool will be needed before it can replace manual or automated stromal dissection techniques in hydrated corneas

Representative scanning electron microscopy images of the post-ablation stromal surface for various levels of preoperative corneal hydration.

<p>CCT = 598 µm (A); CCT = 801 µm (B); CCT = 996 µm (C); and CCT = 1104 µm (D). These images illustrate the difficulty of quantifying corneal surface roughness based on human eye perception. The cornea illustrated in (A) had a preoperative thickness within the normal range and was used as a reference.</p

Quality of the laser cut.

<p>Effect of corneal hydration on the achieved ablation depth in the central and peripheral cornea (A and B), flap separability (C) and surface roughness (D). Surface roughness as a function of flap separability (E) is also illustrated.</p