Femtosecond laser: incident energy thresholds and corneal hydration

International audiencePurpose: The interacting phenomena that happen between femtosecond lasers in edematous or pathological corneas are still not clearly known. Their characterization is fundamental to develop femtosecond laser use for penetrating and lamellar keratoplasty. The plasma threshold corresponds to the minimal energy that must be focalised into the cornea in order to induce ionization of matter. The possibility of making one corneal cut depends on plasma creation. We determined incident energy thresholds to obtain plasma in function of different corneal depths and hydrations. Methods: We use a Nd:Glass femtosecond laser, with a CPA system, and 1065 nm wavelength. The repetition rate is adjustable from 1 to 10 KHz, with maximum pulse energy of 60 µJ. The pulse delivery is done through an objective adapted on an optic table. The samples are fixed to an anterior chamber system and moved 3–dimensionally by micrometric precision motors. Human corneas are obtained from the French Eye Bank. Three corneal groups with 10 corneas each are organized depending on corneal pachymetry: Group 1: 100 µm +/– 75 µm, Group 2: 700 µm +/– 45 µm, Group 3: 500 µm +/– 25 µm. The control group is composed of silicon dioxide samples. The samples are analyzed in confocal microscopy of the experimental set up. Results: The incident energy threshold is similar in the control group and in the group 3; it ranges from 5 J/cm² on surface to 10 J/cm² at 500 µm depth. In cornea Groups 2 and 1, the incident energy threshold increases, respectively, of 5 J/cm² and of 10 J/cm² for each 100 µm of corneal depth that the laser beam travels through. Conclusions:In each given group, the incident energy threshold increases proportionally to the corneal thickness the laser beam traveled through. To the same corneal thickness, the threshold is higher in the most edematous corneas. The knowledge of these results is fundamental in order to determinate cut parameters and side effects in corneas with hydration above the normal