Femtosecond laser photodisruptive effects on the posterior human corneal stroma investigated with atomic force microscopy

PURPOSE. To analyze the effects of femtosecond laser pulses on the posterior human corneal stroma with atomic force microscopy (AFM) and environmental scanning electron microscopy (ESEM). METHODS. A femtosecond laser (IntraLase iFS, Abbott, USA) was programmed to create a full posterior lamellar dissection in 9 human corneal tissues, using 3 different pulse energies (1.00 mu J, 0.75 mu J, and 0.50 mu J). Three corneal tissues were prepared in a similar fashion using a mechanical microkeratome (Mona Evolution 3, Moria, France). Six corneal tissues received an 8.00-mm diameter full cylindrical resection using either the femtosecond laser or the Barron trephine (Katena Products Inc., USA). The posterior corneal lenticules were first examined at AFM (Autoprobe CP, Veeco, USA). Both the posterior lenticules and the trephined corneal samples were scanned by ESEM (FEI Quanta 400, USA). RESULTS. Granules and crater-like features were observed on the stromal interface of all the laser dissected tissues, likely due to a secondary thermal effect of femtosecond laser dissection. Collagen fibers were seen only on samples treated with the 0.50 mu J pulse energy. Images of an even stromal surface were observed on the posterior stroma of mechanically dissected corneal samples. CONCLUSIONS. Mechanical and thermal effects, induced by femtosecond laser pulses on the human corneal stroma, were seen with AFM. Surface regularity of the photodisrupted stroma was inversely and non-linearly related to the pulse energy. The femtosecond laser provided high surface quality for lamellar resection of the posterior stroma comparable to those provided by mechanical devices.