Femtosecond laser: cavitation bubbles analysed by confocal microscopy
Résumé
Purpose: When a femtosecond laser beam causes photodisruption, a cavitation bubble is produced. Many laser shots may be placed close together, and the induced cavitation bubbles coalesce to create a cut. Reduction of the bubble diameter is fundamental to improving the cut quality. Methods: A home–built Nd:Glass femtosecond laser is used with a CPA system and 1065 µm wavelength. The pulse repetition rate ranges from 1 to 10 KHz, with a maximum pulse energy of about 60 µJ. The laser delivery system works through an objective lens on an optical table. The samples are fixed to a 3–dimensional moving anterior chamber system, which is connected to a micrometric stepper motor. Human corneas are obtained from the French Eye Bank and are divided into 2 groups, according to their pachymetry: Group 1 (n=10): 1000 µm +/– 75 µm, and Group 2 (n=10): 700 µm +/– 45 µm. The control group is composed of silicon dioxide samples. The treated corneas are analyzed through confocal microscopy. Results: At plasma threshold, the average cavitation bubble diameter is: 6 µm +/– 7µm in the Group 1 corneas, 25 µm +/– 1.5 µm in the Group 2 corneas and 2 +/– 1 µm in the silicon dioxide samples. In group 2 corneas and silicon dioxide samples, the majority of laser pulses generated cavitation bubbles. In group 1 corneas, far fewer of the laser pulses induced cavitation bubbles. The time it takes for one cavitation bubble to disappear is: 90 +/– 10 seconds for a 25 µm cavitation bubble, 25 +/– 5 seconds for a 6 µm cavitation bubble, and 10 minutes in the silicon samples. Conclusions: In more edematous corneas, the cavitation bubble diameter is heterogeneous. This heterogeneity is due to laser beam attenuation by the corneal edema. This information suggests that, in the absence of laser parameter adaptation, the cut quality in edematous and pathological corneas will be less than in clear corneas.