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Michael Campos, Anthony Dennison, Hong Fu; Effect of numerical aperture of a femtosecond laser system on the required pulse energy for capsulotomy and lens fragmentation. Invest. Ophthalmol. Vis. Sci. 2013;54(15):1823.
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The purpose of this study is to characterize the effect of numerical aperture (NA) of a femtosecond laser system on the required laser pulse energy to create anterior capsulotomy and lens fragmentation.
A femtosecond laser system with a wavelength of 1053 nm, pulse duration of 650 fs, and a pulse repetition rate of 15 kHz was used in this study. The NA of the system can be adjusted to either 0.4 or 0.12. The respective NA values were verified with FWHM spot diameters. For each NA, spot-to-spot separations from 5 to 10μm with 1 μm steps were tested with freshly harvested porcine globes. The corneas and irises were cut away to exclude their associated variations. The threshold energy for capsulotomy was defined as the energy needed to achieve about 80% separation from the capsular bag. The threshold energy for lens was defined as the ability to split the lens with a surgical tool along the vertical cross-chop planes irradiated by the laser focal point. Once a threshold energy value was found, it was verified on two additional porcine globes to confirm that the threshold criteria were met.
For each NA and each procedure, a table of threshold energies and spot-to-spot separations was generated. For both NA = 0.4 and NA = 0.12, the capsulotomy threshold energy as a function of the spot-to-spot separation can be fitted well with a linear function. It was found that the capsulotomy energy thresholds for NA = 0.12 are about 2.6 times those for NA = 0.4 for the respective spot-to-spot separations. For lens fragmentation with NA = 0.12, the laser pulses of 8 µJ energy (the maximum pulse energy available to this setup) were insufficient to split the lens as defined for this study. For NA = 0.4, the lens fragmentation energy thresholds are all below 8μJ. It is interesting to notice that the 10 µm spot-to-spot separation required considerably more energy than that projected by the trend derived from 5-9 µm spot-to-spot separations.
It is shown that a femtosecond laser system with NA=0.4 requires much less energy to do capsulotomy and lens fragmentation than one with NA=0.12. This, and several other factors associated to the system NA, should be taken into account when designing the NA for a femtosecond laser cataract surgery system.
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