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Hong Fu; A Possible Root Cause for Incomplete Cornea Incision in a Femtosecond Laser Cataract Surgery. Invest. Ophthalmol. Vis. Sci. 2013;54(15):1842.
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© ARVO (1962-2015); The Authors (2016-present)
Liquid patient interface has become one of the eye stabilization methods in a femtosecond laser cataract surgery. It has been observed that, when a liquid interface is used in conjunction with a laser beam of low numerical aperture (NA), an intended penetrating cornea incision may not cut through the anterior cornea surface. The purpose of this work is to investigate the root cause responsible for this type of incomplete cornea incisions.
We consider a model with a laser focus inside a two-media structure. We use Gaussian beam waist to describe the laser focus, and use the two-media structure to simulate the water-cornea interface. We analyze the power intensity distribution in the laser focal volume for various focal depths relative to the interface.
Our analysis shows that, if the optical breakdown threshold of medium-1 is lower than that of medium-2, which is the case for water and cornea, there is a layer from anterior cornea surface to certain depth, in which the laser power density never reaches the optical breakdown threshold of medium-2. Let Δ be the thickness of this no-optical-breakdown layer, we present an analytical expression for Δ as a function of the laser wavelength, NA, pulse energy, and the optical breakdown thresholds of the two media. Using typical parameters, we estimate that Δ is about 30-50μm for NA=0.12, and about 3~5μm for NA=0.4, both depending on the laser pulse energy. It is worth pointing out that the thickness of the no-optical-breakdown layer is related to but is not equal to the thickness of the uncut layer, because tissue separation inside the no-optical-breakdown layer can be caused by the photodisruption occurring under but near this layer if the range of the photodisruption is sufficiently long, which can range from about 10 μms to a few tens of μms, depending on the laser pulse energy.
We investigated a possible root cause for a type of incomplete cornea incisions that has been observed when a liquid patient interface is used in conjunction with a low NA femtosecond laser beam: because the optical breakdown threshold of water is lower than that of the cornea, there is always a no-optical-breakdown layer in the cornea. The thickness of this layer can lead to incomplete incision for certain regime of parameters for low NA, and it is negligibly small for high NA.
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