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Hong Fu; A Physics Model for Intraocular Pressure Increase Induced by Patient Interface. Invest. Ophthalmol. Vis. Sci. 2014;55(13):1539.
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A patient interface is an opto-mechanic device used in a femtosecond laser eye surgery. It serves two functions: to stabilize the eye and to couple the laser into the eye to form a sharp focus in the surgical region. It is well known that docking a patient interface onto the eye causes a side effect - an increase in intraocular pressure (IOP). The purpose of this work is to develop a physics model to describe the key mechanisms that cause the IOP increase.
We established a physics model that includes three factors that are known to cause IOP increase: the suction, applanation, and load force. We made two assumptions: first, the eyeball is a sphere filled with incompressible fluid, and second, the eyeball surface is elastic with a uniform Young’s modulus. Under these assumptions, any deformation made to the eyeball, which can be induced by the suction, applanation, or load force, will result in a stretch of the eyeball surface to cause increase in the surface strain. By calculating the potential energy of the elastically stretched eyeball surface, we can derive the IOP change.
We derived that Δ(IOP) ≈ α (ΔVS + ΔVA + ΔVL), where Δ(IOP) is the IOP change, α is a constant proportional to the Young’s modulus and a geometric factor of the eyeball, ΔVS, ΔVA, and ΔVL are the volumetric deformations associated to the suction, applanation, and load force, respectively, which can be evaluated for given patent interface and docking parameters. Comparing with the pig eye IOP change data measured at various docking steps, we estimated that α ≈ 0.3 - 0.7mmHg/mm^3 for pig eyes. Using this model, we calculated Δ(IOP) for flat and curved patient interfaces as functions of glass-cornea contacting diameter for different radii of curvature. Without knowing the specific value of the proportional constant α, the model explains correctly the signs of Δ(IOP) at various docking steps, and points out the root causes for IOP increase as induced by each of the three sources.
We developed a physics model to describe the mechanisms for IOP increase induced by a patient interface. It explains why IOP changes the way as observed in various docking steps, and provides guidelines to the design of a patient interface with minimum IOP increase.
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