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Carolina de Freitas, Florence Cabot, Fabrice Manns, Sonia H Yoo, William W Culbertson, Jean-Marie A Parel; Effect of viscoelastic gel refractive index on femtosecond laser-assisted small-pupil cataract surgery: Optical model. Invest. Ophthalmol. Vis. Sci. 2014;55(13):4177.
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In cataract surgery for patients with high-risk of pupillary miosis, a viscoelastic is injected into the anterior chamber (AC) and an iris-ring (Malyugin ring, MicroSurgical Technology, MST) is inserted. The goal of this study was to assess, using an optical model, if changes in refractive index of the AC filled with viscoelastic can affect the outcome of fs-laser-assisted capsulotomies.
The index of refraction and chromatic dispersion of 6 viscoelastic agents were measured using an Abbe refractometer (RL Instruments, Northbridge, MA): VisCoat and ProVisc (Alcon, USA), Healon 10 (AMO, USA), EyeFill types HD, SC and C (Croma GmBH, Austria). The group refractive index of each viscoelastic was calculated from these measurements for a 1040nm fs laser wavelength and an 830nm OCT wavelength. The shift in fs laser beam focus depth produced by the change in refractive index was estimated for each viscoelastic using a paraxial model of the cornea and anterior chamber. The error in anterior capsule position measured with OCT biometry was estimated by comparing the anterior chamber depth obtained using the group refractive index of the viscoelastic and that of aqueous. An anterior chamber depth of 3.1 mm was assumed. The group refractive index of aqueous was calculated to be 1.342 at 830nm and 1.336 at 1040nm (Atchison and Smith, 2005).
The refractive indices at 589.32 nm and the group refractive indices at 1040nm and 830 nm are shown in Table 1. All materials had approximately the same refractive index and dispersion, except VisCoat, most likely due to changes in composition. The error in depth of the refilled anterior chamber ranged from -1μm to 6 μm. The viscoelastic produced a shift of the femtosecond laser ranging from -7 to 7 μm. The errors compound theoretically to result in a cut that is up to 13μm deeper than expected in the natural anterior chamber
The study demonstrates that change in refractive index due to anterior chamber refilling has a small, but possibly significant effect on capsulotomy during femtosecond laser small-pupil cataract surgery.
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