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S. Norrby, R. Guthoff, O. Stachs, C. Campbell; Exact Optical Intraocular Lens Power Calculation Based on Physical Ocular Properties Only. Invest. Ophthalmol. Vis. Sci. 2008;49(13):5654.
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© ARVO (1962-2015); The Authors (2016-present)
To investigate different approaches to intraocular lens (IOL) power calculation based on physical ocular properties only.
In a pilot study we measured preop corneal topography (both surfaces) and thickness with Pentacam; axial length (AL), anterior chamber depth (ACD) and mean corneal radius (CR) with IOLMaster; pupil size with Colvard pupillometer; and postop ACD with ACMaster, on patients who underwent routine cataract surgery and were implanted with either CeeOn 911A spherical IOL (n=7) or TECNIS Z9000 modified prolate IOL (n=11). By linear regression of preop AL, ACD, and CR against postop ACD, we obtained a formula for estimation of postop IOL position independent of refractive outcome. The corneal topography data was fitted to various shapes (spheric, conic, toric, quadric, and Zernike 4th order). Retinal thickness was calculated in accordance with a published relationship. This information, together with commonly accepted refractive indices and the exact IOL designs, was used to build a model of the pseudophakic eye in OSLO Premium. Best correction in terms of sphere, cylinder and axis was computed using the OSLO optimization utility. The calculated refraction was compared with the outcome, measured by autorefractometry, in terms of spherical equivalent (SE) by paired t-test.
The difference (calculated - outcome) in dioptres (mean ± SD) was: spheric (-0.22 ± 0.84), conic (-0.04 ± 0.91), toric (-0.22 ± 0.83), quadric (-0.03 ± 0.91), and Zernike (-0.09 ± 0.89). None was statistically different from zero. Using the A-constants posted on the ULIB web page the SRKII (-0.29 ± 1.06) and SRK/T (-0.14 ± 0.75) formulas produce similar results.
Exact optical IOL power calculation based on preop physical ocular properties result in a calculated refraction that is not different from that found postop. The standard deviation is rather large, probably due to noise in the measured input. There is indication that corneal surface fits taking asphericity into account (conic, quadric, Zernike) are more accurate (mean), though not more precise (SD), than those that do not (spheric, toric). Current formulas based on thin lens theory (e.g. SRK/T) using only AL and K-values as input appear more precise, though they require the use of constants obtained by statistical regression against refractive outcome. However, these formulas are not valid for eyes that have undergone corneal refractive surgery. For those eyes, our approach should work equally well if the formula for estimating the postop IOL position is modified to omit CR.
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