Study finds, Ratio of torus and equivalent power to refracti
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Spherical and astigmatic powers for phakic intraocular lenses are frequently calculated using fixed ratios of phakic lens refractive power to refractive spherical equivalent, and of phakic lens astigmatism to refractive cylinder. In this study, a Monte-Carlo simulation based on biometric data was used to investigate how variations in biometrics affect these ratios, in order to improve the calculation of implantable lens parameters.

A data set of over 16000 biometric measurements including axial length, phakic anterior chamber depth, and corneal equivalent and astigmatic power was used to construct a multidimensional probability density distribution. From this, researchers determined the axial position of the implanted lens and estimated the refractive spherical equivalent and refractive cylinder.

--50000 artificial data sets were used to calculate the phakic lens spherical equivalent and astigmatism required for emmetropization, and to determine the corresponding ratios for these two values.

--The spherical ratio ranged from 1.0640 to 1.3723 and the astigmatic ratio from 1.0501 to 1.4340.

--Both ratios are unaffected by the corneal spherical / astigmatic powers, or the refractive cylinder, but show strong correlation with the refractive spherical equivalent, mild correlation with the lens axial position, and moderate negative correlation with axial length.

--As a simplification, these ratios could be modelled using a bi-variable linear regression based on the first two of these factors.

Finally, when choosing high refractive power phakic IOLs, fixed spherical and astigmatic ratios should be avoided because their change might result in refractive errors of up to 0.3 D for an 8 D lens. Using a linear regression based on the refractive spherical equivalent and the axial position, both ratios can be estimated with clinically acceptable precision.