Intraocular lens calculation in extreme myopia

Abstract

To study from a theoretical viewpoint the effects in cases of extreme myopia of the different axial length (AL) behavior of current intraocular lens (IOL) calculation formulas and the problems caused by changes in lens geometry in the transition from plus to minus lenses. University Eye Hospital, Wuerzburg, Germany. A comparison of the thick-lens and thin-lens approach was made with allowance for the role of IOL constants and exemplified in model calculations based on design data of the MA60MA IOL in the power range from +5.0 to -5.0 diopters. Ray tracing was used to define model eyes for the MA60MA IOL, and the IOL constants producing the correct lens powers with the Haigis formula were determined. An equation was derived linking lens geometry represented by the positions of the principal planes with the effective thin-lens position and thus with the respective IOL constants. Because IOL geometry changes considerably at the transition from plus to minus powers, a corresponding change is obtained in the necessary IOL constants. If no allowance is made for this effect, refractive errors increasing with AL will be associated with minus-power IOLs. Plus IOLs and minus IOLs have to be characterized by different sets of IOL constants.