Biometry and intraocular lens calculation in extreme myopia.

Extreme myopia with axial lengths above 30 mm is very rare (approcimately = 1.2%). Not only is there a considerable risk for a posterior staphyloma making biometry even more difficult; problems also exist in different axial length behavior of current intraocular lens (IOL) formulas. Furthermore, around approximately = 31-32 mm, the required lenses change from plus to minus power IOLs. Special problems associated with this transition were examined in this study. A comparison of the thick- and thin-lens approach with allowance for the role of IOL constants was made and exemplified in model calculations based on the Alcon MA60MA lens in the power range from +5 to -5 D. An equation was derived correlating lens geometry with the effective thin-lens position and thus with the respective IOL constant. Since a dramatic change in lens geometry occurs 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 axial length will be associated with minus-power IOLs. In conclusion, plus- and minus-IOLs need to be characterized by different sets of IOL constants.