Influence of axial length on IOL constants

In IOL formulas, the lens itself is represented by specific constants. Usually, these are given by the manufacturers as averages that need customization to allow for individual measurement setups. Apart from the instrumentation, patient characteristics such as axial length distribution may influence IOL constants, as demonstrated in this study. The effect of axial length on different IOL constants was studied on model calculations in theoretical ametropic eyes derived from the standard Gullstrand eye as well as in clinical results obtained for different surgical centers within the ULIB project in optical biometry. The model calculations showed definite dependence of IOL constants of different formulas on axial length, which was strongest for the SRK II A-constant. Clinical results are in good qualitative and quantitative agreement with SRK/T A-constants varying within 0.4 D for axial length averages of 23.2 to 24.2 mm. Even if the same instrumentation is used, different IOL constants may be necessary due to different axial length means in the respective patient populations. Thus, for best refractive results, constant individualization should be done at the surgeon level. Published constants like ULIB constants are nevertheless a good starting point.