Pseudophakic correction factors for optical biometry

The IOLMaster of Carl Zeiss Jena, which has recently become available, is a combined instrument for biometry and intraocular lens (IOL) planning for cataract surgery utilizing partial coherence interferometry for measuring axial length. Whereas measurement data from classical ultrasound biometry, e.g. in pseudophakic eyes, need to be corrected by +0.4 to -0.8 mm--depending on the lens material--smaller corrections are expected to be necessary in optical biometry. Correction factors for various modern IOL materials were estimated theoretically and checked in first clinical measurements.

Starting from the dispersion relation of PMMA and manufacturers' phase refractive index data at 546 nm, the group refractive indices of different IOL materials were estimated for the IOLMaster wavelength of lambda=780 nm. Then, for an average eye of 23.48 mm, the center thicknesses of emmetropizing lenses of different material were calculated. Finally, comparing the pseudophakic optical axial lengths thus deduced with the respective phakic value, individual material-specific correction factors were obtained.

Expressing the true axial length ALtrue by the length ALphak measured in phakic mode and a correction factor delta (ALtrue=Alphak+delta), we obtained values of delta=0.12+/-0.01 mm for all IOL materials considered (PMMA, silicone, collamer, AcrySof, MemoryLens). For aphakic eyes, delta=0.20 mm was deduced.

The calculations suffered from the fact that hardly any information relating to optical material specifications of IOLs is available from lens manufacturers. Therefore, calculations had to be based on--nevertheless realistic--assumptions. Early clinical results support our theoretical findings. Thus, optical biometry seems to be more reliable and forgiving in pseudophakic eyes than classical ultrasound.