Pseudophakic correction factors for optical biometry

Introduction: The IOL-Master 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 f or measuring axial length. Whereas measurement data from classical ultrasou nd 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 ma terials were estimated theoretically and checked in first clinical measurem ents. Methods: Starting from the dispersion relation of PMMA and manufactur ers' phase refractive index data at 546 nm, the group refractive indices of different IOL materials were estimated for the IOLMaster wavelength of lam bda =780 nm. Then, for an average eye of 23.48 mm, the center thicknesses o f emmetropizing lenses of different material were calculated. Finally, comp aring the pseudophakic optical axial lengths thus deduced with the respecti ve phakic value, individual material-specific correction factors were obtai ned. Results: Expressing the true axial length AL(true) by the length AL(ph ak) measured in phakic mode and a correction factor Delta (AL(true) Al-phak +Delta), we obtained values of Delta =0.12 +/-0.01 mm for all IOL materials considered (PMMA, silicone, collamer, Acrysof, MemoryLens). For aphakic ey es, Delta =0.20 mm was deduced. Discussion: The calculations suffered from the fact that hardly any information relating to optical material specifica tions of IOLs is available from lens manufacturers. Therefore, calculations had to be based on-nevertheless realistic assumptions. Early clinical resu lts support our theoretical findings. Thus, optical biometry seems to be mo re reliable and forgiving in pseudophakic eyes than classical ultrasound.