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A comparison of axial length, anterior chamber depth, and keratometry measurements taken with the IOLMaster 500 and the NIDEK AL-scan optical biometer

Poster Details

First Author: A.Baneke UK

Co Author(s):    T. Carr   O. Mahroo   C. Hammond        

Abstract Details


Accurate biometry is critical for optimising refractive outcomes following cataract surgery. We aimed to compare measurements taken with two non-contact optical biometers: the IOLMaster 500 and the NIDEK AL-Scan.


Academic department of ophthalmology, St Thomas' Hospital, London.


As part of a wider twin eye study, eighty twin subjects underwent measurement using both the IOLMaster 500 (V. 7.5, Carl Zeiss Meditech) and the NIDEK AL-Scan Optical Biometer (V1.06.01, Nidek Co Ltd). Each subject was examined by the same operator, and scans were performed consecutively in the automated mode. Values for axial length (AL), anterior chamber depth (ACD), and mean keratometry (K) were compared (two-tailed paired t tests). Although measurements were undertaken in both eyes, only the right eye was included in the analysis to avoid confounding due to inter-eye correlation. Also, the predicted intraocular lens (IOL) power needed for emmetropia was compared between instruments (as predicted by that instrument's software using the SRK/T formula with the same A-constant of 118.8 and refractive index of 1.3375). As IOL powers are typically 0.5 D apart, we also investigated whether this might lead to a different choice of IOL for each subject.


Mean (SD) values for AL and ACD and were 23.27 (1.11) mm and 3.26 (0.34) mm for the IOLMaster, and 23.29 (1.11) mm and 3.25 (0.38) mm for the Nidek (differences not significant, p = 0.15 and 0.19 for AL and ACD comparisons respectively). Average mean (SD) K values were 43.99 (1.34) D for IOLMaster, and 44.11 (1.35) D for Nidek at 2.4 mm, and 44.06 (1.34) D for Nidek at 3.3. mm. The mean K value for the IOLMaster was significantly lower when compared to the Nidek K values both at 2.4 mm and 3.3 mm (mean difference 0.13 D and 0.08 D respectively, p < 0.0001 and p = 0.0018 respectively). Predicted lens powers for emmetropia were available for 63 subjects: mean (SD) predicted power was 21.27 (2.51) D for IOLMaster and 21.08 (2.52) D for Nidek (p < 0.0001). When aiming for refractive outcome closest to emmetropia, both machines gave identical IOL choices in 51% of eyes; the Nidek predicted an IOL of 0.5 D greater power in 3%; the IOLMaster predicted an IOL of 0.5 D greater power in 43%, and of 1 D greater power in 3%.


Whilst both machines gave very similar measurements, the mean K values were slightly, but significantly lower for the IOLMaster 500. If aiming for emmetropia, the machines predicted different choices of IOL in 49% of eyes, with the Nidek more often predicting a lower powered IOL, possibly due to measuring a steeper corneal curvature. The results show that the machines are not completely interchangeable, highlighting the importance of auditing outcomes for a particular machine. As our subjects were not patients who went on to undergo cataract surgery, our study cannot determine which machine was more accurate in terms of predicted post-operative outcome, but future studies with cataract patients could answer this question. FINANCIAL INTEREST: NONE

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