The new fully automated Eyestar 900 uses SS-OCT technology to provide biometric measurements with the imaging of the anterior segment structures. The purpose of this study was to evaluate its repeatability and to evaluate the agreement with another well-used biometer, the Lenstar LS 900 (based on OLCR technology). The results obtained showed excellent repeatability (the CoV for all the parameters measured with Eyestar 900 and Lenstar LS 900 were lower than 1% and 2%, respectively) and good agreement between both instruments (the interval of the LoA was < 0.6 D for K1 and K2, 15.78 µm for CCT, 0.21 mm for ACD, 0.34 mm for LT, and 0.08 mm for AL.). To the best of our knowledge, this study is the first to compare the two devices.

The outcomes found in our study revealed that both instruments showed high levels of repeatability (Table 1). For the two biometers, AL is the parameter with the best repeatability (CoV: 0.03%–0.06% and CoR: 0.022–0.040 mm) while CCT was the parameter that had the lowest repeatability (CoR: 4.320–8.113 mm). There are numerous publications done using the Lenstar LS 900 on different type of eyes. Some of these publications agree with our results. Shammas and Hoffer [11] for example, evaluated 37 cataract eyes, reporting CoV values ranging from 0.001% to 0.006% for the different parameters analyzed in our study. These authors concluded that the precision of the measurements was very high. Specifically, in another study, Chen et al. [12] studied 40 eyes and found Sw values of 3.10 μm for CCT, 0.02 mm for ACD and 0.17 D for Kmean; and Zhao et al. [13] reported higher values of Sw for 56 myopic eyes (0.018 mm for AL, 0.052 mm for ACD, 0.181 D for K1, 0.301 D for K2 and 14.244 μm for CCT). In another group of 33 myopic eyes, Shen et al. [14] found Sw values of 0.016 mm for AL, 0.009 mm for ACD, 0.014 mm for LT and 1.982 μm for CCT, respectively. CoV values were small and varied from 0.3%–0.5%, except for AL, which was 0.06% (being in this case the same reported by us). McAlinden et al. [15] reported in 102 patients Sw (and CoR, in brackets) values of 0.02 (0.05) mm for AL, 0.02 (0.06) mm for ACD, 0.11 (0.29) D for K1 and 0.13 (0.36) D for K2. Ruiz-Mesa et al. [16] analyzed Sw, CoR and CoV in 40 normal eyes, reporting higher values than us: 5.58 μm, 14.44 μm and 0.58%, for CCT; 0.04 mm, 0.11 mm, and 0.50%, for ACD; and 0.13 mm, 0.36 mm and 0.16%, for AL. The only other study reporting the repeatability of the Eyestar 900 [7], evaluated the repeatability of the Eyestar 900 in 56 eyes undergoing the preoperative work-up for cataract surgery or corneal refractive surgery and healthy volunteers. They found a good repeatability for the different parameters measured (AL, K, corneal astigmatism, CCT, corneal diameter, ACD, LT and lens tilting) with the CoV value less than 1% in most cases. They concluded that the Eyestar 900 produces highly repeatable measurements. Another study assessed the feasibility and repeatability of Lenstar in a large group of children and adolescents [17]. The results obtained in all these studies agree with those found in our series (Table 1).

In relation to the agreement between both devices, Table 2 shows the mean values and Table 3 shows the mean difference for each parameter evaluated. We found statistically significant differences for CCT and LT but not for other parameters. Figures 1 and 2 show the Bland-Altman plots. In relation to the K results we found mean differences < 0.1 D and 95% LoA interval < 0.6 D both for K1 and K2. The small differences in the agreement found in the present study for both K1, K2 and Kmean values suggest that the differences in the IOL power calculation using these two instruments would also be small. Despite the statistically significant differences between biometers for CCT, the mean difference was about 3 µm and the 95% LoA interval was around 16 µm. In relation to ACD, the mean difference was small (0.012 mm) but the 95% LoA interval was about 0.2 mm. We consider that this difference would not affect but should be taken into account when these biometers are used interchangeably. The mean difference for LT, which was statistically significant, was also small (− 0.024 mm) but the 95% LoA interval was large (0.347 mm). Thus, both biometers can be used interchangeably for LT measurements. Finally, with the AL measurements, our mean difference value was very small (0.004 mm) and also the 95% LoA interval (0.080 mm). If the repeatability of one or both of the instruments evaluated was not good, that could result in poor agreement between the instruments. In this study, we report that both instruments have good repeatability and agreement [18].

Since there are no publications comparing both technologies, some studies have several outcomes using different samples for repeatability and agreement with other SS-OCT biometers available in the market. For example, Sorkin et al. [5] have compared the Eyestar 900 with the Anterion SS-OCT (Heidelberg engineering, Germany) biometer in a sample of 133 eyes of 66 cataract patients (mean age of 71.6 ± 9.8 years and 62.1% females). They found that all differences were statistically significant except for differences in anterior K measurements (lower than 0.05 D) and no changes were noted with analysis considering only their right eyes. On average, the Eyestar 900 measured longer AL (0.014 mm), thicker CCT (7.1 μm), shallower ACD (0.031 mm) and thinner LT (0.127 mm) than the Anterion. The Bland-Altman analysis also showed excellent agreement for AL, ACD, CCT, anterior K1, anterior K2 and LT measurements. They reported a small consistent mean measurement bias in measurements of CCT (7.1 μm thicker in the Eyestar 900) and LT (0.13 mm thicker in the Anterion). These authors discussed that despite finding a mean difference of 0.014 mm in AL between both biometers, this difference would lead to a difference of roughly 0.05 D in IOL power calculation which can be considered clinically insignificant [19, 20]. They suggested that the measurement obtained can be considered clinically interchangeable between biometers. In another study, Lender et al. [6] evaluated three biometers in a sample of 157 eyes of 79 cataract patients (comparing the Eyestar 900 with the IOLMaster 700 [Zeiss, Germany]) and 38 eyes of 19 cataract patients (comparing the Anterion with the IOLMaster 700). They aimed to compare the different ocular parameters and assess the effect of possible differences found on the calculated IOL power for implantation in cataract surgery. Their results revealed that, when comparing the IOLMaster 700 to the Eyestar 900, no difference was found in AL, ACD, K1 or K2 measurements (P > 0.05). In contrast, AL and ACD measurements differed between the IOLMaster 700 and Anterion (P < 0.05), but not for K1 or K2 (P > 0.05). They indicated that the differences in measurements were found to be statistically significant but were minor enough to most likely be clinically insignificant. To establish interchangeability of the biometers, the Bland-Altman analysis indicated good agreement between all three biometers on most parameters, with a minor offset in ACD measurements between the IOLMaster and the Eyestar. In this study, in order to investigate whether the minor differences observed between the devices impacted the suggested IOL power, we input the mean output values of each device in several online formulae calculators and found that the calculated IOL power was 0.50–1.00 D lower with the IOLMaster 700. Finally, Galzignato et al. [7] evaluated the agreement between the Eyestar 900 and two other SS-OCTs: the IOLMaster 700 and the Argos (Inc, Santa Clara, CA). They found good to high agreement among the measurements of the three optical biometers, although some statistically significant differences were detected between the Eyestar 900 and the Argos (mean K, ACD, LT and corneal diameter were higher than the Argos). They also found that the Argos biometer measured a shorter AL in eyes > 25 mm. They concluded that the measurements obtained with the Eyestar 900 are in good agreement with those found with the SS-OCT and Argos devices.

Our study has some limitations, primarily due to including only healthy eyes. This information is useful for general comparisons, but future studies should also consider cataract patients and IOL power calculation for emmetropization. Second, it would have been interesting to include comparison with other optical biometers based on different optical technologies to ascertain possible differences among them. However, one of the most important strengths of our study was that we assessed the agreement with the OLCR-based Lenstar LS 900 biometer.