Since 1999, optical biometry has become the standard technique for axial length measurement.1–3 The main reason for such popularity is not related to higher accuracy, because its results are similar to those achieved by immersion ultrasound biometry,4 but to the great advantage of being a non-contact technique, which is associated with less discomfort for the patient and less risk of corneal complications. Moreover, it is easier to perform for most surgeons and technicians. Until 2009 the IOLMaster (Carl Zeiss Meditec, Jena, Germany), based on partial coherence interferometry, has been the only device measuring axial length by an optical method. Since 2009, newer instruments have been introduced: the LenStar (Haag-Streit AG, Köniz, Switzerland), the Aladdin (Topcon EU, Visia Imaging, San Giovanni Valdarno, Italy), the AL-Scan (Nidek Co. Ltd., Gamagori, Japan), the Galilei G6 (Ziemer, Port, Switzerland), the Argos (Movu, Santa Clara, CA), the Pentacam AXL (Oculus, Wetzlar, Germany), and the IOLMaster 700 (Carl Zeiss Meditec).
Previous studies have shown that both the LenStar and Aladdin provide similar measurements with respect to the IOLMaster 500,5–9 although slight differences in keratometry (K) values, anterior chamber depth (ACD) (corneal epithelium to lens), and axial length values require constant optimization for each instrument when calculating intraocular lens (IOL) power by theoretical formulas.10–12 To date, a few studies investigated the Nidek optical biometer,13–15 but some of them suffer from methodological biases (in one case14 both eyes of most patients were enrolled and in another case15 IOL power calculation was not based on back-calculated constant optimization).
To validate the Nidek AL-Scan, in this prospective study, we aimed to confirm whether the biometric measurements have any statistically significant difference in mean axial length, K values, and ACD.
Patients and Methods
This prospective observational study included consecutive adult patients scheduled for cataract surgery between January and March 2014. Only one eye of each patient was analyzed. The study was approved by the Institutional Review Committee, all patients provided informed consent, and the study complied with the tenets of the Declaration of Helsinki. Exclusion criteria were previous corneal or intraocular surgery, any ocular surface disease (eg, dry eye) leading to irregular tear film, any corneal disease (eg, keratoconus or marginal pellucid degeneration), previous contact lens wear in the past month, and any retinal or optic nerve pathology affecting the postoperative visual acuity. Only eyes with a postoperative visual acuity of 20/40 or better were enrolled.
Each eye was evaluated on the same day using both units. In each group, one unmasked examiner performed all measurements. For both instruments, the mean axial length, mean K, and ACD values were recorded. All K values reported here are derived from the anterior corneal curvature using a 1.3375 keratometric index of refraction.
IOLMaster 500 Measurements
The IOLMaster 500 (software version 5.2) optical biometer uses partial coherence interferometry with a 780-mm laser diode infrared light to measure axial length. The ACD is measured through a lateral slit-illumination and is defined as the measurement from the corneal epithelium to the anterior lens surface. The K readings are calculated by analyzing the anterior corneal curvature at six reference points in a hexagonal pattern at approximately the 2.3-mm optical zone.
After ensuring the correct positioning of the patient against the chin and headrest, the partial coherence interferometry was focused and coarsely aligned with the participant's eye using the overview mode. The patient was directed to focus on the illuminated target. The axial length measurement mode was activated and fine alignment occurred while the patient was asked to observe the red fixation point. Five axial length measurements were recorded and any with a signal-to-noise ratio below 2.0 were repeated. With respect to the keratometry, patients were requested to observe a yellow light and to blink to produce a continuous tear film, thus improving the reflectivity of the cornea. Six peripheral measuring points at a diameter of 2.3 mm were optimally focused on the cornea as demonstrated by a green light from the partial coherence interferometry “traffic light system.” Subsequent depression of the joystick button provided three consecutive K measurements and the mean of these values was used for the IOL calculations. If any of the 6 measurement points were not correctly identified, the measurements were repeated.
The technology of the Nidek AL-Scan (software version 1.03) is also based on partial coherence interferometry, with partial coherence superposition of light waves emitted from an 830-nm super luminescent diode laser, to measure the axial length of the eye. It uses a 970-nm light-emitting diode (LED) for K assessment and a 525-nm LED for determination of corneal diameter. Corneal power is measured by analyzing the images of double mires of spots (360°) at diameters of 2.4 and 3.3 mm reflected from the anterior surface of the cornea. In addition, the unit measures ACD, defined as the measurement from the corneal epithelium to the anterior lens surface.
Patients were carefully aligned for partial coherence interferometry biometry measurements. The equipment was optimally positioned as demonstrated by a clear view of the anterior eye and the appearance of a quality control image, which indicated when the working distance of approximately 45 mm was achieved. The patient was asked to fixate on the red fixation lamp in the measuring window. This engaged alignment software, which shows arrows to clearly indicate the direction in which the instrument must be moved to fine-tune the alignment. When the eye is aligned and in focus, a measurement starts automatically, taking a measurement of consecutive six parameters: six axial length measurements, three K measurements, three central corneal thickness and ACD measurements, one pupil diameter measurement, and one corneal diameter measurement, all under 10 seconds. The same procedure was repeated three times and the average value for each parameter was used for statistical analysis.
Statistical analysis was performed using MedCalc software (version. 22.214.171.124; MedCalc Software, Ostend, Belgium). All values were compared using a paired t test because all data followed a Gaussian distribution according to the Kolmogorov–Smirnov method. Correlation was used to quantify how well the measurements by the two instruments varied. The Pearson product moment correlation coefficient (r) was used to statistically evaluate each correlation. Agreement was evaluated using the method of Bland and Altman,16 who suggest plotting the differences between the measurements (y-axis) against their mean (x-axis). The 95% limits of agreements (LoA) were defined as the mean ± 2 standard deviations (SDs) of the differences between the two units. A P value less than .05 was considered statistically significant.
Based on the result of a recent study,17 the SD of the differences in axial length, K, and ACD between two optical biometers was 0.02 mm, 0.15 D, and 0.07 mm, respectively. Using a two-sided level of significance (α) at 0.05 and power (β) at 90%, a sample size calculation indicates that a minimum of 48 patients would be required to detect a mean difference of 0.01 mm, 0.07 D, and 0.03 mm.
Eighty-six eyes of 86 patients (43 [50%] females, 47 [55%] right eyes) with a cataractous lens were evaluated. The mean age of the patients was 72 ± 9.1 years (range: 45 to 87 years). Table 1 describes the axial length, K values, and ACD measurements taken by the two instruments.
Comparison of Biometry Measures Between the AL-Scan and IOLMaster 500
Axial length mean values were exactly the same for the two instruments (23.46 ± 0.99 mm), did not show any statistically significant difference, and showed excellent agreement and correlation (r = 0.9999, P < .0001). Figure 1 shows the Bland–Altman plot for axial length. The AL-Scan showed slightly steeper K measurements than the IOLMaster at the 2.4-mm zone by 0.08 D, which was statistically significant. The difference was smaller (only 0.03 D) and not statistically significant when measurements were taken at the 3.3-mm diameter. However, agreement was slightly better with measurements at the 2.4-mm diameter than those at the 3.3-mm diameter. Figures 2–3 show the Bland–Altman plots for the 2.4- and 3.3-mm diameters, respectively. The AL-Scan showed deeper ACD measurements (mean difference: +0.13 ± 0.43 mm), which was statistically significant (P = .0001). Figure 4 shows the Bland–Altman plot for ACD.
Bland–Altman plot for the axial length measurements.
Bland–Altman plot for the 2.4-mm corneal power measurements for the IOLMaster (Carl Zeiss Meditec, Jena, Germany) and AL-Scan (Nidek Co, Ltd., Gamagori, Japan).
Bland–Altman plot for the corneal power measurements for the AL-Scan (Nidek Co, Ltd., Gamagori, Japan) at 3.3 mm and IOLMaster (Carl Zeiss Meditec, Jena, Germany) at 2.4 mm.
Bland–Altman plot for the anterior chamber depth measurements.
The current study shows that the two partial coherence interferometry optical biometers provide similar measurements, although agreement is not perfect other than for axial length. Some differences in K and ACD do not allow us to consider the two instruments interchangeable.
There was no statistically significant difference for mean axial length. Agreement was excellent as the 95% LoA showed that the difference would be lower than 0.2 mm in 95% of cases. In this regard, our data confirm the results of previous studies comparing these two devices.13–15 Our data are also similar to those previously reported between the IOLMaster 500 and the LenStar5–7 and between the IOLMaster 500 and the Aladdin.8,9
Some differences were found for the mean K values, which were steeper with the AL-Scan in both optical zones. Although agreement was good and the mean difference may not seem clinically significant, actually this difference in the 2.4-mm diameter is sufficient to require constant optimization to nullify systematic differences between the two devices. Our results are in good accordance with previous studies, where the AL-Scan was found to provide steeper K measurements than the IOLMaster.13–15 On the other hand, in two of three studies those differences were not statistically significant,13,15 whereas we found the difference at the 2.4-mm diameter to be statistically significant. It is hard to explain why the AL-Scan provides steeper K measurements than the IOL-Master. In most published studies, the IOLMaster provided mean steeper K values than other technologies, such as Scheimpflug imaging and Placido-disk corneal topography.5,18,19 These differences had been related to the fact that the IOLMaster 500 analyzes a smaller corneal diameter (approximately 2.3 mm) than other instruments, such as the Lenstar or the Pentacam (which analyze a diameter of approximately 3 mm). However, the difference in the current comparison cannot be related to the analyzed diameter, which is almost the same for the two partial coherence interferometry biometers. Therefore, the difference is likely to depend on the method used to analyze the mires of spots reflected from the cornea.
As regards ACD, the AL-Scan provided deeper mean values compared to the IOLMaster 500. This is likely to depend on the different technology used to measure the ACD; the IOLMaster measures ACD through a lateral slit illumination, whereas the AL-Scan measures ACD using Scheimpflug imaging. Similarly, both the LenStar and, to a lesser extent, the Aladdin gave mean deeper ACD values than the IOLMaster 5005,9 (Table 2). Deeper ACD measurements by the AL-Scan with respect to the IOLMaster had already been reported by Huang et al.13 and Kaswin et al., 15 whereas the opposite result had been found by Srivannaboon et al.14
Anterior Chamber Depth Measures Compared to IOLMaster
This study is limited by the fact that we did not evaluate the accuracy of astigmatic axis measurement or the accuracy of the two devices in IOL power calculation, but these will be the subjects of further investigation. Also, our sample did not include myopic eyes with an axial length greater than 27.50 mm, so our results cannot necessarily be applied to those cases. Finally, we did not investigate some of the potentially advantageous features of the AL-scan such as the attached ultrasound probe for eyes that cannot be measured optically.
Both optical biometers provide close mean measurements of the main biometric parameters of concern in IOL power calculation. Although there is no difference in axial length measurement, the small differences in K values and ACD should not be overlooked and warrant constant optimization for IOL power calculation.
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Comparison of Biometry Measures Between the AL-Scan and IOLMaster 500
||AL-Scan Mean ± SD
||IOLMaster Mean ± SD
||CC r (P)
||23.46 ± 0.99
||23.46 ± 0.99
||−0.03 to +0.03
||0.9999 (< .001)
|K 2.4 mm (D)
||43.84 ± 1.49
||43.76 ± 1.46
||−0.57 to +0.43
||0.9851 (< .001)
|K 3.3 mm (D)
||43.79 ± 1.44
||K@2.4 mm (43.76 ± 1.46)
||−0.58 to + 0.53
||0.9429 (< .001)
||2.96 ± 0.38
||2.83 ± 0.38
||−0.44 to +0.18
||0.9135 (< .001)
Anterior Chamber Depth Measures Compared to IOLMaster