Since the first optical biometer (IOLMaster, Carl Zeiss Meditec AG) was introduced in 1999, several similar instruments have been developed. The first devices combined optical biometry (to measure axial distances) and automated keratometry (to measure corneal curvature and calculate corneal power). These include the Lenstar LS 900 (Haag-Streit AG)1 and AL-Scan (Nidek Co., Ltd).2 Subsequently, optical biometry was combined with Placido-disk corneal topography on 2 devices, the Aladdin (Europe Medical B.V.)3 and OA-2000 (Tomey Corp.),4 and with a rotating Scheimpflug camera on 2 other instruments, the Galilei G6 (Ziemer Ophthalmic Systems AG)5 and Pentacam AXL (Oculus Optikgeräte GmbH).6 More recently, a new generation of optical biometers was developed based on optical coherence tomography (OCT); these include the IOLMaster 700 (Carl Zeiss Meditec AG),7 the Argos (Movu, Inc.),8 and the Anterion (Heidelberg Engineering GmbH). Although the latter group provides excellent anterior segment OCT images, none is able to generate high-definition scans of the retina and optic nerve and give, at the same time, the biometric parameters required to calculate intraocular lens (IOL) power. The recently introduced Revo NX device (Optopol Technology S.A.) combines anterior and posterior segment spectral-domain OCT (SD-OCT) with optical biometry.9 Having one instrument to perform both tasks would be an advantage to clinicians because they would have to purchase only one device rather than two.
The purpose of this study was to examine the repeatability and reproducibility of the measurements provided by the new Revo NX optical biometer and compare them with those of a validated optical biometer,10 the Lenstar LS 900, which is based on optical low-coherence reflectometry (OLCR).
Participants and Methods
This prospective study enrolled volunteers with healthy eyes at Hygeia Clinic, Gdańsk, Poland, between July 2018 and August 2018. The study adhered to the tenets of the Declaration of Helsinki for the use of human participants in biomedical research and was approved by the local research ethics board. All participants signed informed consent forms after receiving an explanation of the purpose of the study.
Exclusion criteria were any ocular disease (including cataract), corrected distance visual acuity worse than 20/25, previous ocular surgery, and trauma. Before being enrolled, all eyes had a complete ophthalmologic examination consisting of subjective refraction, noncontact tonometry, slitlamp evaluation, and ophthalmoscopy.
The Revo NX SD-OCT device has an axial resolution of 5 μm, transverse resolution of 12 μm, and single scan depth of 2.4 mm and obtains 110 000 scans per second. A superluminescent laser diode (830 nm) is used as the light source. The optical biometry program within the device measures the axial length (AL), anterior chamber depth (ACD; measured from the epithelium to lens), lens thickness (LT), and central corneal thickness (CCT). For each measurement, it performs 10 B-scans to calculate a precise average value (Figure 1).
The Lenstar LS-900 optical low-coherence reflectometry (OLCR) biometer is a superluminescent diode laser (820 nm) that measures the AL, LT, CCT, and aqueous depth. The device also provides keratometry values, pupil size, and corneal diameter measurements.
To allow comparison of the ACD values determined by the 2 devices, the ACD for the OLCR device was calculated as the sum of aqueous depth and CCT.11
Measurements were performed with the 2 devices in random order. Only right eyes were analyzed, and all scans were taken between 15:00 and 19:00. Two skilled operators, in random order, performed 3 scans with the SD-OCT device to obtain 3 measurements. Repeatability was calculated for each observer, while reproducibility was assessed by a change in observer. To assess the agreement between the SD-OCT device and the OLCR device, the first observer took 3 measurements consecutively with both devices.
Statistical analysis was performed using Prism software (GraphPad Software, Inc.) and MedCalc software (MedCalc Software bvba). The results are presented as the mean ± SD. The repeatability, repeatability limit, reproducibility, and reproducibility limit were calculated as recommended by McAlinden et al.12 The coefficient of variation (CoV) was determined as the ratio of the repeatability/reproducibility to the mean value (lower CoV stands for better reliability).13 The intraclass correlation coefficient (ICC) was defined as the ratio of variance between subjects to the sum of the pooled intraparticipant variance and the interparticipant variance. An ICC less than 0.75 indicates poor agreement; an ICC from 0.75 to 0.90 is considered as moderate, and an ICC of 0.90 or above is considered high.14 Comparison between the devices was performed using correlation coefficients, the Bland-Altman method, and paired t tests given that all parameters were normally distributed in the Kolmogorov-Smirnov test. A P value less than .05 was considered statistically significant.
The study enrolled 65 participants. Two participants were excluded from the study; no measure for LT could be obtained in 1 eye and 1 participant withdrew consent for participation. Therefore, the results of 63 participants (44 women) were analyzed. The mean age was 41.8 ± 13.8 years. The mean spherical equivalent refraction was −0.11 ± 2.02 diopters (D) (range −5.38 to +4.63 D).
The AL, ACD, LT, and CCT measurements obtained with SD-OCT device had high repeatability for both observers (Table 1). The CoV was less than 0.7% for all parameters, while the ICC was higher than 0.92 for all parameters. Table 2 shows the reproducibility of the outcomes measured by the SD-OCT device. The reproducibility limit values were excellent, showing low variability.
There was a statistically significant difference in AL, ACD, and LT measurements between the SD-OCT device and OLCR device (P < .01). The mean AL and ACD measurements were higher with the SD-OCT device (Table 3). The CCT measurements were not significantly different between the devices (P = .07). The correlation between the systems was very strong for AL (r = 0.9997), ACD (r = 0.9953), LT (r = 0.9626), and CCT (r = 0.9922). Bland-Altman analysis showed narrow 95% limits of agreement for AL, ACD, LT, and CCT measurements. Figures 2 to 5 show the Bland-Altman plots for these parameters.
Information about the repeatability and reproducibility of measurements and agreement with validated devices is essential when a new instrument becomes commercially available. Our data show that the measurements provided by the new Revo NX SD-OCT optical biometer offer high repeatability and reproducibility and a very strong correlation with those by the Lenstar LS 900 OLCR device. Although the agreement was good for all parameters except AL, a statistically significant difference was detected for all of the 4 parameters evaluated in this study (AL, ACD, CCT LT). The largest dissimilarities were observed for the AL (mean difference +0.11 ± 0.02 mm) and ACD (mean difference +0.05 ± 0.04 mm). Such differences have not only statistical significance but also clinical significance and should not be underestimated; a shift of 0.1 mm in AL measurement transposes to approximately a 0.27 D change in the spectacle plane (for normal eye dimensions).15 Variations in biometric parameters have been reported with many optical biometers.16–25 In a study by Goebels et al.,21 AL values obtained with partial coherence interferometry (PCI) were on average 0.07 mm higher than those obtained with OLCR. Ortiz et al.18 reported that the AL determined with PCI was on average 0.04 mm shorter than that obtained with OLCR biometry. In a meta-analysis by Rozema et al.,10 the Lenstar OLCR device measurements were on average 0.02 ± 0.01 mm (SD) higher than the IOLMaster PCI measurements, which was significantly equal. Table 4 shows the results in studies reporting differences in AL and ACD optical biometry measurements.1–3,7,16,18,21–25
The differences between the devices in our study could presumably be associated with the use of a dissimilar group refractive index to convert the optical path length into axial distances.26 Other potential reasons include the minimally different optical wavelength used by the devices or a dissimilar technique for automatic structure border detection. These issues are likely a matter of calibration and should play no major role in the usefulness of the device. The difference in AL and ACD values in our study requires specific constant optimization for IOL power theoretical formulas that include them as predictors of the IOL position.
This study and the SD-OCT optical biometer have limitations. First, we did not evaluate patients with eye disease, including cataract; therefore, our findings can be applied only to healthy eyes. Second, keratometric measurements and IOL calculation were not available in the device at the time of the study. Nevertheless, we believe this feature is an interesting development of OCT because most clinics likely have a validated keratometry device and theoretically could use the obtained data in IOL calculation formulas. Future software versions of this device enabling keratometric measurements will require further validation.
In conclusion, the new Revo NX SD-OCT optical biometer provided repeatable and reproducible AL, ACD, LT, and CCT measurements. The results obtained with the SD-OCT biometer had a very strong correlation with those obtained with the Lenstar OLCR device; however, AL measurements, and to a lesser extent ACD measurements, cannot be considered interchangeable.
What Was Known
- Optical methods are considered to be the gold standard for preoperative biometry.
- At present, it is necessary to buy a separate optical coherence tomographer (OCT) device for anterior and posterior segment imaging and another for optical biometry.
What This Paper Adds
- Optical biometry can be used in a new anterior and posterior segment spectral-domain (SD) OCT device.
- The new SD-OCT biometer provided repeatable and reproducible measurements of axial length, anterior chamber depth, lens thickness, and central corneal thickness.
- The results obtained with the new biometer showed a very strong correlation with a validated optical biometer. However, they cannot be considered interchangeable.
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Dr. Kanclerz receives non-financial support from Visim and Optopol Technology SA. To ensure accurate programming of his formulas, Dr. Hoffer licenses the registered trademark name Hoffer® to Carl Zeiss Meditec AG (IOLMasters), Haag-Streit AG (LenStar/EyeStar), Heidelberg Engineering, Inc. (Anterion), Movu, Inc. (Argos), Nidek, Inc. (AL-Scan), Oculus Optikgeräte GmbH (Pentacam AXL), Tomey Corp. (OA-2000), Topcon Europe Medical B.V./Visia Imaging S.r.l. (Aladdin), Ziemer Ophthalmic Systems AG (Galilei G6), and all A-scan biometer manufacturers. None of the other authors has a financial or proprietary interest in any material or method mentioned.