The measurement of anterior segment parameters is an integral part of a comprehensive ophthalmic examination. The most commonly measured anterior segment parameters are corneal thickness and curvature, white-to-white corneal diameter (CD) distance, aqueous depth (AQD, distance between corneal endothelium and anterior surface of the crystalline lens), angle-to-angle distance, anterior chamber angle, and lens vault and thickness. Accurate and reliable measurements of these parameters are important in the screening, diagnosis, and follow-up of ocular disorders, such as corneal ectasias or glaucoma, as well as to plan cataract and refractive surgery.1,2 The availability of high-resolution noncontact and in vivo imaging and quantitative measurements of various ocular structures with different techniques provides new possibilities and additional information that could help in the early diagnosis and evaluation of progression of different ocular disorders.3,4

A wide diversity of techniques, including Placido disk, Scheimpflug tomography, optical low coherence reflectometry (OLCR), or optical coherence tomography (OCT), have been used to image the anterior segment. Recent anterior segment imaging devices are based on swept source OCT (SS-OCT) as this technology offers rapid scanning, deeper tissue penetration, and outstanding image quality of the anterior segment. This technology has been shown to be beneficial to screening, diagnose, and follow-up of different anterior segment disorders.1,5 Optical biometer based on SS-OCT is also available, providing posterior corneal information in addition to the standard parameters provided by the conventional biometers.6 Previous studies have shown that these instruments have good repeatability for measuring anterior segment parameters, and the agreement among these instruments was good for some of the anterior segment parameters.7–9

Precision studies are needed to quantify the repeatability of new ocular imaging devices and their agreement with the existing instruments. From the calculated repeatability, the minimum number of repeated measurements needed to ensure a certain measurement tolerance can be determined.10,11 In addition, the agreement studies help to decide whether 2 instruments could be used interchangeably.12 In this study, we evaluated the precision of a new SS-OCT biometer to measure anterior segment parameters and its agreement with 3 different imaging devices based on different measurement principles (OLCR, a combination of SS-OCT and Placido disk topography, and a combination of Scheimpflug tomography and Placido disk topography). To our knowledge, there are no previous studies comparing these different measurement principles to evaluate anterior segment parameters. The aim of this study was to evaluate the repeatability of a new SS-OCT–based biometer to measure anterior segment parameters and to assess the agreement with 3 other imaging devices based on different measurements principles.

METHODS Participants

In this prospective study, healthy participants aged between 18 years and 70 years were included. Participants with previous ocular surgery, diabetes, hypertension, retinal disease, glaucoma, or nystagmus were excluded from this study. This study was approved by the Regional Ethics Committee (Swedish Ethical Review Authority) and was conducted in accordance with the tenets of the Declaration of Helsinki. Written informed consent was obtained from all subjects before their enrollment in this study.

Instrumentation

Two different optical biometers (a new SS-OCT biometer and an OLCR biometer) and 2 different anterior segment devices (an anterior segment OCT and a Scheimpflug camera–based topographer) were analyzed in this study. The detailed description of the instruments analyzed is given below:

(1) The biometer Eyestar 900 (Haag-Streit AG, software version v.2.2.0) is based on SS-OCT technique, and it uses a wavelength of 1060 nm with a scan speed of 30 kHz to measure the central corneal thickness (CCT) and AQD. An infrared LED source of 850 nm is used to obtain the dual-zone keratometry on 32 measurement points. The CD is determined from the image of the iris and the radii obtained from keratometry.

(2) The biometer Lenstar LS900 (Haag-Streit AG, software version v.2.5.2) is based on OLCR technique and uses a LSD source of 820 nm to measure CCT and AQD and a LED source of 950 nm to measure the corneal topography on 32 reference points distributed in 2 concentric circles of approximately 2.30 mm and 1.65 mm diameter.

(3) The MS-39 (Costruzione Strumenti Oftalmici, software version v.4.1.0.7) combines a swept source anterior segment OCT and Placido disk rings. The OCT uses a wavelength of 845 nm, and it covers a scan area of 16 × 7.5 mm containing 25 B-scans each consisting of 1024 A-scans. The data from the Placido (wavelength of 635 nm) disk image and OCT scans are merged to calculate the curvature, thickness, and anterior segment values.

(4) The Sirius (Costruzione Strumenti Oftalmici, Firenze, Italy, software version v.3.7) combines a rotating Scheimpflug camera and a Placido disk topographer (700 nm wavelength). The Scheimpflug camera uses a slit-light source with a wavelength of 475 nm (UV-free). This instrument acquires 25 Scheimpflug images and 1 Placido top-view image. Previous studies report technical details of this instrument.13,14

Table 1 summarizes the measurement principle and ocular parameters evaluated in this study with each instrument.

Table 1. - Summary of the measurement principle and parameters analyzed from each instrument Instruments Measurement principle Parameters AQD CCT CD Anterior K Posterior K Sim K Eyestar 900 SS-OCT ✓ ✓ ✓ ✓ N/A ✓ Lenstar LS 900 OLCR ✓ ✓ N/M ✓ N/A N/A MS-39 SS-OCT and Placido disk ✓ ✓ ✓ ✓ ✓ ✓ Sirius Scheimpflug tomography and Placido disk ✓ ✓ ✓ ✓ ✓ ✓

AQD = aqueous depth; CCT = central corneal thickness; CD = corneal diameter; K = keratometry; N/A = not available; N/M = not measured; OLCR = Optical Low Coherence Reflectometry; ✓ = measured and analyzed

Procedure

The instruments were calibrated before the measurements, and the order of each instrument was randomized for each participant. One eye from each participant was measured, and 3 measurements were taken under repeatability conditions in a single session.10

Statistical Analysis

The mean, SD, and range values were calculated for all parameters measured. Repeatability and agreement limits were calculated based on standards adopted by the British Standards Institute and the ISO.15 The repeatability of each instrument was described using the following metrics: within-subject standard deviation (Sw), repeatability limit (Rlim), and coefficient of variability (CoV). The Rlim was calculated as 1.96∙2∙Sw, and it represents the expected limits that 95% of the measurements should be within. The CoV was calculated as the ratio between Sw and the average value (x): CoV = Sw/x. The agreement between each instrument was assessed by a Bland-Altman analysis for repeated measurements. The average difference, 95% limits of agreement (LoA, calculated as the mean difference ±1.96 SD), and the CI of each CI at 95% were calculated.

The required sample size (n) was calculated considering both repeatability and agreement. For repeatability purposes, n was calculated considering the number of repeated measurements (m) and confidence level (CL) for the estimated Sw as CL = 1.962n(m−1).16 Considering a CL of 0.125 and 3 repeated measurements, 62 eyes are required. For agreement purposes, the following formula was used: 1.96 3s2n = desired CI of LoA, where s is the SD of the differences. We considered that the desired CI for the LoA in our study to be 0.02 mm for the AQD. With this value and the s value obtained in a subset of 30 eyes, the minimum n value required was 47 eyes. Then, taking into account both n values, we considered that this should be at least 62 eyes, with our target being 70 eyes.

RESULTS

In total, 74 healthy eyes (37 right eyes) from 74 participants (55 female participants) were evaluated in this study. The mean age of the participants was 32.0 ± 11.29 years, and the mean spherical equivalent was –1.39 ± 2.70 diopters (D).

Repeatability

The descriptive statistics and the repeatability metrics outcomes for the anterior segment parameters are summarized in Table 2. The Rlims for CCT, CD, and AQD were lower than 10 μm, 0.3 mm, and 0.10 mm, respectively. The corresponding CoVs for all these parameters never exceeded 1.2% for all instruments.

Table 2. - Descriptive statistics and repeatability metrics outcomes for the anterior segment parameters Parameters Instruments Mean ± SD RLimit CoV (%) CCT (μm) Eyestar 546.59 ± 30.86 4.32 0.29 Lenstar 543.47 ± 32.52 8.13 0.54 MS39 540.45 ± 31.47 5.51 0.37 Sirius 537.46 ± 30.83 6.75 0.45 CD (mm) Eyestar 12.23 ± 0.44 0.08 0.25 MS39 12.15 ± 0.37 0.27 0.80 Sirius 12.09 ± 0.39 0.28 0.85 AQD (mm) Eyestar 3.00 ± 0.31 0.05 0.60 Lenstar 2.99 ± 0.32 0.10 1.17 MS39 3.10 ± 0.30 0.04 0.47 Sirius 3.04 ± 0.31 0.07 0.81

AQD = aqueous depth; CCT = central corneal thickness; CD = corneal diameter; CoV = coefficient of variation; RLim = repeatability limit

The descriptive statistics and repeatability metrics outcomes for the corneal keratometry parameters are summarized in Table 3. The Rlim for the anterior K1 was lower than 0.25 D for both biometers (Eyestar and Lenstar) and about 0.50 D for both corneal topographers (MS-39 and Sirius). The Rlim for the anterior K2 was lower than 0.50 D for all instruments, but the MS39 that had a Rlim of 0.60 D. Nevertheless, the CoVs for these 2 corneal parameters were lower than 1%. Regarding the repeatability metrics for the posterior keratometry, the Rlim lower than 0.35 D for both corneal topographers (MS39 and Sirius) on both meridians and the corresponding CoVs were less than 2.0%. Finally, the Rlim for both Sim K1 and K2 was lower than 0.5 D for Eyestar, Ms-39, and Sirius. In addition, the CoVs for these corneal parameters were lower than 0.5%. Table S2 (available at https://links.lww.com/JRS/B59) summarizes the descriptive statistics and Rlim outcomes for the average anterior corneal power and astigmatic vector components. For these parameters, the Rlim of all instruments was lower than 0.50 D.

Table 3. - Descriptive statistics and repeatability metrics outcomes for the corneal keratometry parameters Parameter Instrument Mean ± SD RLimit CoV (%) Anterior K1 (D) Eyestar 43.16 ± 1.47 0.22 0.18 Lenstar 43.17 ± 1.45 0.20 0.17 MS39 43.26 ± 1.49 0.55 0.46 Sirius 43.36 ± 1.50 0.43 0.36 Anterior K2 (D) Eyestar 44.17 ± 1.62 0.21 0.17 Lenstar 44.19 ± 1.63 0.45 0.37 MS39 44.31 ± 1.58 0.60 0.49 Sirius 44.41 ± 1.61 0.37 0.30 Posterior K1 (D) MS39 −6.06 ± 0.24 0.05 0.30 Sirius −5.97 ± 0.26 0.32 1.94 Posterior K2 (D) MS39 −6.40 ± 0.28 0.21 1.23 Sirius −6.41 ± 0.39 0.28 1.58 Anterior Sim K1 (D) Eyestar 43.00 ± 1.41 0.32 0.27 MS39 43.16 ± 1.48 0.39 0.33 Sirius 43.28 ± 1.49 0.29 0.24 Anterior Sim K2 (D) Eyestar 44.07 ± 1.59 0.38 0.31 MS39 44.17 ± 1.62 0.42 0.34 Sirius 44.30 ± 1.62 0.33 0.27

CoV = coefficient of variation; K = keratometry; RLimit = repeatability limit; Sim K = simulated keratometry value

Agreement

Figures S1 to S8 (available at https://links.lww.com/JRS/B50, https://links.lww.com/JRS/B51, https://links.lww.com/JRS/B52, https://links.lww.com/JRS/B53, https://links.lww.com/JRS/B54, https://links.lww.com/JRS/B55, https://links.lww.com/JRS/B56, https://links.lww.com/JRS/B57) show the Bland-Altman analysis for the parameters analyzed. Table 4 summarizes the agreement results among all instruments for the anterior segment parameters. For the CCT, the mean differences between the instruments varied from 2.99 to 9.13 μm. The device pair Eyestar-MS 39 showed the shortest LoA interval (15.54 μm); meanwhile, the Eyestar-Sirius showed the largest interval (37.21 μm). The CIs of each LoA never exceeded 4 μm. The agreement results for the CD were similar among the 3 instrument comparison. The mean difference and LoA interval were approximately 0.1 and 0.6 mm, respectively, in all cases. Similarly, all instrument comparison resulted with similar mean differences for AQD (lower than 0.15 mm) and LoA interval (shorter than 0.3 mm). Table 5 summarizes the agreement results among all instruments for the corneal topography parameters. The mean difference for keratometry parameters and Sim K in both the flattest and steepest meridians was lower than 0.3 D for all device comparisons, and the LoA interval ranged between 0.52 D and 1.21 D. Table S3 (available at https://links.lww.com/JRS/B60) summarizes the agreement results among all instruments for the average anterior corneal power and astigmatic vector components. For the average anterior corneal power, the mean difference was lower than 0.22 D, and the LoA interval never exceeded 1 D. For the astigmatic vector components, the mean difference was lower than 0.1 D and the LoA interval never exceeded 0.75 D.

Table 4. - Agreement results among all instruments for the anterior segment parameters Parameter Agreement Mean difference LOA interval CI of LOA CCT (μm) Eyestar: Lenstar 3.12 19.04 1.56 Eyestar: MS 39 6.14 15.54 1.3 Eyestar: Sirius 9.13 37.21 3.56 Lenstar: MS 39 3.02 16.55 1.24 Lenstar: Sirius 6.01 38.3 3.57 MS 39: Sirius 2.99 35.66 3.38 CD (mm) Eyestar: MS 39 0.08 0.55 0.04 Eyestar: Sirius 0.14 0.6 0.05 MS 39: Sirius 0.06 0.66 0.05 AQD (mm) Eyestar: Lenstar 0.01 0.25 0.02 Eyestar: MS 39 −0.09 0.16 0.01 Eyestar: Sirius −0.04 0.18 0.02 Lenstar: MS 39 −0.11 0.24 0.02 Lenstar: Sirius −0.05 0.27 0.02 MS 39: Sirius 0.05 0.16 0.01

AQD = aqueous depth; CCT = central corneal thickness; CD = corneal diameter; LoA = limit of agreement

Table 5. - Agreement results among all instruments for the corneal topography parameters Parameter Agreement Mean difference LOA interval CI of LOA Anterior K1 (D) Eyestar: Lenstar −0.01 0.52 0.04 Eyestar: MS 39 −0.11 0.9 0.07 Eyestar: Sirius −0.2 0.76 0.06 Lenstar: MS 39 −0.09 0.95 0.07 Lenstar: Sirius −0.19 0.76 0.06 MS 39: Sirius −0.1 1.06 0.08 Anterior K2 (D) Eyestar: Lenstar −0.02 0.8 0.06 Eyestar: MS 39 −0.14 0.98 0.07 Eyestar: Sirius −0.24 0.71 0.05 Lenstar: MS 39 −0.12 1.19 0.09 Lenstar: Sirius −0.22 0.96 0.07 MS 39: Sirius −0.1 1.07 0.08 Posterior K1 (D) MS 39: Sirius −0.09 0.73 0.06 Posterior K2 (D) MS 39: Sirius 0.01 1.18 0.11 Anterior Sim K1 (D) Eyestar: MS 39 −0.16 1.21 0.1 Eyestar: Sirius −0.27 1.16 0.1 MS 39: Sirius −0.11 0.78 0.06 Anterior Sim K2 (D) Eyestar: MS 39 −0.1 1.09 0.09 Eyestar: Sirius −0.23 1.07 0.09 MS 39: Sirius −0.13 0.87 0.06

LoA = limit of agreement; Sim K = simulated keratometry value

DISCUSSION

We evaluated the repeatability of a new SS-OCT–based biometer to measure anterior segment parameters and assessed the agreement with 3 other imaging devices based on different measurement principles. To our knowledge, this is the first study assessing the agreement of instruments based on SS-OCT, OLCR, a combination of SS-OCT and Placido disk topography, and a combination of Scheimpflug tomography and Placido disk topography. Our results show that the repeatability of all the anterior segment parameters analyzed was good for all instruments considering that the CoV never exceeded 2%. Both SS-OCT–based instruments showed the best agreement results for CCT and AQD. For the corneal keratometry, the device combination that provided the best agreement varied depending on the parameter analyzed.

Regarding the CCT measurements, all instruments showed a good repeatability with Rlim lower than 10 μm (Sw lower than 3 μm, which is only approximately 0.5% of the average CCT). Previous studies have obtained CoV values similar to the ones that we have obtained in this study.7,8,17–19 To measure CCT, all instrument comparisons, excluding the Scheimpflug based device (Sirius), showed a good agreement with a LoA interval lower than 20 μm. Previous studies comparing CCT measured with other Scheimpflug-based devices with other SS-OCT devices also showed that the LoA interval was large and similar to our agreement results. CCT is an essential factor to consider in screening for corneal ectasias and in interpreting the intraocular pressure (IOP) readings.20,21 Considering that a 25 μm deviation from the average CCT of 550 μm can affect the IOP measurement by 1 mm Hg, we can suggest that the Scheimpflug-based device cannot be used interchangeably with the other devices analyzed.22

CD measurement is one of the easiest and one of the most widely used parameter to calculate the phakic intraocular lens (IOL) size. To measure CD, the repeatability of both SS-OCT instruments and the Scheimpflug-based topographer is good, with the CoV values never exceeding 1%. Previous studies using similar devices also reported CoV values lower than 1%.9,23 Regarding agreement results, all instrument combination showed similar outcomes with a LoA interval of approximately 0.60 mm. These results are clinically significant as the phakic IOL size is chosen to be close to the nearest 0.50 mm.24

The repeatability to measure AQD was also good for the devices analyzed (maximum CoV value of 1.17%), which is in line with previous studies.7–9 The agreement among the different devices to measure AQD was similar as the narrowest and widest LoA intervals were 0.16 and 0.27 mm, respectively. AQD measurement is important in predicting the effective IOL position after cataract surgery, as errors in predicting the lens position are the main factor influencing the accuracy in the IOL power selection.25 For example, it has been reported that a 0.25 mm error in the postoperative lens position can result in a lens power error up to 0.55 D, depending on the axial length of the eye.26

There was a variation among the repeatability to measure keratometric corneal power among the devices analyzed (for the anterior K, the CoV varied between 0.17% and 0.49%, and for the posterior K, the CoV varied between 0.30% and 1.94%). However, for the simulated keratometric values, the repeatability was similar among the instruments (CoVs about 0.30%). These results are similar to those obtained in previous studies.8,9,27 For the keratometric corneal power, both Placido-based topographers (MS-39 and Sirius) showed the widest LoA interval; however, these instruments showed the narrowest LoA interval for the simulated corneal power. In general, for the corneal topography parameters, the LoA interval ranged between 0.52 D and 1.21 D. For the anterior corneal astigmatism, the Rlim for the vector components was lower than 0.50 D for all instruments, and for the agreement between the instruments, the LoA interval never exceeded 0.75 D (Tables S2 and S3, available at https://links.lww.com/JRS/B59, https://links.lww.com/JRS/B60).

We evaluated instruments based on SS-OCT, OLCR, a combination of SS-OCT and Placido disk topography and a combination of Scheimpflug tomography and Placido disk topography to assess the repeatability and agreement to measure the anterior segment parameters. However, some of parameters were not included in the analysis due to the following reasons: 1) CD was not measured with Lenstar in this study, 2) posterior K was not available in Eyestar and it cannot be measured with Lenstar, 3) anterior Sim K values cannot be measured with Lenstar, and 4) posterior Sim K was measurable only with Eyestar. Based on the repeatability results from this study, we can calculate the measurement tolerance for different number of repeated measurements.10,11 From the smallest and largest Rlims for CCT, CD, AQD, and average anterior corneal power, we calculated the measurement tolerance for when 1, 2, or 3 measurements are taken with the respective instruments (Table S1, available at https://links.lww.com/JRS/B58). This table shows that even with just 1 measurement, the anterior segment parameters can be measured with minimal uncertainty. We can use this information to decide the number of repeated measurements needed depending on a desired tolerance. However, this result is only applicable for healthy eyes as this study only included healthy eyes.

The repeatability for CCT, CD, AQD, and keratometer was good with Eyestar and the other 3 devices. The agreement among all the instruments was good for CD and AQD measurements. For CCT and keratometer parameters, some of the devices showed large LoA interval suggesting that not all instruments can be used interchangeably.WHAT WAS KNOWN Optical biometry and corneal topography have converted into the gold standard for measuring ocular parameters. There are no previous studies comparing several instruments designed with different measurement principles to evaluate anterior segment parameters.

WHAT THIS PAPER ADDS The repeatability for central corneal thickness, corneal diameter, aqueous depth, and keratometer was good with Eyestar 900, Lenstar LS90, MS39, and Sirius. The agreement among all the instruments was good for corneal diameter and aqueous depth measurements. For central corneal thickness and keratometer parameters, some of the devices showed large limit of agreement interval suggesting that not all instruments can be used interchangeably. REFERENCES 1. Konstantopoulos A, Hossain P, Anderson DF. Recent advances in ophthalmic anterior segment imaging: a new era for ophthalmic diagnosis? Br J Ophthalmol 2007;91:551 2. Ang M, Baskaran M, Werkmeister RM, Chua J, Schmidl D, Aranha dos Santos V, Garhöfer G, Mehta JS, Schmetterer L. Anterior segment optical coherence tomography. Prog Retin Eye Res 2018;66:132–156 3. Gokul A, Vellara HR, Patel DV. Advanced anterior segment imaging in keratoconus: a review. 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