Abstract
Purpose
To evaluate the agreement in anterior segment measurements obtained with Pentacam HR and Visionix VX650 topographers, in terms of keratometry, pachymetry and anterior chamber parameters in healthy eyes.
Methods
Inclusion criteria consisted of healthy eyes with no history of ocular pathology or surgery. Measurements of central corneal thickness (CCT), minimum corneal thickness (MCT), anterior and posterior keratometry (K1, K2), anterior chamber depth (ACD), and anterior chamber angle (ACA) were taken using the Pentacam HR and Visionix VX650. Differences between devices were assessed with paired Student’s t-tests, and agreement was evaluated using Bland-Altman plots.
Results
Fifty-five eyes from 55 healthy participants mean age 32.55 ± 12.60 years) were examined. Anterior keratometry and MCT showed clinically acceptable agreement between devices, with no significant differences (p > 0.164). Pentacam HR reported steeper posterior keratometry, smaller ACD, lower CCT and larger ACA values compared with VX650 (p < 0.001). For CCT and MCT, ranges of agreement were large, with values of ± 27.07 and 55.41 μm, respectively. However, the ranges of agreement for anterior K1 and K2 were smaller, with values of ± 0.11 and 0.11 mm, respectively. Likewise, the ranges of agreement for ACD and ACA were also clinically significant, with a clear trend of VX650 to provide longer ACD measurements and lower ACA estimates.
Conclusions
Anterior corneal keratometry measured with the Pentacam HR and Visionix VX650 exhibit acceptable inter-device agreement. However, caution should be exercised regarding their interchangeability, especially when making decisions that require precise thickness (refractive surgery screening) or anterior chamber parameters (phakic intraocular lens implantation).
Keywords: Corneal topography, Anterior segment imaging, Pachymetry, Keratometry, Pentacam HR, Visionix VX650
Introduction
Advances in corneal diagnostic technology have enabled highly detailed assessment of corneal anatomy, providing clinicians with essential parameters for both diagnosis and treatment planning. Measurements such as keratometry and pachymetry are fundamental for characterizing corneal morphology, helping with the early detection of conditions such as keratoconus [1, 2], and the fitting of soft, rigid gas-permeable, and orthokeratology contact lenses [3–5].
Anterior chamber depth (ACD), defined as the distance between the corneal endothelium and the anterior lens surface, is another key parameter in anterior segment evaluation, as it contributes to early diagnosis of ocular pathologies and surgical planning [6, 7]. Likewise, the anterior chamber angle (ACA), formed between the posterior corneal surface and the anterior iris surface, provides clinically relevant information for the diagnosis and management of angle-related disorders, particularly glaucoma [8, 9].
The Pentacam HR (OCULUS Optikgeräte GmbH, Wetzlar, Germany) is one of the most widely used corneal imaging devices, employing a rotating Scheimpflug camera to generate high-resolution three-dimensional reconstructions of the anterior segment. In contrast, the Visionix VX650 (Visionix Luneau Technologies, Chartres, France) is a multi-diagnostic platform that not only provides corneal topographic and tomographic data but also integrates complementary evaluations, including intraocular pressure measurement and fundus imaging.
Understanding the level of agreement between devices is essential to ensure accurate interpretation of clinical data and to support consistent decision-making in daily practice.
The purpose of this study was to evaluate the agreement in anterior segment measurements obtained with Pentacam HR and Visionix VX650 topographers, in terms of keratometry, pachymetry, ACD and ACA in healthy eyes.
Methods
Measurements were taken in the Optometry Clinic of the University of Alicante (Alicante, Spain) by the same expert examiner (L.B.B.). The study followed the tenets of Declaration of Helsinki and was approved by the ethics committee of the University of Alicante (UA-2023-01-19_2). All patients signed a written informed consent.
Inclusion criteria required participants to have healthy eyes with no history of ocular pathology or prior ocular surgery. To confirm eligibility, all participants underwent a slit-lamp biomicroscopy examination and best-corrected visual acuity assessment. Contact lens wearers were required to discontinue lens use for at least two weeks before the measurements.
Examinations were conducted within a fixed time window between 11:00 and 12:00 to ensure consistency, using both Pentacam HR and Visionix VX650. Measurements were taken under dim light conditions, and patients were asked to blink immediately before the measurement. For each device, a single measurement per eye was acquired and only measurements meeting the devices’ predefined quality criteria were included in the analysis. Specifically, for the Pentacam HR, only scans classified as “OK” by the system were accepted, while for the Visionix VX650, only acquisitions free of artefacts (e.g., blinking, poor fixation, or misalignment) and rated as valid by the device software were considered. Both eyes were measured but only one randomly selected eye was used for statistical analysis purposes.
Pentacam HR
Pentacam HR is a topographer that employs a rotating Scheimpflug camera with slit illumination to capture images of the anterior segment. It performs up to 100 scans in two seconds, collecting 2,760 measurement points per scan. These data, including over 138,000 elevation values, are combined with the Scheimpflug images to generate a high-resolution three-dimensional model of the anterior segment.
Visionix VX650
Visionix VX650 is a multidiagnostic platform that allows to get measurements of the anterior and posterior segment. Corneal topography is measured with 24 Placido rings capturing 6,144 measurement points, while pachymetry, ACA and ACD are evaluated with a static horizontal Scheimpflug scan. A Hartmann-Shack aberrometer with 1,400 measurement points is used to get refraction and ocular wavefront aberrations. The posterior evaluation is performed using a non-mydriatic fundus camera with a resolution of 6 Mpx, while intraocular pressure is measured via an air-puff tonometer.
Unlike the rotating Scheimpflug tomography system of the Pentacam HR, which acquires multiple cross-sectional images over a 360° rotation, the VX650 relies on a single Scheimpflug acquisition combined with Placido-based corneal topography. This represents a fundamental difference in image acquisition geometry between both devices.
Sample size
Post hoc power analysis was performed using the GRANMO sample size calculator (IMIM, Barcelona, Spain; version 8.0). The analysis was based on a paired design, assuming a bilateral test with an alpha level of 0.05. For a sample size of 55 eyes, the statistical power exceeded 0.98 for detecting clinically relevant differences between devices in anterior K1, posterior K1, and central corneal thickness (CCT).
Statistical analysis
Software IBM SPSS Statistics version 29.0.1.0 (IBM SPSS Inc., Chicago, IL, U.S.) was used to complete the statistical analysis. Kolmogorov-Smirnov test was used to assess normality of the data.
Differences between both devices were assessed using the paired Student-T test. P-values equal or lower than 0.05 were considered statistically significant. Agreement between instruments was assessed using Bland-Altman plots [10]. The limits of agreement (LoA) were defined as the mean difference ± 1.96 times the standard deviation.
Results
A total of 55 eyes from 55 subjects were recruited, including 28 right eyes and 27 left eyes. The sample included 37 women and 18 men with an average age of 32.55 ± 12.60 years (range: 18–67). The mean spherical equivalent was − 1.25 ± 2.30 D and the best-corrected visual acuity was − 0.10 ± 0.06 logMAR.
Table 1 shows the comparison between instruments for the different corneal parameters. Among all variables, only minimum CCT (p = 0.568) and anterior surface keratometry (K1: p = 0.164; K2: p = 0.850) showed no significant differences between devices. In contrast, the VX650 reported significantly higher ACD, smaller ACA, and higher CCT measurements (p < 0.001).
Table 1.
Agreement between Pentacam HR and Visionix VX650 + for corneal and anterior segment parameters
| Parameter | Pentacam | VX650 | Difference | p-value |
|---|---|---|---|---|
| CCT (µm) | 539.64 ± 36.31 | 546.31 ± 41.46 | 6.67 ± 13.81 | < 0.001 |
| MCT (µm) | 536.96 ± 36.13 | 534.78 ± 43.57 | 2.18 ± 27.96 | 0.568 |
| Anterior K1 (mm) | 7.87 ± 0.26 | 7.86 ± 0.28 | 0.01 ± 0.06 | 0.164 |
| Anterior K2 (mm) | 7.70 ± 0.27 | 7.70 ± 0.28 | 0.00 ± 0.06 | 0.850 |
| Posterior K1 (mm) | 6.52 ± 0.25 | 6.74 ± 0.46 | 0.22 ± 0.34 | < 0.001 |
| Posterior K2 (mm) | 6.18 ± 0.26 | 6.38 ± 0.31 | 0.20 ± 0.19 | < 0.001 |
| ACD (mm) | 2.99 ± 0.29 | 3.11 ± 0.32 | 0.12 ± 0.12 | < 0.001 |
| Temporal ACA (º) | 37.97 ± 5.18 | 30.78 ± 3.70 | 7.20 ± 4.50 | < 0.001 |
| Nasal ACA (º) | 37.97 ± 5.18 | 30.30 ± 3.83 | 7.68 ± 4.59 | < 0.001 |
Abbreviations: CCT, central corneal thickness; MCT, minimum corneal thickness; K1, flattest keratometry; K2, steepest keratometry; ACD, anterior chamber depth; ACA, anterior chamber angle. Data is shown as mean ± SD
Figure 1 presents the Bland–Altman plots comparing both instruments across all measured parameters. Table 2 displays the corresponding Bland–Altman statistics, including mean differences, 95% confidence intervals, and limits of agreement (LoA) for each parameter. As shown, mean differences between devices were close to zero for anterior keratometry. For CCT and MCT, the ranges of agreement were wide, with values of ± 27.07 and 55.41 μm, respectively. In contrast, narrower limits of agreement were observed for anterior K1 and K2 (± 0.11 and ± 0.11 mm, respectively). Likewise, agreement for ACD and ACA showed clinically relevant variability, with a clear trend of VX650 to provide longer ACD measurements and lower ACA estimates.
Fig. 1.
Bland-Altman plots showcasing the difference between both instruments. Black line represents the bias and dotted lines represent the lower and upper limits of agreement
Table 2.
Bland–Altman analysis of agreement between the Pentacam HR and Visionix VX650+
| Parameter | Mean Difference (95% CI) | Lower LoA (95% CI) | Upper LoA (95% CI) | Range of agreement |
|---|---|---|---|---|
| CCT (µm) | 6.67 (2.94/10.41) | -20.40 (-26.87/-13.94) | 33.75 (27.28/40.22) | ± 27.07 |
| MCT (µm) | -2.18 (-10.20/4.96) | -57.59 (-70.72/-44.46) | 53.24 (39.21/65.47) | ± 55.41 |
| Anterior K1 (mm) | -0.01 (-0.03/0.00) | -0.12 (-0.15/-0.10) | 0.10 (0.07/0.13) | ± 0.11 |
| Anterior K2 (mm) | -0.01 (-0.02/0.01) | -0.12 (-0.15/-0.09) | 0.11 (0.09/0.14) | ± 0.11 |
| Posterior K1 (mm) | 0.22 (0.12/0.31) | -0.47 (-0.64/-0.31) | 0.90 (0.74/1.07) | ± 0.69 |
| Posterior K2 (mm) | 0.20 (0.16/0.26) | -0.16 (-0.25/-0.07) | 0.58 (0.49/0.67) | ± 0.36 |
| ACD (mm) | 0.12 (0.08/0.12) | -0.04 (-0.07/-0.01) | 0.24 (0.21/0.27) | ± 0.16 |
| Temporal ACA (º) | -7.20 (-8.41/-5.98) | -16.01 (-18.12/-13.91) | 1.62 (-0.49/3.73) | ± 8.81 |
| Nasal ACA (º) | -7.68 (-8.92/-6.44) | -16.67 (-18.82/-14.53) | 1.32 (-0.83/3.46) | ± 8.99 |
CI: confidence interval; LoA: limit of agreement; CCT, central corneal thickness; MCT, minimum corneal thickness; K1, flattest keratometry; K2, steepest keratometry; ACD, anterior chamber depth; ACA, anterior chamber angle
Discussion
Recent technological developments in anterior segment imaging have improved the accuracy and reliability of corneal assessment, allowing clinicians to obtain detailed measurements. Consequently, evaluating the agreement between different instruments has become essential to determine whether their measurements can be used interchangeably in clinical practice.
The present study compared anterior corneal measurements obtained with the Pentacam HR and the Visionix VX650 in eyes with healthy corneas. In previous studies, the Pentacam has demonstrated high repeatability for pachymetric and keratometric parameters in normal eyes [11, 12], whereas the VX650 has shown good repeatability for CCT and most keratometric values, with lower repeatability noted for posterior K2 [13]. Discrepancies between devices were found, likely attributable to differences in terms of measurement principles, the level of resolution of Scheimpflug images and the mathematical algorithms used for reconstruction of surfaces. For example, while the Pentacam employs rotating Scheimpflug imaging for determining the geometry of the anterior cornea, the Visionix system combines Scheimpflug imaging with Placido-ring topography.
Despite these methodological differences, anterior corneal keratometry values showed good inter-device agreement in the present study, with no statistically significant differences. Likewise, the ranges of agreement can be considered as clinically acceptable as they have a magnitude in the level of precision of keratometric readings (around 0.5 D). For posterior corneal curvature, the VX650 produced significantly flatter values compared to the Pentacam. These findings contrast with the results reported by Kumar et al., who observed significantly steeper anterior K1 and K2 measurements with Pentacam, with mean differences of 0.38 and 0.39 D, respectively, and flatter posterior K2 values (difference of 0.36 D). However, measurements obtained by these authors [13] of posterior K1 was significantly steeper with Pentacam, as in our series.
The higher inter-device agreement for anterior parameters compared with posterior measurements may be explained by several factors. The anterior corneal surface is more accessible, facilitating accurate measurement across devices [14]. In contrast, the posterior corneal surface must be imaged through the anterior stroma and endothelium, which may reduce contrast and increase sensitivity to minor misalignment or ocular micro‑movements, leading to light scatter [15]. Indeed, prior studies have documented greater variability and poorer reproducibility for posterior keratometry or elevation measurements in Scheimpflug imaging systems than for anterior surface data [16, 17].
Regarding pachymetry, the Pentacam produced higher CCT values, with a mean difference of 6.67 μm (95% CI: 2.76 to 10.37), consistent in magnitude with the previously reported difference (− 5.69 μm; 95% CI: −7.91 to − 3.46) [13], although the sign differs due to the order of subtraction used in each study. In contrast, no significant differences between devices were found in MCT, with a mean difference observed in this study of 2.18 μm (95% CI: -10.10 to 5.01), closely matching the one reported by Kumar et al. (− 3.73 μm; 95% CI: −7.97 to 0.51) [13]. Although these differences were of small magnitude, the LoA between devices were large and clinically relevant. Discrepancies between devices up to more than 25 μm can be obtained and this error can be relevant for aspects such as detection of ectatic diseases [1] and calculation of residual stromal bed in corneal refractive surgery [13].
From a clinical perspective, the observed limits of agreement for CCT and MCT may have relevant implications for refractive surgery planning and corneal ectasia screening. Differences of this magnitude can influence the classification of borderline corneas and affect key surgical decisions, such as the estimation of residual stromal bed thickness in corneal refractive procedures. Therefore, these parameters should be interpreted with caution when comparing data obtained from different devices, and interchangeability between systems should be avoided in clinical decision-making. For longitudinal follow-up, the use of a single device is strongly recommended to ensure consistency and avoid measurement-related variability.
The differences in agreement observed for pachymetric parameters may be related to differences in the acquisition strategy between both devices. The Pentacam HR, based on rotating Scheimpflug imaging, enables the reconstruction of a full three-dimensional model of the cornea, allowing a more comprehensive spatial assessment and more precise localization of the minimum corneal thickness. In contrast, the VX650 relies on a single Scheimpflug acquisition combined with Placido-based data, which provides more limited cross-sectional information. This difference in spatial sampling may partly explain the wider limits of agreement observed for MCT compared with CCT, as the identification of the thinnest point is particularly sensitive to sampling density and reconstruction strategy.
Finally, significant differences were also observed in the anterior chamber measurements. The VX650 provided smaller ACA values and larger ACD measurements compared with the Pentacam. While Kumar et al. documented almost identical ACD values between devices (mean difference of 0.02 mm), the present findings revealed a larger difference of 0.12 mm, suggesting that these parameters may be more sensitive to methodological variations or population-related factors. Differences in ACA measurements may be related to variations in the underlying calculation methods implemented by each device.
Limitations of the study include the relatively small sample size, which may reduce the generalizability of the findings. Additionally, only healthy eyes were included, limiting the applicability of these results to patients with corneal pathologies, post-surgical corneas or other corneal conditions. Future studies involving larger and more diverse populations, including eyes with corneal ectasia, would help to further validate the inter-device agreement of these instruments.
A notable strength of this study is the standardization of measurement conditions, as all assessments were performed at the same time of day. This approach minimizes the potential influence of circadian variations [18], enhancing the reliability and comparability of the results.
In conclusion, this study demonstrates that anterior corneal keratometry measured with the Pentacam HR and Visionix VX650 exhibit acceptable inter-device agreement, while posterior corneal keratometry, pachymetry and anterior chamber measurements are more variable. Overall, the results emphasize that posterior corneal curvature, corneal thickness and anterior chamber depth and angle measurements are not directly interchangeable between Pentacam and Visionix VX650 system. Therefore, the same instrument should be used consistently for patient follow-up to ensure accurate longitudinal comparisons.
Author contributions
All authors contributed to the conception and design of the study. LBB drafted the manuscript, prepared the figures, and organized the tables. All authors contributed to data interpretation, provided critical feedback on the manuscript, and approved the final version for submission.
Funding
The author Laura Barberán-Bernardos was supported by the Conselleria d’Educació, Cultura, Universitats i Ocupació of the Generalitat Valenciana within the Program ACIF (Subvenciones para la contratación de personal investigador predoctoral), reference number CIACIF/2022/073, cofinanced by European Social Fund, as well as within the program CIBEFP/2024/117 (Subvenciones para estancias de contratados predoctorales en centros de investigación fuera de la Comunitat Valenciana). The rest of authors did not receive any type of funding for the performance of the study.
Data availability
Data are available upon reasonable request to the corresponding author.
Declarations
Ethical approval
The study followed the tenets of the Declaration of Helsinki and was approved by the ethics committee of the University of Alicante (UA-2023-01-19_2).
Consent to participate
All recruited patients were informed about the nature of the study and provided written informed consent prior to their inclusion.
Consent for publication
Not applicable.
Competing interests
The authors declare no competing interests.
Footnotes
Publisher’s note
Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
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Associated Data
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Data Availability Statement
Data are available upon reasonable request to the corresponding author.