With the availability of state-of-the-art instrumentation for evaluation of the anterior segment parameters, comparative studies between devices are as essential as the independent investigation of each equipment. This subject becomes more important when patients may undergo anterior segment measurements through different tomography machines.[15] Furthermore, the development of corneal refractive surgery is greatly changing every day, safety is improving, and the visual quality after the surgery is getting more refined by customized surgical methods.
Scheimpflug imaging technology is increasingly being used as an established technique for measuring the anterior segment parameters. The newly introduced Scansys incorporates the Scheimpflug camera for assessing cornea and anterior segment organs. Before a new device becomes approved for routine clinical examinations, it necessitates knowing whether its measurements are reliable and in agreement with available and popular instruments. This study mainly aimed to compare some analogous parameters between Scansys and Pentacam HR, which either warrants or abolishes their possible interchangeable use in anterior segment examination. To our knowledge, most anterior segment parameters between these two Scheimpflug imaging systems have not been previously compared. In the present study, we found that most studied anterior segment parameters had excellent or good reliability in ICC analysis. However, ACA and horizontal WTW were two parameters with moderate reliability. This finding shows that horizontal WTW and ACA do not have good agreement between the two devices. In addition, analysis of Bland-Altman plots showed that the bias line for all measurements was close to zero, except for posterior flat-k and steep-k readings; however, the discrepancy was not large enough to be clinically important. Analysis of the limit of agreement in Bland-Altman plots also displayed narrow limits and clinically irrelevant differences in 95% LoA for anterior and posterior keratometry readings, CCT, TCT, and ACD. Then, the two devices are essentially equivalent in measuring the mentioned parameters. In terms of comparison among other Scheimpflug-based corneal topography systems, we explored the literature, and some main findings are presented as follows.
Among the corneal parameters, CCT and corneal curvature are two important indices for selecting corneal refractive surgery.[16] Therefore, the accuracy of measurements of these values is of great significance for evaluating the risk of complications such as corneal ecstasies and keratectasia after refractive surgery.[17–19]. The anterior segment evaluation and analysis systems can obtain various anterior segment parameters, including corneal thickness, corneal curvature, anterior chamber depth, and corneal diameter (WTW), in one measurement. It is an essential part of preoperative examinations in patients who are candidates for refractive surgery. Previous studies have confirmed that Pentacam has favorable repeatability and accuracy in measuring CCT and corneal curvature[20]. Some studies evaluated the agreement among Scheimpflug devices in measuring corneal curvature and pachymetry values in healthy eyes.[11, 21–24] In a prospective study, Anayol et al. investigated the agreement of three Scheimpflug-based devices (Pentacam HR, Sirius, and Galilei) for measuring anterior segment parameters in healthy individuals.[25] Regarding CCT, they reported that there was a significant difference between the three devices (P < 0.001), with the Galilei having the highest measurements than Pentacam and Sirius. In addition, Pentacam and Sirius devices showed better agreement with each other than with the Galilei. Also, similar findings were observed for TCT measurements. They reported that the differences between the CCT and TCT measurements of Pentacam and Galilei were − 13.93 and − 5.5 µm, respectively. In terms of corneal curvature, they reported that Galilei and Sirius devices displayed better agreement with each other than with Pentacam. This finding was in contrast with the reports of Wang et al., that revealed good agreement between Pentacam and Sirius in measuring anterior corneal curvature.[26] The authors concluded that these three Scheimpflug-based systems should not be used interchangeably for measuring corneal thickness and curvature data in healthy eyes. Their finding was approved by Nasser et al. in comparing the corneal curvature measurements between Sirius and Pentacam.[27] However, on contrary to their results, we found strong or good agreement between the two devices for measuring corneal thickness (CCT, TCT, and pupil center pachymetry) and front and back corneal curvature data. It is notable that we determined that Scansys represented statistically higher values of CCT and TCT compared to those provided by the Pentacam HR, which was consistent with the previous studies reporting that the Pentacam HR underestimated the measurements of CCT and TCT.[23, 28] Our finding was in line with the study of Jahadi Hosseini et al., in which they reported a good agreement between two Scheimpflug devices (Galilei and Pentacam) in measuring CCT data[28]. Also, when comparing Pentacam and Sirius, Huang et al. reported that the differences between the CCT and TCT measurements were − 3.3 and − 3.4 µm, respectively[29]. Therefore, compared to other devices that incorporate the Scheimpflug camera, the differences between the pachymetry findings of the Scansys and Pentacam HR had minimal clinical significance. Notably, the Bland-Altman analysis of pachymetry values showed excellent agreement between the two instruments, suggesting they could be used interchangeably. This finding is in line with previous studies on other devices with Scheimpflug technology[14, 21, 23, 25]. Results of the present study, otherwise, disagree with the ones published by la Parra-Colín et al., in which reported Pentacam and Sirius should not be used interchangeably for measuring CCT, TCT, and steep-K reading in normal healthy individuals.[21] This conclusion has been also deduced in patients with keratoconus.[12] It is notable that few studies investigated the interchangeability of Scansys and Pentacam HR for anterior segment measurements. In a recent study by Yu et al., the authors investigated the reliability and agreement of the pachymetry measurements in different corneal locations through Scansys and Pentacam HR.[14] Consistent with our findings, they reported that Scansys and Pentacam HR could be used interchangeably regarding corneal thickness measurements.
ACD is an established anterior segment parameter, which is predominantly important for preoperative evaluation in intraocular surgeries.[30, 31] Some studies reported that Scheimpflug corneal imaging devices are interchangeable in measuring ACD and the differences were within the acceptable clinical levels[21, 23, 32, 33]. In contrast, Anoyal et al. considered the differences as significantly different in ACD findings[25]. The authors asserted that interchangeable application of the two instruments might lead to miscalculations in intraocular lenses (IOL) power due to the somewhat wide differences in ACD measurements. We found a strong agreement in ACD findings between Scansys and Pentacam HR.
Horizontal WTW diameter is another anterior segment parameter that is applied for the power calculation of refractive IOLs. However, few studies investigated the agreement between different corneal topography devices, and their results are inconsistent[34, 35]. Some studies reported that corneal topography devices could be used interchangeably regarding horizontal WTW measurements,[32] whereas others argued that practitioners should not consider different devices as interchangeable in clinical practice[36–38]. In terms of Scheimpflug-based imaging devices, Domínguez-Vicent et al. evaluated the agreement between Pentacam HR and Galilei regarding horizontal WTW[39]. The authors reported that as the 95% limits of agreement were large enough to be clinically relevant, these two Scheimpflug imaging systems could not be used interchangeably. We also did not find a good correlation and agreement (r = .418 and ICC = 0.579) in horizontal WTW measurements between Scansys and Pentacam HR.
Despite the advantages, Pentacam HR is an expensive product, which is not affordable for some primary eye care centers. Scansys anterior segment analyzer is a new Scheimpflug imaging system which can acquire data of 107,520 or 230,400 points over the anterior and posterior corneal surfaces. As mentioned earlier, there is a lack of data on the reliability of Scansys in measuring CCT and corneal curvature measurements. This study evaluated the reliability and agreement of Scansys in measuring CCT and corneal curvature compared to the results of Pentacam HR measurements to explore its clinical application value. The different findings of this study compared to previous studies on Scheimpflug imaging devices could be explained by different sample sizes, different examiners and their skills, and different refractive statuses of the participants.
In terms of limitation, it should be noted that we did not include all anterior segment parameters provided by the two devices. In addition, the measurements of other topography systems with different imaging modalities, such as slit scanning and optical biometry devices, were not compared. Another limitation is that we only included healthy eyes, and further studies should be planned to compare other groups of patients, including those with corneal ectasia and corneal diseases excluded from the study.
Our findings confirmed the interchangeability of these two devices to be used for anterior segment evaluation regarding most of the examined parameters. Therefore, results obtained with the two devices can be considered comparable.