Assessment of corneal biomechanics, tonometry and pachymetry with the Corvis ST in myopia

To evaluate the repeatability of Corvis ST corneal biomechanical, tonometry and pachymetry measurements, and agreement of pachymetry measures with the Pentacam HR and RTVue OCT. Three consecutive measurements of the right eye of 238 myopic subjects were acquired with the Corvis ST, Pentacam HR, and RTVue OCT. Repeatability of Corvis ST was evaluated by within-subject standard deviation [Sw] and repeatability limit [r]. The agreement of central corneal thickness (CCT) measurements were compared among the three instruments using the Bland–Altman limits of agreement. Comparisons were further stratified by CCT (Corneathin ≤ 500 µm; Corneanormal = 500–550 µm; Corneathick > 550 µm). Sw was below 1 mmHg in Corneathin, Corneanormal, and Corneathick groups for IOP and bIOP. Sw for SP-A1 were 4.880, 6.128, 7.719 mmHg/mm respectively. Sw for CBI were 0.228, 0.157, 0.076, and correspondingly Sw for TBI and SSI were 0.094 and 0.056, 0.079 and 0.053, 0.070 and 0.053. The Bland–Altman plots for CCT implied poor agreement with mean differences of 29.49 µm between Corvis and OCT, 9.33 µm between Pentacam and OCT, and 20.16 µm between Corvis and Pentacam. The Corvis ST showed good repeatability with the exception of CBI in the various CCT groups. The CCT measured by Corvis ST was not interchangeable with Pentacam HR and RTVue OCT.

www.nature.com/scientificreports/ deformation and then rebounds back to its original shape. Using the Corvis ST, the deformation of the central 8 mm of the cornea along the horizontal meridian is recorded using a high-speed Scheimpflug camera at a rate of 4330 frames per second. Within 31 ms, the Corvis ST acquires 140 images of the cornea with a high resolution of 640 × 480 pixels. High-speed imaging allows for analysis of the detailed movement of the cornea during the deformation process, thus may provide clinically relevant parameters correlated with the biomechanical properties of the cornea. As research using the Corvis ST advances, new parameters have been developed. The latest software version (V.1.6r2015) offers a total of 25 parameters, including dynamic corneal response (DCRs), Vinciguerra screening data and biomechanical/tomographic measurement and an promising parameter of Stress-Strain Index (SSI) which seems to be independent of IOP and corneal thickness. An ideal device for measuring corneal biomechanics should provide precise (repeatable and reproducible) measurements. The current study aimed to evaluate the repeatability of measurements acquired with the Corvis ST and investigate whether repeatability varied across different CCT thicknesses. Finally, the agreement of CCT measurements using the Corvis ST with the Pentacam HR (Oculus Optikgeraẗe GmbH, Wetzlar, Germany, software version.1.2r43) and the RTVue OCT (Optovue Inc., Fremont, California, USA, software version.2018.0.0.18) centered on the corneal apex were compared.

Patients and methods
This was a prospective observational study. Patients presenting for refractive surgery (no previous corneal laser surgery history) at the First Affiliated Hospital of Xi'an Jiaotong University between April and August 2019 were invited to participate in the current prospective study. Exclusion criteria included the following conditions: rigid lens wear, currently pregnant or nursing, and a history of ocular surgery. Soft lens wearers were required to cease contact lens wear for at least 7 days prior to measurements, IOP > 21 mmHg or IOP < 10 mmHg, keratoconus and other corneal pathologies. The study was approved by the Ethics Board of the First Affiliated Hospital of Xi'an Jiaotong University Institute, and was performed in compliance with the tenets of the Declaration of Helsinki. All participants signed an informed consent form before participation in the study.
All subjects underwent comprehensive ophthalmic examinations, including slit-lamp biomicroscopy, fundoscopy, auto-refraction (AR-1, NIDEK, Japan), subjective manifest refraction with an undilated pupil (RT-500, NIDEK, Japan), and Scheimpflug corneal topography with the Pentacam HR (Oculus Optikgeraẗe GmbH, Wetzlar, Germany, software version.1.2r43), and anterior segment OCT (RTVue, Optovue Inc., Fremont, California, USA, software version.2018.0.0.18). All measurements were acquired with pachymetry mode centered on the corneal vertex normal (apex). Three consecutive measurements were performed on each eye using the Corvis ST (Oculus Optikgeraẗe GmbH, Wetzlar, Germany, software version. V.1.6r2015) by the same experienced technician. Only image quality graded as "OK" were collected. All the examinations were performed between 10:00 and 17:00. For each patients, all examinations were conducted within 1 h. Two hundred and thirty eight patients (112 females and 126 males, mean age 25.41 ± 7.44 years, range 18 to 51) were enrolled. Only the right eye of each patient was included in the analyses to avoid the bias of the bilateral eye correlation. To evaluate the possible influence of CCT on the performance of the Corvis ST, subjects were divided into three groups based on the CCT measured with the anterior segment OCT: thin cornea (Cornea thin , CCT ≤ 500 µm), normal thickness cornea (Cornea normal , CCT = 500-550 µm), and thick cornea (Cornea thick , CCT > 550 µm).
Statistical analyses were performed using SPSS version 24.0 (IBM, Armonk, New York, USA). Descriptive data were expressed as mean ± standard deviation (SD). The Shapiro-Wilk test was used to assess if data were normally distributed. To determine the repeatability of the Corvis ST measurements, the within-subject standard deviation (S w ) of three consecutive measurements was calculated. The repeatability limit was also calculated and is defined as 1.96√2 × S w 14,15 . Bland-Altman plots were used to assess the measurement agreement of CCT among the three devices. For the comparison of biomechanical characters among the three corneal thickness groups, a linear mixed model analysis of covariance (ANCOVA) was used to adjust for the effect of age and spherical equivalent (SE).  Table 1. Subjects in the Cornea thin group were significantly older than the other two groups. The eyes in this group were also more myopic and had slightly longer axial lengths (AL). CCT with both OCT and Corvis ST were significant different among the three groups (P < 0.001) (Figs. 1, 2, 3). No significant differences in Km or corneal diameter were detected among those three groups.

The repeatability of Corvis ST measurements.
In all three corneal thickness groups, the majority of parameters including IOP, bIOP, CCT, DA ratio 2, SP-A1, Defle A, A1V, A1T, A2V, A2T, PD, Def A showed good repeatability ( Table 2). In thick corneas, the repeatability of the Corvis ST measurement was slightly better than thin and normal corneas ( Table 2).

Discussion
Changes in corneal biomechanical properties are thought to appear prior to morphological changes in ectasia. Therefore, corneal biomechanical measurement is of significant importance in terms of early detection of corneal ectatic diseases such as keratoconus and post corneal refractive surgery ectasia. However, the measurement precision of new technology is of utmost importance. To the best of our knowledge, this is the first study, with a large sample size, to evaluate the repeatability of latest version of Corvis ST in healthy subjects with myopia seeking refractive surgery and stratifying by CCT. Our results showed that the repeatability for the majority of Corvis ST measurement parameters were good. The repeatability in the thick corneas appears to be marginally better than the normal and thin cornea groups. It is not clear why this difference was elicited in this study. The corneal deformation behavior, in particular, was significantly affected by corneal thickness.  17 showed an intraclass correlation coefficient (ICC) of CCT of 0.97 and Chen et al. 18 reported S w values of 3.57 μm and 8.16 μm for CCT in virgin and post laser-refractive corneas, respectively. In the current study, S w and r for CCT measurements ranged 6.165 to 7.658 μm, and 17.078 to 21.213 μm respectively, in all groups. Similarly, IOP and bIOP also showed good repeatability in all three groups. The S w and r for IOP and bIOP were < 1 mmHg (range 0.777 to 0.997 mmHg) and < 3 mmHg (range 2.153 to 2.761 mmHg) respectively. Lopes et al. 16 found the S w and r for IOP were 0.98 mmHg and 2.7146 mmHg, and the S w and r for bIOP were 0.89 mmHg and 2.47 mmHg. Nemeth et al. 17 found an ICC of 0.865 and coefficient of variation (CoV) of 6.9% for IOP. Chen et al. 18 demonstrated repeatable measurements for IOP in both virgin and post-PRK eyes with S w values of 0.59 and 0.55, and r values of 1.62 and 1.52, respectively. The bIOP algorithm was developed using numerous simulations with the Corvis-ST on human eye models with different CCT, age, topographies, material properties, and IOP values. This has been shown to have significantly reduced the reliance of IOP measurement on the above mentioned parameters 19 , and the bIOP correction has successfully provided close estimates of true IOP in ex-vivo tests conducted on human donor eye globes 20 . Also, Matsuura et al. 21 found that the bIOP measurement from the Corvis ST is independent from CCT, akin to our study findings.
Dynamic corneal responses (DCRs) are defined as "deformation" parameters, while those from which whole eye movements are removed are described as "deflection" parameters. Some DCRs including Def A, DA ratio, defle A and defleA ratio at 2 mm showed good repeatability results in thick corneas. These findings were consistent with previous studies 16,22 . Wu et al. 23 found that the deformation amplitude (DA) exhibited excellent repeatability with a S w of 0.098, which was similar to our result, albeit slightly higher. Nemeth et al. 17 showed good repeatability for IOP and pachymetric values, whereas other measurements. Interestingly, our results demonstrated that a thinner CCT was associated with lower repeatability of measurements but perhaps without clinical significance, as shown by higher S w and r values in general.
Researchers hypothesized that there was a decrease in the SP-A1, the displacement between the undeformed cornea and cornea at the first applanation, in keratoconus compared with normal eyes. They concluded that SP-A1 could be a potential marker when evaluating disease progression [24][25][26] . We found that the repeatability of  www.nature.com/scientificreports/ www.nature.com/scientificreports/ SP-A1 in thin, normal and thick corneas were good. Ambrósio's Relational Thickness to the horizontal profile (ARTh) 24,27 is based on the thickness profile in the temporal-nasal direction. We found that the S w and r values of ARTh gradually reduced with increased CCT. The S w values of ARTh were 59.239, 53.466 and 61.090 in thin, normal and thick cornea groups respectively, which appear poor. The Corneal Biomechanical Index (CBI) is a parameter that aims to provide early detection of keratoconus, with a cut off value of 0.5 24 . In the current study, the S w values for the CBI were 0.228, 0.157 and 0.076 in the thin, normal and thick corneas, respectively. The actual CBIs in this normal group of myopia subjects in the respective CCT groups were 0.55, 0.27 and 0.05. Hence, the thin cornea group in our study had a mean CBI value above the cut-off value for abnormality. The TBI is a combined parameter based on Scheimpflug-based corneal tomography and biomechanical assessments; generated by the leave-one-out cross-validation (RF/LOOCV) and is suggested to provide greater accuracy for detecting ectasia 28 . The TBI may be sensitive for detecting subclinical (fruste) ectasia among eyes with normal topography. In our study, the S w values for TBI ranged from 0.070 to 0.094, demonstrating better repeatability than CBI values.
The stress strain index (SSI), is the corrected biomechanical index which has been shown to be independent of IOP and CCT 29 . We found no difference between values in the thin and normal corneal thickness groups. However, a significant difference between the thin or normal CCT and thick CCT was observed. The repeatability of SSI in three groups was very good, with S w values of 0.056, 0.058 and 0.058 respectively.
In the evaluation of measurement repeatability, different examination intervals have been used among studies. Ali et al. 30 compared Corvis ST measurements at the same time of a day across four different days, or at four times of a day, or taken 2 to 5 min apart. They found similar results for IOP, CCT and A1T with good repeatability. These results are comparable to the present study. However, in their study, DA, PD, A1V, A2V, A2T and HCT, had poorer repeatability than the present study. Ali et al. found A1L and A2L parameters had poor repeatability with all three intervals, which was similar to ours findings. Wang et al. 22 assessed repeatability with 5-min intervals, and had an ICC of 0.98 for IOP, but they also found the A1L, A2L and HCT had poorer ICCs of 0.17, 0.21 and 0.43 which is in accordance with our findings. Interestingly, we observed better repeatability for PD and HCR in all three groups.
Previous studies have demonstrated that the majority of biomechanical metrics offered by the Corvis ST are significantly correlated to CCT, IOP, age, and SE 9,11,[31][32][33][34][35] . However, the Corvis ST parameters are more affected by IOP than by CCT 9,36 . We also demonstrated that the deformation parameters were negatively associated with CCT in normal eyes, which is a similar concept to the idea that a thicker cornea deforms less, and a thinner cornea deforms more 37,38 . The main limitations of this study were the relatively small sample in one of the www.nature.com/scientificreports/ subgroups (thick cornea group) and the exclusion of patients with keratoconus and other corneal diseases. The latter will be a focus of future study.
In conclusion, overall the repeatability of the parameters measured with the Corvis ST were good with S w values generally observed to be better in thicker corneas, with corneal deformation significantly affected by corneal thickness. Possible markers for the early detection of ectasia appear repeatable with the exception of CBI and ARTh. The parameter SSI appears promising parameter with differences observed between thin or normal corneal thickness and thick corneas. The CCT measurements with the Corvis ST were found not to be interchangeable with those acquired with the RTVue OCT and Pentacam HR.   www.nature.com/scientificreports/ Publisher's note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creat iveco mmons .org/licen ses/by/4.0/.