Assessment of Tonometry Methods in Keratoconic Eyes Following Intracorneal Ring Segments Implantation: A Comparative Study

Article information

Korean J Ophthalmol. 2025;39(3):231-240

Publication date (electronic) : 2025 May 7

doi : https://doi.org/10.3341/kjo.2025.0016

Siamak Zarei-Ghanavati ¹, Seyed Mehdi Tabatabaei ², Samaneh Gholamhoseinpour-Omran ¹, Hamed Hosseinikhah-Manshadi ¹^,², Saeed Banan ¹, Mehdi Aminizade ², Kosar Esmaili ², Ebrahim Azaripour ²^,³

¹Eye Research Center, Mashhad University of Medical Sciences, Mashhad, Iran

²Glaucoma Service, Farabi Eye Hospital, Tehran University of Medical Sciences, Tehran, Iran

³Eye Research Center, Amiralmomenin Hospital, Guilan University of Medical Sciences, Rasht, Iran

Corresponding Author: Seyed Mehdi Tabatabaei, MD. Glaucoma Service, Farabi Eye Hospital, Tehran University of Medical Sciences, Qazvin Square, Tehran 1336616351, Iran. Tel: 98-9122307414, Fax: 98-2155410520, Email: Meh.Tabatabaei@gmail.com

Received 2025 February 4; Revised 2025 April 18; Accepted 2025 May 6.

Abstract

Purpose

To compare intraocular pressure (IOP) readings from corneas with intracorneal corneal ring segments (ICRS) using various methods, including Goldmann applanation tonometry (GAT), Tonopen, corneal-compensated IOP from the Ocular Response Analyzer (ORA), and biomechanically corrected IOP from the Corneal Visualization Scheimpflug Technology (Corvis ST).

Methods

This cross-sectional observational study included participants who had undergone ICRS implantation with KeraRing at least 3 months before the study. The mean IOP recorded by different instruments was compared using analysis of variance. Agreement among the methods was assessed with Bland-Altman plots.

Results

A total of 54 eyes from 27 participants were enrolled. The mean IOP measured by Tonopen was significantly lower in the center compared to the peripheral quadrants (p < 0.001). IOP measured by GAT was significantly lower than that measured by Tonopen (13.02 ± 2.31 mmHg vs. 14.50 ± 2.91 mmHg, p = 0.021). There were no significant differences between the IOP measurements provided by Tonopen, ORA, and Corvis ST. The corneal-compensated IOP from ORA and biomechanically corrected IOP from Corvis ST had the highest correlation, with a weak intraclass correlation coefficient of 0.38.

Conclusions

IOP measurements using Tonopen were significantly lower in the central 5-mm zone compared to other quadrants. GAT measurements were significantly lower than those from Tonopen. Different measurement tools did not show a strong correlation. Corvis ST (biomechanically corrected IOP) tended to present lower readings at higher IOP levels in eyes with ICRS.

Keywords: Biomechanically corrected intraocular pressure; Corneal-compensated intraocular pressure; Intracorneal ring segments; Intraocular pressure; Keratoconus

Keratoconus is a progressive, noninflammatory corneal ectatic condition characterized by the gradual steepening and thinning of the cornea. This leads to a significant degree of myopia and irregular astigmatism, resulting in a marked decline in visual acuity [1]. Currently, various therapeutic options are available for visual rehabilitation in keratoconus, including intracorneal ring segments (ICRS) implantation [2]. ICRS are small polymethyl methacrylate (PMMA) devices inserted into the cornea to reshape its geometry and improve refractive qualities, ultimately enhancing the patient’s visual acuity [3,4]. Corneal ring segments do not alter central corneal thickness (CCT) [5]. These segments act as spacers between collagen fibers within the corneal stroma, inducing an arc-shortening effect that flattens the central corneal area. The degree of induced flattening is directly proportional to the thickness of the inserted segment and inversely proportional to its diameter [6,7].

Reliable intraocular pressure (IOP) measurement is crucial for the diagnosis and management of glaucoma [8]. Changes in corneal geometry due to the presence of ring segments in the corneal stroma can potentially affect the accuracy of IOP measurements, especially with applanation tonometry [9]. Goldmann applanation tonometry (GAT) is the most commonly utilized method for measuring IOP. Despite its widespread use, GAT readings are susceptible to influences from various physical characteristics, such as corneal curvature and CCT [10,11]. Additionally, factors like corneal elasticity and tear film quality can affect IOP readings [12,13]. Consequently, the introduction of ICRS may present challenges in achieving precise IOP measurements using GAT, given their potential impact on corneal curvature, elasticity, and tear film distribution. These effects are expected to be more pronounced when the optical zone of the ring segment implant is 5.5 mm compared to 6.5 mm [9].

There are several other methods to measure IOP, including the Tonopen XL (Reichert Ophthalmic Instruments), the Ocular Response Analyzer (ORA; Reichert Ophthalmics), and the more recent Corneal Visualization Scheimpflug Technology (Corvis ST; Oculus). Corneal-compensated IOP (IOPcc) and biomechanically corrected IOP (IOPb) provided by ORA and Corvis ST, respectively, consider the biomechanical properties of the cornea and may be more accurate in cases of ectatic corneal pathologies [14].

The objective of this study is to compare IOP readings obtained from corneas with ICRS using various aforementioned methods. Additionally, the study sought to ascertain the correlation among these measurements.

Materials and Methods

Ethics statement

This study was approved by the Institutional Review Board of Mashhad University of Medical Sciences (No. 960624). Informed consent was obtained from each participant before the study. The investigation adhered to the principles outlined in the 2013 Declaration of Helsinki.

Study design and setting

This cross-sectional study included 27 participants, comprising 16 men and 20 women, with a total of 54 eyes affected by keratoconus. Patient recruitment took place from 2019 to 2021. The main outcome measure was IOP as assessed by different tonometry instruments.

Keratoconus was diagnosed based on clinical and topographic findings. Topographic assessments were performed using the Pentacam (Oculus). Inclusion criteria required confirmation of keratoconus and the completion of ICRS procedures at least 3 months before the study. Participants needed to be free of ocular pathologies such as glaucoma or retinal disease, without a history of other ocular surgeries including collagen crosslinking, and were not using any medications that could potentially alter IOP measurement. Patients with corneal scars were also excluded. All patients underwent thorough ophthalmic examinations, including a complete medical history, corrected distance visual acuity, slit-lamp biomicroscopy, and fundus examination.

KeraRings (Mediphacos) with diameters of 5.0 and 6.0 mm were employed as ICRS. The determination of the number, thickness, and length of the segments was guided by KeraRing nomograms. The tunnel depth was precisely established at 75% of the thickness of the corneal ring site. The tunnel itself was created using a femtosecond laser (Femto LDV Z6, Ziemer Group).

Tonometry measurements were performed by an individual thoroughly familiar with the method, conducted three times for each eye, and the averages were recorded. The time interval between consecutive measurements was 10 minutes. All IOP measurements were performed in the morning between 9 am and 12 pm. IOP measurements were done using four different methods; including GAT, Tonopen XL, ORA, and Corvis ST. The sequence of IOP measurements across different instruments was assigned randomly. Given the Tonopen XL’s small tip and the possibility of result variation with different corneal points and also to differentiate IOP measurements in the central region from peripheral quadrants, the cornea was divided into five sections, and measurements were taken at various points. These regions encompassed the central area, as well as the superior, inferior, nasal, and temporal quadrants. The central region specifically covered the central 5 mm in diameter of the cornea.

The GAT operates on the Imbert-Fick principle, correlating the force needed to flatten an area of the cornea with the pressure within the eye. This is achieved by applying force to flatten a circular area of the cornea with a diameter of 3.06 mm, where corneal rigidity and tear film surface tension are balanced. The resulting force in grams, multiplied by 10, provides the IOP in millimeter of mercury [15,16]. Although GAT is widely regarded as the clinical gold standard for measuring IOP, it has limitations. GAT relies on patient positioning at the slit-lamp, lacks portability, cannot be used in the supine position, and poses a risk of infections due to the necessary direct eye contact [17].

The Tonopen XL, a portable handheld instrument, utilizes the Mackay-Marg principle and micro strain-gauge technology. With a 1.5-mm transducer tip covered by a disposable cap, it gently applanates the cornea, recording 10 readings and calculating the mean for precise IOP measurements [9]. Its application over a smaller corneal area (1.5 mm) makes it suitable for irregular corneas. Nevertheless, it is important to consider that the accuracy of Tono-Pen readings may be influenced by corneal thickness [15,18].

ORA is an automated, noncontact tonometer that provides two measurements of IOP: Goldmann-correlated IOP (IOPg) and IOPcc [19]. It utilizes a precisely metered air pulse to indent the cornea, recording two applanation pressure measurements (P1 and P2). The average of P1 and P2 yields the Goldmann-correlated IOP, calibrated to correspond with GAT. Corneal hysteresis, the difference between inward and outward applanation pressures, forms the basis for IOPcc, which is expected to be less affected by corneal properties compared to other tonometric measurements [20].

Corvis ST is a noncontact tonometer designed for precise measurement of IOP and assessment of the cornea’s biomechanical response to an air pulse. The device records the corneal reaction through Scheimpflug imaging, capturing approximately 4,330 images per second. Corvis ST calculates IOPb. This parameter considers the dynamic corneal response, providing an estimate of corrected IOP that is minimally affected by ocular biomechanical factors such as age, CCT, and radius at the highest concavity [21,22].

Statistical analysis

In this study, we utilized various statistical measures to present data, including mean, standard deviation, median, range, interquartile range, and mean absolute error. To assess the agreement among methods and the mean average of all methods, we employed the Pearson correlation coefficient, intracluster correlation (ICC), and 95% limits of agreement (LoA). The difference between methods and the average of values was evaluated using the paired t-test. Also, a pairwise comparison of the methods was conducted. The agreement of methods was visually demonstrated through Bland-Altman plots, with any trends of disagreement illustrated by a Loess fitted line. Statistical analyses were conducted using IBM SPSS ver. 27 (IBM Corp), and a p-value less than 0.05 was considered statistically significant. We utilized GraphPad Prism ver. 10 (GraphPad Software) for both statistical analysis and creating graphical representations. The normality of the data was verified using the D’Agostino-Pearson test.

Results

In this study, 54 eyes from 27 patients were examined. The mean age was 26.9 ± 7.5 years, and 14 patients (51.9%) were female (Table 1). The mean CCT was 470.0 ± 36.11 μm.

Table 1

Descriptive statistics of measured variables (n = 27)

A notable difference among the four tonometers was observed (p < 0.05) through the application of repeated measures one-way analysis of variance (ANOVA) (Fig. 1A). This statistical test accounts for matched or repeated data within each row. Fig. 1A also incorporates the outcomes of pairwise comparisons, analyzed using the post hoc Tukey test. IOP measured by GAT was significantly lower than that measured by Tonopen (13.02 ± 2.31 mmHg vs. 14.50 ± 2.91 mmHg, p = 0.021).

Fig. 1

Box plots of (A) the disparities among the different intraocular pressure (IOP) measurement tools and (B) comparative analysis of Tonopen measurements across different corneal zones. GAT = Goldmann applanation tonometry; Tonopen-C = Tonopen (Reichert Ophthalmic Instruments) measurement in the central 5-mm zone; ORA = Ocular Response Analyzer (Reichert Ophthalmics); IOPcc = corneal-compensated intraocular pressure; Corvis ST = Corneal Visualization Scheimpflug Technology (Oculus); IOPb = biomechanically corrected intraocular pressure. ^*p < 0.05 (post hoc Tukey test).

Fig. 1B illustrates variations in Tonopen measurements across different zones of the cornea. The mean IOP measurements for the central corneal region, superior quadrant, nasal quadrant, inferior quadrant, and temporal quadrant were 14.5 ± 2.91, 17.07 ± 3.41, 16.85 ± 3.01, 17.72 ± 4.52, and 17.52 ± 3.52 mmHg, respectively. Repeated measures of one-way ANOVA revealed a highly significant difference (p < 0.001) among these distinct zones. Specifically, Tonopen measurements in the central 5-mm zone (Tonopen-C) were significantly lower compared to those from the peripheral quadrants of the cornea (p < 0.001).

Table 2 illustrates the comparison of each IOP value provided by each device with the mean of all values. The correlation and agreement analyses among various IOP measurement methods are also provided in Table 3. For each comparison, we provide the results of Pearson correlation, 95% LoA, and ICC. These metrics collectively offer a comprehensive assessment of the relationships and consistency across the diverse IOP measurement techniques. In Fig. 2A–2F, Bland-Altman plots vividly illustrate the level of agreement within the same comparisons. These graphs provide a visual representation of the agreement between various parameters, enhancing our understanding of the consistency and potential biases in the measured data. Bland-Altman plots also include simple linear regression for illustration. For the comparison between GAT versus Tonopen-C simple linear regression provided an equation of Y = −0.4339 × X + 4.489 (R² = 0.05, p = 0.097). Similar equations for comparison between GAT versus IOPb (Corvis ST), Tonopen-C versus IOPb (Corvis ST), and IOPcc (ORA) versus IOPb (Corvis ST) were Y = 0.7289 × X − 10.45 (R² = 0.26, p < 0.001), Y = 1.039 × X − 13.89 (R² = 0.43, p < 0.001), and Y = 0.8693 × X − 12.03 (R² = 0.42, p < 0.001), respectively.

Table 2

The results of paired t-test for different tonometry methods and the average of all values

Table 3

Correlation and agreement between different IOP measurements

Fig. 2

Bland-Altman plots of agreement between different intraocular pressure (IOP) measurements. For each comparison mean bias and 95% limits of agreement are shown. (A) Goldmann applanation tonometry (GAT) versus Tonopen (Reichert Ophthalmic Instruments) measurement in the central 5-mm zone (Tonopen-C). (B) GAT versus corneal-compensated IOP (IOPcc) of the Ocular Response Analyzer (ORA; Reichert Ophthalmics). (C) GAT versus biomechanically corrected IOP (IOPb) of the Corneal Visualization Scheimpflug Technology (Corvis ST; Oculus). (D) Tonopen-C versus ORA (IOPcc). (E) Tonopen-C versus Corvis ST (IOPb). (F) ORA (IOPcc) versus Corvis ST (IOPb).

Discussion

Precision in measuring IOP is crucial for ophthalmologists, especially when dealing with altered ocular surfaces. This challenge is particularly pronounced in conditions such as keratoconus or after corneal refractive surgeries, where obtaining accurate measurements becomes more complex [16]. In keratoconus patients, characterized by a thinner, steeper, and more astigmatic cornea, ICRS are implanted to stabilize and strengthen the ectatic cornea. This helps reduce corneal steepening and associated astigmatism [2,23,24]. Although ICRS implantation does not directly modify the corneal structure, the introduction of rigid rings—PMMA, which is approximately 2,800 times stiffer than the cornea—can affect corneal deformation during tonometry. This, in turn, can influence biomechanical metrics assessed by the Corvis ST and potentially alter IOP measurements [25].

GAT is widely considered the gold standard for measuring IOP. However, the presence of ICRS can impact the accuracy of GAT readings. Changes in corneal geometry due to ICRS can affect corneal curvature, elasticity, and tear film distribution, which in turn can distort IOP measurements [26,27]. These effects are notably more pronounced with smaller optical zones, such as the 5.5-mm diameter, compared to larger zones like the 6.5-mm diameter [28].

Studies comparing different methods of IOP measurements have yielded mixed results. Lanza et al. [29] reported no statistically significant difference between GAT and IOPcc in a normal population. However, other studies have found that IOPcc tends to overestimate IOP compared to GAT [30,31]. Similarly, there is ongoing debate regarding GAT and Corvis ST measurements in nonkeratoconus patients [29,32]. The findings of the present study indicate that the IOP measured by GAT was significantly lower than Tonopen (central corneal zone), with a mean IOP of 13.02 ± 2.31 and 14.5 ± 2.91 mmHg, respectively. The IOP measurements of GAT and Tonopen exhibited the least correlation. This finding aligns with the observations made by Arribas-Pardo et al. [27]. They reported that, in comparison to GAT, Tonopen XL exhibited a slightly higher IOP. Although the difference was statistically significant, the difference between the means was lower than 1 mmHg which is not clinically significant. In contrast to our results, Rateb et al. [16] reported a mean difference of 0.39 ± 2.59 mmHg between GAT and Tonopen readings, with no statistically significant difference observed. One potential explanation for this disparity could be that they employed MyoRing (Dioptex GmbH) which is a complete intrastromal ring implant, providing uniform support across the corneal circumference as the ICRS in their study while KeraRing is segmental and may cause nonuniform changes in the corneal curvature. Also, Rateb et al. [16] used rebound tonometry in their study. They concluded that this method can result in clinically irrelevant underestimation of IOP while Arribas-Pardo et al. [26] found rebound tonometry as an alternative to GAT after ICRS implantation. The Bland-Altman plot demonstrates that this difference is even more pronounced at higher IOP levels. This suggests that, in patients with ICRS, Tonopen may overestimate IOP levels compared to GAT, particularly in higher average IOP levels. By increasing pressure, the effect of IOP on corneal biomechanical properties may become more prominent especially when comparing Tonopen and GAT which needs a smaller area to be flattened for the measurement of IOP. Mendez-Hernandez et al. [9] reported a mean difference of 2.0 mmHg with GAT when comparing it to ORA (IOPcc). Consequently, in keratoconic eyes with ICRS, they recommended subtracting approximately 2 to 6 mmHg from pressures measured by GAT to obtain equivalent ORA IOP values [9]. However, our study did not observe a similar difference in results. In a recent study, Elfwwal et al. [28] conducted a comparison of IOP measurements using GAT with IOPcc of ORA in patients who underwent KeraRing implantation. The findings indicated a slightly higher IOP measurement by GAT, but the difference was deemed statistically insignificant.

IOP measurements of Tonopen, IOPcc of ORA, and IOPb of Corvis ST did not show significant differences. Among all comparisons, the IOPcc of ORA and IOPb of Corvis ST had the best correlation with an ICC of 0.38, which is not a strong correlation. Although the IOPcc of ORA and IOPb of Corvis ST demonstrated the highest Pearson correlation among the devices tested, the ICC was relatively low (0.38). This apparent contradiction may be explained by the fundamental difference between correlation and agreement. Pearson correlation assesses the linear relationship between two variables, while ICC measures absolute agreement, considering both correlation and consistency of measurements. The low ICC in this case suggests that although IOPcc and IOPb change in parallel (i.e., increase or decrease together), their absolute values may differ significantly on a case-by-case basis. This discrepancy may be due to differences in the algorithms used by ORA and Corvis ST to compensate for corneal biomechanics, as each device incorporates distinct parameters and models of deformation. As a result, while the trends in IOP change may be similar, the actual IOP values reported by each device may vary, leading to limited agreement. Results of the present study show that IOP measurements of GAT, Tonopen (central corneal zone), IOPcc of ORA, and IOPb of Corvis ST are not statistically correlated in patients with ICRS. However, the absolute mean difference of pairwise comparisons is less than 1 mmHg in all comparisons (except for GAT vs. Tonopen, which is 1.48 mmHg). Although the mean differences were small between different tonometry methods, the LoA were relatively wide indicating these devices cannot be used interchangeably without adjustment.

Simple linear regressions illustrated on Bland-Altman plots comparing the IOPb of Corvis ST with other measurement tools share the same pattern: as the average IOP increases, the difference between the two measurements also increases. This shows that the IOPb of Corvis ST tends to underestimate higher levels of IOP in the context of ICRS which can show a systematic error. Therefore, we suggest considering other methods of IOP measurement in such patients. Further studies with targeted groups of participants are needed to investigate this issue. Additionally, although all four tonometry methods showed wide LoA, this variability is particularly relevant in the context of glaucoma screening and monitoring in keratoconic eyes post-ICRS. ORA and Corvis ST provide biomechanically compensated IOP values, which may offer a more physiologically accurate estimation. However, due to the wide LoA, no single method can be definitively recommended as superior. In clinical practice, awareness of these differences is crucial. If longitudinal IOP monitoring is needed, consistency in the method used is advised to reduce interpretive variability. Where available, biomechanically compensated measurements (e.g., IOPcc from ORA) may offer an advantage in accuracy, though cost and accessibility must also be considered.

We also compared Tonopen measurements across different corneal zones in the corneas of patients who underwent KeraRing implantation. IOP measurements from the central 5-mm zones were significantly lower compared to measurements from other quadrants. Also in the eyes without keratoconus, such difference can be seen as Tonopen measurements are affected by the thicker cornea in the periphery [33]. Whether this difference is related to the corneal thickness or the ring segment needs a comparison between the eyes with and without ring segments. Therefore, it is advisable to avoid IOP measurements over peripheral corneal regions as well as over the ring segment [34].

One limitation of the present study is the variation in KeraRing segments, encompassing different diameters, arch lengths, and thicknesses. These factors were not controlled in our investigation. Furthermore, our study enrolled patients with a time interval of 3 months or more since ICRS implantation for IOP measurements. Although research suggests that it takes approximately 6 months for corneal viscoelastic properties to stabilize and for the corneal segments to exert their effects, providing ample time for the washout of any influence from topical medications on IOP [35], this duration might still introduce limitations. Another constraint lies in the small sample size and the comparative study design, assessing four different tonometer measurements simultaneously without comparing presurgical and postsurgical results. Additionally, the tomographic and biomechanical parameters of the patients were not included in this study. Also, the measurements with different devices were conducted in a random sequence. The applanation tonometers like Tonopen and GAT can cause a message effect and encounter an error in measurement of IOP by noncontact tonometers. Albeit, there was a 10-minute gap between measurements to reduce the message effect. Assessing IOP measurements with different instruments across various stages of keratoconus could be a topic for further research. Another limitation of our study is that we did not account for the impact of CCT on our IOP measurements. We did not evaluate or adjust for the potential influence of corneal thickness, which could have affected the accuracy of the IOP readings obtained with the devices we used. Several studies have shown that CCT significantly influences the accuracy of IOP measurements across different tonometry devices. Noncontact tonometry (NCT) has been identified as the most sensitive to CCT variations, while dynamic contour tonometry is the least affected. It is important to consider corneal thickness when interpreting IOP measurements, especially with devices like the GAT and noncontact tonometry, which may overestimate or underestimate IOP based on corneal properties [36]. Another limitation of our study is that IOP measurements were not performed at the same time for all participants, although all measurements were conducted in the morning between 9 am and 12 pm. This variability could have influenced the results, as IOP is known to fluctuate throughout the day. For example, Merola et al. [37] observed that in patients with keratoconus, IOP measurements using devices like the GAT and Tonopen could peak at 6 amand vary during the day. Such diurnal variation shows the potential for differences in mean and variability of IOP readings depending on the exact timing of measurements. Additionally, specific tomographic and biomechanical parameters that could influence the results were not included in the present study. Finally, the absence of consistent preoperative GAT data is another limitation. Including such measurements would have allowed a direct assessment of the impact of ICRS implantation on IOP values and the relative accuracy of each tonometry method preoperatively and postoperatively.

In conclusion, our study on patients who underwent ICRS implantation revealed that IOP measurements from the central 5-mm zones were significantly lower compared to measurements from other quadrants, emphasizing the importance of considering corneal zones in IOP assessments for these individuals. Furthermore, the IOP measured by GAT was significantly lower than that measured by Tonopen, with the two exhibiting the least correlation. This disparity is particularly accentuated at higher IOP levels. IOP measurements from Tonopen, IOPcc of ORA, and IOPb of Corvis ST neither showed significant differences nor exhibited strong correlation. Corvis ST (IOPb) tended to underestimate higher levels of IOP in the context of ICRS.

Notes

Conflicts of Interest

None.

Acknowledgements

None.

Funding

None.

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Variable	Value	95% CI	Median (range)
Age (yr)	26.9 ± 7.5	23.9–29.9	26.0 (13.0–44.0)
Sex		-	-
Male	13 (48.1)
Female	14 (51.9)
IOP (mmHg)
GAT measurement	13.02 ± 2.31	12.39–13.65	10.0 (8.0–18.0)
Tonopen-C	14.50 ± 2.91	13.71–15.29	11.0 (10.0–21.0)
ORA measurement
IOPcc (mmHg)	13.53 ± 2.73	12.79–14.28	12.0 (8.3–20.3)
Corneal hysteresis (mmHg)	8.67 ± 1.24	8.33–9.01	5.50 (6.0–11.50)
Corneal resistance factor (mmHg)	7.24 ± 1.37	7.06–7.80	5.30 (4.70–10.0)
Goldmann-correlated IOP (mmHg)	10.46 ± 2.79	9.70–11.23	12.30 (4.90–17.20)
Corvis ST measurement (IOPb) (mmHg)	13.72 ± 1.37	13.35–14.10	5.50 (12.0–17.50)

Variable	IOP measurement tool

	GAT	Tonopen-C	ORA (IOPcc)	Corvis ST (IOPb)
ΔMean ± SD	0.66 ± 1.96	−0.86 ± 2.26	0.21 ± 2.00	−0.01 ± 1.09
p-value	0.018	0.009	0.461	0.942
ΔMedian (range)	0.89 (−4.18 to 4.8)	−0.42 (−5.77 to 3.25)	0.54 (−4.02 to 4.82)	−0.21 (−2.07 to 3.57)
ΔInterquartile range	−0.49 to 1.93	−2.39 to 0.32	−1.06 to 1.35	−0.64 to 0.58
MAE ± SD	0.66 ± 1.96	−0.86 ± 2.26	0.21 ± 2	−0.01 ± 1.09
95% LoA	−3.18 to 4.5	−5.29 to 3.57	−3.71 to 4.13	−2.15 to 2.13
Correlation	0.554	0.659	0.693	0.716
Intracluster correlation	0.484	0.508	0.592	0.714

Comparison	Pearson correlation		Mean bias (95% LoA)	ICC (95% CI)

	r (95% CI)	p-value
GAT vs. Tonopen-C	0.05 (0.22 to 0.31)	0.720	−1.48 (−8.59 to 5.63)	0.04 (−0.19 to 0.28)
GAT vs. ORA (IOPcc)	0.15 (−0.12 to 0.41)	0.270	−0.52 (−6.97 to 5.94)	0.15 (−0.12 to 0.40)
GAT vs. Corvis ST (IOPb)	0.36 (0.1 to 0.57)	0.007	−0.70 (−5.05 to 3.64)	0.30 (0.05 to 0.52)
Tonopen-C vs. ORA (IOPcc)	0.18 (−0.09 to 0.43)	0.180	0.97 (−6.10 to 8.04)	0.18 (−0.08 to 0.41)
Tonopen-C vs. Corvis ST (IOPb)	0.30 (0.03 to 0.52)	0.030	0.78 (−4.76 to 6.30)	0.22 (−0.04 to 0.45)
ORA (IOPcc) vs. Corvis ST (IOPb)	0.47 (0.23 to 0.65)	<0.001	−0.19 (−4.92 to 4.54)	0.38 (0.12 – 0.59)