Journal of Refractive Surgery

Biomechanics 

Repeatability of Ocular Biomechanical Data Measurements With a Scheimpflug-Based Noncontact Device on Normal Corneas

Gabor Nemeth, MD, PhD; Ziad Hassan, MD; Adrienne Csutak, MD, PhD; Eszter Szalai, MD, PhD; Andras Berta, MD, PhD, DSci; Laszlo Modis, Jr., MD, PhD, DSci

Abstract

PURPOSE:

To analyze the repeatability of a new device measuring ocular biomechanical properties, central corneal thickness (CCT), and intraocular pressure (IOP) and to investigate these parameters and their correlations in healthy eyes.

METHODS:

Three consecutive measurements were performed on each eye using the CorVis ST device (Oculus Optikgeräte, Inc., Wetzler, Germany). Ten specific parameters, CCT, and IOP were measured. Biometric data were recorded with IOLMaster (Carl Zeiss Meditec, Jena, Germany).

RESULTS:

This study comprised 75 eyes of 75 healthy volunteers (mean age: 61.24 ± 15.72 years). Mean IOP was 15.02 ± 2.90 mm Hg and mean CCT was 556.33 ± 33.13 μ m. Intraclass correlation coefficient (ICC) was 0.865 for IOP and 0.970 for CCT, and coefficient of variation was 0.069 for IOP and 0.008 for CCT. ICC was 0.758 for maximum amplitude at highest concavity and 0.784 for first applanation time, and less than 0.6 for all other parameters. The device-specific data showed no significant relationship with age and axial length. Flattest and steepest keratometric values and IOP showed a significant correlation with the 10 device-specific parameters.

CONCLUSIONS:

The CorVis ST showed high repeatability for only IOP and pachymetric values. Single measurements are not reliable for the 10 device-specific parameters. The device allows for conducting clinical examinations and screening for surgeries altering ocular biomechanical properties with some form of averaging of multiple measurements.

[ J Refract Surg. 2013;29(8):558–563.]

From the Department of Ophthalmology, Medical and Health Science Center, University of Debrecen, Debrecen, Hungary.

The authors have no financial or proprietary interest in the materials presented herein.

AUTHOR CONTRIBUTIONS

Study concept and design (LM, GN); data collection (AB, AC, ZH, ES); drafting of the manuscript (ZH, GN, ES); critical revision of the manuscript (AB, AC, LM); statistical expertise (GN, ES)

Correspondence: Gabor Nemeth, MD, PhD, Department of Ophthalmology, University of Debrecen, Debrecen, Hungary, Nagyerdei blvd. 98, H-4012 Debrecen, Hungary. E-mail: nemeth222@yahoo.com

Received: January 03, 2013
Accepted: April 09, 2013

Abstract

PURPOSE:

To analyze the repeatability of a new device measuring ocular biomechanical properties, central corneal thickness (CCT), and intraocular pressure (IOP) and to investigate these parameters and their correlations in healthy eyes.

METHODS:

Three consecutive measurements were performed on each eye using the CorVis ST device (Oculus Optikgeräte, Inc., Wetzler, Germany). Ten specific parameters, CCT, and IOP were measured. Biometric data were recorded with IOLMaster (Carl Zeiss Meditec, Jena, Germany).

RESULTS:

This study comprised 75 eyes of 75 healthy volunteers (mean age: 61.24 ± 15.72 years). Mean IOP was 15.02 ± 2.90 mm Hg and mean CCT was 556.33 ± 33.13 μ m. Intraclass correlation coefficient (ICC) was 0.865 for IOP and 0.970 for CCT, and coefficient of variation was 0.069 for IOP and 0.008 for CCT. ICC was 0.758 for maximum amplitude at highest concavity and 0.784 for first applanation time, and less than 0.6 for all other parameters. The device-specific data showed no significant relationship with age and axial length. Flattest and steepest keratometric values and IOP showed a significant correlation with the 10 device-specific parameters.

CONCLUSIONS:

The CorVis ST showed high repeatability for only IOP and pachymetric values. Single measurements are not reliable for the 10 device-specific parameters. The device allows for conducting clinical examinations and screening for surgeries altering ocular biomechanical properties with some form of averaging of multiple measurements.

[ J Refract Surg. 2013;29(8):558–563.]

From the Department of Ophthalmology, Medical and Health Science Center, University of Debrecen, Debrecen, Hungary.

The authors have no financial or proprietary interest in the materials presented herein.

AUTHOR CONTRIBUTIONS

Study concept and design (LM, GN); data collection (AB, AC, ZH, ES); drafting of the manuscript (ZH, GN, ES); critical revision of the manuscript (AB, AC, LM); statistical expertise (GN, ES)

Correspondence: Gabor Nemeth, MD, PhD, Department of Ophthalmology, University of Debrecen, Debrecen, Hungary, Nagyerdei blvd. 98, H-4012 Debrecen, Hungary. E-mail: nemeth222@yahoo.com

Received: January 03, 2013
Accepted: April 09, 2013

An accurate and reliable measurement of the anterior segment parameters of the eye is important in planning ophthalmological and refractive surgeries and in postoperative monitoring. The diagnostic techniques used currently have the potential to measure only static parameters of the anterior segment. However, the cornea has been identified as a substance with viscoelastic properties. 1 Until recently, the only device conducting in vivo measurements of the ocular biomechanical properties has been the Ocular Response Analyzer (ORA; Reichert Ophthalmic Instruments, Depew, NY), which became commercially available in 2005. 2,3 With the introduction of the ORA, an emphasis has been placed on the biomechanical measurements of the cornea in glaucoma diagnosis and in the assessment of the outcomes of refractive surgeries and corneal collagen cross-linking therapies. 4–11 The first publications reported differences in the parameters measured with the ORA in healthy and keratoconic eyes and subsequent to refractive surgeries. 12–14

The CorVis ST (Corneal Visualization Scheimpflug Technology, CorVis ST; Oculus Optikgeräte, Inc., Wetzlar, Germany) uses a high intensity air impulse for biomechanical measurements applying an ultrahigh-speed Scheimpflug camera. The equipment has the potential to measure the amplitude of maximal applanation and time taken to reach this applanation. The CorVis ST also monitors the speed of the cornea during first and second applanation measurement and the distance of the two apexes at highest concavity time.

Our aim was to define “normal” data in healthy eyes for the parameters obtained by this new device. We also investigated the specific parameters determined with the CorVis ST, assessed repeatability of these data, and analyzed correlations between CorVis ST data and other biometrical parameters and age.

Patients and Methods

Our examinations were conducted in healthy eyes. Exclusion criteria were any anterior segment disease, spherical or astigmatic refraction error greater than 2.0 diopters (D), any previous intraocular surgery, and contact lens wearing.

The first measurements were performed using the CorVis ST device, a noncontact tonometer and pachymeter that measures 10 specific ocular biomechanical parameters. The CorVis ST uses an ultrahigh-speed Scheimpflug camera (4,330 frames/second) covering 8.0 mm horizontally. The light source is an ultraviolet free blue LED light with a wavelength of 455 nm. In the slow motion video, the deformation response of the cornea to a high intensity air impulse is seen within a range of 30 milliseconds. The air impulse employs a metered, symmetrical, and fixed maximal internal pump pressure of 25 kilopascal. The measurement periods of 30 milliseconds were also recorded on a video. The video and the data obtained during measurements can be easily exported from the device for further statistical analysis.

Due to the air impulse, the cornea goes through three distinct phases (first applanation, highest concavity, and second applanation). During these phases, several parameters are recorded: maximum deformation amplitude (the highest concavity of the cornea), time taken to reach it, first and second applanation time, cord length (length of the flattened cornea), maximum corneal velocity in and out, peak distance (the distance of the two apexes at highest concavity), and a radius value that represents the central concave curvature at highest concavity ( Table 1 and Figure 1 ). Central corneal thickness (CCT) and intraocular pressure (IOP) are also determined.

Description of the 10 Specific Biomechanical Parameters Measured by CorVis ST

Table 1: Description of the 10 Specific Biomechanical Parameters Measured by CorVis ST

Normal picture obtained with CorVis ST (Oculus Optikgeräte, Wetzlar, Germany). After an air impulse, the cornea goes through three phases (first applanation, highest concavity, and second applanation) while several parameters are recorded: maximum deformation amplitude (highest concavity of the cornea), time taken to reach it, first and second applanation time, cord length, maximum corneal velocity in and out, peak distance, which is the distance of the two apex at highest concavity, and a radius value, which represents the central concave curvature at highest concavity.

Figure 1. Normal picture obtained with CorVis ST (Oculus Optikgeräte, Wetzlar, Germany). After an air impulse, the cornea goes through three phases (first applanation, highest concavity, and second applanation) while several parameters are recorded: maximum deformation amplitude (highest concavity of the cornea), time taken to reach it, first and second applanation time, cord length, maximum corneal velocity in and out, peak distance, which is the distance of the two apex at highest concavity, and a radius value, which represents the central concave curvature at highest concavity.

Patients were seated with their chin on the chinrest and their forehead against the equipment. Using the joystick, the examiner targets the center of the cornea, thus enabling the patients to see a red light on which they have to fixate. The adjusting direction needed to center on the corneal apex is seen on the screen. At an accurate setting, the air impulse automatically starts. Subsequent to the measurements, the data are exported to a computer.

Three measurements were conducted in the right eyes of all patients using the CorVis ST with software version 1.00r24 rev. 772. The measurements were taken by the same investigator and on the same day period. Patients could move their chin from the chinrest between scans. Subsequently, ocular biometric parameters (axial length with signal noise ratio > 10.0 and keratometric values) were recorded with IOLMaster (Carl Zeiss Meditec, Jena, Germany).

Statistical analysis was performed with MedCalc 10.0 (MedCalc Software, Ostend, Belgium) and Microsoft Excel (Microsoft Corporation, Redmond, WA) software. Descriptive statistical results were described as mean, standard deviation, and 95% confidence interval (95% CI) for the mean. Multiple regression analyses were performed adjusting CorVis ST data for age, axial length, keratometric values, and IOP. Spearman’s rank correlation test was used to study the correlation between age and IOP. A P value less than .05 was considered statistically significant. The coefficient of repeatability (1.96*standard deviaton), the mean coefficient of variation for all parameters, the intraclass correlation coefficient (ICC) and its 95% CI value, and the value of Cronbach’s alpha were also determined. The ICC is a measure of the reliability of measurements and the obtained data determine the intrasession repeatability. It was suggested that the ICC values should be at least 0.9 to ensure a reasonable reliability. 15

Results

Our measurements were conducted in 75 right eyes of 75 healthy volunteers (mean age: 61.24 ± 15.72 years; 95% CI: 57.62 to 64.86 years; range: 22.2 to 87.3 years). Specific parameters were measured with the CorVis ST and values representing repeatability are shown in Tables 23 . Kolmogorov–Smirnov test reject normality for all parameters.

Data Obtained by CorVis ST in a Normal Population (n = 75)

Table 2: Data Obtained by CorVis ST in a Normal Population (n = 75)

Repeatability Data Obtained by CorVis ST in a Normal Population (n = 75)

Table 3: Repeatability Data Obtained by CorVis ST in a Normal Population (n = 75)

Axial length was 23.28 ± 1.26 mm (95% CI: 22.98 to 23.57 mm; range: 21.19 to 27.7 mm), flattest keratometric value was 43.58 ± 1.58 D (95% CI: 43.22 to 43.95 D; range: 39.24 to 47.2 D), and steepest keratometric value was 44.43 ± 1.53 D (95% CI: 44.08 to 44.78 D; range: 40.13 to 48.01 D) measured by IOLMaster.

Repeatability Data

IOP and CCT showed the highest repeatability with an ICC of 0.865 and 0.970. Maximum amplitude at the highest concavity and time from starting to first applanation showed moderate repeatability with an ICC of 0.758 and 0.784, respectively. All other parameters showed poor repeatability ( Table 3 ).

Correlation Data

The 10 specific CorVis ST parameters showed no significant relationship with age (adjusted r 2 of coefficient of determination = 0.11; P = .10) or axial length (adjusted r 2 of coefficient of determination = 0.05; P = .21) determined by multiple regression. Flattest (adjusted r 2 of coefficient of determination = 0.2; P = .008) and steepest (adjusted r 2 of coefficient of determination = 0.24; P = .002) keratometric values showed a statistically significant, positive correlation with the 10 specific CorVis ST parameters. A significant, positive correlation was found between IOP and CorVis ST data (adjusted r 2 of coefficient of determination = 0.96; P < .001). There was no significant correlation between age and IOP measured by CorVis ST (r 2 = 0.03; P = .71).

Discussion

According to the literature, the ICC for corneal hysteresis and IOP is between 0.84 and 0.92 when applying the ORA. 2,3 Our data obtained with the CorVis ST showed excellent repeatability for only IOP and pachymetric values. Highest concavity data and first applanation time showed good repeatability, but all other parameters had poor or unacceptable repeatability, although second applanation time, highest concavity time, and radius data had low coefficient of variation values. Possible reasons for the poor repeatability could be limitations of the technology or unknown issues requiring further research.

A significant correlation was observed between corneal hysteresis, corneal resistance factor, CCT, and IOP with the ORA. 16 Corneal hysteresis and corneal resistance factor values measured with the ORA showed a significant correlation with visual acuity and certain corneal parameters (eg, negative correlation with the highest keratometric readings), 17,18 although others did not prove keratometric correlation. 19 Our data obtained with the CorVis ST show that significant correlation was observed between keratometric values and CorVis ST parameters, indicating that corneal curvature influences the measured data.

The age dependency of the biomechanical parameters is also well known from the literature, 5,20–23 confirming that the elastic properties of the cornea change with age. 23 Others found that corneal hysteresis and corneal resistance factor are independent of age. 24,25 Toubul et al. 24 hypothesized that viscoelasticity decreases with age because the corneal resistance factor is positively correlated with IOP and because it is known that IOP increases with age; thus, corneal hysteresis and corneal resistance factor variations are compensated for with the IOP elevation. According to our data, the CorVis ST was not capable of detecting an age-related difference in a normal population.

The ORA has been reviewed in a recent publication. 26 Changes in corneal hysteresis and corneal resistance factor after LASIK, 4–6 epiLASIK, 27 and clear corneal phacoemulsification 28 are well known. Corneal collagen cross-linking causes no changes in the two main parameters measured with the ORA 5,10 ; however, biomechanical differences were observed in corneas after corneal collagen cross-linking when applying the latest parameters of the latest ORA software. 10 Using a uniaxial tensile test, it could be observed that corneal collagen cross-linking treatment lasting for 30 minutes increases the stiffness of porcine corneal tissue, but treatment lasting for 60 minutes reduces it. 29 Several publications reported changes in ocular biomechanical properties in keratoconic eyes. 17,30–33 Statistically, corneal hysteresis and corneal resistance factor are significantly lower in keratoconic eyes compared to normal eyes, but both parameters have low sensitivity and specificity for distinguishing between the groups and are not suitable for establishing the diagnosis. 32,33

Although the CorVis ST also analyzes corneal deformations due to air impulse applanation, the parameters obtained by the two devices cannot be compared.

Our data reveal that the CorVis ST has excellent repeatability for IOP and pachymetric values only. Maximum amplitude at highest concavity and A1 time showed good repeatability, but all other specific parameters had poor repeatability. The CorVis ST has the potential for investigating specific ocular biomechanical properties with some form of averaging of multiple measurements. Further studies are needed to evaluate the number of measurements required to attain reasonable repeatability.

References

  1. Soergel F, Jean B, Seiler T, et al. Dynamic mechanical spectroscopy of the cornea for measurement of its viscoelastic properties in vitro. Ger J Ophthalmol . 1995;4:151–156.
  2. Kynigopoulos M, Schlote T, Kotecha A, Tzamalis A, Pajic B, Haefliger I. Repeatability of intraocular pressure and corneal biomechanical properties measurements by the ocular response analyser. Klin Monbl Augenheilkd . 2008;225:357–360. doi:10.1055/s-2008-1027256 [CrossRef]
  3. Moreno-Montanes J, Maldonado MJ, Garcia N, Mendiluce L, Garcia-Gomez PJ, Segui-Gomez M. Reproducibility and clinical relevance of the ocular response analyzer in nonoperated eyes: corneal biomechanical and tonometric implications. Invest Ophthalmol Vis Sci . 2008;49:968–974. doi:10.1167/iovs.07-0280 [CrossRef]
  4. Pepose JS, Feigenbaum SK, Qazi MA, Sanderson JP, Roberts CJ. Changes in corneal biomechanics and intraocular pressure following LASIK using static, dynamic, and noncontact tonometry. Am J Ophthalmol . 2007;143:39–47. doi:10.1016/j.ajo.2006.09.036 [CrossRef]
  5. Spoerl E, Terai N, Haustein M, Bohm AG, Raiskup-Wolf F, Pillunat LE. Biomechanical condition of the cornea as a new indicator for pathological and structural changes [article in German]. Ophthalmologe . 2009;106:512–520. doi:10.1016/S0161-6420(01)00715-1 [CrossRef]
  6. Chen S, Chen D, Wang J, Lu F, Wang Q, Qu J. Changes in ocular response analyzer parameters after LASIK. J Refract Surg . 2010;26:279–288. doi:10.3928/1081597X-20100218-04 [CrossRef]
  7. Ayala M. Corneal hysteresis in normal subjects and in patients with primary open-angle glaucoma and pseudoexfoliation glaucoma. Ophthalmic Res . 2011;46:187–191. doi:10.1159/000326896 [CrossRef]
  8. Detry-Morel M, Jamart J, Pourjavan S. Evaluation of corneal biomechanical properties with the Reichert Ocular Response Analyzer. Eur J Ophthalmol . 2011;21:138–148. doi:10.5301/EJO.2010.2150 [CrossRef]
  9. Greenstein SA, Fry KL, Hersh PS. In vivo biomechanical changes after corneal collagen cross-linking for keratoconus and corneal ectasia: 1-year analysis of a randomized, controlled, clinical trial. Cornea . 2012;31:21–25. doi:10.1097/ICO.0b013e31821eea66 [CrossRef]
  10. Spoerl E, Terai N, Scholz F, Raiskup F, Pillunat LE. Detection of biomechanical changes after corneal cross-linking using Ocular Response Analyzer software. J Refract Surg . 2011;27:452–457. doi:10.3928/1081597X-20110106-01 [CrossRef]
  11. Kaushik S, Pandav SS, Banger A, Aggarwal K, Gupta A. Relationship between corneal biomechanical properties, central corneal thickness, and intraocular pressure across the spectrum of glaucoma. Am J Ophthalmol . 2012;153:840–849. doi:10.1016/j.ajo.2011.10.032 [CrossRef]
  12. Luce DA. Determining in vivo biomechanical properties of the cornea with an ocular response analyzer. J Cataract Refract Surg . 2005;31:156–162. doi:10.1016/j.jcrs.2004.10.044 [CrossRef]
  13. Shah S, Laiquzzaman M, Bhojwani R, Mantry S, Cunliffe I. Assessment of the biomechanical properties of the cornea with the ocular response analyzer in normal and keratoconic eyes. Invest Ophthalmol Vis Sci . 2007;48:3026–3031. doi:10.1167/iovs.04-0694 [CrossRef]
  14. Saad A, Lteif Y, Azan E, Gatinel D. Biomechanical properties of keratoconus suspect eyes. Invest Ophthalmol Vis Sci . 2010;51:2912–2916. doi:10.1167/iovs.09-4304 [CrossRef]
  15. Portney LG, Watkins MP. Foundations of Clinical Research; Applications to Practice , 2nd ed. Upper Saddle River, NJ: Prentice-Hall; 2000:557–584. doi:10.1016/j.jcrs.2010.06.004 [CrossRef]
  16. Kamiya K, Hagishima M, Fujimura F, Shimizu K. Factors affecting corneal hysteresis in normal eyes. Graefes Arch Clin Exp Ophthalmol . 2008;246:1491–1494. doi:10.1007/s00417-008-0864-x [CrossRef]
  17. Gkika M, Labiris G, Giarmoukakis A, Koutsogianni A, Kozobolis V. Evaluation of corneal hysteresis and corneal resistance factor after corneal cross-linking for keratoconus. Graefes Arch Clin Exp Ophthalmol . 2012;250:565–573. doi:10.1007/s00417-011-1897-0 [CrossRef]
  18. Lim L, Gazzard G, Chan YH, et al. Cornea biomechanical characteristics and their correlates with refractive error in Singaporean children. Invest Ophthalmol Vis Sci . 2008;49:3852–3857. doi:10.1167/iovs.07-1670 [CrossRef]
  19. Franco S, Lira M. Biomechanical properties of the cornea measured by the Ocular Response Analyzer and their association with intraocular pressure and the central corneal curvature. Clin Exp Optom . 2009;92:469–475. doi:10.1111/j.1444-0938.2009.00414.x [CrossRef]
  20. Shen M, Wang J, Qu J, et al. Diurnal variation of ocular hysteresis, corneal thickness, and intraocular pressure. Optom Vis Sci . 2008;85:1185–1192. doi:10.1097/OPX.0b013e31818e8abe [CrossRef]
  21. Kotecha A, Elsheikh A, Roberts CR, Zhu H, Garway-Heath DF. Corneal thickness- and age-related biomechanical properties of the cornea measured with the Ocular Response Analyzer. Invest Ophthalmol Vis Sci . 2006;47:5337–5347. doi:10.1167/iovs.06-0557 [CrossRef]
  22. Foster PJ, Broadway DC, Garway-Heath DF, et al. Intraocular pressure and corneal biomechanics in an adult British population: the EPIC-Norfolk Eye Study. Invest Ophthalmol Vis Sci . 2011;52:8179–8185. doi:10.1167/iovs.11-7853 [CrossRef]
  23. Ortiz D, Piñero D, Shabayek MH, Arnalich-Montiel F, Alió JL. Corneal biomechanical properties in normal, post-laser in situ keratomileusis, and keratoconic eyes. J Cataract Refract Surg . 2007;33:1371–1375. doi:10.1016/j.jcrs.2007.04.021 [CrossRef]
  24. Touboul D, Roberts C, Kérautret J, et al. Correlations between corneal hysteresis, intraocular pressure, and corneal central pachymetry. J Cataract Refract Surg . 2008;34:616–622. doi:10.1016/j.jcrs.2007.11.051 [CrossRef]
  25. Kirwan C, O’Keefe M, Lanigan B. Corneal hysteresis and intraocular pressure measurement in children using the reichert ocular response analyzer. Am J Ophthalmol . 2006;142:990–992. doi:10.1016/j.ajo.2006.07.058 [CrossRef]
  26. Terai N, Raiskup F, Haustein M, Pillunat LE, Spoerl E. Identification of biomechanical properties of the cornea: the ocular response analyzer. Curr Eye Res . 2012;37:553–562. doi:10.3109/02713683.2012.669007 [CrossRef]
  27. Ryan DS, Coe CD, Howard RS, Edwards JD, Bower KS. Corneal biomechanics following epi-LASIK. J Refract Surg . 2011;27:458–464. doi:10.3928/1081597X-20110112-01 [CrossRef]
  28. de Freitas Valbon B, Ventura MP, da Silva RS, Canedo AL, Velarde GC, Ambrósio R Jr, . Central corneal thickness and biomechanical changes after clear corneal phacoemulsification. J Refract Surg . 2012;28:215–219. doi:10.3928/1081597X-20111103-02 [CrossRef]
  29. Lanchares E, del Buey MA, Cristóbal JA, Lavilla L, Calvo B. Biomechanical property analysis after corneal collagen cross-linking in relation to ultraviolet A irradiation time. Graefes Arch Clin Exp Ophthalmol . 2011;249:1223–1227. doi:10.1007/s00417-011-1674-0 [CrossRef]
  30. Wolffsohn JS, Safeen S, Shah S, Laiquzzaman M. Changes of corneal biomechanics with keratoconus. Cornea . 2012;31:849–854. doi:10.1097/ICO.0b013e318243e42d [CrossRef]
  31. Galletti JG, Pförtner T, Bonthoux FF. Improved keratoconus detection by ocular response analyzer testing after consideration of corneal thickness as a confounding factor. J Refract Surg . 2012;28:202–208. doi:10.3928/1081597X-20120103-03 [CrossRef]
  32. Fontes BM, Ambrósio R Jr, Velarde GC, Nosé W. Ocular response analyzer measurements in keratoconus with normal central corneal thickness compared with matched normal control eyes. J Refract Surg . 2011;27:209–215.
  33. Fontes BM, Ambrósio R Jr, Jardim D, Velarde GC, Nosé W. Corneal biomechanical metrics and anterior segment parameters in mild keratoconus. Ophthalmology . 2010;117:673–679. doi:10.1016/j.ophtha.2009.09.023 [CrossRef]

Description of the 10 Specific Biomechanical Parameters Measured by CorVis ST

ParameterDescription of Specific CorVis ST Parameters
Maximum deformation amplitudeMaximum deformation amplitude of the cornea at the highest concavity, in millimeters
A1 timeTime from air-puff starting until the first applanation, in milliseconds
A1 lengthLength of the flattened cornea at the first applanation, in millimeters
A1 velocitySpeed of corneal apex at the first applanation, in meters/second
A2 timeTime from starting until the second applanation, in milliseconds
A2 lengthLength of the flattened cornea at the second applanation, in millimeters
A2 velocitySpeed of the corneal apex at the second applanation, in meters/second
HC timeTime from air-puff starting until the highest concavity of the cornea is reached, in milliseconds
Peak distanceDistance of the two apex of the cornea (two “knees”) at the time of the highest concavity, in millimeters
RadiusRadius of curvature of a circle that fits to corneal concavity at the time of the maximum deformation, in millimeters

Data Obtained by CorVis ST in a Normal Population (n = 75)

ParameterMean95% CIRange
IOP (mm Hg)15.0214.35 to 15.6810.83 to 26.17
Pachy ( μ m)556.33548.71 to 563.95480.67 to 648.67
Def. amp. max (mm)1.061.04 to 1.080.86 to 1.25
A1 time (ms)7.277.19 to 7.346.53 to 8.34
A1 length (mm)1.751.72 to 1.791.32 to 2.04
A1 velocity (m/s)0.1490.14 to 0.150.06 to 0.19
A2 time (ms)21.6021.48 to 21.7120.15 to 24.04
A2 length (mm)1.911.83 to 1.980.77 to 2.66
A2 velocity (m/s)−0.34−0.35 to −0.32−0.49 to −0.18
HC time (ms)16.8416.76 to 16.9315.63 to 17.86
Peak distance (mm)3.032.86 to 3.201.19 to 5.22
Radius (mm)7.947.75 to 8.146.02 to 11.36

Repeatability Data Obtained by CorVis ST in a Normal Population (n = 75)

ParameterICC95% CI for ICCCronbach’s alpha95% Lower CI for Cronbach’s alphaCVAverage SDCoefficient of Repeatability a
IOP (mmHg)0.8650.811 to 0.9070.9510.9320.0691.025.693
Pachy ( μ m)0.9700.956 to 0.9790.9900.9860.0084.5164.939
Def. amp. max (mm)0.7580.670 to 0.8290.9040.8680.0430.040.181
A1 time (ms)0.7840.704 to 0.8480.9160.8840.0170.120.624
A1 length (mm)0.0620.170 to 0.0720.2870.0200.1330.230.283
A1 velocity (m/s)0.3540.212 to 0.4960.6220.4800.1480.020.049
A2 time (ms)0.3050.161 to 0.4530.5680.4040.0230.260.965
A2 length (mm)0.2400.099 to 0.3900.4860.2940.1960.360.623
A2 velocity (m/s)0.5470.416 to 0.6650.7830.701−0.1140.030.118
HC Time (ms)0.2610.119 to 0.4090.5140.3320.0210.340.709
Peak dist. (mm)0.2160.077 to 0.3660.4530.2480.2300.721.437
Radius (mm)0.5600.433 to 0.6740.7920.7150.0680.541.626

10.3928/1081597X-20130719-06

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