Contact Lens & Anterior Eye 38 (2015) 89–93
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Evaluation of corneal deformation analyzed with a Scheimpflug based device Michele Lanza a,b,∗ , Michela Cennamo b , Stefania Iaccarino b , Vito Romano a , Mario Bifani a , Carlo Irregolare b , Alessandro Lanza a a b
Multidisciplinary Department of Medical, Surgical and Dental Sciences, Second University of Naples, Naples, Italy Centro Grandi Apparecchiature, Second University of Naples, Naples, Italy
a r t i c l e
i n f o
Article history: Received 16 November 2013 Received in revised form 30 September 2014 Accepted 28 October 2014 Keywords: Corneal biomechanics Corneal pachimetry Naive eyes Tomographic evaluation Biomechanical evaluation
a b s t r a c t Purpose: To evaluate the correlation between corneal biomechanical and morphological data in healthy eyes. Methods: A complete clinical eye examination of naïve eyes was followed by tomographic (Pentacam, Oculus, Wetzlar, Germany) and biomechanical (Corvis ST, Oculus, Wetzlar, Germany) evaluation. Linear regression between central corneal thickness (CCT), corneal volume (CV) and anterior corneal curvature measured with Sim’K (SK), versus corneal deformation parameters measured with Corvis ST have been run using SPSS software version 18.0. Results: Seventy-six eyes of 76 healthy subjects (44 women and 32 men) with a mean age of 36.84 ± 10.74 years and a mean refractive error of −0.55 ± 1.68 D (measured as spherical equivalent) were evaluated. Corneal deformation parameters were weakly correlated with corneal morphological parameters and with spherical equivalent. Although the correlations between deformation amplitude versus SK and between SK versus Velocity of Applanation 2, were higher than the others (R2 = 0.28 and 0.26 respectively), none of them was statistically significant (p > 0.01). Conclusions: According with these findings, Corvis ST seems to be able to provide an analysis of corneal deformation independent from corneal morphological characteristics. If these data will be confirmed in further studies, this device could be useful in the management and screening of eyes with corneal diseases. © 2014 British Contact Lens Association. Published by Elsevier Ltd. All rights reserved.
1. Introduction Until few years ago, the corneal parameters traditionally studied were central corneal thickness (CCT), corneal curvature (K) and transparency, measured using different devices such as keratometers, auto-keratometers, corneal topographies, corneal tomographies, slit lamps and confocal microscopes. In 2005, Reichert introduced a new instrument, the ocular response analyzer (ORA; Reichert Ophtalmic Instrument, Depew, NY, USA), a device able to measure, in vivo, other corneal properties such as corneal hysteresis (CH) and corneal resistance factor (CRF), using a collimated air pulse to applanate the central cornea [1]. Corneal biomechanical properties measured with ORA have been widely
∗ Corresponding author at: Multidisciplinary Department of Medical, Surgical and Dental Sciences, Seconda Università di Napoli, Via de Crecchio 16, 80100 Napoli, Italy. Tel.: +39 0815666768; fax: +39 0815666775. E-mail address:
[email protected] (M. Lanza).
studied in healthy subjects and in patients affected by different kinds of ocular diseases [2–16], so today they have a role in the diagnosis, follow up and management of many of them [7,9,11]. Different papers, however, showed that CH and CRF are somehow affected by corneal morphological parameters [2,10,13,14,17,18], that is why new kinds of technologies, like optical coherence tomography, are currently utilized in corneal biomechanical evaluation [19–21]. It would be very important to have an accurate evaluation of corneal biomechanic because it could help us in better managing pathologic conditions due to a disease (i.e. keratoconus) or to a iatrogenic cause (i.e. refractive surgery), moreover, it would allow us to have a more precise measurement of intraocular pressure (IOP), especially in eyes affected by corneal disease since, the presently gold standard, Goldmann applanation tonometry (GAT), has been largely proven to be affected by corneal properties [6,9,12,22]. Corvis ST (Oculus, Wetzlar, Germany) (CST) is a new clinical device introduced to investigate corneal deformation properties. It uses an ultra high-speed Scheimpflug camera that records the
http://dx.doi.org/10.1016/j.clae.2014.10.002 1367-0484/© 2014 British Contact Lens Association. Published by Elsevier Ltd. All rights reserved.
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Fig. 1. Screenshot of Corvis ST display, showing information recorded immediately upon the air impulse (A); screenshot of Corvis ST display, showing information recorded during the corneal deformation obtained by the air impulse (B); screenshot of Corvis ST display, showing time of applanation 1 (ellipse), length ofapplanation 1 (rectangle), velocity of applanation 1 (hexagon) at first applanation (C); screenshot of Corvis ST display, showing time of applanation 2 (ellipse), length of applanation 2 (rectangle), velocity of applanation 2 (hexagon) at second applanation (D); screenshot of Corvis ST display, showing deformation amplitude at the highest concavity at corneal apex (ellipse) (E).
M. Lanza et al. / Contact Lens & Anterior Eye 38 (2015) 89–93
deformation process in 4330 frames/s along an 8 mm horizontal corneal coverage, while an air puff indentation causes a corneal deformation (Fig. 1); whereas ORA measures the corneal changes in shape with an electro-optical collimation detector system in the central 3.0 mm diameter area, throughout 20 ms measurement [1]. Repeatability, reproducibility and correlations of the parameters provided by CST have been studied by Hon and Lam [23], differences among CST, no contact tonometry (NCT) and GAT have been analyzed by Hong and co-workers [21] but this is one of the first papers evaluating the correlations between corneal deformation provided by CST and corneal morphological parameters in healthy subjects.
2. Materials and methods Seventy-six eyes of 76 healthy subjects (44 women and 32 men) with an age ranging from 23 to 65 years (mean = 36.84 ± 10.74 years) and a refractive error (measured as spherical equivalent) ranging from −7 D to +3 D (mean = −0.55 ± 1.68 D) underwent a complete ophthalmic evaluation, a corneal tomography with Pentacam and a CST scan. Patients affected by systemic and/or ocular diseases that could interfere with the corneal evaluation such as diabetes, connective tissue disorders, dry eye, uveitis, corneal opacities and glaucoma, were excluded from the study. Subjects wearing contact lenses were asked to discontinue using them at least 3 days before being evaluated. Oculus Pentacam is a corneal tomographer utilizing a rotating Scheimpflug camera and a monochromatic slit light source (blue led at 475 nm) that rotate together around the optical axes of the eye to calculate a three-dimensional model of the anterior segment, including data from anterior and posterior corneal topography, pachymetry, and measurements of anterior chamber depth, lens opacity and lens thickness. Within 2 s, the system rotates 180◦ and acquires 25 or 50 images (depending on the user’s settings) that contain 500 measurement points on the front and back corneal surfaces to draw a true elevation map. For this study, the 25 images per scan option was chosen. The parameters evaluated in this study were: CCT at pupil centre, corneal volume (CV) and anterior corneal curvature measured with Sim’K (SK). Corvis ST uses 4330 images/s Scheimpflug camera to record the corneal behaviour during an air puff indentation. It provides many different parameters related to two main moments: the first applanation, when cornea flattens because of the air puff, and the second applanation, when cornea flattens again after reaching the highest concavity shape in order to come back to its original one. The parameters included in this study were the following: Time of applanation 1 (AT1): time from the start until an air-puff causes the corneal flattening (first applanation) (Fig. 1). Length of applanation 1 (AL1): length of the flattened cornea in the first applanation (Fig. 1). Velocity of applanation 1 (AV1): velocity of corneal deformation during the first applanation (Fig. 1). Time of applanation 2 (AT2): time from the start until the flattened cornea rebounds from its highest concavity (second applanation) (Fig. 1). Length of applanation 2 (AL2): length of the flattened cornea in the second applanation (Fig. 1). Velocity of applanation 2 (AV2): velocity of corneal deformation during the second applanation Deformation amplitude at the highest concavity (HCDA): maximum deformation amplitude (from the start to the highest concavity) at the corneal apex (Fig. 1). Three good quality Corvis ST measurements have been taken and every scan has been performed after 5 min from the previous one, so as to avoid an underestimation or overestimation of the corneal biomechanical parameters. All subjects started with
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Table 1 Mean, standard deviation and range of the parameters evaluated in our study population. SE: spherical equivalent; SK: anterior corneal curvature measured with Sim’K; CCT: central corneal thickness. Parameters
Mean ± SD
Age (years) SE (D) SK (D) Corneal volume (mm3 ) CCT (m) Applanation time 1 (ms) Applanation length 1 (mm) Applanation velocity 1 (m/s) Applanation time 2 (ms) Applanation length 2 (mm) Applanation velocity 2 (m/s) Deformation amplitude to the highest concavity (mm)
36.84 −0.50 43.34 61.00 550 7.35 1.74 0.15
± ± ± ± ± ± ± ±
10.74 1.67 1.22 3.36 32.76 0.41 0.27 0.03
21.46 ± 0.41 1.91 ± 0.45 −0.33 ± 0.11 1.02 ± 0.10
Range From 23 to 65 From −7 to +3 From 40.9 to 45.90 From 54.3 to 68.60 From 488 to 631 From 6.85 to 9.12 From 1.29 to 2.28 From 0.04 to 0.22 From 20.24 to 22.20 From 0.95 to 2.70 From 0.35 to 0.47 From 0.73 to 1.29
the Pentacam evaluation and then underwent the CST in order to reduce bias in morphological measurements, since the air puff could introduce errors in corneal evaluation if Scheimpflug scan is performed after it. Two different and trained physicians used the two devices, (MC used Pentacam and SI used CST) and they were not aware of the results the other one was obtaining. Despite the fact that all patients underwent bilateral evaluation, only the right eye results were included in the statistical analysis to eliminate any potential intra-subject effect if both eyes of the same patient were considered. 2.1. Statistical analysis Normality of distribution was tested with Kolmogorov–Smirnov test, linear regression among spherical equivalent (SE), CCT, CV and SK versus corneal deformation parameters measured with Corvis ST have been run using SPSS software version 18.0 (IBM Corp., Armonk, New York). The study was performed in accordance with the ethical standards stated in the 1964 declaration of Helsinki and approved by the local clinical research ethics committee; informed consent was obtained from all subjects before examination. 3. Results Table 1 shows mean, range and standard deviation of age, spherical equivalent refraction, CCT, CV, SK, AT1, AL1, AV1, AT2, AL2, AV2 and HCDA. Table 2 shows the Pearson index and the statistical significance of the correlation between SE and corneal morphological parameters versus corneal deformation parameters. The Pearson values ranged from extremely poor to low; even if some correlations were higher than others (SK versus HCDA = 0.28 and SK versus AV2 = 0.26), none of them had a statistical significance (p > 0.01) (Table 2). 4. Discussion It is well known that the study of corneal biomechanical properties is important for the diagnosis and follow up of several ocular conditions. Many papers evaluated corneal parameters measured using ORA [1–16]. Corvis ST, the first non-contact tonometer incorporating Scheimpflug technology, has recently been introduced as a clinical device in ophthalmology to measure also corneal deformation properties [23]. Data from this study show no statistical correlation between SE, CCT, CV, SK and AT1, AL1, AV1, AT2, AL2, AV2, HCDA in human healthy eyes; this could have two explanations: it could mean that the device is not able to provide
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Table 2 Pearson correlation index (R2 ) and statistical significance (p) between corneal deformation parameters versus corneal morphological parameters and spherical equivalent (SE). CCT: central corneal thickness; CV: corneal volume; SK: anterior corneal power measured with Sim’K. CCT
Applanation time 1 (ms) Applanation length 1 (mm) Applanation velocity 1 (m/s) Applanation time 2 (ms) Applanation length 2 (mm) Applanation velocity 2 (m/s) Deformation amplitude at the highest concavity (mm)
CV
SK
SE
R2
p
R2
p
R2
p
R2
p
0.227 5E-05 0.038
>0.05 >0.05 >0.05
0.0752 0.0148 0.0009
>0.05 >0.05 >0.05
0.1044 0.0065 0.1348
>0.05 >0.05 >0.05
0.0315 0.0007 0.0416
>0.05 >0.05 >0.05
0.087 0.062 0.106
>0.05 >0.05 >0.05
0.002 0.027 0.0068
>0.05 >0.05 >0.05
0.2113 0.1069 0.2618
>0.05 >0.05 >0.05
0.0669 0.0643 0.0395
>0.05 >0.05 >0.05
0.187
>0.05
0.0286
>0.05
0.2816
>0.05
0.1696
>0.05
any reliable measurement of corneal deformation or that corneal deformation parameters detected by CST are not affected by the morphological and refractive parameters analyzed. Our results regarding the correlation analysis, differ from the ones reported by Hon and Lam [23] that found a negative correlation between HCDA and CCT. This could be due to different reasons: this study has a larger population; participants were asked to stop using contact lens 3 days before the evaluation, whereas Hon et al. [23] required to discontinue lens wear 24 h before; Pentacam scan has been performed always before CST evaluation, in order to reduce bias in morphological measurements, whereas Hon et al. [23] assessed the scan order randomly; the air puff, indeed, could introduce errors in morphological corneal evaluation if the Scheimpflug scan is performed just after it. Results provided by Hon et al. [23] about repeatability of IOP measurements with CST, differ from the ones reported by Hong et al. [21]; this could be due to the diversity of the population enrolled in the studies or to some methodological differences. Although this is one of the first papers about this topic and these results need to be confirmed by further studies also including affected eyes, it was important to start testing these correlations in healthy eyes in order to avoid bias coming from a non-homogeneous comparison, and then evaluate affected eyes, highlighting the differences with healthy ones. It is known that changes in corneal deformation are not only related to corneal structural organization and to IOP, but also to corneal biomechanic [6]; biomechanical properties measured with ORA have been demonstrated to be not completely independent from corneal morphological characteristics in previous papers [2,10,13,17,18]; this means that currently is not available any device able to provide information about corneal behaviour that are not related to parameters known to introduce bias in some case (i.e. IOP measurement) or to be insufficient to allow an early diagnosis of certain diseases (i.e. keratoconus, corneal ectasia). The clinical application of these findings could help ophtalmologists in: • Having more precise values of IOP, especially in case of eyes affected by corneal disease or in ones that undergo a change in shape, as after corneal refractive surgery. • Better understanding the evolution of corneal degenerative disease such as keratoconus, that cause a change both in corneal shape and in biomechanic. • Better screening corneas undergoing refractive surgery, in order to avoid complications like ectasia.
Moreover this paper reports a range of the parameters provided by CST, that may be useful to recognize suspect values.
In conclusion, the results of this study suggest that CST could be able to perform a corneal deformation analysis not affected by other parameters like CCT, CV, SK, and SE in healthy eyes; if these data will be confirmed by further studies, this device could be useful for the management and early diagnosis of corneal disease. Further studies are needed to confirm these data and to provide additional information about the utility of CST in diseased eyes. References [1] Luce DA. Determining in vivo biomechanical properties of the cornea with an ocular response analyzer. J Cataract Refract Surg 2005;31:156–62. [2] 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;92:469–75. [3] Pepose JS, Feigenbaum SK, Qazi MA, 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. ˜ [4] Ortiz D, Pinero 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;33:1371–5. [5] Kerautret J, Colin J, Touboul D, Roberts C. Biomechanical characteristics of the ectatic cornea. J Cataract Refract Surg 2008;34:510–3. [6] Liu J, Roberts CJ. Influence of corneal biomechanical properties on intraocular pressure. measurement quantitative analysis. J Cataract Refract Surg 2005;31:146–55. [7] Kotecha A. What biomechanical properties of the cornea are relevant for the clinician? Surv Ophthalmol 2007;52:109–14. [8] Martinez-de-la-Casa JM, Garcia-Feijoo J, Fernandez-Vidal A, MendezHernandez C, Garcia-Sanchez J. Ocular response analyzer versus goldmann applanation tonometry for intraocular pressure measurements. Invest Ophthalmol Vis Sci 2006;47:4410–4. [9] 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;37:553–62. [10] Hurmeric V, Sahin A, Ozge G, Bayer A. The relationship between corneal biomechanical. properties and confocal microscopy findings in normal and keratoconic eyes. Cornea 2010;29:641–9. [11] McMonnies CW. Assessing corneal hysteresis using the ocular response analyzer. Optom Vis Sci 2012;89:E343–9. [12] 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;47:5337–47. [13] Narayanaswamy A, Chung RS, Wu RY, Park J, Wong WL, Saw SM, et al. Determinants of corneal biomechanical properties in an adult Chinese population. Ophthalmology 2011;118:1253–9. [14] Fontes BM, Ambrósio Jr R, Alonso RS, Jardim D, Velarde GC, Nosé W. Corneal biomechanical metrics in eyes with refraction of −19.00 to +9.00 D in healthy Brazilian patients. J Refract Surg 2008;24:941–5. [15] Kara N, Altinkaynak H, Baz O, Goker Y. Biomechanical evaluation of cornea in topographically normal relatives of patients with keratoconus. Cornea 2013;32:262–6. [16] Congdon NG, Broman AT, Bandeen-Roche K, Grover D, Quigley HA. central corneal thickness and corneal hysteresis associated with glaucoma damage. Am J Ophthalmol 2006;141:868–75. [17] Shah S, Laiquzzaman M, Cunliffe I, Mantry S. The use of the Reichert ocular response analyser to establish the relationship between ocular hysteresis, corneal resistance factor and central corneal thickness in normal eyes. Cont Lens Anterior Eye 2006;29:257–62.
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