Corneal Biomechanics as a Function of Intraocular Pressure and Pachymetry by Dynamic Infrared Signal and Scheimpflug Imaging Analysis in Normal Eyes TUKEZBAN HUSEYNOVA, GEORGE O. WARING, IV, CYNTHIA ROBERTS, RONALD R. KRUEGER, AND MINORU TOMITA
PURPOSE:
To evaluate corneal biomechanical deformation response using Ocular Response Analyzer (ORA) and Corvis ST data. DESIGN: Prospective observational case-control study. METHODS: A total of 1262 eyes of 795 patients were enrolled. Three groups were established, according to the corneal compensated intraocular pressure (IOPcc): Group I (10-13 mm Hg), Group II (14-17 mm Hg), and Group III (18-21 mm Hg). Each group included 3 subgroups, based on central corneal thickness (CCT): Subgroups 1 (465-510 mm), 2 (510-555 mm), and 3 (555-600 mm). In addition, similar groups of CCT were divided into subgroups of IOPcc. Corneal hysteresis (CH) and corneal resistance factor (CRF) were derived from ORA. The parameters of highest concavity with the parameters of first and second applanation were recorded from Corvis ST. RESULTS: CH and CRF, applanation 1 time, and radius of curvature at highest concavity showed significant differences between CCT subgroups for each IOPcc group (P < .0001). CH, applanation 1 and 2 time, and applanation 2 velocity, as well as deformation amplitude (DA), showed significant differences by IOP subgroups for all CCT groups. IOPcc is correlated negatively with CH (r [ L0.38, P < .0001). There are positive correlations of IOPcc with applanation 1 time, applanation 2 velocity, and radius and negative correlations with applanation 2 time (r [ L0.54, P < .0001), applanation 1 velocity (r [ L0.118, P < .0001), and DA (r [ L0.362, P < .0001). CONCLUSION: ORA and Corvis ST parameters are informative in the evaluation of corneal biomechanics. IOP is important in deformation response evaluation and must be taken into consideration. (Am J Ophthalmol 2014;157:885–893. Ó 2014 by Elsevier Inc. All rights reserved.) Accepted for publication Dec 30, 2013. From Shinagawa LASIK Center, Chiyoda-ku, Tokyo, Japan (T.H., M.T.); Medical University of South Carolina, Storm Eye Institute, Charleston, South Carolina (G.O.W.); The Ohio State University, Columbus, Ohio (C.R.); Cleveland Clinic Cole Eye Institute, Cleveland, Ohio (R.R.K.); and Department of Ophthalmology, Wenzhou Medical College, Wenzhou, China (M.T.). Inquiries to Minoru Tomita, Shinagawa LASIK Center, Yurakucho ITOCiA 14F, 2-7-1 Yurakucho, Chiyoda-ku, Tokyo 100-0006, Japan; e-mail: [email protected] 0002-9394/$36.00 http://dx.doi.org/10.1016/j.ajo.2013.12.024
Ó
2014 BY
T
HE CORNEA IS A COMPLEX BIOMECHANICAL COMPOS-
ite whose behavior depends on its structural subcomponents and their organizational motifs.1 The microstructure of the corneal stroma is composed of 300500 lamellar sheets. Each of these sheets consists of thin, unbranched collagen fibrils that stretch from limbus to limbus.2 The fibrils in each sheet are arranged parallel to one another and are evenly spaced. A gel-like material, known as ground substance, fills the spaces between the fibrils and lamellae.3 The biomechanical properties of corneal tissue determine how it will respond and deform when placed under stress. This process depends on the viscoelastic properties of the cornea. Knowledge of biomechanical properties is important in the fields of intraocular pressure (IOP) measurement, glaucoma, corneal pathology such as keratoconus, and corneal refractive surgery. We will describe 2 devices that evaluate the biomechanical response of the cornea to an air puff– induced deformation: the Ocular Response Analyzer (ORA; Reichert Ophthalmic Instrument, Inc., Buffalo, New York. USA) and Corvis ST (Oculus, Wetzlar, Germany). The Ocular Response Analyzer was the first device commercially available to provide an in vivo measurement of corneal biomechanical response using dynamic infrared signal analysis. ORA is a noncontact tonometer developed to provide a more accurate assessment of IOP than Goldmann applanation tonometry (GAT), and through this assessment also provides a measure of biomechanical features of the cornea through monitoring and analyzing corneal response during an air pulse. There are 4 main parameters of the ORA. During the measurement, 2 applanation pressures (P1, P2) are obtained by an electro-optic system. The difference between these 2 pressure values is termed corneal hysteresis (CH) (Figure 1). CH provides information not on elastic properties—that is, how stiff or soft the cornea is—but rather on the rate-dependent viscoelastic response.4 Corneal resistance factor (CRF) is strongly associated with central corneal thickness by design as well as being uncorrelated to corneal compensated intraocular pressure (Luce DA. IOVS 2006;47:ARVO E-Abstract 2266). Thus, CRF may correlate with elastic properties5 even though it is viscoelastic by nature. Both CH and CRF are the analyzed responses of the cornea with applied air jet–induced deformation.6 ORA also reports 2 IOP values: the Goldmann-correlated IOPg, derived from the mean of P1 and P2; and the corneal
ELSEVIER INC. ALL
RIGHTS RESERVED.
885
FIGURE 1. Ocular Response Analyzer (ORA) process signals.
compensated IOP (IOPcc). The IOPcc was designed to be less sensitive to corneal properties than traditional applanation tonometry and was calibrated empirically to be relatively unaffected by laser in situ keratomileusis (LASIK) (Luce DA. IOVS 2006;47:ARVO E-Abstract 2266). The dynamic Scheimpflug imaging analysis (Corvis ST) uses a high-speed camera at a rate of 4300 frames per second to capture a series of horizontal Scheimpflug images during corneal deformation with an air puff jet. It is a noncontact tonometer (NCT) and imaging device that measures not only intraocular pressure but also corneal thickness and provides additional information about biomechanical responses of the cornea using dynamic Scheimpflug imaging analysis. The biomechanical response of the cornea is characterized by the following parameters: time (time to reach applanation), length (the length of the flattened segment in a Scheimpflug image), and velocity (corneal velocity of movement during applanation), all at the moment of both the first and second applanation events, as well as the following characteristics at the point of highest concavity: time, deformation amplitude (DA), distance between bending points of the cornea (Dist), and concave radius of curvature (RadCurv) (Figure 2). The approach taken in this study was to evaluate corneal biomechanical parameters by ORA and Corvis ST, and to investigate their relationship with IOPcc and central corneal thickness (CCT).
PATIENTS AND METHODS THIS WAS A PROSPECTIVE OBSERVATIONAL CASE-CONTROL
study. The study conformed to the ethics codes established by the Ethical Board Committee of Japan. The study was carried out with approval from the Institutional Review Board (Matsumoto Clinic, Tokyo, Japan), and all subjects consented. Because of the study population, the study was conducted on Asian eyes. The study was performed to evaluate the biomechanical parameters of the cornea using the ORA and Corvis ST in normal eyes and to determine the influence 886
of both IOPcc and CCT on corneal biomechanical response to an air puff, defined by the parameters produced. Exclusion criteria included corneal astigmatism >4.0 diopters (D), CCT >600 mm, IOP >21 mm Hg or IOP
PURPOSE:
To evaluate corneal biomechanical deformation response using Ocular Response Analyzer (ORA) and Corvis ST data. DESIGN: Prospective observational case-control study. METHODS: A total of 1262 eyes of 795 patients were enrolled. Three groups were established, according to the corneal compensated intraocular pressure (IOPcc): Group I (10-13 mm Hg), Group II (14-17 mm Hg), and Group III (18-21 mm Hg). Each group included 3 subgroups, based on central corneal thickness (CCT): Subgroups 1 (465-510 mm), 2 (510-555 mm), and 3 (555-600 mm). In addition, similar groups of CCT were divided into subgroups of IOPcc. Corneal hysteresis (CH) and corneal resistance factor (CRF) were derived from ORA. The parameters of highest concavity with the parameters of first and second applanation were recorded from Corvis ST. RESULTS: CH and CRF, applanation 1 time, and radius of curvature at highest concavity showed significant differences between CCT subgroups for each IOPcc group (P < .0001). CH, applanation 1 and 2 time, and applanation 2 velocity, as well as deformation amplitude (DA), showed significant differences by IOP subgroups for all CCT groups. IOPcc is correlated negatively with CH (r [ L0.38, P < .0001). There are positive correlations of IOPcc with applanation 1 time, applanation 2 velocity, and radius and negative correlations with applanation 2 time (r [ L0.54, P < .0001), applanation 1 velocity (r [ L0.118, P < .0001), and DA (r [ L0.362, P < .0001). CONCLUSION: ORA and Corvis ST parameters are informative in the evaluation of corneal biomechanics. IOP is important in deformation response evaluation and must be taken into consideration. (Am J Ophthalmol 2014;157:885–893. Ó 2014 by Elsevier Inc. All rights reserved.) Accepted for publication Dec 30, 2013. From Shinagawa LASIK Center, Chiyoda-ku, Tokyo, Japan (T.H., M.T.); Medical University of South Carolina, Storm Eye Institute, Charleston, South Carolina (G.O.W.); The Ohio State University, Columbus, Ohio (C.R.); Cleveland Clinic Cole Eye Institute, Cleveland, Ohio (R.R.K.); and Department of Ophthalmology, Wenzhou Medical College, Wenzhou, China (M.T.). Inquiries to Minoru Tomita, Shinagawa LASIK Center, Yurakucho ITOCiA 14F, 2-7-1 Yurakucho, Chiyoda-ku, Tokyo 100-0006, Japan; e-mail: [email protected] 0002-9394/$36.00 http://dx.doi.org/10.1016/j.ajo.2013.12.024
Ó
2014 BY
T
HE CORNEA IS A COMPLEX BIOMECHANICAL COMPOS-
ite whose behavior depends on its structural subcomponents and their organizational motifs.1 The microstructure of the corneal stroma is composed of 300500 lamellar sheets. Each of these sheets consists of thin, unbranched collagen fibrils that stretch from limbus to limbus.2 The fibrils in each sheet are arranged parallel to one another and are evenly spaced. A gel-like material, known as ground substance, fills the spaces between the fibrils and lamellae.3 The biomechanical properties of corneal tissue determine how it will respond and deform when placed under stress. This process depends on the viscoelastic properties of the cornea. Knowledge of biomechanical properties is important in the fields of intraocular pressure (IOP) measurement, glaucoma, corneal pathology such as keratoconus, and corneal refractive surgery. We will describe 2 devices that evaluate the biomechanical response of the cornea to an air puff– induced deformation: the Ocular Response Analyzer (ORA; Reichert Ophthalmic Instrument, Inc., Buffalo, New York. USA) and Corvis ST (Oculus, Wetzlar, Germany). The Ocular Response Analyzer was the first device commercially available to provide an in vivo measurement of corneal biomechanical response using dynamic infrared signal analysis. ORA is a noncontact tonometer developed to provide a more accurate assessment of IOP than Goldmann applanation tonometry (GAT), and through this assessment also provides a measure of biomechanical features of the cornea through monitoring and analyzing corneal response during an air pulse. There are 4 main parameters of the ORA. During the measurement, 2 applanation pressures (P1, P2) are obtained by an electro-optic system. The difference between these 2 pressure values is termed corneal hysteresis (CH) (Figure 1). CH provides information not on elastic properties—that is, how stiff or soft the cornea is—but rather on the rate-dependent viscoelastic response.4 Corneal resistance factor (CRF) is strongly associated with central corneal thickness by design as well as being uncorrelated to corneal compensated intraocular pressure (Luce DA. IOVS 2006;47:ARVO E-Abstract 2266). Thus, CRF may correlate with elastic properties5 even though it is viscoelastic by nature. Both CH and CRF are the analyzed responses of the cornea with applied air jet–induced deformation.6 ORA also reports 2 IOP values: the Goldmann-correlated IOPg, derived from the mean of P1 and P2; and the corneal
ELSEVIER INC. ALL
RIGHTS RESERVED.
885
FIGURE 1. Ocular Response Analyzer (ORA) process signals.
compensated IOP (IOPcc). The IOPcc was designed to be less sensitive to corneal properties than traditional applanation tonometry and was calibrated empirically to be relatively unaffected by laser in situ keratomileusis (LASIK) (Luce DA. IOVS 2006;47:ARVO E-Abstract 2266). The dynamic Scheimpflug imaging analysis (Corvis ST) uses a high-speed camera at a rate of 4300 frames per second to capture a series of horizontal Scheimpflug images during corneal deformation with an air puff jet. It is a noncontact tonometer (NCT) and imaging device that measures not only intraocular pressure but also corneal thickness and provides additional information about biomechanical responses of the cornea using dynamic Scheimpflug imaging analysis. The biomechanical response of the cornea is characterized by the following parameters: time (time to reach applanation), length (the length of the flattened segment in a Scheimpflug image), and velocity (corneal velocity of movement during applanation), all at the moment of both the first and second applanation events, as well as the following characteristics at the point of highest concavity: time, deformation amplitude (DA), distance between bending points of the cornea (Dist), and concave radius of curvature (RadCurv) (Figure 2). The approach taken in this study was to evaluate corneal biomechanical parameters by ORA and Corvis ST, and to investigate their relationship with IOPcc and central corneal thickness (CCT).
PATIENTS AND METHODS THIS WAS A PROSPECTIVE OBSERVATIONAL CASE-CONTROL
study. The study conformed to the ethics codes established by the Ethical Board Committee of Japan. The study was carried out with approval from the Institutional Review Board (Matsumoto Clinic, Tokyo, Japan), and all subjects consented. Because of the study population, the study was conducted on Asian eyes. The study was performed to evaluate the biomechanical parameters of the cornea using the ORA and Corvis ST in normal eyes and to determine the influence 886
of both IOPcc and CCT on corneal biomechanical response to an air puff, defined by the parameters produced. Exclusion criteria included corneal astigmatism >4.0 diopters (D), CCT >600 mm, IOP >21 mm Hg or IOP
