Corneal Deformation Parameters Research Articles

Patient-specific air puff-induced loading using machine learning.

Introduction: The air puff test is a contactless tonometry test used to measure the intraocular pressure and the cornea's biomechanical properties. Limitations that most challenge the accuracy of the estimation of the corneal material and the intraocular pressure are the strong intercorrelation between the intraocular pressure and the corneal parameters, either the material properties that can change from one person to another because of age or the geometry parameters like central corneal thickness. This influence produces inaccuracies in the corneal deformation parameters while extracting the IOP parametric equation, which can be reduced through the consideration of the patient-specific air puff pressure distribution taking into account the changes in corneal parameters. This air puff pressure loading distribution can be determined precisely from the fluid-structure interaction (FSI) coupling between the air puff and the eye model. However, the computational fluid dynamics simulation of the air puff in the coupling algorithm is a time-consuming model that is impractical to use in clinical practice and large parametric studies. Methods: By using a supervised machine learning algorithm, we predict the time-dependent air puff pressure distribution for different corneal parameters via a parametric study of the corneal deformations and the gradient boosting algorithm. Results: The results confirmed that the algorithm gives the time-dependent air puff pressure distribution with an MAE of 0.0258, an RMSE of 0.0673, and an execution time of 93s, which is then applied to the finite element model of the eye generating the corresponding corneal deformations taking into account the FSI influence. Using corneal deformations, the response parameters can be extracted and used to produce more accurate algorithms of the intraocular pressure and corneal material stress-strain index (SSI). Discussion: Estimating the distribution of air pressure on the cornea is essential to increase the accuracy of intraocular pressure (IOP) measurements, which serve as valuable indicator of corneal disease. We find that the air puff pressure loading is largely influenced by complex changes in corneal parameters unique to each patient case. With our innovative algorithm, we can preserve the same accuracy developed by the CFD-based FSI model, while reducing the computational time from approximately 101000s (28h) to 720s (12min), which is about 99.2% reduction in time. This huge improvement in computational cost will lead to significant improvement in the parametric equations for IOP and the Stress-Strain Index (SSI).

Evaluation of Corvis ST tonometer with the updated software in glaucoma practice.

To evaluate the agreement of biomechanically corrected intraocular pressure (b-IOP) and central corneal thickness (CCT) measurements obtained with the updated Corvis ST tonometer versus Goldmann applanation tonometry (GAT) and optical-based corneal pachymetry (OB-CCT) in controls, patients with ocular hypertension (OHT) and primary open angle glaucoma (POAG). Additionally, we examined the differences in corneal deformation parameters provided by the updated Corvis ST among the three groups. For each participant, GAT IOP, OB-CCT and measurements with a Corvis ST with updated software were obtained. Bland-Altman analysis was used to assess the agreement between the two measurement methods. A consecutive series of 80 eyes from 80 participants (30 with POAG, 25 with OHT and 25 normal controls) were included in this prospective study. The mean GAT IOP of all eyes was 17.2±3.6 mm Hg, and the mean b-IOP was 15.9±3.7 mm Hg (Spearman's rho=0.767, P<0.001). The 95% limits of agreement (LoAs) ranged from -3.1 mm Hg to 5.5 mm Hg for GAT IOP and b-IOP. b-IOP was not correlated with OB-CCT (Spearman's rho=-0.13 P=0.917). Meanwhile there was a weak positive corelation between OB-CCT and GAT IOP-b-IOP difference (Spearman's rho=0.378, P=0.001). The mean OB-CCT was 549.5±36.4 µm, and the Corvis-CCT was 556.1±41.5 µm (Spearman's rho=0.900, P<0.001). No statistically significant difference in the new indices provided by the updated Corvis ST was detected among the three groups. Compared with control eyes, POAG eyes had a significantly reduced applanation time 2 after adjusting for OB-CCT and GAT IOP (P=0.048). Corvis b-IOP and CCT correlate well with GAT IOP and OB-CCT. b-IOP is not affected by CCT, which might be an advantage, especially in thick or thin corneas. Corvis ST yields shorter applanation time 2 measurements in patients with POAG, which might reflect altered corneal viscoelasticity.

Association Between Severity of Myopia and Deformation Characteristics of the Cornea Based on Propensity Score Matching Analysis.

To investigate the relationship between severity of myopia and corneal deformation characteristics after removing confounding factors using propensity score matching (PSM) analysis. A prospective study was conducted from April 1 to December 30, 2019 in Tianjin Eye Hospital. Participants were divided into low (spherical equivalent [SE] > -6.00 diopters) and high (SE ⩽ -6.00 diopters) myopia groups. Corneal deformation parameters were obtained using corneal visualization Scheimpflug technology. PSM analysis was conducted to minimize the effect of confounding factors (age, intraocular pressure, and corneal thickness) on corneal deformation. Correlation analysis and logistic regression models were applied to investigate the relationships between SE and corneal deformation parameters. A total of 2,126 eyes from 1,063 patients with a mean age of 23.81 ± 5.68 years were enrolled. The left and right eyes were analyzed separately. After PSM analysis, logistic regression indicated that the peak distance was a significant indicator for high myopia in both eye models (left: odds ratio [OR] = 1.978, 95% CI: 1.303 to 3.004; right: OR = 2.089, 95% CI: 1.362 to 3.202; P < .001). The maximum amplitudes of deformation and deflection and peak distance were significantly negatively correlated with SE in both eyes, whereas the whole eye movement was significantly positively correlated with SE (P < .01), as well as the deformation amplitude at the second applanation (P < .001). Eyes with high myopia exhibited larger deformation and deflection amplitude compared to eyes with low myopia. The high distance between bending points of the cornea at the highest concavity (peak distance) may be a feature of high myopia. [J Refract Surg. 2021;37(5):344-350.].

Influence of corneal shape parameters on corneal deformation responses measured with a Scheimpflug camera.

To investigate the effect of corneal shape parameters on corneal deformation responses measured with a Scheimpflug camera. A total of 241 eyes of 241 participants were enrolled in this study. The anterior and posterior corneal curvature radii (CCR), anterior and posterior corneal Q-values, and corneal diameters of the participants were measured using the Pentacam HR. A total of 17 corneal deformation parameters including time, velocity, deflection amplitude, length, and area during ingoing applanation, highest concavity, and outgoing applanation were recorded by corneal visualization using Scheimpflug technology (Corvis ST). The effect of corneal shape parameters on corneal deformation responses was evaluated using multivariate regression models. Multivariate regression analyses showed that six, five, four, and three corneal deformation parameters were significantly correlated with anterior CCR, posterior CCR, anterior Q-value, and posterior Q-value, respectively. Steeper anterior corneal curvature was associated with faster velocity during ingoing applanation and greater deformation during outgoing applanation. Steeper posterior corneal curvature was correlated with faster velocity during outgoing applanation and greater deformation during ingoing applanation. Eyes that had steeper corneal curvatures were associated with less stiff corneas. More negative anterior Q-value corresponded with faster velocity and greater deformation during ingoing applanation. Eyes that had more prolate posterior corneal surfaces showed more resistance to corneal deformation at the highest concavity. However, corneal diameter was not selected in any corneal deformation parameters models. Corneal deformation response is significantly influenced by anterior and posterior corneal curvature and corneal asphericity, but not corneal diameter.

Corneal pulsation and biomechanics during induced ocular pulse. An ex-vivo pilot study.

The purpose of this study was to ascertain the relationships between the amplitude of the corneal pulse (CP) signal and the parameters of corneal biomechanics during ex-vivo intraocular pressure (IOP) elevation experiments on porcine eyes with artificially induced ocular pulse cycles. Two experiments were carried out using porcine eyes. In the first one, a selected eye globe was subjected to three IOP levels (15, 30 and 45 mmHg), where changes in physical ocular pulse amplitude were controlled by infusion/withdrawal volumes (ΔV). In the second experiment, six eyes were subjected to IOP from 15 mmHg to 45 mmHg in steps of 5 mmHg with a constant ΔV, where corneal deformation parameters were measured using Corvis ST. In both experiments, at each IOP, the CP and IOP signals were acquired synchronically using a non-contact ultrasonic distance sensor and a pressure transmitter, respectively. Based on the amplitudes of the CP and IOP signals ocular pulse based corneal rigidity index (OPCRI) was calculated. Results indicate positive correlations between ΔV and the physical ocular pulse amplitude, and between ΔV and the corneal pulse amplitude (both p < 0.001). OPCRI was found to increase with elevated IOP. Furthermore, IOP statistically significantly differentiated changes in OPCRI, the amplitudes of CP and IOP signals and in most of the corneal deformation parameters (p < 0.05). The partial correlation analysis, with IOP as a control variable, revealed a significant correlation between the length of the flattened cornea during the first applanation (A1L) and the corneal pulse amplitude (p = 0.002), and between A1L and OPCRI (p = 0.003). In conclusion, this study proved that natural corneal pulsations, detected with a non-contact ultrasonic technique, reflect pressure-volume dynamics and can potentially be utilized to assess stiffness of the cornea. The proposed new rigidity index could be a simple approach to estimating corneal rigidity.

Corneal Stiffness and Its Relationship With Other Corneal Biomechanical and Nonbiomechanical Parameters in Myopic Eyes of Chinese Patients.

To investigate corneal stiffness and analyze its possible influence on other corneal biomechanical and nonbiomechanical parameters in myopic eyes of Chinese patients. A total of 387 healthy (right) myopic eyes were included in this study. Corneal visualization Scheimpflug technology was used to measure the corneal stiffness parameter at the first applanation (SP-A1), deformation amplitude ratio at 1.0 and at 2.0 mm, biomechanically corrected intraocular pressure (bIOP), and corneal deformation parameters during the first applanation (A1: A1-time, A1-length, and A1-velocity), second applanation (A2; A2-time, A2-length, and A2-velocity), and highest concavity (HC; HC-time, HC-radius, HC deformation amplitude, and HC peak distance). The Pentacam was used to evaluate central corneal thickness, mean corneal curvature (Km), anterior corneal central elevation, and corneal asphericity (Q value) of the anterior cornea. The mean SP-A1 and bIOP values were 90.46 ± 15.39 mm Hg/mm and 13.5 ± 1.85 mm Hg, respectively. The SP-A1 increased with age (β = 0.41 [95% CI, 0.28%-0.54%]; P < 0.0001). Corneal stiffness was positively correlated with central corneal thickness, uncorrected intraocular pressure, and bIOP and was also significantly correlated with all corneal deformation parameters (P < 0.05), except A1-length, HC-time, and HC peak distance. Corneal stiffness was negatively correlated with central corneal elevation of the anterior surface (r = -0.124, P = 0.014) and mean corneal curvature (r = -0.114, P = 0.025) and positively correlated with the Q value of the anterior surface (r = 0.109, P = 0.032). Corneal stiffness increased with age. Stronger corneal stiffness may be related to flattening of the cornea and higher intraocular pressure.

Measurement repeatability of the dynamic Scheimpflug analyzer.

To evaluate the repeatability of corneal deformation parameters measured using a dynamic Scheimpflug analyzer and the impact of baseline clinical factors on the repeatability of each parameter. Retrospective, cross-sectional study. Forty-eight eyes (48 healthy subjects; mean age, 49.0±19.5years) underwent repeated examinations with the Scheimpflug analyzer to evaluate the test-retest variability. The intraclass correlation coefficient (ICC) and repeatability coefficient as indicators of variability were computed for 35 parameters measured with the Scheimpflug analyzer. The associations between the magnitude of the test-retest variability and baseline factors, such as age, axial length (AL), intraocular pressure (IOP), and central corneal thickness (CCT), were analyzed. The test-retest repeatability was excellent for 22 (62.9%) of 35 parameters (ICC≥0.75), good for seven (20%), (ICC≥0.6), fair for four (11.4%), (ICC≥0.4), and poor for two (5.7%) parameters (ICC<0.4). Age was associated positively with the magnitude of variability in 13 (37.1%) parameters; measurement variability was affected significantly by AL (5 parameters, 14.3%) and CCT (7 parameters, 20%) but, except for one parameter not by IOP. Most parameters of the dynamic Scheimpflug analyzer showed favorable measurement reliability in healthy subjects. However, six parameters showed poor-to-fair repeatability. Age, AL, and CCT significantly affected the repeatability of several parameters. These results should be considered when clinicians use this device in clinical practice.

Biomechanical Changes After In Vivo Collagen Cross-Linking With Rose Bengal-Green Light and Riboflavin-UVA.

To compare corneal biomechanical properties after in vivo and ex vivo cross-linking (CXL) using rose bengal-green light (RGX) or riboflavin-UVA (UVX). Corneas of 30 rabbits were treated in vivo by the two CXL modalities monolaterally (Group 1) or bilaterally (Group 2). Rabbits in Group 1 were euthanized 1 month after treatments and in Group 2 two months after treatment. Ex vivo CXL was also performed. Eyes were measured by Scheimpflug air puff corneal deformation imaging (Corvis ST) under constant IOP. Corneal deformation parameters were assessed. Inherent corneal biomechanical properties were estimated using inverse finite element modeling. Peak to peak distance decreased 16% 2 months after RGX, and 4% and 20% 1 and 2 months after UVX, respectively. The equivalent Young's modulus (Eeq) increased relative to the control during the post treatment period for both RGX and UVX. The Eeq increased by factors of 3.4 (RGX) and 1.7 (UVX) 1 month and by factors of 10.7 (RGX) and 7.3 (UVX) 2 months after treatment. However, the Eeq values for ex vivo CXL were much greater than produced in vivo. The ex vivo Eeq was greater than the 1-month in vivo values by factors of 8.1 (RGX) and 9.1 (UVX) and compared with 2 month by factors of 2.5 (RGX) and 2.1 (UVX). These results indicate that corneal stiffness increases after CXL, and further increases as a function of time after both RGX and UVX. Also, while biomechanical properties determined after ex vivo CXL are indicative of corneal stiffening, they may not provide entirely accurate information about the responses to CXL in vivo.

Evaluation of Corneal Deformation Parameters Provided by the Corvis ST Tonometer After Trabeculectomy.

The aims of this study were to evaluate how the corneal deformation parameters provided by the Corvis ST tonometer (CST) were influenced by pressure-lowering ocular surgery, and to determine the correlations of intraocular pressure (IOP) and axial length (AL) with CST corneal deformation parameters. This prospective 1-month study enrolled 22 subjects (22 eyes) who underwent trabeculectomy combined with mitomycin C. The corneal deformation parameters were measured using the CST. IOP was measured before and after surgery by a Goldmann applanation tonometer and the CST. The central corneal thickness and AL were also recorded. The correlations of the corneal deformation parameters with central corneal thickness, AL, and IOP changes were determined by linear regression analysis. IOP decreased significantly after surgery. AL was significantly shorter at 1 week after surgery. There were significant changes in the CST parameters time 1, velocity 1, velocity 2, peak distance highest concavity, and deformation amplitude highest concavity at 1 week and 1 month after surgery and in time highest concavity at 1 week after surgery. The change in time 1 was significantly correlated with preoperative IOP and the IOP reductions, and was positively correlated with the decrease in AL at 1 week. The increase in velocity 1 was negatively correlated with preoperative IOP, and IOP reductions. The change in time highest concavity was negatively correlated with IOP before surgery and the decrease in IOP at 1 week. CST is a good choice for measuring IOP, especially when aiming for normalization of IOP after glaucoma surgery.

Factors Associated With Corneal Deformation Responses Measured With a Dynamic Scheimpflug Analyzer

The purpose of this study was to clarify correlations between corneal deformation parameters measured with a dynamic Scheimpflug analyzer and baseline factors such as axial length, intraocular pressure (IOP), age, central corneal thickness (CCT), and corneal curvature. Ninety-six eyes of 96 healthy subjects (mean 55.2 ± 16.1 years of age) were examined using a dynamic Scheimpflug analyzer. Eighteen of 35 deformation parameters were selected for analyses based on measurement reliability and clinical relevance. The associations between corneal deformation parameters and axial length, IOP, age, CCT, and average corneal power were evaluated using multivariate regression analyses. Deformation parameters were correlated significantly with axial length (n = 13), IOP (n = 13), age (n = 8), and CCT (n = 6) in the multivariate models. Longer axial length corresponded with greater corneal deformability, less viscous damping, and less movement of the entire eye. Higher IOP was associated with greater corneal resistance and less movement of the entire eye. Older age was associated with less corneal deformability and greater movement of the entire eye. Corneal curvature was correlated significantly with only three deformation parameters. This study clarified the substantial impact of axial length, age, and IOP on the biomechanical responses of the cornea and the entire eye. In contrast, corneal curvature did not affect most of the deformation parameters. The current results confirmed the importance of corneal biomechanics, especially in eyes with longer axial length and in older subjects.

First Clinical Impressions on the Integrated Corneal Tomography and Corneal Deformation with Scheimpflug Imaging

ABSTRACT The purpose of this study is to report the preliminary findings and the retrospective clinical impressions of the integrated corneal tomography and corneal deformation parameters with a dynamic ultra-high-speed Scheimpflug camera equipped with a noncontact tonometer. Twelve eyes of six patients were evaluated by the Scheimpflug imaging system of the Pentacam HR and Corvis ST (OCULUS Optikgeräte GmbH, Wetzlar, Germany) devices. The parameters used were the Belin/Ambrósio deviation (BAD-D) and Corvis biomechanical index (CBI), as well as the recently published tomographic and biomechanical index (TBI), combined with a detailed clinical data and evaluation of normal eyes and patients with different stages of corneal ectasia. How to cite this article Haddad JS, Barnwell E, Rocha KM, Waring IV GO, Ambrosio Jr R. First Clinical Impressions on the Integrated Corneal Tomography and Corneal Deformation with Scheimpflug Imaging. Int J Kerat Ect Cor Dis 2017;6(2):101-109.

Detection of Keratoconus With a New Biomechanical Index.

To evaluate the ability of a new combined biomechanical index called the Corvis Biomechanical Index (CBI) based on corneal thickness profile and deformation parameters to separate normal from keratoconic patients. Six hundred fifty-eight patients (329 eyes in each database) were included in this multicenter retrospective study. Patients from two clinics located on different continents were selected to test the capability of the CBI to separate healthy and keratoconic eyes in more than one ethnic group using the Corvis ST (Oculus Optikgeräte GmbH, Wetzlar, Germany). Logistic regression was employed to determine, based on Database 1 as the development dataset, the optimal combination of parameters to accurately separate normal from keratoconic eyes. The CBI was subsequently independently validated on Database 2. The CBI included several dynamic corneal response parameters: deformation amplitude ratio at 1 and 2 mm, applanation 1 velocity, standard deviation of deformation amplitude at highest concavity, Ambrósio's Relational Thickness to the horizontal profile, and a novel stiffness parameter. The receiver operating characteristic curve analysis of the training database showed an area under the curve of 0.983. With a cut-off value of 0.5, 98.2% of the cases were correctly classified with 100% specificity and 94.1% sensitivity. In the validation dataset, the same cut-off point correctly classified 98.8% of the cases with 98.4% specificity and 100% sensitivity. The CBI was shown to be highly sensitive and specific to separate healthy from keratoconic eyes. The presence of an external validation dataset confirms this finding and suggests the possible use of the CBI in everyday clinical practice to aid in the diagnosis of keratoconus. [J Refract Surg. 2016;32(12):803-810.].

Correlations between the deformation parameters measured by Corvis ST and corneal morphology parameters in myopic patients

Background The novel dynamic biomechanical device Corvis ST can measure a variety of corneal deformation parameters and record the dynamic process of cornea deformation.Understanding various corneal deformation parameters is of important clinical significance for morphological and functional prediction for myopic eyes in corneal refractive surgery.However, the range and meaning of corneal deformation parameters in myopic eyes are still unknown. Objective This study was to establish the range of corneal defamation parameters by Corvis ST and explore its clinical significance in myopic eyes. Methods A descriptive study was designed under the approval of Ethic Committee of Tianjin Eye Hospital and informed consent of patients.This study protocol complied with Helsinki Declaration.Four hundred and seventy-seven eyes of 257 myopic patients who were going to receive corneal refractive surgery were included in Tianjin Eye Hospital from June 2014 to January 2015.The corneal deformation parameters including the first applanation time (1st A-time), first applanation lengh (1st A-length), first applanation velocity (1st A-velocity), second applanation time (2nd A-time), second applanation length(2nd A-length), second applanation velocity (2nd A-velocity), highest concavity time (HC-time), highest concavity deformation amplitude (HC-DA), highest concavity peak distance (HC-PD), highest concavity radius (HC-R), non-contact intraocular pressure (IOPnct) and the central corneal thickness (CCTst) were measured by Corvis ST.Corneal structural parameters including CCT and corneal curvature were measured by 3-D anterior segment analysis system( Pentacam), and corneal biomechanical parameters including corneal resistance factor (CRF), corneal hysteresis (CH), intraocular pressure mimic Goldmann (IOPg) and intraocular pressure of corneal compensation (IOPcc) were measured by Ocular Response Analyzer (ORA). The distributions of the data were tested with the Shapiro-Wilk test and 95% confidence intervals (CI) were calculated.The correlations of the variables were evaluated by Pearson correlation coefficient and Spearman rank correlation coefficient.Multiple linear regression was used to analyze the changes of 1st A-time, 2nd A-time and HC-DA with corneal morphology and biomechanical parameters. Results Only 1st A-time, 2ndA-time, HC-time, HC-DA, CCTst and IOPnct showed the normality distribution in corneal deformation parameters.The positive correlations were found between 1st A-time, 1st A-length, 2nd A-length, 2nd A-velocity, HC-R with CCT (r=0.338, rs=0.129, rs=0.282, rs=0.374, r=0.306, all at P<0.01), while 1st A-velocity, 2nd A-time, HC-D and HC-PD showed the negative correlations with CCT (rs=-0.235, r=-0.130, r=-0.259, r=-0.226, all at P<0.01). CRF and CH showed positive correlations with 1st A-time, 2nd A-length, 2nd A-velocity, HC-time, HC-R and negative correlations with HC-PD, △A-length (all at P<0.05).1st A-time, 2nd A-time and HC-DA changed with CRF and IOPcc with the regression formations of 1st A-time =6.185+ 0.066CRF+ 0.034IOPcc (F=300.123, P=0.000), 2nd A-time=23.397-0.074IOPcc-0.044CRF (F=227.979, P=0.000) and HC-DA= 1.523-0.017IOPcc-0.017CRF (F=152.662, P=0.000). The CCTst, IOPst and IOPnct values by Corvis ST were significantly lower than CCT by Pentacam ([548.23±26.31]μm, IOPg ([15.02±2.72]mmHg and ([16.02±2.56]mmHg) by ORA respectively (t=11.00, 2.919, 6.815, all at P<0.01). Conclusions The corneal deformation parameters by Corvis ST can quantitatively describe corneal biomechanical properties in myopic eyes.Both IOP and the corneal steep curvature of the 3 mm center area (K2) affect the deformation response of cornea.The reliability of Corvis ST in the measurement of CCT and IOP remains to be further studied. Key words: Cornea/physiopathology; Cornea/anatomy h Myopia/physiopathology; Biomechanical phenomena/physiology; Elasticity/physiology; Corneal deformation parameters

Role of Age and Myopia in Simultaneous Assessment of Corneal and Extraocular Tissue Stiffness by Air-Puff Applanation.

To assess the correlation of age and myopia with corneal and extraocular tissue stiffness derived from air-puff applanation using a composite viscoelastic model. This retrospective, cross-sectional study evaluated 155 normal eyes (age range: 5 to 50 years) measured on Corvis ST (Oculus Optikgeräte GmbH, Wetzlar, Germany). Manifest refraction spherical equivalent was also analyzed. A linear viscoelastic model that segregated corneal and extraocular tissue stiffness from the applanation deformation waveform was implemented. Corvis ST measured the total deformation (deformation amplitude waveform), which was simply the sum of corneal and extraocular tissue deformation. Age- and myopia-based multivariate analyses of variance between deformation parameters were performed after adjusting for intraocular pressure and central corneal thickness. Corvis ST corneal hysteresis was also calculated from the corneal deformation waveform. All myopia and age groups were matched for intraocular pressure and central corneal thickness. Extraocular tissue stiffness significantly increased with age (P = .03). Some other extraocular tissue deformation parameters also correlated with age, indicating age-related stiffening (P < .05). Corneal and extraocular tissue stiffness decreased with increasing myopia, but the trend was not significant (P = .10). Corvis ST corneal hysteresis increased with increasing age (P = .01) but not with increasing myopia (P = .61). Extraocular deformation parameters indicated stiffening of the extraocular tissues with age. Corneal deformation parameters were unaffected by age and myopia. Further studies with larger sample sizes are needed to clearly understand the effect of myopia on corneal and extraocular tissue stiffness. [J Refract Surg. 2016;32(7):486-493.].

Changes in Corneal Biomechanical Properties after Long-Term Topical Prostaglandin Therapy.

ObjectiveTo compare corneal biomechanical properties, measured by a newly developed tonometer (Corneal Visualization Scheimpflug Technology, Corvis ST), in untreated primary open angle glaucoma (POAG) patients, POAG patients with long-term topical prostaglandin analog (PGA) therapy and in normal controls. Further is to investigate the potential effects of PGA on corneal biomechanics.MethodsIn this case-control study, 35 consecutive medication naïve eyes with POAG, 34 POAG eyes with at least 2 years treatment by PGA and 19 normal eyes were included. Intraocular pressure (IOP), central corneal thickness (CCT) and corneal biomechanical parameters, including deformation amplitude (DA), applanation time (AT1 and AT2), applanation length (AL1 and AL2), applanation velocity (AV1 and AV2), and peak distance and radius were measured using Corvis ST. Axial length and corneal curvature were measured with partial coherence interferometry (IOLMaster, Zeiss, Germany). General linear model analysis was performed to investigate the corneal biomechanical property changes among the normal controls, newly diagnosed POAG patients and POAG patients with long-term PGA treatment, and among the subgroups of different types of PGA treatment, including bimatoprost, latanoprost and travoprost. Furthermore, pairwise comparisons using Bonferroni correction for least squares means were employed.ResultsAT1 (p<0.0001), AV1 (p<0.0001), AT2 (p = 0.0001), AV2 (p<0.0001) and DA (p = 0.0004) in newly diagnosed glaucoma patients were significantly different from those in normal subjects and in patients underwent at least 2 years topical PGA therapy after adjusting for age and gender. After adjusting for age, gender, IOP, CCT, axial length and corneal curvature, a significant difference was detected for DA between glaucoma patients without PGA treatment and patients with long-term PGA therapy (p = 0.0387). Furthermore, there were no statistical significant differences in all of the corneal biomechanical parameters among the 3 types of PGA therapy subgroups, namely bimatoprost, latanoprost and travoprost.ConclusionsSignificant changes in corneal deformation parameters were found among untreated POAG patients, POAG patients with long-term topical PGA therapy and normal controls. Long-term topical PGA treatment might have a direct effect on corneal biomechanical properties in addition to the indirect effect owing to the PGA-induced IOP reduction and CCT decrease on corneal dynamic properties.

In vivo human corneal deformation analysis with a Scheimpflug camera, a critical review.

Corneal morphological analysis has greatly improved in recent years, providing physicians with new and reliable parameters to study. Moreover, today corneal functional too is a routine analysis, thanks to biomechanical evaluation allowed by an ocular response analyzer (Reichert Ophthalmic Instrument, Depew, NY, USA). Corvis ST (OCULUS Optikgeräte GmbH, Wetzlar, Germany), that relies on the ultrahigh speed Scheimpflug camera, is a new device providing corneal deformation parameters measured ny scanning the cornea response to an air puff; it is an instrument able to measure intraocular pressure too. This device could open up a whole new prospective in screening, detecting and managing corneal diseases, intraocular pressure measurement and in evaluating surgical procedures involving the cornea. This paper provides a comprehensive explanation of Corvis ST measurement principles and parameters and a literature review of scientific studies.

Corneal Deformation Parameters Provided by the Corvis-ST Pachy-Tonometer in Healthy Subjects and Glaucoma Patients.

To evaluate precision and accuracy of Corvis-ST, a new noncontact tonometer equipped with a high-speed Scheimpflug technology, which measures intraocular pressure (IOP), central corneal thickness (CCT), and corneal deformation parameters (CDPs). Relationships among IOP, CCT, and CDPs were also assessed. This prospective, randomized study included 1 eye of 164 subjects (79 controls and 85 glaucoma patients), which underwent IOP measurements taken 3 times with Goldmann applanation tonometry (GAT) and Corvis-ST in random order. Precision was evaluated using intraclass correlation coefficients. Intermethod agreement was assessed using Bland-Altman method. Relationships amongst IOP, CCT and CDPs were evaluated using linear regression analysis. Corvis-ST intraclass correlation coefficients ranged between 0.95 and 0.99 for IOP and CCT; between 0.17 and 0.97 for CDPs. Mean (GAT-Corvis-ST) IOP was 1.4±2.7 mm Hg, which appeared related to IOP and CDPs (P<0.05). CDPs significantly differed between glaucoma patients and controls (P<0.05). GAT and Corvis-ST IOP was related to CCT (P<0.05) and CDPs (P<0.001). Corvis-ST precision was excellent for IOP and CCT; moderate for CDPs. Corvis-ST underestimated GAT IOP, especially at higher IOP and at lower corneal deformability levels. GAT and Corvis-ST IOP increased in thicker and less deformable corneas. Glaucoma patients showed significantly less deformable corneas than controls.

Evaluation of corneal deformation analyzed with a Scheimpflug based device.

To evaluate the correlation between corneal biomechanical and morphological data in healthy eyes. 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. 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 (R(2) = 0.28 and 0.26 respectively), none of them was statistically significant (p>0.01). 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.

Changes in corneal deformation parameters after lenticule creation and extraction during small incision lenticule extraction (SMILE) procedure.

PurposeTo investigate the effects of lenticule creation and subsequent corneal lenticule extraction on corneal deformation parameters during small incision lenticule extraction (SMILE) procedure.Materials and MethodsIn this prospective study, 18 eyes of 10 patients (27.90±7.11 years, −5.64±2.45 diopters) scheduled for SMILE procedure were enrolled. Changes in the corneal deformation parameters, including deformation amplitude (DA), applanation time(AT1 and AT2), applanation length(AL1 and AL2), corneal velocity(CV1 and CV2), peak distance(P.Dist.), radius and intraocular pressure values were measured preoperatively, immediately after lenticule creation and subsequent to corneal lenticule extraction in all eyes with the Corvis Scheimpflug Technology (Corvis ST, OCULUS, Wetzlar, Germany). Repeated measures analysis of variance (ANOVA) with bonferroni-adjusted post hoc comparisons was performed to investigate changes following each step of the procedure.ResultsAll surgical procedures were uneventful. A significant difference was detected among the three time points (pre-operation, post-lenticule creation and post lenticule extraction) for AT1 (P<0.001), AT2 (P = 0.001), DA(P<0.001), and IOP(P = 0.002). Bonferroni-adjusted post hoc comparisons indicated that there was no significant change in AT1, AT2, DA, or IOP after lenticule creation (post hoc P>0.05), but there was a significant change in these parameters following subsequent corneal lenticule extraction (post hoc P<0.01), when compared to values obtained pre-operatively. The scheimpflug camera of the Corvis ST demonstrated the intralamellar small gas bubbles formed from the vaporisation of tissue after lenticule creation and a gray zone was observed between the cap and the residual stromal bed after lenticule extraction.ConclusionsThere is a significant change in corneal deformation parameters following SMILE procedure. The changes may be caused predominantly by stromal lenticule extraction, while lenticule creation with femtosecond laser may not have an obvious effect on corneal deformation properties.

Evaluation of corneal deformation analyzed with Scheimpflug based device in healthy eyes and diseased ones.

This study was designed to evaluate the correlation between corneal biomechanical and morphological data in healthy eyes, eyes that underwent myopic photorefractive keratectomy (PRK), keratoconus affected eyes, and keratoconus affected eyes that underwent corneal collagen crosslinking (CCC). Complete clinical eye examination of all eyes was followed by tomographic (Pentacam, Oculus, Wetzlar, Germany) and biomechanical (Corvis ST, Oculus, Wetzlar, Germany) evaluation. Differences among Corvis ST (CST) parameters in the different groups have been performed. Linear regression between central corneal thickness (CCT), intraocular pressure (IOP), and anterior corneal curvature measured with Sim'K (KM), versus corneal deformation parameters measured with Corvis ST in the different groups, has been run using SPSS software version 18.0. We evaluated 64 healthy eyes of 64 patients with a mean refractive error of −0.65 ± 1.68 D (measured as spherical equivalent), 17 eyes of 17 patients that underwent myopic PRK for a mean refractive defect of −4.91 ± 2.05 D (measured as spherical equivalent), 16 eyes of 16 patients affected by keratconus (stage 2-3 of Amsler Classification), and 13 eyes of 13 patients affected by keratoconus that underwent CCC. Our data suggest that corneal curvature would have a greater influence on corneal deformation than CCT; in fact KM values are more strongly associated with more CST parameters both about corneal change in shape and both about the corneal ability to come back at original shape.