Assessment of corneal biomechanical properties using corneal visualization Scheimpflug technology at different stages of keratoconus
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Objective To compare the corneal biomechanical properties of the corneas of keratoconic eyes and normal eyes using corneal visualization Scheimpflug technology (Corvis ST),and to investigate the role of corneal biomechanical parameters in the diagnosis of keratoconus.Methods Ninety keratoconic eyes from 65 patients and 90 normal eyes from 90 participants were enrolled in this comparative study.Based on the Amseler-Krumeich keratoconus stages,the keratoconic eyes were divided into a mild group (46 eyes),moderate group (23 eyes) and severe group (21 eyes).Tomography and biomechanical parameters of all eyes were obtained with the Pentacam and Corvis ST,respectively.All parameters were compared between the keratoconic and normal groups.The correlation between deformation amplitude and anterior segment parameters was also analyzed.An independent t test,Wilcoxon rank sum test,ANOVA,nonparameter test were used.The receiver operating characteristic (ROC) curves were plotted to distinguish keratoconus from the normal cornea.Results The tomography and biomechanical parameters of the keratoconic eyes were significantly different from those of normal eyes except for the anterior chamber angle,first applanation length,highest concavity time,and peak distance.The deformation amplitude (area under the curve:0.865)was the best predictive parameter,with a sensitivity of 84.5％,specificity of 75.6％ and cut-off point of 1.14 mm.The diagnostic efficiency of the deformation amplitude increased with an increase in the severity of keratoconus.In both the normal and keratoconic groups,the deformation amplitude was negatively correlated with intraocular pressure,central corneal thickness,and corneal volume at 3 mm.The respective r values of the deformation amplitudes of the normal and keratoconic groups in regard to:intraocular pressure,-0.707 and-0.213 ; central corneal thickness,-0.219 and-0.357 ;and corneal volume at 3 mm,-0.212 and-0.27.All P values were ＜0.05.Conclusion Corvis ST offers an alternative method for measuring corneal biomechanical properties.The deformation amplitude has a high sensitivity for the diagnosis of keratoconus.The negative correlations with intraocular pressure and central corneal thickness deserve clinical attention.
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Biomechanics; Keratoconus ; Corneal visualization Scheimpflug technology ; Deformation amplitudeKeywords:
Scheimpflug principle
Corneal pachymetry
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Before the appearance of evident keratoconus, corneal biomechanical changes may be detectable. Here, these properties are analyzed to detect any difference that could help in the early recognition of keratoconus to allow patients to benefit from early treatments and to avoid refractive procedures in these corneas.The purpose of this study was to compare corneal biomechanical characteristics as determined by Corvis Scheimpflug Technology tonometry between normal eyes and asymmetric keratoconic eyes.Retrospective data from normal eyes (n = 100), keratoconic eyes (n = 18), and their topographically normal fellow eyes (n = 18) were analyzed. Differences in the variables among the groups were determined. For the parameters that showed significant differences, the receiver operating characteristic curve and the area under the curve (AUC) were used to assess the diagnostic accuracy of each variable. The optimal cutoff points were determined when comparing normal and fellow eyes. Also, a new linear combination of variables was performed to obtain better discriminative values.The following variables differed significantly between normal and fellow eyes: length of the flattened cornea in the second applanation, peak distance, curvature radius at highest concavity, and central corneal thickness. When each variable was independently considered, AUCs, sensitivity, and specificity were insufficiently high for good discrimination between the two groups. However, using a linear combination of variables, an optimal cutoff point (0.157) was obtained with an AUC of 0.78, sensitivity of 0.84, and specificity of 0.69.A best predictive linear combination of corneal biomechanical variables was tested including diameter of the flattened cornea in the second applanation and central corneal thickness. This combination was considered as the best in terms of its prediction capacity, simplicity and clinical application. This formula may be useful in clinical practice to discriminate between normal eyes and incipient keratoconus.
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To study the biomechanical properties of the corneas of both eyes in patients with evident keratoconus manifestation in one eye.Our study consisted of nine patients with keratoconus and 25 volunteers in the control group. Both eyes of all participants were measured twice with a Pentacam Scheimpflug system: first with the standard Scheimpflug system and subsequently with the original version of the same technique in combination with a new device that can generate experimental artificial intraocular pressure (IOP) elevation. Diagnoses of keratoconus or non-keratoconus were made using the Pentacam Scheimpflug system software (StatSoft, Inc., Tulsa, OK).The artificially elevated IOP caused bulging of the anterior corneal surface in both eyes of keratoconic patients and a small flattening of the cornea in the eyes of the control group. Corneal ectasia, expressed in terms of diopters, during IOP loading in both keratoconic and nominally unaffected eyes was 4.12 D and 1.37 D, respectively. The changes were statistically significant (p < 0.05). Our dynamic study revealed a distinct weakness of the corneal tissue and an inability of keratoconic and contralateral eyes to resist IOP elevation.The IOP loading method permits evaluation of the biomechanical properties of the cornea across its entire surface. All nine contralateral unaffected eyes in patients with a diagnosis of unilateral keratoconus presented weak biomechanical properties and in fact should be considered as forme fruste keratoconus.
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To compare the corneal deformation response, central corneal thickness (CCT), and intraocular pressure (IOP) measurements and their test-retest variability obtained with an ultrahigh-speed Scheimpflug camera between normal and keratoconus eyes.Three consecutive measurements were obtained using Corvis ST. The following parameters were analyzed: A1 and A2 length (length of flattened cornea at first and second applanation), A1 and A2 velocity (deformation velocity until first and second applanation), corneal deformation amplitude (deformation amplitude of cornea at the highest concavity), peak distance (distance of two apices of cornea at time of highest concavity), and radius of corneal curvature at the time of maximum deformation. Repeatability coefficient and intraclass correlation coefficient were measured. Linear mix models were used to adjust for the effect of age, CCT, and IOP on corneal deformation response parameters.Twelve normal subjects and 12 keratoconus patients were included. Data from only one eye of each participant were randomly selected for analysis. Significant differences were found in corneal deformation amplitude (p < 0.001) and radius of corneal curvature (p < 0.001) between normal and keratoconus eyes after adjusting for age, CCT, and IOP. Although there was no significant difference of intraclass correlation coefficient between the groups, repeatability coefficient values of A1 and A2 length, A1 velocity, and peak distance were significantly smaller in normal eyes as compared with keratoconus eyes (p ≤ 0.023).Corvis ST showed adequate repeatability for measurement of corneal deformation amplitude, CCT, and IOP in normal and keratoconus eyes. It may be used to understand ocular pathologies associated with altered biomechanical properties.
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Objective The aim of this study was to compare corneal hysteresis (CH) in normal and keratoconic corneas in high cylindrical astigmatism (≥3 D). Background There is a great interest in corneal biomechanics as it aids in the study of corneal viscoelasticity. The ocular response analyzer (ORA) measures the corneal biomechanics through the analysis of the corneal dynamic behavior when it is deformed by an air puff. Studies using ORA in the evaluation of corneal biomechanics revealed a significant difference between keratoconic and healthy corneas. Patients and methods This study included 22 normal and 24 keratoconic eyes, which were diagnosed by means of clinical examination and corneal pentacam topography. CH and corneal resistant factor were measured using ORA. Results The mean CH was found to be 10.92 ± 1.7 mmHg in normal individuals, whereas in keratoconic patients it was 7.7 ± 1.29 mmHg. However, the corneal resistance factor (CRF) was 11.077 ± 1.96 mmHg in normal individuals compared with 6.3 ± 1.59 mmHg in keratoconic patients. The difference between the normal and affected groups was statistically significant in both CH and CRF (P Conclusion CH and CRF were significantly higher in normal eyes than in keratoconic eyes. The correlation of CCT with CH and CRF when assessing the corneal biomechanics showed that lower CH and CRF implies a significantly reduced viscoelastic response in keratoconus than in normal eyes. Thus, this weakness in corneal viscoelasticity is associated with the lower CCT identified in keratoconus eyes.
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To compare the corneal hysteresis (CH) and corneal resistance factor (CRF) measured with the Ocular Response Analyzer (ORA) in normal and keratoconic eyes.It was a case-control study. Random selected 96 normal eyes and 46 keratoconic eyes in the same period were included in this study. Normal eyes were divided into 2 groups: high corneal astigmatism (> or = 3.00 D) and low-to-moderate corneal astigmatism (< 3.00 D). Keratoconic eyes were also divided into 3 groups based on Amsler-Krumeich classification: mild (stage I), moderate (stage II) and severe (stage III/IV). CH and CRF were compared between groups and the areas under ROC curves of the CH and CRF were calculated.The mean CH and CRF were (7.1 +/- 1.6) mm Hg and (6.3 +/- 1.5) mm Hg in keratoconic eyes compared with (10.1 +/- 1.3) mm Hg and (10.5 +/- 1.6) mm Hg in normal eyes. The difference were statistically significant(t = -11.813, -14.943; P < 0.001). In normal eyes, there was no difference of CH or CRF between the high corneal astigmatism and low-to-moderate corneal astigmatism (t = 0.373, 0.095; P > 0.05). In keratoconic eyes, there was a significant negative correlation between CH and the keratoconus grade (r = -0.627, P < 0.001) and the same relationship was found between CRF and the keratoconus grade (r = -0.587, P < 0.001). In multiple linear regression analysis,CH was correlated with central corneal thickness (CCT) and corneal curvature (r = 0.320, -0.375; P < 0.05) and CRF was correlated with corneal curvature in keratoconic eyes (r = -0.441, P < 0.01) , while they were only correlated with CCT in normal eyes (r = 0.367, 0.459; P < 0.001). The areas under ROC curves of the CH and CRF were 0.9282 and 0.9731 (Z = 20.462, 38.305; P < 0.0001), the difference between them was significant (Z = 7.134, P = 0.008).The CH and CRF were significantly lower in keratoconic eyes than in normal eyes, especially on CRF. The long-term follow-up of CH and CRF may provide information for evaluation of progression of keratoconus. They may be included as indicators for detecting keratoconus.
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To perform advanced analysis of the corneal deformation response to air pressure in keratoconics compared with age- and sex-matched controls.The ocular response analyzer was used to measure the air pressure-corneal deformation relationship of 37 patients with keratoconus and 37 age (mean 36 ± 10 years)- and sex-matched controls with healthy corneas. Four repeat air pressure-corneal deformation profiles were averaged, and 42 separate parameters relating to each element of the profiles were extracted. Corneal topography and pachymetry were performed with the Orbscan II. The severity of the keratoconus was graded based on a single metric derived from anterior corneal curvatures, difference in astigmatism in each meridian, anterior best-fit sphere, and posterior best-fit sphere.Most of the biomechanical characteristics of keratoconic eyes were significantly different from normal eyes (P < 0.001), especially during the initial corneal applanation. With increasing keratoconus severity, the cornea was thinner (r = -0.407, P < 0.001), the speed of corneal concave deformation past applanation was quicker (dive; r2 = -0.314, P = 0.01), and the tear film index was lower (r = -0.319, P = 0.01). The variance in keratoconus severity could be accounted for by the corneal curvature and central corneal thickness (r = 0.80) with biomechanical characteristics contributing an additional 4% (total r = 0.84). The area under the receiver operating characteristic curve was 0.919 ± 0.025 for keratometry alone, 0.965 ± 0.014 with the addition of pachymetry, and 0.972 ± 0.012 combined with ocular response analyzer biomechanical parameters.Characteristics of the air pressure-corneal deformation profile are more affected by keratoconus than the traditionally extracted corneal hysteresis and corneal resistance factors. These biomechanical metrics slightly improved the detection and severity prediction of keratoconus above traditional keratometric and pachymetric assessment of corneal shape.
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Abstract Purpose To compare the morphological and biomechanical properties of normal cornea and keratoconus at different stages. Methods A total of 408 patients (517 eyes) with keratoconus were included in this study. According to the Topographic Keratoconus (TKC) grading method, keratoconus was divided into stage I (TKC = 1, 130 eyes), stage II (TKC = 1–2, 2, 164 eyes), stage III (TKC = 2–3, 3, 125 eyes) and stage IV (TKC = 3–4, 4, 98 eyes). A total of 158 normal subjects (158 eyes) were recruited as the normal group. The corneal morphological parameters and biomechanical parameters were obtained with Scheimpflug tomography (Pentacam) and corneal visualization Scheimpflug technology (Corvis ST), and the receiver operating characteristic (ROC) curves were drawn. Results Each corneal morphological and most biomechanical parameters of the keratoconic eyes were significantly different from those of the normal eyes in this study ( p < 0.001). ROC curve demonstrated that most parameters in this study showed high efficiency in diagnosing keratoconus (the area under the ROC (AUC) was > 0.9), with the Belin-Ambrósio deviation (BAD-D) and Tomographic and Biomechanical Index (TBI) showing higher efficiency. The efficiency of BAD-D and TBI was high in differentiating keratoconus at different stages (AUC > 0.963). The comparison of ROC curves of keratoconus at different stages did not reveal statistically significant differences for TBI. Conclusion BAD-D and TBI can effectively diagnose stage I keratoconus. Moreover, the efficiency of TBI is the same in diagnosing keratoconus at all stages, while the diagnostic efficiency of other parameters increases with the increase in keratoconus stages.
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To investigate corneal biomechanical parameters in healthy and keratoconic eyes using the Ocular Response Analyzer dynamic bidirectional applanation device (ORA) and the Corvis ST dynamic Scheimpflug analyzer (CST).Department of Ophthalmology, Carl Gustav Carus University Hospital Dresden, Germany.Prospective, monocentric, case-control study.Corneal biomechanical parameters were obtained in 60 eyes of 60 healthy participants (Group I) and 60 eyes of 60 keratoconus patients (Group II) with different grades of severity using the ORA and the CST. Participants were matched by age (Group I: 38.3 years ± 12.8 [SD], Group II: 37.3 ± 11.2 years) and intraocular pressure (Group I: 13.7 ± 1.7 mm Hg, Group II: 13.6 ± 1.5 mm Hg).For the ORA, the receiver operating characteristic curve analysis showed an area under the curve (AUC) of 0.950 for the keratoconus score, a sensitivity of 87% and a specificity of 93%. The AUC for the corneal resistant factor and corneal hysteresis was 0.930 and 0.868 with a sensitivity of 87% and a specificity of 87%, and sensitivity of 80% and a specificity of 80%, respectively. For the CST, the corneal biomechanical index showed the highest AUC (0.977) with a sensitivity of 97% and a specificity of 98%. The AUC of integrated radius (0.974; 90% sensitivity, 93% specificity) was followed by maximum inverse radius (0.962; 92% sensitivity, 93% specificity). Most parameters were able to discriminate healthy eyes from different keratoconus stages and early stages of keratoconus from moderate stages.Both devices allowed for good differentiation between healthy eyes and keratoconic eyes and between different severity grades of keratoconus. Several parameters of ORA and CST revealed high sensitivity and specificity values for keratoconus detection.
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Purpose Corneal hysteresis is a viscoelastic property characterized by the difference in behavior under loading and unloading. The aim of the study was to determine corneal hysteresis in different eyes: healthy eyes, keratoconic eyes and keratoconic eyes after intracorneal ring implants. Methods This study comprised 95 eyes of 59 patients. The study population was divided into 3 groups. We analyze 30 healthy human eyes, 35 keratoconic eyes (Amsler grade II-III) and 30 keratoconic eyes after intracorneal rings implantation. Corneal biomechanical properties of these eyes were measured with the Ocular Response Analyser (ORA-Reichert) according to two parameters: corneal hysteresis (CH) and corneal resistance factor (CRF. The unpaired T student test was used for statistical analysis. Results Mean CH and CRF in normal eyes (10.78 ± 1.6 (SD) mm Hg, 11.91 mm Hg ± 1.2) was clearly superior than in keratoconus group (8.01 ± 1.5 mm Hg, 7.32 ±1.8 mm Hg). However the difference of these parameters between keratoconus group without intracorneal rings and after corneal ring implantation (7.27mm Hg, 6.32 mm Hg) was not statistically significant (p ≤ 0.01). Mean follow-up period was of one year. Conclusion Corneal hysteresis and CRF values were significantly lower in keratoconic eyes than in normal eyes. But there is no marked differences if we compare these two biomechanical parameters in keratoconic corneas, before and after corneal ring implants. Improvements in the software accuracy of the ORA devices are needed to characterize corneal biomechanics.
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