Corneal Visualization Scheimpflug Technology Research Articles

Transglutaminase-induced Cornea Collagen Crosslinking Effect on Central Corneal Thickness and Keratocyte Cell Density

Abstract
 Introduction & Objectives : This research is aimed to evaluate the effect of transglutaminase-induced corneal collagen crosslinking (CXL) on central corneal thickness (CCT) and keratocyte cell density in vivo.
 Methods : Twenty-eight white New Zealand rabbits were divided into four groups: the transglutaminase- induced CXL group, the epithelial-off CXL group, the transepithelial CXL group, and the control group. The ocular surface was treated with a 1 U/mL microbial transglutaminase solution, and both the epithelial-off and transepithelial groups were exposed to clinical ultraviolet A-riboflavin (UVA/RF). The efficacy of each group was evaluated on the 14th day after the procedures. Central corneal thickness was evaluated with Corneal Visualization Scheimpflug Technology (Corvis ST) and keratocyte cell density wad evaluated with histopathology examination.
 Results : Transglutaminase-induced CXL group exhibited the highest mean biomechanical CCT (370.14 ± 38.85) in comparison to the UVA/RF epithelial-off group (368.00 ± 25.48), the UVA/RF transepithelial group (369.86 ± 23.43), and the control group (365.14 ± 28.74). Still, there was no significant differences in both biomechanical CCT (p=0.990). Transglutaminase-induced CXL group had the highest mean of keratocyte cell density (43.26 ± 10.65) compared to UVA/RF epithelial-off (29.99 ± 4.79), UVA/RF transepithelial group (42.03 ± 6.55), and control group (34.36 ± 6.76). There was a significant difference between the group, with a p-value of 0.008.
 Conclusion : Transglutaminase shown that it produces favorable results for CCT and keratocyte density after CXL, which are two factors contributing to corneal rigidity. The outcomes are equivalent to riboflavin used as a conventional element in CXL.

Corneal biomechanics in normal and subclinical keratoconus eyes

BackgroundThe diagnosis of keratoconus, as the most prevalent corneal ectatic disorder, at the subclinical stage gained great attention due to the increased acceptance of refractive surgeries. This study aimed to assess the pattern of the corneal biomechanical properties derived from Corneal Visualization Scheimpflug Technology (Corvis ST) and evaluate the diagnostic value of these parameters in distinguishing subclinical keratoconus (SKC) from normal eyes.MethodsThis prospective study was conducted on 73 SKC and 69 normal eyes. Subclinical keratoconus eyes were defined as corneas with no clinical evidence of keratoconus and suspicious topographic and tomographic features. Following a complete ophthalmic examination, topographic and tomographic corneal assessment via Pentacam HR, and corneal biomechanical evaluation utilizing Corvis ST were done.ResultsSubclinical keratoconus eyes presented significantly higher Deformation Amplitude (DA) ratio, Tomographic Biomechanical Index (TBI), and Corvis Biomechanical Index (CBI) rates than the control group. Conversely, Ambrósio Relational Thickness to the Horizontal profile (ARTh), and Stiffness Parameter at the first Applanation (SPA1) showed significantly lower rates in SKC eyes. In diagnosing SKC from normal eyes, TBI (AUC: 0.858, Cut-off value: > 0.33, Youden index: 0.55), ARTh (AUC: 0.813, Cut-off value: ≤ 488.1, Youden index: 0.58), and CBI (AUC: 0.804, Cut-off value: > 0.47, Youden index: 0.49) appeared as good indicators.ConclusionsTBI, CBI, and ARTh parameters could be valuable in distinguishing SKC eyes from normal ones.

Comparing corneal biomechanic changes among solo cataract surgery, microhook ab interno trabeculotomy and iStent implantation

Minimally invasive glaucoma surgery has expanded the surgical treatment options in glaucoma, particularly when combined with cataract surgery. It is clinically relevant to understand the associated postoperative changes in biomechanical properties because they are influential on the measurement of intraocular pressure (IOP) and play an important role in the pathogenesis of open-angle glaucoma (OAG). This retrospective case–control study included OAG patients who underwent cataract surgery combined with microhook ab interno trabeculotomy (µLOT group: 53 eyes of 36 patients) or iStent implantation (iStent group: 59 eyes of 37 patients) and 62 eyes of 42 solo cataract patients without glaucoma as a control group. Changes in ten biomechanical parameters measured with the Ocular Response Analyzer and Corneal Visualization Scheimpflug Technology (Corvis ST) at 3 and 6 months postoperatively relative to baseline were compared among the 3 groups. In all the groups, IOP significantly decreased postoperatively. In the µLOT and control groups, significant changes in Corvis ST-related parameters, including stiffness parameter A1 and stress‒strain index, indicated that the cornea became softer postoperatively. In contrast, these parameters were unchanged in the iStent group. Apart from IOP reduction, the results show variations in corneal biomechanical changes from minimally invasive glaucoma surgery combined with cataract surgery.

Changes in Stress-Strain Index in School-Aged Children: A 3-Year Longitudinal Study.

To determine three-year change of the corneal biomechanical parameter stress-strain index (SSI) in schoolchildren aged 7- 9 years and their correlation with refractive error and axial length (AL). This is a prospective cohort study. Data of the AL, refractive error, and corneal biomechanical parameter SSI were collected at baseline and a 3-year follow-up for 217 schoolchildren. SSI, AL, and refractive error were measured via corneal visualization Scheimpflug technology (Corvis ST), IOLMaster biometry, and cycloplegic refraction. Three years of changes in SSI and its association with refractive error and AL were analyzed. Participants were divided into persistent nonmyopia (PNM), newly developed myopia (NDM), and persistent myopia (PM). The three-year difference in SSI among the three groups was analyzed. After three years of follow-up, the corneal biomechanical parameter SSI decreased in all participants (P < 0.01). There was a negative correlation between the change in SSI and the change in AL (r = -0.205, P=0.002) and a positive correlation between the change in refractive error (r = 0.183, P=0.007). After three years of follow-up, there was a decrease in the SSI for the NDM, PM, and PNM participants, with a median change of -0.05 for PNM and -0.13 and -0.09 for the NDM and PM, respectively. There was a significant decrease in corneal biomechanical properties for NDM patients compared with PNM patients (P < 0.01). In 7- to 9-year-old schoolchildren, SSI decreased after three years of the longitudinal study, and the change in SSI was correlated with the change in AL and refractive error. There was a rapid decrease in corneal biomechanical properties among newly developed myopic patients.

Comparison of Corvis ST Parameters between Primary Open-Angle Glaucoma and Primary Angle-Closure Glaucoma.

We compared corneal visualization Scheimpflug technology (CST) parameters between eyes with primary open-angle glaucoma (POAG) and primary angle-closure glaucoma (PACG). A retrospective analysis was performed on data from 89 eyes with POAG and 83 eyes with PACG that had CST examinations. CST parameters were compared between eyes with POAG and those with PACG using a linear mixed model (LMM). No differences were observed in age, central corneal thickness, intraocular pressure, or use of antiglaucoma eye drops between the two groups. Patients with PACG had a significantly shorter axial length (AL), a higher proportion of females, CST parameters, longer applanation 2 (A2) time, deeper A2 deformation amplitude, shorter peak distance, longer whole eye movement, and longer whole eye movement time than patients with POAG. The highest concavity (HC) length and PD showed a significant positive correlation with AL. However, A1 length, A1 deformation amplitude, A2 time, A2 velocity, A2 length, A2 deformation amplitude, HC time, whole eye movement, and whole eye movement time were negatively correlated with AL. The biomechanical properties of the cornea differed between POAG and PACG. In some parts, AL differences between the POAG and PACG groups might contribute to the variation in CST parameters.

The role of corneal biomechanics in visual field progression of primary open-angle glaucoma with ocular normotension or hypertension: a prospective longitude study.

Introduction: To analyze effects of dynamic corneal response parameters (DCRs) on visual field (VF) progression in normal-tension glaucoma (NTG) and hypertension glaucoma (HTG). Methods: This was a prospective cohort study. This study included 57 subjects with NTG and 54 with HTG, followed up for 4years. The subjects were divided into progressive and nonprogressive groups according to VF progression. DCRs were evaluated by corneal visualization Scheimpflug technology. General linear models (GLMs) were used to compare DCRs between two groups, adjusting for age, axial length (AL), mean deviation (MD), etc. VF progression risk factors were evaluated by logistic regression and receiver operating characteristic (ROC) curves. Results: For NTG, first applanation deflection area (A1Area) was increased in progressive group and constituted an independent risk factor for VF progression. ROC curve of A1Area combined with other relevant factors (age, AL, MD, etc.) for NTG progression had an area under curve (AUC) of 0.813, similar to the ROC curve with A1area alone (AUC = 0.751, p = 0.232). ROC curve with MD had an AUC of 0.638, lower than A1Area-combined ROC curve (p = 0.036). There was no significant difference in DCRs between the two groups in HTG. Conclusion: Corneas in progressive NTG group were more deformable than nonprogressive group. A1Area may be an independent risk factor for NTG progression. It suggested that the eyes with more deformable corneas may also be less tolerant to pressure and accelerate VF progression. VF progression in HTG group was not related to DCRs. Its specific mechanism needs further studies.

Change in the corneal material mechanical property for small incision lenticule extraction surgery.

Purpose: To assess the distribution characteristics and related factors of stress-strain index (SSI) values and discuss changes in biomechanical parameters, including SSI, after small incision lenticule extraction (SMILE) surgery. Methods: This study included 253 patients who underwent SMILE (253 eyes). SSI and other biomechanical parameters were measured using corneal visualization Scheimpflug technology before and 3months after surgery. The data collected included SSI, central corneal thickness (CCT), and eight other dynamic corneal response parameters. The Kolmogorov-Smirnov test, Pearson and partial correlation analyses, and paired-sample t-tests were used for statistical analyses. Results: Both pre-op SSI and ΔSSI follow a normal distribution, while post-op SSI does not follow a normal distribution. The decline in SSI after SMILE surgery was not statistically significant, and the data dispersion of SSI after SMILE surgery was close to that before surgery (p > 0.05). No statistical correlation was noted between SSI values and age and pre-op CCT (all p > 0.05). However, both pre- and post-op SSI values decreased with increasing degree of myopia (all p < 0.05), and weakly correlated with preoperative intraocular pressure and biomechanically corrected intraocular pressure (all p < 0.05). Other biomechanical parameters changed significantly after surgery (all p < 0.001). After SMILE, the magnitude of the deformation at the highest concave, deformation ratio, and integral radius increased significantly (all p < 0.001), while the Ambrosio relational thickness horizontal, stiffness parameter A1, and Corvis biomechanical index decreased significantly (p < 0.001). Conclusion: SSI, which reflects essential corneal material attributes, differs from other corneal biomechanical parameters and remains stable before and after SMILE surgery, and can be used as an indicator to evaluate changes in corneal material properties after SMILE surgery.

Safety of transglutaminase-induced corneal collagen cross-linking on the central cornea thickness and intraocular pressure in vivo

Background: Corneal collagen cross-linking (CXL) is a procedure for making bonds that connect polymer chains to one another. Corneal CXL aims to slow or stop the progression of keratoconus by using photooxidative therapy so as to increase stromal rigidity. Transglutaminase enzymes are currently widely used in the food industry. Recent studies have shown that mRNA, fibronectin, and transglutaminase were found to be more abundant in human corneal keratocytes treated with UVA and riboflavin. Transglutaminase is considered to reduce discomfort caused by UVA irradiation. Methods: A total of 21 white New Zealand rabbits were divided into three groups, namely, transglutaminase-induced CXL group, epithelial-off CXL group, and transepithelial CXL group. The ocular surface was treated with a 1 U/mL microbial transglutaminase solution, and both the epithelial-off and transepithelial groups were exposed to clinical ultraviolet A-riboflavin (UVA/RF). The efficacy of each group was evaluated on the 14th day after the procedures. The central corneal thickness (CCT) and intraocular pressure (IOP) were evaluated using Corneal Visualization Scheimpflug Technology (Corvis ST). Results: The transglutaminase-induced CXL group exhibited the highest mean CCT (370.14 ± 38.85) in comparison with the UVA/RF epithelial-off group (368.00 ± 25.48) and the UVA/RF transepithelial group (369.86 ± 23.43). The transglutaminase-induced CXL group had the highest IOP mean (8.50 ± 3.02) compared with the UVA/RF epithelial-off (6.50 ± 3.07) and UVA/RF transepithelial groups (7.00 ± 1.90). There were no significant differences in CCT (p = 0.990) or IOP (p = 0.563) between the groups. Conclusions: The findings of this study suggest that there are no significant differences between the transglutaminase-induced CXL group and the UVA/RF CXL group. The safety of transglutaminase-induced CXL could be comparable to that of UVA/RF CXL in terms of altering CCT and IOP, which are two factors contributing to corneal rigidity.

Corneal Biomechanical Characteristics in Osteogenesis Imperfecta With Collagen Defect.

To identify the characteristic corneal biomechanical properties of osteogenesis imperfecta (OI), and to compare the corneal biomechanical properties between OI and keratoconus. We included 46 eyes of 23 patients with OI, 188 eyes of 99 keratoconus patients, and 174 eyes of 92 normal controls to compare corneal biomechanical parameters between OI corneas, keratoconus, and normal controls by using Corneal Visualization Scheimpflug Technology (Corvis ST). Patients with OI had significantly higher Corvis biomechanical index (CBI) (P < 0.001), higher tomographic and biomechanical index (TBI) (P = 0.040), lower Corvis Biomechanical Factor (CBiF) (P = 0.034), and lower stiffness parameter at first applanation (SP-A1) (P < 0.001) compared with normal controls. In contrast, OI group showed lower CBI (P < 0.001), lower TBI (P < 0.001), higher CBiF (P < 0.001), and higher SP-A1 (P = 0.020) than keratoconus group. Notably, the stress-strain index (SSI) was not significantly different between the OI and normal controls (P = 1.000), whereas keratoconus showed the lowest SSI compared with OI group (P = 0.025) and normal controls (P < 0.001). Although the corneal structures of OI patients are less stable and easier to deform as compared to those of the control group, there is no significant difference in material stiffness observed between the OI and normal controls. In contrast, the corneas of keratoconus showed not only lower structural stability and higher deformability but also lower material stiffness compared with those of OI cornea and normal controls. The biomechanical alterations are different between OI corneas and keratoconus.

Comparison of two analyzer measurements focusing on material stiffness among normal, treatment-naïve, and treated glaucoma eyes

To investigate differences in biomechanical properties focusing on stiffness parameters between normal, treatment-naïve primary open-angle glaucoma (POAG), and treated POAG eyes. Retrospective case–control study, This study included 46 treatment-naïve POAG eyes, 46 POAG eyes treated with prostaglandin analogues, and 49 normal eyes used as controls; matched in terms of age and axial length. Corneal hysteresis (CH) and corneal resistance factor (CRF) were measured using an ocular response analyzer (ORA). Fifteen biomechanical parameters were measured with the Corneal Visualization Scheimpflug Technology (Corvis ST), including biomechanical glaucoma factor (BGF) and two stiffness parameters of ‘SP A1’ and ‘stress–strain index (SSI)’, which were compared among the three groups. Additionally, the area under the curve (AUC) values of the receiver-operating curve to discriminate control and treatment-naïve POAG eyes were calculated for BGF and CH. Treatment-naïve POAG eyes had higher ‘SSI’ than normal eyes even after controlling for IOP (p < 0.05, Tukey-Cramer test). Treated POAG eyes had significantly lower CRF, and higher BGF than treatment-naïve POAG eyes. There were also significant differences in CH or SP A1 among the three groups. BGF and CH had similar AUC values (0.61 and 0.59). Treatment-naïve POAG eyes had stiffer corneas compared to normal eyes, which seemed to result from the material/structure of the cornea rather than higher intraocular pressure. Antiglaucoma topical medication alters biomechanical properties measured with Corvis ST. These results are important for understanding the pathogenesis and improving the management of POAG.

Corneal biomechanical properties in vernal keratoconjunctivitis and its subtypes: a preliminary study.

To explore the corneal biomechanical properties (CBPs) of patients with vernal keratoconjunctivitis (VKC) and the discrepancies among three subtypes of VKC including palpebral, limbal, and mixed forms. Forty eyes of 20 VKC patients and twenty eyes of ten non-VKC patients were included in this case-control study. Patients with VKC were further divided into three subtypes (six patients in Palpebral form, five patients in limbal form, and nine patients in mixed form). The CBPs of all patients were obtained from the Corneal Visualization Scheimpflug Technology (Corvis ST). First applanation (A1) length, Ambrosio relational thickness in horizontal (ARTh), and stiffness parameter at first applanation (SP-A1) were significantly lower in the VKC group while A1 velocity was significantly higher in the VKC group (p < 0.05), compared to the non-VKC group. Furthermore, A1 velocity presented a positive correlation with disease course (p < 0.05). In addition, VKC patients of limbal form had lower central corneal thickness (CCT), SP-A1, and higher deformation amplitude ratio (DA ratio), compared to the other two subtypes (p < 0.05). Besides, patients in limbal form had higher A1 velocity, integrated radius, and corneal biomechanical index (CBI) compared with mixed form, and lower A1 length than palpebral form (p < 0.05). The corneas of VKC patients were softer and more protruded compared with the control group, and the property of steepness was closely related to disease course. VKC patients in limbal form were more inclined to be keratoconus than the other two subtypes due to their CBPs` discrepancies.

Effect of myopia and astigmatism deepening on the corneal biomechanical parameter stress-strain index in individuals of Chinese ethnicity.

Purpose: To investigate the differences in corneal biomechanical parameter stress-strain index (SSI) among different degrees of myopic eyes in Chinese individuals and to analyze the relevant factors of the SSI. Methods: This study analyzed the right eyes of 240 participants (240 eyes) aged 18-34years. The participants were divided into low-, moderate-, high-, and ultra-high myopia groups according to their spherical equivalent (SE), with 60 eyes included in each group. Spherical, cylinder, and SE were measured via automatically integrated optometry. Intraocular pressure (IOP) was measured using a non-contact tonometer. AL was measured using an IOLMaster device. Corneal curvature and central corneal thickness (CCT) were measured using a Pentacam. SSI and biomechanical corrected IOP (bIOP) were measured via corneal visualization Scheimpflug technology (Corvis ST). The statistical analyses included one-sample Kolmogorov-Smirnov tests and normal distribution histogram methods, Levene variance homogeneity tests, Pearson's correlation analyses, multiple linear stepwise regression analyses, one-way ANOVA, and LSD t-tests. Results: The mean (±SD) age of the 240 participants was (24.97 ± 4.16) years. The SSI was positively correlated with spherical, cylinder, SE, CCT, IOP, and bIOP and negatively correlated with K1 and AL (r = 0.475, 0.371, 0.497, 0.169, 0.291, 0.144, -0.154, and -0.464, respectively; all p < 0.05), but were not correlated with age, K2, or Km (all p > 0.05). Multiple linear regression analysis performed with SSI as the dependent variable, and spherical, cylinder, K1, CCT, and IOP as independent variables produced the following regression equation: SSI = 0.989 + 0.017 spherical + 0.042 cylinder +0.018 IOP (R 2 = 0.402, F = 31.518, p < 0.001). The SSI values in the low-, moderate-, high-, and ultra-high myopia groups were 0.945 ± 0.135, 0.940 ± 0.128, 0.874 ± 0.110, and 0.771 ± 0.104, respectively. The values decreased sequentially, and the differences between pairs were statistically significant (all p < 0.05), except for that between the low- and moderate-myopia groups (p > 0.05). Conclusion: SSI decreased with increasing myopia and astigmatism in the Chinese participants. The SSI was significantly lower in high and ultra-high myopia, especially ultra-high myopia. These findings indicate that increased corneal elasticity may be related to the pathogenesis of high and ultra-high myopia.

Analysis of the diagnostic accuracy of Belin/Ambrósio Enhanced Ectasia and Corvis ST parameters for subclinical keratoconus.

To investigate the diagnostic accuracy of the parameters in the Belin/Ambrósio Enhanced Ectasia Display built in Pentacam, which is designed for the screening of subclinical keratoconus (SKC) built in Pentacam, and the parameters in Corneal visualization Scheimpflug technology (Corvis ST). A retrospective study: The fellow eyes of unilateral keratoconus cases were diagnosed with SKC. Patients presented to Shanxi Eye Hospital with SKC from October 2020 to November 2021 were included as the SKC group, and myopic patients undergoing corneal refractive surgery at the Refractive Surgery Department in our hospital within the same period were included as the control group. The Belin/Ambrósio and Corvis ST parameters were extracted from the system and analyzed using independent samples t test. Receiver operating curves (ROCs) were also created to test the diagnostic accuracy of each parameter. There were 70 patients (70 eyes) in the SKC group and 137 patients (137 eyes) in the control group. For Corvis ST parameters, Radius (P = 0.021), PachySlope (P = 0.040), SP-A1 (P = 0.002), A2 Deformation Amp. (P = 0.028), A2 Deflection Length (P < 0.001), Max ICR (P = 0.005), DA Ratio Max (1mm) (P = 0.023), IR (P = 0.016), CBI (P = 0.003) and TBI (P < 0.001) were statistically different between the two groups. For Belin/Ambrósio parameters, PPI min. Axis, ART min, ART max, ART avg, Pachy min, Front K2, Astig, BAD-Df, BAD-Db, BAD-Dp, BAD-Dt, BAD-Da, BAD-D, PPI min, PPI max, PPI max. Axis, PPI avg and Dist.Apex-Thin.Loc. were significantly different between the two groups (all p < 0.001). TBI and BAD-D showed the best diagnostic accuracy, with AUCs of 0.944 and 0.965, respectively. Some Belin/Ambrósio and Corvis ST parameters differed between SKC eyes and eyes with normal cornea. TBI and BAD-D showed the ideal diagnostic performance for SKC. In clinical practice, conventional corneal topography could not be replaced by Corvis ST.

Comparison of different correction formulas and measurement methods for the accurate determination of intraocular pressure after SMILE and FS-LASIK surgery

BackgroundTo compare the accuracy of non-contact tonometry, Pentacam, and corneal visualization Scheimpflug technology (Corvis ST) for the measurement of intraocular pressure (IOP) after small incision lenticule extraction (SMILE) or femtosecond laser-assisted in situ keratomileusis (FS-LASIK) surgery.MethodsA total of 49 patients (98 eyes) undergoing FS-LASIK or SMILE surgery at West China Hospital, Sichuan University from January to March 2021 were enrolled in this prospective, comparative, self-controlled study. IOP values were measured with non-contact tonometer, Pentacam, and Corvis ST before surgery and 1 month after surgery. Pentacam-derived postoperative IOP values were corrected using five correction formulas (Ehlers, Shah, Dresden, Orssengo-Pye, and Kohlhaas), while Corvis ST-derived values were corrected using a single formula.ResultsIn the SMILE group, no significant differences were observed between the IOP values corrected with the Shah formula before and after surgery (t = 0.17, P = 0.869), whereas postoperative IOP values corrected with the other formulas were significantly different from the corresponding preoperative measurements (P < 0.05). In the FS-LASIK group, postoperative IOP values corrected with the Ehlers, Shah, or Corvis ST formulas were significantly different from the corresponding preoperative IOP measurements (P < 0.05), but no significant differences were observed between pre- and postoperative IOP values corrected with the Dresden (t = − 0.08, P = 0.941), Orssengo-Pye (t = − 0.52, P = 0.604), or Kohlhaas (t = 1.22, P = 0.231) formulas.ConclusionsPentacam’s Shah correction formula seemed to be the most appropriate method for accurately measuring postoperative IOP in patients undergoing SMILE surgery, while the Dresden, Orssengo-Pye, and Kohlhaas correction formulas of Pentacam were identified as the most reliable methods for estimating IOP in patients after FS-LASIK surgery.

Assessment of corneal biomechanics in anisometropia using Scheimpflug technology.

Purpose: To investigate the relationship between corneal biomechanical and ocular biometric parameters, and to explore biomechanical asymmetry between anisometropic eyes using the corneal visualization Scheimpflug technology device (Corvis ST). Methods: 180 anisometropic participants were included. Participants were divided into low (1.00≤△Spherical equivalent (SE) < 2.00D), moderate (2.00D≤△SE < 3.00D) and high (△SE ≥ 3.00D) anisometropic groups. Axial length (AL), keratometry, anterior chamber depth (ACD) and corneal biomechanical parameters were assessed using the OA-2000 biometer, Pentacam HR and Corvis ST, respectively. Results: The mean age of participants was 16.09 ± 5.64 years. Stress-Strain Index (SSI) was positively correlated with SE (r = 0.501, p < 0.001) and negatively correlated with AL (r = -0.436, p < 0.001). Some other Corvis ST parameters had weak correlation with SE or AL. Corneal biomechanical parameters except for time of first applanation (A1T), length of second applanation (A2L), deformation amplitude (DA), first applanation stiffness parameter (SPA1) and ambrosia relational thickness-horizontal (ARTh) were correlated with ametropic parameters (SE or AL) in multiple regression analyses. A1T, velocity of first applanation (A1V), time of second applanation (A2T), A2L, velocity of second applanation (A2V), corneal curvature radius at highest concavity (HCR), peak distance (PD), DA, deformation amplitude ratio max (2mm) (DAR), SPA1, integrated radius (IR), and SSI showed significant differences between fellow eyes (p < 0.05). There was no significant difference in asymmetry of corneal biomechanics among the three groups (p > 0.05). Asymmetry of some biomechanical parameters had weak correlation with asymmetry of mean corneal curvatures and ACD. However, asymmetry of corneal biomechanical parameters was not correlated with asymmetry of SE or AL (p > 0.05). Conclusion: More myopic eyes had weaker biomechanical properties than the contralateral eye in anisometropia. However, a certain linear relationship between anisometropia and biomechanical asymmetry was not found.

Effects of SMILE with different residual stromal thicknesses on corneal biomechanical properties of rabbits in vivo

Femtosecond laser small incision lenticule extraction (SMILE) with different residual stromal thicknesses (RST) is set to investigate its effect on corneal biomechanical properties of rabbits in vivo. In this study, 24 healthy adult Japanese rabbits were randomly divided into group A and B. The RST of group A was set 30% of the corneal central thickness (CCT), and the RST of group B was 50% of the CCT. The thickness of the corneal cap in both groups was set one third of CCT. Corneal visualization Scheimpflug technology (Corvis ST) and Pentacam three-dimensional anterior segment analyzer were used to determine corneal biomechanical and morphological parameters before surgery, and 1 week, 1 month and 3 months after surgery. Pearson correlation analysis was used to analyze factors affecting corneal biomechanical parameters after SMILE. The results showed that the corneal stiffness of group A was significantly higher than that of group B at 1 week and 1 month after surgery, and most biomechanical parameters returned to preoperative levels at 3 months postoperatively. The results of correlation analysis showed that postoperative CCT and RST were the main factors affecting corneal biomechanical parameters after SMILE. There was no significant difference in corneal posterior surface height (PE) between 3 months after surgery and before surgery in both two groups. It indicates that although the ability to resist deformation of cornea decreases in SMILE with thicker corneal cap and less RST, there is no tendency to keratoconus, which may be related to the preservation of more anterior stromal layer.

Age-related changes in dynamic corneal backscatter observed in Scheimpflug imaging.

Corneal visualization Scheimpflug technology (Corvis ST) has the potential to indirectly provide information on corneal structure via the analysis of statistical properties of the backscatter. The aim of this work was to ascertain whether there are age-related changes in the dynamics of corneal backscatter during an air-puffed induced corneal deformation. Retrospective data from Corvis ST measurements of 151 young subjects (19-30 years) and 82 older subjects (50-87 years) were considered. Each measurement consisted of 140 frames (sampling frequency: 4330 fps). For every frame the cornea was first segmented, then regions of interest, encompassing temporal, central and nasal parts of cornea were selected, to which the parameters of Weibull distribution (scale and shape) were fitted, leading to time series of the estimated parameters. Apparent differences were found between the parameters of Weibull distribution between the two considered groups that manifest themselves mostly in the nasal region of the cornea. However, those differences cannot be attributed to the age alone. For this, a normalization method is proposed that leads to a much better separation between the groups in all considered regions. Clinical Relevance- The parameters of the corneal backscat-ter are widely used to asses corneal clarity (so-called corneal densitometry). Recently the parameters of Weibull distribution fitted to the corneal backscatter data have been used to support diagnosis of keratoconus. This work contributes to the assessment of corneal clarity by identifying the apparent age-related differences in those parameters when dynamic raw data is considered highlighting the need for such parameters to be appropriately normalized. Further it shown that the shape parameter of Weibull distribution unlike the scale parameter carries that information already for the raw non-normalized data.

Corneal Stiffness and Modulus of Normal-Tension Glaucoma in Chinese

To assess and compare the corneal biomechanics of normal-tension glaucoma (NTG), high-tension glaucoma (HTG), and normal controls based on stiffness and modulus. The correlations among central corneal thickness (CCT), visual field, retinal nerve fiber layer (RNFL) thickness, and corneal biomechanics in glaucoma eyes were also evaluated. A prospective, cross-sectional, comparative study. This study included 334 eyes of 108 NTG patients, 113 HTG patients, and 113 control subjects at Zhongshan Ophthalmic Center, Sun Yat-Sen University. Corneal biomechanics were evaluated using a corneal indentation device (CID) and corneal visualization Scheimpflug technology (Corvis ST). Visual field and RNFL thickness were obtained using standard automated perimetry and spectral-domain optical coherence tomography. One-way analyses of variance with Bonferroni post hoc tests and a multivariable linear regression analysis with adjustment were conducted. Correlations among corneal biomechanical parameters, CCT, visual field, and RNFL thickness were analyzed. The corneal stiffness of the NTG patients (71.0 ± 10.9 N/m) was significantly lower than that of the HTG patients (77.3 ± 15.6 N/m; P=.001) and the CCT- and IOP-matched normal controls (75.6 ± 11.0 N/m; P=.023). The patients in the NTG group had lower corneal stiffness than those in the control group (β=-4.88, 95% CI -9.002, -0.758; P=.020) after adjusting for confounders. Stiffness was positively correlated with CCT in the NTG group (P=.028) but not in the HTG group (P= .509). There was no significant correlation (P > .05) between corneal biomechanics, visual field, or RNFL thickness. The corneas of NTG patients were softer than those of HTG patients and controls, as assessed by CID, which were associated with thinner CCT. These might suggest different ocular biomechanical properties in NTG and HTG.

Association Between Ocular Biomechanics Measured With Corvis ST and Glaucoma Severity in Patients With Untreated Primary Open Angle Glaucoma.

PurposeTo compare the ocular biomechanical differences between normal controls and patients with untreated primary open angle glaucoma, including normal-tension glaucoma (NTG) and high-tension glaucoma (HTG), and to investigate the association between ocular biomechanics and glaucoma severity in each group.MethodsOne hundred fifty-three eyes of 153 subjects, including 51 controls, 47 NTG, and 55 HTG cases, were enrolled in this cross-sectional study. Each participant underwent biomechanical measurements by using the Corneal Visualization Scheimpflug Technology. Glaucoma severity was evaluated by mean deviation (MD), pattern standard deviation (PSD), ganglion cell complex (GCC), and retinal nerve fiber layer (RNFL) thickness.ResultsDeformation amplitude (P = 0.001) significantly increased, whereas first applanation time (P < 0.0001), highest concavity time (P = 0.001), stiffness parameter at first applanation (P = 0.009), and time of whole eye movement (WEM, P = 0.008) decreased significantly in NTG eyes compared with controls. Besides, NTG had the highest first applanation velocity than controls (P < 0.0001) and HTG (P = 0.044). Shorter time of WEM was independently correlated with worse MD (P = 0.02) and higher values of PSD (P = 0.03) in NTG. Axial length was positively related to PSD (P = 0.02) and negatively related to GCC (P < 0.0001) and RNFL (P < 0.0001) thickness in HTG.ConclusionsNTG corneas are more deformable than healthy ones and HTG. Time of WEM, which relates to orbital compliance, is significantly associated with glaucomatous visual field defect in NTG, whereas axial length is correlated with glaucoma severity in HTG.Translational RelevanceOcular biomechanics may partly account for the differences of pathogenic mechanisms between NTG and HTG.

Measuring Human Corneal Stromal Biomechanical Properties Using Tensile Testing Combined With Optical Coherence Tomography.

Purpose: To investigate the ex vivo elastic modulus of human corneal stroma using tensile testing with optical coherence tomography (OCT) imaging and its correlation with in vivo measurements using corneal visualization Scheimpflug technology. Methods: Twenty-four corneal specimens extracted from stromal lenticules through small incision lenticule extraction were cut into strips for uniaxial tensile tests. In vivo corneal biomechanical responses were evaluated preoperatively using the corneal visualization Scheimpflug technology (CorVis ST). The correlation of the elastic modulus with clinical characteristics and dynamic corneal response parameters were analyzed using Spearman’s correlation analysis. Results: The mean low strain tangent modulus (LSTM) of the human corneal stroma was 0.204 ± 0.189 (range 0.010–0.641) MPa, and high strain tangent modulus (HSTM) 5.114 ± 1.958 (range 2.755–9.976) MPa. Both LSTM (r = 0.447, p = 0.029) and HSTM (r = 0.557, p = 0.005) were positively correlated with the stress-strain index (SSI). LSTM was also positively correlated with the A1 deflection length (r = 0.427, p = 0.037) and A1 deflection area (r = 0.441, p = 0.031). HSTM was positively correlated with spherical equivalent (r = 0.425, p = 0.038). Conclusions: The correlation of corneal elastic modulus with A1 deflection parameters and SSI may indicate a relationship between these parameters and tissue elasticity. The HSTM decreased with the degree of myopia. Combining tensile test with OCT may be a promising approach to assess corneal biomechanical properties.