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Corneal visualization Scheimpflug technology(Corvis ST)is currently the most commonly used clinical device for assessing in vivo corneal biomechanics. The new parameter stress-strain index(SSI)has been a hot topic of clinical research in recent years, which not only directly reflect corneal biomaterial stiffness, but also closely correlates with the progression of certain diseases. SSI was generated based on the predictions of corneal behavior using finite element(FE)numerical modeling to simulate the effects of intraocular pressure and Corvis ST jets. The SSI algorithm does not change with central corneal thickness(CCT), intraocular pressure, or biomechanically corrected intraocular pressure(BIOP), but it is clearly associated with altered collagen fibres in the corneosclera. The principles of SSI, the relationship between age and SSI, the relationship between axial length and SSI, the relationship between myopia and SSI, and the application of SSI are summarized and concluded.
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Purpose: To report age?related variations in corneal stress–strain index (SSI) in healthy Indians. Methods: It was a retrospective study where healthy Indian individuals aged between 11 and 70 years who had undergone corneal biomechanics assessment using Corvis ST between January 2017 and December 2021 were enrolled. Composite corneal biomechanical parameters and corneal SSI were abstracted from Corvis ST and compared across different age groups using one?way analysis of variance (ANOVA). Also, Pearson’s correlation was used to evaluate the association between age and SSI. Results: Nine hundred and thirty?six eyes of 936 patients with ages between 11 and 77 years with mean ± SD intraocular pressure (IOP) and pachymetry of 16.52 ± 2.10 mmHg and 541.13 ± 26.39 ?s, respectively. Composite corneal biomechanical parameters such as deformation amplitude ratio max at 1 mm (P < 0.001) and 2 mm (P < 0.001), biomechanically corrected IOP (P = 0.004), stiffness parameter at A1 (P < 0.001, Corvis biomechanical index (P < 0.018), and SSI (P < 0.001) were found to be significantly different as a function of age group. We noted a statistically significant positive association of SSI with age (P < 0.001), spherical equivalent refractive error (P < 0.001), and IOP (P < 0.001) and a significant negative association with anterior corneal astigmatism (P < 0.001) and Anterior chamber depth (ACD) (P < 0.001). Also, SSI was positively associated with SPA1 and bIOP, whereas negatively associated with integrated radius, max inverse radius, and Max Deformation amplitude (DA) ratio at 1 mm and 2 mm. Conclusion: We noted a positive association of corneal SSI with age in normal healthy Indian eyes. This information could be helpful for future corneal biomechanical research.
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AIM: To assess the differences in corneal biomechanical parameters and their correlation with other ocular biometric parameters in myopic patients using a corneal visualisation scheimpflug technology(Corvis ST).METHODS: A total of 132 myopic patients who received treatment in the department of refractive surgery of Eyegood Ophthalmic Hospital from May to December 2021 were selected, and the data of right eye were taken. The subjects were classified into low and moderate myopia(-0.50 to -6.00D), high myopia(>-6.00D to <-8.00D), and super high myopia(≥-8.00D)according to the spherical equivalent(SE). The basic parameters of corneal biomechanical deformation measured with Corvis ST included the amount of corneal displacement at highest degree of concavity(DA), radius of curvature at highest concavity(HCR), and distance between the two peaks of the cornea at highest concavity(PD); stiffness parameters at applanation 1(SP-A1), integrated radius(IR), deformation amplitude ratio(DA ratio), Ambrosio relational thickness horizontal(ARTh), Corvis biomechanical index(CBI). Other ocular biometric parameters measured included SE, intraocular pressure(IOP), axial length(AL), central corneal thickness(CCT), white-to-white(WTW)and retinal nerve fiber layer(RNFL)thickness. The differences in corneal biomechanical parameters were compared among the three groups of myopic patients, and the correlation between corneal biomechanical parameters and other ocular biometric parameters and age was further analyzed.RESULTS: The SP-A1 in the low and moderate, high, and super high myopia groups were 106.8±16.2, 115.6±21.9, and 106.9±11.5, respectively. The SP-A1 in the high myopia group was higher than that in the low and moderate and severe myopia groups(all P<0.05). All corneal biomechanics were significantly correlated with CCT(P<0.01); all corneal biomechanics were correlated with IOP except ARTh(P<0.05); AL was significantly and positively correlated with PD(r=0.270, P=0.002), weakly and positively correlated with HCR(r=0.177, P=0.043), and weakly and negatively correlated with IR(r=-0.183, P=0.036); WTW was positively correlated with PD, DA and DA ratio(r=0.363, P<0.001; r=0.252, P=0.003; r=0.200, P=0.02); there was no correlation between corneal biomechanical parameters and age, SE, and RNFL.CONCLUSION: Corneal stiffness was higher in high myopia patients than in patients with low to moderate and super high myopia. IOP and CCT were both important factors affecting corneal biomechanics. Some of the biomechanical parameters also have certain correlation with AL and WTW. As the growth of axial length, the PD of corneal deformation and HCR were larger, and IR was smaller, but HCR and IR had little significance; the longer the WTW, the greater the corneal deformation amplitude. In clinical practice, attention should be paid to the corneal biomechanical parameters in patients with super high myopia.
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AIM:To evaluate the changes in corneal biomechanics of patients with moderate refractive error after receiving small-incision lenticule extraction(SMILE)and femtosecond laser-assisted laser in situ keratomileusis(FS-LASIK)using the corneal visualization Scheimpflug technology(Corvis ST).METHODS:Prospective cohort study. A total of 65 moderate myopia patients(65 eyes)who were scheduled to undergo refractive surgery at the Ophthalmic Refractive Surgery Center of the Ningxia Eye Hospital from November 2020 to November 2021 were included in the study, and there were 30 eyes in the SMILE group and 35 eyes in the FS-LASIK group. The changes in corneal biomechanical parameters, including integrated radius(IR), inverse concave radius(ICR), deformation amplitude ratio 2mm(DAR2), stiffness parameter at first applanation(SP-A1), ambrosio relational thickness(ARTh)and the central curvature radius at highest concavity(HC-Radius)were observed by Corvis ST between both groups preoperatively and 1 and 3mo postoperatively.RESULTS: There were no statistical significance in biomechanical parameters between two groups of patients 1 and 3mo postoperatively(P&#x003E;0.05). IR, ICR and DAR2 of each groups of patients 1 and 3mo postoperatively were significantly increased than those preoperatively, and SP-A1, ARTh and HC-Radius were significantly decreased than those preoperatively(all P&#x003C;0.05). The biomechanical parameters at 1mo and 3mo postoperatively showed no statistical significance(P&#x003E;0.05). In addition, a positive correlation was found between central corneal thickness(CCT)and ARTh and SP-A1 of the two groups of patients at 3mo postoperatively(FS-LASIK group: r=0.727, 0.819, SMLIE group: r=0.683, 0.434, all P&#x003C;0.05), while a negative correlation was found between CCT and IR and ICR at 3mo postoperatively.(FS-LASIK group: r=-0.697, -0.622, SMLIE group: r=-0.447, -0.491, all P&#x003C;0.05).CONCLUSION:For patients with moderate myopia, both SMILE and FS-LASIK can reduce corneal biomechanical stability. Both surgeries showed no significant differences in the effect on biomechanical, and the biomechanical has been stabilized at 1mo postoperatively. A correlation was found between postoperative CCT and ARTh, SP-A1, IR and ICR.
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AIM:To investigate the correlation between corneal biomechanical parameters measured by the corneal visualization Scheimpflug Technology(Corvis ST)and corneal high-order aberrations(HOAs)in children with mild to moderate myopia.METHODS:A cross-sectional study. A total of 255 pediatric patients with myopia enrolled from April to July 2021 in Tianjin Medical University Eye Hospital were continuously collected, and all the right eyes were taken for analysis. Corneal biomechanical parameters were obtained from Corvis ST. Pentacam three-dimensional anterior segment analyzer was used to measure total corneal higher-order aberrations(RMSh), third order aberrations(RMS3)and fourth order aberrations(RMS4).RESULTS:RMS3 was positively correlated with the second applanation time(A2T)(r=0.175, P=0.009)and negatively correlated with the axis length(AL)(r=-0.155, P=0.014). RMS4 was negatively correlated with the highest concavity radius(HCR)(r=-0.165, P=0.009). RMSh was negatively correlated with HCR and AL(r=-0.152, P=0.037; r=-0.175, P=0.005).CONCLUSION:There is a correlation between corneal biomechanical parameters and HOAs in children with myopia. Cornea with higher stiffness and stronger deformation resistance has smaller RMS3, RMS4 and RMSh.
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@#AIM:To evaluate the repeatability and comparison of corneal visualization scheimpflug technology(Corvis ST)parameters in keratoconus eyes before and after accelerated transepithelial corneal cross-linking(ATE-CXL).<p>METHODS: Thirty eyes of 30 progressive keratoconus patients were included in the prospective study. Three repeated corneal biomechanical measurements were performed preoperatively and one month postoperatively by Corvis ST. The interclass correlation coefficient(ICC)and 95% confidence interval(<i>CI</i>), Cronbach'α, repeatability coefficient(RC), and coefficient of variation(CV)were used to evaluate the repeatability of Corvis ST parameters. Paired <i>t</i>-test or Wilcoxon rank test was used to evaluate the differences between preoperative and postoperative data.<p>RESULTS: At preoperative, 26 of 39(66.67%)parameters showed good to excellent repeatability, 6(15.38%)showed moderate, and 7(17.95%)showed poor repeatability. Similarly, 34(87.18%)parameters showed good to excellent repeatability, 3(7.69%)showed moderate, and 2(5.13%)showed poor repeatability after ATE-CXL. After ATE-CXL 1mo, the intraocular pressure(IOP), biomechanical corrected IOP(bIOP), first applanation time(A1T), Radius, deformation amplitude at the first applanation(A1DA), deflection length at the maximum deformation(HCDLL)and stiffness parameter at first applanation(SP A1)parameters increased, while the steep keratometry(Ks), flat keratometry(Kf), mean keratometry(Kmean), second applanation time(A2T), DA Ratio Max(2 mm)and integrated radius parameters decreased(all <i>P</i><0.05). <p>CONCLUSION:The repeatability of the Corvis ST parameters before and 1mo ATE-CXL follow up were both acceptable, and the corneal stiffness was improved after 1mo ATE-CXL.
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The decrease of corneal stiffness is the key factor leading to keratoconus, and the corneal collagen fiber stiffness and fiber dispersion are closely related to the corneal biomechanical properties. In this paper, a finite element model of human cornea based on corneal microstructure, namely collagen fiber, was established before and after laser assisted in situ keratomileusis (LASIK). By simulating the Corvis ST process and comparing with the actual clinical results, the hyperelastic constitutive parameters and corneal collagen fiber stiffness modulus of the corneal material were determined before and after refractive surgery. After LASIK, the corneal collagen fiber stiffness modulus increased significantly, and was highly correlated with central corneal thickness (CCT). The predictive relationship between the corneal collagen fiber stiffness modulus and the corresponding CCT before and after surgery was: = exp(9.14 - 0.009CCT ), = exp(8.82 - 0.008CCT ). According to the results of this study, the central corneal thickness of the patient can be used to estimate the preoperative and postoperative collagen fiber stiffness modulus, and then a personalized corneal model that is more consistent with the actual situation of the patient can be established, providing a theoretical reference for more accurately predicting the safe surgical cutting amount of the cornea.
Sujets)
Humains , Phénomènes biomécaniques , Cornée , Topographie cornéenne , Analyse des éléments finis , Kératomileusis in situ avec laser excimère , MyopieRésumé
Objective To study the changes in biomechanical properties of human cornea after laser in situ keratomileusis (LASIK) and predict corneal stiffness after the LASIK surgery. Methods According to the measurement results from corneal visualization scheimpflug technology (Corvis ST), the corneal tangent stiffness coefficient (STSC) and energy absorbed area (Aabsorbed) were calculated. The change patterns of corneal stiffness and viscosity after refractive surgery were analyzed. Results The difference of corneal STSC and Aabsorbed before and after LASIK had a statistical significance (P<0.05). The obtained formula for predicting corneal stiffness after refractive surgery was: Sbefore surgery =1.055bIOPbefore surgery + 0.015CCTbefore surgery，Safter surgery =0.937Sbefore surgery +0.019CCTafter surgery. Conclusions LASIK surgery not only changes corneal thickness, but also reduces corneal stiffness and viscosity. Prediction of corneal stiffness after surgery can provide guidance for the design of clinical surgery and improve the safety of surgery.
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AIM: To investigate and analyze the changes of corneal biomechanics of normal eyes,forme frusta keratoconus eyes,subclinical keratoconus eyes and clinical keratoconus eyes by Corneal visualization Scheimpflug technology (Corvis ST),and provide clinical basis for early diagnosis of keratoconus.METHODS: Case-control study.We randomly selected 40 normal eyes as normal group,15 forme frusta keratoconus eyes as forme frusta keratoconus group,23 subclinical keratoconus eyes as subclinical keratoconus group,and 40 clinical keratoconus eyes as keratoconus group.The biomechanical parameters of each group were measured by Corvis ST.The receiver operating characteristic(ROC) curves was plotted to distinguish keratoconus from the normal cornea.RESULTS: There was no significant difference in the parameters of biomechanics between normal group and forme frusta keratoconus group (P>0.05).Compared to normal group and subclinical keratoconus group,the parameters second applanation length(AL2),first velocity of applanation (AV1),central curvature radius at highest concavity (HC-radius),deformation amplitude (DA) were revealed statistically significant differences(P<0.05).The biomechanical parameters of the keratoconic group were significantly different from those of normal group except for the second velocity of applanation (AV2),time from the start until the highest concavity(HC-time),peak distance (PD).ROC curve showed that the DA(area under the curve:0.891±0.028) was the best predictive parameter to distinguish keratoconus from the normal eyes.CONCLUSION: The corneal biomechanical parameters of forme frusta keratoconus group are not changed compared with normal group.The changes between normal group and subclinical keratoconus group should combine with other technology to further improve subclinical keratoconic screening.Compared with normal corneas,keratoconus has a great change in biomechanics,which DA diagnosis of the highest efficiency.
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Objectives: The objective was to determine the repeatability of intraocular pressure (IOP) measurements made through a soft contact lens (CL) using the Scheimpflug noncontact tonometry in healthy subjects. Methods: This prospective, randomized, single‑center study included one eye of 88 subjects (40 male and 48 female). Only participants without glaucoma or any other ocular pathology were included in this study. Three consecutive IOP measurements by the Scheimpflug noncontact tonometry were performed with and without daily disposable hydrogel CLs (−0.50 DS) (Dailies‑nelfilcon A, 69% water, 8.7 mm base curve, 14 mm diameter, center thickness 0.10 mm) by a single operator. To avoid any bias arising from diurnal variation, all measurements were made at a similar time of day (11 am ± 1 h). The repeatability of IOP measurements using the Scheimpflug noncontact tonometry with and without CLs was evaluated using Pearson’s correlation analysis. Bland–Altman plotting was used to assess the limits of agreement between the measurements with and without CLs. Results: The mean (± standard deviation) IOPs with and without CL were 13.80 ± 2.70 and 13.79 ± 2.54 mm of Hg respectively. The mean difference was 0.01 ± 0.16 (95% confidence interval, +1.97 to − 2.00) mm Hg. Statistical analysis via paired t‑test showed no statistical difference between the two groups with (P = 0.15). A good correlation was found for IOP measurements with and without CL (r = 0.93, P < 0.001). Good test‑retest reliability was found when IOP was measured with and without CL. Conclusion: There was no significant difference between IOP measured with and without CLs by Scheimpflug noncontact tonometry.