The Effect of Axial Length Elongation on Corneal Biomechanical Property

Corneal Biomechanical Changes in Patients with Inherited Retinal Diseases

Purpose

Inherited retinal diseases (IRDs) are a group of degenerative disorders of the retina, that can be potentially associated with changes in the anterior segment, but their prevalence and impact are not known. Exploring these concomitant ophthalmologic changes with biomechanical assessment may help identify other non-retina causes of vision loss in these patients, such as corneal ectasia or susceptibility to glaucoma. This study aimed to measure and compare corneal biomechanics in patients with and without IRDs.

Methods

A total of 77 patients (154 eyes) with IRD were recruited as the study group. The control group consisted of 77 healthy adults (154 eyes) with matched age and sphere equivalents. All participants underwent a comprehensive assessment including corneal tomography (Pentacam®) and biomechanical assessment (Corvis ST®). A total of 4 second-generation biomechanical parameters and 3 indexes were collected: Ambrosio Relational Thickness (ARTh), Deflection Amplitude Ratio Max (DARM), Integrated Radius (IR) and Stiffness Parameter at Applanation (SP-A1), the final deviation value D of the Belin/Ambrosio Enhanced Ectasia Display (BAD-D), Corvis Biomechanical Index (CBI) and Tomographic Biomechanical Index (TBI).

Results

For IRD patients, there was a higher DARM (p < 0.001), lower ARTh (p < 0.001), higher CBI (p < 0.001), higher TBI (p<0.001), and higher BAD-D (p < 0.001) compared to the control group. Regarding discrimination of healthy subjects and IRD patients, ARTh was the most sensitive parameter.

Conclusion

The results showed that IRD patients tend to have softer corneal behaviour, compared to eyes without pathology, which may predispose patients to corneal ectasia or glaucoma development. ARTh could be used to screen IRD patients if a non-retina cause of vision loss is suspected.

Potential dynamic corneal response parameters for myopia: relationships between axial length with whole eye movement at the first and second applanations and the highest concavity

PURPOSE

To investigate the correlation between whole eye movement (WEM) parameters measured using Corvis ST and axial length (AL) to explore whether AL affects WEMs.

METHODS

This single-center, cross-sectional study included data from healthy subjects and patients preparing for refractive surgery at the Qingdao Eye Hospital of Shandong First Medical University. Data were collected from July 2021 to April 2022. We first determined the correlations of WEMs at the time of corneal first applanation (A1_WEM), highest concavity (HC_WEM), and second applanation (A2_WEM), as well as the maximum value of WEM (WEM_Max) with AL. Subsequently, we established a series of regression models to analyze the relationships between different WEM values and AL.

RESULTS

AL was negatively correlated with HC_WEM, A2_WEM, and WEM_Max (r = - 0.28, - 0.23, and - 0.22, respectively; P < 0.001). The correlation between AL and A1_WEM was not significant (P = 0.77). According to the adjusted regression models, AL was negatively associated with HC_WEM (Model 2: β = -7.39, P < 0.001) and WEM_Max (Model 4: β = -3.52, P = 0.02), while the associations of AL with A1_WEM (Model 1: P = 0.61) and A2_WEM (Model 3: P = 0.23) were not significant.

CONCLUSIONS

AL is an independent negative influencing factor for HC_WEM. WEM is a potentially useful parameter that reflects the biomechanical properties of the eye behind the cornea in myopia.

Corneal stress‒strain index in relation to retinal nerve fibre layer thickness among healthy young adults

BACKGROUND/OBJECTIVES

To determine the relationship between corneal stress-strain index (SSI) and retinal nerve fibre layer (RNFL) thickness.

SUBJECTS/METHODS

1645 healthy university students from a university-based study contributed to the analysis. The RNFL thickness was measured by high-definition optical coherence tomography (HD-OCT), axial length (AL) was measured by IOL Master, and corneal biomechanics including SSI, biomechanical corrected intraocular pressure (bIOP), and central corneal thickness (CCT) were measured by Corvis ST. Multivariate linear regression was performed to evaluate the relationship between the SSI and RNFL thickness after adjusting for potential covariates.

RESULTS

The mean age of the participants was 19.0 ± 0.9 years, and 1132 (68.8%) were women. Lower SSI was significantly associated with thinner RNFL thickness ( β =8.601, 95% confidence interval [CI] 2.999-14.203, P  = 0.003) after adjusting for age, CCT, bIOP, and AL. No significant association between SSI and RNFL was found in men, while the association was significant in women in the fully adjusted model. The association was significant in the nonhigh myopic group ( P for trend = 0.021) but not in the highly myopic group. Eyes with greater bIOP and lower SSI had significantly thinner RNFL thickness.

CONCLUSIONS

Eyes with lower SSI had thinner RNFL thickness after adjusting for potential covariates, especially those with higher bIOP. Our findings add novel evidence of the relationship between corneal biomechanics and retinal ganglion cell damage.

Comparative analysis of corneal parameters in simple myopic anisometropia using Scheimpflug technology

Purpose

This study aims to investigate the differences in binocular corneal parameters and their interrelation with binocular biometric parameters asymmetry in patients with simple myopic anisometropia, thereby elucidating the influence of myopia process on various corneal parameters.

Methods

In this cross-sectional study, 65 patients with anisometropia in monocular myopia were included. They were divided into low anisometropia group: 3.00D<Δ spherical equivalent (SE)≤-1.00D (Δ represents the difference between the two eyes, i.e., myopic data minus emmetropic data) and high anisometropia group: ΔSE ≤ -3.00D. Corneal and ocular biometric parameters were measured using Pentacam, Corvis ST, and IOL Master 700. Statistical analyses focused on the binocular corneal parameters asymmetry, using the contralateral emmetropia as a control.

Results

The mean age of participants was 18.5 ± 1.3 years, with the average SE for myopia and emmetropia being -2.93 ± 1.09D and -0.16 ± 0.41D, respectively. The central corneal thickness (CCT), flat keratometry (Kf), keratometry astigmatism (Ka), total corneal aberration (6 mm) (TOA), surface variance index (ISV), vertical asymmetry index (IVA), stress-strain index (SSI), and first applanation stiffness parameter (SPA1) and ambrosia relational thickness-horizontal (ARTh) showed significant differences between anisometropic fellow eyes (p < 0.05). There were significant differences in ΔIVA, Δ the difference between the mean refractive power of the inferior and superior corneas (I-S), Δ deviation value of Belin/Ambrósio enhanced ectasia display (BAD-D), Δ deformation amplitude ratio max (2 mm) (DAR)and Δ tomographic biomechanical index (TBI) (p < 0.05) in two groups. Asymmetry of corneal parameters was correlated with asymmetry of ocular biometric parameters. Anisometropia (ΔSE) was positively correlated with ΔIVA (r = 0.255, p = 0.040), ΔBAD-D (r = 0.360, p = 0.006), and ΔSSI (r = 0.276, p = 0.039) and negatively correlated with ΔDAR (r = -0.329, p = 0.013) in multiple regression analysis. Δ mean keratometry (Km), Δ anterior chamber depth (ACD), and Δ biomechanically corrected intraocular pressure (bIOP) were also associated with binocular corneal differences.

Conclusion

Compared to contralateral emmetropia, myopic eyes have thinner corneas and smaller corneal astigmatism. Myopic corneas exhibit relatively more regular surface morphology but are more susceptible to deformation and possess marginally inferior biomechanical properties. In addition, there is a certain correlation between anisometropia and corneal parameter asymmetry, which would be instrumental in predicting the development of myopia.

Intraocular lens power calculation: angle κ and ocular biomechanics

PURPOSE

To study the effect of ocular biomechanics on the prediction error of intraocular lens (IOL) power calculation.

SETTING

Centro Hospitalar Universitário do Porto, Porto, Portugal.

DESIGN

Prospective longitudinal study.

METHODS

This study included 67 subjects. Before cataract surgery subjects underwent biometry with IOLMaster 700 and biomechanical analysis with Corvis Scheimpflug technology. The targeted spherical equivalent was calculated with SRK-T and Barrett Universal II. Associations between prediction error (PE), absolute prediction error (AE), and biometric and biomechanical parameters were performed with stepwise multivariate linear correlation analysis.

RESULTS

Using the SRKT formula, there was association between PE and Corvis Biomechanical Index (CBI, B = -0.531, P = .011) and between AE and the horizontal offset between the center of the pupil and the visual axis (angle κ, B = -0.274, P = .007). Considering the Barret Universal II formula, PE was independently associated with anterior chamber depth ( B = -0.279, P = .021) and CBI ( B = -0.520, P = .013) and AE was associated with angle κ ( B = -0.370, P = .007).

CONCLUSIONS

A large angle κ may reduce the predictability of IOL power calculation. Ocular biomechanics likely influence the refractive outcomes after IOL implantation. This study showed that eyes with softer corneal biomechanics had more myopic PE. This may relate to anteriorization of the effective lens position. Dynamic measurements may be the way to progress into future formulas.

Characterization of Corneal Biomechanics Using CORVIS ST Device in Different Grades of Myopia in a Sample of Middle Eastern Ethnicity

Purpose

To characterize corneal biomechanical properties using the CORVIS-ST device in myopic individuals.

Methods

This prospective cross-sectional study included patients with myopia. Our study included 154 eyes of 154 myopic patients aged between 18 and 40 years, with stable refraction for at least 2 years. A full ophthalmological examination and corneal tomography were performed using a Pentacam HR device. Corneal biomechanical parameters were assessed using the CORVIS-ST device in mild, moderate, severe, and extreme myopia groups.

Results

Statistically significant differences were observed in the DA ratio (p = 0.033), SP-A (p=0.009), CBI (p=0.041), SSI (p=0.000), and Peak distance (p = 0.032). In correlation with different Corvis ST biomechanical variables, SE was found to be correlated with DA ratio(r=-0.191, p=0.018), SP-A(r=0.199, p=0.013) and SSI(r=-0.336, p=0.000), while in multiple regression analysis, SE was found to be independently correlated with SSI and peak distance(p=0.036,0.038 respectively) while the grade of myopia was found to be independently correlated with SP-A(p=0.034).

Conclusion

SSI, Peak distance, and SP-A were independently related to SE and myopia grade, confirming the hypothesis that eyes with higher myopia are more deformable and less stress resistant.

Refractive associations with corneal biomechanical properties among young adults: a population-based Corvis ST study

PURPOSE

To assess the associations of corneal biomechanical properties as measured by the Corvis ST with refractive errors and ocular biometry in an unselected sample of young adults.

METHODS

A total of 1645 healthy university students underwent corneal biomechanical parameters measurement by the Corvis ST. The refractive status of the participants was measured using an autorefractor without cycloplegia. Ocular biometric parameters were measured using the IOL Master.

RESULTS

After adjusting for the effect of age, sex, biomechanical-corrected intraocular pressure and central corneal thickness, axial length was significantly associated with A1 velocity (A1v, β = -10.47), A2 velocity (A2v, β = 4.66), A2 deflection amplitude (A2DeflA, β = -6.02), HC deflection amplitude (HC-DeflA, β = 5.95), HC peak distance (HC-PD, β = 2.57), deformation amplitude ratio max (DA Rmax, β = -0.36), Ambrósio's relational thickness to the horizontal profile (ARTh, β = 0.002). For axial length / corneal radius ratio, only A1v (β = -2.01), A1 deflection amplitude (A1DeflA, β = 2.30), HC-DeflA (β = 1.49), HC-PD (β = -0.21), DA Rmax (β = 0.07), stress-strain index (SSI, β = -0.29), ARTh (β < 0.001) were significant associates. A1v (β = 23.18), HC-DeflA (β = -15.36), HC-PD (β = 1.27), DA Rmax (β = -0.66), SSI (β = 3.53), ARTh (β = -0.02) were significantly associated with spherical equivalent.

CONCLUSION

Myopic eyes were more likely to have more deformable corneas and corneas in high myopia were easier to deform and were even softer compared with those in the mild/moderate myopia.

Corneal stress-strain index in myopic Indian population

AIM

The purpose is to study the corneal stress-strain index (SSI) in myopic refractive error among Indian subjects.

METHODS

A retrospective study where young myopic subjects aged between 11 and 35 years who had undergone corneal biomechanics assessment using Corvis ST between January 2017 and December 2021 were enrolled. Subjects with central corneal thickness (CCT) <500 μ, intraocular pressure (IOP) >21 mmHg, history of any systemic and ocular disease or any previous ocular surgery, high astigmatism, corneal disease such as keratoconus were excluded. Subjects with missing data or having poor quality scan were excluded. Corneal biomechanical properties and corneal SSI were assessed using Corvis ST. For statistical purposes, eyes were divided into four different groups and were analyzed using one-way ANOVA.

RESULTS

Nine hundred and sixty-six myopic eyes with mean ± standard deviation age, IOP, and CCT of 26.89 ± 4.92 years, 16.94 ± 2.00 mmHg, and 540.18 ± 25.23 microns, respectively, were included. There were 311, 388, 172, and 95 eyes that were low, moderate, severe, and extreme myopic. Deformation amplitude ratio at 1 mm and 2 mm were similar across different myopic groups. A significant increase in max inverse radius, ambrosia relational thickness, biomechanically corrected IOP, integrated radius was noted with an increase in myopic refractive error. Corvis biomechanical index, corneal SSI was found to be decreased significantly with an increase in myopic refractive error. We noted a significant positive association between myopic refractive error and SSI (P < 0.001).

CONCLUSION

Corneal SSI was found to be reduced in extreme myopic eyes.

Effect of central corneal curvature on corneal material stiffness parameter acquired by dynamic corneal responses

The stress-strain index (SSI) is a measure of corneal material stiffness, which is obtained using the Corvis ST algorithm based on dynamic corneal response parameters. The reduced SSI corresponds to the longer axial length (AL). In a previous study, we found SSI increases as the corneal curvature flattens, whereas a flatter corneal curvature indicates a longer AL (emmetropia or myopia). Therefore, in this cross-sectional study, we aimed to address these contradictory findings. First, we characterized the features of SSI, curvature radius of the anterior corneal surface (CR), and AL and analyzed their correlation with advanced myopia. Next, we compared the relationship between AL and SSI after adjusting for the effect of CR. We found a significant positive correlation between SSI and CR, which contradicts the developmental law of axial myopia. Furthermore, after accounting for the effect of CR, we observed a stronger correlation between SSI and AL than that in the unadjusted model. In conclusion, CR is an independent influencing factor for SSI in addition to AL, which masked the decrease in SSI caused by prolonged AL in axial myopia.

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

Purpose

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).

Methods

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.

Results

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).

Conclusion

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.

Corneal Biomechanical Properties Demonstrate Anisotropy and Correlate With Axial Length in Myopic Eyes

Purpose

The purpose of this study was to investigate the ex vivo and in vivo biomechanical characteristic of cornea in myopic eyes.

Methods

Fifty-one corneal stromal lenticules were obtained from myopic eyes during the SMILE procedure and were tested by a biaxial tensile system within 24 hours postoperatively. The material properties of the lenticules were described using stress-strain curves and were compared among axial length (AL) <26 mm and AL ≥ 26 mm group. Pre-operative stress-strain index (SSI) parameters were used to evaluate the biomechanical properties of the cornea in vivo.

Results

Compared with AL < 26 mm, the tangent modulus significantly decreased in horizontal and vertical directions when AL ≥ 26 mm (P < 0.05); SSI also significantly decreased when AL ≥ 26 mm (P < 0.05). Anisotropic parameter is positively correlated with AL (r = 0.307, P < 0.05). Compared with AL < 26 mm, anisotropic parameter significantly increased when AL ≥ 26 mm (P < 0.05). SSI was negatively correlated with AL (r = -0.380, P < 0.05) in the AL < 26 mm group but not in the AL ≥ 26 mm group (P > 0.05). Compared with 26 mm ≤ AL < 27 mm group, the tangent modulus significantly decreased in the horizontal direction (P < 0.05) but not in the vertical direction when 27 mm ≤ AL < 28 mm (P > 0.05).

Conclusions

The biomechanical properties of cornea decreased with the increase of AL. Tangent modulus significantly decreased in the horizontal direction compared with vertical direction. AL should be taken into account during calculation of corneal biomechanical parameters in order to improve validity.

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 4 years. 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.

Analysis of potential impact factors of corneal biomechanics in myopia

PURPOSE

To investigate potential impact factors associated with corneal biomechanical properties in Chinese myopia and further to investigate quantifying corneal biomechanics in clinical work.

METHODS

Three hundred fifty-five eyes from 181 healthy myopic subjects with a mean age of 25.1 ± 9.4 were recruited in this study. Each patient carried out a comprehensive ophthalmic examination, including corneal hysteresis(CH), corneal resistance factor(CRF), central corneal thickness(CCT), axial length(AL), intraocular pressure(IOP), spherical equivalence(SE) and corneal curvature (K). CH and CRF were measured with the ocular response analyzer(ORA). To analyze the corneal biomechanical characteristics of myopia patients and their association with other parameters.

RESULT

The multiple linear regression analysis showed that CH was positively associated with CCT, and corneal curvature (all with P < 0.05) and negatively associated with SE and AL)(all with P < 0.05); CRF was positively correlated with CCT, corneal curvature and IOP(all with P < 0.05), but negatively correlated with SE and AL(all with P < 0.05). The CH and CRF values in children group were both higher than adults group (≥ 18 years old) (P < 0.05), but it attributed to that the CCT of children was thicker than adults. Excluding factor of CCT, there was no significant difference in CH and CRF between children group and adult group. Excluding factor of CCT, there was no significant difference in CH and CRF among different stage of age (age 18-48).

CONCLUSION

The CCT played the most important role of affecting the CH and CRF. The SE, corneal curvature, AL and IOP had a certain influence on corneal biomechanics. Whether the CH and CRF values of individual patient are normal in clinical work, it should refer to the CH and CRF values corresponding CCT sectional range and SE.

The relationship between baseline axial length and axial elongation in myopic children undergoing orthokeratology

PURPOSE

To investigate the correlation between the baseline axial length (AL) and axial elongation in myopes undergoing orthokeratology (ortho-k).

METHODS

This was a retrospective study. During the 1-year follow-up, 1176 children (aged 8-14 years) were included and divided into an ortho-k group (n = 588) and a single-vision spectacle group (n = 588). The ortho-k group participants (8-11 years of age) who completed the 3-year follow-up (n = 150) were further divided into three subgroups stratified by their baseline AL: subgroup 1 (AL < 24.5 mm), subgroup 2 (24.5 ≤ AL < 26 mm) and subgroup 3 (AL ≥ 26 mm). AL was measured at baseline and during the annual visit.

RESULTS

The ortho-k group exhibited slower 1-year axial elongation (39% reduction) than the spectacle group. The 1-year axial elongation was negatively correlated with initial age in both groups. A negative association between 1-year axial elongation and baseline AL was observed in the ortho-k group but not in the spectacle group. However, this relationship only existed in ortho-k participants 8-11 years of age. For the younger ortho-k participants who completed the 3-year follow-up, the annual axial elongation was significantly higher in subgroup 1 for the first and second years but not in the third year compared with subgroups 2 and 3.

CONCLUSION

Axial elongation was negatively correlated with baseline AL in the ortho-k group. Children aged 8-11 years with longer baseline AL (≥24.5 mm) demonstrated slower annual axial elongation during the first 2 years of ortho-k treatment, which may provide insight into establishing individual guidelines for controlling myopia using ortho-k in children with different baseline characteristics.

Effect of corneal stiffness decrease on axial length elongation in myopia determined based on a mathematical estimation model

Purpose: To investigate the relationship between the corneal material stiffness parameter stress-strain index (SSI) and axial length (AL) elongation with varying severities of myopia, based on a mathematical estimation model. Methods: This single-center, cross-sectional study included data from healthy subjects and patients preparing for refractive surgery in the Qingdao Eye Hospital of Shandong First Medical University. Data were collected from July 2021 to April 2022. First, we performed and tested an estimated AL model ( A L M o r g a n ) based on the mathematical equation proposed by Morgan. Second, we proposed an axial increment model ( Δ A L ) corresponding to spherical equivalent error (SER) based on A L e m m e t r o p i a ( A L M o r g a n at SER = 0) and subject's real AL. Finally, we evaluated the variations of Δ A L with SSI changes based on the mathematical estimation model. Results: We found that AL was closely associated with A L M o r g a n (r = 0.91, t = 33.8, p < 0.001) with good consistency and SER was negatively associated with Δ A L (r = -0.89, t = -30.7, p < 0.001). The association of SSI with AL, A L e m m e t r o p i a , and Δ A L can be summarized using the following equations: A L = 27.7 - 2.04 × S S I , A L e m m e t r o p i a = 23.2 + 0.561 × S S I , and Δ A L = 4.52 - 2.6 × S S I . In adjusted models, SSI was negatively associated with AL (Model 1: β = -2.01, p < 0.001) and Δ A L (Model 3: β = -2.49, p < 0.001) but positively associated with A L e m m e t r o p i a (Model 2: β = 0.48, p < 0.05). In addition, SSI was negatively associated with Δ A L among subjects with AL ≥ 26 mm (β = -1.36, p = 0.02). Conclusion: AL increased with decreasing SSI in myopia.

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 18–34 years. 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 (R² = 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.

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 (2 mm) (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.

The relationship between axial length/corneal radius of curvature ratio and stress–strain index in myopic eyeballs: Using Corvis ST tonometry

Purpose: This study aimed to investigate the correlation of axial length/corneal radius of curvature ratio with stress–strain index (SSI).

Methods: Retrospective analysis was conducted to compare the right eyes of those with high myopia (HM, n = 132; age and 10–48 years) with those without high myopia (NHM, n = 135; age and 7–48 years), where the baseline axial length, corneal radius of curvature ratio, and central corneal thickness were analyzed; the differences in two groups were compared; and the relationship of axial length and axial length/corneal radius of curvature ratio with SSI were explored.

Results: Compared with AL &lt; 26mm, SSI significantly decreased when AL ≥ 26 mm (p = 0.001), while there was no correlation with AL in the NHM group (r = -0.14, p = 0.12) or HM group (r = -0.09, p = 0.32). AL/CR was significantly associated with SSI in both the NHM (r = -0.4, p &lt; 0.001) and HM (r = -0.18, p = 0.04) groups. In the NHM group, AL/CR was significantly associated with SSI (unstandardized beta = -0.514, se = 0.109, p &lt; 0.001) with the adjustment of age and gender. Additionally, a significant association of SSI with AL/CR was also found after adjusting for age and gender (unstandardized beta = -0.258, se = 0.096, and p = 0.0082) in the HM group.

Conclusion: SSI showed a significant negative correlation with AL/CR in patients without high myopia and in patients with high myopia. However, SSI exhibited no decrease with the worsening of myopia, but it gradually remained stable at a low level. The findings of this study validate, to some extent, the possibility of analyzing the dynamic changes in ocular wall stiffness during the development of myopia by measuring in vivo corneal biomechanical parameters.