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

Abstract

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.

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

“…An interesting finding was that our model's intermediate quantity AL emmetropia , which represented the axial extension of the "non-myopic component" in the model, was positively correlated with SSI. As per the mathematical estimation model, ΔAL is responsible for the larger weight factor of the negative correlation between AL and SSI, which clarifies why the correlation between SSI and AL is not significant as compared to SER, especially as AL is extended (Liu et al, 2022) (Chu et al, 2022) (Liu et al, 2021). However, according to the AL emmetropia formula, SSI is correlated with AL emmetropia , which indicates that SSI is correlated with the corneal curvature of the front surface, and rationally, as a material index, SSI should be independent of corneal geometry to estimate the material stiffness as per the original publication (of SSI) (Eliasy et al, 2019).…”

“…The stress-strain index (SSI) is a corneal biomechanical parameter provided by Corvis ST Tonometry to evaluate the stiffness of corneal materials, which can intuitively and quantitatively reflect the stressstrain relationship in corneal materials (Eliasy et al, 2019). Previous research has confirmed that a decreased SSI is associated with high myopia based on SER (Liu et al, 2022). As a morphological variable related to axial myopia, theoretically, AL should correlate more with SSI than SER because AL elongation may be caused by a decrease in scleral stiffness.…”

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

“…In the current study, we found that SE was independently correlated with SSI, and we found a statistically significant difference in SSI between the mild, moderate myopia, and extreme myopia groups, consistent with the findings of Han et al, 18 who found that SSI values were lower in patients with higher grades of myopia; however, in their study, they included only two groups: the low myopia group with SE ≥ 3.00D and the high myopia group with SE > −6.00D, which is also consistent with other studies that involved extreme myopia patients among the Chinese population. 23 , 27 ,…”

“…12 A better understanding of corneal biomechanical characteristics in different grades of myopia might help improve the selection of suitable candidates for refractive procedures and guide the understanding of the management of biomechanic-modulating treatments. 13,14 Many studies 15,[16][17][18][19][20][21][22][23][24][25][26][27] have been conducted to detect the associations between corneal biomechanics measurements using Corvis ST in China, India, and Iran with few studies incorporating extreme myopia cases and few have addressed the value of SSI. The current study is the first to describe corneal biomechanical properties in different grades of myopia in our country and found that SSI and Peak distance were independently correlated with refractive error spherical equivalent, and SP-A was independently correlated with the grade of myopia.…”

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

“…Recent cross-sectional studies found that eyes with higher myopia have lower corneal stiffness [ 9 , 10 ] and may be less stable and more susceptible to stress [ 11 – 13 ]. Similarly, in a cross-sectional study conducted by Liu et al, it was observed that the SSI value exhibited a sequential decrease when categorized according to the degree of myopia, namely, low, moderate, high, and ultra-high [ 14 ]. The difference in the rate of change of spherical equivalent (SE) refractive error may be related to the change in corneal stiffness and needs to be investigated in a longitudinal study.…”