This study described the distribution of corneal tissue material stiffness parameter SSI in different age groups and related factors in a healthy Chinese population. We found that SSI was relatively stable before age of 35, and then increased significantly with age. SSI was positively correlated with age, IOP, and anterior radius of curvature, meanwhile, it was negatively correlated with axial length. No significant effect was found in gender, ACV, corneal thickness, and bIOP.
In our study, a nonlinear relationship was found between age and SSI, showing that SSI increased with age significantly after age of 35. Age has been shown to be an important factor affecting corneal biomechanical in previous study8. Wang found a positive correlation between age and second applanation (A2L) in healthy Chinese adults18. In corneas of patients aged from 50 to 95 years, the stress-strain ratio increased with age7. However, He found that there was no significant correlation between biomechanical parameters and age in healthy Chinese adolescents at 4–18 years of age19. Valbon found that deformation amplitude (DA) and other biomechanical parameters of healthy eye in population under 40 years old were not correlated with age20. Trend of corneal biomechanics with age could be accounted for changes of the molecular structure in cornea. Daxer and Malik observed that non-enzymatic crosslinking, collagen glycation, fibril diameter, and the number of collagen molecules increased with age over 40 years in corneal X-ray21–23. These could explain the reason that SSI increased with age after 35.
SSI was found to be negatively correlated with axial length, which indicated that the corneal material stiffness decreased when axial length increasing. Previous studies indicated that the cornea and sclera were mainly composed of the same types of collagen24.In addition, when the collagen fibers of the sclera became longer and were damaged in myopia progression, the overall arrangement of collagen fibers of corneal stroma also restructured25–27.In infant monkeys and chicks, corneal astigmatism changes were also associated with induced eye growth28, 29. Chang's study pointed out that the axial elongation led to corneal flattening and thickness reduction30, suggesting that the increase of axial length may affect the biomechanics of the cornea and previous studies have proved this. Myopia in glaucoma and normal eyes would lead to biomechanical parameters changes such as CDA, OCA and cornea stiffness (CS)31, 32. Especially the cornea in high myopia had worse biomechanics compared to emmetropia15. Long found in Chinese children, SP - A1 declined gradually between presbyopia, emmetropia and myopia group33. However, in Lim's study, CH and CRF were not significantly correlated with refractive errors by ORA measurement in corneal biomechanics in children aged 7–9 years. Lim believed the possible reason was the brief loading– unloading cycle of the ORA contrasts with the slower profile of scleral creep experiments and myopic deformation17.
Consistent with the study of Eliasy, neither bIOP or CCT were significantly correlated with SSI8, but SSI was found to be positively correlated with IOP. It was not surprising, since SSI reflected the corneal stiffness, and IOP measurement was affected by corneal stiffness10, 34. While it was emphasized that bIOP can exclude the influence of corneal thickness and age on intraocular pressure measurement35, and can reflect more accurate intraocular pressure36–38. Besides IOP, previous studies have shown that, parameters of corneal biomechanics in vivo were mostly affected by bIOP such as deflection area at the highest concavity and deflection amplitude (HC DefArea and HC DefA), SP - A138 CH and CRF39. However, SSI based on finite element (FE) numerical modeling simulating the effects of Corvis ST air puff and bIOP to predict of corneal behavior and excluded the influence of bIOP, indicating that this is an ideal method for in vivo measurement of corneal tissue material stiffness8.
There was no significant correlation between SSI and anterior chamber volume in multiple regression model. The results of this study were consistent with Gabor Nemeth and Hwang, that the anterior chamber volume was not significantly related to biomechanical parameters14, 40. Cui’s study pointed out that a decrease in ACV led to a reduced corneal stiffness in the PAC suspects. This could be explained by the different population in the studies. Cui believes that the stiffness of cornea would reduce in PAC and compensate for the high intraocular pressure caused by the shallow anterior chamber13. But in healthy people, there was no significant effect of ACV on corneal stiffness.
This study found that the radius of anterior curvature is positively correlated with SSI. When the cornea was stressed, the collagen fibers would slide relative to each other. The curvature of the cornea affects the pressure between the layers of the cornea, thereby affecting the sliding of collagen fibers41 and this may account for the result. There has been controversy on the relationship between corneal curvature and biomechanics. It was found that the radius of anterior and posterior curvature was positively correlated with biomechanics14, 42. On the contrary, several studies found that there was no significant correlation between the curvature and CH or CRF40, 43, 44. The correlation between corneal curvature and biomechanics needs further study.
Our research had several limitations. First, the SSI was currently estimated with normal corneal topography, which could not be applied in corneas with biomechanical decline caused by pathological changes. The calculation method of corneas with keratoconus or ectasia needs to be further developed. Second, the relationship between refractive error and SSI was not analyzed. Previous studies have pointed out that refractive error was highly correlated with the axial length of the eye45, and it has also been shown that the extension of the eye axis had a significant correlation with the sclera and cornea tissue structure27. Therefore, we considered that the axis of eye was more significant as an evaluation factor.