The present study aimed to assess the corneal biomechanics in eyes with different degrees of myopia, especially the severe myopic eyes, using the Corvis ST with a large sample size. In the current prospective study, we found that the severe myopia group had a significantly longer DA than the low, moderate and high myopia group. Moreover, DA was significantly negatively correlated with SE (r=-0.20, P = 0.001)and positively correlated with AL (r = 0.16, P = 0.009). DA is the deformation amplitude at highest concavity and is regarded as a surrogate for corneal stiffness.[18, 21, 28] A higher DA value would represent a softer cornea due to a lower resistance to deformation.[28, 29] Our findings showed that the higher the degree of myopia or the longer the axial length, the softer of the cornea is, which is consistent with previous studies.[20, 21, 30–33] But Miki et al. did not divide subjects into different groups according to SE or AL with relatively few participants. On the other hand, our research focus on corneal biomechanical properties in severe myopia in a large number of samples with rigorous screening criteria. Xiang et al. suggested that eye elongates particularly fast before the onset of myopia. It’s conceivable that changes of corneal and scleral biomechanics precede the occurrence of myopia. But our date does not prove whether the corneal biomechanical properties changed already before the eye becomes myopic. In clinical practice, if a larger DA indicates a greater risk of axial elongation, preventive methods can be considered. Further, assessment of corneal biomechanics may also be crucial for surgical planning (and preoperative screening) to reduce the risk of corneal ectasia after refractive surgery.
A2-velocity in the severe myopia group was significantly greater than in the other three groups. A2-time in the high and moderate group was significantly shorter than in the low myopia group. The severe and high myopia groups had a smaller A2-length than other groups. These findings are not in line with those from Wang et al., their study did not show the difference of A2-velocity between groups. Another study, consistent with our results, showed high myopia had greater A2-velocity compared to either low to moderate myopia or emmetropia using the Corvis ST. A2-time, A2-length and A2-velocity were significantly positively correlated with SE and negatively correlated with AL, this agreed with the ANOVA in the present study. A previous study found that A2-time may be an indicator of the total viscoelasticity of the cornea. The higher the degree or the longer the AL of the myopic eye, the less time is needed to go back to the origin after applanation as its viscoelasticity increased. In a recent research, myopia had greater A2-velocity indicating that cornea tends to be more deformable and softer than the emmetropia and hyperopia in children. Therefore, A2-velocity may be another indicator of corneal elasticity, as Lee et al. and Miki et al. suggested, which indicated that less viscous damping capacity was seen in eyes with a longer axial length. Further research about the use of A2-velocity is warranted.
In accordance with a previous study by Wang et al., we found that the HC radius was significantly smaller in eyes with severe myopia than high myopia. Frings et al. demonstrated that in eye following corneal refractive surgery, there was a tendency to have a smaller radius at highest concavity. In other words, the softer the cornea, the easier it is to deform, resulting in a smaller HC-radius. In addition, our findings showed that the other factors, which were age, IOP, CCT and CV, may be potential factors influencing corneal deformation. We found that the cornea becomes softer with longer DA values with increasing age, in agreement with previous studies.[21, 38] However, others supported a view that the cornea becomes stiffer in elderly individuals because of more glycation-induced cross-linking.[39, 40] The relatively narrow range of age in this study may be a reason for the conflicting results. Further study is required to determine the influence of age on corneal biomechanics. The central corneal thickness might be associated with corneal deformation with thinner corneas representing higher corneal deformation,[18, 41] which is consistent with the findings in the present study. Our correlation analysis showed a lower IOP was associated with less stiffness of the cornea with a larger DA (r=-0.78, P < 0.001). This phenomenon was in accordance with previous studies. Alonso et al. indicated that the variation in corneal deformation may depend on the IOP rather than simply affected by the corneal structure. Tian et al. also found that DA was negatively correlated with IOP in both primary open-angle glaucoma (POAG) and normal eyes. In other words, we should take the corneal biomechanical into consideration when it comes to the accurate measurement of IOP.
It has been suggested that myopic eyes have a lower ocular rigidity than emmetropic eyes,[8, 9] and the diameter of collagen fiber bundles of sclera in highly myopic eyes is less and the sclera is significantly thinner.[44, 45] Less of proteoglycan and glycosaminoglycan synthesis[46–48] and a reduction in the extracellular matrix may explain the remodeling of the sclera during the development of myopia. In this way, the scleral mechanical properties weaken whilst the deformability increases. Since the corneal stroma is the continuation of the sclera, the expansion of the sclera may result in the reduction of corneal stiffness. Recently, translating the unique scleral features into measurable corneal biomechanical properties in myopic eyes has received great attention. If a change in corneal biomechanical properties potentially indicate a risk for myopia development, such measurements may become clinically useful in the management of myopia. On the other hand, various studies have demonstrated that corneal refractive surgery may change corneal biomechanics,[31, 37, 51–53] which can lead to iatrogenic keratectasia. Therefore, a better understanding of the biomechanical characteristics of the cornea is of great importance in the field of corneal refractive surgery.
The main limitations of our study are as follows: first, we did not assess corneal deformation in emmetropia. Second, a longitudinal study is required to clarify whether the parameters of corneal biomechanical properties, such as DA and A2-velocity, can be indicators of myopia progression and axial elongation. Whether changes in corneal biomechanics is the consequence or the cause of myopia development requires further longitudinal study to investigate.
In conclusion, the myopic eyes with a greater degree and longer AL exhibited longer DA and A2-velocity under stress, suggesting that the cornea in higher myopia tends to be more elastic and deformable. Further research is warranted in this field as this may have clinical significance the management of high myopia and also in the refractive surgery field.