Myopia is a public health problem[9] with a high prevalence especially in the Far East[10]. Onset and progression of myopia has been associated with genetic and environmental factors[11, 12]. Previous studies have noted that myopia is related to an increase in corneal curvature and decrease in corneal thickness[13]. Animal studies have shown a change in length of the eye and shape of the anterior cornea during the process of myopia modeling[14, 15]. It is also known that high myopes have lower corneal hysteresis than emmetropes [16]. However, it is difficult to detect the ocular biomechanical properties in vivo[17]. The Corvis ST provides information on corneal deformation parameters by visualizing the dynamic reaction of the cornea to a single puff of air [18].
In our study, one of the new Corvis ST parameters, SSI was evaluated in myopic eyes. We demonstrated that the SSI was positively correlated with spherical equivalent(r = 0.313, p<0.01) (Fig. 1). When comparing eyes with low and high myopia, there was no significant difference in CCT, BIOP, SP, ARTH, and CBI, but there was a significant difference in SSI(t = 8.960, p<0.01)and Integrated radius༈t=-3.509, p<0.01) (Fig. 2,Table 4) values.
Inmaculada Bueno-Gimeno et al used Ocular Response Analyzer (ORA) and suggested that corneal biomechanical properties seem to be compromised in myopia from an early age, especially in high myopia[19]. Another study showed a weak but significant correlation between corneal hysteresis (CH) and refractive error with CH being lower in both moderate and high myopia compared to eyes with emmetropia and low myopia eyes[20]. Wu et al [21]reported a difference in corneal biomechanical properties between 835 low myopic eyes and 1027 high myopic eyes. Low CH and low CRF, high IOPcc and high IOPg were suggested to be associated with high myopia. Nevertheless, the association of biomechanics of myopia is controversial. Some studies reported no significant relationship between myopia and CH[22, 23]The results of our study revealed a strong negative correlation between SSI and HCDefA(r=-0.721, p<0.01)、HCDefArea༈r=-0.665, p<0.01)、PeakDist༈r=-0.597, p<0.01)、Integrated radius༈r=-0.555, p<0.01) and DefAmax༈r=-0.564, p<0.01) (Table 6, Fig. 3). Wang et al[24] found that eyes with high myopia had a larger corneal deformation amplitude compared with mild to moderate myopia while A2 time and HC radius were positively correlated with equivalent spherical (SE). Eyes with high myopia also showed longer DA and smaller HC radius. Similar results were reported by Miaohe et al[5] .These findings are consistent with our results.
Previous studies have shown that the biomechanical properties of cornea are related to CCT. Eyes with thick CCT exhibited strong corneal resistance to external force and are less prone to deformation[25].Higher intraocular pressure may mask abnormal corneal biomechanical properties and resulting in apparently normal HCDA measurements [26]. The introduction of SSI resolved this issue since it estimates material stiffness [27]. Eliasy et al[7]used the numerical models in the SSI parametric study that covered wide variations in IOP ,CCT,geometry and material parameters which covered and slightly extended beyond the ranges reported in clinical studies.Through consideration of a Corvis parameter—SP-HC which is more strongly correlated with corneal stiffness than IOP, SSI is intended to be independent of intraocular pressure and corneal geometry. As a new index it could help in the detection of patients with higher risk or susceptibility for ectasia development or progression after refractive surgery and could aid in surgery planning.
In this study, we saw a weak correlation between BIOP(r = 0.23, p<0.01)、CCT(r = 0.125, p<0.01) and SSI (Table 3). It indicated that despite correction, the effect of corneal biomechanics cannot be completely independent of IOP and thickness, which is consistent with our clinical experience and previous studies. Effects of IOP and corneal biomechanics on eye behavior are difficult to separate; IOP also effects the immediate corneal stiffness. It is generally believed that sex has no significant effect on corneal biomechanics[28].Our study also suggested that the corneal biomechanical properties of myopia may have nothing to do with sex. Correlation between stress-strain behavior and age was reported[8, 29], although this study found that there was no strong significant correlation between age(r = 0.198, p<0.01) and SSI, which may be related to the concentration of individual age included.
It is noteworthy that the mean CCT value measured by CorvisST (553 ± 29.96um) was slightly lower than that measured by Pentacam corneal topography (554 ± 31.04um)(t = 4.970 p<0.01).However, it has been shown that Corvis-ST CCT measurements have good repeatability[30].
The main limitation of the current study is lack of eye axis parameters and a control group with emmetropia, inspite of a large sample size with myopic participants,which will be improved and supplemented in the future research.