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The importance of corneal biomechanical properties in tonometry, ocular diagnosis and ocular disease management was well known. The ORA, which is the first in-vivo instrument for the measurement of corneal biomechanical parameters was well evaluated by many studies [15, 19, 59, 60, 69, 74, 84, 272-277]. In recent years, Corvis ST (CST) was introduced as a pachy-tonometer. It is able to perform in-vivo Scheimpflug imaging of the corneal deformation under air-pulse pressure [212, 245, 265]. In the current study, the relationship between CST parameters and ORA was analysed. This study analysed the influence of IOP, CCT, gender, age and laterality on the corneal biomechanical parameters by CST and ORA.

This study showed highly significant association between both CRF and CH, which was demonstrated in many studies since the launch of ORA a decade ago [220]. The parameters were derived from the same infra-red wave analysis but empirically calculated using different mathematical algorithms [69]. The results showed that all corneal biomechanical parameters from ORA except HcT and HpD, were significantly correlated with the parameters from CSTc. Both ORA parameters have significant moderate correlation with A1T. Other CST parameters (A1L, A2T, A2L, A2V and HcR) have weak but significant correlation with CH and CRF. However, DA showed an inverse relationship with CRF. Previous studies highlighted A1T and DA as the most reliable and reproducible parameters for describing corneal biomechanics [211, 245, 269]. These CST parameters may be important to represent the viscoelastic properties of the cornea in-vivo.

Similarly, the CRF was also noted to be more representative than CH on corneal viscoelasticity. A study on spectral analysis of the waveform of both ORA and CST found no statistically significant difference between CH and CRF versus DA [278]. The study did not explore direct correlations between corneal biomechanical parameters from both instruments.

The CH was noted to be affected by age, CCT and IOP. However, the CRF increased with thicker CCT and was not influenced by other demographic variables and parameters. In a British population cohort, the mean CH and CRF declined with age and were higher in women than in men [58, 63, 96, 97]. Similar observations were made by other studies that indicated that the ageing cornea could reduce corneal viscoelasticity (92, 96, 124, 125). Despite a few contradictory findings in laboratory studies using donor cornea, the inverse association between age and corneal

viscoelasticity remains strong. However, the CRF was noted to be less influenced compared to CH . In this study, aging positively influenced A1T, A2T, HcR and DA which indicate higher cornea resistance to dynamic external force (air-pulse pressure of the CST).

Similar positive association between age with A2T and DA was noted in a study on younger subjects undergoing refractive surgery [20, 70]. However, a study on a young to middle-aged healthy Brazilian cohort revealed an increase in HcT with age [70]. The author postulated that the HcT inversely represent the viscoelasticity profile in patients as a result of physiological cross-linkage of the corneal stoma collagen fibrils in ageing eyes. The variability on the association of the CST parameters with age maybe resulted from the heterogeneity of the study cohort as well as the CST software version employed. The present study population has a wide age range with multi-ethnic distribution and was examined with a more updated CST software version.

The effect of IOP on the corneal biomechanical properties by ORA was previously analysed in normal and glaucomatous eyes [15, 17, 163]. The ORA’s estimate of corneal-compensated IOP (IOPcc) is a mathematically derived tonometry value and claims to be less affected by corneal biomechanical properties and IOP measurement in comparison to other tonometers [157]. Earlier studies have found that IOPcc was not associated with corneal curvature ,central corneal thickness [72, 73, 143]or axial length [72]. Therefore, IOPcc was chosen to represent the IOP for the analysis on factors affecting corneal biomechanics. The present study showed that CH was negatively affected by IOPcc.

Other studies t noted an inverse correlation between IOP and CH [65, 68, 85-87]. Kamiya et al. found that eyes with thinner CCT as well as higher IOP values are more predisposed to having lower CH [68]. By contrast, there is significant positive contribution of IOPcc on CRF in univariate analysis. CRF increases with rising IOP, indicating that resistance against the deformation of the cornea is higher in eyes with higher IOP values. However, upon inclusion of age, gender, IOPcc and CCT in the multivariate analysis, the significant effect of IOPcc disappeared and was shown to be affected by only CCT. The lack of influence of IOPcc on CRF may be due to the strong inter-correlation between IOPcc and CCT. Galletti et al. confirmed the confounding effects of IOP and CCT on ORA biomechanical parameters and suggested new values termed “transformed CH” (CHcorr) and “transformed CRF” (CRFcorr) [88]. The IOP significantly affected seven out of ten CST parameters in the presentstudy. Higher IOP caused significant increment of A1T, A1L, A2V and HcR. The effect of these CST

parameters indicates a ‘soft’ cornea and showed reduction of the corneal biomechanical properties with increase IOP.

Previous studies have reported that a thinner CCT is an independent risk factor for open-angle glaucoma in patients with ocular hypertension (OHT) [75-79]. A study by Shah et al. [80] found significant and positive relationships between ORA parameters, CH and CCT, CRF and CCT and CRF and CH in normal eyes. In both glaucoma and normal subjects, CCT is positively correlated with CH [81-83]. As the cornea contains more collagen fibres and ground substances, resistance against deformation and damping capacity rises. Moreover, the stronger the corneal tension, the faster the cornea regains its original position following deformation. In addition, IOP represents an additional force that restores the cornea to its original position [84] Central corneal thickness was a significant predictor of A2L, A2V, and also HcR.