Corneal biomechanics: Where are we?

Corneal biomechanics emerged as a very hot topic for research in Ophthalmology. In this issue, Sharifipour and coworkers present an observational cross-sectional study in healthy individuals that evaluated the correlations of corneal viscoelastic properties measured by the Ocular Response Analyzer (ORA, Reichert, Buffalo, NY) with central corneal thickness (CCT) in different age groups. In this study, corneal hysteresis (CH) and corneal resistance factor (CRF) had a significant negative correlation with age (P < 0.001). Other studies had already demonstrated that CH and CRF have a negative correlation with age and a positive correlation with CCT.While statistical methodology was done properly in these studies, it is very important to note that CH and CRF do not express stiffness or the stressestrain behavior of corneal tissue. It is well recognized that corneal stiffness increases with age due to enzymatic pathways such as transglutaminase and lysyl oxidase, generating natural collagen crosslinking. In agreement with such concept, Elsheikh and coworkers analyzed human corneal specimens ranging in age between 50 and 95 years with an inflation test, finding a significant increase in stiffness associated with age and also with the load rate. The ORAwas introduced in 2005 as the first instrument for assessing corneal biomechanics in vivo. The ORA is a noncontact tonometer (NCT) that monitors corneal deformation using the infrared reflex of the corneal apex. The applanation is detected as a peak on this reflex and is correlated with the pressure of the air puff. The inward phase (P1) and in the outward phase (P2) applanations are registered. The air puff maximal pressure is related to P1 as the system has an integrated loop to control the pump, which generates a collimated air pulse or puff with a symmetrical configuration. P1 and P2 pressure measurements are the basis for the first generation variables as reported by the original ORA software. The difference between the two pressures is called CH. Hysteresis is derived from the Greek, meaning ‘‘lagging behind’’. Corneal resistance factor is also derived from the inward and outward pressure values, based on a formula (P1 e kP2), where k is a constant that was empirically developed to augment the correlation with CCT. The concept was to develop a parameter (CRF) that reflects the resistance to deformation, but this is still related to IOP. Shah and coworkers