# Goldmann Tonometry and Corneal Biomechanics
## 1 Introduction
The ocular response analyzer (ORA) uses a jet of air as the applanating force to the apex of the cornea; it can measure corneal hysteresis and the corneal resistance factor and provides higher IOP measurements as compared to GAT [27]. Pneumatonometer is a portable instrument which overestimates IOP at high values and at high CCT [24]. The Icare HOME tonometer is a portable device, contact rebound tonometer. It also has been recently shown to overestimate IOP compared to GAT [28]. The dynamic contour tonometer (DCT) or Pascal tonometer differs from GAT having a concave tip equipped with a tiny piezoelectric sensor: it takes about a hundred measurements per second and also evaluates IOP fluctuations with systemic pressure variation; no statistically significant effect of corneal curvature, astigmatism, axial length, and age on the difference between DCT and intracameral IOP has been detected [29; 30]. A recent study has been carried out on an IOP sensor mounted on contact lens, not yet available for clinical use, that can be worn by patients for up to 24 h, also measuring the circadian rhythm. The data show an overnight rise in IOP [31]. Finally, the Corvis tonometer, a rebound tonometer, able to take into account corneal biomechanical characteristics, also has been shown to measure higher IOP values compared to GAT [32]. Although observations outlined above underline that most of the tonometers on the market \"overestimate\" the IOP measured by the Goldmann tonometer, the Goldmann explanation tonometry (GAT) is internationally considered as the gold standard for IOP measurement in clinical practice without the need to deform appreciably the cornea [33; 34]. The functioning of this device is based on the measurement of the force required for the explanation of a specific portion (of 3.06 mm in diameter) of the central cornea, and on the use of this measured force to estimate the value of the internal pressure, on the basis of a calibration procedure which depends on a number of standard parameters. The accuracy of tonometry therefore depends on the mechanical resistance of the eye structure to the explanation, which is in turn influenced by the central corneal thickness, the curvature, and the mechanical properties of the cornea [35; 36; 37; 38; 24]. Recently, the need to make the IOP measurement faster and more comfortable has led to the development of new techniques [39; 40; 41]. Analyzing corneal thickness, some researchers found a statistically significant correlation between corneal thickness and GAT [24; 42; 43], while many others argued that thicker corneas led to an overestimation of the IOP, and on the contrary thinner corneas led to IOP underestimation [24; 35; 44; 45; 46; 47]. corneal curvature has been recognized as a parameter that reduces the precision in the measurement of IOP [35; 47; 48]. In summary, Goldman tonometry appears imprecise. It would be very important to know how the geometrical and mechanical properties of the cornea influence and modify the pressure inside the eye, making more or less accurate the measurement made by tonometry. Noted the presence of several studies concerning the correlation between IOP measurement and corneal thickness [19; 41; 49; 50], only a few suggest that corneal biomechanics may affect IOP measurements [19; 35; 51], inducing significant errors in the diagnosis of glaucoma. Thus, the purpose of this study was first to determine any difference between IOP measured indirectly by the GAT and directly by cannulation of the eye, and then to investigate the existence of statistically significant correlations between such differences and (i) the corneal thickness; and (ii) the corneal elastic modulus. In order to be able to evaluate the mechanical properties of the corneal tissue and its influence on the tonometry measurement, in vitro tests were performed in pig eyes.