# Goldmann Tonometry and Corneal Biomechanics
## 4 Discussion
Corneal hysteresis is not \(E\), although it is directly related to it, being defined as the difference between the pressure at which the cornea bends inward during an air jet explanation and the pressure at which it bends out. [PERSON] et al. [67] also measured a lower stiffness in glaucomatous eyes. A negative correlation was found between delta-pressure and corneal thickness, \(CCT\), meaning that thinner corneas result in greater IOP measurement errors by GAT, and in particular larger underestimation of IOP. The possible influence of corneal thickness in the measurement of IOP was identified and briefly discussed by [PERSON] and [PERSON] in 1957 [33]. Subsequent studies, performed by [PERSON] et al. [44], described in great detail the effect of corneal thickness in the measurement of internal pressure, and the interest in this phenomenon grew again with the advent of refractive surgery. The procedure by which the refractive surgery is performed in fact determines a thinning of the cornea, and was widely discussed, in relation to tonometry, in several experimental works [68]. [PERSON] et al. [45] and [PERSON] et al. [35] found that a high value of corneal thickness leads to an overestimate of IOP, which is line with our findings. It is known that porcine corneas are thicker (about 1 mm [69]) than human ones (about 600 \(\upmu\)m [70]). Average thickness for our porcine corneas was 1.4 mm, meaning that they probably underwent swelling. Nevertheless, since measurements were taken a-posteriori from images of excised strips, a rapid swelling may have occurred as a consequence of the cut of the protective endothelium and epithelium. Although our thickness measurements therefore are affected by an error, the negative correlation with delta-pressures is still valid and suggests that an even larger error would have been measured for thinner corneas. The accuracy of the GAT depends on the mechanical strength to explanation, which is in turn influenced by central corneal thickness (CCT), by the curvature, and the mechanical properties of cornea and sclera. The measurement of the intraocular pressure by means of tonometry is, therefore, affected by several sources of error, which may lead to an erroneous diagnosis of ocular hypotension or hypertension or glaucoma. In this paper we demonstrated the correlation between such errors and corneal stiffness and corneal thickness. Our study presents a few limitations. First of all, it was conducted in vitro, so missing several of the in vivo factors which may affect the results. Although this is an obvious drawback, it also allows to perform controlled experiments and to obtain reproducible results. Tests were conducted on pig eyes which are very similar to human eyes but present some differences. In particular, pig corneas are much thicker than human corneas and they may possess different material properties relative to the human corneas. The effect of thickness on the results of the present study has already been discussed above. The interpretation of experimental data on porcine eyes [71; 72], showed that the porcine stroma has, on average, mechanical properties very similar to the ones of the human stroma. Tests conducted on human corneas would clearly be of great importance to confirm our results obtained on porcine corneas. Regarding the methods, we evaluated the apex displacement of corneas from 2D images, whereas a 3D strain distribution may have resulted in more precise calculations. We measured the cornea thickness after the experiments on excised strips of the tested corneas. A pachymetry used on corneas during the test would probably give more precise results. Finally, we did not investigate the correlation with the radius of curvature, \(R\), since our pig eyes, originating from animals of the same age of the same facility, were roughly of the same dimension with very low variability in \(R\) data (see Table 3).