The limitations of this study should be mentioned. First, the FEA did not account for the elastic anisotropy and nonlinearity of the ocular tissues, which has been reported in the cornea and sclera.
43–46 Therefore, the Lamb wave speed measurements would be more related to the out-of-plane shear modulus (along the depth axis) than the Young's modulus. On the other hand, OCE experiments were conducted at a single excitation frequency of 800 Hz, preventing the calculation of dispersion curves and the subsequent estimation of viscoelastic properties of the eye, as previously reported using transient techniques.
30,31 The reasons behind this decision lay in the great difficulty of transient methods in generating smaller wavelengths (∼ 1 mm) with sufficient displacement amplitude capable of propagating along the whole field of view. Moreover, as shown in
Figures 3g to
3i, the effect of different thicknesses in the cornea, limbus and sclera segments produce wave speed differences of < 1 m/s assuming the same Young's modulus for all regions. Even though such discrepancies are nontrivial, they are much smaller than the speed variations detected in this study when increasing IOP. In addition, eyeball samples showed some evidence of swelling, which may change the mechanical properties and thickness of the cornea and sclera. Our previous research found changes in the wave speed (Δ
c < 0.5 m/s) in porcine corneas after an hour of hydration with PBS.
47 Because our study is measuring larger variation of wave speed (Δ
c > 8 m/s) in eyes exposed at the same level of hydration under variations of IOP, we could consider this effect to be of low impact when ocular tissues are measured individually. Nevertheless, swollen corneas may have a different biomechanical response than physiological hydrated corneas; therefore, results in
Figure 7 could be different under in vivo conditions, which may be explored in our future work. Finally, this whole study has been conducted in ex vivo porcine eyeballs and a direct extrapolation of our results to human eyes may require considering the impact of physiological hydration and collagen fibril distribution in the human anterior segment.