Optical coherence elastography assessment of corneal viscoelasticity with a modified Rayleigh-Lamb wave model

Abstract

The biomechanical properties of the cornea play a critical role in forming vision. Diseases such as keratoconus can structurally degenerate the cornea causing a pathological loss in visual acuity. UV-A/riboflavin corneal collagen crosslinking (CXL) is a clinically available treatment to stiffen the cornea and restore its healthy shape and function. However, current CXL techniques do not account for pre-existing biomechanical properties of the cornea nor the effects of the CXL treatment itself. In addition to the inherent corneal structure, the intraocular pressure (IOP) can also dramatically affect the measured biomechanical properties of the cornea. In this work, we present the details and development of a modified Rayleigh-Lamb frequency equation model for quantifying corneal biomechanical properties. After comparison with finite element modeling, the model was utilized to quantify the viscoelasticity of in situ porcine corneas in the whole eye-globe configuration before and after CXL based on noncontact optical coherence elastography measurements. Moreover, the viscoelasticity of the untreated and CXL-treated eyes was quantified at various IOPs. The results showed that the stiffness of the cornea increased after CXL and that corneal stiffness is close to linear as a function of IOP. These results show that the modified Rayleigh-Lamb wave model can provide an accurate assessment of corneal viscoelasticity, which could be used for customized CXL therapies.

Optical coherence elastography – OCT at work in tissue biomechanics [Invited]

“…Point dynamic loading is attractive as a source of shear waves, and an attractive noncontact localized method is the micro-air puff developed and applied extensively by the Larin group [45, 57, 63, 81, [97][98][99][100][101][102][103][104][105][106][107][108][109][110][111][112][113][114][115][116], mainly to the cornea, but also to skin [108,116], cardiac and skeletal muscles [104], cartilage [101,106], fat and soft tissue tumors [97], and kidneys [112]. Similar to ARF-based excitation, an air puff can deliver a localized and well-controlled impulse stimulus, which can be accompanied by measurement of the tissue displacement or measurement of the air puff-induced elastic wave.…”

“…As well as recognizing that viscosity is not accounted for in the simple approach [181], boundary conditions must be carefully considered in the conversion of elastic wave speed into quantitative values of tissue viscoelasticity. For example, finite-element modelling (FEM) supported by experimental data showed that taking account of the stress at the tissue surface, e.g., imparted by a fluid, significantly reduced the group velocity of the elastic wave and, thus, could produce errors in quantification of elastic properties [109,111]. Geometrical factors, such as tissue curvature and thickness, also have profound effects on the measured elastic wave speed [105].…”

“…Spectral decomposition can provide dispersion curves which could be used to extract both elastic and viscous properties of the sample [45,57,109,110]. Such quantification requires application of either time-consuming FEM, or other analytical models, which are all constrained by specific assumptions [45], but may provide a more accurate and complete biomechanical assessment.…”

“…), or attempting to exploit the time dependence to extract more information. However, such studies of viscoelasticity in OCE are in their infancy [45,109,110,150]. At the same time, as described in Section 3, soft tissue responds in a nonlinear fashion to stress and the tangent modulus depends on stress, or equivalently, strain.…”

“…Although viscoelasticity has been discussed in Section 6 [150] and Section 7 [45,64,[109][110][111], it is has featured necessarily little in this review, because there is little work explicitly focusing on the viscosity of soft tissue [150], although that is not the case for the wider field of optical elastography [182,199].…”