Abstract
PurposeTo demonstrate a Scheimpflug-based imaging procedure for investigating the depth- and time-dependent strain response of the human cornea to inflation testing of whole eye globes.MethodsSix specimens, three of which with intact corneal epithelium, were mounted in a customized apparatus within a humidity and temperature-monitored wet chamber. Each specimen was subjected to two mechanical tests in order to measure corneal strain resulting from application of cyclic (cyclic regimen) and constant (creep regimen) stress by changing the intra-ocular pressure (IOP) within physiological ranges (18–42 mmHg). Corneal shape changes were analyzed as a function of IOP and both corneal stress-strain curves and creep curves were generated.ResultsThe procedure was highly accurate and repeatable. Upon cyclic stress application, a biomechanical corneal elasticity gradient was found in the front-back direction. The average Young's modulus of the anterior cornea ranged between 2.28±0.87 MPa and 3.30±0.90 MPa in specimens with and without intact epithelium (P = 0.05) respectively. The Young's modulus of the posterior cornea was on average 0.21±0.09 MPa and 0.17±0.06 MPa (P>0.05) respectively. The time-dependent strain response of the cornea to creep testing was quantified by fitting data to a modified Zener model for extracting both the relaxation time and compliance function.ConclusionCyclic and creep mechanical tests are valuable for investigating the strain response of the intact human cornea within physiological IOP ranges, providing meaningful results that can be translated to clinic. The presence of epithelium influences the results of anterior corneal shape changes when monitoring deformation via Scheimpflug imaging in inflation experiments of whole eye globes.
Highlights
The corneal tissue is the main refractive element of the human eye
Cyclic and creep mechanical tests are valuable for investigating the strain response of the intact human cornea within physiological intraocular pressure (IOP) ranges, providing meaningful results that can be translated to clinic
Modeling the corneal topography changes caused by IOP loading and unloading of the whole eye globe, as done via inflation testing, represents one of the most appropriate procedures to investigate ex vivo the biomechanical properties of the corneal tissue
Summary
The corneal tissue is the main refractive element of the human eye. Understanding the corneal shape and thickness and their relation with the tissue’s mechanical properties is important to refractive or therapeutic corneal treatments. The mechanical properties of the intact tissue are fundamentally related to the anterior and posterior corneal shapes. Mapping corneal deformation non-invasively in response to variable intraocular pressure (IOP) changes can be highly valuable to understand the viscoelastic properties of the intact tissue in situ. In this context, corneal topographers permit rapid and detailed characterization of the cornea. The use of the intact eye and measure of corneal shape changes by corneal topography provide closer situation to understand the natural behaviour of the cornea that can be readily translated to clinical practice [1,2]. Different mathematical models [3,4,5,6,7] have been proposed to fit data measured by computerized topographers, which included the use of surface models like sphere, conic or biconic, with the last model providing most of the basic optical properties of the cornea
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