Analysis of the viscoelastic properties of the human cornea using Scheimpflug imaging in inflation experiment of eye globes.
OBJECTIVE
To 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.


METHODS
Six 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.


RESULTS
The 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.


CONCLUSIONS
Cyclic 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.
CNR-IPCF, Unit of Support of Cosenza, Ponte P. Bucci, 87036 Rende, Italy; Vision Engineering Italy S.r.l., Via Adda 7, 00198 Rome, Italy.
Giuseppe
G
Lombardo
Fondazione G.B. Bietti IRCCS, Via Livenza 3, 00198 Rome, Italy.
Sebastiano
S
Serrao
Fondazione G.B. Bietti IRCCS, Via Livenza 3, 00198 Rome, Italy.
Marianna
M
Rosati
Fondazione G.B. Bietti IRCCS, Via Livenza 3, 00198 Rome, Italy.
Marco
M
Lombardo
D000368:Aged; D001696:Biomechanical Phenomena; D003315:Cornea / Q000502:physiology*; D063171:Corneal Pachymetry; D004548:Elasticity; D006801:Humans; D021621:Imaging, Three-Dimensional / Q000379:methods*; D008875:Middle Aged; D015203:Reproducibility of Results; D013314:Stress, Mechanical; D014783:Viscosity