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Research Papers

Mechanical Characterization of Porcine Corneas

[+] Author and Article Information
F. Boschetti

Laboratory of Biological Structure Mechanics, Dipartimento di Ingegneria Strutturale, Politecnico di Milano, Piazza Leonardo da Vinci 32, Milano, Italy, 20133;  IRCCS Istituto Ortopedico Galeazzi, Milano, Italy, 20161

V. Triacca

 Institute of Bioengineering, École Polytechnique Fédérale de Lausanne, Lausanne, Switzerland 1015

L. Spinelli

SKE S.r.l., via Durando 38/A, Milano, Italy, 20161

A. Pandolfi1

Dipartimento di Ingegneria Strutturale, Politecnico di Milano, Piazza Leonardo da Vinci 32, Milano, Italy, 20133pandolfi@stru.polimi.it

Cycle/degree: inverse tangent of the ratio between the distance of two parallel lines and the maximum distance at which an eye can perceive the two lines as distinct.

1

Corresponding author.

J Biomech Eng 134(3), 031003 (Mar 23, 2012) (9 pages) doi:10.1115/1.4006089 History: Received August 14, 2011; Revised February 05, 2012; Posted February 21, 2012; Published March 21, 2012; Online March 23, 2012

An experimental program has been carried out in order to investigate the mechanical behavior of porcine corneas. We report the results of inflation tests on the whole cornea and uniaxial tests on excised corneal strips, performed on 51 fresh porcine eyes. Uniaxial tests have been performed on specimens cut from previously inflated corneas. The cornea behavior is characterized by means of elastic stiffness, measured on both average pressure-apex displacement and average uniaxial stress-strain curves; and by means of transversal contraction coefficient, peak stress, and failure stress measured on uniaxial stress-strain curves. Uniaxial tests performed on excised strips allowed to measure the anisotropy in the corneal stiffness and to compare the stiffness of the cornea with the one of the sclera. Viscous properties of the cornea have been obtained through uniaxial relaxation curves on excised corneal strips. The relevant geometrical parameters have been measured and, with the aid of the elastic thin shell theory, a stress-strain curve has been derived from the average inflation test data and compared with similar data available in the literature. The experimental system has been developed in view of future applications to the mechanical testing of both porcine and human corneas.

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Copyright © 2012 by American Society of Mechanical Engineers
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References

Figures

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Figure 1

Top (left) and bottom (right) geometry of the mechanical clamp designed for holding the edge of the corneal specimens

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Figure 2

Example of a corneal profile image as acquired form the inflation test instrumentation

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Figure 3

(a) Geometry of a generic porcine cornea and meaning of the parameters used in the description of the surfaces. Location of the three basis points A, B, and C for the construction of the interpolating circumference. (b) Schematic of the circular thin shell cup used in the analytical calculations.

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Figure 4

Testing equipment for uniaxial tests on porcine corneas

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Figure 5

Images of the specimens arranged for the evaluation of the contraction index

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Figure 6

Average pressure-apex displacement curve as obtained from 51 tests on porcine corneas. Horizontal bars show the experimental standard deviation in the measured apical rise.

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Figure 10

Average nasal-temporal and superior-inferior uniaxial stress-strain curves for 47 specimens of porcine cornea, n = 22 SI and n = 25 NT

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Figure 11

Example of relaxation curve. The sudden increase of the stress corresponds to the application of a deformation increment.

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Figure 12

Average uniaxial stress-strain curve for 18 sclera samples, compared to the average curve for the 47 cornea samples

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Figure 13

Average slopes K1 and K2 of the two distinct regions of behavior for the porcine cornea material in inflation tests

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Figure 14

Stress-strain curve as obtained from the thin shell theory and the average (n = 16) experimental measurements (see Appendix). Stress in MPa. Calculations were done for three different thicknesses, to cover the experimental range.

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Figure 7

Average loading, up to 40 MPa, and unloading curves for inflation tests, showing 25% of hysteresis

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Figure 8

Average uniaxial stress-strain curve for 47 specimens of porcine corneal strips

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Figure 9

Example of uniaxial stress-strain curve up to failure

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