Comparative study of corneal strip extensometry and inflation tests

Abstract

Strip extensometry tests are usually considered less reliable than trephinate inflation tests in studying corneal biomechanics. In spite of the evident simplicity of strip extensometry tests, several earlier studies preferred inflation tests in determining the constitutive relationship of the cornea and its other material properties, such as Young's modulus and the hysteresis behaviour. In this research, the deficiencies of the strip tests are discussed and a mathematical procedure presented to take account of these deficiencies when obtaining the corneal material properties. The study also involves testing 10 pairs of porcine corneas using both strip extensometry and trephinate inflation techniques and the results are subjected to mathematical back analysis in order to determine the stress-strain behaviour. The behaviour obtained from the strip extensometry tests and using the new mathematical analysis procedure is shown to match closely the inflation test results.

Figure 1

Strip extensometry specimen with end parts.

Figure 2

Isometric view of a strip specimen.

Figure 3

Orthogonal views of a strip specimen.

Figure 4

Strain distribution on the cross-section of a strip extensometry specimen.

Figure 5

Section across the longitudinal centre line of the strip specimen showing its thickness.

Figure 6

Variation of the specimen's cross-sectional area from a minimum at the centre to a maximum at the ends.

Figure 7

Thickness variation along the length of the strip specimen.

Figure 8

Trephinate inflation test rig.

Figure 9

Strip extensometry test rig.

Figure 10

Selection of pressure-apical rise results of the trephinate inflation tests.

Figure 11

Material constitutive relationship based on inflation test results.

Figure 12

Load–elongation behaviour of a selection of strip tests.

Figure 13

Material constitutive relationship obtained using both inflation and strip tests.

Figure 14

Effect of introducing new mathematical procedures on accuracy of material constitutive relationship obtained using strip tests. (Results (1) are obtained using the commonly adopted equation (2.1); results (2) obtained after considering effect of length variation; results (3) obtained after also considering effect of flattening the initial curvature; results (4) obtained after also considering effect of thickness variation.)