An Inverse Method to Determine Mechanical Parameters of Porcine Vitreous Bodies Based on the Indentation Test

Abstract

The vitreous body keeps the lens and retina in place and protects these tissues from physical insults. Existing studies have reported that the mechanical properties of vitreous body varied after liquefaction, suggesting mechanical properties could be effective parameters to identify vitreous liquefaction process. Thus, in this work, we aimed to propose a method to determine the mechanical properties of vitreous bodies. Fresh porcine eyes were divided into three groups, including the untreated group, the 24 h liquefaction group and the 48 h liquefaction group, which was injected collagenase and then kept for 24 h or 48 h. The indentation tests were carried out on the vitreous body in its natural location while the posterior segment of the eye was fixed in the container. A finite element model of a specimen undertaking indentation was constructed to simulate the indentation test with surface tension of vitreous body considered. Using the inverse method, the mechanical parameters of the vitreous body and the surface tension coefficient were determined. For the same parameter, values were highest in the untreated group, followed by the 24 h liquefaction group and the lowest in the 48 h liquefaction group. For C₁₀ in the neo-Hookean model, the significant differences were found between the untreated group and liquefaction groups. This work quantified vitreous body mechanical properties successfully using inverse method, which provides a new method for identifying vitreous liquefactions related studies.

Keywords: indentation test; inverse method; liquefaction; mechanical properties; vitreous body.

Figure 1

The overall experimental setup.

Figure 2

The effect of indentation speeds on the indentation load–depth curves.

Figure 3

The finite element model. (a) The whole model; (b) meshes of the whole model; (c) meshes in the contact area between the indenter and the upper surface of the vitreous body.

Figure 4

Process for determining the mechanical properties.

Figure 5

Average results of the indentation tests from different groups. The solid lines represent the averaged results, while the light−colored areas represent for corresponding standard deviation ranges.

Figure 6

Comparison of experimental results and simulated results of the indentation loads using the identified mechanical properties of different groups. (a) Untreated group; (b) 24 h liquefaction group; (c) 48 h liquefaction group.

Figure 7

Results of identified mechanical parameters in different groups. (a) C₁₀ of neo−Hookean model; (b) D₁ of neo−Hookean model; (c) the surface tension coefficient α. * means p < 0.05, while ** means p < 0.01.