Mechanical behaviour of healthy versus alkali-lesioned corneas by a porcine organ culture model

Abstract

Background: Cornea is a composite tissue exhibiting nonlinear and time-dependent mechanical properties. Corneal ulcers are one of the main pathologies that affect this tissue, disrupting its structural integrity and leading to impaired functions. In this study, uniaxial tensile and stress-relaxation tests are developed to evaluate stress-strain and time-dependent mechanical behaviour of porcine corneas.

Results: The samples are split in two groups: some corneas are analysed in an unaltered state (healthy samples), while others are injured with alkaline solution to create an experimental ulcer (lesioned samples). Furthermore, within each group, corneas are examined in two conditions: few hours after the enucleation (fresh samples) or after 7 days in a specific culture medium for the tissue (cultured samples). Finally, another condition is added: corneas from all the groups undergo or not a cross-linking treatment. In both stress-strain and stress-relaxation tests, a weakening of the tissue is observed due to the imposed conditions (lesion, culture and treatment), represented by a lower stiffness and increased stress-relaxation.

Conclusions: Alkali-induced corneal stromal melting determines changes in the mechanical response that can be related to a damage at microstructural level. The results of the present study represent the basis for the investigation of traditional and innovative corneal therapies.

Keywords: Alkali-induced lesions; Biomechanical behaviour; Porcine cornea; Riboflavin/UV-A corneal phototherapy; Statistical analysis; Tensile tests.

Fig. 1

Results from mechanical tests on corneas samples: equilibrium stress-strain data of healthy (H, red lines) (a) and lesioned (L, black lines) (b) groups, of non-treated (N, continuous and dot lines) (c) and treated (Y, dashed and dashed-dot lines) (d) groups, of fresh (F, continuous and dashed lines) (e) and cultured (C, dashed-dot and dot lines) (f) groups. Mean data are reported for each group; data with associated standard deviation (±SD) are available in Supplementary Materials

Fig. 2

Results from mechanical tests on corneas samples: normalized stress-time data of healthy (H, red lines) (a) and lesioned (L, black lines) (b) groups, of non-treated (N, continuous and dot lines) (c) and treated (Y, dashed and dashed-dot lines) (d) groups, of fresh (F, continuous and dashed lines) (e) and cultured (C, dashed-dot and dot lines) (f) groups. Mean data are reported for each group; data with associated standard deviation (±SD) are available in Supplementary Materials

Fig. 3

Main effects plot representing the significant results of Table 1 for the output stress in tensile tests (a). Interaction plot for the output stress representing the significant results of Table 1 (b)

Fig. 4

Main effects plot representing the significant results of Table 2 for the output stress relaxation (a). Interaction plot for the output force representing the significant results of Table 2 (b)

Fig. 5

Porcine cornea preparation procedure: porcine eyes showing the filter paper placed on the center of the ocular surface (a); application of a plastic ring on the L cornea (b); application of isoosmolar 0.1% riboflavin drops into the plastic ring for 30 min (c); details of UV-A lamp during riboflavin/UV-A corneal phototherapy (d). To obtain the cultured condition, the corneas were isolated from the bulbs and preserved in a specific culture medium for corneal deturgescence (Carry-C^® medium, Alchimia, Padua, Italy) for 7 days in an incubator at 37 °C with 5% CO₂ [24]

Fig. 6

Mechanical tests on corneas rectangular specimens and lateral view of the gripped specimens with glued patches of balsa wood (a). Experimental activities based on the assumed strain history (b) and the measured force-time data (c)