Corneal Resistance to Shear Force After UVA-Riboflavin Cross-Linking

Abstract

Citation: Søndergaard AP, Ivarsen A, Hjortdal J. Corneal resistance to shear force after UVA-riboflavin cross-linking. Invest Ophthalmol Vis Sci. 2013;54:5059-5069. DOI:10.1167/ iovs.12-10710 PURPOSE. We evaluated whether UVA-riboflavin collagen cross-linking (CXL) increases transverse stromal shear moduli ex vivo, whether the shear moduli are greater in the anterior compared to the posterior stroma, and whether the shear moduli are affected by CXL.METHODS. The resistance to unidirectional transverse shear of human (n ¼ 18) and porcine (n ¼ 42) corneas was measured in a custom engineered biaxial biomechanical setup at different hydrations. The corneas were separated into untreated, riboflavin solution-treated, and CXLtreated groups. The depth dependency of shear moduli within groups was assessed in femtosecond laser cut sheets. Dry weights were obtained for solids correction. RESULTS.In porcine full-thickness buttons and 300 lm anterior sheets, a significantly increased unidirectional transverse shear modulus was detected in riboflavin-treated and CXL-treated groups compared to the respective untreated groups. There was no significant difference in shear modulus between riboflavin-and CXL-treated groups. In all groups, the shear moduli were greater in the anterior sheets compared to posterior sheets. Similar results were detected in human corneas. CONCLUSIONS.A method for unidirectional transverse shear resistance measurement was developed. The shear moduli were greater in the anterior compared to the posterior sheets. Increase in shear moduli was observed in the riboflavin and CXL groups compared to the untreated group, indicating that the immediate effects of the riboflavin or CXL treatment may be due partly to ground substance/dextran-5-phosphate interaction.Keywords: cornea, riboflavin, UVA, cross-linking, CXL, collagen, swelling pressure, shear force, shear modulus, biomechanics T he corneal stroma is a very dense matrix composed of several hundred sheets of highly organized collagen lamellae, stacked on each other preferably in parallel with respect to the corneal surface. Within a given lamella, the majority of fibrils lie parallel to each other and to the corneal surface, but at large angles with those in adjacent lamella. Overall, they are found at all angles within the corneal plane. Anterior and midstromal lamellae bifurcate and interweave, 1 whereas posterior lamellae tend to lie noninterwoven in stacked layers.2 The collagen fibrils in the central cornea have preferred directions in the inferior-superior and nasal-temporal meridians in the posterior two-thirds, whereas the anterior third is arranged more isotropically. 3,4 This microstructure of the corneal stroma suggests that its elasticity will depend on direction during mechanical testing (anisotropy) and that the in-plane properties will be different from the transverse properties.5-7 The cornea shears readily in the tangential plane without separating, even at high shear deformations. 8,9 Thus, the stroma possesses self-cohesion between c...

Impact of Hydration Media on Ex Vivo Corneal Elasticity Measurements

“…Therefore, the restoration of the corneal thickness to the normal physiological thickness range before characterization testing is imperative. Some researchers explicitly make mention of measuring the corneal thickness before characterization testing 20, 22–24, 30–32, 37, 38 but only a few take measures to address corneal thickness restoration before characterization testing 30–32, 38 . Based on the results of this present study, measures should also be taken to address the corneal hydration (and thickness), through the use of hydration media, during measurements as they affect corneal biomechanical properties.…”

“…However, direct measurement of standard mechanical property parameters of the corneal tissue, such as Young’s modulus of elasticity, has only been conducted in the ex vivo experimental setting. Ex-vivo biomechanical testing methods currently applied to measure corneas include tensile stretching (or strip extensiometry) 7–20 , bulge/inflation testing 18, 21–27 , nanoindentation testing (atomic force microscopy, AFM) 28–35 , indentation testing 36 , shear testing 4, 37 , and acoustic radiation force elastic microscopy 38 . Despite the existence of a variety of characterization methods available, the published values derived from such techniques lack reproducibility, evidenced by the large range of reported corneal Young’s modulus of elasticity values in literature (0.57kPa – 41MPa) 7–18, 21–36, 39 .…”

Instrumented indentation for determination of mechanical properties of human cornea after ultraviolet‐A crosslinking

“…Despite the simple approach in our study and the low contact pressures (2 to 4 kPa), the creep generated by the indentation load could be observed and quantified. 43 These changes could explain our observation of lower corneal creep after crosslinking. A possible explanation for this decrease in creep on crosslinked corneas is densification of the extracellular matrix due to the crosslinking.…”

High-irradiance CXL combined with myopic LASIK: flap and residual stroma biomechanical properties studied ex-vivo

“…We provided 10% and 20% strain values for two reasons. First, these are the ones typically reported by other researchers,14 in an effort to provide some form of comparison. In addition, the stress–strain plot had adequate linearity in these areas to enable data collection.…”