A Constant-Force Technique to Measure Corneal Biomechanical Changes after Collagen Cross-Linking

Abstract

PurposeTo introduce a constant-force technique for the analysis of corneal biomechanical changes induced after collagen cross-linking (CXL) that is better adapted to the natural loading in the eye than previous methods.MethodsFor the biomechanical testing, a total of 50 freshly enucleated eyes were obtained and subdivided in groups of 5 eyes each. A Zwicki-Line Testing Machine was used to analyze the strain of 11 mm long and 5 mm wide porcine corneal strips, with and without CXL. Before material testing, the corneal tissues were pre-stressed with 0.02 N until force stabilization. Standard strip extensiometry was performed as control technique. For the constant-force technique, tissue elongation (Δ strain, %) was analyzed for 180 seconds while different constant forces (0.25 N, 0.5 N, 1 N, 5 N) were applied.ResultsUsing a constant force of 0.5 N, we observed a significant difference in Δstrain between 0.26±0.01% in controls and 0.12±0.03% in the CXL-treated group (p = 0.003) over baseline. Similarly, using a constant force of 1 N, Δstrain was 0.31±0.03% in controls and 0.19±0.02% after CXL treatment (p = 0.008). No significant differences were observed between CXL-treated groups and controls with 0.25 N or 5 N constant forces. Standard stress-strain extensiometry failed to show significant differences between CXL-treated groups and controls at all percentages of strains tested.ConclusionWe propose a constant-force technique to measure corneal biomechanics in a more physiologic way. When compared to standard stress-strain extensiometry, the constant-force technique provides less variability and thus reaches significant results with a lower sample number.

Establishing Corneal Cross-Linking With Riboflavin and UV-A in the Mouse Cornea In Vivo: Biomechanical Analysis

“…Twodimensional extensometry 30 as applied in this study resembles the natural condition in a more realistic way than onedimensional extensometry, which has been used for the biomechanical analysis of porcine, rabbit, and human corneas. 6,16 We could confirm the finding of a previous study 17 that viscoelastic testing approaches, such as stress-relaxation or creep tests, are more sensitive to measure the effect of CXL. Although both elastic (stress-strain) and viscoelastic (stressrelaxation) testing showed significant differences between CXL treated and control groups, these differences are distinctly more obvious in the stress-relaxation than in the stress-strain test.…”

“…The golden standard technique consists of measuring the stress-strain curve and determining the Young's modulus. 1,2,6,11,12,16 Viscoelastic testing, including creep and relaxation tests, [17][18][19] has recently emerged as an option. However, a methodologic problem with ex vivo testing is that the tissue is subjected to hydration and other degrading processes soon after enucleation.…”

Assessment of UVA-Riboflavin Corneal Cross-Linking Using Small Amplitude Oscillatory Shear Measurements

“…The oscillatory shear technique does not require any specific modification of the device and is straightforward to perform in conditions of controlled humidity that ensure unchanged properties of the cornea sample over time, since the importance of corneal stroma hydration during measurements for tissue biomechanical properties has been reported in the literature. 39,40 In contrast, strip extensometry requires that cutting of a strip of tissue ensuring accurate cut and equal size 9,12 which, in our experience, can be challenging. It also requires a modification of the strip extensometer to mount the strips of tissue and prevent slippage.…”

“…32 Strip extensometry involves cutting the corneal tissue in equal sized strips, then mounting it on an appropriately modified extensometer 9,12,33 and eventually measuring the Young modulus until the sample breaks up. 34 Although the technique is standardized, it can prove difficult to obtain consistent measurements due to several issues, such as ensuring equal tissue strip sizes to measure and avoiding slippage as the tissue is stretched.…”

“…34 Although the technique is standardized, it can prove difficult to obtain consistent measurements due to several issues, such as ensuring equal tissue strip sizes to measure and avoiding slippage as the tissue is stretched. 12 Furthermore, measurements must be performed quickly enough to avoid dehydration of the corneal sample, which can alter the results of the measurement itself. Hence, two crucial issues are maintaining a constant water content of the sample and the nonlinear deformation imposed, leading to fracture.…”