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David Tabibian, Olivier Richoz, Eberhard Spoerl, Arthur Hammer, Florence Hoogewoud, Farhad Hafezi; A new constant-force technique to measure corneal biomechanical changes after corneal collagen cross-linking. Invest. Ophthalmol. Vis. Sci. 2013;54(15):5265.
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© ARVO (1962-2015); The Authors (2016-present)
Corneal collagen cross-linking (CXL) leads to an increase in biomechanical stiffness, as measured by stress-strain experiments in laboratory settings. Stress-strain measurements, as currently performed, analyze the force necessary to produce a progressive corneal elongation. They were directly adopted from inorganic material testing, using non-physiological values, which do not allow for accurate assessment of physiological and pathological changes in a biological tissue. Here, we propose a new measurement method using a low and constant-force testing that is more accurate for the analysis of biomechanical modifications of the cornea.
Stress-strain measurements were performed using an extensometer (Zwick Roell, Model ZO.05, Zwick GmbH&Co. Ulm, Germany) on porcine corneal strips of 12 mm length, 5 mm width treated with standard CXL. Untreated strips served as controls. Standard technique: following a stabilization time of 10 seconds with a pre-charge of 0.02 N, a progressive elongation with constant speed of 1mm/min was used to analyze Young’s module at 2% 4% 6% 8% 10%. New technique: after the same 10 s stabilization time with 0.02 N, tissue elongation was analyzed under various constant forces (0.25 N, 0.5 N, 1 N, 5 N) rather than constant speed.
Standard technique: none of the elasticity modules reached statistical significance between the two groups. The elasticity module at 6% of strain was the most reproducible with a 1.01±0.06 MPa for the non-CXL group and 1.5±0.21 MPa for the CXL group (P=0.06). The elasticity module at 2% of strain was the least reproducible between the two groups, 0.26±0.05 MPa for non- CXL group and 0.29±0.09 MPa (P=0.18) for CXL group (P=0.78). New technique: with 0.5 N, an elongation of 0.26±0.01 % in the controls and of 0.12±0.03 % in the CXL group (P=0.003) was observed. For the 1 N group, elongation was 0.31±0.03 % in controls and 0.19±0.02 % after CXL (P=0.008). The 0.25 N group was the least reproducible and without significant difference between controls 0.17±0.04% (P=0.22) and the CXL group 0.26±0.06 %
Biomechanical stress-strain measurements using low and constant forces work at a more physiological level than standard elongation measurements. The level of accuracy is higher, thus less corneas are needed to reach significance. We propose this technique as new standard for measuring corneal biomechanics ex vivo.
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