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Hugh J Morris, Yi Juin Liew, Benjamin Cruz Perez, Xueliang Pan, Paul A Weber, Jun Liu; Effects of Corneal Edema on Tonometry Measurements and Corneal Biomechanical Properties as Evaluated by Inflation Testing and Uniaxial Rheometry. Invest. Ophthalmol. Vis. Sci. 2014;55(13):3728.
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© ARVO (1962-2015); The Authors (2016-present)
To examine the effect of corneal edema on the biomechanical properties of the cornea and tonometry measurements of intraocular pressure (IOP).
Three groups of paired canine eyes were obtained immediately after euthanasia. For the first two groups, one globe from each pair was randomly chosen for immediate testing of the fresh cornea and the other eye was immersed in PBS overnight at 4 degC to induce corneal swelling. For group 1 (12 pairs), dynamic viscoelastic testing was performed on the nasal-temporal strips of the corneas. For group 2 (7 pairs), inflation testing was performed on whole globes pressurized from 5 to 25 mmHg and the through-thickness strains were measured using ultrasound speckle tracking (Tang & Liu, J Biomech Eng 2012, 134(9)). For group 3 (4 pairs), Goldmann Applanation Tonometry (GAT) and Tonopen measurements were performed before and after swelling at true IOPs of 10, 15, 20, 30 and 40 mmHg. Paired t-tests were used to compare the complex modulus and tan(delta) obtained from uniaxial testing, as well as the radial strains in the inflation tests. Linear mixed models were used to evaluate whether tonometric readings were changed after corneal swelling.
Uniaxial rheometry of corneal strips revealed a significant decrease in complex modulus after swelling (4.95±1.42 MPa vs. 2.53±0.70 MPa, p<0.001), but no changes in tan(delta) (0.139±0.017 vs. 0.136±0.011, p=0.51). The average radial strains in the central layer of the cornea at 15 mmHg were significantly reduced in swollen corneas (2.7±0.9% vs. 1.4±0.6%, p=0.008). GAT readings were 10.9±2.4 and 17.7±2.9 mmHg in fresh corneas and 13.2±1.4 and 21.5±2.8 mmHg after swelling at true IOPs of 20 and 30 mmHg. Tonopen readings showed similar changes. Linear mixed models showed a significant increase in tonometry readings after corneal swelling (p<0.001).
The results suggest that corneal hydration is an important parameter to control during corneal mechanical testing. In addition, caution should be used when interpreting tonometry readings in eyes with significant edema. While uniaxial testing on corneal strips revealed a reduction in modulus, when the cornea was left intact, smaller strains during inflation tests and increased tonometry readings both indicated a stiffer response. Future studies are needed to explain the mechanisms of such changes.
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