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Jun Liu, Elias Pavlatos, Xueliang Pan, Keyton Clayson; Influence of physiological parameters on corneal deformation in response to ocular pulse. Invest. Ophthalmol. Vis. Sci. 2017;58(8):4334.
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© ARVO (1962-2015); The Authors (2016-present)
To measure corneal deformation during ocular pulse cycles at different ocular pulse amplitudes (OPA), heart rate (HR), and baseline intraocular pressure (IOP) levels.
Ten porcine globes were tested within 72 hours postmortem. Globes were immersed in 0.9% saline during tests. Two 20G needles were inserted into the anterior chamber, one connected to a syringe pump (PHD Ultra, Harvard Apparatus) to control IOP and the other to a pressure sensor (P75, Harvard Apparatus) to measure IOP. Oscillatory IOP changes were produced to simulate the ocular pulse, and ultrasound scans of the central 5.7 mm cornea in the nasal-temporal direction were obtained at a frame rate of 128 Hz (Vevo 2100, VisualSonics). In each eye, the ocular pulse parameters were varied separately from the nominal condition: OPA = 1 or 5 vs 3 mmHg, HR = 45 or 100 vs 72 BPM, baseline IOP = 10 or 20 vs 15 mmHg. Corneal through-thickness strains were calculated using an ultrasound speckle tracking algorithm (Tang & Liu, J Biomech Eng 2012, 134(9)). The responses under different testing conditions were compared using linear mixed models with repeated measures.
Ocular pulse induced cyclic compression of the cornea in phase to pressure change (Fig 1a). The slope of the compressive strain over pressure, representing corneal compliance, was estimated using linear regression of data from five ocular pulse cycles (Fig 1b). The strain/pressure slope was -0.038 ± 0.009 for the nominal condition. OPA and heart rate had a minimal influence on corneal compliance (Fig 2), while baseline IOP had a significant influence (-0.109 ± 0.043, at 10 mmHg, p<0.01; -0.024 ± 0.006 at 20 mmHg, p<0.01).
High frequency ultrasound speckle tracking can reliably measure small corneal deformation in response to the ocular pulse, and represents a potential clinical tool for measuring corneal stiffness in vivo. Changes in OPA and heart rate had a minimal influence on the strain/pressure slope, while baseline IOP had a significant effect due to the nonlinear mechanical behavior of the cornea. Future work will seek to establish a method to derive corneal properties independent of baseline IOP.
This is an abstract that was submitted for the 2017 ARVO Annual Meeting, held in Baltimore, MD, May 7-11, 2017.
Fig 1. (a) Ocular pulse cycles and resultant cyclic compressive strains; and (b) strain/IOP slope for one cornea.
Fig 2. Average strain/IOP slope under each testing condition (n=10).
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