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Hong Chen, Xueliang Pan, Paul A Weber, Jun Liu; Corneal strains during small cyclic IOP fluctuations. Invest. Ophthalmol. Vis. Sci. 2014;55(13):3727.
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To demonstrate the feasibility of using high-resolution ultrasound speckle tracking to measure corneal strains induced by small cyclic fluctuations of the intraocular pressure (IOP).
Fourteen canine globes were tested within 3 hrs postmortem. Whole globes were placed in an eye holder and immersed in 0.9% saline. IOP was monitored with a pressure sensor (P75 low range blood pressure transducer, Harvard Apparatus) via a 27G needle inserted into the anterior chamber. After preconditioning (five cycles of pressurization from 5 to 35 mmHg), IOP was maintained at a baseline of 10 mmHg for 15 mins. Cyclic (0.2 Hz) IOP fluctuations of about 2.5-5.0 mmHg in magnitude were then introduced in the eye using a programmable syringe pump (PHD Ultra, Harvard Apparatus). Ultrasound radiofrequency scans were acquired at each 200 ms interval for five consecutive IOP cycles using a Vevo 660 high-resolution ultrasound system with a 55-MHz probe. An ultrasound speckle tracking technique was used to measure accumulative radial strains for each cycle (Tang & Liu, J Biomech Eng 2012, 134(9)). A stiffness-equivalent parameter, i.e., the change in IOP divided by the peak strain, was proposed to assess the range of the responses in the measured corneas.
Cyclic radial strains were robustly measured in all eyes with a cyclic small change in IOP. Figure 1 shows five cycles of corneal radial strains in phase with the IOP changes that were recorded simultaneously in one eye. In the normal dog corneas, strains appeared somewhat heterogeneous through the thickness and across a 3 mm region of the central cornea (Figure 2). The average peak strains in the tested eyes were 0.33± 0.30% with a maximum of 1.01% and minimum of 0.04%. The stiffness equivalent parameter in the measured normal canine corneas spanned from 0.06 to 1.56 MPa.
This study demonstrated the feasibility of reliable measurements of corneal strains in response to a cyclic small IOP fluctuation. The large range in the peak strains suggested large inter-subject differences in corneal response to similar IOP fluctuations. This approach represents a potential clinical tool for non-invasive evaluation of corneal biomechanics.
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