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M. R. Ford, A. S. Roy, W. J. Dupps, Jr.; Elastographic Measurement of the Corneal Stretch Ratio. Invest. Ophthalmol. Vis. Sci. 2008;49(13):667.
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To measure stretch ratio in the corneal stroma with an optical coherence tomography (OCT)-based biomechanical tool and a clinically feasible stress perturbation.
A whole rabbit eye was mounted beneath a high resolution custom fourier domain OCT scanner. The eye was infused to 11mmHg through an in-line pressure transducer and 21 Gauge needle. A glass microscope slide attached to a mechanical micrometer stage was used to compress the cornea while OCT images were acquired at a rate of 20,000 A-lines per second in 3-dimensions with oversampling (fig 1.A). Captured images (fig 1.B) were then processed to extract displacement information from corneal speckle using a normalized cross correlation coefficient algorithm. The stretch ratio for each pixel was calculated from the displacement data. The stretch ratio, lambda, defined as =lz/l0z, where z is the direction of the stress, l0 is the initial length, and l is the final length. The lengths are defined relative to the estimated center of curvature of the posterior corneal surface.
The figure shows the distinct stretch ratio patterns with heterogeneity throughout the corneal stroma. The vertical stretch ratio (fig 1.C) indicates larger stretch ratios in the posterior stroma with values less than 1 due to applanation (decreasing distance from the reference point).
This OCT-based elastographic technique provides a non-destructive modality for measuring differences in biomechanical properties throughout the corneal stroma under clinically feasible stress conditions. The ability to detect spatial differences in the corneal stretch ratio may be important in screening for preclinical corneal ectatic disease and predicting the biomechanical response to incisional and photoablative corneal surgeries.
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