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Jonathan L Grimm, Ning-Jiun Jan, Donald J Brown, Ian A Sigal; Experimental characterization of the nonlinear displacements and deformations of human lamina cribrosa (LC) microstructure during acute increases in IOP. Invest. Ophthalmol. Vis. Sci. 2014;55(13):4254.
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To measure eye-specific displacements and deformations of the human LC microstructure during acute increases in IOP and test the hypothesis that the biomechanical effects of IOP are nonlinear.
Six human eyes from four donors aged 23 to 82 were scanned using second harmonic generated imaging (Zeiss 510 Meta LSM, 1.76 to 4.96 µm/pixel lateral resolution, 2 µm depth resolution) at various levels of IOP between 10 and 50 mmHg (3 pressure steps for 5 eyes and 7 pressure steps for 1 eye). After an IOP change the eyes were allowed to equilibrate for 30 min before imaging. An image registration technique [Sigal et al. IOVS In-Press] was used to find the deformation mapping between maximum intensity projection images at different levels of IOP. The mappings were analyzed to determine the magnitude and distribution of the IOP-induced displacements and deformations. A linear regression was fit to predict the deformations per mmHg as a function of the pressure allowing for box-cox transformations.
For Eye 1 the median LC deformation magnitudes were 0.41, 0.25, 0.17, 0.22, 0.10 and 0.12 μm/mmHg, respectively for pressure differences 10-15, 10-20, 10-35, 10-30, 10-35, and 10-40 mmHg (Figure 1). The other eyes showed a similar pattern of decreasing deformation per mmHg as IOP increases. Deformation per mmHg decreased significantly as IOP change increased (p<0.001, R2=.95).
Previously we reported substantial effects on the LC of acute increases in IOP (10 to 50 mmHg). Here we found ~55% of the effects of IOP between 10 and 20 mmHg, and ~45% between 20-40 mmHg (twice the IOP increase), denoting substantial nonlinearity in LC mechanics. The roles of LC and sclera properties on this nonlinearity remain to be determined. To the best of our knowledge, these are the first measurements of nonlinear LC microstructure response to IOP.
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