Abstract
Purpose :
Corneal tensile tests show a nonlinear stress-strain behavior due to collagen fibril stretching. Such nonlinearity is the cause for observed dependence of corneal mechanical modulus on intraocular pressure (IOP). However, since the cornea is mechanically anisotropic, its mechanical response to IOP is direction dependent and not well characterized. This is particularly relevant for Brillouin microscopy, a highly directional noncontact method to characterize corneal mechanics, which has not reported IOP dependent corneal modulus in the radial direction. Performing gold-standard dynamic mechanical analysis (DMA) and Brillouin microscopy in tangential and radial directions, we show that IOP-induced stress-strain behavior is strongly direction dependent and Brillouin measurements along the tangential direction are IOP dependent.
Methods :
Porcine corneas were placed in an artificial anterior chamber (AAC) where IOP levels were controlled (10, 20, 30 mmHg), radial strain was measured with an ultrasonic pachymeter, and tangential stress was theoretically calculated using Laplace Law. Using these values and a DMA, we performed strain-controlled compression of corneal buttons in the radial direction, and stress-controlled tension of corneal strips in the tangential direction. From stress-strain curves, we extracted IOP-equivalent dependence of corneal Young’s modulus in both directions. Brillouin shift of corneas was mapped while in the AAC to obtain the longitudinal modulus in radial and tangential directions.
Results :
As IOP changes, radial strains are limited to compressions of <5%, where the stress-strain curve is nearly linear, with minimal IOP dependence of the modulus. In the tangential direction, stresses corresponding to IOP values lie in the nonlinear region of the stress-strain curve, with a strong dependence of modulus on IOP. Brillouin measurements are consistent with these findings: in the tangential direction, Brillouin shift showed significant increase with IOP much higher than in the radial direction and in time dependent controls at constant IOP.
Conclusions :
Corneal moduli dependence on IOP is anisotropic: strong in the tangential direction, small in the radial direction. Consistently, Brillouin measurements in the radial direction have shown a minimal IOP-dependence of corneal modulus. Here we demonstrate that in the tangential direction, Brillouin microscopy can measure the expected dependence of modulus with IOP.
This abstract was presented at the 2024 ARVO Annual Meeting, held in Seattle, WA, May 5-9, 2024.