The effect of enzymatic GAG degradation on transverse shear properties of porcine cornea.

The structural integrity of cornea depends on properties of its extracellular matrix, mainly a mixture of collagen fibers and soluble proteoglycans (PGs). PGs are macromolecules of negatively charged sulphated glycosaminoglycans (GAGs) covalently attached to a protein core. GAGs appear as bridges between adjacent collagen fibers and could facilitate force transfer between them. Furthermore, GAGs are responsible for corneal hydration by attracting and maintaining water molecules into the extracellular matrix. Based on these observations, GAGs are expected to be essential for biomechanical properties of cornea. The primary objective of the present study was to determine the effects of GAGs on shear properties of cornea. For this purpose, GAGs were enzymatically removed from porcine corneal disks by keratanase II enzyme. After confirming the successful removal of GAGs by histochemical methods, torsional rheometry was performed to characterize the shear stiffness of GAG-depleted samples as a function of axial strain. It was found that the shear modulus of all samples was a function of applied shear strain and compressive strain. Beyond the range of linear viscoelastic response, the average complex shear modulus decreased with increasing the shear strain. Increasing the axial strain from 0% to 40% significantly increased the average complex shear modulus of corneal disks in all groups. Finally, the enzyme treatment with keratanase II enzyme significantly decreased the shear stiffness. The experimental measurements were discussed in terms of microstructural and compositional properties of corneal extracellular matrix and it was concluded that GAGs play a significant role in defining shear properties of cornea.