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C.S. Nickerson, H.L. Karageozian, J. Park, J.A. Kornfield; The Mechanical Properties of the Vitreous Humor . Invest. Ophthalmol. Vis. Sci. 2004;45(13):37.
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Purpose: Understanding the physical structure and properties of the vitreous is of fundamental and therapeutic interest, providing insight into transport of molecules to the retina and defining the ideal properties of vitreous replacement materials. However, the mechanical properties of the vitreous humor remain elusive despite attempts to characterize them due to the lubricating properties of its constituent molecules and its fragile network structure. We have overcome these difficulties using a novel "cleat" tool geometry for dynamic shear rheometry which suppresses slip and permits sample loading with minimal disturbance of tissue structure and properties. Methods: We have determined the mechanical properties of vitreous samples from fresh porcine, ovine, and bovine eyes. 2.0–3.0g samples were gently removed from freshly enucleated eyes and loaded immediately on a Rheometrics RFS–II fluids rheometer which had been modified to overcome wall slip by adding rows of cleats to a parallel–plate geometry. Shear moduli were monitored as samples were subjected to oscillatory strain (3%) at a fixed frequency (10 rad/sec) in a hydrated atmosphere at 20°C. Results: The elastic character of the tissue is manifested in the storage modulus, G', and the viscous character in the loss modulus, G". Ultimate, steady values of the storage and loss moduli of fresh pig vitreous are, on average, G'= 21.7 Pa +/– 10, and G"= 4.5 Pa +/– 3, respectively. The average values are significantly higher in bovine specimens (G' = 47 +/– 19 Pa, and G"=11 Pa +/– 4, respectively), while ovine eyes appear to have properties intermediate between these two. Shear moduli of samples taken from the anterior portion of the vitreous are as much as x3 times higher than those taken from the posterior portion in all 3 species. Initial human data will also be presented. One additional observation was that all vitreous samples show signs of recovery of weak molecular ordering. Viable explanations for this behavior in terms of the physical association of constituents in the vitreous will be discussed. Conclusions: We have accurately determined the mechanical properties of the vitreous humor in absolute rheological units and quantified mechanical differences in the anterior and posterior regions of the vitreous. The values obtained are higher than those previously reported and quantify interspecies differences. These values can be used in polymer diffusion and network models to gain insight into the molecular structure and permeability of the vitreous humor and can also be used as target values for synthetic vitreous substitutes.
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