Internal Tension: A Novel Hypothesis Concerning the Mechanical Properties of the Vitreous Humor

Abstract

The vitreous humor is a soft viscoelastic gel composed of a complex network of biomolecules (primarily collagen and polyhyaluronic acid, “hyaluronan”). While extensive progress has been made in identifying the components of the vitreous, lack of sufficient experimental methods has hampered previous efforts to quantitatively define its mechanical properties. To address this issue, we have developed a novel “cleat” tool geometry for dynamic shear rheometry. Using this geometry we find that the shear moduli of vitreous decline by roughly a factor of five to steady‐state values within an hour after removal from the eye. Steady‐state values (Porcine: G′ = 2.6 ± 0.9 Pa, and G″ = 0.65 ± 0.41 Pa, n = 9; Bovine: G′ = 6.5 ± 2.0 Pa, and G″ = 2.0 ± 0.6 Pa, n = 17) are significantly larger than previously reported. The decrease in modulus also correlates with a decrease in mass (65 ± 11 %, n = 8) that occurs spontaneously after the vitreous is extracted. Rheological findings, taken in combination with biochemical analyses suggest that hyaluronan, a ubiquitous polyelectrolyte in connective tissue, contributes to vitreous stiffness by inducing tension in the network in vivo.

A finite element infant eye model to investigate retinal forces in shaken baby syndrome

“…However, Weber and Landwehr consider the vitreous body as part of the system, and as such, state that the material is more rubber-elastic. The non-invasive material determination technique results in stiffer vitreous material properties that are a closer match, to what is described [16,18] to be an infant vitreous, than the other compliant viscoelastic materials [17,56,57].…”

“…The stiffest vitreous material, to approximately estimate an infant vitreous, that is measured and not based on an estimated guess [26][27][28], is from the work of Weber and Landwehr [39,55]. Other vitreous materials have critical issues with how the vitreous is handled, adversely compromising the stiffness, when measuring the viscosity [17]. However, Weber and Landwehr consider the vitreous body as part of the system, and as such, state that the material is more rubber-elastic.…”

“…Further, the vitreous gel is resting on a surgical towel exposed to air with no weeping of fluid indicated. This fluid loss tends to lower the shear modulus of the material [17]. The vitreous posterior to the lens is more firmly attached in infants, and becomes less so with increasing age [18].…”

Rheology of the vitreous gel: Effects of macromolecule organization on the viscoelastic properties

“…Although the results give several important insights into the properties of the vitreous, it is unclear whether the measured local properties report the bulk response. Several other attempts to measure bulk rheological properties of vitreous humor using methods such as shear rheometery (Nickerson et al, 2005a(Nickerson et al, , b, 2008Lee et al, 1992aLee et al, , b, 1994Tokita, 1984;Bettelheim and Wang, 1976) and indirect viscoelastic measurements (Zimmerman, 1980;Walton et al, 2002) exist. While these studies advanced the body of knowledge pertaining to the physical properties of vitreous, they are either incomplete from a rheological standpoint or have tested the fluid under conditions that do not resemble conditions in vivo.…”

Synthesis and characterization of in situ forming anionic hydrogel as vitreous substitutes

“…Therefore, the mechanical properties of a gel could be adjusted by varying the concentration of the polymer and the density of the crosslinker, and thus match the mechanical properties of the natural vitreous. It is promising that the published storage modulus of the porcine and bovine vitreous 24,25 were in the range of our test values.…”