Syneretic response to pressure in bovine and rhesus monkey vitreous

Vitreous substitutes: a comprehensive review

Vitreoretinal disorders constitute a significant portion of treatable ocular disease. Advances in vitreoretinal surgery have included the development and characterization of suitable substitutes for the vitreous. Air, balanced salt solutions, perfluorocarbons, expansile gases, and silicone oil serve integral roles in modern vitreoretinal surgery. Vitreous substitutes vary widely in their properties, serve different clinical functions, and present different shortcomings. Permanent vitreous replacement has been attempted with collagen, hyaluronic acid, hydroxypropylmethylcellulose, and natural hydrogel polymers. None, however, have proven to be clinically viable. A long-term vitreous substitute remains to be found, and recent research suggests promise in the area of synthetic polymers. Here we review the currently available vitreous substitutes, as well those in the experimental phase. We classify these compounds based on their functionality, composition, and properties. We also discuss the clinical use, advantages, and shortcomings of the various substitutes. In addition we define the ideal vitreous substitute and highlight the need for a permanent substitute with long-term viability and compatibility. Finally, we attempt to define the future role of biomaterials research and the various functions they may serve in the area of vitreous substitutes.

In vitro evaluation ofin situ gels as short term vitreous substitutes

Dysfunction of the vitreous humor, present in the posterior cavity of the eye, leads to its detachment from the retina and vision loss. In this study, biopolymers were evaluated as in situ gels for short term vitreous substitution. Biophysical characterization revealed that the viscosity of the vitreous was >4000 cP at a shear rate of 0.15/s and it formed a gel with elastic modulus G' greater than the viscous modulus G''. Biopolymers of gellan and hyaluronic acid (8:2 w/w, 1% concentration) were low viscosity liquids at 37 degrees C and gelation was triggered both by the addition of 0.18 mM CaCl(2) as well as ocular temperature, thus making them feasible as in situ gels. Gelation was confirmed by viscoelastic moduli where G' was greater than G'', similar to the vitreous and unlike that of silicone oil, a common vitreous substitute. The gels had a viscosity >5000 cP at a shear rate of 0.512/s, excellent light transmittance and absence of syneresis. Contact angle studies with water and simulated ocular fluids showed that gellan hyaluronic acid gels had similar wetting properties to that of vitreous with contact angles of 27 degrees +/- 1 degrees , 36.7 degrees +/- 1.6 degrees , and 33.7 degrees +/- 0.5 degrees for water, simulated tear fluid, and simulated aqueous humor, respectively. The results of this study suggest that biopolymers of gellan and hylauronic acid are suitable as in situ gels, have biophysical properties similar to that of the vitreous, and may be promising as alternatives to silicone oil as short-term vitreous substitutes.

Regional mapping of molecular components of human liquid vitreous by dynamic light scattering

The distribution of particle sizes was studied in six regions of human liquid vitreous samples. Particle sizes were calculated from dynamic light scattering measurements as hydrodynamic diameter of molecules. In general, particle sizes increased progressively from anterior (near lens) to posterior (near retina). A much weaker trend of increase in particle size was found from nasal to temporal regions. Relative concentrations of particles of certain sizes were calculated from dynamic light scattering parameters. Viscosity coefficients of liquid vitreous samples were measured in a semi-micro viscometer. Hyaluronan concentrations increased from anterior to posterior and the reverse was true for viscosity, implying that the lower concentration of hyaluronan near the lens was offset by increased molecular size. A similar concentration gradient was found with the 6 nm diameter particles, and to a lesser degree with the 15 nm particles. The identities of these proteins must be resolved by techniques of proteomics.