Mechanical Properties of the Internal Limiting Membrane and Intraoperative Utility of Brilliant Blue g (Bbg) and Indocyanine Green (Icg) Assisted Chromovitrectomy

The human retina is a light-sensitive tissue lining the inner surface of the posterior segment of the eye. It is a complex, layered structure containing the photoreceptors and several layers of neurons. Proper retinal function requires a smooth interface between the retina and the adjoining vitreous body (vitreous), a gelatinous and transparent substance occupying the cavity of the posterior ocular segment. The innermost of the retinal layers, a basement membrane called Inner limiting membrane (ILM), represents the boundary between the retina and the vitreous. Excessive contact between the ILM and the vitreous leads to vitreoretinal traction and represents a common cause of ocular pathology: The vitreous adheres to the ILM and shearing forces are conveyed to the retina. Vitreoretinal traction concentrates around the Macula lutea, the small area in the center of the retina which is responsible for central vision. It may be associated with significant visual disturbance as it creates retinal folds, provokes retinal edema and epiretinal metaplastic membranes through the liberation of inflammatory messenger substances and may ultimately result in the formation of macular holes. Macular holes tend to lead to sudden and often complete loss of central vision.

Removal of the vitreous (vitrectomy) may improve vitreoretinal traction, but a complete relief of traction is only achieved if the ILM is removed from the retina in the area around the macula: At the end of vitrectomy, the surgeon grasps the ILM with a fine forceps and carefully peels it off the underlying retinal layers. This procedure is extremely delicate, as the ILM is transparent, extremely thin and in direct contact with highly vulnerable retinal structures. Vital dyes have been employed to make the ILM more visible and because some dyes have been described to improve "grip" of the ILM during its extraction. The most commonly used dye, Indocyanine green (ICG) is not approved for intravitreal use and a discussion on possible toxic side effects is ongoing. The approved alternative substance Brilliant blue G is employed only by a minority of vitreoretinal surgeons. It is our hypothesis, that the popularity of ICG is due to superior staining capacity and a stiffening effect which may make ILM removal easier for the surgeon.

The first goal of this interdisciplinary and translational project, integrating Medical physics, Biomedical engineering, Nanosciences, Biochemistry, Neurobiology, Medical Image analysis and Clinical ophthalmology is to assess the biophysical properties of the ILM and the possible effects of existing dyes in terms of staining behavior and their influence on ILM "grip". In a second step, the project will analyze how new application protocols and the introduction of novel components to the molecular structure of vital dyes can improve staining as well as "ILM-grip" while guaranteeing favorable toxicity profiles Hypotheses

1. A novel "Heavy BBG" (BBG D2O) stains the ILM better than conventional BBG. A replacement of part of the water molecules with Deuterium Oxide (D20) in the BBG solvent increases the dye's specific gravity. After injection into the vitreous, this new preparation (BBG D2O) would collect on the retinal surface, increasing local concentration and retinal exposure. We hypothesize that this alteration of the BBG molecular structure could improve staining properties without compromising its favorable toxicity profile. This new BBG has already been introduced by the manufacturer, but its intraoperative usefulness has not been objectively examined.
2. The use of intraoperative light filters improves the recognizability of the contrasts generated by vital dyes. Most endoillumination lighting systems are equipped with light filters, originally intended to reduce intraoperative light toxicity. Anecdotal reports by numerous surgeons indicate, however, that the use of some filters improves the recognizability of the stained ILM. The green filter is regarded as particularly useful for this purpose. A systematic analysis of the effects of light filters on the usefulness of intravitreal dyes has not been undertaken to date.
3. ICG and TB's photochemical properties improve "ILM grip" through an ILM cross-linking effect resulting in an alteration of ILM material properties. We expect increased intra-operative ILM compressive and tensile stiffness and reduced ILM thickness in Atomic force microscopy examinations, explaining why many surgeons describe facilitated intraoperative manipulation and extraction of the ILM. BBG is not known to dispose of photochemical properties and should not influence ILM material properties.
4. Novel Modified ICG preparation stains equally well but is less toxic An alteration of the molecular structure in a way such that photochemical properties of the substance would be largely eliminated would strongly reduce oxidative stress and retinal toxicity. Synthetization of such a preparation is being prepared by our group. Staining properties are different from the original preparation in that the absorption maximum is shifted towards shorter wavelengths and the staining effect is bluish rather than green. The substance's affinity to the ILM its staining strength and its toxicity have not been studied, to date.