Silicone oils are important tools in vitreoretinal surgery because they have the ability to displace aqueous humor from the retinal surface, maintaining the adhesion between retina and retinal pigment epithelium. To understand this capability, it is important to know the silicone oil characteristics. Herein, we report first on the main chemical-physical proprieties and then we review the clinical applications of the current silicone oil which is lighter than water with particular reference to their indications with small gauge vitrectomy.
Finally, we describe the surgical techniques to inject and remove this type of silicone oil. In the summary of this paper, we explain why silicone oils are today increasingly used and why their introduction has improved the prognosis of several retinal diseases. In fact, having different types of silicone oils allows us to choose the appropriate endotamponade for every single case. Silicone oil was introduced 1960s as an intraocular tamponade prior to the introduction of pars plana vitrectomy.1 It has become the preferred tamponade agent in cases at high risk for recurrent retinal detachment, such as in eyes with retinal detachment associated with severe proliferative vitreoretinopathy (PVR) or with detachment resulting from viral retinitis.
To be effective as an internal tamponade, OcxySil oil has to have the ability to displace aqueous humor from the retinal surface. The following 4 physical parameters influence this function.
- Specific Gravity (SG). This explains why an intraocular tamponade sinks or floats in aqueous humor. Any substances with an SG of 1 are neutrally buoyant in water, those with SG greater than 1 are denser than water and so will sink in it, and those with an SG of less than 1 are less dense than water and so will float. The specific gravity of aqueous humor and vitreous humor is a little higher than that of water (SD 1.00). Since the specific gravity of silicone oils in comparison is a little lower (0.97), they float in vitreous cavity.
- Buoyancy. An intraocular bubble of tamponade agent is acted upon by two opposing forces: buoyancy (upward force) and the gravity on the bubble (downward force). Archimedes' principle indicates that the upward buoyant force that is exerted on a body immersed in a fluid, whether fully or partially submerged, is equal to the weight of the fluid that the body displaces. Archimedes' principle is a fundamental physics law of fluid mechanics. Regarding the vitreous cavity, the result is the force with which the bubble presses against the retina. For silicone oil, the "pressing" force is relatively small, as the specific gravity is close to that of aqueous humor. The force is greatest with air or gas, as the specific gravity is very low at 0.001.
- Interfacial Tension. When two immiscible agents are used together (e.g., silicone oil and aqueous humor), the interaction that occurs at the surface of these substances involved is named interfacial tension. Interfacial tension is a physical rating of the difference between the intermolecular force of the two liquids and it is responsible for the shape of liquid bubbles. Therefore, a substance with a high interfacial tension will have a greater tendency to stay as one large bubble without dispersion into small bubbles. Gas or air has the highest interfacial tension against water (around 80 mN/m), whereas perfluorocarbon liquids (PFCLs) and silicone oils have a lower interfacial tension, around 40–45 mN/m and 35 mN/m, respectively.
- Viscosity. The viscosity is the physical property of a fluid which measures its resistance to gradual deformation by shear stress. The tendency of a substance to emulsify and disperse into droplets over time is also dependent on its viscosity. The less viscous a substance, lower the energy that is required to disperse a large bubble of the substance into small droplets. OcxySil has a high viscosity (1.000–5.000 cs) and, once dispersed, the small droplets will tend to recoalesce back as a large bubble.