Unlike other vitreous substitutes, silicone oil may remain in the eye almost permanently. Silicone oil is a term used for a group of clear inert hydrophobic polymer compounds based on siloxane chemistry26. The viscosity is determined by the lengths of the polymer, ranging from 1000 to 12 500 cSt. Silicone oil has a density of 0.975 – less than that of water – and thus floats on vitreous fluid. Its refractive index is 1.4035, which is slightly higher than that of the vitreous (1.33). The interfacial tension of silicone oil is high (40 dyn/cm2), but less than that of the interface between gas and water (70 dyn/cm2).

Silicone oil is used when a more permanent tamponade is required which cannot be provided by gas. This may be important when an initially closed retinal break is expected to reopen, particularly in cases of proliferative vitreoretinopathy. In addition, silicone oil is considered for patients who cannot maintain the position required for effective intraocular gas tamponade. A third rational to use silicone oil is to limit postoperative vitreous hemorrhage in diabetic eyes as blood and oil do not mix, and the optical axis is kept clear by the oil, allowing retinal examination and laser photocoagulation if needed.

The complications of the use of silicone oil are mainly related to its migration into the anterior chamber. This may especially occur in pseudophakic patients with zonulolysis and in aphakic eyes, and cause keratopathy if left untreated. Aphakic eyes need an inferior peripheral iridectomy to enable the aqueous to pass from the posterior chamber to the anterior chamber when the silicone oil occludes the pupil27. Aphakic patients should be advised not to stay in supine position to avoid the oil entering the anterior chamber.

Silicone oil causes refractive changes. Phakic patients become more hyperopic when filled with silicone oil because the refractive index of silicone oil is higher that that of vitreous, and a negative lens effect occurs with the concave silicone oil surface at the back of the lens. Aphakic eyes experience a myopic shift, which is caused by the convex anterior surface of silicone oil.

Silicone oil removal is usually performed 3–12 months after filling, depending on a stable looking retina and other factors such as lens status, elevation of intraocular pressure, and development of cataract or epiretinal membranes.