The action of silicone oil is due especially to two of its properties: density and surface tension. Its density is less than 1: for this reason, the silicone oil bubble always presses upon the upper part of the eye, i.e. most of the time at twelve o'clock; so there is a constant support upwards which closes the tears and reattaches the retina. The action of silicone oil is different on the lower part of the eye, even if the cavity is filled with silicone oil: a buckle is then required. The surface tension being weak, a vitrectomy is necessary to obtain a single bubble which gives a higher pressure than several small bubbles. The cumulative effect of the density below 1 and the low surface tension explains that the recurrences always occur in the lower part of the retina. Retinotomies or retinectomies are presently trying to counteract this ineffectiveness of silicone oil.