Victor Kazaikin, MD (Ekateringurg, Russia), Khristo P. Takhchidi, MD, PhD (Moscow, Russia), Gennadiy V. Chashchin, (Ekaterinburg, Russia), Michail B. Matevosyan, (Ekaterinburg, Russia)

#### PURPOSE:

To estimate physical forces most crucial for tamponading properties of silicon oil under static condition of the eyeball.

#### METHODS:

The value of tamponade pressure to the retina was calculated for silicon oil specific gravity equal to 0.94 and 0.97 g/cm3 and vitreous cavity filling with silicon oil by 100% and 98%. The value of superficial tension forces at the retina/silicon oil/aqueous humor border was found experimentally. During calculations it was assumed that vitreous cavity volume was 5 ml and aqueous humor specific gravity was 1.005÷1.01 g/cm3. Tamponade pressure was calculated according to Archimede’s law. Superficial tension forces were calculated using methods of maximal bubble pressure and geometric drop dimensions measurement. The experiment was performed using cadaver eye retina.

#### RESULTS:

Let us consider a physical model of vitreous cavity in which aqueous humor forms a free surface, and assume that SO injected into the vitreous cavity has such a volume that it only contacts the retina at “1” point without any pressure to it. Then maximal possible tamponade force of the SO (Ftamp), that is, in case of 100% tamponade, is equal to aqueous humor

weight in the volume of non-submerged part of the SO bubble (Vnon-sub). Calculations show that Ftamp equals 198 din for SO with specific gravity 0.97 g/cm3 and 346 din for silicon oil with specific gravity 0.94 g/cm3. In case of 98% filling these values drop to 97 and 242 din, respectively.During investigation it was disclosed that superficial tension factor at the

water/SO border (Fwo) equals 29 din/cm. Wetting angle at the water/retina/SO border was 100° for water. In other words, superficial tension forces Fwo is directed away from the retina and tears it off the retinal pigment epithelium.

#### CONCLUSION:

1. A small change of SO specific gravity and the extent of tamponade results in significant change of SO tamponading force. 2. A tearing off force is applied to the retina at the aqueous humor/retina/SO. This force equals 29 din/cm and is directed at 100° to the interface.

3. Retina adhesion force is comparable with hydrostatic pressure and superficial tension forces arising during SO tamponade.