The Small Saphenous Vein lies in the fatty tissue between the skin and the muscle. It is surrounded by a special envelope of connective tissue called fascia. In this fascia, a nerve runs alongside the small saphenous vein called the Sural nerve. It is this nerve that we have to be very careful with when we operate on or treat the Small Saphenous Vein. It supplies sensation to the outer part of the foot and so when damaged, the outer part of the foot and back of the lower leg (including the heal) can become numb.
The Small Saphenous Vein starts just behind the lateral malleolus which is the lumpy bone on the outside of the ankle. Veins taking blood from the foot feed into the Small Saphenous Vein. The vein then runs up the back of the leg, more or less in the middle, until it gets to the back of the knee. In most people, the vein then dives deep, and joins the deep-vein at the back of the knee called the Popliteal Vein. This is how blood is transmitted from the side of the foot and back of the lower leg into the deep veins where it can be pumped forcibly back to the heart.
The point at which the Small Saphenous Vein dives deep is variable and is usually between 2 cm below and 2 cm above the knee skin crease. Sometimes the Small Saphenous Vein can travel higher in the back of the leg before running deep. Occasionally, it does not join the deep-vein at all but continues up the back of the leg, winding around the inner aspect of the thigh and joining the deep veins in the groin either by itself or by entering the great saphenous vein first. The vein that ascends from the back of the knee and winds around the inner thigh is called the Giacomini Vein. Therefore in these patients the Small Saphenous Vein becomes the Giacomini Vein.
Because this vein is very straight and connects directly with the deep veins, it is called a truncal vein.
The other reason it is called a truncal vein is because it is one of the two original veins that forms in the limb bud of the fetus. When a human grows as a fetus, the leg initially forms as a "limb bud". This starts off by looking like a small flipper before it elongating and becomes a proper leg. At this point, there is one major vein at the lower part of the edge of the limb bud which becomes the Great Saphenous Vein, and one had the lower part of the edge of the limb bud which becomes the Small Saphenous Vein.