All veins in the leg - superficial, perforator and deep - have valves. These valves are composed of two leaflets and allow blood to flow in only one direction to prevent it from falling back (refluxing) into the leg after the leg muscles have helped to propel the blood toward the heart. If these valves become damaged and fail to function properly (become incompetent), blood can flow down the veins (in the wrong direction) and puddle in the lower leg. This leads to vein enlargement (varicose veins), pain, leg swelling, hyperpigmentation (skin discoloration), and even skin ulcers (breakdown of the skin), generally in the part of the leg around the ankle. Superficial and perforator vein incompetence can often be improved by removing the affected veins. The deep veins, however, are generally essential to allow blood to get out of the leg, and so some other approach must be considered.

The valves in the deep veins can become incompetent because of vein wall laxity (the vein diameter becomes so dilated that the two leaflets of the valve lying within the vein cannot meet properly), elongation and laxity of the valve leaflets themselves, or direct destruction of the valves such as can occur when a patient is afflicted with deep vein thrombosis (clots within the veins which lead to scarring and destruction of the valves). The first two causes leave the valve structure intact and are considered primary venous insufficiency. When direct destruction of the valve results from a damaging event (such as an injury), secondary venous insufficiency is present.