A swollen lower limb is often due to deep vein thrombosis or lymphedema, according to the National Center for Biotechnology Information. Lymphedema occurs when the body's lymph drainage system fails to function properly.
Thrombosis can develop due to the slow flowing of blood, or venous stasis, around the body's venous valve sinuses, according to the NCBI. This occurs between or within the superficial or deep leg veins, and results in a spreading clot that can cause valve damage, venous obstruction and possible thromboembolism. Other causes of swelling below the knee include a torn gastrocnemius muscle, acute arterial ischemia, superficial thrombophlebitis, arthritis and cellulitis.