Hello and Welcome to ‘Ask A Doctor’ service. I have reviewed your query and here is my advice. The dent that can be felt behind the knee might be related to some different conditions. The most common cause of it is Backer s cyst. Backe s cyst is caused from the collection of the fluid that normally is within joint s spaces, that bulges out to the back of the knee joint and can be felt as a dent or lump. The lump felt may be painful or it may be asymptomatic. Other possible causes might be bleeding, blood clot in the deep veins of the legs, tumor of the skin, bone or soft tissue. In my opinion you should consult a surgeon, because to put the right diagnosis it is needed a more detailed history of your complaints, physical examination and to get ultrasonography done for it. Hope I have answered your query. Let me know if I can assist you further.