Anyone had thrombosis in the leg or know anything about it.

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Someone close to me has had a problem with their knee for the past week and it was diagnoised as torn cartilage in the knee. Yesterday his leg swelled up to a solid mass. He has just called to say the hospital suspect thrombosis and are keeping him in. Can anyone give me details about this and I don't want a medical dictionary reply, I am worried about how serious this is. Thanks in advance.