What Causes Unexplained Pain In Leg?

without warning my husband had severe pain in his leg just below the knee...no injury....just out of the blue there came pain so severe he said it felt like someone had hit him with an ax......he is afraid to walk on it because he feels he may fall.....he is 61 and no known health problems.....sorry but we have no money for pay for your service as I am on disability and he is retired and on a fixed income, but thank you anyway