An intricate series of nerves runs from the spinal cord past the back and hips to the knees. Occasionally, a pain impulse from any of the higher-residing structures, such as a pinched nerve, herniated disc, arthritis of the hip, or blood clot in the leg, may be carried to the knee. As a result, the patient feels a phantom pain in the knee for a problem that exists outside of the knee. Thus, the pain impulse is "referred" to the knee.Causes The obturator nerve, which runs down the leg, has three branches: one that goes to the hip, one that goes to the inside of the knee, and one that attaches on the outside edge of the joint. If a pain stimulus from one end of the nerve is misdirected, the pain is felt elsewhere. Pinched nerves, blood clots, herniated discs in the back, Perthes Disease, osteoarthritis, or other hip problems may trigger referred pain.Risk Factors No study has concluded that any group is more at risk of referred pain than any other. Those affected with some forms of arthritis, including osteoarthritis of the hips, rheumatoid arthritis, and gout, seem to be at a higher risk of referred pain.
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