The body is a complicated thing. All is often not what it seems to be. Take pain for example.
Pain itself is a many faceted experience. Its perception can be heightened or dampened by our emotional state. Like other forms of sensation, it travels along our nervous system to reach our brains. This is why spinal injured people have no sensation and no pain. If the message doesn't get through, the pain is not felt.
The system of nerves that carry the messages of pain are also susceptible to mistakes. This is known as referred pain.
Our nerves leave our spine and provide sensation to a specific area of the body. This has been mapped out as displayed in the picture on the right, each area corresponding to the level of the spine the nerves come out of.
The L4 dermatome passes over the front of the knee and the back of the knee is covered by S1 and S2.
When the nerve roots are aggravated as they leave the spine, the body can not determine where the pain has come from. The result is pain felt throughout the area covered by that nerve.
This means that if you had a disc bulge in your spine at L4 on the left, you would feel a band of pain from the outside of your left hip sweeping down over the front of your knee.
Your knee pain could in fact be coming from your back.
Any new leg pain that is associated with back pain, pins and needles, numbness or an increase in pain with coughing or sneezing, should be discussed with your doctor.
A pain in the knee is not always a pain in the knee.