Knee pain down the outside of their leg?

May be from back: Pain that follows this pattern may be coming from the spine, especially if it radiates bellow the knee. Your doctor will need more history and a physical exam to know where the pain is coming from for sure. There are other things that can cause this pattern of pain as well.

Many causes : Dr. Shahbandar is correct. When describing pain, it is best to describe the pain (sharp, burning, shooting, etc.) and add a few qualifiers such as timing, location & activities that make it worse. This could be referred pain, radiating pain, a tumor, muscular, bone, nerve entrapment, neuropathy, vascular, etc. Since there are a myriad of causes a doctors evaluation is advised.