This type of pain pattern in the leg, buttock to knee, is commonly referred from the facet joints.(Referred pain means that the pain is felt in an area distant from the actual origin of pain.) But, the age of 21 is a little young to be experiencing facet arthritis pain, even though the MRI shows the facet arthropathy and resulting spinal stenosis. But, what caused all these changes in your spine? This is common age to have spondylolysis, a small fracture or defect in a spine bone called the pars. These small boney defects are easily missed on plain x-rays and MRI's. I would recommend a special bone scan called the SPECT (single photon emission computed tomography). This 3D image of the spine clearly shows the bones and any bone problems. Like facet arthritis, spondylolysis can also cause referred pain into the leg. If the pain does not go below the knee and does not have associated numbness and tingling, nerve pain is a less likely cause of your leg pain. When presented with leg pain, doctors should always try to differentiate between referred pain and nerve pain (neurogenic pain). An accurate diagnosis will lead to your solution. Dr. Christina Lasich, MD