The commonest cause of lateral (outer side) knee pain in runners is iliotibial band friction syndrome (ITBFS). This inflammatory condition is where the iliotibial band (ITB, a tendon) rubs on a bony prominence at the lateral knee when the joint is 25-30 degrees flexed. This is approximately the flexion when the foot strike the ground.
The result is a collection of inflammatory fluid near the ITB (see MRI image below). The pain associated with this can be severe. Marathon and other longer distance athletes are especially prone. Downhill running, or running on a sloped surface (putting one leg lower than the other) also increases the risk.
There will be exquisite tenderness at the lateral aspect of the knee. Runners are especially prone when there is a rapid increase in training volume or there are adverse biomechanical factors in the runner.
These factors include excessive pronation (this results in inward rotating of the tibia, tightening the ITB); pelvic imbalance, due to weakness of hip muscles; and an excessively tight ITB.
Treatment of these factors via physiotherapists, podiatrists and massage therapists are usually successful. Occasionally a corticosteroid injection via a Sports Physician is required to settle the pain.