As with all the joints in the lower limb, knee pain is often caused by problems elsewhere, usually the low back, hip or foot.
It can be caused by arthritis, and is one of the most common sporting injuries. Knee pain patients often struggle to walk especially on stairs (and very often worse going down the stairs!). Some swelling may occur around the knee, which may indicate inflammation within the joint.
A torn cartilage can be the cause of knee pain, often associated with a locking or clicking of the joint, but usually it is the other soft tissues around the knee which are the problem. Ligaments, tendons and muscles are all working very hard to keep the knee stable, and they can injure if loaded wrongly.
Knee arthritis is caused by wear of the cartilage in the knee and can occasionally get so severe that a knee replacement is necessary.