Sometimes the cartilage on the back of the kneecap or in the groove of the thighbone is damaged. Damaged cartilage can’t spread pressure evenly. Uneven pressure wears down the cartilage even further.

Sometimes a muscle or ligament in the knee is pulled the wrong way. Or the kneecap may be pushed too hard. Then the kneecap may move partly out of the groove (subluxation). It may even move completely out (dislocation).

Patellar tendinitis (“jumper’s knee”) happens when the quadriceps muscles are overused or tight. During movement, the patellar tendon absorbs more shock than usual. The tendon becomes irritated or damaged.

Plica bands are tissue fibers that some people have near the kneecap. They usually cause no problems. But sometimes they can become irritated or inflamed.