Damage to the cartilage under the kneecap.

The cartilage under the kneecap is a natural shock absorber. Chondromalacia patellae may develop when the knee is overused or injured.

The most common symptom is knee pain that worsens when walking up or down stairs. Kneeling, squatting or sitting with a bent knee for long periods of time also may hurt.

It's usually treated with rest, pain relievers and physical therapy. If these treatments don't work, surgery may be needed.