Bursitis

Tendinitis -- Inflammation of the tendon with change in activities, can be related to overuse or deconditioned tissue

Dislocation of the kneecap

Fracture of the kneecap or other bones

Iliotibial band syndrome -- Injury to the thick band that runs from your hip to the outside of your knee

Patellofemoral syndrome -- Pain in the front of your knee around the kneecap

Torn ligament. -- An anterior cruciate ligament (ACL) injury, or medial collateral ligament (MCL) injury

Torn cartilage (a meniscus tear) 

Strain or sprain