Knee pain can be caused by:
Anterior knee pai
Arthritis -- including rheumatoid arthritis, osteoarthritis, and gout
Baker's cyst -- a fluid-filled swelling behind the knee that may occur with swelling (inflammation) from other causes, like arthritis
Bursitis -- inflammation from repeated pressure on the knee, such as kneeling for long periods of time, overuse, or injury
Connective tissue disorders such as lupus
Dislocation of the kneecap
Iliotibial band syndrome -- a hip disorder from injury to the thick band that runs from your hip to the outside of your knee
Infection in the joint
Knee injuries -- an anterior cruciate ligament injury or medial collateral ligament injury may cause bleeding into your knee, which makes the pain worse
Osgood-Schlatter disease
Tendinitis -- a pain in the front of your knee that gets worse when going up and down stairs or inclines
Torn cartilage (a meniscus tear) -- pain felt on the inside or outside of the knee joint
Torn ligament (ACL tear) -- leads to pain and instability of the knee
Strain or sprain -- minor injuries to the ligaments caused by sudden or unnatural twisting