Arthritis
Arthritis most often causes problems with the knee joint, but can affect other structures like muscles, tendons and ligaments. Osteoarthritis, the most common form of arthritis in the knee, is caused by the gradual degradation of the cartilage in the joint. Rheumatoid arthritis causes the joint to become inflammed and can often cause destruction of the surrounding cartilage. A deformity often leads to arthritis in the knee, but more often, obesity or excess weight, or repetitive stress injuries from sports are the culprits. Symptoms include pain, swelling, stiffness or locking joints.

Cartilage Injuries
Chondromalacia patella, a softening of the knee cap cartilage, occurs most often in runners, skiers, cyclists and soccer players.

Iliotibial Band Syndrome
Inflammation of a tendon and its subsequent rubbing over the outer knee bone is most often caused by stress of long-term overuse, such as sports training. Symptoms include aches or burning sensations at the side of the knee, sometimes the pain can radiate up the side of the thigh.

Ligament Injuries
The anterior cruciate ligament (ACL) and posterior cruciate ligament (PCL) can become sprained and cause a great deal of pain. Injury to the cruciate ligaments is sometimes referred to as a "sprain." The ACL can become stretched or torn by a sudden or direct impact, such as in an auto accident or football tackle. Injuries to the medial collateral ligaments are often caused by a blow to the outer side of the knee that stretches and tears the ligament on the inner side of the knee. Football and hockey players often incur these kinds of injuries. The injury is often accompanied by a "pop" sound, followed by a buckling of the knee sideways.