The human knee is a marvel. Although it is an incredibly complex and sturdy structure, it also is more prone to injury than any other bony structure besides the spine.

Common causes of knee pain:

• Arthritis of the knee – Arthritis most often causes problems with the knee joint, but also can affect other structures such as 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 inflamed 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 culprit. Symptoms include stiffness or locking joints.
• Cartilage injuries – including chondromalacia patella, a softening of the knee cap cartilage. This disorder 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 the stress of long-term overuse, such as sports training. Symptoms of iliotibial band syndrome 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 twisting motion. The PCL is most often injured by a direct impact, such as in an automobile 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.
• Meniscus injuries – Quick twists or rotations of the upper leg or repetitive rotations of the knee while bearing weight can tear the meniscus.
• Osgood-Schlatter disease – This condition is caused by repetitive stress or tension on the upper tibia, or leg bone. The patellar tendon and surrounding soft tissues at the point where the tendon attaches to the tibia can become inflamed. Pre-adolescent boys involved in sports that include frequent running or jumping are particularly prone to this condition.
• Tendon injuries – Tendon injuries in the knee can be caused by anything from tendonitis to a ruptured or torn tendon. Overuse can cause the tendon to stretch like a rubber band, later becoming inflamed.