Arthritis is a generic condition indicating inflammation of a joint. Inflammation is the general response of the body to injury of any type. The signs of inflammation are pain, swelling, warmth, and sometimes redness. There are many causes of arthritis. These include a sudden, or acute, injury such as might occur when playing sports. Acute trauma is actually the most common cause of arthritis. In most cases, however, the injury does not cause permanent damage, the inflammation subsides over a few weeks, and the joint functions normally for the rest of that person's life. Arthritis can also develop after many years of certain activities performed over and over, which can be considered a form of chronic injury.

The most common form of arthritis, which necessitates knee replacement, is osteoarthritis. In this condition, the articular cartilage, which is the smooth gliding surface of the joint, has worn away. This can occur for a number of reasons. If there is a genetic defect in the quality of the articular cartilage, then this condition is called primary osteoarthritis. Primary osteoarthritis may affect all of the articular cartilage in the body, not just the knees. More frequently, osteoarthritis affects only a few joints, usually in the legs. In these cases, the genetic composition of the articular cartilage is normal, but there is another factor that causes damage to the cartilage in that specific joint or joints. This is called secondary osteoarthritis.

The most common causes of osteoarthritis are related to the alignment of the leg (too bow-legged or too knocked-kneed) and how well the two halves of the knee joint fit together, which is called congruency. Similar to the tires on an automobile, if the leg is not properly aligned, then the surfaces of the knee joint will wear unevenly. Alternatively, if the two sides of the joint do not fit together properly, they will wear unevenly. In some cases people are born with imperfect alignment, and in other cases they develop conditions which result in imperfect alignment of their leg or legs. In other cases, there may have been a disease or an injury that altered the alignment or congruency of the joint (such as an infection, a fracture, or a torn ligament). Increased weight puts more stress on the joint and over time this can cause damage and contribute to a wearing out of the cartilage. Exercise or hard work, without any other factor, does not result in arthritis. Moderate exercise actually improves the condition of joints.

Another common type of arthritis that can affect the knees is rheumatoid arthritis. This type of arthritis is caused by dysfunction of the immune system. Abnormal anti-bodies are produced that get deposited in the lining tissue of the joints (the synovium). This causes chronic inflammation and slow destruction of cartilage. All of the synovial joints of the body may be affected by rheumatoid arthritis and the level of arthritis is roughly equal on both sides of the body, i.e., both the right and the left knees, hands, etc.