Etiology The causes of knee pain include injury, mechanical problems, various types of arthritis, and other diseases. Injuries Knee injuries can involve any ligament, tendon, or capsule containing fluid (bursa) around the knee joint, as well as the bones, cartilage, and ligaments that make up the knee joint. Some of the more common knee injuries include: 1. anterior cruciate ligament (ACL) injuries. an ACL injury is a tear of the ACL. ACL injuries are especially common in basketball, soccer or other sports that require sudden changes in direction. 2. Meniscus tears. The meniscus consists of tough, flexible cartilage that acts as a shock absorber between the tibia and the femur. If the knee joint is suddenly twisted under weight, the meniscus may tear. 3. Bursitis of the knee joint. Some knee injuries lead to inflammation within the bursa. Bursae are small sacs that contain fluid and provide cushioning outside the knee joint, allowing tendons and ligaments to move smoothly across the joint surface. 4. Patellar tendonitis. Tendonitis is an irritation reaction and inflammation in one or more tendons. Tendons are the dense fibrous tissues that attach muscles to bone. Running, skiing, biking, and sports or activities that require jumping can easily cause inflammation of the patellar tendon. The patellar tendon connects the quadriceps muscle in the front of the thigh to the tibia. Mechanical problems Certain mechanical problems can cause knee pain, including: 1. Free bodies. Sometimes, damage or degeneration of the bone or cartilage can cause a small piece of bone or cartilage to separate and float in the joint cavity. This may not cause any problems unless the free body prevents the knee from moving, which is like a pencil stuck in a doorfold. 2. Iliotibial bundle syndrome. Iliotibial bundle syndrome is caused when the ligament extending from the outside of the pelvis to the outside of the tibia (iliotibial bundle) becomes tight and rubs against the outside of the femur. Long-distance runners are particularly susceptible to iliotibial fasciculus syndrome. 3. Patellar dislocation. Patellar dislocation occurs when the triangular bone (patella) covering the front of the knee joint slips out of its original position, usually outside the knee joint. In some cases, the patella will remain in the dislocated position and the dislocation will be visible to the naked eye. 4. Hip or foot pain. If you have hip or foot pain, you may be able to change your walking posture to reduce the burden on these painful joints. But this gait change can put more stress on the knee joint. Sometimes, a hip or foot lesion can cause pain in the knee joint. Various types of arthritis There are more than 100 types of arthritis that have been identified. The conditions most likely to involve the knee include: 1. Osteoarthritis. This is sometimes called degenerative arthritis. Osteoarthritis is the most common form of arthritis. It is a wear-and-tear disease in which the cartilage in the knee joint degenerates with use and age. 2. Rheumatoid arthritis. Rheumatoid arthritis is the most debilitating form of arthritis and is an autoimmune disease that can involve almost every joint in the body, including the knee. Although rheumatoid arthritis is a chronic disease, the severity of its condition varies greatly, and may even recur. 3. Gout. This type of arthritis is caused by deposits of uric acid crystals in the joints. Gout most often affects the big toe and can also involve the knee joint. 4. Pseudogout. It is often confused with gout. Pseudogout is caused by calcium-containing crystals in the joint fluid. The knee joint is the most commonly involved joint in pseudogout. 5. Infectious arthritis. Sometimes the knee joint may become infected, causing swelling, pain, and redness. There is usually no trauma before the pain begins. Infectious arthritis is often accompanied by fever. Other disorders Patellofemoral tenderness (patellofemoral pain syndrome) is a general term for pain between the patella and the thigh bone (femur) below it. The condition is common in athletes, young people with mild patellar ectasia and older people with patellofemoral arthritis. Risk factors Certain factors increase the risk of developing knee disease, including: 1. Being overweight. Being overweight or obese can increase the stress on the knee joint, even when performing everyday activities such as walking or walking up and down stairs. This accelerates the wear and tear of joint cartilage and puts you at an increased risk of developing bone and joint problems. 2. Biomechanical problems. Specific structural abnormalities – such as one leg being shorter than the other, a misaligned knee, or even flat feet – can make you more susceptible to knee disease. 3. Inadequate muscle flexibility or strength. Lack of strength and flexibility are both common causes of knee injuries. Tight or weak muscles can make the knee get less support, because the muscles can not withstand enough, the pressure on the joint. 4. Certain sports. Certain sports put more stress on the joint than others. Hard ski boots and the potential for falls during alpine skiing, bouncing and turning during basketball, and the repeated impact on the knee joint during running or jogging can increase the risk of knee injury.