The location and severity of knee pain may vary, depending on the cause of the problem. Signs and symptoms that sometimes accompany knee pain include:

• Swelling and stiffness
• Redness and warmth to the touch
• Weakness or instability
• Popping or crunching noises
• Inability to fully straighten the knee

Knee pain can be caused by injuries, mechanical problems, types of arthritis and other problems.

Injuries

A knee injury can affect any of the ligaments, tendons or fluid-filled sacs (bursae) that surround your knee joint as well as the bones, cartilage and ligaments that form the joint itself. Some of the more common knee injuries include:

• ACL injury. An ACL injury is a tear of the anterior cruciate ligament (ACL) — one of four ligaments that connect your shinbone to your thighbone. An ACL injury is particularly common in people who play basketball, soccer or other sports that require sudden changes in direction.
• Fractures. The bones of the knee, including the kneecap (patella), can be broken during motor vehicle collisions or falls. People whose bones have been weakened by osteoporosis can sometimes sustain a knee fracture simply by stepping wrong.
• Torn meniscus. The meniscus is formed of tough, rubbery cartilage and acts as a shock absorber between your shinbone and thighbone. It can be torn if you suddenly twist your knee while bearing weight on it.
• Knee bursitis. Some knee injuries cause inflammation in the bursae, the small sacs of fluid that cushion the outside of your knee joint so that tendons and ligaments glide smoothly over the joint.
• Patellar tendinitis. Tendinitis is irritation and inflammation of one or more tendons — the thick, fibrous tissues that attach muscles to bones. Runners, skiers, cyclists, and those involved in jumping sports and activities may develop inflammation in the patellar tendon, which connects the quadriceps muscle on the front of the thigh to the shinbone.

Mechanical problems

Some examples of mechanical problems that can cause knee pain include:

• Loose body. Sometimes injury or degeneration of bone or cartilage can cause a piece of bone or cartilage to break off and float in the joint space. This may not create any problems unless the loose body interferes with knee joint movement, in which case the effect is something like a pencil caught in a door hinge.
• Iliotibial band syndrome. This occurs when the tough band of tissue that extends from the outside of your hip to the outside of your knee (iliotibial band) becomes so tight that it rubs against the outer portion of your femur. Distance runners and cyclists are especially susceptible to iliotibial band syndrome.
• Dislocated kneecap. This occurs when the triangular bone (patella) that covers the front of your knee slips out of place, usually to the outside of your knee. In some cases, the kneecap may stay displaced and you'll be able to see the dislocation.
• Hip or foot pain. If you have hip or foot pain, you may change the way you walk to spare these painful joints. But this altered gait can place more stress on your knee joint. In some cases, problems in the hip or foot can cause knee pain.

Types of arthritis

More than 100 different types of arthritis exist. The varieties most likely to affect the knee include:

• Osteoarthritis. Sometimes called degenerative arthritis, osteoarthritis is the most common type of arthritis. It's a wear-and-tear condition that occurs when the cartilage in your knee deteriorates with use and age.
• Rheumatoid arthritis. The most debilitating form of arthritis, rheumatoid arthritis is an autoimmune condition that can affect almost any joint in your body, including your knees. Although rheumatoid arthritis is a chronic disease, it tends to vary in severity and may even come and go.
• Gout. This type of arthritis occurs when uric acid crystals build up in the joint. While gout most commonly affects the big toe, it can also occur in the knee.
• Pseudogout. Often mistaken for gout, pseudogout is caused by calcium-containing crystals that develop in the joint fluid. Knees are the most common joint affected by pseudogout.
• Septic arthritis. Sometimes your knee joint can become infected, leading to swelling, pain and redness. Septic arthritis often occurs with a fever, and there's usually no trauma before the onset of pain. Septic arthritis can quickly cause extensive damage to the knee cartilage. If you have knee pain with any of these symptoms, see your doctor right away.

Other problems

Patellofemoral pain syndrome is a general term that refers to pain arising between the kneecap (patella) and the underlying thighbone (femur). It's common in athletes; in young adults, especially those who have a slight maltracking of the kneecap; and in older adults, who usually develop the condition as a result of arthritis of the kneecap.

A number of factors can increase your risk of having knee problems, including:

• Excess weight. Being overweight or obese increases stress on your knee joints, even during ordinary activities such as walking or going up and down stairs. It also puts you at increased risk of osteoarthritis by accelerating the breakdown of joint cartilage.
• Lack of muscle flexibility or strength. A lack of strength and flexibility can increase the risk of knee injuries. Strong muscles help to stabilize and protect your joints, and muscle flexibility can help you achieve full range of motion.
• Certain sports or occupations. Some sports put greater stress on your knees than do others. Alpine skiing with its rigid ski boots and potential for falls, basketball's jumps and pivots, and the repeated pounding your knees take when you run or jog all increase your risk of knee injury. Jobs that require repetitive stress on the knees such as construction or farming also can increase your risk.
• Previous injury. Having a previous knee injury makes it more likely that you'll injure your knee again.

Not all knee pain is serious. But some knee injuries and medical conditions, such as osteoarthritis, can lead to increasing pain, joint damage and disability if left untreated. And having a knee injury — even a minor one — makes it more likely that you'll have similar injuries in the future.