Two bones, the femur and the tibia, come together to form the joint that is commonly known as the knee. There are different types of joints in our body (hinge, ball and socket, pivot, gliding, saddle, and conyloid). The knees are hinge joints which swing open and closed, similar to the movement of a hinged door. The knee bends and straightens (flexion and extension).
The knee joint is protected by a kneecap (patella) that sits to the front. Articular cartilage serves as a cushion, covering the ends of the tibia and femur and under the patella. The knee joint is further cushioned and stabilized by menisci -- crescent-shaped bands of thick, rubbery cartilage attached to the tibia. The medial meniscus is on the inner side of the knee joint, while the lateral meniscus is located on the outside of the knee.
Ligaments also help stabilize the knee. Collateral ligaments are located on the sides of the knee and limit sideways motion. The anterior cruciate ligament (ACL) connects the tibia to the femur at the center of the knee. The ACL limits rotation and forward motion of the tibia. The posterior cruciate ligament (PCL) sits behind the ACL and limits backward motion of the tibia.
When everything is working properly, you have a healthy, stable knee joint which allows you to walk, run, jump, sit, stand, and more. When the knee isn't functioning properly, knee pain will often be the first sign that something is wrong.
Causes of Knee Pain
There are many causes of knee pain. Knee pain can result from overuse of the joint, repetitive motion, strains, and sprains. In these situations, treating knee pain related to acute injury with rest, ice, compression, and elevation, (R.I.C.E) should be enough to resolve the problem.
There can be more complicated causes of knee pain, though. Knee arthritis is a very painful condition. With arthritis, the cartilage that covers the ends of the bones in the knee joint becomes damaged. Inflammation causes cartilage to erode in certain types of inflammatory arthritis, such as rheumatoid arthritis. With osteoarthritis, joint cartilage wears away, eventually leading to bone rubbing on bone. Bone-on-bone can be excuciating.
According to the Journal of the American Medical Association, more than 10 million Americans have knee osteoarthritis. The condition is recognized as one of 5 leading causes of disability in non-institutionalized adults by the Centers for Disease Control and Prevention (CDC). Other conditions that can cause knee pain, include:
- Bursitis (inflammation of the bursa) - There are six bursa sacs which, together with fluid produced by the lining of the joint, lubricate the knee. Injury and overuse are common reasons bursa become irritated and inflamed.
- Water-on-the-knee or joint effusion - Excess fluid can accumulate inside the knee joint after injury. The fluid can also be produced by the tissues that are affected by the arthritis.
- Patellofemoral pain syndrome - Overuse of the knee joint can cause symptoms that particularly cause pain in the front of the knee.
- Torn meniscus - One of the menisci can be torn due to injury. This is common among athletes.
- Ligament tear, sprain, or rupture - The ligaments that support the knee joint can be damaged due to injury.
- Quadricep muscle or hamstring strain - The large muscle in front of the thigh (quad) and the muscle in back of the thigh (hamstring) support and move the knee joint. Weakness in these muscles due to injury or atrophy can make the knee less stable and more painful.
- Infection in the joint - A germ (bacterium, virus, or fungus) can enter the body and travel to a joint. The infected joint becomes red, swollen, and painful.
- Baker's cyst - A fluid-filled cyst can develop behind the knee. The Baker's cyst is caused by excess fluid that pushes through the back part of the joint capsule, protruding into the back area of the knee. The cyst can be very painful.
Diagnosing Knee Pain
A physical examination of both of your knees is necessary to determine the cause and extent of knee pain. Your doctor will start with a visual examination of your knees, looking for redness, swelling, or any other visible changes that might offer clues. If there is redness or excess fluid, a joint fluid analysis (arthrocentesis) may be performed.
Your range of motion will be tested, passively (the doctor moves your leg around) and actively (you move your leg). Which movements provoke pain may be telling, as will any sounds that occur (popping, clicking, grating) with movement.
There are specific tests the doctor can perform that may provide information about the ligaments and menisci. Imaging studies (x-rays, CT scan, and MRI) are used to evaluate the knee joint and surrounding structures, too.
Surgically, an arthroscopy is an option for detecting injury or damage. Through a small incision, the surgeon uses an arthrocope to view the inside of the knee, and at the same time a decision can be made about the necessity for surgical repair. Abnormalities found during any of the diagnostic procedures determine the course of treatment.
Treating Knee Pain
Minor knee injuries often resolve with rest, anti-inflammatory medications (NSAIDs), over-the-counter pain relievers (acetaminophen), ice, and stablizing the joint. Major injuries, such as ligament or meniscus tears may require surgical intervention.
For knee arthritis, there are many avenues of treatment, depending on the severity of joint damage. Oral medications (analgesics, NSAIDs), topical pain relievers, dietary supplements, injections (cortisone or hyaluronan), appropriate exercise, weight loss, knee bracing, knee taping, knee supports, mobility aids are some of the options.
When all treatment options fail to relieve knee pain and the severity of arthritis is such that it affects your ability to perform usual daily activities, it may be time to consider knee replacement surgery.
Anatomy of the Knee. Southern California Orthopedic Institute. Accessed 11/05/2011.
Medial and Lateral Meniscus Tears. Cedars-Sinai. Accessed. 11/05/2011.
Patient Information: Knee Pain. UpToDate. May 2011.
Osteoarthritis of the Knee. Parmet et al. Journal of the American Medical Association.
JAMA. 2003;289(8):1068.doi: 10.1001/jama.289.8.1068.
Osteoarthritis. Centers for Disease Control and Prevention. Updated September 1, 2011.