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Picture of the Knee
The knee is one of the body’s largest and most complex joints. The knee joins the thigh bone (femur) to the shin bone (tibia). The smaller bone that runs alongside the tibia (fibula) and the kneecap (patella) are the other bones that make the knee joint.
Tendons connect the knee bones to the leg muscles that move the knee joint. Ligaments join the knee bones and provide stability to the knee:
- The anterior cruciate ligament prevents the femur from sliding backward on the tibia (or the tibia sliding forward on the femur).
- The posterior cruciate ligament prevents the femur from sliding forward on the tibia (or the tibia from sliding backward on the femur).
- The medial and lateral collateral ligaments prevent the femur from sliding side to side.
Two C-shaped pieces of cartilage called the medial and lateral menisci act as shock absorbers between the femur and tibia.
Numerous bursae, or fluid-filled sacs, help the knee move smoothly.
- Chondromalacia patella (also called patellofemoral syndrome): Irritation of the cartilage on the underside of the kneecap (patella), causing knee pain. This is a common cause of knee pain in young people.
- Knee osteoarthritis: Osteoarthritis is the most common form of arthritis and often affects the knees. Caused by aging and wear and tear of cartilage, osteoarthritis symptoms may include knee pain, stiffness, and swelling.
- Knee effusion: Fluid buildup inside the knee, usually from inflammation. Any form of arthritis or injury may cause a knee effusion.
- Meniscal tear: Damage to a meniscus, the cartilage cushions the knee, often occurs with twisting the knee. Large tears may cause the knee to lock.
- ACL (anterior cruciate ligament) strain or tear: The ACL is responsible for a large part of the knee’s stability. An ACL tear often leads to the knee “giving out” and may require surgical repair.
- PCL (posterior cruciate ligament) strain or tear: PCL tears can cause pain, swelling, and knee instability. These injuries are less common than ACL tears, and physical therapy (rather than surgery) is usually the best option.
- MCL (medial collateral ligament) strain or tear: This injury may cause pain and possible instability to the inner side of the knee.
- Patellar subluxation: During activity, the kneecap slides abnormally or dislocates along the thigh bone. Knee pain around the kneecap results.
- Patellar tendonitis: Inflammation of the tendon connecting the kneecap (patella) to the shin bone. This occurs mostly in athletes from repeated jumping.
- Knee bursitis: Pain, swelling, and warmth in any knee bursae. Bursitis often occurs from overuse or injury.
- Baker’s cyst: Collection of fluid in the back of the knee. Baker’s cysts usually develop from a persistent effusion in conditions such as arthritis.
- Rheumatoid arthritis: An autoimmune condition that can cause arthritis in any joint, including the knees. If untreated, rheumatoid arthritis can cause permanent joint damage.
- Gout: A form of arthritis caused by a buildup of uric acid crystals in a joint. The knees may be affected, causing episodes of severe pain and swelling.
- Pseudogout: A form of arthritis similar to gout caused by calcium pyrophosphate crystals depositing in the knee or other joints.
- Septic arthritis: An infection caused by bacteria, a virus, or fungus inside the knee can cause inflammation, pain, swelling, and difficulty moving the knee. Although uncommon, septic arthritis is a serious condition that usually gets worse quickly without treatment.
- Physical examination: By examining the location of knee pain and looking for swelling or abnormal movement, a doctor gathers information about potential causes of damage or stress on the knee.
- Drawer test: With the knee bent, a doctor can pull (anterior drawer test) and push (posterior drawer test) the lower leg while holding the foot stable to check the stability of the ACL and PCL knee ligaments.
- Valgus stress test: Pushing the calf outward while holding the thigh stable, a doctor can check for injury to the medial collateral ligament (MCL). Pushing the calf inward (varus stress test), a doctor can look for injury to the lateral collateral ligament (LCL).
- Knee X-ray: A plain X-ray film of the knee is typically the best initial imaging test for most knee conditions.
- Magnetic resonance imaging (MRI scan): Using high-energy magnetic waves, an MRI scanner creates highly detailed images of the knee and leg. An MRI scan is the most used method of detecting ligament and meniscal injuries.
- Arthrocentesis of the knee (joint aspiration): A needle is inserted into the joint space inside the knee, and fluid is drawn out. Various forms of arthritis may be diagnosed through knee arthrocentesis.
- Arthroscopy: A surgical procedure that allows knee examination with an endoscope.
- RICE therapy: Rest (or reducing daily activities), Ice, Compression (as with bandage support) and Elevation. RICE is a good initial therapy for many knee conditions.
- Pain medicines: Over-the-counter or prescription pain relievers such as acetaminophen (Tylenol), ibuprofen (Motrin), and naproxen (Aleve) can treat most knee pain.
- Physical therapy: An exercise program can strengthen the muscles surrounding the knee, increasing the knee’s stability.
- Cortisone injection: Injecting steroids into the knee can help reduce pain and swelling.
- Hyaluronan injection: Injection of this “goo” material into the knee may reduce pain from arthritis and delay the need for knee surgery in some people.
- Knee surgery: Surgery may be done to correct various knee conditions. Surgery can replace or repair a torn ligament, remove an injured meniscus, or entirely replace a severely damaged knee. Surgery may be done with a large incision (open) or smaller incisions (arthroscopic).
- Arthroscopic surgery: An endoscope (flexible tube with surgical tools on its end) is inserted into the knee joint. Arthroscopic surgery has a shorter recovery and rehabilitation period than open surgery.
- ACL repair: A surgeon uses a graft (cut from your own or a donor’s body) to replace the torn ACL.
Anatomy of the Knee
Knee anatomy involves more than just muscles and bones. Ligaments, tendons, and cartilage work together to connect the thigh bone, shin bone, and knee cap and allow the leg to bend back and forth like a hinge.
The largest joint in the body, the knee, is also one of the most easily injured. Problems with any part of the knee’s anatomy can result in knee pain, stiffness, and difficulty walking.
This article details knee anatomy. It explains the different parts of the knee joint, how the knee works, and common knee problems.
Bones Around the Knee
Three important bones come together at the knee joint:
- The tibia (shin bone)
- The femur (thigh bone)
- The patella (kneecap)
A fourth bone, the fibula, is located just next to the tibia and knee joint and can play an important role in some knee conditions.
The tibia, femur, and patella are all covered with a smooth layer of cartilage where they contact each other at the knee joint.
There is also a small bone called a fabella, often located behind the knee joint. The fabella is a type of bone called a sesamoid bone (meaning it sits within a tendon). It’s of little consequence to the knee joint function and is only found in about 25% of the population.
Cartilage of the Knee
There are two types of cartilage in the knee joint:
- Articular cartilage is the smooth lining that covers the end of the bone. When the smooth articular cartilage is worn away, knee arthritis is the result. Cartilage is generally a resilient structure that resists damage, but it has a difficult time healing when injured. It can also wear down over time with age.
- The other type of cartilage in the knee joint is called the meniscus. The meniscus is a shock absorber that sits between the end of the thigh bone and the top of the shin bone.
Ligaments of the Knee
Ligaments are structures that connect two bones. Four major ligaments surround the knee joint.
Two of these ligaments are in the center of the joint, and they cross each other. These are called the cruciate ligaments and consist of the anterior and posterior cruciate ligaments.
One ligament is on each knee joint—the medial collateral ligament on the inner side and the lateral collateral ligament on the outer side. Ligament injuries typically result in complaints of instability of the knee joint.
Muscles and Tendons
Muscles propel the knee joint back and forth. A tendon connects the muscle to the bone. When the muscle contracts, the tendons are pulled, and the bone is moved.
The knee joint is most significantly affected by two major muscle groups:
- The quadriceps muscles provide strength and power with knee extension (straightening).
- The hamstring muscles allow for strength and power in flexion (bending).
The patellar tendon on the front of the knee is part of the quadriceps mechanism. Other smaller muscles and tendons surround the knee joint as well.
Joint Capsule and Lining
The synovium is the lining of the joint space. The synovium is a layer of tissue that defines the joint space.
The synovial cells produce a slippery, viscous fluid called synovial fluid within the joint. In conditions that cause joint inflammation, an abundance of synovial fluid can be produced, leading to swelling of the knee joint.
A bursa is a structure in your body that is placed between two moving parts. There is a prominent bursa in your knee in front of your knee and underneath the skin.
The bursa functions as a means to allow for smooth movement between these two structures (skin and bone). There are hundreds of bursae spread throughout your body.
The bursa in front of the kneecap is prone to swelling, especially when people injure their knees or perform activities that involve kneeling on hard surfaces. Inflammation of the bursa, called prepatellar bursitis, is common in people who do flooring or cleaning work and have to spend much time kneeling.
Knee Joint Function
Knee function is determined in large part by the anatomy of the joint. The primary function of the knee is to hinge at the lower extremity.
However, the knee does not only bend back and forth. There are also rotational movements at the knee joint.
For the knee joint to function correctly, there needs to be good stability of the joint throughout its range of motion. If there are restrictions in mobility or instability of the knee joint, the function will not be normal.
A normally functioning knee joint will allow the following:
- Lower extremity support when standing
- Strength and power with movements such as standing up, squatting, or climbing
- Efficient movement when walking or running
- Power to propel your body more when you move
- Shock absorption when walking or landing from a jumping position
These are just some of the important functions that the knee joint allows. For any of these functions to behave normally, all of the aforementioned structures must work together and function normally.
Common Knee Conditions
Knee pain, decreased range of motion, and functional problems can be due to a number of conditions, including:
- Arthritis: Arthritis occurs when there is inflammation and damage to the cartilage of the knee joint. Arthritis can lead to swelling, pain, and difficulties with activities.
- Ligament injuries: Some of the most common sports-related injuries to the knee joint are ligament injuries. The anterior cruciate and medial collateral ligaments are the most commonly injured ligaments.
- Meniscus tears: Tears of the meniscus, the cushion between the bones, can occur due to an injury or wear and tear. Not all tears cause pain or functional problems.
- Tendonitis: Inflammation to the tendons that surround the joint can lead to a common condition known as tendonitis. Some of the tendons around are more prone to developing inflammation.
A Word From Verywell
The knee joint is a complex structure that involves bones, tendons, ligaments, muscles, and other structures for normal function. When there is damage to one of the structures that surround the knee joint, this can lead to discomfort and disability. Understanding the normal function of the knee joint can help you address some of these common conditions.
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
- American Academy of Orthopaedic Surgeons: OrthoInfo. Common knee injuries.
- U.S. National Library of Medicine: MedlinePlus. Meniscus tears–aftercare.
- American Academy of Orthopedic Surgeons: Ortho Info. Synovial Chondromatosis.
- National Institute of Health: News in Health. Beating Bursitis: Take Care of Your Joint Cushions.
- U.S. National Library of Medicine: MedlinePlus. Knee Injuries.