The knee joint is the largest joint in the human body and is one of the most commonly injured joint. The knee functions like a hinge, and allows for flexion and straightening with a little bit of rotational twisting. The tissues in the knee joint include bones, cartilage, meniscus, ligaments, tendons, synovium, and bursae. Pain is resultant when these tissues are injured.
There are 4 bones in the knee joint:
- Tibia or shin: The tibia is the larger bone of the lower leg and connects to the bottom of the femur by ligaments. This supports 85% of the weight but through the lower extremity.
- Femur or thigh: The femur is the largest and strongest bone in the body. It supports the body during any weight bearing activity such as kneeling, standing, walking, or running.
- Patella or kneecap. The patella sits at the anterior-most (front) of the knee joint. The patella anatomically lies in the front of the knee, in the groove formed by the two cartilage covered expansions (condyles) of the lower femur (patellofemoral joint).
- Fibula: The fibula is the smaller leg bone in the lower leg. It is on the outer side of the leg and connects to the tibia bone at the knee and ankle joint. It plays a role in the rotation of the lower leg. Many knee ligaments attach to the fibula bone and the large hamstring muscle, the biceps femoris, attaches to the fibula bone.
Where all of the bones connect to each other they are covered with gliding cartilage called articular cartilage allowing them in a normal knee to move smoothly on each other.
Muscles of the Knee Joint
The quadriceps muscles and hamstring muscles provide most of the power and control for the knee joint.
- The quadriceps muscles. This is a group of four muscles: (vastus lateralis, vastus intermedius, vastus medialis, and rectus femoris) located on the front of the thigh. These muscles are connected to the knee joint via the quadriceps tendon. When these muscles contract and shorten, the leg is straightened. See Chronic High (Proximal) Hamstring Tendinopathy
- The hamstring muscles. This is a group of 3 muscles (biceps femoris, semimembranosus, and semitendinosis) located on the back of the thigh and knee joint. When activated these muscles act to bend the knee. See How Do Hamstring Tears Occur?
Both muscle groups are vital to forward motion. Injuries to these muscle groups can range from mild strains and bruises to debilitating tears.
Cartilage of the Knee Joint
Slippery and flexible, hyaline (articular) cartilage within the knee joint allows, has less friction than two pieces of glass placed together. This allows the joint to move with minimal friction in a healthy knee. There are two primary types of cartilage in the knee:
- Articular (Hyaline) cartilage. This cartilage covers the bones where they meet at the knee joint. The end of the femur (condyles) and the back of the patella (knee cap). It is simultaneously smooth and strong, allowing bones to move over one another with minimal friction. See Understanding Jumper’s Knee
- Meniscus (Fibrocartilage). The menisci are two C-shaped cartilaginous structures within the knee. The medial (inside) and lateral (outer) menisci act as cushions for weight bearing activities, decreasing the effect of impact between the femur and tibia. See Understanding Meniscus Tears
The articular cartilage and menisci are integral to the proper function of the knee joint. They can be injured in an acutely or gradually through repetitive stresses. The loss of cartilage in the knee is termed arthritis. This can be both inflammatory, as in rheumatoid arthritis, or primary, as in the case of osteoarthritis. See Symptoms of an Acute Patellar Injury
Tendons, Ligaments, and Other Soft Tissues of the Knee Joint
The knee joint relies on a variety of ligaments, tendons, and soft tissue structures to maintain flexibility, stability, and strength. Ligaments are ropy, fibrous bands of tissue that connect bones to other bones.
- The Anterior Cruciate Ligament (ACL). The ACL connects the tibia to the femur and functions to prevent the tibia from sliding forward on the femur. The ACL is commonly injured in sporting activities and rarely injured in isolation. See Anterior Cruciate Ligament (ACL) Tears
- The Posterior Cruciate Ligament (PCL). The PCL also connects the tibia to the femur. It functions to prevent the tibia from sliding backward on the femur. The PCL works with the ACL for stabilization of the knee. It is commonly injured in hyperextension type knee moments. See Posterior Cruciate Ligament (PCL) Injuries
- The Lateral Collateral Ligament (LCL). The LCL, which is also known as the fibular collateral ligament, is located on the outside (lateral side) of the knee. It connects the outside, bottom edge of the femur to the outside, top edge of the fibula. The LCL helps stabilize the knee joint by limiting outward (varus) force across the knee. See Lateral Collateral Ligament (LCL) Injuries
- The Medial Collateral Ligament (MCL). The MCL is located on the inside (medial side) of the knee, connecting the inside, bottom edge of the femur with the inside, top edge of the tibia. The MCL helps to stabilize the knee by limiting inward (valgus) force across the knee. The MCL works with the LCL to prevent unwanted side-to-side motion. The MCL is the most commonly injured knee ligament. See Medial Collateral Ligament (MCL) Tears and Sprains
Tendons are flexible tissues that attach muscle to bone.
- The hamstring tendons. There are three hamstring tendons that cross the knee joint on the back of the knee. Two are on the inside (medial) part of the knee attaching to the shin bone (Semimembranosus and Semitendinosus) and one is on the outside (lateral) part of the knee, attaching to the fibula (Biceps femoris). See Symptoms of Chronic High (Proximal) Hamstring Tendinopathy
- The quadriceps tendon. This tendon is composed of contributions from the four quadriceps muscles (the vastus lateralis, vastus intermedius, vastus medialis, and rectus femoris). It attaches the powerful quadriceps muscles to the top part of the patella.
- The patellar tendon. This tendon (also called patellar ligament) attaches the bottom part of the patella to the top part of the tibia.
- A synovial membrane. All of the joints in the body are surrounded by a balloon that holds the joint fluid in. These balloons hold the joint fluid (synovial fluid) in the joint. This fluid is integral to the health of the joint, providing lubrication and delivering nutrients.
Injury to the bones of the knee joint can cause moderate to extreme pain, depending on the severity of the bruise or fracture. Considering how integral the knee joint is to movement, injury to these bones may also cause reduced mobility during the healing process. Injuries to the leg bones are most common in contact sports such as football, hockey, or lacrosse. See What to Do When an Acute Patellar Injury Is Suspected
Common Sports Injuries of the Knee Joint
Some of the most common athletic injuries to the knee joint include:
- Torn Meniscus. Either of the menisci in either knee can be torn as a result of an aggressive twisting motion, or traumatic contact with the knee. Torn menisci are among the most common athletic injuries pertaining to the knee. The meniscus can also degenerate over time as a person ages. A person with a torn meniscus may feel knee pain during rest and knee pain that worsens with deep squatting or going down stairs. See Symptoms of Meniscal Tears
- ACL Tear. A torn anterior cruciate ligament often results from a one-time trauma, such as sudden twisting or an impact to the knee. Individuals who suffer an ACL tear will likely be unable to continue competition. They will also likely require an orthopedic consultation. See Anterior Cruciate Ligament (ACL) Tears
- Prepatellar bursitis. The most common form of knee bursitis, prepatellar bursitis is the inflammation of a bursa at the front of the knee. This is also known as nursemaid’s knee. Pre-patellar bursitis results from inflammation and swelling. Knee bursitis often develops from repetitive motion or pressure on the knee, such as frequent kneeling on or bending of the joint. Pain from a case of knee bursitis will typically lessen with rest but return with activity. Bursitis can also be caused by bacterial infection.
- Tendonitis and tendinopathy. This can affect any tendon in the knee but the most common is the patellar tendon and the quadriceps tendon. Recent research has shown that tendinopathy does not involve inflammatory cells. Therefore, a better way to think of tendinopathy is degeneration of the tendon. Degeneration of a tendon can affect knee strength and mobility as well as cause pain. See Chronic High (Proximal) Hamstring Tendinopathy
- Distal Iliotibial band syndrome. The iliotibial band, more commonly known as the IT band, originates at the outside of the hip, crosses the knee and inserts on the outside of the shin bone. People with IT band friction syndrome typically experience sharp pain at the outside of the knee joint, particularly when the foot hits the ground. See IT Band Syndrome Causes and Risk Factors
- Patellofemoral syndrome. This condition occurs when the patella (kneecap) does not track correctly within the patellofemoral groove. Patellofemoral syndrome most commonly results from congenital alignment or hip abductor weakness. Pain from patellar malalignment will be present during athletic activity and particularly exercise that result in repetitive bending of the knee or running activities. See What You Need to Know About Runner’s Knee
Other Conditions of the Knee Joint
Injuries to the knee can also occur as a result of aging and normal degradation of the body. Common causes of knee joint pain that result from causes besides athletic injury are:
- Gout. This condition arises from high uric acid levels within the body and results in inflammation and deposition of uric acid crystals within the synovium (joint capsule). This results in knee stiffness and pain within the knee joint. Gout pain typically comes on quickly, and can become severe in a matter of hours.
- Pseudogout. Similarly to gout, pseudogout causes sudden and severe inflammation and pain in the affected joint. Pseudogout results in calcium pyrophosphate deposition within the synovium (joint capsule).
- Osteoarthritis. This is the most common diagnosis in all of medicine. This form of arthritis is very common, and occurs when the hyaline articular cartilage wears away from the ends of a bone. Without the cushioning provided by that cartilage, the bones of the knee joint can rub together during motion and cause pain within the joint. See Knee Cartilage Repair, Regeneration, and Replacement
While most acute knee injuries are immediately apparent, other injuries and painful knee conditions develop gradually over time, and an individual may not notice the problem until the joint damage produces knee pain.