The knee is a complex structure and one of the most stressed joints in the body. It is the largest joint, vital for movement, and vulnerable to injury.
The knee is the most commonly injured joint by adolescent athletes with an estimated
Fast facts on knee anatomy
- The knee is the largest and most complex joint in the body.
- The knee joins together the thigh bone, shin bone, fibula (on the outer side of the shin), and kneecap.
- The knee is a synovial joint, meaning it contains a fluid-filled capsule.
- Participation in sports and recreational activities are risk factors for knee injury.
- A direct blow to the knee is a serious injury and requires immediate medical attention.
The knee is a hinge joint that is responsible for weight-bearing and movement. It consists of bones, meniscus, ligaments, and tendons.
The knee is designed to fulfill a number of functions:
- support the body in an upright position without the need for muscles to work
- helps to lower and raise the body
- provides stability
- acts as a shock absorber
- allows twisting of the leg
- makes walking more efficient
- helps propel the body forward
Below, we will explain the basic components of knee anatomy.
The femur (thigh bone), tibia (shin bone), and patella (kneecap) make up the bones of the knee. The knee joint keeps these bones in place.
The patella is a small, triangle shaped bone that sits at the front of the knee, within the quadriceps muscle. It is lined with the thickest layer of cartilage in the body because it endures a great deal of force.
There are two types of cartilage in the knee:
Meniscus: these are crescent-shaped discs that act as a cushion, or “shock absorber” so that the bones of the knee can move through their range of motion without rubbing directly against each other. The menisci also contain nerves that help improve balance and stability and ensure the correct weight distribution between the femur and tibia.
The knee has two menisci:
- medial – on the inner side of the knee, this is largest of the two
- lateral – on the outer side of the knee
Articular cartilage: found on the femur, the top of the tibia, and the back of the patella; it is a thin, shiny layer of cartilage. It acts as a shock absorber and helps bones move smoothly over one another.
Ligaments are tough and fibrous tissues; they act like strong ropes to connect bones to other bones, preventing too much motion and promoting stability. The knee has four:
- ACL (anterior cruciate ligament) – prevents the femur from sliding backward on the tibia, and the tibia from sliding forward on the femur.
- PCL (posterior cruciate ligament) – prevents the femur from sliding forward on the tibia, or the tibia from sliding backward on the femur.
- MCL (medial collateral ligament) – prevents side to side movement of the femur.
- LCL (lateral collateral ligament) – prevents side to side movement of the femur.
These tough bands of soft tissue provide stability to the joint. They are similar to ligaments, but instead of linking bone to bone, they connect bone to muscle. The largest tendon in the knee is the patellar tendon, which covers the kneecap, runs up the thigh, and attaches to the quadriceps.
Although they are not technically part of the knee joint, the hamstrings and quadriceps are the muscles that strengthen the leg and help flex the knee.
The quadriceps are four muscles that straighten the knee. The hamstrings are three muscles at the back of the thigh that bend the knee.
The gluteal muscles – gluteus medius and minimus – also known as the glutes are in the buttocks; these are also important in positioning the knee.
The joint capsule is a membrane bag that surrounds the knee joint. It is filled with a liquid called synovial fluid, which lubricates and nourishes the joint.
There are approximately 14 of these small fluid-filled sacs within the knee joint. They reduce friction between the tissues of the knee and prevent inflammation.
Knees are most often injured during sports activities, exercising, or as a result of a fall. Pain and swelling, difficulty with weight bearing, and instability are the most common symptoms experienced with a knee injury.
Sprains and strains
Sprains and strains are injuries to the ligaments. The ACL and MCL are the ligaments most often injured.
These injuries usually happen in sports such as soccer, football, and basketball where the knee might experience a sudden twisting motion, a rapid change in direction, or an incorrect landing from a jump.
Often a pop or snap is heard followed by swelling. Symptoms also include tenderness along the joint line and pain with walking.
A meniscal tear generally happens during sports where the knee twists, pivots, or an individual is tackled.
Even normal wear from aging can weaken the meniscus, causing it to tear with a simple awkward turn during normal day to day activities. Symptoms of a tear are usually pain, stiffness, swelling, locking, and decreased range of motion.
A fracture is most often caused by trauma, such as falls, motor vehicle accidents, and sports-related contact. The most common bone broken around the knee is the patella (kneecap).
The most common overuse injury is “runner’s knee,” a loose term that refers to several disorders, including patellofemoral pain syndrome (PFPS). These painful conditions are common among athletes such as runners and cyclists.
Pain is experienced behind or around the kneecap and can travel to the thigh or shin. The pain worsens with activity and is relieved by rest.
All knee injuries should be evaluated by a doctor as soon as possible. Basic treatment for common knee injuries includes rest, ice, elevation, and an over-the-counter pain reliever such as Ibuprofen.
It is also important to begin strengthening and stretching exercises 24-48 hours after minor injuries, or as advised by a doctor. There should be a gradual return to normal activities. In some cases, such as an ACL tear, arthroscopic surgery is necessary to repair the damage.
Additionally, short-term rest and avoiding putting weight on the leg may be necessary for proper healing. Crutches might be recommended for a limited time to prevent additional injury.
Physical therapy is often indicated to assist a person’s recovery to a pain-free range of motion. An important extension of therapy involves continuing a home exercise program. Without adequate rehabilitation, a person will tend to have on going problems with an injured knee. The goal of treatment is to restore stability, strength, and mobility.
The following tips may help prevent common knee injuries:
- Warm up by walking and stretching gently before and after playing sports.
- Keep the leg muscles strong by using stairs, riding a stationary bicycle, or working out with weights.
- Avoid sudden changes in the intensity of exercise.
- Replace worn out shoes. Choose ones that fit properly and provide good traction.
- Maintain a healthy weight to avoid added pressure on the knees.
- Always wear a seatbelt.
- Use knee guards in sports where knees could get injured.
Maintaining strong, flexible leg muscles and seeking prompt medical attention for all knee injuries is essential to assure accurate diagnosis and appropriate treatment of the injury. Additionally, keeping the supporting leg muscles strong and practicing injury prevention will help keep the knee healthy across the lifespan.