The knee is the joint where the bones of the upper leg meet the bones of the lower leg, allowing hinge-like movement while providing stability and strength to support the weight of the body. Flexibility, strength, and stability are needed for standing and for motions like walking, running, crouching, jumping, and turning (pivoting activities).
Several kinds of supporting and moving parts, including bones, cartilage, muscles, ligaments, and tendons, help the knees do their job. Each of these structures is subject to disease and injury. When a knee problem affects your ability to do things, it can have a big impact on your life. Knee problems can interfere with many things, from participation in sports to simply getting up from a chair and walking.
Anatomy of the Knee
The knee joint’s main function is to bend and straighten for moving the body. The knee is more than just a simple hinge. It also twists and rotates. In order to perform all of these actions and to support the entire body while doing so, the knee relies on a number of structures, including bones, ligaments, tendons, and cartilage.
Bones
The knee joint involves three bones.
The thighbone or femur comprises the top portion of the joint.
One of the bones in the lower leg (calf area), the tibia, provides the bottom portion of the joint.
The kneecap or patella rides along the front of the femur.
The remaining bone in the calf, the fibula is not directly involved in the knee joint but is close to the outer portion of the joint (sometimes this bone is included in the knee joint and therefore the joint will be described as involving four bones).
Ligaments
Ligaments are fibrous bands that connect bones to each other.
The knee includes four important ligaments, all four of which connect the femur to the tibia:
The anterior cruciate ligament (ACL) and posterior cruciate ligament (PCL) provide front and back (anterior and posterior) and rotational stability to the knee.
The medial collateral ligament (MCL) and lateral collateral ligament (LCL) located along the inner (medial) and outer (lateral) sides of the knee provide medial and lateral stability to the knee.
Tendons
Tendons are fibrous bands similar to ligaments.
Instead of connecting bones to other bones as ligaments do, tendons connect muscles to bones.
The two important tendons in the knee are (1) the quadriceps tendon connecting the quadriceps muscle, which lies on the front of the thigh, to the patella and (2) the patellar tendon connecting the patella to the tibia (technically this is a ligament because it connects two bones).
The quadriceps and patellar tendons as well as the patella itself are sometimes called the extensor mechanism, and together with the quadriceps muscle they facilitate leg extension (straightening).
Cartilage
Cartilaginous structures called menisci (one is a meniscus) line the top of the tibia and lie between the tibia and the two knuckles at the bottom of the femur (the femoral condyles).
Menisci provide both space and cushion for the knee joint.
Bursae
Bursae (singular is bursa) are fluid-filled sacs that help to cushion the knee. The knee contains three important groups of bursae.
The prepatellar bursa lies in front of the patella.
The anserine bursa is located on the inner side of the knee about 2 inches below the joint.
The infrapatellar bursa is located underneath the patella.