The knee is a meeting place for four bones — the femur (thigh bone), tibia (shinbone), fibula (calf bone), and patella (kneecap). It requires several ligaments to keep these bones in place and maintain its ability to flex and bend.
The knee joint capsule, also known as the articular capsule of the knee, is an important structural component. Like many other joints in the body, the knee has a dense, fibrous, connective tissue that seals the joint space between the bones. In the knee, these bones are the femur and tibia. The patella sits outside the capsule.
Inside this capsule are pads that cushion the joint. One of those pads is a bursa, a sac filled with a gelatinous liquid that helps prevent friction between bones in the joint. When the fluid becomes irritated or infected, it is known as bursitis, a condition that can make joint movement painful and stiff in the morning.
Also between the heads of the femur and tibia are the menisci. These are the medial meniscus and lateral meniscus. These are horseshoe-shaped pads of cartilage, a kind of strong, flexible tissue. They lie opposite each other on the inner (medial) and outer (lateral) edges of the tibia. They act as shock absorbers in the knee and help with weight distribution among the bones.
The menisci are prone to injury from trauma — most commonly in athletes when the knee is twisted while in a flexed position— and degeneration (arthritis), such as in older people. Damage to this cartilage often creates pain and tenderness in the joint along with clicking in the joint. Severe damage often requires surgery.
Ligaments are tough bands of fibrous tissue that connect bones. In the knee and other joints, they absorb pressure from twisting, impact, and other potentially harmful movements to ensure the bones stay in place.
There are four major ligaments of the knee. Two are located outside the knee joint capsule. They are:
- Medial collateral ligament (MCL): This broad, flat, ligament is on the outside of the knee and connects the head of the femur to the head of the tibia. It is commonly injured in sports involving impact when the knee is bent, such as football, skiing, or skateboarding.
- Lateral collateral ligament (LCL): The LCL connects the head of the tibia to the end of the femur. It is important in keeping the knee joint intact when the knee is injured from the inside. The LCL is most commonly injured in sports that involve many quick stops, such as soccer and basketball, or high-impact sports, such as football and ice hockey.
The two ligaments that are located inside the knee joint capsule are the cruciate ligaments. They form a cross in the middle of the knee, inside the knee joint capsule. These ligaments are:
- Anterior cruciate ligament (ACL): The ACL stretches from the front of the head of the tibia to the back of the head of the femur to prevent the tibia from moving forward. Injury to the ACL is common in twisting movements, such as freestyle rollerblading. ACL tears also commonly result from high-impact sports, such as football and basketball. A torn ACL often requires reconstructive surgery and extensive physical rehabilitation.
- Posterior cruciate ligament (PCL): This ligament wraps around the ACL as it travels from the back of the head of the tibia to the back of the head of the femur. It is most commonly injured by direct impact when the knee is flexed, such as striking a dashboard in a car accident.
The knee also contains the patellar ligament, which helps hold the kneecap in place. This tough, flat ligament connects the kneecap to the head of the tibia below it. At the top of the kneecap, the tendon of the quadriceps femoris muscle rises above the kneecap. Because the bottom of the kneecap is connected to bone and the top to a muscle, the knee has greater flexibility than if it were connected to bone on both ends.