As the knee is a meeting place for four bones — the femur, tibia, fibula, and patella — it requires several ligaments to keep the 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 knee cap, or 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 slimy 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.

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 patellar ligament 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.