The knee is the largest hinge joint in the body. Besides flexing and extending, it also rotates slightly. This movement is made possible by muscles that move the largest bones in the leg, which all meet near the knee.

The femur, or thighbone, is the longest and largest bone in the human body. It is the only bone in the upper leg. The head of the femur creates the ball-and-socket joint of the hip, and the lower portion creates the upper portion of the knee. The bone’s shape resembles a walking stick.

As the femur can traditionally withstand up to 25 times the weight of an adult, fracturing it requires tremendous force, such as a high-speed car accident. However, degenerative bone diseases, such as osteoporosis, can weaken the integrity of the bone and leave it more prone to fractures. Broken hips, in which the upper part of the femur is fractured, are more common in the elderly for this reason.

The second largest bone in the leg — and the human body — is the tibia, also called the shinbone. This long, straight bone connects with the knee and the ankle. Fractures to this bone require less force than those that cause fractures to the femur. Falls from great heights, sporting injuries, or car accidents can cause them.

The knee joint is where the tibia and femur meet.

Running parallel to the tibia is the fibula, the thinner and weaker bone of the lower leg. It is also known as the calf bone, as it sits slightly behind the tibia on the outside of the leg. Although it does not directly affect the knee’s movement, the fibula is connected via ligaments to the two ends of the tibia. It also helps to strengthen the tibia and provides support in the slight rotation of the knee.

The most common fracture of the fibula occurs at the bottom of the bone near the ankle. This can occur during direct impact, such as a fall, or from a twisting action, such as tripping while running.

The fourth bone of the knee is the patella. Commonly referred to as the kneecap, this nearly heart-shaped bone at the center of the knee helps extend the knee and protect the joint from impact. A tendon at the top of the patella and a ligament at the bottom hold the bone in place. As the knee bends, the patella slides along a groove in the femur. Sometimes, due to numerous complications, the kneecap comes out of its groove and becomes dislocated, an injury known as patellar subluxation.

Fibrous bands called ligaments hold these bones together. The ligaments’ ability to flex and bend allow for greater movement of the joint yet provide stability. Two of the ligaments between the femur and tibia, the anterior cruciate ligament (ACL) and the posterior cruciate ligament (PCL), create a cross and provide stability to the joint.

Surrounding the connection of the bones, various elements such as bursa (fluid-filled sacs), fat pads, and cartilage pads (strong, flexible tissue called menisci), protect the bones and keep the knee joint moving fluidly. Injury, infection, and degeneration to these parts can be painful and cause mobility problems, but corrective surgery, rehabilitation, and medication can help ease problematic symptoms.