Your knee consists of three main bones: the femur (thigh bone), the tibia (shin bone), and the patella (kneecap). The femur supports your weight during standing, walking and running, and is covered in cartilage where it meets the tibia and patella. The tibia is the primary weight-bearing bone and connects to the bottom of the femur. Lastly, the patella sits on the front of the joint.

The muscles in the front of your thigh are your quadriceps, while the muscles in the back of your thigh are your hamstrings. To straighten your knee, you contract the quadriceps. To bend your knee, you contract the hamstrings.

Ligaments are the strong bands of tissue that keep the ends of the bone connected, and there are four major ones in the knee: the medial collateral ligament (MCL), the lateral collateral ligament (LCL), the anterior cruciate ligament (ACL), and the posterior cruciate ligament (PCL). The MCL and LCL are on the sides of your knee and prevent it from moving too much in a side-to-side direction, while the ACL and PCL are in the front and back, respectively, and regulate the front-to-back bending of the knee.

These tough bands of soft tissue are kind of like ligaments, but instead of connecting bone to bone, they connect muscles to bones, which provides stability to the joint. The largest tendon in the knee is the patellar tendon, which connects the patella to the tibia. When ligaments in the knee are torn or ruptured, tendons are sometimes used as grafts to replace the damaged ligament.

Articular cartilage is a tough, rubbery, shiny material that covers the ends of the bones and serves to absorb shock, while providing a smooth surface to facilitate motion.

When healthy, it provides a mechanism that has almost no friction, so the joint can bend freely.

The menisci are rubbery, crescent-shaped sections of fibrocartilage around the bone that fill the space between the round femur and the flat tibia. They help to absorb shock, spreading stress around the joint, while also providing a softer, smoother surface to protect the articular cartilage from wear.

The bursae are fluid-filled sacs that act as gliding surfaces to reduce friction between the bones, tendons and muscles. They are filled with synovial fluid, a thick liquid that acts as a lubricant inside the joint.