There are four main parts to any given joint. They are bones, ligaments, tendons, and cartilage. Ligaments attach one bone to another, tendons attach a bone to a muscle and cartilage (a white spongy tissue) aids in shock absorption and making sure the joint moves smoothly. The three bones which make up the knee joint are the big thigh bone, the femur, the lower leg bone, the tibia, and a disc shaped bone, the patella. The patella is part of the mechanism that allows us to walk. The patella is held in place by a tendon which is an extension of one of the quadriceps muscles on the front of the thigh. This tendon is called the patellar tendon. The patella fits into a groove called the patellar groove on the front of the femur. As we walk, the patella moves along this groove. If the patella does not glide smoothly in the patellar groove, it can greatly affect the overall function of the knee. (see patello-femoral problems) Inside the knee joint, two ligaments, the anterior cruciate (pronounced: crew-she-it) ligament (ACL) and the posterior cruciate ligament (PCL) make a sort of cross, with the anterior (which means front) cruciate ligament crossing in front of the posterior (which means back) cruciate ligament. These two ligaments help to keep the knee stable by holding the tibia in place and prevent it from moving too far front or back. Ligaments are very important structures in maintaining the stability of a joint. Two other ligaments, the collateral ligaments, work together on each side of the knee joint to keep the tibia from moving too far to either side. These two ligaments are called the medial (which means middle) collateral ligament and the lateral (which means toward the outside) collateral ligament.

Two semi-circular cartilage structures sit between the femur and the tibia. These structures are the medial and lateral menisci. The menisci play a huge role in stabalizing the knee. While the ligaments keep the tibia from moving too far to any given side, this movement would be extremely hard for the ligaments to stop were it not for the menisci. Because the end of the femur is rounded, and the top of the tibia is flat, this joint would move like a ball sitting on a table, not too good for stability. This is where the menisci come in. In addition to providing excellent shock-absorbancy, they act to form a pseudo-ball and socket joint, which is quite stable. You see, the menisci allow the femur to sort-of "sink" and "settle" into them, like sticking a ball to a table using clay. This way the bones are not allowed to move more than they should.