The knee is the largest joint in the body and it is also one of the most complex. The knee joint is made up of four bones, which are connected by muscles, ligaments, and tendons. The femur is the large bone in the thigh. The tibia is the large shin bone. The fibula is the smaller shin bone, located next to the tibia. The patella, otherwise known as the knee cap, is the small bone in the front of the knee. It slides up and down in a groove in the femur (the femoral groove) as the knee bends and straightens.
Ligaments are like strong ropes that help connect bones and provide stability to joints. In the knee, there are four main ligaments. On the inner (medial) aspect of the knee is the medial collateral ligament (MCL) and on the outer (lateral) aspect of the knee is the lateral collateral ligament (LCL). The other two main ligaments are found in the center of the knee. These paired ligaments are called the anterior cruciate ligament (ACL) and the posterior cruciate ligament (PCL). They are called cruciate ligaments because the ACL "crosses" in front of the PCL. Smaller ligaments help hold the patella in the center of the femoral groove.
Two structures called menisci sit between the femur and the tibia. These structures act as "cushions" or "shock absorbers". They also help provide stability to the knee. There is a medial meniscus and a lateral meniscus. When either meniscus is damaged it is often referred to as a "torn cartilage".