The knee is the largest joint in the human body and a common source of athletic-related injuries. Classified as a pivotal hinge joint, the knee allows for straightening and bending in one direction, with some twisting (screw home mechanism).
The knee joint is made up of bones, cartilage, ligaments, tendons, bursae, and meniscus. A injury to any one of these structures can cause knee pain.
The knee joint is comprised of three bones that interact to allow for hinge-like motion:
The tibia (shin bone). The tibia is the primary weight bearing bone of the lower leg and connects to the bottom of the femur, forming the hinge of the knee joint.
The femur (thigh bone). The femur is both the longest and strongest bone in the human body. It supports the upper body weight during standing, walking, or running. Injuries to the femur can take longer to heal than injuries to other bones. The femur is covered in cartilage where it meets with the tibia (below the femur) and patella (front of the femur).
The patella (kneecap). The patella sits at the anterior-most (front) of the knee joint. The patella anatomically lies in the front of the knee, in the groove formed by the two cartilage covered expansions (condyles) of the lower femur (patellofemoral joint).
The quadriceps muscles and hamstring muscles provide most of the power and control for the knee joint.
The quadriceps muscles. This is a group of four muscles: (vastus lateralis, vastus intermedius, vastus medialis, and rectus femoris) located on the front of the thigh. These muscles are connected to the knee joint via the quadriceps tendon. When these muscles contract and shorten, the leg is straightened.
The hamstring muscles. This is a group of 3 muscles (biceps femoris, semimembranosus, and semitendinosis) located on the back of the thigh and knee joint. When activated these muscles act to bend the knee.
Both muscle groups are vital to forward motion. Injuries to these muscle groups can range from mild strains and bruises to debilitating tears.
Tendons connect the knee bones to the leg muscles that move the knee joint.
Ligaments join the knee bones and provide stability to the knee. The four main ligaments in the knee connect the femur (thighbone) to the tibia (shin bone), and include the following:
Anterior cruciate ligament (ACL) - The ligament, located in the centre of the knee, that controls rotation and forward movement of the tibia (shin bone).
Posterior cruciate ligament (PCL) - The ligament, located in the centre of the knee, that controls backward movement of the tibia (shin bone).
Medial collateral ligament (MCL) - The ligament that gives stability to the inner knee.
Lateral collateral ligament (LCL) - The ligament that gives stability to the outer knee.
Numerous bursae, or fluid-filled sacs, help the knee move smoothly.