Overview
The knee is made up of three bones:
- Femur - the thigh bone, which approaches the knee from above.
- Tibia - the larger lower-leg bone, which approaches the knee from below.
- Patella - the kneecap, which sits at the front of the knee. Together, these bones form the three compartments of the knee.
The Three Compartments
The medial compartment is where the femur and tibia meet on the inside of the knee, toward the opposite knee. The lateral compartment is where the femur and tibia meet on the outside of the knee, away from the opposite knee. The patellofemoral compartment is where the patella meets the groove on the front of the femur, called the trochlea. These are the three compartments we commonly refer to when describing arthritis and other conditions within the knee.
Articular Cartilage
The surfaces of the bones within the knee are covered by articular cartilage, an extremely smooth surface that allows the bones to glide against one another with very little friction. The progressive breakdown and loss of this cartilage is what we call arthritis.
Joint Fluid
The knee contains a small amount of joint fluid that lubricates the joint and helps the articular surfaces glide smoothly.
Meniscus
There are two menisci: the medial meniscus and the lateral meniscus. Each is a C-shaped structure attached to the top of the tibia. The menisci help the rounded femur articulate with the relatively flat surface of the tibia. They also function as shock absorbers and contribute to the stability of the knee.
Ligaments
The knee has four major ligaments that connect bone to bone and provide stability. The medial collateral ligament (MCL) and lateral collateral ligament (LCL) are located along the sides of the knee. The anterior cruciate ligament (ACL) and posterior cruciate ligament (PCL) are located inside the knee. Together, these ligaments help control movement and provide stability throughout knee motion.
Extensor Mechanism
The quadriceps tendon connects the quadriceps muscles to the patella. Below the patella, the patellar tendon connects the patella to the tibia. Together with the patella and quadriceps muscle, these structures form the extensor mechanism, which allows the knee to straighten.
Muscles
The quadriceps muscles are located at the front of the thigh and straighten the knee. The hamstring muscles cross the knee from behind and help bend the knee.