The knee is made up of four bones. The femur or thighbone is the bone connecting the hip to the knee. The tibia or shinbone connects the knee to the ankle. The patella (kneecap) is the small bone in front of the knee and rides on the knee joint as the knee bends. The fibula is a shorter and thinner bone running parallel to the tibia on its outside. The joint acts like a hinge, but with some rotation.
Smooth frictionless movement of the knee joint is allowed by the synovium, which produces a fluid that lubricates and nourishes the inside of the joint and the articular cartilage, a smooth tissue that lines the joint bone surfaces.
The different parts of the knee joint include:
The femur (thighbone) is the largest and the strongest bone in the body. It is the weight-bearing bone of the thigh and provides attachment for most of the muscles of the knee.
Two femoral condyles make up the rounded end of the femur. Its smooth articular surface allows the femur to move easily over the tibial (shinbone) meniscus.
The tibia, the second largest bone in the body, is the weight-bearing bone of the leg.
The fibula, although not a weight-bearing bone, provides attachment sites for the lateral collateral ligament (LCL) and the biceps femoris tendon.
The articulation of the tibia and fibula also allows a slight degree of movement, providing an element of flexibility in response to the actions of muscles attaching to the fibula.
The patella (kneecap), attached to the quadriceps tendon above and the patellar ligament below, rests on the anterior articular surface of the lower end of the femur and protects the knee joint. It acts as a fulcrum for the quadriceps by holding the quadriceps tendon off the lower end of the femur.
The medial and the lateral meniscus are thin C-shaped layers of fibrocartilage, incompletely covering the surface of the tibia where it articulates with the femur. The menisci act as shock absorbers, protecting the articular surface of the tibia as well as assisting in rotation of the knee. The majority of the meniscus has no blood supply and for that reason, when damaged, the meniscus is unable to undergo the normal healing process that occurs in the rest of the body. The menisci act as shock absorbers, protecting the articular surface of the tibia as well as assisting in rotation of the knee. As secondary stabilizers, the intact menisci interact with the stabilizing function of the ligaments and are most effective when the surrounding ligaments are intact.
The anterior cruciate ligament (ACL) is the major stabilizing ligament of the knee. The ACL is located in the center of the knee joint and runs from the femur to the tibia, through the center of the knee. The ACL prevents the femur from sliding backward on the tibia (or the tibia sliding forwards on the femur). Together with the posterior cruciate ligament (PCL), the ACL stabilizes the knee in a rotational fashion. Thus, if one of these ligaments is significantly damaged, the knee will be unstable when planting the foot of the injured extremity and pivoting, causing the knee to buckle and give way.
The PCL prevents the femur from moving too far forward over the tibia. The PCL is the knee’s basic stabilizer and is almost twice as strong as the ACL. It provides a central axis about which the knee rotates.
Collateral ligaments prevent hyperextension, adduction and abduction.