Structure of the Knee Joint
The knee joint is the largest and most superficial joint of the lower limb. It is a synovial hinge joint and one of the most complex joints of the body. Unlike most hinge joints, which allow only flexion and extension, the knee joint is able to combine flexion and extension with gliding, rolling and rotation around a vertical axis.
The knee joint is essential for everyday activities such as standing, walking and climbing stairs. It is also the major joint for athletic actions such as running, jumping and kicking. All these activities require the knee joint to be extremely mobile. This mobility is what makes the knee joint extraordinary, but also is what makes it one of the most injury-prone joints in the body.
-
Bones
-
Three major bones of the lower limb are connected together by the knee joint. The largest of the three is the femur (thigh bone). The femur meets up with the tibia (the larger of the two shin bones) to form the hinge of the joint.
In front of these two bones is the patella (known as the kneecap). The patella is about 3 inches long and 2 inches wide. It sits on top of the other bones and slides when the leg moves. It also protects the knee.
Muscles
-
The quadriceps and hamstring muscles combine to support and move knee. The four quadriceps muscles connect to the front of the knee and create leg extension when they contract. The hamstring muscles oppose the quadriceps along the back part of the leg and help to flex the knee. .
-
Ligaments
-
The ligaments are often considered the most important parts of the knee joint. They wrap around the whole joint and provide strength and stability. There are four major ligaments in the knee; each connect the femur and tibia.
The most well known of the ligaments is the anterior cruciate ligament (ACL). It works to control and limit rotation and forward movement of the tibia. It runs down the center of the front of the knee. Partnered with the ACL on the back part of the knee is the posterior cruciate ligament (PCL). It runs down the center of the back of the knee and limits backward movement of the tibia.
The medial collateral ligament (MCL) and the lateral collateral ligament (LCL) provide stability to the inner (medial) and outer (lateral) part of the knee.
Tendons
-
Tendons connect the muscle to bone. They also help support the action and leverage of the muscle. In the knee, the major tendon connects the quadriceps to the patella. It helps the muscle extend the leg.
Joint
-
The knee joint consists of an external fibrous capsule and an internal synovial membrane. The membrane lines all the internal sufaces of the joint cavity that are not covered with articular cartilage and contains synovial fluid.
The knee has a full range of motion. It is able to extend to a straight and 'locked' postion, flex between 120 and 160 degrees (depending on the position of the hip) and rotate medially 5 to 10 degrees and medially 30 degrees.
Injuries to the knee joint are extremely common because it is a low-placed, mobile and weight-bearing joint that serves as a meeting point of the two long levers of the upper and lower leg bones. Its stability is almost completely dependent on the surrounding ligaments and muscles, which are often too weak to counteract the forceful blows that the knee is susceptible to.
-
