Axis Knee

Anatomy Of The Knee

The knee joint is the largest joint in the body; however, it is highly susceptible to injuries. This has been attributed to its complex anatomical structure. A significant number of knee injuries involve the anterior cruciate ligament (ACL), the ACL injury often being caused by non-contact situations.

Understanding the anatomy of the knee joint, specifically the ACL, is fundamental to understanding the mechanisms of ACL injuries, development of signs and symptoms, treatment and knee strengthening exercises.

THE BONES

The knee joint is composed of 4 essential bones: the femur (thigh bone), tibia (shin bone), fibula (calf bone) and patella (kneecap). The femur is attached to the tibia by the tough ligaments and a knee capsule. The tips of these bones are covered by articular cartilages that keep them from grinding each other during movements. Most knee movements occur between the femur, patella and tibia.

Femur

The femur is the major weight bearing bone of the upper leg. This important and highly demanding function is evidenced by its size and structure. The femur is the largest and strongest bone in the human body. It provides attachment sites to muscles that move the knee.

Tibia and fibula

The bones of the lower leg are the tibia and the fibula. They run parallel to each other, starting from the knee to the ankle. The fibula articulates with both ends of the tibia, allowing a slight degree of movement.

Next to the femur, the tibia is the largest bone in the body. It is the weight bearing bone of the lower leg. The upper end of the tibia joins with the femur and its lower end joins with the talus, the bone that forms the lower part of the ankle. The fibula is located on the outer side of the leg. Unlike the tibia, this bone is non-weight bearing. Instead, it functions as an ankle joint stabilizer and as an attachment site for one of the four major knee ligaments and the biceps femoris tendon.

Patella

The patella, also known as the kneecap, protects the knee joint. It holds the quadriceps tendon on the lower end of the femur, acting as a fulcrum for the quadriceps muscles. The quadriceps is a group of four individual muscles on the anterior part of the thigh. The lower patella connects to the tibia through the patellar tendon.

MENISCI

Incompletely covering the surface of the tibia that joins with the femur are the C-shaped fibrocartilages known as the medial and lateral menisci. The menisci function as shock absorbers that equally spread the weight of the body, reducing friction between the tibia and the femur during knee movements. They assist in knee rotation and play a function in stabilizing the ligaments.

Most areas of the menisci are not supplied by blood, which contains oxygen and nutrients needed for healing. When these structures are damaged, repair is nearly impossible. The menisci deteriorate with age. Menisci damage is a common cause of knee pain and injuries.

KNEE JOINT CAPSULE

The knee capsule is a thick, water-tight sac that surrounds the knee joint. The outer part of the capsule is lined with a fibrous and thick membrane. Within the capsule is the synovial membrane, which produces the fluid that lubricates the joints. This fluid also provides nutrients to the articular cartilage.

MUSCLES

The movements and the stabilization of the knee joint are supported by the quadriceps and the hamstrings. The quadriceps is actually composed of four individual muscles located on the anterior upper leg. These muscles fuse, forming the quadriceps tendon. The quadriceps straightens the knee by pulling the patella up on contraction.

The hamstrings are the muscles that attach to the tibia, specifically at the back of the knee. It consists of three individual muscles. The hamstrings functions by flexing or bending the knee joint. This muscle group also provides stability on both sides of the knee.

THE LIGAMENTS

The stability of the knee largely depends on the four major knee ligaments: the medial collateral ligament (MCL), lateral collateral ligament (LCL), anterior cruciate ligament (ACL) and posterior cruciate ligament (PCL).

Ligaments are the tough but slightly elastic bands of connective tissues that hold two or more bones together. Excessive movements, such as hyperextension or hyperflexion, at the knee joint are restrained by these ligaments, stabilizing the knee joint and keeping the bones in their correct alignment during movements.

Medial and lateral collateral ligament

The MCL resists excessive forces coming from the knee’s outer surface. The LCL resists the forces coming from the inner surface of the knee. These ligaments are located on the outside of the knee joint and are able to heal on their own.

Anterior cruciate ligament and posterior cruciate ligament

The cruciate ligaments are located within the center of the knee, where they are constantly bathed by the synovial fluid. These ligaments form a cross right in the center of the knee joint.

The ACL and PCL are closely interconnected and if one of them is injured, the damage can completely disrupt the function of the entire knee joint. Without timely and appropriate treatment, a cruciate ligament injury may result in degenerative arthritis.

The ACL is considered one of the most important structures of the knee. Functioning as the knee’s major stabilizer, the ACL runs from the thigh bone to the shin bone through the center of the knee, and provides stability and reduces stress across the knee. It prevents hyperextension of the knee by restraining the excessive forward movement of the tibia in relation to the femur.

Working with the PCL, the ACL limits the rotational movements of the knee. If one of these ligaments is damaged, instability of the knee occurs whenever the foot of the affected leg is planted on the ground or during pivoting. A major injury involving the ACL usually requires extensive surgery and prolonged therapy.

In contrast to ACL, the PCL is not frequently injured. The PCL is almost two times stronger than the ACL and is more resistant to damages of excessive forces. Not much is known about this ligament; however, it is believed that the PCL functions as a central axis during knee rotation.

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