Anterior Cruciate Ligament of the Knee: Structure and Functions

The anterior cruciate ligament is one of the four main ligaments of the knee joint. It lies in the center of the joint and keeps the tibia from shifting forward. Understanding its structure is useful for anyone who has heard this term from a doctor or from athletes.

What the anterior cruciate ligament is and where it is located

The knee joint connects the femur, the tibia, and the patella. Four main ligaments work between the thigh and the lower leg. The medial collateral ligament runs along the inner side of the knee and prevents it from bending inward; the lateral one runs along the outer side and limits movement outward. The anterior and posterior cruciate ligaments lie within the joint cavity, in its center.

The anterior cruciate ligament originates from the posterolateral aspect of the inner surface of the lateral femoral condyle, crosses the joint cavity, and attaches to the anterior part of the intercondylar eminence of the tibia. It is an intra-articular structure: it is stretched within the capsule, not outside it, as the collateral ligaments are.

What the cruciate ligaments of the knee do

The ligament's job is mechanical: it is a tether that limits the movement of the bones. The anterior cruciate ligament prevents the tibia from moving forward relative to the femur and restrains the lateral tibial condyle. The posterior one works in the opposite direction — it keeps the tibia from shifting backward.

The knee flexes and extends, with a total range of these movements of 151 degrees. In the flexed position, the collateral ligaments relax, and rotation becomes possible: 15 degrees in total, up to 35 passively. The ligaments in this system serve as limiters, like stops in a hinge.

How the anterior and posterior cruciate ligaments are structured

The ligaments got their name because they cross each other within the joint cavity. The posterior cruciate ligament originates from the anteromedial aspect of the lateral surface of the medial femoral condyle, crosses the joint, and attaches to the lateral part of the intercondylar eminence of the tibia. The anterior one runs toward it.

Most ruptures of the anterior cruciate ligament occur in the middle portion of the fibers or at the attachment site to the femur.

Causes and signs of ligament injury

The injury occurs from a blow to the side of the knee, from hyperextension of the joint, from a sudden stop and change of direction while running, from landing after a jump, or from a twist. A separate mechanism is a force directed forward on the back of the knee with the tibia flexed and rotated inward. Most often this is basketball, football, alpine skiing, handball, or wrestling. In women, anterior cruciate ligament ruptures occur more often than in men.

The ligament does not rupture in isolation: the medial collateral ligament and the medial meniscus are affected along with it — this combination is called the Turner triad. An avulsion fracture of the bone plate at the attachment site can also occur.

• a popping sound at the moment of injury;
• noticeable swelling of the knee within 6 hours;
• pain when trying to bear weight on the leg;
• inability to continue training;
• a feeling of instability, the knee "giving way."

Blood pours into the joint cavity — hemarthrosis develops, the knee enlarges, and the patellar ballottement sign appears. Some people do not notice the moment of injury and later complain of instability. With a mild injury, the only complaint is the knee "giving way" under load.

How the injury is diagnosed: examination, MRI

The main examination technique is the drawer test: the doctor shifts the tibia forward or backward. With an anterior cruciate rupture, the tibia moves forward excessively; with a posterior rupture, it shifts backward easily. In old ruptures, the sign may be unclear: adipose tissue grows around the injury site, which partly stabilizes the joint.

The diagnosis is refined with an X-ray, sometimes with contrast injected into the joint cavity, or with computed tomography or magnetic resonance imaging. MRI confirms the rupture and shows other knee injuries. If necessary, arthroscopy is performed: a probe is inserted into the joint cavity and the joint is examined from the inside.

When to see a doctor

After a serious knee injury, see a doctor immediately: an examination is needed to determine which structures have been damaged. The doctor determines the timeline for returning to activity and sports based on the examination results.

See a doctor urgently. A ligament injury by itself is not a reason to call an ambulance. 103 or 112 — if after a knee injury the foot is cold and blue: the joint may have dislocated, and the vessels to the foot may have been damaged.

This material is for reference: it explains the structure of the ligament but does not replace a doctor's consultation.

This content is for reference and does not replace a doctor's consultation. Medical disclaimer