Knee Joint Ligaments

Four Components of Knee Anatomy

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Knee ligaments are thick strands of tissue made of collagenous fibers that connect the upper leg bones to the lower ones. There are four main knee joint ligaments: the lateral collateral ligament (LCL), medial collateral ligament (MCL), anterior cruciate ligament (ACL), and the posterior cruciate ligament (PCL). Collectively, their main function is to restrain knee movement in order to provide joint stability.

Injury to any one of these knee ligaments means that the tissue has been stretched or torn. Symptoms and treatment will vary depending on the degree of the injury.

Knee Joint Anatomy

Your knee joint is the largest joint in the body and is made up of three bones—your femur (thighbone), your tibia (shinbone), and your patella (kneecap). Knee joint ligaments help control and support their movement.

Your LCL and MCL, the two collateral ligaments in your knee, work to control the sideways movement of your knee joint. 

The two cruciate ligaments in your knee—your ACL and PCL—work to control the backward and forward movement of your knee joint.

Lateral Collateral Ligament (LCL)

The lateral collateral ligament is located on the outside of the knee joint, and it connects your femur to your fibula (a lower-leg bone that is smaller than the tibia). The LCL prevents excessive adduction of the knee (i.e., movement toward the central axis of the body).

LCL injuries often occur as a result of a blow or hit to the inside of the knee, which pushes the knee outward. Symptoms may include pain located on the outside of the knee, along with swelling and knee instability (feeling like the knee is "giving out").

common causes of knee instability
Verywell / Brianna Gilmartin

Medial Collateral Ligament (MCL)

The medial collateral ligament is located on the inside of the knee joint, and it connects the femur to your tibia. This ligament prevents excessive valgus angulation of the knee (i.e., a knock-kneed position).

Injury to the MCL often occurs when the knee suffers a blow or hit to the outside, pushing it inward. Symptoms are similar to LCL injuries, but the pain and swelling are located on the inside of the knee, not the outside.

Anterior Cruciate Ligament (ACL)

The anterior cruciate ligament runs diagonally down the middle of the knee and connects the femur to the tibia. This ligament prevents your shinbone from sliding too far forward during running, hopping, and activities that involve changing direction quickly (cutting).

ACL stretches and tears (either partial or complete) are one of the most common injuries to the knee. ACL injuries usually occur during a physical activity that involves either suddenly stopping or changing direction, such as football. While most ACL injuries are non-contact injuries that occur from landing on the leg funny or an abnormal twist, sustaining a direct hit to the knee may also result in an ACL injury.

After injuring their ACL, a person may hear a sudden "pop" and feel their knee buckling or giving way.

Other symptoms of an ACL injury may include:

  • Knee swelling and pain
  • Decreased or loss of range of motion
  • Tenderness along the joint line
  • Pain with walking
  • Knee joint effusion (fluid within the joint space) one to two hours after the injury

Interestingly, females are more likely to suffer from an ACL injury than males. Research suggests this may be due to a number of factors—differences in strength and stiffness of the knee muscles, neuromuscular control, and collagen concentration (due to estrogen), to name a few.

Posterior Cruciate Ligament (PCL)

The posterior cruciate ligament is the strongest and largest ligament in the knee. It runs diagonally down the back of your knee, connecting your femur to your tibia. The main function of the PCL is to prevents your tibia from moving too far backward.

PCL injuries are produced by some sort of external trauma, such as a "dashboard injury" in which a person hits their flexed knee against the dashboard during a motor vehicle accident.

A direct blow to the front of the shinbone or a fall onto your knee with your foot pointing down can also cause a PCL injury. Symptoms of a PCL injury often include stiffness, swelling, and pain located at the back of the knee and/or pain with kneeling.

In general, PCL injuries occur far less commonly than ACL injuries. The sports most commonly associated with PCL injuries are football, soccer, rugby, and skiing.

Treatment of Knee Ligament Injuries

The treatment of your knee ligament injury depends on the degree of the injury and what specific ligament is injured. For example, torn MCLs often do not require surgery. Many ACL tears do not require surgery either. Everyday activities like walking and jogging do not require an intact ACL. ACL repair is recommended if an individual has the sensation that their knee is unstable, or if they are looking to return to athletic activity that involves side-to-side motion.

Likewise, most PCL and LCL injuries can be treated non-surgically, but if there are combined injuries (meaning more than one ligament is injured), surgery may be indicated.

Non-surgical strategies for knee ligament injuries often include one or more of the following:

  • R.I.C.E. (rest, ice, compression, and elevation)
  • Immobilizing the knee and protecting the injured ligament with a brace
  • Taking an over-the-counter pain medication, such as Tylenol (acetaminophen) or a nonsteroidal anti-inflammatory drug (NSAID), such as Motrin (ibuprofen)

In many instances, physical therapy (whether you end up undergoing knee surgery or not) plays an essential role in your knee healing and recovery.

A physical therapist can teach you knee stretches and strengthening exercises to help restore the normal strength and range of motion (ROM) to your knee.

A Word From Verywell

Your knee is a major weight-bearing joint in the body and its complex anatomy is what allows you to walk, run, and rising from sitting. If you think you may have injured one or more of your knee's ligaments, please see your doctor promptly. With a proper treatment plan, you can hopefully get back to your daily routine as safely and as quickly as possible.

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