Knee Ligaments

Author: Chloe Wilson - BSc(Hons) Physiotherapy

The knee ligaments are one of the vital components of knee stability and control. There are two pairs, the collateral ligaments (around the sides of the joint) and the cruciate ligaments (inside the joint). Ligaments are thick fibrous bands like ropes and their job is to provide stability by holding bones together. 

The knee joint is made up three bones, the femur (thigh bone), tibia (shin bone) and the patella (kneecap). The ligaments hold the bones together, control how the knee moves to keep it stable and prevent injury.

Ligaments are frequently damaged by sudden twisting movements e.g. changing direction quickly when running, or a force through the knee e.g. a fall or tackle.

After a ligament injury, the knee can feel painful, weak and unstable. This may only last a few days, but if left untreated, can persist for many months. If this happens, wearing a brace can help.

Here we will look at the different knee ligaments, how they work, and what happens when they get damaged.

Collateral Knee Ligaments

The collateral knee ligaments are found on either side of the knee joint. They are responsible for providing sideways stability by holding the femur and tibia bones together.  There are two collateral ligaments, medial and lateral.

1) Medial Collateral Ligament (MCL)

Diagram showing the knee ligaments

The medial collateral ligament is found on the medial (inner) side of the knee. It is a broad flat ligament approximately 10cm long attaching to the femur and the tibia. It resists forces from the outside of the leg (known as valgus forces).

It gets damaged when there is a force through the outer side of the knee, overstretching the ligament.  This results in pain and swelling on the inner side of the joint and the knee may feel unstable depending on the severity of the injury. Visit the MCL injuries for more information including treatment options.

2) Lateral Collateral Ligament (LCL)

The lateral collateral ligament is found on the outside of the knee, attaching to the femur and the fibula.  It resists forces from the inner side of the knee (known as varus forces). The lateral collateral ligament is much shorter than the medial collateral ligament making it much less common to injure the LCL than the MCL.

Cruciate Knee Ligaments

These are the most important knee ligaments in providing stability of the knee. There are two cruciate ligaments, anterior (ACL) and posterior (PCL). They sit deep inside the middle of the joint attaching to the tibia and femur. They cross over each other, hence their name, and resemble the St Andrews Cross (X).

Diagram showing the knee ligaments.  The ACL is shown in dark blue, the PCL in light blue

The cruciate knee ligaments are each about as thick as a pencil and are extremely strong, with a breaking strain of about 60kg. They get their name from where they attach to the tibia, i.e. the ACL attaches to the anterior (front) surface of the tibia and the PCL to the posterior (back) surface.

Their job is to control the forwards and backwards movement of the knee joint. They are also important in providing proprioception – the body's ability to know where it is and to make subtle adjustments to maintain balance. Each ligament is about 2cm long and any force which stretches it an additional 1.7mm (8% total length) will result in complete tear.

1) Anterior Cruciate Ligament (ACL)

The anterior cruciate ligament sits deep in the middle of the knee joint. It attaches to the front of the tibia and the back of the femur. It stops the tibia sliding too far forward in relation to the femur and is the primary structure for proprioception.

The ACL is commonly injured in sporting activities such as football and skiing, usually from awkward twisting movements, sudden stopping or landing awkwardly. It can take up to a year to recover from an ACL injury so prevention has become an important factor in sports training.  To find out more visit the ACL injuries section for in-depth information on injuries, surgery and rehab.

The posterior cruciate ligament gets damaged when it is overstretched

2) Posterior Cruciate Ligament (PCL)

The posterior cruciate ligament attaches to the back of the tibia and the front of the femur. It is shorter than the ACL (3/5 of the length) but is twice as strong, and is therefore not damaged as often as the ACL. It stops the tibia moving too far back in relation to the femur.

It is most commonly damaged by a sudden force through the top of the shin, from a car accident or fall, or by hyperextending the knee.  You can find out more in the PCL injury section.

Want to Know More?

To find out more about knee ligaments injuries or for help diagnosing your pain click on the relevant links.

NB What is the difference between a ligament and a tendon?
A ligament joins bone to bone whereas a tendon joins bone to muscle.

Go to Anatomy Section or Knee Pain Guide

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