The knee ligaments can be split into 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.
They are frequently damaged by sudden twisting movements or a force through the knee eg a fall or tackle. After a ligament injury, the knee can feel painful, weak and unstable. If this happens, wearing a brace can help.
The knee joint is made up 3 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.
Here we will look at the different knee ligaments, how they work, and what happens when they get damaged.
These knee ligaments are found on either side of the joint. They are responsible for providing sideways stability by holding the bones together. There are two collateral ligaments, medial and lateral.
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).
When it is damaged it causes pain on the inner side of the joint - visit the MCL injuries for more information including treatment options.
The lateral collateral ligament is found on the outside of the knee, attaching to the femur and the fibula. It is much shorter than the MCL. It resists forces from the inner side of the knee (known as varus forces). It is less common to injure the LCL than the MCL.
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).
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, ie ACL attaches to the anterior surface of the tibia.
Their job is to control the forwards and backwards movement of the knee joint. They are also important in providing proprioception – the bodies 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.
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 propriception.
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. For more information visit the
ACL injuries section.
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.
NB What is the difference between a ligament and a tendon?
A ligament joins bone to bone whereas a tendon joins bone to muscle.