The Anterior Cruciate Ligament (ACL) is one of two main internal stabilisers of the knee. Along with the Posterior Cruciate Ligament (PCL), the cruciate ligaments work in concert to reduce shear (to the front and into rotation) of the tibia on the femur. This is only one of many functions of the ACL and is one of the main reasons why the ACL becomes damaged.
As you can see from the above diagram, the ACL has close links to the medial meniscus (cartilage), which is in turn attached to the medial collateral ligament. Remember this, it will become important later….
Main Functions of the ACL
The main function of the ACL is to reduce anterior translation and rotation of the tibia on the femur. It also has an important role in the brain’s understanding of where the knee is in space.
How Does It Go Wrong?
There are a number of ways ACL injuries occur:
intrinsic - i.e. occur due to movement or loading of the individual knee in a way that overloads the ACL to the point of damage or
extrinsic - i.e. trauma caused by a blow causing overload of the ACL to the point of damage.
Classically, the ACL becomes damaged during deceleration movements with the lower leg is turned outwards relative to the knee, which is why physios keep banging on about hip, knee and foot being in alignment as this reduces this type of shear. This can come from sudden changes in direction, poor landing from a height or pivoting with a fixed foot.
Extrinsic (traumatic) can be caused by force striking (usually) the outside of the knee. In the most severe cases, due to the close links between the structures, an ACL tear can also involve the medial meniscus and medial collateral ligament - also known as the ‘Unhappy Triad’ injury.
What can I do to reduce the risk of ACL injuries?
Much of this needs to be taken care of through management of well aligned movement patterns. If the resting position of the lower limbs tends towards either knees facing forwards with feet turned out or feet facing forward with kneed facing inwardly, stress on the ACL is increased. There are two main contributors to this, either poor hip and trunk control and/or tightness in the calf (especially gastrocnemius) muscle. There seems to be some unpublished data suggesting a predeterminant of ACL injury can be recent poorly/incompletely rehabilitated ankle injuries which then place more load upon the knee.
Read our blog post on how to avoid skiing-related ACL injuries.
What happens if it goes wrong?
An ACL injury is generally accompanied (but not always) by significant swelling. A feeling of the knee giving way (especially on going down slopes or hills) is also a good indicator, however there is usually a significant loss of range of movement and pain that are more obvious indicators.
What should I do?
That depends upon the severity of the injury and what you would like your lifestyle to include. A surgical opinion is highly recommended, however there is quite a trend currently to eschew surgery and rehabilitation in favour of non-operative management.
In my experience this tends to prolong the inevitable surgery and rehab if you are interested in an active lifestyle with multidirectional sports/activities. Rehabilitation can be a long, frustrating process (between 9 and 12 months) however whilst it won’t ever return the knee to the ‘perfect’ pre-operative state, it will give you a strong, functional knee that will allow you to continue the vast majority of activities with minimal restriction.
Words by Paul Martin.