Anatomy and Function

The Anatomy of the Anterior Cruciate Ligament (ACL) 

First things first. Ligaments are short bands of tough, fibrous connective tissue that connect two bones. Consequently, ligaments provide stability at each of our joints. The anterior cruciate ligament (ACL) is one of the four ligaments within the knee. Importantly, the ACL connects the femur (thigh bone) to the tibia (shin bone). 

Want to be technical? It arises from the anteromedial aspect of the tibial plateau and passes upwards and backwards to attach to the posteromedial aspect of the lateral femoral condyle. The ligament itself consists of two components; the anteriomedial bundle and the posterolateral bundle (named after their attachments to the tibia). 

The cruciate name? Formally because the ACL and PCL cross over each other within the knee joint, forming an ‘X’. 

The Function of the Anterior Cruciate Ligament (ACL) 

The Anterior Cruciate Ligament (ACL) is arguably the most important ligament within the knee, and here’s why:

Firstly, it provides approximately 85% of our total restraining force against anterior translation (forward movement of the shin relative to the thigh). Additionally, it prevents excessive rotation of the tibia both medial and lateral. 

As a result the ACL checks extension and hyperextension. 

Long story short, it prevents the knee from ‘giving way’. 


Anterior Cruciate Ligament (ACL) Injuries

The ACL is most commonly injured when an internal or external force creates an anterior translation or rotation movement at the knee joint that outweighs what the ligament can restrain. 

Consequently, damage to the knee in this category are usually seen when: 

  • Stopping suddenly.
  • Landing poorly from a jump. 
  • Suddenly slowing down and changing direction (cutting). 
  • Pivoting with the foot firmly planted.
  • Direct contact to the knee is made against the momentum of the rest of the body. 

An ACL Injury is devastating. 

Unquestionably due to the considerable stability that it provides the knee being lost. 

Evidently, ligament damage can be categorised as a partial or complete rupture, but in both instances the signs and symptoms can typically include: 

  • An audible pop or a ‘popping’ sensation felt in the knee.
  • Most often severe pain and an inability to continue with activity comfortably.
  • A feeling of instability or ‘giving way’ with weight bearing.
  • Knee joint swelling.
  • Loss of range of motion.

There are a few known risk factors that increase risk of an ACL injury. These aren’t limited to, but include: 

  • Anatomical variants such as excessive knee range of motion and structural valgus alignment.
  • Poor limb symmetry of muscle strength.
  • Poor movement patterns while the knee is loaded.
  • Poor conditioning.
  • Participation in pivot, change of direction and contact sports.
  • Gender (hormonal influences).
  • Poor environment: including surface, footwear, and equipment used.

Finally, assessment, treatment options, and rehabilitation for ACL injuries are other topics within themselves. The following parts to this series will explore each. Stay tuned.