The Anterior Cruciate Ligament Explained


The Anterior Cruciate Ligament (ACL) lies deep within the knee joint, connecting the thigh bone (femur) to the shin bone (tibia).


The function of the ACL is complex for it provides both mechanical stability and proprioceptive feedback to the knee. In its stabilising role, it prevents excessive forward motion of the shin relative to the thigh; it also prevents excessive rotation at the knee joint and hyperextension of the knee joint.


The ACL ligament can be injured in several different ways, most commonly by landing from a jump onto a bent knee then twisting, or landing on a knee that is hyperextended. In collision sports, direct contact of the knee from opponents can cause damage to the ACL. When skiing the injury normally occurs during a fall when the bindings do not release. Because of the amount of force required to injure the ACL it is not uncommon for other structures inside the knee to be damaged, such as the meniscus (cartilage) or the medial collateral ligament (the ligament on the inside of the knee).

Signs and Symptoms

At the moment of injury the person may experience a popping sensation deep within the knee. There will be pain, proportional to the force and degree of damage to other structures within the knee joint. The knee will become painful and swollen due to what is known as a haemarthrosis (bleeding within the joint).


During the acute stage of the injury (the first 48-72hours) exact diagnosis can sometimes be difficult due to the pain and the gross swelling inside the knee. Once the initial treatment to decrease the swelling (Rest Ice Compression Elevation regime) has taken effect a clinical diagnosis may be possible.

The management of an ACL injury is completely dependent upon the degree of damage and the subsequent functional impairment, the age of the patient and the level of sporting activity. If the diagnostic investigations only reveal a partial tear and there is minimal instability then a conservative approach with a physiotherapist is indicated. If the ligament is completely ruptured and functional instability is present (i.e. giving way) then surgical reconstruction may be necessary to restore normal function and a return back to sport.

Once the acute stage of the injury has been treated rehabilitation to the knee can start.

The aims of the rehab program are outlined below:

  • To regain full range of motion at the knee joint
  • To regain full strength to the injured leg
  • To regain endurance and power to the injured leg
  • To retrain balance and proprioception
  • Sports specific rehab to allow a safe return to sporting activity