ACL Reconstruction General Information

The Anterior Cruciate Ligament (ACL) is tough fibrous rope like structure in the centre of the knee that connects the lower end of the thigh bone (femur) to the upper end of the shin bone (tibia). It is approximately 25mm in length with the diameter of a pen. Its primary function is to prevent the femur from moving  abnormally apart from the tibia especially during athletic activities. 


The ACL is the main support structure of the knee that prevents:

  • The tibia from translating forward on the femur
  • Valgus, varus and hyperextension of the knee
  • Rotation of the thigh bone (femur) on the shin bone (tibia)


When this ligament suffers a tear it doesn’t heal on its own and will often lead to a feeling of instability in the knee.

An ACL rupture is one of the most common knee injuries and one of the most common sports injuries. It is recommend to reconstruct a torn ACL to prevent future osteoarthritis and further damage to the knee joint.

ACL rupture in females is three times more common than males. This is due to:

  • Oestrogen hormones which lead to weaker collagen
  • Anatomical tight notch
  • A wider pelvis which leads altered mechanics and increases valgus stress on landing from a jump

70% of pure ACL ruptures are from a non-contact injury since contact injuries are more likely to result in other ligament injuries such as torn posterior cruciate ligaments and collaterals (medial cruciate ligament, lateral cruciate ligament) as well as the rupture of the ACL.


Signs and Symptoms

  • Most injuries to the ACL are sports related and result from a twisting injury to the knee, a forceful jump with the knee in flexion, a sudden change of direction, a direct blow to the knee or landing awkwardly.
  • Pain when bearing weight on the affected leg or at rest
  • Swelling of the knee joint which can occur within 24 hours of the injury

  • Most likely there will be some instability when walking and the feeling of being unstable or ‘wobbly’ on the leg as well as the sensation of the knee feeling not as tight or compact as it was previously

  • 30%-50% of patients report hearing and feeling a ‘pop’ which occurs when the ligament tears



A physical examination can reveal instability of the knee but the best current method of identifying a torn or damaged ACL is by using an MRI scan which has a high sensitivity and specificity in diagnosing ACL injury and any associated meniscal injury. In extremely difficult cases, sometimes the final diagnoses can only be made during arthroscopy surgery.


For ACL surgical interventions and treatment options please see ACL Reconstruction Treatment Options.