Anterior Cruciate Ligament (ACL)

The anterior cruciate ligament (ACL) is one of the four primary knee ligaments. Ligaments are tough fibrous pieces of connective tissue between bones. The ACL runs diagonally across the middle of the knee. It prevents the thigh bone from sliding out and forward in front of the shin bone as well as providing rotational stability. The ligament is commonly injured when suddenly changing directions while playing sport or hyper extending the knee. Many people report feeling or hearing a pop followed by pain within the knee. The knee tends to swell up quite quickly because of the ligament’s blood supply. It may be difficult to walk on the knee after such an injury.


The initial treatment is to rest and ice the knee and try and keep the swelling down. A thorough medical examination after such injury can determine which structures have been damaged and what further investigations are needed. This may include x-rays to exclude any broken bones but an MRI scan is better to assess the soft tissue structures and extent of the injury. Not all cruciate injuries have to have surgery. Nonsurgical treatment may be a good option for people with low demand activities or people who do not complain of any instability about the knee. This treatment will involve physiotherapy and possibly lifestyle modifications. However, younger patients who are keen to continue with pivoting sports or any patient who continues to complain of instability type symptoms during their normal daily activities are better with a surgical option.

Surgery options

There are different surgical options for an ACL injury, depending on the patient’s individual circumstances and extent of the injury. The goal of ACL surgery is to stabilise the knee, minimise the risks of reinjury and prevent long-term arthritis. Younger patients (less than 25 years of age) have the highest risk of reinjuring the ACL. Other risk factors include hyper lax knee joints or other knee ligaments injured at the same time. Depending on the severity of the cruciate tear, different graft options are available. The commonest grafts used are patellar tendon, hamstrings or quadriceps tendon. In some circumstances, the ACL is torn from the thighbone but otherwise intact. Depending on the age of the patient and condition of the ACL, repairing the ligament back onto the bone is possible and has become a viable option in the last few years. However, most of the time a repair is not possible. Often, the ACL remnant may be preserved and supplemented with a graft such as a hamstring tendon.


In high demand younger patients, graft taken from the central third of the patellar (kneecap) tendon has the lowest re- rupture rate and is therefore usually chosen. It does have a higher incidence of pain towards the front of the knee while kneeling. Other factors include whether there is a lot of rotational instability after the injury and supplementing the ACL graft with a lateral ligament repair (anterolateral ligament) has recently been shown in the 2020 Stability study to reduce the risks of re-rupture of hamstring grafts from 11% to 4% in younger patients. There are numerous variables and risk factors that have to be considered in each case to minimise the risks of re-rupture and long-term arthritis. There is an overall graft re-tear rate of 2% to 8%, higher in younger patients. The rehabilitation period is usually 9 months or more and over 80% of patients will return to play.

With so many variables, each patient and injury is different and so the ACL surgery must be individualised.

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