Anterior Cruciate Ligament Injury

ACL Injury Overview

The anterior cruciate (ACL) is one of two ligaments in the centre of the knee which have been described as primary stabilisers. The ACL prevents anterior movement or translation of the femur (thigh bone) on the tibia (shin bone) whilst allowing a functional range of movement during activity.

In the UK there has been a significant increase in ACL injuries seen. This in part is due to a better appreciation of the injury amongst therapists and clinicians, and in part due to the increase in participation in sporting and leisure activities. One particularly worrying observation is the greater numbers of children and adolescents we are seeing with this injury. Children are playing sports at a higher intensity and at a younger age than in the past. This is in many ways laudable but it is putting stresses and strains on the immature skeleton and soft tissues which may have long term consequences. 

Mechanism of injury and diagnosis

The classic history is of a non-contact pivoting injury during which the patient often experiences a popping sensation in the knee. It is usually not possible to continue with the activity and in the majority of cases the knee becomes very swollen within minutes. This swelling is known as a haemarthrosis and is the result of bleeding into the knee cavity from blood vessels in the torn ligament. Haemarthrosis is an indication of serious internal derangement within the knee an always warrants further investigation, best carried out by a knee specialist. At the very least an X-ray of the knee should be undertaken within 24 hours of the injury to rule out a fracture or broken bone or dislocation of the patella. To confirm the diagnosis of ACL rupture an MRI scan should then be carried out. The MRI will also allow the visualisation of any associated injuries. These include tears of one of the meniscal cartilages, damage to the articular surfaces within the joint or damage to one or more of the other ligaments about the knee.
It is a sad fact of modern emergency medical services that a patient with an obvious haemarthrosis is seen in the A&E department by a junior doctor who rules out a fracture with an X-ray and then discharges the patient without making the appropriate referral or follow up arrangements.


Examples of specific injury mechanisms that I see frequently include;

  • Rugby
  • Football
  • Cricket
  • Netball
  • Badminton
  • Skiing

In addition there are situations outside sporting activity of high risk usually involving jumping down (often backwards) from a relatively low height onto an uneven surface.

Is surgery always needed?

There is a spectrum of severity of ACL injury. Each patient is different and treatment should reflect this. Patients with only moderate laxity and low expectations may be best treated with non-operative means and a program of rehabilitation with a physiotherapist will often allow return to ‘normal’ activity. The treatment program may take up to 16 weeks and initially should be directed at reduction of swelling and maintenance of muscle tone, especially in the quadriceps. Further treatment concentrates on proprioceptive (balance) training and strengthening.
If there is a high degree of laxity and symptomatic instability, then surgery should be considered. Sporting individuals who need to pivot and wish to return to their sport will most times require surgery.
Combined ligament injury will tend to produce a more unstable knee and again it is more likely that surgery will be needed.

What does surgery involve?

The majority of patients who need surgery will have pre-operative physiotherapy aimed at restoring range of movement and muscle tone. The successful outcome of the surgery depends upon these goals being achieved before operation. To operate on a stiff knee is to invite complications which may result in long term loss of function (arthrofibrosis). There are some situations where the pre-operative rehabilitation is hindered by a secondary factor such as a meniscal tear which is blocking the extension of the knee and in such cases an arthroscopy (keyhole surgery) may be needed to address the problem and allow the patient to return to rehabilitation in preparation for definitive reconstruction of the ACL
Methods of surgery have evolved beyond recognition over the last 10 years. I now use hamstrings taken from the same leg to recreate the ACL. Surgery is performed through very small incisions using a camera in the knee. Most patients elect to stay in hospital for one night but I have done many ACL reconstructions as day cases with surgery early in the morning and return to home the same day.

After leaving hospital

I advise my patients to rest with the leg elevated for one week following surgery. Icing and exercises are undertaken on an hourly basis during this period and crutches are used for up to 10 days. Crutches can be discarded once gait pattern is restored and good quadriceps function achieved. An outline of the subsequent physiotherapy protocol is available on this website.

Return to sport

With modern surgical techniques there is a 90% chance of return to sport at the end of the rehabilitation period. This is dependant to a large extent on any major associated injury sustained at the time of injury (especially articular cartilage damage – see article)

Possible complications of surgery

Complications are rare but include bruising and swelling, re-rupture of the ligament, arthrofibrosis (stiffness of the soft tissues in the knee) and infection of either the wounds or of the knee itself.