ACL Injury

The ACL Is a strong band of tissue that connects the back of the Femur (thigh bone) to the  front of the Tibia (shin bone) within the knee. It is one of the four main stabilising ligaments that provide stability to the knee whilst allowing it to maintain a large range of movement. It sits in the centre of the knee. 

What does the ACL do?

The ACL resists forward movement of the tibia on the femur and also provides a degree of rotational stability. This function is important in keeping the knee stable in activities such as going down stairs or cutting or pivoting manoeuvres often experienced in sporting activity.

How is the ACL injured?

The ACL is often injured (ruptured or torn mean the same thing) when the knee is excessively twisted or pivoted. This can happen in sporting activity such as whilst performing a sidestep or quick change of direction. This is called a non -contact injury and is the most frequent mechanism and  occurs in sports such as football rugby or netball. A slow twist whilst skiing such as when a ski doesn’t come off in a fall whilst skiing is also a common presentation. It can also occur in trips and falls or in tackles or accidents, a contact injury.

What are the symptoms of an ACL injury?

At the time of the injury people will often feel or hear a pop from the knee. They will often fall to the ground and be unable to carry on with the activity. They will often experience pain from the knee. Because the ACL contains blood vessels when these are torn they bleed into the joint causing fairly rapid onset of swelling. Immediately afterwards people may struggle to put weight through the knee.

The acute swelling and pain initially felt will progressively settle over a few weeks. The more long term effects of the injury may then be felt. These may include a sensation of instability in other words a loss of confidence in the knee or a feeling that the knee may give way. This is often felt when trying to turn or change direction on the knee. Occasionally the knee actually  gives way and this risks causing further damage to the knee such as tears to the meniscus or damage to the joint surface. This instability often prevents people from returning to playing sport.

How is an ACL injury diagnosed?

The damage can often be diagnosed by the typical history or story of how the injury was sustained and the immediate symptoms of a pop or giving way and immediate onset of swelling. An experienced knee surgeon or physio can detect the injury by examining the knee. There will be increased movement to pulling the shin bone anterior on the thigh bone (lachman test) and increased movement on attempting to pivoting the knee. The doctor will also be able to detect the swelling. 

An MRI scan is frequently requested. This is very accurate in confirming the injury as it visualises any damage to the ligaments of the knee very clearly. It is also requested to see if there are any other associated injuries such as meniscal tear or cartilage damage that can occur at the same time as the injury.

Treatment for ACL injury

Initial treatment focuses on getting over the acute injury. This involves reducing the initial swelling with rest and ice.  Physiotherapy is also important to activate the quadriceps muscle and to help prevent them from significantly weakening. This can also help get the swelling down. Exercises will also be commenced to work on regaining a full range of movement in the joint which is very important if surgery is to be considered. After this exercises to build back full strength in the knee are instituted. Regular use of an exercise bike at this stage can be very useful.

Surgery to reconstruct The ACL is frequently considered. The ACL unfortunately doesn’t heal itself as it usually pulls away from the femur and displaces from its attachment due to gravity. A reconstruction involves taking tissue from elsewhere to make a new ligament.


Not every patient needs to undergo a reconstruction as some people can cope without an ACL As a very rough rule of thumb about one third of patients can cope without an ACL without altering their activities to any great extent. A further one third can cope if they alter their activities and the final third   have regular episodes of instability or giving way with daily activity risking further damage to the knee. It is therefore reasonable to wait a period of time after injury to see how one copes with the injury after a period of physiotherapy (conservative treatment) and choosing surgery if the knee isn’t stable enough to perform desired activities. 

It is also reasonable to choose early surgery after the knee has settled from the injury and regained range of movement. This is more likely to be considered if the person regularly performs twisting and pivoting sports and the knee feels unstable to clinical testing by a knee surgeon.

Recovery from ACL injury

The acute phase of injury usually takes anything from 4 to 10 weeks to settle. If a non operative course with physiotherapy is chosen the knee is not going to be as good or as stable as it can get until at least 4 months post injury. If surgery is opted for providing there are no other injuries such as meniscal tears that need treatment then we would anticipate the use of crutches for 2 -4 weeks. Use of an exercise bike at 6 weeks. Straight line running at 3 months and gentle twisting activities at 6 months. The earliest I would be happy for a full return to sport would be 9 months but for many people it is closer to a year. This is all very dependant on the amount of effort put into postoperative rehabilitation.

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