Anterior Cruciate Ligament


The Anterior Cruciate Ligament (ACL) is one of the four main supporting ligaments of the knee along with the posterior cruciate ligament, the medial collateral ligament and the lateral collateral ligament. It is the ligament that prevents forward movement of the tibia away from the femur. It is for this reason that a tear or rupture of the ligament can be a very serious sporting injury. Because of its role as a stabilising ligament it is also the most common injury to the knee of athletes.

A ligament is defined as a connective hard tissue that connects two bones, and in most cases their function is to support of enable the movement of the joint. In the case of the ACL its role within the knee as mentioned previously is to prevent the forward movement of the tibia. It works closely with the posterior cruciate ligament to keep the knee stable, as the PCL keeps the tibia from moving backwards. The ramifications and effects on movement of the knee will be discussed further later on.

There are different severities of a tear that can occur in an ACL injury. The severities are divided into three categories, a first-degree tear, second-degree tear, and a third degree tear or rupture. A first degree tear is defined as having 0-50% of the ligament fibres torn, second degree as having 50-80% of the fibres torn, and a third degree having 80-100% torn. The picture shows a complete rupture of the ACL.


Classification and Prevalence

The tear of an ACL can be classified as a direct or indirect hard tissue injury. This is because it can be caused by either a direct blow to the knee or by an awkward landing or twisting of the lower limb. It is the most common injury to knee ligaments in sports, and studies have found that in the US, the incidence in people between the ages of 16-45 is 1 in 1750. Studies have also shown that females are almost 6 times more likely to suffer a non-contact ACL tear than males.

The prevalence of this injury could be due to several factors. The first is because of the huge role the ligament plays in supporting the knee joint during stopping and landing. Due to the stress placed on the ligament during these actions, a tear can occur easily if an awkward or unusual landing angle is taken, or if the stopping is very abrupt. The other factor that makes it common injury is the prevalence of forced twisting of the knee or blows from the exterior to the knee in contact sports. In sports such as rugby league or union, an exterior blow to the knee can occur quite easily in a tackle. Because of the position of the ACL (inside the knee) a blow from any direction has the potential to cause a tear in the ligament, although a blow to the anterior or posterior is far more likely to cause damage.

The increase incidence of the injury in women is also an interesting statistic. Studies have found some possible reasons for this, with most of them being genetic and biomechanical reasons. The main reason more tears of the ACL in women is due to their use of the quadriceps muscles for jumping, as opposed to the hamstrings in men. The contraction of the quadriceps pulls the tibia forward, and hence places stress on the ACL. This combined with the stress of landing on the ligament increases the chance of an ACL tear. The other main reason for increased prevalence of the injury in females is the angle of their hips over their knees. Because women tend to have wider hips than males the weight distribution over the knees changes, and due to this more stress is placed on the ligaments in the knees, rather than the muscles in the legs.

Signs and Symptoms

There are some telltale signs of an ACL tear. The first is when the injury occurs, as a popping sound usually occurs at the time of the injury. After the injury occurs there is usually sudden and possibly major swelling of the knee, as well as a feeling of instability. But the major sign a ligament tear has occurred is pain in and around the knee. The pain can range from moderate to severe.

Common Causes and Prevention

As mentioned previously the main causes of an ACL tear are the stresses of landing and stopping (indirect roughly 80% of all occurrences), or a blow to the knee form an exterior force (direct roughly 20% of occurrences). The injuries occurring from stopping usually occur due to a very abrupt deceleration, followed by a change in direction. The direct injuries usually occur due to a blow to the front of the knee joint, and requires a large amount of force to cause a tear.

Because of the nature of the injury, prevention lies more in technique and avoiding certain situations, rather than equipment needs or physical changes. Proper technique in jumping, stopping, landing, and change in direction need to be taught, this can significantly reduce an athlete’s chance of an ACL tear. This is the main thing that needs to be addressed if the incidence of ACL injuries is to be lowered. Another preventative measure for a normal person is to avoid sports that involve large amounts of contact in the lower limbs. This would include rugby league and union, and to a lesser extent AFL. These sports involve a ‘tackling’ element where there is possibility for the arms, shoulders and torso of other players to come in contact with the knee. Of course this is unavoidable for athletes involved in these sports, so prevention is a hard thing to implement. The only measure that can be taken is that players take caution when tackling and being tackled, to try and prevent unusual twisting of the knee.

Although a large part of preventing ACL injuries is technique, playing surfaces can play a role in prevention of the injury. Hard surfaces such as floorboards and concrete can increase an incident of a tear due to the increased trauma on the legs when stopping and changing direction. This kind of trauma is very evident in basketball and netball. Due to this, sport organisers should make a point of trying to reduce the amount of sport played on these surfaces, or making the surface safer. Measures would include having a cushioning layer underneath floorboards in basketball, and whenever possible, not playing netball on a concrete surface. Loose surfaces can also contribute to the occurrence of an ACL injury, as this can cause the knee to twist when a player changes direction and the surface slips or moves. There are several ways to avoid this, some of which include not allowing competition to continue if the surface is overly wet or muddy, and also calling off the competition if the top of the surface is very loose. If these measures are taken in regard to playing surfaces, the incidence of ACL injuries can be greatly reduced.

Treatment and Rehabilitation

There are two options for treatment of an ACL injury, surgical or non-surgical. The choice of treatment usually relies on how serious the tear is. For first and second-degree tears, surgery may not be necessary, but for almost all third degree tears, surgery is required and in some cases a complete knee reconstruction is required. The immediate first aid treatment of the injury is the RICER principle, rest, ice, compression, elevation, and referral. This is used in an effort to prevent large amounts of bleeding cause by the ruptures in blood vessels around the ligament.

For the non-surgical option, treatment is based around management of the injury, and trying to prevent further harm to the area. This usually involves physical therapy and a knee-brace. The brace prevents twisting and cutting motions in the knee, which is in an effort to prevent further injury to the knee. The non-surgical option is usually recommended for people not involved in sporting activities.
The other method of treatment is the surgical option. This involves the grafting of tendon from either the hamstring or patella, to reconstruct the damaged ligament. The tendon can either be taken form the patients own leg, or from a posthumous donor. Taking the graft from their own leg usually results in large amounts of pain in the area, although grafts from a donor increase the risk of infection. These grafts are then used to replace the ACL and create stability in the knee.

Because of the severity of the injury, the rehabilitation process is arduous and lengthy. The process can be divided into phases. The first phase involves the regular short-term treatment most injuries, which includes methods to reduce the pain and swelling around the area, as well regaining some movement. This phase covers the first two weeks after the injury. The second phase covers weeks 3-4, and this is when the pain is starting to subside, but the knee is not ready for movement. This is why joint protection is emphasised at this point in rehab. Light exercise of the knee can also start at this phase. Phase 3 covers weeks 4-6, and includes the same exercises as phase 2, but with added ones of greater difficulty. By this stage the aim is to have the knee bend to 130 degrees, and a large emphasis is placed on regaining balance. The fourth phase aims to have complete range of movement regained by the knee, as well as increased resistance in rehab exercises. Phase 5 focuses on improvements in strength and balance, and this is at weeks 8 to 10. Phase 6 is the last phase in the process and marks the point at which the athlete can return to activity. They will be able to participate in light jogging and moderate intensity agility drills. After around 3-6 months the doctor will put the patient through a series of tests to ascertain the strength and stability of the knee. Depending on the results of this test the patient may be able to return to a normal lifestyle.
A typical knee brace


There are many tests that can indicate an ACL tear has occurred. These include the pivot-shift test and the Lachman test. Both test knee stability with the Lachman test being the preferred method of testing. The other way to confirm a torn ACL is a MRI of the area. This provides a detailed image of the area, with a professional easily being able to identify if the ACL has been torn.
MRI of a torn ACL

Outcomes On Future Performance

The performance of an athlete can be affected well after an injury to the ACL. The strength of the knee can take well over 6 months to regain full strength, but even after this there is now an increased risk of tearing the ligament again. This is especially in the case of females, with the chance of recurrence increasing to 1 in 4. The injury can also have a mental affect on the athlete, as they may be back to full strength but still be worried about another injury occurring. This may prevent the athlete from playing at their best for some time.

Revision Exercises

After watching the above video, answer these questions relating to ACL injuries.

1. What is the role of the ACL in supporting the knee?

2. How is the ACL usually injured, and what measures can be taken to reduce the risk of the injury?

3. What are the signs and symptoms of an ACL tear and how should they be treated?

4. Outline what the Rehabilitation process involves.

5. Will the athlete be able to return to sporting activities, and if so, will their performance be affected?