What are ACL Injuries?
The anterior cruciate ligament (ACL) is one of the key stabilising ligaments of the knee joint. It is located deep within the knee and connects the femur (thigh bone) to the tibia (shin bone). The ACL plays a critical role in maintaining knee stability by preventing excessive forward movement of the tibia relative to the femur, as well as controlling rotational forces during movement.
This ligament is particularly important during activities that involve cutting, pivoting, or rapid changes in direction. When the ACL is injured, the knee can become unstable, especially during dynamic movements, which can significantly impact physical activity and sports participation.
Causes
ACL injuries most commonly occur during sports and high-demand physical activities and are frequently seen in sports such as football (soccer), basketball, netball, and skiing. The injury typically occurs through sudden pivoting or twisting movements, especially when the foot is planted, as well as during rapid deceleration, awkward landing from a jump, or direct impact and collisions, although non-contact injuries are more common. Several factors may increase the risk of ACL injury, including poor landing mechanics, muscle imbalances, fatigue, and anatomical differences. These injuries are particularly common in young and active individuals.
Symptoms
Symptoms of an ACL injury are often immediate and can be quite distinctive.
Common features include:
- A sudden “pop” or snapping sensation at the time of injury.
- Swelling : of the knee is rapid within hours, due to bleeding within the joint.
- Pain : particularly with weight-bearing.
- Instability : a feeling of the knee “giving way,” especially during turning or pivoting movements.
- Range of motion : is reduced with difficulty fully straightening or bending the knee.
In some cases, patients may initially be able to walk but later develop increasing instability or discomfort with activity.
Diagnosis
Diagnosis of an ACL injury involves a combination of clinical assessment and imaging. A detailed history is taken to understand the mechanism of injury and symptom progression.
During a physical examination, specific tests are performed to assess the integrity of the ACL. The Lachman test evaluates the forward movement of the tibia relative to the femur and is highly sensitive for detecting ACL tears. The pivot shift test is used to assess rotational instability of the knee, while the anterior drawer test evaluates anterior translation of the tibia. Together, these tests help determine whether the ACL is intact, partially torn, or completely ruptured. However, in the acute setting, examination findings may sometimes be limited due to pain and swelling.
Imaging
Imaging is important to confirm the diagnosis and identify associated injuries. MRI scans are the gold standard for evaluating ACL injuries. They provide detailed images of the ligament and can confirm whether it is torn, as well as assess the extent of the injury. MRI is also useful for detecting associated damage, such as meniscal tears, cartilage injuries, bone bruising, or injury to other ligaments. X-rays may be performed initially to rule out fractures or avulsion injuries but do not directly show ligament damage.
Treatment
Conservative (non-operative)
Treatment of ACL injuries depends on several factors, including the patient’s age, activity level, degree of instability, and the presence of associated injuries. Conservative management may be appropriate for some individuals, particularly those with lower activity demands or partial tears. This approach typically involves physiotherapy with a structured rehabilitation program aimed at strengthening the muscles around the knee, especially the quadriceps and hamstrings. Neuromuscular training is also important to improve balance, coordination, and movement control. Patients may be advised to modify activities by avoiding high-risk movements such as pivoting or sudden changes in direction, and in some cases, a knee brace may provide additional support. With appropriate rehabilitation, many patients can achieve good functional outcomes without the need for surgery.
Surgical Treatment
ACL reconstruction surgery is commonly recommended for individuals who are young, active, or involved in sports that require pivoting movements, as well as for those experiencing persistent knee instability. The procedure involves reconstructing the torn ligament using a graft, which may be taken from the patient’s own tissue (such as the hamstring tendon or patellar tendon) or from a donor. The graft is positioned to replicate the function of the original ACL and secured within the knee using fixation devices.
Surgery is typically followed by a comprehensive rehabilitation program that progresses through phases of recovery, including restoring range of motion, rebuilding strength, and gradually returning to sport.
Recovery
Outcomes following ACL reconstruction are generally very good, with most patients able to return to their previous level of activity. However, adherence to rehabilitation and appropriate timing of return to sport are critical to minimise the risk of re-injury. Full recovery and return to high-level activity may take 9–12 months or longer.