Ligaments are bands of strong, flexible tissue that connect bones or cartilage, providing stability to joints. Knee ligament injuries can occur when your knee is hit forcefully, you land on it badly, or it sharply changes direction. Symptoms include swelling and pain, reduced movement, a popping sound, a feeling of instability and sometimes bruising.…
Anterior cruciate ligament (ACL) injury
- The anterior cruciate ligament (ACL) is a band of strong connective tissue in the knee, connecting the shinbone to the thighbone.
- ACL injury can occur with all levels of physical activity, but is commonly experienced by athletes and physically active people.
- The ACL can be injured in a number of ways, leading to pain, swelling and instability in the knee joint.
- ACL injuries are treated with rest, icepacks and compression bandages, medication, physiotherapy. In more serious injuries, surgery may be required.
- With treatment, a mild ACL injury usually heals within six weeks. More severe injuries, such as a tear, may require up to 3-4 months for a full recovery.
What is the anterior cruciate ligament?
Ligaments are a type of strong, connective tissue that connect bones to other bones, providing stability to joints. The knee joint has four major ligaments that stabilize it. Two of those, the anterior and posterior cruciate ligaments, cross one another as they connect the shinbone (tibia) to the thighbone (femur). They are found deep within the knee joint and allow for a wide range of movement within the knee while maintaining the joint's stability.
Injury of the ACL often occurs together with damage to other knee ligaments, or other structures within the knee, such as menisci (see the report on Meniscal tears).
ACL injury can be caused by a range of activities, such as soccer, football or basketball. A sudden deceleration, twisting of the knee in the opposite direction or a direct blow to the side of the knee can all cause an ACL sprain or tear.
Risk factors for an ACL injury include:
- Gender - ACL injuries are more common in female athletes than in male athletes performing the same activity,  and;
- Bone anatomy - natural differences in people's knee anatomy can increase their risk of knee ligament injury.
Depending on the degree of damage to your ACL, the injury is described as being:
- Grade I (mild) sprain - the ligament is stretched but not torn, with minimal pain and swelling;
- Grade II (moderate) sprain - the ligament is partly torn. There may be swelling, bruising and pain, or;
- Grade III (severe) sprain - the ligament is completely torn, making the joint unstable and causing swelling and sometimes bruising. Often there is no pain at this grade of injury, as the nerve fibers that transmit pain are also torn.
Signs and symptoms
Symptoms of ACL injury include:
- Swelling, pain and tenderness in the knee or around it;
- Reduced movement of the knee;
- A popping or snapping sound, which is sometimes heard as a ligament is completely torn;
- A feeling of instability when walking, and;
- Bruising, which may develop some time after sustaining a ligament injury.
Methods for diagnosis
During a physical examination, your doctor will check for signs of swelling, instability and reduced movement.
Magnetic resonance imaging (MRI) is commonly used to confirm the diagnosis, assess the grade of ACL injury and check for additional knee injuries.
Types of treatment
Following a suspected knee injury, it is important to immediately stop any sporting or physical activity. Rest, elevation, use of icepacks and compression bandages on the affected knee can help reduce discomfort and further injury. Seek medical attention for further treatment.
Medications for knee ligament injuries include pain-relief medications such as acetaminophen. Non-steroidal anti-inflammatory drugs (NSAIDs), such as ibuprofen, can be used to treat inflammation.
Surgery may be needed to repair a knee ligament injury when the ACL is completely torn, or if there are additional injuries to other knee structures.
During surgery, ligaments can be reconstructed by using a piece of tendon from another part of the leg, a process known as autograft. If a section of tendon is obtained from a donor, the procedure is called an allograft.
Physiotherapy can improve knee movement and help the ACL to recover. Physiotherapy may also be useful before and after knee surgery. While your knee heals, crutches could help you when you need to stand or walk.
For ACL injury, open and closed-chain movements are helpful. Open-chain movements include leg curls, bench presses and seated leg extensions. Closed-chain movements include squats and lunges. It is important that you do not perform any exercises unless your physiotherapist or doctor specifically advises it.
Potential complications of an ACL injury include:
- Avulsion fractures, which occur when a fragment of bone breaks away from the attached tendon or ligament;
- Vascular injury - if the knee dislocates, blood vessels can be damaged, and;
- Nerve injuries - damage to the nerves travelling through the leg.
Surgery for an ACL injury carries its own potential complications. These include:
- Infection of the joint;
- Reduced ability to extend the leg;
- Knee instability due to graft failure in the knee;
- Deep vein thrombosis - although a rare complication, deep vein thrombosis can occur as a result of prolonged immobilization, and;
- Injury to blood vessels and nerves can sometimes occur during surgery. Injury to the nerves can result in complex regional pain syndrome, which can cause severe pain and delay recovery.
With treatment, a mild sprain of the ACL usually heals within six weeks. A moderate sprain can take more than six weeks to heal and a severe injury, such as a tear, could require 3-4 months for a full recovery.
You can reduce the risk of an ACL injury by following proper technique during sporting activities. A knee brace may prevent further damage to a weakened knee. Physiotherapy can speed up recovery of the injured ligament and strengthen local muscles to prevent repeat injuries.
- Smith, H.C., Vacek, P., Johnson, R.J. et al. (2012) Risk factors for anterior cruciate ligament injury: a review of the literature - part 2: hormonal, genetic, cognitive function, previous injury, and extrinsic risk factors. Sports Health. 4:155-161. doi:10.1177/1941738111428282.
FAQ Frequently asked questions
What is an anterior cruciate ligament injury?
Known as an ACL injury, this is damage to the anterior cruciate ligament, one of the strong ligaments within the knee that connects the shinbone (tibia) to the thighbone (femur). It can occur during all levels of physical activity, but is …
What are the symptoms of anterior cruciate ligament injury?
Symptoms of anterior cruciate ligament injury include: • Swelling, pain and tenderness in the knee or the surrounding area; • Reduced movement of the knee; • A popping or snapping sound, which is sometimes heard as a ligament is …
What causes anterior cruciate ligament injuries?
An injury to the anterior cruciate ligament can be caused by a range of activities, such as soccer, football or basketball. A sudden deceleration, twisting of the knee in the opposite direction, or a direct blow to the side of the knee can …
How is an injury to the anterior cruciate ligament diagnosed?
During a physical examination, your doctor will check for signs of swelling, instability and reduced movement. A type of scan called magnetic resonance imaging (MRI) is commonly used to confirm the diagnosis, assess the grade of …