ACL Injuries: What are the Possible Causes?

ACL Injuries: What are the Possible Causes?

Brief knee anatomy: The tough bands of fibrous tissues that connect two bones across a joint is called a ligament. Inside the knee joint, you can find the anterior cruciate ligament (ACL) and the posterior cruciate ligament (PCL). The two ligaments connect the femur (thigh bone) to the tibia (the lower leg bone).  The PCL and the ACL form an “X” inside the knee and functions by stabilizing it against back-to-front and front-to-back (shear) forces and when the ACL ligament is stretched beyond its normal limit or torn, an ACL injury occurs.


In the majority of the cases, an ACL injury can be attributed to any of the following injury patterns:

  • A drastic stop, pivot, change in direction, and twist at the knee joint – These knee movements are often considered a routine part in sports like rugby, gymnastics, football, basketball, soccer, and skiing. Understandably, athletes who are involved in the aforementioned sports are highly susceptible to ACL tears.
  • Extreme knee hyperextension – In some cases, during athletic landings and jumps, the knee extends beyond its limit or is straightened more than it should. Similar scenarios may sometimes result in ACL tears. For instance, ACL tears can occur when landing awkwardly while playing basketball or missing a dismount when landing in gymnastics.
  • Direct contact – The ACL can get injured during contact sports especially when there is a direct impact to the outside of the lower leg or the knee. A misdirected soccer kick that hits the knee, a sliding tackle while playing soccer and a sideways football tackle are some of the likely ways the ACL can get injured. A sideways fall while skiing also commonly injures the ACL.

Just like sprains, ACL injuries are classified using the following grading system:

Grade I – This pertains to mild injuries (sprain) with only microscopic ACL tears. While the tiny tears may cause the ligament to stretch out of shape, the tears will not affect the knee joint’s overall ability to support the individual’s weight.

Grade II – This is considered a moderate injury where the ACL is partially torn or stretched. The knee can get a bit unstable and may even “give way” periodically when the person walks or stands up.

Grade III – This is considered a severe injury where the knee is very unstable and the ACL is completely torn and the knee feels unstable and wobbly.


Prevalent symptoms that indicate an ACL injury can include:

  • Feeling a “pop” in the knee (when the ACL tears)
  • Deformity and significant swelling (usually manifest within a few hours after the injury)
  • Severe knee pain
  • Feeling unstable and unable to bear weight (especially if the ACL has been torn completely and there is no tension across the ligament injured)
  • A black and blue discolouration (around the affected knee secondary to bleeding from inside the knee joint)
  • A feeling that the injured knee will “give way,” “give out,” or buckle (especially when standing) many weeks or months later in untreated cases


To accurately diagnose the condition, the doctor will need to physically examine the affected knee and look for signs of deformity, tenderness, discolouration, and swelling. The knee’s range of motion will also be checked if it’s not too swollen or too painful. The strength of the ligaments will also be assessed by the Lachman’s test or anterior drawer test. To do this, the doctor will ask the patient to bend the knee while the leg is gently pulled forward. If the ACL is torn, a protruding “lower lip” of the knee or an “underbite” appearance will manifest. If the physical exam of the knee will suggest a significant injury of the ACL, a magnetic resonance imaging (MRI) scan of the knee will be ordered.

In other cases, an arthroscopy (camera-guided surgery) might be required to inspect the ACL damage.

Treatment Interventions

For ACL sprains that are classified under Grade I and Grade II, the initial treatment will often follow the RICE rule:

  • Resting the affected joint
  • Icing the injured area (to minimize swelling)
  • Compressing the swelling (using an elastic bandage)
  • Elevating the injured knee

In some cases, wearing a knee brace may also be suggested. To ease the pain and minimize the swelling, taking nonsteroidal anti-inflammatory drugs (NSAIDs) will be prescribed. Needle aspiration of the knee to remove blood from the joint also relieves pain very quickly. When the pain has subsided, a rehabilitation program designed to strengthen the muscles around the knee will be recommended. Generally, treatment for ACL injuries will depend on the patient’s activity level. For instance, surgery might be the ideal treatment route for those who need to return to playing sports that will require jumping and pivoting. Grade III injuries are also initially treated with RICE, bracing, and rehabilitation. Once the swelling has subsided, the torn ACL is surgically reconstructed using a piece of the patient’s own tissue (autograft) or a donor tissue (allograft). In the case of autografts, the torn tendon may be replaced by the patient’s own patellar tendon or a tendon section taken from a large leg muscle.

Recently, most knee reconstructions are carried out through arthroscopic surgery. The procedure has become the preferred option since it will require smaller incisions and will cause less trauma and scarring.