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What is an ACL tear?

Anterior cruciate ligament (ACL) tears are some of the most common forms of injuries encountered by athletes. They occur when one of the four main ligaments in the knee, called the ACL, tears, leaving the knee less stable. The ACL is one of four ligaments that connect the thighbone (femur) to the tibia (shin). These ligaments are what give the knee and leg flexibility and allow them to move easily. A person who has an ACL tear can experience reduced mobility and difficulty rotating the knee. It is one of the most common sports injuries, with nearly 300,000 cases occurring each year.

The majority of people who experience an ACL tear are athletes. Females are more susceptible to this than males. Some researchers believe this is due to variable conditioning methods in females. Athletes are usually more prone to this form of injury because of the pressure put on the ACL during athletic training, such as fast turns when running, or being tackled from behind in football or hockey. There are different levels of severity in ACL tears, and some require surgery to treat. Without proper treatment from a trained doctor, other parts of the knee can become injured as well, causing further problems down the road.

Anatomy of the knee

The knee is the bone that connects the thighbone to the shinbone and provides mobility. It consists of the following:

  • The medial collateral ligament (MCL)
  • The posterior cruciate ligament (PCL)
  • The anterior cruciate ligament (ACL)
  • The lateral collateral ligament (LCL)
  • The patella (the kneecap)
  • The femur (the thighbone)
  • The tibia (the shinbone)

In order for the knee to function properly, it uses each of the ligaments to give it flexibility and allow the leg to move and work smoothly. The ACL is the most important of the ligaments because it is the one that provides the most stability to the knee and allows a person to walk properly.


Depending on the severity of the ACL tear, symptoms can change. Most people who have an ACL tear report hearing a loud pop when the rip occurs. They also feel the sensation of the knee giving out from underneath them.   

The following are common ACL tear symptoms. Every person is different and not everyone will experience all of these symptoms when a tear occurs:

  • Swelling in the knee
  • Pain on the outside or back of the knee
  • Restricted mobility in the knee
  • The feeling of instability when standing on the leg in question

ACL tear symptoms may be confused with those of other sports injuries, such as a tear in the meniscus, which often goes along with an ACL tear. Generally, people with a tear are in so much pain that they stop all activity. If this happens, it is important to see a trained doctor in a timely manner. 


There are many causes of ACL tears. While most causes are associated with some sort of sports trauma, there are other ways a person can tear the ACL:

  • Intense impacts or collisions – This happens in sports when an athlete is impacted from behind. When this occurs, the knee moves in an abnormal direction, making it easy for the ACL to rip.
  • A rapid change in direction – When an athlete or other person is running and quickly makes a turn to avoid a tackle or to change directions, the person risks ripping the ligaments. This is because the ligaments are suddenly jolted in an abnormal direction and the strain can cause them to snap.
  • Suddenly stopping – If an athlete comes to a fast stop, the ligament wants to continue to help the body move. This puts an unusual type of strain on the ligament, often resulting in a rip or partial tear.
  • Landing wrong from a jump – This can happen to anyone who is jumping. As a person lands, the ligament experiences the same sort of sensation as it does when a person stops suddenly. If a person lands at an awkward angle, it can cause the ligament to rip because the knee tries to continue downward and can tear it.  


ACL tears can be divided into three gradation levels depending on their severity:

  • Grade 1 – This is the mildest form of an ACL tear. A person who has a grade 1 tear is still able to walk and maintain some stability from the knee. While the ligament is stretched, it does not tear enough not to provide some function.
  • Grade 2 – This is usually referred to as a partial tear. When a grade 2 ACL tear occurs, the ligament is stretched to a point where it becomes loose, making it more difficult to fully support the knee as it should.
  • Grade 3 – This is the most severe form of tear and results in a complete rip in the ligament. When this happens, the knee is unstable. Unfortunately, this is the most common of all ACL tears.

From pro athletes to weekend warriors, we treat many sports injuries including torn ligaments, concussions, broken bones and overextended muscles.

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