ACL Injury

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ACL injury

An ACL injury is a tear or sprain of the anterior cruciate (KROO-she-ate) ligament (ACL) — one of the major ligaments in your knee. ACL injuries most commonly occur during sports that involve sudden stops or changes in direction, jumping and landing — such as soccer, basketball, football and skiing.

Ligaments are strong bands of tissue that connect one bone to another. The ACL, one of two ligaments that cross in the middle of the knee, connects your thighbone (femur) to your shinbone (tibia) and helps stabilize your knee joint.

When the ligament is damaged, there is usually a partial or complete tear of the tissue. A mild injury may stretch the ligament but leave it intact.

Many people hear or feel a “pop” in the knee when an ACL injury occurs. Your knee may swell, feel unstable and become too painful to bear weight.

Signs and symptoms of an ACL injury usually include:

  • A loud “pop” or a “popping” sensation in the knee
  • Severe pain and inability to continue activity
  • Rapid swelling
  • Loss of range of motion
  • A feeling of instability or “giving way” with weight bearing


Seek immediate care if any injury to your knee causes signs or symptoms of an ACL injury. The knee joint is a complex structure of bones, ligaments, tendons and other tissues that work together. It’s important to get a prompt and accurate diagnosis to determine the severity of the injury and get proper treatment.

Did you know that the ACL

  • Is one of 4 main ligaments of the knee.
  • Connects the tibia and the femur.
  • Prevents anterior translation of the tibia on the femur or posterior translation of the femur on the tibia.
  • Limits medial rotation of the femur on the tibia in closed-chain movement.
  • Provides stability to the knee joint:
    • Helps prevents tibia moving forward.
    • Helps prevents tibia rotating.
    • Helps prevent knee opening up sideways (varus/valgus)
Right knee, seen from an angle between anteriorly and laterally.

Important things

  • There are approximately 200,000 ACL related surgeries preformed in the US annually.
  • Up to 50% of all ACL injuries come with damage to other structures of the knee such as the menisci, other ligaments, & supporting cartilage.
  • Up to 50% those with ACL injuries develop osteoarthritis years later.
  • Surgery and recovery from and ACL injury is very challenging.
  • Sport such as basketball, football, soccer & rugby carry high risk for ACL injury.
  • In some sports, females have a higher risk of ACL injury, while in others, both sexes are equally affected.
  • ACL tears newly occur in about 69 per 100,000 per year with rates in males of 82 per 100,000 and females of 59 per 100,000.
  • When breaking down rates based on age and sex, females between the ages of 14-18 had the highest rates of injury with 227.6 per 100,000. Males between the ages of 19-24 had the highest rates of injury with 241 per 100,000.
  • While adults with a complete tear have a higher rate of knee osteoarthritis, treatment strategy does not appear to change this risk.
  • Rates of re-rupture among college athletes were highest in male football players with 15 per 10,000, followed by female gymnasts with 8 per 10,000 and female soccer players with 5.2 per 10,000.

How does the ACL get injured?


  • Often in high paces sporting activities e.g. football, basketball or handball.
  • Position of the knee before the ACL is injured:
    • Knee slightly bent (flexed), tibia pushed forward, knee twisted and valgus stress.
    • Often happens when athletes change direction very quickly and with force.


  • Direct high energy blow to the outside of the knee.
    • Causes knee to open up sideways and twist (what the ACL tries to prevent).

Primary Risk Factors for Tearing Your ACL