woman crouching to run


ACL Injuries

What is an ACL?

ACL stands for anterior cruciate ligament. It is one of two ligaments in the center of the knee (the other is the PCL), which helps connect the femur to the tibia. The ACL originates in a large notch at the end of the femur and runs diagonally to insert on the top surface of the tibia.

What does the ACL do?

The ACL prevents the tibia from sliding too far forward from underneath the femur. It also helps prevent hyperextension of the knee, and resistance to rotational forces about the knee. ACL knee surgery is necessary once the tibia has slid too far or the knee joint has experienced hyperextension.

How does an ACL knee injury occur?

An ACL knee injury typically occurs during cutting, twisting or pivoting activities or with hyperextension injuries. During these types of activities, the knee is sharply twisted or extended beyond its normal range. ACL surgery can generally help correct when this phenomenon has happened to the knee.

How do I know if I have injured my ACL?

Although an ACL injury can present itself in many ways, from very subtle to severe, here are some of the common signs and symptoms of an acute ACL injury.

  • "Pop" - Patients may report that they heard or felt a "pop" in their knee during a strain or twist-type injury surrounding the knee. Although this is a common presentation, this is not experienced by all patients.
  • Immediate swelling after the injury occurs - This typically indicates there is some form of bleeding in the knee joint, which suggests either injury to the ligaments, cartilage, or both. However, not all patients experience swelling.
  • Pain - Knee pain is a very non-specific sign and can represent many different conditions. However, the majority of patients who have injured their ACL will experience a substantial amount of pain.
  • Instability - Many patients will describe having the feeling of "buckling" or "giving way" sensations in their knee.

Are there different degrees of an ACL knee injury?

Yes. An ACL knee injury is graded on the amount of ligament tearing and, therefore, severity of the injury. ACL surgery repairs tearing and moves the knee back to its normal range of movement.

  • Grade I - Minor trauma or "sprain" to ligament. Some ligament fibers are stretched but none are torn.
  • Grade II - More severe trauma or "partial tear." Here some fibers are actually torn.
  • Grade III - Severe trauma to the ligament, which completely tears all the ligament's fibers. Also known as a "complete tear," Grade III represents the most severe ACL knee injury.

How is an ACL typically torn in sporting events?

An ACL is typically torn in sports with pivoting motions: soccer, football, basketball, skiing, golf or tennis. In these types of sports, the foot is planted with the knee bent, and a sudden change in direction typically occurs, which stresses the ligament.

Contact sports also put the ACL at an increased risk of injury. When the knee is struck from the outside, the resultant force may drive the tibia forward, while the femur stays in place or is driven backward. When the knee is struck from the outside, the ACL can be injured alone or in conjunction with the MCL (medial collateral ligament). Also, an ACL knee injury or damage can result from a severe hyperextension of the knee joint. ACL surgery is necessary once hyperextension, tearing of the ligament, or movement within the knee joint has occurred.