One of the most common knee injuries is a sprain or tear in the anterior cruciate ligament or ACL.

How the Knee Works

There are four primary ligaments in the knee: medial collateral ligament (MCL), anterior cruciate ligament (ACL), lateral collateral ligament (LCL) and posterior cruciate ligament (PCL). Together they hold the bones of the joint in place and keep the knee stable.

Cruciate ligaments are found inside the knee joint. They cross each other forming an “X” with the anterior cruciate ligament in front and the posterior cruciate ligament in back. The cruciate ligaments control the back and forth motion of your knee.  The ACL runs diagonally in the middle of the knee. It prevents the tibia from sliding out in front of the femur and provides stability to the knee, both front to back and in rotation.

ACL Injuries

Grade 1 Sprains: The ligament is mildly damaged and slightly stretched, but is still able to keep the knee joint stable.

Grade 2 Sprains: The ligament is stretched to the point where it becomes loose, often referred to as a partial tear.

Grade 3 Sprains: Referred to as a complete tear of the ligament; the ligament has been split into two pieces, and the knee joint is unstable.

ACL Tear Causes

ACL tears are common in high demand sports where an athlete stops suddenly, changes direction rapidly, or frequently jumps.  In addition to high demand athletics, ACL injury is common in falls or direct collision of the knee into an unforgiving object.  Trauma from auto accidents or injury from stepping on uneven surfaces also are known causes of ACL injury.  Often injury to the ACL is found in conjunction with other injuries such as damage to the articular cartilage, meniscus or other ligaments.  Most ACL injuries are complete tears. Partial ACL tears are seen less frequently.

Symptoms of ACL Injury

  • Swelling
  • Pain
  • Loss of full range of motion
  • Discomfort when walking
  • Tenderness
  • Feeling of instability


Treatment for ACL injuries are dependent on the extent of a patient’s injury, their lifestyle, and the demands their occupation and recreational choices place on their knees. Surgery is often required to return to sports and an active lifestyle; however, a less active individual may return to their lifestyle without surgery.

Most ACL tears cannot be stitched back together, instead the ligament must be reconstructed. A surgeon will replace the torn ligament with a tissue graft that acts as a foundation for a new ligament to grow upon. This is performed arthroscopically using small incisions, which means less pain and a faster recovery time for the patient than with an open surgical repair. The graft may come from the patient utilizing hamstring tendon, quadriceps tendon or can come from a tissue bank using a cadaver graft. The surgeon determines the best graft source based on a number of factors.

Ligament regrowth takes time and it may require six months or more before returning to a sport or other athletic activity.