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What is the Anterior Cruciate Ligament (ACL) of the Knee? 

The anterior cruciate ligament (ACL) is an important ligament in the knee for knee stability. The ligament attaches the femur or thigh bone to the tibia or lower leg bone. The ligament prevents rotation and translation motions of the leg during movement. In this manner, the ACL provides stability to the knee joint.

The ACL can be injured in many different ways. Sometimes the individual is hit by someone or something else injuring the knee but other times landing or twisting the leg can result in an ACL tear. When landing from a jump or pivoting suddenly the knee can shift towards the midline - moving towards the other side of the body. This is called a valgus force. This position stresses the ligament and is commonly associated with a tear. Most people with an ACL tear notices that the knee becomes swollen after the injury.


The ACL is an important area of orthopaedic research because it is a common injury.

How is the ACL injured?

How is the ACL treated?

This MRI image shows the ACL and PCL intact.

These are two different MRI sequences demonstrating that the ACL is disrupted or torn.

The ACL can be treated surgically or without surgery. Typically, more active patients have difficulty living with the instability associated with an ACL tear. For those patients, the ACL is reconstructed using a graft of tissue that is similar in properties to the ACL. This graft of tissue takes the place of the ACL and provides stability to knee.


Grafts are of two main types: autograft and allograft. Autograft is taken from your own body from an area where the tissue is not needed or redundant. For exmple, a single piece of patella bone, patellar tendon, and tibia bone plug graft is a common option. Other common options involve hamstring and quadriceps tendon grafts. Allograft tissue is obtained from a human cadaver but goes through multiple sterilization processes to prepare the graft for surgical implantation.

A video demonstrating a bone-tendon-bone reconstruction can be found here.

A video demonstrating the hamstring autograft can be found here.

A video demonstrating the allograft reconstruction can be found here.

Patients treated without surgery go through a focused physical therapy protocol to strengthen the muscles around the knee. These muscles help compensate for the ACL deficient knee. These patients commonly avoid activities that require pivoting and jumping. 

The image on the left is identifying the torn ACL between the flexed knee femur and tibia in a LEFT knee. The two images on the right demonstrate some common techniques we use to reconstruct the ACL. 

Operative ACL Reconstruction Images  (Performed in a Right Knee)

Torn ACL

Probe in the Tear

Removing the ACL

The first and second images demonstrate the ACL tear in the middle of the knee. The third image demonstrates removing the torn ACL tissue.

ACL Attachment on the Femur


ACL Attachment on the Tibia

Tunnel for the ACL Graft

The first and second images demonstrate the native ACL attachments that will be used to make the anatomic bone tunnels. The third image demonstrates the tibial (or lower leg bone) tunnel after drilling.


Suture for Femur Tunnel

Suture for Tibial Tunnel

Reconstructed ACL




Button Deployed for Graft Fixation

The first image shows the two passing sutures that will be used to deliver the new ACL graft into the knee. The middle image is the final reconstructed ACL graft in the anatomic position. The third image shows the suspensory fixation button deployed on the outside of the femur (or thigh bone).


1. Skelley, NW, Castile, RM, Canon, PC, Weber, CI, Lake, SP, and Brophy, RH. Regional Variation in the Mechanical and Microstructural Properties of the Human ACL. American Journal of Sports Medicine. 2016 Nov;44(11):2892-2899. doi: 10.1177/036354651665448.

2. Castile, RM, Skelley, NW, Babaei, B, Brophy, RH, and Lake, SP. Microstructural Properties and Mechanics Vary Between Bundles of the Human Anterior Cruciate Ligament During Stress-Relaxation. Journal of Biomechanics. December 2015. doi: 10.1016/j.jbiomech.2015.11.016.

3. Skelley, NW, Castile, RM, York, TE, Gruev, V, Lake, SP, and Brophy, RH. Differences in the microstructural properties of the anteromedial and posterolateral bundles of the ACL. American Journal of Sports Medicine. January 2015. doi: 10.1177/0363546514566192

Knee Anterior Cruciate Ligament Tear

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