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The Importance of the ACL

Written by Bobby Zhu '26

Edited by Naphat Permpredanun '24

For many sports lovers, the term ACL injury or ACL tears often strikes fear in many’s hearts. You may have heard about it during the 2019 NBA Finals when the Golden State Warriors were playing against the Toronto Raptors. Klay Thompson, a Warriors guard, landed wrong after coming down from a jump, tearing his ACL [1]. Or if you were a football fan way back in 2008, you may remember that Tom Brady had his ACL torn after getting hit by a Kansas City Chief defender [2]. Both were out for the remainder of the games they needed to play for that season and didn’t come back until almost a year later. Now, with these two season-ending heartbreaks, what really is the ACL and why does it take so long to recover from tearing it?

The ACL’s unabbreviated name is the anterior cruciate ligament. As a ligament, it’s role in the body is to stabilize the bones by connecting them together. The ACL does so by forming an X-like shape with the PCL (Posterior Cruciate Ligament), which is the strongest ligament in the knee that prevents the tibia from moving past the femur [3]. In conjunction with the PCL, the ACL helps with the rotational stability of the knee and prevents the tibia from sliding out in front of the femur [4], both of which are bones that connect at the knee joint.

The importance of rotational stability in the knee is highlighted in sports like basketball, football, soccer, and lacrosse, which heavily uses the ACL. As these activities involve a tremendous amount of twisting and turning while also being extremely physically demanding [5]5 the, Anterior Cruciate Ligament is more likely to be torn. Imagine constantly trying to stretch stiff rubber band. Eventually, tears will form and if stretched with enough force, the rubber will snap. In essence, a huge load is placed on the knee, which can be overwhelming towards the ligaments, resulting in a torn ACL. However, the good news is that there are procedures that can help repair the ACL, called ACL reconstruction. These are surgeries where the tissues are grafted into where the ACL tendon used to be, effectively replacing the torn ACL.

There are multiple ways to graft tissue onto another site, but the most common method for ACL reconstruction surgery is to use tissue from the patient’s body. This method of grafting tissue is known as auto-grafting, and surgeries that utilize autografting mostly target the patellar tendon, the hamstring tendon, or the quadriceps tendon. The benefits of such a procedure lie in the body’s immune system response [6]. By utilizing tissue that comes from the patient’s body, there is a reduction in the likelihood that the body will reject that tissue since the immune system recognizes it. In addition, it is less costly to utilize your own tissue and your body’s own tissue is less likely to re-rupture in cases of young patients that do high activity sports. The main downsides to autografting are the danger of scarring over where the autografted tissue was taken out of and more post-operative pain as the body is not used to missing a chunk of tissue from a region [5]. Yet, most doctors, like Dr. Cruz from University Orthopedics, utilize autografts as the better method for getting consistently good results from ACL reconstruction.

Now, why does it take so long for athletes with an ACL injury to recover? A year's loss to recovery is quite a long time in the sports world, and many things can happen in that time span. Unfortunately, the ACL takes so long to recover after ACL reconstruction is because the graft needs time to heal. When the graft is first introduced to the knee joint, some graft cells die as they are not supplied with sufficient nutrition. Then after some time, when blood vessels invade the graft site, remodeling it, the dead graft cells can be cleared away. However, this weakens the “new” ACL a bit as the grafted ACL no longer supports the dead cells. Still, it is in the best interest of the ACL to undergo such a phase as more nutrients can be brought to the region allowing for more normal functioning. The disadvantage is that the ACL is prone to rupturing, stretching, and elongation. This is around when patients wear their brace to protect the newly made ACL from being reinjured again. As the weeks pass by, the ACL graft slowly regains its strength and its functionality. By the end of 3 months, the ACL returns to its original strength [7]. However, before the patient can return to play, they need to undergo therapy to reintroduce their body to the function of the ACL and prevent pain or swelling. The ACL is an area of proprioception [8], which describes the brain knowing where a body part is without having to see it. To reestablish this connection, more time is needed, resulting in a total of 9 month’s time of recovery. After all the recovery, the ACL can finally be used to its fullest potential, and athletes can once again play the sport they love.



[1] Case report: Klay Thompson's injury journey [Internet]. Sports Injury Bulletin. [cited 2023Apr29]. Available from:,joint%20(see%20figure%201).

[2] Tom Brady - ACL injury [Internet]. Team ACL. 2022 [cited 2023Apr29]. Available from:,City%20Chiefs%20defender%20Bernard%20Pollard.

[3] [cited 2023Apr29]. Available from:,the%20pelvis%20to%20the%20knee).

[4] Anterior cruciate ligament (ACL) injuries - orthoinfo - aaos [Internet]. OrthoInfo. [cited 2023Apr29]. Available from:

[5] Ballestrini C. Anterior cruciate ligament tear: Orthopedics & sports medicine [Internet]. UConn Health. 2017 [cited 2023Apr29]. Available from:,have%20high%20ACL%20injury%20rates.

[6] Team O. Choosing an ACL graft - what is your best option? [Internet]. Dr. Frank Petrigliano. 2022 [cited 2023Apr29]. Available from:,No%20risk%20of%20disease%20transmission

[7] Dr. Bu Balalla. How long does it take to recover after ACL reconstruction? [Internet]. Dr. Bu Balalla. Dr. Bu Balalla; 2018 [cited 2023Apr29]. Available from:,undergo%20necrosis%20(i.e.%20die).

[8] Falk. 5 reasons ACL rehab takes so long [Internet]. Kinetic SMP. 2023 [cited 2023Apr29]. Available from:

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