What Is the Anterior Cruciate Ligament?

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  • Written By: K. Willis
  • Edited By: Angela B.
  • Last Modified Date: 09 October 2019
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The anterior cruciate ligament (ACL) is a band of tough, fibrous connective tissue in the middle of the human knee joint. It is one of four main ligaments in the knee. The anterior cruciate ligament is connected to both the tibia, or shin bone, and the femur, or thigh bone.

Three other ligaments also connect to the tibia and femur. The medial collateral ligament runs along the inside of the knee joint and prevents it from bending inward. The lateral collateral ligament runs along the outside of the knee joint and stops the knee joint from bending outward. The posterior cruciate ligament works in conjunction with the ACL and prevents the tibia from moving out of alignment and sliding backward under the femur.

The anterior and posterior cruciate ligaments cross over one another inside the knee. The anterior and posterior ligaments are responsible for maintaining the correct anatomical position of the femur and tibia. Another function of the anterior cruciate ligament is to allow stable rotation of the knee joint. If this ligament sustains an injury such as a tear, the knee joint becomes far less stable.


A tear of the anterior cruciate ligament is one of the most common sports-related knee injuries. This injury can occur when the knee suffers a sudden direct blow, when landing after a jump or when the joint twists while the foot in firmly placed on the ground. The ligament then becomes overextended and cannot contend with the force being exerted on it. The end result of this is a tear in the ligament. ACL injuries are more likely to occur in females than males, which is believed to be because of the anatomical, muscular and hormonal differences found between males and females.

There is often an audible popping sound when the anterior cruciate ligamentis torn. Severe pain and a feeling of instability usually accompany this injury. Extensive swelling usually occurs along with widespread tenderness. Movement of the joint will be restricted, including the inability to completely straighten the knee.

An extensive or complete tear of the anterior cruciate ligament usually requires surgery to replace the ligament. Surgery is followed by a period of immobilization to allow healing. This is followed by an intense rehabilitation regime designed to regain maximum flexibility, mobility and rotation of the joint. Rehabilitation can take between six and nine months before normal activity can be resumed.


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Post 4

@cardsfan27 - I agree with you. I remember many years ago that someone's career may be over if they tore their ACL, but today it is obviously not the case. I have also heard that some people are born without ACL's and I wonder how this is possible.

I know there is a wide receiver in the NFL for the Pittsburgh Steelers that does not have an ACL in one knee and he is one of the greatest and most accomplished receivers in the game. It astounds me how that is possible, but I guess that he does not have to worry about having to miss a season due to tearing it.

Post 3

@jmc88 - You may be right there. The star wide receiver for the New England Patriots tore his ACL and was out for about that same amount of time.

Sports outlets have come out and said that him healing that quickly is almost inhuman and that it is astounding that he was not only able to heal that quickly, but also play at such a high level.

Watching him this year this injury did not slow him down at all and he is still able to make fast and quick lateral movements on that knee. He may be an anomaly in medical science, but I know what I see and he proves that just because someone tears their ACL it does not mean their athletic career is over.

Post 2

@TreeMan - I think you are correct in that assumption. My uncle played football back in the early 1970's and he said he sacked the Quarterback and tore the guy's ACL and he walks with a limp today.

However, you see athletes nowadays and hear about them tearing their ACL, yet coming back and still being able to perform at a high level.

Now if an athlete tore their ACL, they can expect to be out for maybe 18 months, but they will be able to probably make a full recovery, with maybe a bit limitation to lateral movement, however this does not seem to affect some people.

I still remember a football player for the University of Miami tearing his ACL badly, but returning in 7 or 8 months and becoming a star in his first season of the NFL. It may depend on how fast a person heals and how good the doctors are that perform the surgery.

Post 1

I have heard stories about people tearing their Anterior Cruciate Ligament and not being able to walk the same again.

I have met a few people that told me they tor their ACL and they walk a little bit funny. This could be because they are a little older and tore their ACL back in the 1970's.

I would think that technology and medical science has advanced enough that they are able to properly fix an ACL tear and still allow the person to still walk normal and live a normal life.

I know that they are able to fix athletes knees to the point they can perform incredible acts of athleticism, despite severely damaging their knee in the past, so I would think that they have this surgery down to a science and the people I know that tore theirs back in the 1970's just tore those too early for medical science.

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