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ACL Tears

What Is The Anterior Cruciate Ligament?

Monday, September 24, 2018 - 13:19

Contributing Author: Guy Slowik FRCS

The anterior cruciate ligament (ACL) is one of the most important of four strong ligaments connecting the bones of the knee joint. It is often injured.

Ligaments are strong, dense structures made of connective tissue that stabilize a joint. They connect bone to bone across the joint.


Anterior Cruciate Ligament


The function of the ACL is to provide stability to the knee and minimize stress across the knee joint:

  • It restrains excessive forward movement of the lower leg bone (the tibia) in relation to the thigh bone (the femur).
  • It limits rotational movements of the knee.

A tear of the anterior cruciate ligament (ACL) results from overstretching of this ligament when certain movements of the knee put too great a strain on the ACL


Anterior Cruciate Ligament Knee Joint



  Its usually due to a sudden stop and twisting motion of the knee, or a force or "blow" to the front of the knee

  The movements of the knee that can result in a tear of the ACL are described as follows:

  • Hyperextension of the knee, that is, if the knee is straightened more than 10 degrees beyond its normal fully straightened position, is a very common cause of an torn ACL. This position of the knee forces the lower leg excessively forward in relation to the upper leg.
  • Pivoting injuries of the knee with excessive inward turning of the lower leg can also damage the ACL.


Basically any athletic or non-athletic related activity in which the knee is forced into hyperextension and/or internal rotation may result in an ACL tear.

Often those are non-contact activities with the mechanism of injury usually involving:

  • Planting and cutting - the foot is positioned firmly on the ground followed by the leg (and body for that matter) turning one direction or the other. Example: Football or baseball player making a fast cut and changing direction.
  • Straight-knee landing - results when the foot strikes the ground with the knee straight.Example: Basketball player coming down after a jump shot or the gymnast landing on a dismount.
  • One-step-stop landing with the knee hyperextended - results when the leg abruptly stops while in an over-straightened position.Example: Baseball player sliding into a base with the knee hyperextended with additional force upon hyperextension.
  • Pivoting and sudden deceleration resulting from a combination of rapid slowing down and a plant and twist of the foot placing extreme rotation at the knee. Example: Football or soccer player quickly slowing down followed by a quick turn in direction.

The severity of the injury to the knee will depend on:

  • The position of the knee at the time of the injury
  • The direction of the blow
  • The force of the blow

At least half of all ACL tears are associated with other soft tissue injuries in the knee, usually the medial meniscus or medial collateral ligament (see further below about the anatomy of the knee). When the ACL, medial meniscus and medial ligament are all torn the triad (3 injuries) is known as O'Donohugh's triad.

About 40% of people who who tear the ACL describe a "popping sensation" at the time of injury (which may be the tear of the ACL or of the medical meniscus). The knee usually swells and is painful.

The tear of the ACL can be a partial tear or a complete tear.

Instability or a sensation the knee is "giving out" may be a major complaint following this injury.


Often, but not always, depending on a person's activity level, a torn ACL needs to be fixed.

Unfortunately a simple repair by suturing the torn ligament together again is not effective. A successful repair involves completely replacing the torn ligaments, and there are a number ways that this can be done.

To read more about surgery needed for ACL tears and what's involved please go to Surgery for ACL tears

To read whether you personally should consider surgery for an ACL tear go to Do I need Surgery for an ACL tear?


Sports that cause ACL tears:

Hyperextension (forceful over-straightening of the knee) is most often caused by accidents associated with:

  • Skiing
  • Volleyball
  • Basketball
  • Soccer
  • Football

Because the ACL becomes taut with inward rotation of the tibia, activities placing any excessive inward rotation of the tibia (turning the lower leg inward - usually seen from a plant and twist mechanism) are seen in sports such as:

  • Football
  • Tennis
  • Basketball
  • Soccer

Injury to the ACL may occur in other sports such as:

  • Wrestling
  • Gymnastics
  • Martial arts
  • Running

Non-Athletic-Related Injuries

Non-sport related injuries to the ACL result from similar contact and non-contact stresses on the ligament. Examples vary from being struck on the outer side of the knee to landing on the knee forcing it into an over-straightened position with the knee turned inward.

Motor vehicle accidents in which the knee is forced under the dashboard may also cause rupture of the ACL.

Repeated trauma and wear and tear can be a knee problem at any age causing small tears in the ligament, which over time become complete tears.


Nice to Know:

Male/female comparisons

  • Studies from the National Collegiate Athletic Association (NCAA) have shown that female athletes injure the ACL more frequently than their male counterparts.
  • The NCAA data also reports that female basketball and soccer players have a significantly higher incidence of knee injuries in general, and ACL injuries in particular, than their male counterparts.
  • This greater incidence of ACL injuries in women probably originates from several interrelated factors such as hamstring-quadriceps strength imbalances, joint laxity, and the use of ankle braces.



Facts about ACL tears:

  • The anterior cruciate ligament (ACL) provides almost 80% of the stability to the knee joint (counteracting forward movement of the tibia on the femur)
  • More than 11.2 million visits are made to physicians' offices because of a knee problem. It is the most often treated anatomical site by orthopedic surgeons.
  • Of the four major ligaments in the knee, the anterior cruciate ligament and the medial collateral ligament are most often injured in sports.
  • Reconstruction of a torn ACL is now a common procedure, with over 50,000 hospital admissions per year.
  • ACL ruptures occur at a rate of 60 per 100,000 people per year. With society's increasing interest in physical fitness, primary care physicians are seeing more athletic injuries. Along with these injuries are the commonly experienced ACL ruptures in athletes and non-athletes alike. Today's athletes have greater than a 90% chance of returning to their pre-injury level of sports participation.
  • ACL reconstruction is a highly successful operation. With good rehabilitation, 90% to 95% of individuals who undergo this surgery can expect to return to full sports participation within six months.


Understanding The Knee

The knee is a hinge joint made up of three bones held firmly together by ligaments that stabilize the joint. The bones that meet at the knee are the upper leg bone (the femur), the lower leg bone (the tibia), and the knee cap (the patella). A smooth protective layer called cartilageThe smooth tissue that lines the bone ends inside a joint., which allows the bones to glide smoothly upon each other, lines the bones inside the joint. In arthritis, this smooth lining becomes damaged.

Anterior Cruciate Ligament




Ligaments are dense structures of connective tissue that fasten bone to bone and stabilize the knee. Inside the knee joint are two major ligaments:

  • The anterior cruciate ligament (ACL)
  • The posterior cruciate ligament (PCL)


These cross in the center of the knee (that's why they're called cruciate ligaments -a crucifix is a cross). They control the backward and forward motion of the knee. The ACL in particular restrains excessive forward motion of the knee as well as the inward twisting or rotation of the knee. The ACL is frequently injured in severe twisting injuries of the knee.

Collateral ligaments of the knee

Two other major ligaments are actually located outside the knee joint, on the outer and inner side of the knee. They act to stabilize the knee's sideways motion. The ligament on the inner side of the knee is called the medial collateral ligament (MCL) (medial means inner side). The ligament on the outer side of the knee is the lateral collateral ligament (LCL) (lateral means outer side).

The patellar tendon (the 'ligament' of the knee cap) connects the lower part of the kneecap (patella) to the upper part of the tibia   The larger of the two bones of the lower (between the knee and ankle). Also referred to as the shin bone., specifically to the lump one can feel just below the knee on the lower leg bone (the tibia). Part of this tendon is commonly used in reconstructing a torn ACL.



The meniscus is a half-moon-shaped structure placed between the weight-bearing bone ends in the knee. There are two menisci in each knee, one on the inner side called the "medial meniscus" and one on the outer side called the "lateral meniscus."

Meniscus in the knee


  • The two menisci act as shock absorbers within the knee and also help spread the weight load.
  • The meniscus is a type of cartilage, though it is different than the cartilage that lines the bones.
  • The menisci may be torn during twisting movements of the knee. A meniscus is frequently torn at the same time an ACL tears during injury.



Muscles control the movement of the knee joint. Rehabilitation of these muscles is most important following an ACL injury or reconstruction.

The major muscles of the knee joint involved with bending and straightening the knee are:


The quadriceps muscle is made up of the four large muscles at the front of the thigh (these muscles are the rectus femoris, the vastus lateralis, the vastus intermedius, and the vastus medialis). Together they form a large fleshy mass covering the front and sides of the thigh bone. This is the main muscle group that straightens the knee (called extension of the knee).


The hamstring muscles are the muscles at the back of the upper leg. They flex (bend backward) the lower leg. Individually, the muscles of the hamstrings are the biceps femoris, semitendinosus, and semimembranosus.

The biceps femoris is a large muscle comprised of two heads called the long head and short head, converging to a single tendon as it inserts below the knee joint. This common tendon is located on the outer back corner of the knee and forms the outer hamstring. Another member of the hamstring muscles is the semitendinosus muscle originating from the back of the pelvis and crossing below the back of the knee joint. This muscle, along with the tendon from another hamstring muscle called the semimembranosus and yet another inner groin muscle called the gracilis muscle, form the inner hamstring.

Tendons are tough tissues that connect the muscles to the bone. The hamstring tendons are frequently used in reconstruction of the ACL.

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