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.

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 to the anterior cruciate ligament (ACL) results from overstretching of this ligament within the knee.

• It's usually due to a sudden stop and twisting motion of the knee, or a force or "blow" to the front of the knee.

   

• The extent of the tear can be a partial or a complete tear.

   

• Individuals experiencing a tear to the ACL may or may not feel a pop at the time of the injury.

   

• It is often injured together with other structures inside the knee joint.

   

• After the initial injury, the knee may swell and become painful.

   

• 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.

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 cartilage, which allows the bones to glide smoothly upon each other, lines the bones inside the joint. In arthritis, this smooth lining becomes damaged.

Ligaments

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.

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, 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.

Meniscus

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."

• 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

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:

• Quadriceps

   

• Hamstrings

   

Quadriceps

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).

Hamstrings

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.