Anterior cruciate ligament (ACL)

What is the anterior cruciate ligament (ACL)?

The ACL helps stabilize the knee by connecting the front part of the shin bone (tibia) to the back part of the thigh bone (femur), and by keeping the shin bone from sliding (translating) too far forward from the thigh bone.

The ACL is a strong ligament that allows her knee to remain stable while a child turns, cuts, twists and pivots. It’s especially crucial in sports where it provides stability during stop/go/pivot motions—such as basketball, soccer, skiing, gymnastics, volleyball and football.

What are the ligaments of the knee?

The knee is the largest and most complex joint in your child’s body. It depends on four ligaments—as well as muscles and tendons—to function properly:

The two ligaments on the sides of the knee:

• Medial (inside) collateral ligament (MCL)
• Lateral (outside) collateral ligament (LCL)
• The two crossed (“cruciate”) ligaments inside the knee a
• Posterior cruciate ligament (PCL)
• Anterior cruciate ligament (ACL)

Causes

What causes ACL injuries in children?

Most ACL tears happen without contact between the knee and another object and occur during sports like basketball, soccer, skiing, gymnastics, volleyball and football.

An ACL tear can happen when an athlete:

• plants the foot and pivots sharply

• locks a knee after jumping and lands on a straight leg, instead of one that is flexed (for example, when skiing).

• stops abruptly

Risk factors

What factors increase a child’s risk of an ACL injury?

Teenagers have the highest risk of ACL injuries since they are the most athletically active demographic group.

Teenagers who are especially at risk include those who play contact sports (such as  football and hockey) sports that involve cutting and pivoting, (such as soccer, basketball, gymnastics, volleyball and baseball).

• Girls are five to eight times more likely than boys to tear their ACLs for several possible reasons:

• Girls tend to use their ligaments more than their muscles in sports.

• At puberty, boys grow in height and develop their lower leg muscles at the same time. Girls grow too, but they don’t tend to develop lower leg strength unless they train for it.

• Girls land in a more straight-legged way than boys, thus reducing the effectiveness of their quads (quadriceps femoris muscles) as shock absorbers.

• Girls’ knees tend to bend inward in jumping/pivoting/landing moves, which distributes their weight more unevenly than boys.

• Girls have wider hips (in relation to their knees), which puts more stress on  the ACL.

• Whereas boys tend to use their hamstrings when jumping, girls tend to use their quads, which put more stress on the ACL.

Signs and symptoms

What are the symptoms of ACL tears in children?

When the ACL is torn, a child will usually feel her knee give way (often with an audible “pop”) and be in a considerable amount of pain.  Most children will not be able to walk or put weight on the injured leg.

Over the next several hours, the following symptoms may occur:

• Swelling of the knee
• Severe pain when walking or bending the knee

The swelling and pain are usually most severe in the first two days after the tear, and then they begin to subside.

Can other knee structures be injured along with the ACL?

Other structures in your child’s knee can be injured at the same time as her ACL tear, such as the meniscus (the cartilage cushion in her knee that contains thick bundles of collagen fibers).