ACL Tears

What Causes An ACL Tear?

The ACL may tear when certain movements of the knee place a great strain on the ACL. These movements are:

• 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.

Activities placing the knee into hyperextension and /or the tibia into excessive inward rotation can be from either an outside force or non-contact in nature.

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

Most ACL injuries occur during athletic activity. 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.

About 40% of all individuals experience a “popping” sensation at the time of the injury, which is actually the tearing of the ligament tissue. At least half of all anterior ligament tears also cause injury to one of the menisci of the joint, which may also produce a tearing sensation.

Hyperextension (forceful over-straightening) 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 (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.