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ACL INJURY RATES

The anterior cruciate ligament (ACL)

is one of the most commonly injured

ligaments in the knee. Approximately

150,000 ACL injuries occur in the United

States each year. Female athletes par-

ticipating in basketball and soccer are

two to eight times more likely to suffer

an ACL injury compared to their male

counterparts. Recent data from the

Women’s National Basketball Association

indicates white European-American

players may be at increased risk for

ACL injury compared with African-

American, Hispanic or Asian players.

Athletes who have suffered an ACL

injury are at increased risk of developing

arthritis later on in life, even if they have

surgery for the injury. ACL injuries

account for a large health care cost

estimated to be over half-billion dollars

each year.

WHY DO ACL INJURIES

OCCUR?

Researchers believe there are external

and internal factors associated with

ACL injury. External factors include

any play where the injured athlete’s

coordination is disrupted just prior to

landing or slowing down (deceleration).

Examples of a disruption include being

bumped by another player, landing in

a pothole, or a ball deflection. Other

external factors which have been stud-

ied include the effect(s) of wearing a

brace, shoe-surface interface (how

certain types of athletic footwear

perform on different surfaces), and

the playing surface itself.

Internal factors include differences

in the anatomy of men and women,

increased hamstring flexibility, increased

foot pronation (flat-footed), hormonal

effects, and variations in the nerves

and muscles which control the position

of the knee. Anatomical differences

between men and women, such as a

wider pelvis and a tendency towards

“knock knee” in women, may predispose

women to ACL injury. Differences in ACL

injury rates between men and women

seem to begin shortly after puberty

because the nerve/muscle system

(coordination) adapts at a slower pace

than the anatomical and hormonal

changes. It is possible that the incidence

of injuries in women increases at this

age because the nerve/muscle system

(coordination) adapts to these changes

at a slower rate than in men. Women

also tend to have knees that are less

stiff than men, placing more forces on

the ligaments. In addition, the female

hormone estrogen may relax or allow

stretching of the ACL, thereby predis-

posing female athletes to ACL injury.

Nerve/muscle factors pertain to the

interaction and control of the knee by

the quadriceps and hamstrings muscles

in the legs. Researchers are very inter-

ested in studying this particular factor

since it may be the easiest to modify.

HOW DO ACL INJURIES

OCCUR?

Careful study of videos of athletes

tearing an ACL show that approximately

70 percent of these injuries are non-

contact and 30 percent occur during

contact. The noncontact injuries usually

occur during landing or sharp decelera-

tion. In these cases, the knee at the

time of injury is almost straight and

may be associated with valgus (inward)

collapse (see Figure 1). The athlete

often lands with a flat-foot position

and the leg is placed in front or to the

side of the trunk.

ANTERIOR CRUCIATE

LIGAMENT (ACL)

INJURY PREVENTION

AOSSM SPORTS TIPS

ACL INJURY PREVENTION

Figure 1

Figure 2

PREVENTION OF

ACL INJURY

everal prevention programs have been

developed in an attempt to decrease

the incidence of noncontact ACL injuries.

The focus of current prevention

programs is on proper nerve/muscle

control of the knee. These programs

focus on plyometrics, balance, and

strengthening/stability exercises.

Plyometrics is a rapid, powerful move-

ment which first lengthens a muscle

(eccentric phase) then shortens it

(concentric phase). The length-short-

ening cycle increases muscular power.

An example would be an athlete jump-

ing off a small box and immediately

jumping back into the air after contact

with the floor.

Balance training commonly involves

use of wobble or balance boards.

On-field balance exercises may include

throwing a ball with a partner while

balancing on one leg.

To improve single-leg core strength

and stability, athletes perform exer-

cises such as jumping and landing on

one leg with the knee flexed and then

momentarily holding that position.

PLYOMETRIC EXERCISES

High-intensity plyometrics may be key

in reducing the number of ACL injuries.

To be most successful, plyometric

training should be performed more

than once per week for a minimum

of six weeks. Athletes are taught prop-

r landing techniques which emphasize

landing on the balls of the foot with the

knees flexed and the chest over the

knees (see Figure 2). The athlete

should receive feedback on proper

knee position to prevent inward buck-

ling. Many of the newer programs are

being adapted by coaches as an integral

part of warm-up during practice, such

as jumping over a soccer ball and

landing in the correct position.

REFERENCES

Boden BP, Dean GS Feagin JA et al.

Mechanisms of anterior cruciate ligament

injury. Orthopedics. 200;23:573-578.

Hewett TE, Lindenfeld TN, Riccobene JV et

al. The effect of neuromuscular training on the

incidence of knee injury in female athletes:

A prospective study. American Journal of

Sports Medicine. 1999;27:699-706.

Mandelbaum BR, Silvers HJ, Watanabe D et

al. Effectiveness of a neuromuscular and

proprioceptive training program in prevent-

ing anterior cruciate ligament injuries in

female athletes: Two year follow up.

American Journal of Sports Medicine.

2005;33:1003-1010.

Expert Consultant:

Barry P. Boden, MD

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