Pediatric Annals

CME Article 

Preventing Knee Ligament Injuries in Young Athletes

Cynthia LaBella, MD; Rebecca Carl, MD

Abstract

Knee injuries, especially anterior cruciate ligament (ACL) injuries, are a serious concern for physically active children and adolescents. The ACL is one of four major ligaments that provide structural stability to the knee. The main function of the ACL is to prevent the tibia from sliding forward relative to the femur; this ligament also prevents excessive knee extension, varus and valgus positioning at the knee joint, and tibial rotation. 1,2 An intact ACL protects the menisci, the cartilage shock absorbers of the knee, against shearing forces from twisting and pivoting motions.

Abstract

Knee injuries, especially anterior cruciate ligament (ACL) injuries, are a serious concern for physically active children and adolescents. The ACL is one of four major ligaments that provide structural stability to the knee. The main function of the ACL is to prevent the tibia from sliding forward relative to the femur; this ligament also prevents excessive knee extension, varus and valgus positioning at the knee joint, and tibial rotation. 1,2 An intact ACL protects the menisci, the cartilage shock absorbers of the knee, against shearing forces from twisting and pivoting motions.

Cynthia LaBella, MD, is Medical Director, Institute for Sports Medicine, Children’s Memorial Hospital; and Assistant Professor of Pediatrics, Northwestern University. Rebecca Carl, MD, is Attending, Primary Care Sports Medicine, Institute for Sports Medicine, Children’s Memorial Hospital; and Assistant Professor of Pediatrics, Northwestern University.

Dr. LaBella and Dr. Carl have disclosed no relevant financial relationships.

Address correspondence to Rebecca Carl, MD, 2300 Children’s Plaza, Box 69, Chicago, IL 60614; e-mail rcarl@childrensmemorial.org.

Knee injuries, especially anterior cruciate ligament (ACL) injuries, are a serious concern for physically active children and adolescents. The ACL is one of four major ligaments that provide structural stability to the knee. The main function of the ACL is to prevent the tibia from sliding forward relative to the femur; this ligament also prevents excessive knee extension, varus and valgus positioning at the knee joint, and tibial rotation. 1,2 An intact ACL protects the menisci, the cartilage shock absorbers of the knee, against shearing forces from twisting and pivoting motions.

Physicians caring for young athletes have noted an increase in ACL injuries during the past 2 decades.3 This is partly because greater numbers of children and adolescents are participating in organized sports4 and partly because of advances in medical imaging and increasing awareness that skeletally immature patients can tear their ACLs, thereby leading to a greater rate of diagnosis.

Female athletes are at greater risk for ACL injuries than their male counterparts. The passage of Title IX in 1972 dramatically increased the numbers of girls and young women playing sports at every level.5 The rate of girls’ participation in high school sports has climbed more than 900%.6 Although involvement in athletics yields many benefits for female adolescents, including enhanced academic success, lower rates of depression, fewer sexual partners, and decreased rates of pregnancy, participation in sports also increases the risk of injury.6,7

Epidemiology of Anterior Cruciate Ligament (ACL) Injuries

Although there have been no large-scale population studies in children, epidemiologic studies have assessed rates of ACL tears in adolescents and adults. An examination of patient records for the national health care system in Norway revealed 85 reconstructions are performed there per 100,000 individuals in the 16- to 39-year-old age group.8

ACL injuries occur most commonly in sports that involve jumping, twisting, and cutting. In college athletics, participants in basketball, soccer, men’s football, and women’s gymnastics have the highest rates of ACL injury per athlete exposures (see Figure 1, page 712). In women’s basketball, women’s gymnastics, women’s lacrosse, and women’s soccer, ACL injuries make up the largest percentages of total injuries; in women’s collegiate basketball and soccer, ACL injuries represent 4.9% of a team’s injuries.9 High school athletes have similar injury patterns. Complete ligament tears occur at the highest rates in girls’ volleyball, girls’ basketball, and girls’ soccer.10 Most ACL tears (more than 70%) do not involve any direct contact with another player. The most frequently reported movements are a pivot or twist, landing from a jump, or a sudden deceleration.11,12

NCAA Data for ACL Injury Rates. The 10 Sports with the Highest Injury Rates Are Shown Here.

Figure 1. NCAA Data for ACL Injury Rates. The 10 Sports with the Highest Injury Rates Are Shown Here.

In the United States, the ACL injury rate among high school female athletes is between 1% and 4%, which amounts to 20,000 to 80,000 ACL injuries per year.13,14 This is significantly higher than in the general population, where the rate of ACL injury is 1 in 3,000.15

Of particular concern is the fact that female athletes are four to six times more likely to injure their ACL than male athletes in similar sports.9,16 A study of insurance claims for youth soccer injuries showed that, by the age of 14 years, girls have an ACL injury rate five times higher than boys.17

Powell and colleagues conducted a prospective cohort study of high school basketball and soccer athletes that recorded 39,032 player-seasons and 8,988 injuries. Among basketball athletes, the knee injury rate for girls was 44% higher than that for boys, the knee surgery rate was more than twice as high, and the ACL surgery rate was more than four times higher for girls.18

Compared with boys, girls are also more likely to have surgery and less likely to return to sports after an ACL injury, causing them to miss out on the well-documented health benefits of physical activity and the social benefits of sports participation.25 A study of high school athletes found that among female basketball players, knee injuries were the most common cause of permanent disability, accounting for up to 91% of season-ending injuries and 94% of injuries requiring surgery.11 Kujala et al. studied sports injury patterns in volleyball, soccer, and basketball, and found knee injuries were the most common cause of permanent disability.12,16

Unfortunately, an ACL injury at an early age is a life-changing event. In addition to surgery and many months of rehabilitation, treatment costs can be substantial ($17,000 to $25,000 per injury). 19,20 Additionally, the time lost from school and sports participation can have considerable effects on the athlete’s mental health and academic performance.

Freedman et al. examined the academic transcripts of college students who underwent ACL reconstruction surgery.21 Compared with an age-matched control group, those who had surgery had a significant drop in grade point average of 0.3 points during the semester of injury (P = .04).21,22

Beyond these more immediate effects, an ACL injury also has long-term health consequences. Regardless of the type of treatment, athletes with ACL injury are up to 10 times more likely to develop degenerative osteoarthritis, a condition that can not only limit one’s ability to participate in sports, but also often leads to chronic pain and disability.23,24 Lohmander et al. evaluated female soccer players and found that previous knee injury was correlated with subsequent development of osteoarthritis in the injured joint.24 Gelber et al. followed a cohort of more than 1,300 young patients (average age was 22 years) and demonstrated that individuals who had sustained knee injuries at baseline evaluation were at significantly increased risk for developing osteoarthritis.23 In neither study did ACL reconstruction appear to lower the chances of a patient to develop osteoarthritis.

Risk Factors

Extrinsic Risk Factors

A few studies have investigated associations between factors extrinsic to the athlete (eg, playing surface, equipment, weather, or field conditions) and ACL injury. Olsen et al. found that female athletes playing handball on an artificial surface had higher rates of ACL tears than athletes playing on wood flooring.25 Lambson et al. correlated a particular cleat design with ACL injuries in a study of approximately 3,000 high school football players. 26 Dry weather appears to increase the risk of ACL injuries for Australian football players who play on grass.27,28

Intrinsic Risk Factors

A greater body of research has been devoted to identifying intrinsic risk factors for ACL injury. To explain the gender disparity in knee injury patterns, researchers have studied various anatomic, hormonal, and neuromuscular differences between girls and boys.

Studies examining whether higher body mass index (BMI) is associated with an increased rates of ACL injuries have yielded conflicting results.29,30

In addition, individuals with a smaller intercondylar notch, the space between the femoral condyles where the ACL lives, appear to have a higher risk of ACL tear.31 Other anatomic factors that have been studied include tibial plateau slope, ACL size relative to BMI, and Q-angle (the angle formed by a line connecting the center of the patella to the anterior superior iliac spine and the vertical axis of the patella); none of these factors has been consistently correlated with ACL injury risk.9 Joint hypermobility is the only anatomic factor that has been consistently associated with increased risk for ACL injury. In a study of West Point cadets, generalized ligamentous laxity was a risk factor for ACL tear; retrospective studies have shown that individuals with a history of ACL injury have higher rates of knee joint laxity and generalized hypermobility.9,32,33

Estrogen, relaxin, and testosterone receptors have all been identified on the human ACL. Several studies examining the interrelationship between ACL injury and the menstrual cycle have shown increased laxity of the ligament following ovulation. However, in other studies, there was no relationship between hormone levels and ligamentous laxity.34

A growing body of evidence suggests that the key risk factor for girls, and, fortunately, the most modifiable, is poor neuromuscular control of knee motion during athletic tasks, such as landing from a jump or cutting. The key neuromuscular risk factors appear to be 1) an imbalance between quadriceps and hamstrings; 2) dynamic knee valgus; 3) asymmetry in leg strength; and 4) poor coordination between trunk and limb movements.

To protect the ACL from excessive strain, the hamstring and quadriceps muscle groups must contract together during high-risk activities, such as landing, changing direction, and sudden deceleration.35 During athletic tasks, female athletes exhibit greater quadriceps activation forces and lower hamstring activation forces than their male counterparts.36 This is related to the observation that girls tend to land from a jump with less knee flexion compared with boys.36 Additionally, instead of using their muscles to control lower extremity joint position effectively during athletic maneuvers, girls tend to rely on their ligaments to stop joint motion.36, 37,38 This results in “dynamic knee valgus,” a combination of lower extremity joint positions, including hip adduction, knee abduction, tibia external rotation and ankle eversion: the knee collapses inward and the distal tibia is positioned outward with actions, such as landing and cutting.38 This is a high-risk position for tearing the ACL (see Figure 2).

High-Risk Position, Which Is Believed to Increase Risk of ACL Injury.Athlete Is in an Upright Position. Knees Are Forward over the Toes. Her Hips Are Adducted and Internally Rotated Resulting in the Appearance of Knees Deviating Medially (dynamic Valgus).

Figure 2. High-Risk Position, Which Is Believed to Increase Risk of ACL Injury.Athlete Is in an Upright Position. Knees Are Forward over the Toes. Her Hips Are Adducted and Internally Rotated Resulting in the Appearance of Knees Deviating Medially (dynamic Valgus).

In a prospective study of female soccer, basketball, and volleyball players who underwent biomechanical testing prior to their season, nine out of 205 went on to tear their ACLs. Dynamic knee valgus was a significant predictor of ACL injury.39 Boden compared video footage of athletes sustaining ACL tears to footage of athletes performing similar maneuvers without injury. Injured individuals landed significantly more often on the heel or in a flat-footed position and with increased knee abduction (dynamic knee valgus).40

Neuromuscular Training Programs

The correlation between specific neuromuscular patterns and ACL injury risk has led to the development of neuromuscular training programs designed to improve athletes’ knee awareness and neuromuscular control during landing and cutting maneuvers. Neuromuscular training programs combine progressive core and lower extremity strengthening and plyometric jumping exercises with proprioception and balance training, safe landing (see Figure 3, page 718), deceleration techniques, and education on how to recognize and avoid unsafe knee positions. With plyometric activities, an athlete starts with a muscle group in a stretched position, then rapidly contracts the muscle group; jumping and landing on two feet with good control is an example of a simple plyometric exercise. These programs have been shown to reduce knee injuries in adolescent females by up to 88%.41

Athletic Position, Which Is Believed to Reduce the Risk of ACL Injury. A. Frontal View (left). Athlete Has More Hip Flexion. Knees Are Aligned with the Hips. Weight Is Evenly Distributed Across Both Feet. B. Lateral View (right). Athlete Is not Leaning Forward, Hips Are Posterior with Hamstrings Engaged. Knees Do not Extend over Toes.

Figure 3. Athletic Position, Which Is Believed to Reduce the Risk of ACL Injury. A. Frontal View (left). Athlete Has More Hip Flexion. Knees Are Aligned with the Hips. Weight Is Evenly Distributed Across Both Feet. B. Lateral View (right). Athlete Is not Leaning Forward, Hips Are Posterior with Hamstrings Engaged. Knees Do not Extend over Toes.

Henning trained collegiate women basketball players to avoid particular cutting and deceleration maneuvers and found a decrease in the incidence of knee injuries. Sadly, he passed away before his findings could be published.9

Hewett et al. conducted a prospective cohort study to investigate the effect of a 6-week, pre-season neuromuscular training program on the incidence of knee ligament sprains in female adolescent soccer, volleyball, and basketball athletes. Compared with a control group of untrained female athletes playing the same sports, the trained athletes had a significantly lower incidence of knee ligament injuries.20 Heidt et al. conducted a non-randomized, controlled study of the effects of a pre-season conditioning program on injury rates among adolescent female soccer athletes. They found the incidence of lower extremity injuries was significantly lower among the 42 athletes in the trained group compared with the 258 athletes in the untrained group.42 ACL injuries were also lower in the trained group, but this difference was not statistically significant.

Myklebust et al. conducted a prospective cohort study over three seasons to investigate the effect of pre-season neuromuscular training on the incidence of ACL injuries in 942 female team handball players between 16 and 35 years.43 Neuromuscular training was instituted during the second and third seasons. A significantly lower rate of noncontact ACL injuries was observed during the second intervention season compared with the first season (control).

In a prospective, non-randomized, controlled trial, Mandelbaum et al. studied the effect of a pre-practice warm-up containing neuromuscular exercises called “Prevent injury, Enhance Performance” (PEP) on the incidence of ACL injuries among female soccer players between 14 and 18 years. The players’ coaches implemented the warm-up. There were significantly fewer non-contact ACL injuries per 1,000 exposures in the intervention group compared with the control group (0.09 vs. 0.49, respectively).41 The same group performed a subsequent study during which National Collegiate Athletic Association (NCAA) Division I women’s soccer teams were randomized to receive instruction on the PEP program or to continue with their standard warm-up. Compared with the intervention group, control-group athletes were more than three times as likely to sustain a noncontact ACL injury; this finding approached statistical significance (P = .066).44

In 2006, LaBella et al. conducted a prospective cluster-randomized controlled trial of the effects of a coach-led neuromuscular warm-up on lower extremity injury rates among female basketball and soccer players in a large urban public high school system (see Figure 4, page 719). In this study, the intervention group athletes exposed to the warm-up for more than 50% of practices had 75% fewer knee ligament injuries (P < .05) and 84% fewer non-contact ACL tears (P = .05) than the control group.45

Select Exercises from the Knee Injury Prevention Program (KIPP). A. Forward Lunge. B. Side Lunge. C. Broad Jump. D. Single Leg Jump. E. Prone Lift. F. Side Plank.

Figure 4. Select Exercises from the Knee Injury Prevention Program (KIPP). A. Forward Lunge. B. Side Lunge. C. Broad Jump. D. Single Leg Jump. E. Prone Lift. F. Side Plank.

Other studies, however, have not shown a decrease in injury rates with training programs. 42 Soderman implemented a balance board training program for 221 professional women soccer players and found that control group athletes were less likely to have a significant knee injury. The sample size in this study was small, and the intervention involved balance training only.42 Although ACL injuries are common injuries, they occur infrequently enough that intervention studies require a large number of subjects to detect a statistically significant difference.

A 2005 meta-analysis39 found that the successful training programs included a combination of progressive plyometric exercises, core and lower extremity strengthening, and feedback to help athletes recognize and correct unsafe movement patterns. Programs that included only balance training or only strength training were not effective in reducing ACL injuries. Although strengthening exercises were not found to be a pre-requisite for prevention, they enhanced program effectiveness. The Table (page 716) summarizes the findings of several studies investigating the results of neuromuscular prevention programs.

A Sampling of Studies Examining the Effect of Neuromuscular Training Programs on ACL and Other Knee Injuries

Table. A Sampling of Studies Examining the Effect of Neuromuscular Training Programs on ACL and Other Knee Injuries

Directions for Future Research

The evidence demonstrating that neuromuscular factors contribute to the risk of knee injury in female athletes, and that training can modify these factors, is compelling. Future research is needed to determine which specific exercises are essential, the minimum frequency of exposure, the duration of the protective effect, and how to disseminate these programs effectively to athletes and coaches. To date, these programs have only been studied in soccer, handball, and to a lesser extent, basketball. Well-powered studies with athletes from a variety of sports and skill levels will help determine who can benefit from these knee injury prevention programs. As we learn more about the factors that increase the risk of ACL tears, we may be able to screen young athletes to identify those who would benefit most from these prevention programs.

Conclusions

ACL injuries are an increasing concern for young athletes and their families and can have a substantial long-term effect on health. Fortunately, research performed during the past 2 decades has enhanced our understanding of myriad factors that place young, especially female, athletes at risk for ACL tears and how neuromuscular training can decrease this risk. Pediatricians, sports medicine specialists, parents, and coaches who work with young athletes should collaborate to increase awareness of these injuries and improve access to neuromuscular training programs.

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A Sampling of Studies Examining the Effect of Neuromuscular Training Programs on ACL and Other Knee Injuries

Lead AuthorSportSubject Ages (in years)Number of SubjectsType of InterventionSummary of FindingsWebsite for program (if applicable)
Myklebust et al.46Handball16–35Int: 840; Con: 942Balance, plyometrics with feedback to athleteFor 2nd intervention season, 2000–2001Non-contact ACL injuries: Int: 0.08/1000 AE; Con: 0.17/1000 AE; (P = .04)
Olsen et al.47Handball15–17Int: 958; Con: 879Strengthening, balance, plyometrics with feedback to athleteAcute knee injuries: Int: 0.5/1000 AE; Con: 0.9/1000 AE; (P = .001)
Mandelbaum et al.41Soccer14–18Int: 1,885; Con: 3,818Flexibility, strengthening, balance, plyometricsACL injuries: Int: 0.05/1000 AE; Con: 0.47/1000 AE; (P = .001)www.aclprevent.com/aclprevention.htm
Gilchrist et al.44SoccerCollege-age Mean: 19.9Int: 1,429; Con: 854Flexibility, strengthening, balance, plyometrics with feedback to athleteNon-contact ACL injuries: Int: 0.04/1000 AE; Con: 0.14/1000 AE; (P = .06)www.aclprevent.com/aclprevention.htm
LaBella et al.48Basketball and soccer14–18Int:737; Con: 755Strengthening, balance, plyometrics with feedback to athleteNon-contact ACL injuries: Int: 0.036/1000 AE; Con: 0.217/1000 AE; (P = .08)www.childrensmemorial.org/depts/sportsmedicine/program.aspx
Hewett20Basketball, soccer, volleyball14–18Int: 366; Con: 463Flexibility, strengthening, plyometrics with feedback to athleteNon-contact ACL injuries: Int: 0/1000 AE; Con: 0.22/1000 AE; (P = <.05)www.cincinnatichildrens.org/svc/alpha/s/sports-med/dna-training/dna.htm
Kiani49Soccer13–19Int: 737; Con: 755Strengthening, balance, plyometrics with feedback to athleteNon-contact ACL injuries: Int: 0/1000 AE Con: 0.05/1000 AE;(P = .12)

CME Educational Objectives

  1. Discuss the epidemiology and natural history of anterior cruciate ligament (ACL) injuries in young athletes.

  2. Determine the key risk factors for ACL injuries.

  3. Identify the components of a successful neuromuscular program aimed at knee injury prevention.

Authors

Cynthia LaBella, MD, is Medical Director, Institute for Sports Medicine, Children’s Memorial Hospital; and Assistant Professor of Pediatrics, Northwestern University. Rebecca Carl, MD, is Attending, Primary Care Sports Medicine, Institute for Sports Medicine, Children’s Memorial Hospital; and Assistant Professor of Pediatrics, Northwestern University.

Dr. LaBella and Dr. Carl have disclosed no relevant financial relationships.

Address correspondence to Rebecca Carl, MD, 2300 Children’s Plaza, Box 69, Chicago, IL 60614; e-mail .rcarl@childrensmemorial.org

10.3928/00904481-20101013-10

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