Introduction

The anterior cruciate ligament (ACL) is one of four major ligaments of the knee, providing stability to the tibiofemoral joint in weight-bearing activity. The primary role of the ACL is to prevent anterior translation of the tibia on the femur; however, it also acts as a secondary restraint to tibial internal rotation. The ACL is a thick band of connective tissue that runs anteriorly, medially, and distally from its origin on the posterior medial surface of the lateral femoral condyle to its insertion on the anterior portion of the tibial plateau. It consists of two bundles, each serving to stabilize the knee at varying angles of knee motion. The smaller anteriomedial bundle lengthens and becomes taut with flexion, while the larger posterolateral band works primarily to stabilize the knee against rotary loads near full extension.

Since the passage of Title IX in 1972 there has been an exponential increase in female sports participation and thus a significant rise in knee injuries in the female athlete. ACL injuries are quite common, accounting for 20% of all knee injuries and 45% of internal knee lesions occurring in sport, with an estimated incidence of 80,000–200,000 injuries per year in the United States alone. In addition, ACL injuries account for 20,000 injuries per year in high school female athletes and 10,000 per year in collegiate female athletes. Debilitating knee injuries can lead to lost participation in sport for an entire season, long-term physical therapy, or surgical intervention. Due to its fundamental role in dynamic stabilization of the knee, ACL injuries occur most commonly in sports that require change of direction or rapid deceleration, such as cutting, pivoting, or landing, with the highest incidence occurring in athletes between the ages of 15 and 25 who participate in these types of sports. The majority of ACL injuries (70%) are noncontact in nature, with females 6 to 8 times more likely to experience a noncontact injury as compared to their male counterparts participating in the same sports. The increase in female ACL injuries is further reflected in the rise in ACL reconstructions performed on females, with Buller et al. reporting a 404% increase in the sex-adjusted rate of women undergoing outpatient ACL reconstruction between 1996 and 2006.

Risk Factors

The combination of increased female sports participation and high noncontact injury risk has led to an increased volume of research and growing body of evidence on risk factors for ACL injuries in female athletes, but the general consensus is that the predisposition for elevated risk in females is a multifactorial issue. The risk factors for noncontact injuries can be divided into four categories: environmental, anatomic, hormonal, and neuromuscular. Potential environmental risk factors that have been identified include the type or level of competition, protective bracing, footwear to playing surface interface, and meteorological conditions. The remainder of the most commonly recognized risk factors for ACL injury are intrinsic in nature. Anatomical influences that are most often considered include decreased intercondylar notch width, ACL geometry, posterior tibial slope, and Q angle; however, it is acknowledged that while these factors may help identify those most at risk, anatomical issues are the most difficult to modify in hopes of injury prevention. Hormonal contributions to ACL injury risk have also been consistently documented, looking at the effects of hormones on laxity and tensile strength, as well as the relationship between the phases of the menstrual cycle and noncontact injury; however, the current body of evidence is inconclusive. Perhaps the most studied risk factor is neuromuscular differences between females and males, as this is the area where intervention can be most effective. The proposed neuromuscular risk factors can be categorized into altered movement patterns, altered muscle activation patterns, and inadequate muscle stiffness.

It is vital to continue to study potential risk factors, as ACL injuries lead to significant short-term consequences including time lost from school, work, and sport, as well as long-term ramifications to the knee joint health. Fifty percent of those individuals diagnosed with an ACL or meniscal injury will develop posttraumatic knee osteoarthritis (OA), with associated pain and dysfunction, within 10−20 years of injury. Historically it was advised that undergoing ACL reconstruction would prevent the development of posttraumatic OA; however, current evidence suggests that reconstruction does not reduce the rate of OA or improve long-term outcomes. In addition to the significant impact of ACL injury on daily function and activity participation, ACL injuries also lead to a substantial cost burden to society via both direct and indirect costs. The symptomatic, unstable knee after ACL injury can lead to lost wages, decreased productivity, and disability in the short term, as well as a societal cost burden related to knee OA and total knee arthroplasty in the long term. There are two treatment options for ACL injuries: reconstructive surgery or nonoperative management via a structured rehabilitation program. Early ACL reconstruction, however, is the standard of care in the United States, with nearly 130,000 ACL reconstructions performed each year. The mean typical lifetime cost to society for a typical patient undergoing ACL reconstruction is estimated at over $38,000 per patient. In addition, the economic impact of posttraumatic knee OA must also be considered, as ACL injury contributes a rise of 30,000–38,000 patients with symptomatic knee OA and 20,000–25,000 total knee arthroplasties each year.

The long-term consequences of knee joint health as a result of ACL injury, in combination with the high incidence of injuries in young female athletes, has precipitated the need for safe, effective, and feasible prevention programs. The development of the prevention programs continues to rely on identifying modifiable risk factors in order to implement appropriate pre- and in-season training regimens that will decrease ACL injury rates. Development of successful prevention programs could also have an important economic impact by leading to cost savings for society. In a 2013 study of the societal and economic impact of ACL tears, Mather et. al. estimated that reducing the ACL tear risk by half would save society $1.1 billion annually.

Risk Factors

The combination of increased female sports participation and high noncontact injury risk has led to an increased volume of research and growing body of evidence on risk factors for ACL injuries in female athletes, but the general consensus is that the predisposition for elevated risk in females is a multifactorial issue. The risk factors for noncontact injuries can be divided into four categories: environmental, anatomic, hormonal, and neuromuscular. Potential environmental risk factors that have been identified include the type or level of competition, protective bracing, footwear to playing surface interface, and meteorological conditions. The remainder of the most commonly recognized risk factors for ACL injury are intrinsic in nature. Anatomical influences that are most often considered include decreased intercondylar notch width, ACL geometry, posterior tibial slope, and Q angle; however, it is acknowledged that while these factors may help identify those most at risk, anatomical issues are the most difficult to modify in hopes of injury prevention. Hormonal contributions to ACL injury risk have also been consistently documented, looking at the effects of hormones on laxity and tensile strength, as well as the relationship between the phases of the menstrual cycle and noncontact injury; however, the current body of evidence is inconclusive. Perhaps the most studied risk factor is neuromuscular differences between females and males, as this is the area where intervention can be most effective. The proposed neuromuscular risk factors can be categorized into altered movement patterns, altered muscle activation patterns, and inadequate muscle stiffness.

It is vital to continue to study potential risk factors, as ACL injuries lead to significant short-term consequences including time lost from school, work, and sport, as well as long-term ramifications to the knee joint health. Fifty percent of those individuals diagnosed with an ACL or meniscal injury will develop posttraumatic knee osteoarthritis (OA), with associated pain and dysfunction, within 10−20 years of injury. Historically it was advised that undergoing ACL reconstruction would prevent the development of posttraumatic OA; however, current evidence suggests that reconstruction does not reduce the rate of OA or improve long-term outcomes. In addition to the significant impact of ACL injury on daily function and activity participation, ACL injuries also lead to a substantial cost burden to society via both direct and indirect costs. The symptomatic, unstable knee after ACL injury can lead to lost wages, decreased productivity, and disability in the short term, as well as a societal cost burden related to knee OA and total knee arthroplasty in the long term. There are two treatment options for ACL injuries: reconstructive surgery or nonoperative management via a structured rehabilitation program. Early ACL reconstruction, however, is the standard of care in the United States, with nearly 130,000 ACL reconstructions performed each year. The mean typical lifetime cost to society for a typical patient undergoing ACL reconstruction is estimated at over $38,000 per patient. In addition, the economic impact of posttraumatic knee OA must also be considered, as ACL injury contributes a rise of 30,000–38,000 patients with symptomatic knee OA and 20,000–25,000 total knee arthroplasties each year.

The long-term consequences of knee joint health as a result of ACL injury, in combination with the high incidence of injuries in young female athletes, has precipitated the need for safe, effective, and feasible prevention programs. The development of the prevention programs continues to rely on identifying modifiable risk factors in order to implement appropriate pre- and in-season training regimens that will decrease ACL injury rates. Development of successful prevention programs could also have an important economic impact by leading to cost savings for society. In a 2013 study of the societal and economic impact of ACL tears, Mather et. al. estimated that reducing the ACL tear risk by half would save society $1.1 billion annually.

A Review of Prevention Programs

Numerous ACL prevention protocols and training regimens have been developed to address the biomechanical deficits associated with the common mechanisms of injury. These programs have been implemented and analyzed across sport, including skiing, basketball, handball, volleyball, and soccer, with results ranging in overall reduction of ACL injuries from 72% to 89%.

One of the earliest attempts to address the issue of ACL injury was implemented by Charles Henning in the late 1980s to address what he deemed to be a deleterious quad-cruciate interaction, described as the knee being at or near extension in a weight-bearing position. He and his colleagues proposed that the most common mechanisms of ACL injuries included (1) planting/cutting, (2) landing in or at knee extension, and (3) a one-step stop deceleration. Henning implemented a prevention study in two division I basketball programs over a course of 8 years geared at changing players’ biomechanical technique: stressing increased knee flexion upon landing, using accelerated, more obtuse, rounded turns, and decelerating with a multistep stop to decrease anterior shear. He reported an 89% reduction in the incidence of ACL injuries in his intervention group (IG). Unfortunately, the untimely death of Dr. Henning led to this work never being published. However, the authors would be remiss not to allude to his work, as Dr. Henning is deemed one of the forefathers of ACL injury-prevention efforts.

Caraffa et al. implemented a proprioceptive balance training program using 600 semiprofessional and amateur soccer players over the course of three seasons. The study consisted of a 20-minute training program using a Biomechanical Ankle Platform System (BAPS) (Camp Jackson, Michigan) balance board and was divided into five phases of increasing difficulty. ACL injuries were tracked in this semiprofessional cohort for 3 years. Ten ACL injuries were recorded in the IG (incidence rate [IR] = 0.15 injuries/team per year) compared to 70 in the control group (CG) (IR = 1.15 ACL injuries/team per year). These results demonstrated an overall 87% decrease in ACL injuries compared with the CG.

Hewett et al. completed a prospective analysis of 1263 male and female athletes from various sports using a preseason neuromuscular training program. They used a 6-week preseason intervention program entitled Cincinnati Sportsmetrics, consisting of a three-phase progression that included flexibility, plyometrics, and weight training, with an emphasis on proper biomechanical alignment and technique in attempt to decrease deleterious landing forces. The incidence of serious knee injury was 2.4–3.6 times higher in the untrained group (IR = 0.43) than in the trained group (IR = 0.12). When the rate of noncontact ACL injuries was examined, five untrained female athletes sustained ACL injuries (relative injury incidence 0.26), no trained females sustained an ACL injury (0), and one male athlete sustained an ACL injury (0.05).

Ettlinger et al. implemented the guided discovery technique in Vermont, which focused on avoiding high-risk positioning (i.e., phantom foot ) during potentially dangerous skiing situations, and promoting a quick response to unfavorable environmental conditions. During the 1993–1994 ski season, 4700 ski instructors and patrollers completed the comprehensive training program at 20 ski areas throughout the United States. As a result, the rate of serious knee injuries decreased by 62% among the trained personnel compared with those who did not participate in the training program.

Heidt et al. studied 300 female adolescent soccer players (14–18 years of age) over a 1-year period. Forty-two athletes participated in the Frappier acceleration training program, a 7-week preseason training program consisting of strength training, flexibility, sports-specific cardiovascular exercise, plyometrics, and sports cord drills. They found that the trained group had fewer ACL injuries (2.4%) than the age matched CG (3.1%).

Myklebust et al. addressed ACL injury prevention in female team handball players through a prospective cohort study over the course of 3 years (one referent season and two intervention seasons). The program consisted of five phases and encompassed balance exercises on the court, neuromuscular control, balance mat exercises, and wobble board exercises. The researchers reported a reduction of ACL injury rates from 0.14/1000 playing hours in the referent control season to 0.13 and 0.09 in the subsequent intervention seasons.

The Santa Monica Sports Medicine Foundation (SMSMF) created the Prevent Injury and Enhance Performance (PEP) program to address the high incidence of ACL injuries in young female soccer athletes. Similar to previous prevention programs, it emphasized landing softly on the forefoot, engaging knee and hip flexion on landing and with lateral (cutting) maneuvers, avoiding excessive genu valgum at the knee on landing and squatting, increasing hamstring, gluteus medius, and hip abductor strength, and addressing proper deceleration techniques. The study included two age cohorts: 14–18-year-old female club soccer players and 18–23-year-old female collegiate soccer players.

In the first year of the study, two ACL tears were reported for the IG (IR + 0.05 ACL injuries/1000 AE). Thirty-two ACL tears were reported for the CG (IR = 0.47 ACL tears/1000 AE), resulting in an 88% overall reduction in ACL injury in the IG compared to the CG. In year 2 of the study, four ACL tears were reported in the IG (IR = 0.13/1000 AE) and 35 ACL tears were reported in the CG (IR = 0.51/1000 AE), resulting in a 74% reduction of ACL injury.

Following the initial intervention, a randomized controlled trial was conducted using 61 division I NCAA women’s soccer teams. There were seven ACL injuries in the IG (IR of 0.14/1000 AE) and 18 in CG (IR = 0.25/1000 AE, P = .15). A 100% reduction in practice contact and noncontact ACL injuries occurred, with no injuries reported in the intervention athletes compared with six reported in the control athletes (IR = 0.10, P = .01). Noncontact ACL injuries occurred at over 3 times the rate in CG ( n = 10; 0.14) compared with IG ( n = 2; 0.04, P = .06). Control athletes with a previous history of ACL injury had a recurrence 5 times more often than those in the IG (0.10 vs. 0.02; P = .06); this difference reached significance when limited to noncontact ACL injuries during the season (0.06 vs. 0.00; P < .05). There was a significant difference in the rate of ACL injuries in the last 6 weeks of the season with a 100% reduction in contact and noncontact ACL injuries (IG 0.00 vs. CG 0.18; P < .05). This supports the notion that it takes approximately 6–8 weeks for a biomechanical intervention program to impart a neuromuscular effect. On the basis of these findings, researchers contend that a prophylactic training program focusing on developing neuromuscular control of the lower extremity through strengthening exercises, plyometrics, and sports-specific agilities may address the proprioceptive and biomechanical deficits that are demonstrated in the high-risk female athletic population and may significantly reduce the incidence of severe ACL injuries in the female athlete.

The Biomechanics of Anterior Cruciate Ligament Injury: Gender Influences

Although previous studies had looked at the role of leg dominance in ACL injury, none of these stratified for the relationship between leg dominance and ACL injury risk with respect to sport, specifically in soccer. Researchers conducted a retrospective observational study to assess the relationship between leg dominance and ACL injury to determine whether this relationship was gender dependent. Given that most soccer athletes have a preferred kicking limb, researchers suggested there may be an imbalance in lower extremity strength given the difference in muscle activation seen in the kicking limb compared with the supporting limb. The mechanism for this relationship, according to the authors, could likely be explained considering the existing body of research on neuromuscular control. A total of 93 soccer players (41 male and 52 female) who had undergone ACL reconstruction participated in the study. The mechanism of injury was divided into two categories: contact and noncontact.

The ACL-injured limbs were equally distributed between right (72) and left (73) when both contact and noncontact injuries were considered. The right lower extremity was the preferred kicking limb in 84 subjects, while the left lower extremity was the preferred kicking limb in 9. Slightly more than half of the ACL injuries occurred in the dominant lower extremity (53/99). When the data were stratified to look at noncontact ACL injuries specifically, an interesting trend emerged. Approximately half of the injuries occurred in the preferred kicking (dominant) limb (30) and half occurred in the contralateral limb (28). However, when the data were stratified for gender, there was a significant difference in the distribution of noncontact injury with respect to dominance. Exactly 74.07% of males (20/27) were injured on the dominant kicking leg, compared with 32.26% (10/31) of females who were injured ( P < .002). The authors hypothesized that the biomechanical deficits may largely involve the sagittal plane in men and frontal plane in women. However, it is critical to remember that 26% of men and 32.26% of women did not biomechanically mirror their gender cohort. Injury-prevention programs for the female cohort must give particular attention to the hip abductors, external rotators of the hip, hip extensors, core/trunk control, and proprioceptive deficits. Understanding this research with regard to the results of the lower-limb leg dominance tests suggests that the deficits that precede the ACL injury in females may be largely due to a sagittal plane deficit, decreased lateral hip control, decreased abduction, adduction activation due to a muscular imbalance, and decreased hamstring activation resulting in a decreased knee and hip angle during defending and landing from a jump/header.

For the male cohort, the vulnerability of the dominant leg to ACL injury may also be explained by biomechanics and neuromuscular control. During baseline manual muscle testing during preseason physicals, professional male soccer athletes demonstrated a 2:1 ratio of quadriceps to hamstring strength. This may be due to the imbalance that exists between the strength of the quadriceps and the hamstring and the iliopsoas and the gluteal muscles in the sagittal plane. In addition, pelvic positioning can perhaps contribute to this phenomenon. During striking, the pelvis assumes an anterior pelvic tilt on pressing, which transitions to a posterior pelvic tilt at the point of initial contact with the ball. At this point, the insertion for the biceps femoris musculature has migrated caudally, thus altering the length tension relationship of the biceps femoris of the hamstring group. The quadriceps has a mechanical advantage—and may impart a powerful anterior shear force that precludes the ACL injury in the male. Overall, this retrospective research suggests that when gender is taken into account, lower-limb dominance does serve as an etiological factor with regard to ACL injuries that were sustained while playing soccer. These findings have also been found in competitive skiers.