Introduction

The anterior cruciate ligament (ACL) is a crucial stabilizer of the tibiofem oral joint, preventing anterior translation of the tibia on the femur during weight-bearing activities. The ACL works collectively with the posterior cruciate ligament (PCL) to stabilize the knee during dynamic movement. The PCL is attached to the posterior portion of the intercondylar eminence of the tibia and passes forward to attach to the medial condyle of the femur. The medial collateral ligament (MCL) is attached to the medial femoral condyle and the medial surface of the tibia. The lateral collateral ligament (LCL) is attached to the lateral femoral condyle and the lateral portion of the head of the fibula. The MCL and the LCL are extracapsular ligaments and provide stability to the knee joint in the frontal plane during varus and valgus loads.

Since the passage of the Title IX Educational Amendment, there has been an exponential increase of female participation in sports at both the collegiate (fivefold increase over the last 30 years)¹ and high-school (tenfold increase over the last 30 years)² levels. Although participation in organized sports has many physical and psychological benefits, including decreases in obesity, hypertension, diabetes mellitus, and coronary heart disease, this increase has subsequently led to an increase in sports-related injuries.³ While identifying risk factors with regard to sports-related injury, researchers have found an increased rate of ligamentous knee injuries, especially of the ACL, in female athletes compared with their male counterparts participating in similar activities.^4–9 Among athletes in pivoting and jumping sports, adolescent females face a fourfold to sixfold increased risk of ACL injury compared with their male counterparts.^6,10,11

The ACL is at risk for injury during activities that require pivoting, decelerating, or landing from a jump, such as soccer, basketball, volleyball, and team handball, as well as American football and downhill skiing.¹² An estimated 80,000 to 250,000 ACL injuries occur annually in the United States alone.^12,13 The highest incidence of these injuries occurs typically in young athletes between the ages of 15 to 25, which constitutes nearly 50% of all reported ACL injuries.³ Furthermore, the incidence among female athletes exceeds their male counterparts by a twofold to eightfold frequency^12,13,14 Arendt and Dick examined the increased incidence of ACL injury among NCAA Division I athletes participating in basketball and soccer over a 5-year period.¹⁵ These two sports were chosen due to the fact that there is a strong similarity between the men’s and women’s games with regard to rules, training and development, style of play, type of playing surface, and the intensity of the competition. The injury rate was recorded and analyzed per athlete-exposure, where one practice session or game was defined as one exposure. The average ACL injury rate was 0.31 per 1000 athlete-exposures for female soccer and 0.29 per 1000 athlete-exposures for female basketball, compared with 0.13 for male soccer and 0.07 for male basketball per 1000 athlete-exposures. These epidemiological data for ACL injury rates statistically signify the blatant discrepancy that exists between genders.⁶

ACL rupture is a severe ligamentous knee injury, leading to functional instability in the short term and degenerative joint disease in the long term. Injury to the ligament can lead to prolonged absence from both work and sport and can initiate the early onset of degenerative osteoarthritis.^15,16 Although ACL reconstructive procedures are readily available, the injury is painful and costly and can be debilitating. In the United States, at least 50,000 ACL reconstructions are performed each year at a cost of about $17,000 per procedure.^14,17 The direct medical cost for reconstructive surgeries alone is just under $1 billion per year ($850,000,000). This figure does not include initial treatment costs of all ACL injuries, the rehabilitation costs after reconstruction, or the costs of conservative treatment and rehabilitation of those injuries that are not repaired.¹⁸ Complete ACL injuries can lead to chronic knee pathology, including instability, secondary injury to the menisci and articular cartilage, and an early onset of osteoarthritis. Approximately 66% of all patients with complete ACL injury incur damage to the menisci and the articular cartilage of the femur, patella, and/or tibia. This injury, coupled with the risk of secondary injury, can significantly decrease the ability of patients to complete their activities of daily living and affect their quality of life. The surgical reconstruction of a ruptured ACL can significantly reduce the risk of secondary injury. Seitz et al^18a noted that 65% of ACL deficient patients sustained a secondary meniscal injury within 2.5 years of the initial date of injury.

Data show that despite surgical treatment of this injury, patients frequently develop posttraumatic arthritis of the knee.* Despite the most earnest efforts of orthopaedic surgeons to preserve the integrity of the knee joint during ACL reconstructive surgery, ACL reconstructed individuals continue to report with early onset of osteoarthritis. Lohmander et al completed a 12-year longitudinal study to follow up on female athletes who previously underwent ACL reconstruction after sustaining an injury while playing soccer.¹⁰ They found that 55 women (82%) had radiographic changes in their index knee and 34 (51%) fulfilled the criterion for radiographic knee osteoarthritis. The mean age for the subjects involved with this study was 31.

Gillquist et al noted that the prevalence of radiographic knee gonarthrosis is significantly higher in the injured knee compared with the unaffected contralateral limb.¹⁶ The implications of this research are ominous—hence the increased need for the prevention of these injuries from occurring in the first place.⁷

A multidisciplinary meeting was held in Hunt Valley, Maryland, in 1999 involving biomechanists, physicians, certified athletic trainers, and physical therapists to delineate specific risk factors thought to be directly correlated to the increased incidence of ACL injuries in the female athlete.⁵ The identified risk factors included anatomy, hormones, environment, and biomechanics. This meeting spurred the development of various ACL injury-prevention programs and led to increased interest and financial funding in this area of research. This group of researchers reconvened in Atlanta, Georgia, in January 2005 to reevaluate the identified risk factors and to determine what progress has been made since the inaugural meeting in 1999.

Anterior cruciate ligament Injury-Prevention Studies

A growing number of injury-prevention programs targeted at reducing the risk of ligamentous knee injury in general and ACL injury in particular have been reported in the literature. Although a number of risk factors for ACL injury have been proposed, only the biomechanical risk factors have been examined in sufficient depth to support the design and evaluation of prevention interventions.^2,14

During passive motion, tension in the ACL decreases from 0 to 35 degrees and then increases again with further flexion.⁵ Thus, a combination of maximal ACL tension and anterior tibial translation force occurs with quadriceps firing and joint compressive loading at or near full extension. Contraction of the hamstrings decreases ACL strain in all positions. However, co-contraction of the hamstrings is not enough to overcome the strain produced by the quadriceps.⁷

As the knee moves into extension, female athletes take a significantly longer time to activate their hamstrings than do their male counterparts.^8,9 At initial contact, males take approximately 150 ms to achieve their peak flexion angle compared with females, who take approximately 200 ms. Landing from a jump, in-line deceleration, and pivoting all involve eccentric contraction of the quadriceps to prevent the extended knee from collapsing into flexion. In laboratory studies, multiple authors^4–6 have demonstrated significant anterior translation of the tibia with quadriceps contraction, particularly at 0 to 45 degrees of flexion. This anterior translation force is even greater when the quadriceps contraction is combined with a joint compressive force.⁴ These findings are the basis for ACL prevention strategies that emphasize proper biomechanics to address proper landing kinematics (hip and knee flexion while avoiding genu valgum), increase peak flexion angles, and improve hamstring activation and strength.

Most prevention programs attempt to alter dynamic loading of the tibiofemoral joint through neuromuscular and proprioceptive training. The studies to date that focused on biomechanical modifications have resulted in the reduction of lower-extremity injuries in athletes. However, the studies vary widely both in their approach to injury prevention and the validity of the study design. Most studies to date have been nonrandomized, and very few have been conducted as randomized, controlled trials.

Nevertheless, a number of common elements tie these programs together. Most include one or more of the following: traditional stretching, strengthening, awareness of high-risk positions, technique modification, aerobic conditioning, sports-specific agility, proprioceptive and balance training, and plyometrics. The relation of these components to specific risk factors for ACL injury has been summarized in Table 5-1.

Table 5-1 Potential Biomechanical Deficits and Suggested Interventions