Risk Factors for Anterior Cruciate Ligament Injuries in the Female Athlete

Introduction

It is well recognized that female athletes and military recruits have a higher incidence of noncontact anterior cruciate ligament (ACL) injuries than males participating in the same activity. A study done at our institution and published in 1994 was the first to recognize this gender disparity in female soccer players, who had nearly six times the rate of serious knee ligament injuries of male players ( P < .01) during indoor games. In 2007, Prodromos and colleagues conducted a meta-analysis of 33 articles and reported that the average ACL injury rate for females was significantly greater than that for males in basketball, soccer, and handball ( P < .0001 for all comparisons). Several studies that compiled data from the National Collegiate Athletic Association (NCAA) were in agreement with these findings. For instance, Hootman and colleagues analyzed 16 years of NCAA data and reported that female basketball and soccer players had a threefold higher ACL injury incidence compared with male players. Recently, Beynnon and associates compiled first-time noncontact ACL injury data from 8 colleges and 18 high schools and found that female collegiate athletes had the highest risk among the groups studied. The incidence rates were generally higher for females than for males (relative risk, 2.10) and for collegiate athletes compared with high school athletes (relative risk, 2.38).

In regard to ACL injuries that occur in the U.S. military, Gwinn and associates reported that female midshipmen at the U.S. Naval Academy from 1991 to 1997 had a fourfold increase compared with their male counterparts in intercollegiate-level soccer, basketball, and rugby collectively ( P = .006). In addition, women had nearly 11 times the incidence of ACL ruptures as men during obstacle course running ( P = .004). Mountcastle and coworkers reported female/male incidence ratios for noncontact ACL injuries of 4.95 for gymnastics, 3.72 for the obstacle course test, 3.01 for basketball, 1.71 for handball, and 1.27 for soccer at the U.S. Military Academy at West Point over a 10-year period.

A few investigators have noted that, although ACL ruptures represent a serious injury that results in months of lost playing time, the overall incidence in female athletes compared with other injuries, such as ankle sprains, is low. Hootman and associates concluded, “using the standard of <.05 as rare, the actual probability of ACL injury would be considered a rare event.” We recently systematically reviewed eight studies of the effect of ACL injury prevention programs found noteworthy differences in the ACL injury rates in female adolescent athletes, ranging from .03 to .08 per 1000 athlete-exposures to 0.21 to 0.49 per 1000 athlete-exposures. Three neuromuscular retraining intervention programs significantly reduced the noncontact ACL injury incidence rates: the Sportsmetrics, Prevent Injury and Enhance Performance, and Knee Injury Prevention programs. The number of athletes who needed to train to prevent one ACL injury in these three programs ranged from 70 to 98, and the relative risk reduction ranged from 75% to 100%. Although the number of athletes needed to treat appeared somewhat high, in our experience many individuals participate in sports year-round with very little time off. They achieve within a few years a high number of exposures (in both practice and games) that place them at risk for ACL injury. This is especially true for athletes who begin competition at a very young age. For these athletes, by the time they are in high school, they will already have been exposed to well over 1000 hours of participation. These high rates of cumulative exposures validate ACL neuromuscular intervention programs.

Despite the many investigations that have been conducted on potential risk factors for noncontact ACL ruptures, definitive conclusions cannot be reached for either males or females regarding which factors may predispose an athlete to this serious injury. Investigators have attempted to develop risk-screening models using knee valgus motion, range of motion, body mass, tibial length, and quadriceps/hamstring ratio ; however, others have refuted their findings. In one investigation, researchers found that the Landing Error Scoring System failed to predict ACL injuries in a cohort of 5047 high school and collegiate athletes. This system rates body mechanics and technique of a vertical drop-jump on a 17-point scale, with a higher score indicating poor landing technique, and it has been shown in other studies to correlate with several measures of ACL loading and injury risk.

The reasons for the increased incidence of this injury in female athletes over that in male athletes cannot be scientifically defined at present. In many studies published to date, investigators examined a small sample size of each gender (and therefore the studies had insufficient power to avoid a type II statistical error ), focused on one or just a few of the possible risk factors, or examined neuromuscular characteristics in a controlled laboratory environment instead of more realistic playing conditions. Controlled laboratory experiments have variations that may influence results, such as performing a task with or without shoes or with arm movements controlled or uncontrolled. The major potential risk factors that have been proposed for this injury are shown in Table 13-1 . This chapter reviews investigations related to these risk factors and emphasizes findings that may have a role in the gender disparity in noncontact ACL injury rates. We note that Elliot and associates proposed other potential risk factors that should be explored in future studies. These include lifestyle habits and psychosocial influences on female athletes, such as fatigue from chronic sleep deprivation, poor nutrition and eating disorders, substance use and abuse, overtraining, stress, and depression.

Genetics

Regardless of gender, a question has been raised for many years about a potential genetic predisposition for ACL rupture. At the time of this writing, five studies had examined family history of ACL injuries. Webster and associates noted a significant association among both ACL semitendinosus-gracilis graft rupture and contralateral ACL tears and a family history (first-degree relative) of ACL injury. In their cohort of 561 knees that had undergone ACL reconstruction, 26 procedures failed. Of these patients, 8% had a family history and 4% did not (odds ratio, 2.4; 95% confidence interval [CI], 1.1-5.3; P = .04). Forty-one patients tore their contralateral ACL; of these, 13% had a family history and 7% did not (odds ratio, 2.2; CI, 1.2-4.4; P = .02).

Bourke and associates assessed the survival rate of 755 ACL reconstructions followed for a minimum of 15 years postoperatively. A positive family history of ACL rupture doubled the odds of tearing either the ACL graft or the contralateral ACL ( P = .003). This occurred regardless of gender or graft type (patellar tendon or hamstring tendon autograft).

Flynn and associates conducted a questionnaire-based study of 171 patients who sustained either contact or noncontact ACL ruptures to determine whether a familial predisposition existed for their injuries. The patients were matched with 171 uninjured control subjects according to age, gender, and primary sport. Patients who had sustained ACL ruptures were twice as likely as the control subjects to have a first-, second-, or third-degree relative who also had an ACL tear. Posthumus and associates noted a significant difference in the self-reported family history of ligament injuries among 38 females who sustained ACL injuries and 84 controls (incidence rates of 50% and 21.5%, respectively; P = .01). In their investigation, there was no significant difference in this factor between males with ACL injuries and controls. Harner and coworkers, in a small series of 31 patients with noncontact bilateral ACL ruptures, found the patients had a significant increase in the incidence rate of immediate family members who had sustained an ACL injury compared with a control group (35% and 4%, respectively; P < .01).

More recently, investigators have examined the potential of genotypic variants in collagen genes as an inherent risk factor for knee ligament injuries. The genes COL1A1 , COL5A1 , and COL12A1 encode for the major α-chains that make up collagen types I, V, and XII, respectively. Studies have identified sequence variants within these genes that are believed to be associated with ACL injuries in Caucasian South African patients, Swedish patients, and Polish/Eastern European patients. The variants have been hypothesized to alter the structure and behavior (mechanical properties) of ligaments and other musculoskeletal soft tissues. In addition, they may be associated with increased genu recurvatum, anterior knee laxity, and general joint laxity. Posthumus and associates recommended that further work be conducted in this area before genetic testing for ACL injury risk is initiated.

Extrinsic: Footwear, Playing Surface, Climate Conditions

Extrinsic risk factors for ACL injuries include footwear, playing surface, and climate conditions. The effect of footwear is assessed by calculating shoe-surface interaction, which is defined by the force of the friction between the athlete's shoe and the surface it comes into contact with during movement (quantified by the coefficient of friction [COF]). Changing the COF of the shoe-surface interaction causes athletes to change their movement techniques, essentially their biomechanics, to accommodate for the change in COF. The type of shoe, playing surface, and climate conditions all affect the COF of the shoe-surface interaction. To date, all major studies on shoe characteristics and weather conditions and their association with ACL injuries have been conducted only in male athletes.

There are many different types of playing surfaces, including natural grasses, artificial turfs (first, second, third, and fourth generation), wood floors, and artificial rubberized floors. In general, artificial turfs have higher COF than natural grasses, and artificial rubberized floors have a higher COF than wooden floors. Investigators have reported no differences in ACL injury rates in female soccer players who played on third- and fourth-generation artificial turfs compared with those who played on natural grass. However, researchers in two studies reported significantly higher ACL injury rates in female athletes who played floorball or handball on artificial rubberized floors compared with those who played on wooden floors. Dowling and Andriacchi hypothesized that it is “highly likely that the COF of the rubberized floor is high and may be the cause of the observed increase in ACL injury among female athletes on this surface.”

One other extrinsic risk factor is the use of a prophylactic knee brace to prevent ACL injuries. However, only a few investigations have been published on this topic, all of which involved male football players, and no conclusions are possible regarding a potential benefit of the use of these braces for reducing the ACL injury rate in female athletes. It is important to note that no investigation to date has taken into account other risk factors (such as hormonal or neuromuscular factors) along with extrinsic factors that could confound possible conclusions.

Anatomic

Many authors have proposed that inherent anatomic differences between women and men are responsible or partially responsible for the disparity in noncontact ACL injury rates. Proposed anatomic risk factors include quadriceps femoris angle (Q-angle), foot pronation, intercondylar notch size, ACL size, tibial slope and surface geometry, anterior and generalized joint laxity, and body mass index (BMI). However, no investigation has demonstrated that any anatomic factor alone is responsible for the increased risk of noncontact ACL injuries in female athletes. The major problem is that no study to date has entered anatomic indices along with hormonal and neuromuscular factors into an appropriate statistical model to determine the effects of all of these potentially important risk factors on ACL injury rates.