In Stride with Sports Medicine

For the past decade, the words “anterior cruciate ligament injury” have been virtually inseparable from the word “gender.” But a group of researchers who specialize in studying ACL injury risk says that needs to change.

Despite epidemiological evidence that female athletes have a much higher risk of ACL injury than their male counterparts, experts who convened in April for a consensus conference on the topic contend that the current focus on gender as a risk factor may be overshadowing other important variables that are not gender-specific. Attendees at the consensus conference, the fourth in a series of research retreats that have each been titled “ACL Injuries: The Gender Bias,” resolved that the next such event will simply be titled “ACL Research Retreat.”

This philosophical shift, which will be put into writing when the conference proceedings are published in the Journal of Athletic Training this fall, is intended not to discourage comparisons of male and female subjects but to encourage researchers to go a step further in their analyses.

“You don't want to take it out of the equation, but this isolated focus on gender needs to be readdressed,” said Scott G. McLean, PhD, assistant professor of athletic training and movement science and director of the Injury Biomechanics Laboratory at the University of Michigan. “From a screening standpoint and a preventive standpoint, there are drastic within-sex differences that may provide greater insights into the injury mechanism.”

Multiple studies have demonstrated that certain anatomical, postural, and neuromechanical characteristics are more common to one gender than the other. But, in part because of the challenges of conducting large-scale prospective studies in this area, few researchers have conclusively linked any of those characteristics to actual ACL injuries; more often, a correlation has been hypothesized to exist when the potential risk factor and the epidemiological injury data have gender in common.

For example, because research has found that ACL size tends to be smaller in female subjects than male subjects, it's been theorized that ligament size is a risk factor for ACL injury. However, a study from Ohio State University presented at the research retreat tested that theory directly by drawing comparisons based on injury history rather than on gender.

The Ohio State group used MRI to compare ligament volumes in 27 subjects with a history of ACL injury and in 27 uninjured control subjects who were matched for height, age, and weight in addition to gender (34 of the 54 subjects were male). They found that, indeed, the injured individuals on average had significantly smaller ligaments than the controls—suggesting that this anatomical variable could be a risk factor for male athletes as well as their female counterparts.

“We've described a lot of differences between genders at this point, but we don't know whether those differences relate to injury risk,” said Sandra J. Shultz, PhD, ATC, an assistant professor of exercise and sport science and co-director of the Applied Neuromechanics Research Laboratory at the University of North Carolina-Greensboro, which hosted the research retreat. “We're not even sure the risk factors are the same for females and males.”

More precise identification of risk factors, in theory, will lead to more effective interventions for injury prevention. Because research has found that female athletes are more likely than male athletes to adopt jump landing techniques characterized by excessive knee valgus and an erect posture, many ACL prevention programs focus on correcting these jump-landing strategies. A 2005 prospective outcomes study from the University of Cincinnati showed that knee valgus and knee abduction moment during landing were predictive of ACL injury in 205 female athletes, but little is known about whether the same is true for male athletes.

A study of youth soccer players from UNC-Chapel Hill presented at the research retreat suggests that landing mechanics actually vary considerably within each gender. The investigators used their Landing Error Scoring System to assess landing mechanics in 72 players before and after a nine-month intervention involving a customized 10-minute warm-up prior to each soccer practice. Of the 43 players whose LESS score improved following the intervention, 11 were male, suggesting that boys too may utilize “at risk” landing techniques.