NEWARK, Delaware—A torn anterior cruciate ligament (ACL) is among an athlete's most-dreaded injuries, often requiring surgery and months of rehab, as has been the case with Philadelphia Eagles quarterback Donovan McNabb. Less appreciated is that between 72% and 95% of ACL injuries are "noncontact" in that they occur in athletes who haven't collided with a person or object but who landed off-balance during jumping or running.

In a first study of its kind, Charles Buz Swanik, PhD, with the department of health, nutrition and exercise science at the University of Delaware in Newark, and colleagues report that differences in brain function may predispose some people to noncontact knee injuries. These include significant differences in reaction time, processing speed, plus visual and verbal memory. The study is reported in The American Journal of Sports Medicine.¹

"We had some data from previous research which suggested that these noncontact knee injuries occur when a person gets distracted or is ‘caught off guard,'" Dr. Swanik said. "This made me wonder if we could measure whether these individuals had different mental characteristics that made them injury prone."

Preseason neurocognitive tests might predict those at risk for ACL tear

The study objective was to determine whether baseline neurocognitive performance was lower in athletes who subsequently suffered noncontact ACL injuries. To identify subjects, the researchers administered neurocognitive tests to nearly 1500 athletes at 18 universities during the preseason. They then used a case-control design to compare neurocognitive test scores in 80 athletes (35 men, 45 women) with noncontact ACL injuries and 80 matched controls. Controls were matched for height, weight, age, gender, sport, position, and years of experience at the college level. The study included athletes in football, soccer, lacrosse, basketball, volleyball, field hockey, gymnastics, wrestling, fencing, and softball.

All injuries were diagnosed by team physicians, and complete ACL tears were confirmed during reconstructive surgery. Exclusion criteria included any direct blow as part of the mechanism of injury, previous knee sprain or head injury.

Neurocognitive testing was carried out using Immediate Post-Concussion Assessment and Cognitive Testing (ImPACT) software, which according to Dr. Swanick is a reliable method for examining deficits in reaction time, processing speed, working memory, and attention and concentration. The program includes six neurocognitive tests that yield four composite scores for verbal memory, visual memory, reaction time, and processing speed.

The researchers report that athletes with noncontact ACL injuries had significantly slower reaction times (P = .002), had slower processing speeds (P = .001), performed significantly worse on visual memory (P = .000) and verbal memory (P = .045) tests compared with controls.

"Decreased reaction times, processing speed, or visual spatial disorientation may expose athletes to injurious situations by disrupting the neuromuscular control necessary for dynamic restraint," Dr. Swanik said. He noted that maintaining dynamic restraint during athletics requires both cortically programmed muscle preactivation and reflex-mediated contractions. "The cerebral cortex relies on feed-forward processing to initiate a motor program for coordinated execution of the desired task."

Better brains, fewer sprains?

So would improving brain function prevent some noncontact ACL injuries?

"This study means that there may be an alternative application for neurocognitive testing in the area of injury prevention," Dr. Swanik noted. "It's hard to say at this point how much we can alter these characteristics with training, but certainly the brain has great potential for learning and adaptation. Controlling stress and anxiety must be considered, as both cause changes in muscle tone and concentration and the narrowing of our attentional field."

A follow-up study is now under way at the University of Delaware's human performance laboratory with support from the University of Delaware Research Foundation.

"We're trying to identify people who are or are not ‘caught off guard' during different landing tasks," Dr. Swanik said. "Then we'd like to match the neurocognitive characteristics of people who are easily distracted or have awkward landings. This would allow us to search for injury-prone or perhaps accident-resistant people."

ACL injury risk higher in women

An estimated 200,000 ACL injuries occur annually in the US, mostly in young, healthy, active individuals. According to Dr. Swanik, it is not uncommon for a football team to suffer one or two ACL injuries every season, and the incidence is likely even greater on women's sports teams.

"Young women are actually at the highest risk for these injuries, particularly in soccer and basketball," Dr. Swanik said.



Reference

 1.  Swanik CB, Covassin T, Stearne DJ, Schatz P. The relationship between neurocognitive function and noncontact anterior cruciate ligament injuries. Am J Sports Med. 2007;35:943-948.