Researchers Look For Clues To Causes Of ACL Tears In Women

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In their quest to find out why women are at greater risk of tearing knee ligaments than men, scientists have been able to rule out the influence of female hormones on specific movement patterns that might lead to injury.

The finding is a good start at ruling out one theory, but the search for the reasons behind the disparity in anterior cruciate ligament (ACL) tears between men and women is complicated by numerous factors beyond hormonal differences. And there is reason to worry about the ramifications of these injuries: The estimated 38,000 young women and girls who injure their knees each year are at increased risk of developing arthritis at a relatively young age.

One recent study determined that neither the menstrual cycle nor use of oral contraceptives affected the impact on knee and hip joints during high-risk jumping and landing tasks. The results were published in the American Journal of Sports Medicine.

"We had started to get a sense that the menstrual cycle is not an important factor, but we needed to know for sure," said Ajit Chaudhari, assistant professor of orthopedics at Ohio State University Medical Center and lead author of the study. "Overall, the differences between males and females still probably do play a part. The problem is that there are lots of factors that go into these different risks for men and women. There isn't going to be a magic bullet."

By the numbers, more men tear their ACLs than women. But women, whose participation in sports consistently increases each year, are three to five times more likely than men to suffer this injury, Chaudhari said.

"We can't really wait to know why. But once we at least understand the differences better, then we can make existing prevention programs better," he said.

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For the study concerning hormonal differences, Chaudhari and colleagues compared analyses of the results of vertical leaps, horizontal jumps and drops from a 30-centimeter box on the left legs of 25 women - 13 of whom were using oral contraceptives - and 12 men. Men were tested once, and women were tested twice for each phase of the menstrual cycle.

The researchers found no significant differences in the load on knee and hip joints among the three groups, leading them to conclude that the difference in injury rates is probably more likely caused by persistent differences in strength, neuromuscular coordination or ligament properties.

"Why is there so much variability? Training, or lack of training, are likely factors," Chaudhari said.

Training for prevention of injuries remains a relatively new, but important, component of many athletics programs. "You can train people in certain aspects of how to land from a jump and how to plant your foot when you're changing direction, for example," Chaudhari said. But it remains tricky: "Compared to throwing a baseball or a football, there is no similar training in how to land from a jump."

An additional obstacle to research: There is no way to study landing motion during an actual athletic event. Researchers can analyze videos of sports activity, but measurements are not precise. "The lab is different from the playing field. Tests are a reasonable surrogate, but ethically, we still have to avoid the injury," Chaudhari said.

Chaudhari, as director of the Biomechanical Research Laboratory at Ohio State's Medical Center, is continuing to study this phenomenon, examining gait differences after an ACL tear and hoping to determine whether participation in injury prevention programs leads to an improvement in overall balance. He also plans to turn to imaging studies, to see whether identifying the mechanism of injury by comparing healthy knees to injured knees could lead scientists to more easily determine who is at risk for tears.

"Conventional wisdom suggests that if we know women tear their ACLs more often than men, then we can point to their different characteristics," he said. "The flaw there, though, is that not all women tear their ACLs. So being a woman might not be a risk factor. But women might have more true risk factors."

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