A study comparing images of the knees in people who did and didn't have previous injuries to the anterior cruciate ligament suggests that people who tore their ACLs are more likely to have a smaller ligament than do similarly sized people who have never injured a knee.
Researchers calculated the total volume of the ligaments based on magnetic resonance images of human knees. The ACLs among those with previous injuries were, on average, about 10 percent smaller than were ACLs among those without an injury.
In those with previous injuries, the uninjured ACL in the opposite knee was measured for the study. Their ligaments were compared to the ACLs in uninjured people of similar height and weight.
Those who had torn their ACLs had experienced noncontact injuries, meaning the injury occurred during some sort of movement of the body rather than because of a blow to the knee.
Researchers caution that the retrospective study does not mean that a smaller ACL will necessarily result in injury. Instead, they say the research offers more clues about the variety of factors - such as activity level, neuromuscular coordination, gender and muscle strength - that appear to be contributors to ACL injury.
In this group of participants, weight was the strongest predictor of ACL volume.
"If you compared two people of the same weight, based on our data set, we would expect the injured person had the smaller ACL," said Ajit Chaudhari, assistant professor of orthopedics at Ohio State University and lead author of the study.
Knowing that the knee's anatomy can influence susceptibility should help researchers who are trying to figure out why ACL injuries occur and who is most likely to experience these injuries, Chaudhari said. Most study results guide the assignment of a percentage of risk to one or more factors associated with torn ACLs, but to date no research had looked solely at the size of the ligament in injured and non-injured knees.
The research is published in a recent issue of the American Journal of Sports Medicine.
The anterior cruciate ligament, responsible for rotational stability in the knee, is located behind the kneecap and is one of four ligaments that join the thigh and shin bones. Noncontact tears of the ACL tend to occur in athletes when they pivot, stop quickly or land from a jump. Previous research suggests that college-age women athletes are at three- to 10-times higher risk of tearing their ACLs than their male counterparts, depending on the activity, but scientists have not determined why this is.
"Comparisons of the volumes of ACLs between men and women have been done, which have suggested that ACL volume may matter. Studies have also found that female ACLs had fewer fibers than male ACLs that were tested. But even with those findings, any differences between males and females could be a coincidence. There are so many variables that interact that you can't really tell what's causative unless you compare people who have had an injury to those who haven't had an injury," said Chaudhari, also director of Ohio State's Sports Biomechanics Laboratory.
He and colleagues took MR images of the knees of 54 participants, who were divided into two groups. Volunteers with previous injuries were matched with uninjured participants of the same age, gender, height and weight.
The previously injured participants' healthy knees were imaged for the study. Chaudhari said the fibers of a torn ACL tend to fray like a rope, meaning the volume of injured ACLs could not be measured in a meaningful way. Chaudhari recently presented related research that indicated that there is no significant difference in the size of two ACLs in the same body.