Athletes with smaller ACLs may be more susceptible to injury, study finds

People with torn ACLs are more likely to have a smaller ligament than similarly-sized people who have never injured a knee, according to a study recently published in the American Journal of Sports Medicine.

Researchers calculated the total volume of the ligaments based on MRIs of human knees. The ACLs among those with previous injuries were, on average, about 10% 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.

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.

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. Researchers used the MR images to determine the outline of each ACL under the guidance of an orthopedic surgeon experienced in operating on injured knees.

Of the 27 injured participants, 16 had smaller ACLs than their matched controls. Overall, the injured group had an average ACL volume of 1,921 cubic millimeters, while the control group had an average volume of 2,151 cubic millimeters.

Chaudhari said that based on what is currently known about the fibrous makeup of the ligament, it's no surprise that a smaller ACL is more susceptible to injury.

"If you have a weaker ACL, it's more likely to tear if all other factors are equal," he said. "If being larger in size means the ACL has more fibers, then that would make it stronger. If the individual building blocks are of similar strength, then it comes down to how much total tissue there is."

It's too soon to consider knee imaging as a way to screen potential athletes, Chaudhari said, because of the high expense and the fact that knowing the size of the ACL still doesn't tell the whole story of how the knee will react to activity.

"I would certainly not say in any way, shape or form that people should start using ACL size as a determinant of whether they should play any sport," he said.