The Hidden Science of ACL Injuries

One of the most common and debilitating knee injuries for both highly paid professionals and recreational athletes alike is a tear to the anterior cruciate ligament (ACL), yet the mechanics of how it occurs remain surprisingly unclear.  

The uncertainty means that each year, millions of people worldwide – 400,000 in the United States alone – face potential reinjury and complications such as pain, instability and, over time, osteoarthritis.  

Settling the science about how ACL injuries occur is at the heart of what Lou DeFrate, PhD, has worked on for more than two decades. As the Laszlo Ormandy Distinguished Professor of Orthopaedic Surgery at the Duke University School of Medicine and director of the Michael W. Krzyzewski Human Performance Laboratory (K-Lab), DeFrate and colleagues recently received a $3.9 million grant from the National Institutes of Health to study ACL injuries.  

The team, including orthopedic surgeon Jocelyn Wittstein, MD, is using sophisticated imaging technology to assess the impact of specific motions within the knee as they relate to the ACL and how it functions.   

The study aims to enroll about 200 adult participants under the age of 40 who will be assessed during jumping and cutting motion activities. The researchers will work to identify any differences between men and women, the uninjured knees of patients with prior ACL injuries and the knees of people without injury, and performances pre- and post-fatigue.  

“Some prior research has suggested that perhaps men and women have different mechanisms of injury,” DeFrate said. “For example, women may land with this inward collapse motion, which may put them at more risk for injury, while men may be landing on straighter knees.   

“But our prior research shows no such difference,” he said. “Using a forensic biomechanics approach, we looked at the bone bruises, reproducing the position of the knee near the time of injury, and our assessment was that men and women in fact have a similar mechanism of injury.”  

In an additional finding that also proved controversial, DeFrate and colleagues asserted that ACL tears occur from landing on a straight knee rather than from a buckling motion. That cringe-inducing buckling that people often see when someone injures their ACL, he said, happens after the tear.   

“But we need to do more studies to investigate if there are any subtle differences between men and women, how they land, the shapes of their knees and so forth,” he said. “If you really want to prevent an ACL injury from happening, you have to understand what's causing it.”  

Why Are Girls at Higher Risk for ACL Tears? 

In her clinical practice as an orthopedic surgeon, Wittstein, associate professor in the Department of Orthopaedic Surgery, sees firsthand the impact of ACL injuries on athletes, particularly young people who are otherwise in top shape.   

“Tears and injuries to the ACL are one of the most common injuries that adolescents sustain,” Wittstein said. “And it’s far more common in girls than boys. Some studies show up to eight times as common.”  

She said it’s imperative to know why girls are more prone to ACL injuries – whether it’s hormones, or bone structure, or some other underlying issue: “Are there other biomechanical and sex-related risks that we might be able to modify? If we can find anything that we can modify to reduce risk, this will help a lot of people in the future.”  

One of Wittstein’s patients, Addison Pepitone, a competitive rock climber, suffered an ACL tear this past May when the teenager fell 15 feet from a climbing wall. After surgery, weeks on crutches and months in a brace, she recently received the greenlight from Wittstein to resume climbing without a brace.   

“My progress was definitely hard in the beginning,” Addison said. “But I think the more I went to physical therapy, I could see my progress get better. It feels good to walk and do everyday tasks because that was hard in the beginning. So, I feel like my focus now is definitely on getting back to climbing.”  

Wittstein said she is eager for more and better research into risk factors for ACL tears, so she can better counsel Addison and other patients about what they can do to stay healthy. And as a surgeon, she would also welcome insights that might lead to ways to surgically modify the knee.  

“There are so many things that are unique to each person - their sex, age, geometry of their knee, hormone levels, whether fatigue poses a risk,” she said. “There are so many factors that we're just going to kind of investigate them all.”  

Technology Meets Anatomy  

Back in DeFrate’s laboratory, a biplanar radiography system aims at a central point on a platform that measures force. Jefferson Bercaw, a PhD candidate in the DeFrate lab, repeatedly hops down from a stepping stool to demonstrate the activity study participants will perform during the research.   

DeFrate said the images will be analyzed with machine learning technology to measure 3D ligament/tendon elongations and joint kinematics.  

One goal is to use MRI and biplanar radiography together to determine what factors potentially elevate ACL elongation in females compared to males. Specifically, they will investigate whether movement patterns and joint geometry interact to elevate injury risk in females.  

Other experiments will also use these imaging technologies to assess what factors elevate injury risk in people who have suffered prior ACL injuries compared to people who have never had an ACL tear.  

A third research goal will be to see whether fatigue plays a part in injury. A clue that the team wants to pursue is why about 70% of ACL injuries occur without any contact – often when an athlete stops or starts abruptly, or pivots quickly.  

“We will see how the knee moves as you jump through the air, as well as how the ACL stretches,” DeFrate said. “And our hypothesis is that if the ACL is stretching a lot more, it's a lot closer to its breaking point. So, you're at more risk of failure.  

“If we can demonstrate that through these experiments and find out what's causing the injury to happen, then we could target those motions, and we wouldn't have this terrible injury happening so often.”  

Sarah Avery is director of the Duke Health News Office.  

Photos and video by Shawn Rocco who is a photographer and multimedia producer in the Duke Health News Office.