Tears of the knee’s anterior cruciate ligament (ACL), a serious injury for athletes, may not just be the result of one instant of giving the joint more than it can handle, but rather the cumulative effect of less-severe, repeated stresses.
It’s a finding from University of Michigan researchers that could have implications for athletes of all ages, in sports that include soccer, football, volleyball and basketball—even downhill skiing. The researchers say serious childhood knee injuries might be avoided if coaches treated knees similarly to how Little League Baseball manages strain on the throwing arm and shoulder with pitch counts.
ACL tears are often seen in professional sports when an athlete lands, while simultaneously attempting to change directions quickly. These ‘non-contact” injuries often come as athletes make moves they’ve made hundreds of times before.
“The dogma in this area of research has been that the tear occurred because of the tensile force in the ACL during that one landing that caused it to fail,” said James Ashton-Miller, the Albert Schultz Collegiate Research Professor and a professor of mechanical and biomedical engineering at U-M.
“While that can happen, we’re saying that a healthy, young ACL—that can typically withstand 500 pounds of tension before it fails—can actually fail under lesser loads,” he explained. “Why? Because the ligaments are progressively weakened by micro-damage accumulated during each sub-maximal loading cycle before it can be repaired.”
ACL tears are a disruptive injury, with recovery timetables that often reach nine months before an athlete can expect to reach their previous performance level. Even for non-athletes, it can mean major lifestyle and work disruptions. In the U.S. alone, there are between 100,000 and 200,000 such injuries per year.
Among children, the numbers are troublesome. A 2017 study in the journal Pediatrics showed that the number of tears in children had increased 2.3 percent per year over the previous two decades.
“Incredibly, we’re seeing that the peak age for ACL failures is 14 in women,” said Edward Wojtys, the William S. Smith Collegiate Professor of Orthopaedic Surgery at U-M, who co-led the research with Ashton-Miller. “This can lead to people in the mid-20s suffering from osteoarthritis and other chronic health conditions later in life, such as obesity and cardiovascular disease. Female athletes are at most risk.”
To do this study, U-M engineers custom-built a 7-foot-tall, squared-frame device that uses weight to simulate the micro-damage that occurs with repeated loading of the knee during twisting jump landings. They performed their testing on cadaver knees and then compared the results with ACLs removed from living patients in ACL replacement surgeries at the University of Michigan Medical Center.
Damage signatures in the surgically-removed ACLs, which had already failed, were similar to those seen in the cadaver knees tested by the U-M team. Those similarities, seen from the molecular level on up, suggest that ACL failure is likely a result of repetitive loading.
Mark Banaszak Holl, a former U-M professor of macromolecular science and engineering, now head of chemical engineering at Australia’s Monash University, analyzed the ACLs damage signatures after the experiments. The similarities tell a cautionary tale about how athletes train, he says.
“The new hope is that a change in athlete training and preparation could then limit the number of risky submaximal loading cycles,” Banaszak said. “Allowing sufficient time for soft tissue recovery during or between training bouts could prevent the accumulation of ACL micro-damage and eventual failure.
“Now that we know the nature of the problem and the structural changes occurring, we can look for non-invasive ways of determining who is predisposed to injuring their ACL.”
The paper, titled “An anterior cruciate ligament failure mechanism,” is published in The American Journal of Sports Medicine.
The work was funded by the National Institutes of Health and the National Institute of Arthritis and Musculoskeletal and Skin Diseases.
Ashton-Miller is also a research professor of kinesiology and gerontology.