Repetitive knee stress and failure to accommodate sufficient rest between periods of strenuous exercise may be key factors behind the rapid rise in anterior cruciate ligament (ACL) injuries in world sport, suggest researchers from the University of Michigan and Monash University.

While it is already well recognized that a single supramaximal force can cause ACL failure, it has been assumed that sub-maximal forces could not cause ACL failure.

According to this new research, published recently in The American Journal of Sports Medicine, a series of submaximal forces can cause damage to accrue in the ACL, in a process called low-cycle material fatigue, and that same damage is found in ACLs which have failed.

The problem of ACL injuries is described by researchers as a ‘ticking time bomb’ because of the number of these injuries and the degeneration in the knee that they can cause, a media release from Monash University explains.

Competitive sporting environments are producing generations of athletes that risk developing knee osteoarthritis no matter how they are treated, the researchers opine.

Material fatigue is behind this steep rise in “non-contact” ACL injuries, they continue in the release. Nearly three-quarters of ACL injuries are “non-contact” failures, even in contact sports like Australian Rules Football and the American National Football League (NFL).

“ACL injuries are the number one cause of time spent away from NFL practice and play, with recovery periods up to 12 months long,” says lead researcher and orthopaedic surgeon, Dr Edward Wojtys, in the release.

“Incredibly, we’re seeing that the peak age for ACL failures is 14. 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.”

Led by Wojtys and biomechanical engineer Professor James Ashton-Miller, both from the University of Michigan, the team tested one healthy knee from each of seven adult cadavers in a controlled laboratory study using a purpose-built machine, which took 3 years to develop.

Each knee was subjected to repetitive pivot landings loaded under four-times body weight — approximately the weight under which someone jumps and lands — which twisted the knee to cause ACL failure within 100 landings.

What they couldn’t see was what was happening to the ACL itself inside the knee, especially near the thigh bone where the team knew most ACLs fail.

Professor Mark Banaszak Holl, Head of Chemical Engineering at Monash University, joined this project in order to answer that question — along with two PhD students Jinhee Kim and Junjie Chen, the release continues.

Using atomic force microscopy to characterize the ACL collagen at the nanometer scale, or one billionth of a meter, Banaszak Holl and his team were able to identify a consistent unraveling of the molecular structure of the ACL’s collagen in the injured cadaver knees.

According to Banaszak Holl, the results of this latest research throw into serious doubt the argument that most ACL injuries are caused by single force events. This could lead to different ways of managing athletes of all ages and levels.

“What we found from the tested cadaver knees was chemical and structural evidence of micro-damage of the ACL femoral enthesis, which is where the ligament attaches to the thigh bone and where the ACL typically tears,” he states.

“These results were consistent with ACLs removed from patients undergoing knee reconstructions.

“The new hope is that a change in athlete training and preparation could then limit the number of risky submaximal loading cycles. 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.”

“Until we have more answers, it’s not worth pushing our bodies and ligaments to the extreme because of the potential long-term damage it can cause,” Ashton-Miller comments in the release. “There are limits to what the human body can tolerate. Being more selective about the training that is done is clearly warranted.

“Fewer repetitions known to significantly stress the ACL will ultimately lead to fewer injuries,” he adds. “This should be the main focus of sporting clubs and athletes of all ages and levels of professionalism.”

[Source(s): Monash University, Science Daily]