Just over two years ago, I reported that a concussion makes a non-contact lower extremity injury, like an anterior cruciate ligament tear, far more likely in the head injury’s aftermath.
Depending on the study, the subsequent period of vulnerability is at least 90 days, a year, or the entire time remaining in a college career. To explain, an author of one of the cited studies wrote, “Given the demanding environment in which athletes are required to execute complex maneuvers, it is possible that mild neurocognitive deficits may result in judgment errors and loss of coordination during play.”
It all makes sense. Just an instant of clumsiness leads to an inadvertent twist or buckle of the knee.
Now, however, there is new research to show that the troubled trail between the brain and ACL goes both ways.
A study published online in December in the journal NeuroImage: Clinical revealed a key neurologic pathway in the brain shrinks in the wake of ACL reconstruction, and remains that way for as long as six years following the surgery. Specifically, MRI exams of 10 athletes, with an average age of 22, found that the corticospinal tract that controls the injured knee shrank an average of 15%.
The result is fewer signals from the brain to the muscles surrounding the affected knee, according to the authors, “with larger deficits the further the patient is removed (in time) from the injury.”
How could this be, considering the typical intensity of rehabilitation following ACL reconstruction?
There are several theories.
First, any injury to a joint that causes swelling triggers what is known as “central inhibition,” where the brain powers down the muscles that straighten a joint because keeping an injured joint flexed or bent is more comfortable.
Second, when the ACL is reconstructed, the graft may very well end up being as strong or stronger than the original ligament. However, the original ligament has nerve endings that help tell the brain what the knee is doing. The graft, lacking that innervation, is incapable of sending such feedback to the brain. Absent that information, the brain consequently becomes more cautious with the reconstructed knee and the unused neurologic pathways atrophy or shrink.
Traditionally, an ACL tear has been treated as a musculoskeletal injury requiring surgical reconstruction and post-operative rehabilitation that focuses on restoring lost muscular strength. Nonetheless, in many patients, quadriceps strength deficits persist, function remains impaired to the point that 40% of athletes end up not returning to their sport, and 25% of those who do resume competition eventually suffer another ACL tear involving the graft or the opposite knee.
With numbers like that, a change in therapeutic tactics is in order.
The authors — from the University of Michigan, the University of Connecticut, and Ohio University — recommended less focus on strengthening and endurance, and greater emphasis on “motor skill training, feedback/cognitive training, and coordination interventions.”
As for the highly unsatisfactory outcome of a subsequent ACL tear, a study in the current issue of The Journal of Orthopaedic and Sports Physical Therapy discovered a simple preventive strategy: time. Swedish researchers looked at 159 athletes, aged 15-30, who had undergone ACL reconstruction between March 2013 and December 2017.
Of those, nearly two-thirds were female. Overall, 18 of the 159 suffered another tear, 10 to the graft and eight to the opposite knee. However, 10 occurred to the 33 athletes who returned sooner than nine months after surgery. Ultimately, that translated to those who didn’t wait at least nine months being seven times more likely to suffer another ACL injury than those who did wait nine months or longer.
Going into the study, the authors also assumed that quadriceps strength or a lack thereof would correlate with a second ACL tear. However, that ended up not being the case. If you have the misfortune of tearing your ACL, then, and are determined to prevent it from happening again, only time is on your side.