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A new study finds that the brain can be affected by a knee injury. Getty Images
  • A new report in NeuroImage: Clinical was the first to document brain changes in people who underwent ACL reconstruction.
  • The study finds a knee injury affects brain structure and can have negative impacts on it.
  • Various types of injuries, and to specific parts of the body, can affect the brain differently, but the effects can be similar.

An anterior cruciate ligament (ACL) injury and subsequent reconstruction causes structural changes in the brain of patients, a new study finds.

Their report in the journal NeuroImage: Clinical was the first to document brain changes in people who underwent ACL reconstruction. The changes in the brain may play a role in performance and reinjury, said Lindsey Lepley, PhD, an assistant professor of athletic training at the University of Michigan and co-lead author.

In short, a knee injury affects brain structure and can have negative impacts on it, the authors say.

“Most people don’t think about an ankle sprain or twisted knee altering the brain, but that is exactly what is happening,” noted Charles Buz Swanik, PhD, a professor in the department of kinesiology and applied physiology at University of Delaware, who wasn’t affiliated with the research.

Scientists already know that it’s common to lose some joint function permanently after ACL surgery. Injuring the ligament is common as well.

Lepley’s team looked at MRI brain scans of 10 patients who had ACL reconstructions. Part of the corticospinal tract — which sends messages between the brain and muscles — was atrophied. The side of the tract that controls the knee was about 15 percent smaller than the uninjured side. This means that patients who’ve had reconstruction have less information getting from the brain to the muscle, the authors say.

“In essence, the brain not only alters the way it communicates with the rest of the body… but the structural makeup of the basic building blocks of the brain are also changed after ACL injury,” said Adam Lepley, PhD, a study co-author and assistant professor of kinesiology at the University of Michigan. The team thinks the alteration is a protective mechanism so the body can limit unwanted movement around a joint injury.

Previous research showed changes in cortical signals after ACL injuries. It has also demonstrated that people with a history of ACL injuries tend to rely more on sensory input over visual stimuli to complete tasks when compared to those who weren’t injured.

Dr. Claudette Lajam, an orthopedic surgeon with NYU Langone Orthopedic Center, said that injuries to a stabilizing ligament like the ACL cause a breakdown in the knee’s proprioception, or sense of movement.

“Special nerve fibers that live in the ACL send information about knee position to the brain. When the ligament is torn, the brain has trouble coordinating muscle movement to prevent the knee from giving way further,” Lajam said. “This can result in muscle imbalances and improper feedback to the brain about what is going on in the knee. Left unchecked, it becomes a vicious cycle and can cause muscle atrophy and changes in the nerve connections to the muscles surrounding the knee.”

That’s why rehabilitation after injury and surgery is so vital, Lajam noted.

The same thing occurs during joint replacement — the body must relearn muscle coordination. Unlike an unexpected ACL tear, patients can plan joint replacement ahead of time. They can stabilize and strengthen their bodies before surgery so recuperation can go more quickly.

Alan Needle, PhD, an associate professor at Appalachian State University in North Carolina, said researchers are still trying to understand how an ACL injury affects the brain. They believe there are initial impacts from the injury as well as from long-term changes. For instance, when your knee is swollen and painful after the initial injury (or after surgery), it can overload the nervous system’s sensory components. That can cause the system to turn off the muscle, something known as arthrogenic muscle inhibition.

In long-term injuries, changes to the joint’s sensory characteristics means that the nervous system gets less input and doesn’t necessarily respond. Because the brain is constantly adapting to everything — something known as neuroplasticity — it adjusts to the input and generally will pay less attention to the injured joint and remap itself. This is just a theory, Needle pointed out. More needs to be done to document the concept.

Changes to the tract have been seen in ACL injuries and ankle sprains. There’s some evidence that similar processes occur in shoulder injuries as well as in patients with low back pain, Needle said.

“Since your brain is having a harder time activating your muscles, you end up using more parts of your brain to produce simple motion,” he explained. This is why patients perform well right after rehab. As time goes on, they can wind up reverting to poor motor patterns that may make reinjury more likely.

Various types of injuries, and to specific parts of the body, can affect the brain differently, but the effects can be similar, Needle said. Differences may occur in types of tissue affected, or how it was treated, but how the body responds may be similar. For example, pain and swelling can affect a person’s ability to activate a muscle.

Researchers are still figuring out if corticospinal tract damage is permanent.

“I would like to say that this is reversible,” Needle said. “The plasticity that occurs in the corticospinal tract is functionally driven, meaning there was no structural impairment such as a stroke that caused things to remap. Therefore, increasing activation should improve the quality of the corticospinal tract.”

The authors hope that a systematic approach will be taken during treatment to not only improve swelling or range of motion. Clinicians should consider other movement patterns and muscle activation so patients have better outcomes.

“There is evidence of using visual retraining, different motor learning modalities like external focus of attention, and biofeedback, which can help ‘rewire’ the brain to help the body adapt to a new normal,” Lesley Lepley said. Her lab has used biofeedback, motor learning interventions, eccentric exercise, and electromagnetic modalities to improve outcomes. They’ve had a positive impact, but research on their efficacy is in the preliminary stages.