When patients undergo anterior cruciate ligament (ACL) reconstruction, it's often because they're eager to resume participating in a sport or activity they love. But before that can happen, whether they're an athlete trying to get back to the top of their game or a dog dad wanting to get back to tug-of-war, they first have to undergo months of recovery and rehabilitation, then pass a series of "return-to-sports" tests.

"We've come a long way from the days of, 'It's been six months, so you're probably fine,'" says Dr. Patrick McCulloch, vice chair of the Houston Methodist Hospital Department of Orthopedic Surgery. "The objective testing that we do for every patient prior to giving them the green light to resume high-risk knee activities has helped reduce the risks of reinjury or new injuries."

But that's not to say more can't be done to reduce the risks. Almost 30% of active individuals suffer a second ACL injury within two years of ACL reconstruction, in either the same (ipsilateral) knee or other (contralateral) knee. In addition, up to 50% of post-ACL reconstruction patients undergo meniscus surgery after returning to action.

A new Houston Methodist study attempting to identify and understand factors that make testing after ACL reconstruction better at preventing follow-up injuries examined fatigue's effect on performance metrics for the ipsilateral and contralateral limbs.

Fatigue's role in ACL injury has not been incorporated into objective test batteries for return-to-sports decisions following reconstructive surgery, according to study authors, Dr. McCulloch, physical therapists Justin Tallard and Corbin Hedt, and Brad Lambert, Ph.D., manager of the Orthopedics Biomechanics Research Laboratory at Houston Methodist.

The study, published in the International Journal of Sports Physical Therapy, found significant variability in performance on several tests following fatigue protocol and a notably greater loss in jump performance in the contralateral limb while in a fatigued state. The findings show the importance of testing the endurance of both limbs when assessing a patient's readiness to return to action.

Focusing on limb symmetry may miss the mark

The current return-to-sports test protocol focuses almost exclusively on symmetry, comparing the affected (ipsilateral) limb to the unaffected (contralateral) limb in a series of quadricep muscle strength, single-leg hop and agility tests. If the performances of the surgically repaired limb matches that of the unaffected limb, it passes the test. However, many experts think the use of the unaffected limb as the "healthy" control may not be appropriate, given its deficits in bilateral muscle strength, endurance, power and rate of force development in the months following ACL injury.

"During rehab, the patient is working really hard on the injured leg and doing a lot of strength training to get it up to speed, but the other leg doesn't just stay at the same level as it was before the injury," Dr. McCulloch notes. "It's no longer participating in the same sports and activities it was before, and it's not getting the same level of conditioning that the injured leg is getting during rehab."

The study results, which show the unaffected leg is especially vulnerable to fatigue, may be a warning sign.

"By changing this one variable — introducing fatigue — our researchers have been able to demonstrate that perhaps this isn't the right, or only, measuring stick by which to judge the readiness of the injured leg," says Dr. McCulloch. "It also tells us we probably need to start focusing more on both legs during rehab, because once you've torn one ACL, you're at increased risk of tearing the other one."

This research is a good example of what a high-quality, integrated sports medicine program can bring to the table.

"For a long time, there's been a focus on the surgical technique and management of patients with anterior cruciate ligament tears," Dr. McCulloch says. "And while we are established leaders in that field, this research was initiated by our physical therapists and is focused on making sure we're doing all the right things during rehab. We are fortunate to have a physical therapy department that's constantly engaged in research and publication and always keeping the long-term health of the patient in mind."