After knee surgery to repair an anterior cruciate ligament (ACL), people are at high risk of repeat injury to the same structure, but simple screening tests before going back to sports can reduce this risk, according to a Norwegian study.

ACL reconstruction surgery replaces a vital ligament in the center of the knee after a significant injury.

Nearly one third of young, active people who get the surgery will tear the ligament again within the first few years after surgery, researchers write in the British Journal of Sports Medicine

"Being a teenager and participating in sports are well known risk factors for reinjury," said lead author Dr. Hege Grindem, a senior researcher at the Norwegian Research Center for Active Rehabilitation in Norway.

"Our study also shows that returning to sport with poor muscle strength and returning to sport too early are important factors," Grindem told Reuters Health by email.

Knee injuries put people at greater risk for osteoarthritis, a serious condition in which the cartilage between bones wears down and causes joint pain.

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The study team collected data on 100 patients, with an average age of 24 years, who had undergone ACL reconstruction surgery.

Over the course of two years, patients took an online survey, reporting each month whether they had returned to playing sports and whether they had suffered another knee injury.

Four out of five had previously participated in jumping and pivoting sports and three out of four returned to playing sports within two years of the surgery.

The study team also used a series of tests to evaluate patients' knee function and the strength of the quadriceps muscle on the front of the thigh, noting particularly whether patients had equal strength in both quadriceps.

The researchers established "return to sport" criteria based on getting a passing score on all of these tests.

Overall, 24 patients experienced another knee injury following surgery, though not all were injuries to the ACL.

Patients who started doing pivoting and jumping sports again after their injury were 4.32 times more likely to experience knee injuries than those who did not.

Only one in four patients passed the return to sport criteria before beginning playing sports again.

Among those who failed the return to sport criteria, 38 percent suffered another knee injury, compared with 5.6 percent of people who passed the tests.

Having more symmetrical strength in both quadriceps before starting to play sports again significantly reduced the risk of getting another knee injury.

The likelihood of being reinjured decreased by half for each month that patients waited before going back to playing sports, up to the nine-month point, when it leveled off.

"ACL injury is a significant event that has both short and long term consequences for an athlete," said Kate Webster, an associate professor at LaTrobe University in Australia who studies ACL injuries, by email.

Webster, who was not involved in the study, noted that in the short term, patients may not be able to participate in sports. In the longer term, young athletes with multiple injuries may be much more likely to develop osteoarthritis, which can affect their ability to stay active throughout their lives.

"People need to know that rushing back to sport after ACL reconstruction entails significant risks," Grindem warned.

"If they aim to return to sports with frequent pivoting, participation should be delayed until at least nine months have passed from surgery and they have regained thigh muscle strength comparable to the uninjured leg," Grindem said.