Video (02:34) : Dr. Jeffrey Macalena of the University of Minnesota explains what happens to a knee when an injury of the sort that Teddy Bridgewater had takes place.

Medical advances have made it possible for injured athletes such as Vikings quarterback Teddy Bridgewater to return from dislocated knees and ligament tears, according to physicians interviewed Wednesday, but they nonetheless face long and uncertain roads to recovery.

Doctors at Northwestern University analyzed orthopedic injuries suffered by 559 NFL athletes, and found the ACL (anterior cruciate ligament) tear to be one of the most career-damaging. Those who underwent ACL surgeries needed 378 days to recover, on average, and their careers wound up shorter than those of players recovering from other orthopedic injuries, according to results being printed Thursday in the American Journal of Sports Medicine.

“We all look at Adrian Peterson” who had a record-setting season one year after an ACL injury, said Dr. Harry Mai, a lead author. “But that is the absolute best-case scenario. This paper shows that after (most ACL injuries), there is a significance decrease in performance and career length.”

While ACL injuries might be more familiar to sports fans, doctors said the knee dislocation that Bridgewater suffered on Tuesday is more troublesome, because a knee bone moving out of joint can damage the surrounding nerves and blood vessels and threaten the entire limb. Reports that Bridgewater did not suffer such damage suggested optimism for his return to play.

“That’s the scary thing,” said Dr. David Jewison, an orthopedic specialist and a physician for the University of Minnesota football and hockey programs. “It appears that didn’t happen with him, which is great news.”

Bridgewater was reportedly dropping back to pass in practice and wasn’t tackled or hit when he suffered the injury.

Graphic: Graphic: An in-depth look at Bridgewater's ACL injury Graphic: Graphic: An in-depth look at Bridgewater's ACL injury

A very rare injury

The ACL crisscrosses with the posterior cruciate ligament to form the knee joint, stabilizing it and preventing bones and cartilage from grinding.

The ACL commonly tears when athletes hyperextend their knees — which means they bend them too far backward — or twist the joint. The risk is greater when these motions happen in combination — a common sequence for a quarterback who drops back to pass and rotates side to side to survey the field.

More than 100,000 Americans suffer the injury each year.

While ACL tears commonly occur without contact, knee dislocations almost always result from violent collisions such as car wrecks, said Dr. Bruce Levy, a Mayo Clinic orthopedic surgeon who specializes in dislocations.

A noncontact knee dislocation — what Bridgewater apparently suffered — would require a freak confluence of body rotation and weight bearing while a person planted his foot firmly on the ground.

“An obese patient could just miss a step or a curb and have what we call a low-velocity knee dislocation,” he said. “It’s extremely rare in somebody who is not morbidly obese ... but if it did occur in a high-level athlete, it would have to happen at just the right moment when the knee is in a compromised position and there is just enough force and weight.’’

“Even in sports a truly dislocated knee is uncommon,” Levy added.

Bridgewater was taken by ambulance to a nearby hospital following the injury. The response of medical staff at the Vikings facility could have made a difference, doctors said. First-responders often try to put patients’ dislocated knees back in place, because that reduces the pain and the chance that it will damage nerves and blood vessels.

Surgery? Not always

Former Vikings quarterback Daunte Culpepper suffered a grisly knee dislocation along with multiple ligament tears that shortened his career. However, NFL running back Willis McGahee and quarterback Carson Palmer enjoyed success following dislocations.

Improvements in surgical techniques and post-surgery rehab have turned a surefire career-ending injury into something that athletes such as Bridgewater can overcome, said Timothy Hewett, who directs the Mayo Clinic Sports Medicine Research Center. “Twenty-five years ago, they would have cast him and he would have come back with a bunch of scarring, and it would have taken months to decrease the stiffness in the joint. We know now that you immediately move it.”

Surgery for a torn ACL usually involves replacing the severed ligament with tendons from the patient’s own legs, which then develop ligament-like qualities over time. About a third of people with ACL tears don’t have surgery, though, and can bike and run and do anything that doesn’t involve planting and pivoting.

“The majority of those patients will do just fine, actually,” said Dr. Joel Boyd, a TRIA orthopedic surgeon and head physician for Gopher football. “There’s tons of people walking around without an ACL.”

An ACL is necessary for a player in the NFL, which has seen an uptick in injuries. As many as 13 have occurred so far this preseason, Hewett said, compared to 35 total in most seasons.

Hewett suspects that diminishing contact between players and team trainers and medical staff could be to blame, because the trainers can have them do cross-training activities that reduce injury risks. The most ACL tears in a season occurred after the 2011 lockout, when preseason training was shortened, he noted.

Minnesota Vikings quarterback Teddy Bridgewater, left, and Shaun Hill ran through drills during a player offseason workout at Winter Park, Tuesday, April 26, 2016 in Eden Prairie, MN.

Jerry Rice

The Northwestern research is sobering for pro athletes hoping for AP-like returns from ACL injuries, but on the bright side it showed that 80 percent return to play. Most get to 80 percent of their pre-injury performance levels, Mai said. “You won’t get back to full strength, but you’ll get close.”

And while it might sound grim for NFL players to only play 1.6 seasons on average after ACL injuries, Mai said the average player only lasts 3.8 seasons in the league anyway.

Mai, who is now at Harbor-UCLA Medical Center, and doctors at Northwestern will soon focus more on the ACL injury. The group includes the Chicago Bears’ team doctor.

In their sampling, Hall of Fame receiver Jerry Rice remains the only player to play a game in the same season after sustaining an ACL tear. Star players tend to have better odds of returning to play, Mai said, though that could be because other players were already at risk of being cut.

Quarterbacks’ careers are not shortened by ACL injuries, but the number of games they play declines as some end up as backups. Among players who were Pro Bowlers before their injuries, 85 percent returned to play football. Only 30 percent made it back to the Pro Bowl.

Bridgewater played in the Pro Bowl last winter, after two seasons with the Vikings.