Breaking Down Sam Bradford's Knee Injury and Road to Recovery from a Torn ACL

According to Pro Football Talk's Twitter account, St. Louis Rams quarterback Sam Bradford went down with a torn ACL on Sunday. He will likely miss the remainder of the season—a heartbreaking end to his best offensive year to date.

As with all serious knee injuries, the entire football community certainly hopes for a speedy and full recovery.

That said, whereas many significant injuries in the NFL involve bone-crushing hits, Bradford fell victim to a very subtle mechanism of injury.

A closer look at the play, courtesy of Fox Sports, via AOL's Sporting News, makes clear exactly what happened:

When Bradford starts to head toward the sideline, the direction he is facing and the direction of his momentum are identical and in sync.

Then, when Mike Mitchell hits Bradford, the play changes entirely.

At contact, Mitchell gently shoves Bradford on his left side toward the back, forcing the Rams signal-caller to turn ever so slightly to his left.

As a result, when Bradford takes his next step—thereby placing 100 percent of his weight on his left foot—his knee points one way, and he continues to move in another. Instead of falling forward, he falls to his right, thereby twisting his left knee inward and down.

When that happened, the ACL—short for anterior cruciate ligament—stretched and tore, unable to resist the motion.

Why the ACL and not something else?

Simple.

The above inward twisting of the knee is the exact type of motion the ACL prevents. However, it can absorb only so much stress, and when weight and momentum changes are large enough in magnitude, it gives way.

Torn ACLs almost always require surgical reconstruction.

During surgery, a surgeon will usually remove a piece of muscle tendon from elsewhere in the body—the patellar tendon, for example—and use it to replace the ruptured ACL. The muscle tendon then slowly transforms into tissue more closely resembling the native ligament through a fascinating process called ligamentization.

Rehabilitation involves slowly progressing through a series of range-of-motion, strength and agility exercises while the body cements the new ACL—called a "graft"—into place within the knee.

In the past, the entire process frequently required well over one year to return to play, and even then, there were no guarantees.

Nowadays, surgical techniques—and, just as importantly, physical therapy science—are advancing at blazing speeds. As such, NFL recovery times are continuing to shorten.

A very unscientific survey of recent injuries comes up with an average of about nine to 10 months.

Minnesota Vikings running back Adrian Peterson famously returned from a torn ACL in about nine months, and Washington Redskins quarterback Robert Griffin III only needed eight.

With that in mind, it's not unreasonable to look ahead to Bradford returning for the start of the 2014 season.

Then again, the opposite scenario is also possible, and for each short recovery, there is a long one. Every athlete is unique.

Excellent surgical and rehab care—which Bradford will, undoubtedly, receive—as well as his genetically predetermined, innate healing ability and shear willpower will dictate the months that lie ahead.

Dr. Dave Siebert is a resident physician at the University of Washington. Find more of his work at the Under the Knife blog.

Follow @DaveMSiebert