Adrian Peterson's ACL Tear: A Closer Look at His Injury and Incredible Recovery

Make no mistake about it, some athletes never return from injuries such as Adrian Peterson's ACL tear.  Last year, if someone would have told those in the NFL community that Peterson would be in the running for the 2012 MVP, most, if not all, would have laughed.

Except for Peterson.

The Minnesota Vikings' future Hall of Fame running back has defied medical textbooks, anatomical knowledge and clinical reason during his comeback from reconstructive surgery after suffering a torn ACL and MCL in his left knee last December.

It is impossible to put into words just how unbelievable Peterson's story continues to become week after week.  Believe me, I tried.

Many times.

Words like "jaw-dropping" or "superhuman" do not even scratch the surface.

To gain an appreciation of what Peterson is doing, let's take a look at exactly what an ACL is, what happens when the ACL is torn and the surgery and extensive rehabilitation that are necessary before an athlete even thinks of returning to football.

Officially named the anterior cruciate ligament, the ACL makes its home within the center of the knee joint as one of its four main stabilizing ligaments.  Like other ligaments, the ACL is a band of tissue that connects two bones and serves to coordinate their motion as well as prevent certain abnormal movements.

Specifically, the ACL connects the bottom end of the femur (thigh bone) to the top end of the tibia (shin bone).  By doing so, it runs, generally speaking, from the upper-outer to lower-inner corners of the joint.

By connecting the femur and tibia, the ACL helps prevent the following movements:

• Lower leg moving forward in relation to the thigh
• Lower leg being twisted inwards toward the midline of the body

By preventing these two motions, the ACL helps a player balance, cut and change directions on his knee without difficulty.  Without a functional ACL, knee instability and "giving way" would make a career as an NFL running back impossible.

The ACL can only resist so much abnormal movement.  If an outside hit forces the knee to move forward or twist inward strongly enough, the ACL gives way and tears.  Classically, this occurs when a tackle forces the knee of a planted leg inwards.

That is exactly what happened to Peterson.

As can be clearly seen in the video, Peterson firmly plants his leg on the ground just as a tackle from Washington Redskins safety DeJon Gomes forces his knee primarily inward and also slightly forward.  Since Peterson had his leg planted at the time of the tackle, the leg could not give in to the hit.  As a result, his knee and ACL absorbed all of the force of the tackle, resulting in a complete tear.

When an ACL tears, an audible "pop" can sometimes be heard, and the injury often produces extreme pain.  Next, the injured knee immediately and extensively swells.  This occurs because blood vessels within the ACL tear along with the ligament, resulting in bleeding within the knee joint.

Bleeding within a joint is known as a "hemarthrosis," and many surgeons prefer to wait until a hemarthrosis or swelling resolve before operating.  In some cases, this alone takes weeks.

Additionally, some studies suggest that operating immediately after an ACL tear results in decreased knee range of motion following surgery.

Don't tell that to Peterson or Dr. James Andrews.

Dr. Andrews operated on Peterson's knee just six days after the injury.  Though exact medical details are, as always, unavailable to the public, speculation suggests that Dr. Andrews determined Peterson to be a prime candidate for early surgery.

In other words, Peterson was already well ahead of the recovery timeline just one week after the injury.

ACL reconstruction surgery involves taking a small piece of muscle tendon and using it to replace the torn ACL.  The replacement muscle tendon, known as a "graft," can come from the player's hamstring muscle tendons, patellar tendon (the part of the knee a doctor hits with a reflex hammer) or even from a deceased donor.

Following surgery, a player embarks on a long road of physical therapy and rehabilitation.  Rehab allows the graft to slowly but surely be worked into place within the knee.  It is not "supposed" to be there, so to speak, so progression to walking, running, sprinting and, eventually, cutting, must be done gradually as the body cements the graft into place.

This progression takes immense dedication and effort just to ensure return to normal activity.

That said, making a living as an NFL running back certainly does not constitute "normal activity."  An NFL athlete must not only be able to run, sprint and change directions quickly, he must do so at the highest level in order to be effective.

Nevertheless, each player progresses through rehab at a different rate.  Peterson likely graduated from each level of therapy once he proved that his knee was ready rather than according to a strict schedule.  When he showed that his knee was strong enough to perform the tasks of an NFL running back with low risk of re-injury, he returned to the field.

Many experts say that this entire process usually takes seven to nine months.  To return to NFL-caliber play, it could take even longer.

It appears that Peterson's knee did not listen to the experts.

Just nine months after his surgery, Peterson started Week 1 for the Minnesota Vikings this season, and the rest is history.  He now threatens the all-time single season rushing record, all while coming back from an injury infamous for cutting short the careers of athletes of all levels.

Again, it remains impossible to overstate how impressive Peterson's return has been.  His 2012 season is a testament to his work ethic, determination and natural athletic talent as well as to the operative skill of Dr. Andrews.

To put it another way, WebMD's article on ACL surgery states:

After an ACL injury and surgery, the knee is never "normal." But most people regain enough strength and range of motion to return to their usual activities.

In a way, that applies to Peterson, as well.  His knee is definitely not back to "normal."

Apparently, it is better than ever.

Dave Siebert is a medical/injury Featured Columnist for Bleacher Report who will be graduating from medical school in June, 2013.  He plans to specialize in both Family Medicine and Primary Care (non-operative) Sports Medicine.  The injury and anatomical information discussed above is based on his own knowledge and was supplemented by WebMD's article on ACL repair surgery and Dr. Jonathan Cluett's article on ACL rehab.

Follow @DaveMSiebert