Examining Darnell Dockett's Torn ACL and the Non-Contact Injury 'Perfect Storm'

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Examining Darnell Dockett's Torn ACL and the Non-Contact Injury 'Perfect Storm'
Stephen Brashear/Associated Press

The injury bug bit the Arizona Cardinals on Monday when defensive end Darnell Dockett went down during practice with a torn ACL. According to NFL.com's Mike Coppinger, Dockett left the practice field on a cart due to the injury and will undergo surgery at some point in the future.

Coppinger adds that NFL Media Insider Ian Rapoport reported Dockett did not tear his MCL. MCL injuries frequently come along with ACL tears, and the lack of an MCL tear may allow the lineman to go under the knife sooner.

Shortly after the injury occurred, ESPN's Adam Schefter pointed out that Dockett has missed only two NFL games in the past 10 seasons. Such an incredible statistic unfortunately proves that even the most durable players in the NFL can fall victim to an ACL injury. Frequently, all it takes is a perfect storm of awkward positioning, a planted foot and a sudden shift in momentum.

To analyze what possibly happened in Dockett's case—a clear video of the injury is not available—let's crack open the anatomy textbooks and take a closer look.

The Anatomy, the Injury and the Non-Contact "Perfect Storm"

Ordinarily, the ACL prevents the lower leg from over-rotating relative to the thigh. It also keeps it from moving too far forward. To fulfill those roles, the ACL runs down the interior of the joint. It connects the bottom end of the femur, or thigh bone, to the tibia, or shin bone.

Without an intact ACL, the knee may buckle when an athlete attempts to pivot and change directions.

Credit: Wikimedia Commons with edits by the author
This diagram shows the basic anatomy of the knee as viewed from the front and outside. The red circle highlights the location of the ACL.

In football, the ACL can tear as a result of a direct hit to the outside of the knee, but in reality, non-contact tears occur more frequently. Last December, The MMQB's Jenny Vrentas noted that of 32 ACL injuries with available video footage, 24—or 75 percent—did not involve direct contact to the knee.

As mentioned, non-contact tears often result from a combination of awkward positioning, shifting momentum and a planted foot. An article by Cardinals writer Darren Urban suggests a similar situation potentially unfolded in Dockett's case.

"It was a routine play and Darnell was going hard," defensive tackle Dan Williams said. "I don’t know the extent of the injury, but it’s part of football. His foot just got stuck in the ground. We’ll just have to wait and see."

When Dockett engages the offensive line, a push to one side or the other—or an attempt at a swim move to bypass the an opponent—could turn his upper body. If his foot sticks in the ground, as Williams suggests, his lower body may not be able to match his upper body's momentum shift.

If the result of such a scenario is a net strong inward force on the knee, the ACL can give way.

Additional force on the outside of the knee or lower leg can also play a role, but no mention of direct contact surfaced following Dockett's injury.

Non-Contact Injury Examples

Two recent examples of non-contact ACL injuries can help paint a clearer picture. Both occurred within the past few months, representing two of this preseason's numerous ACL injuries.

To start off, here is a video of Dallas Cowboys linebacker Sean Lee's ACL tear that took place in May. A frame-by-frame look at the play demonstrates what probably went wrong.

Credit: Screenshots from the NFL.com video linked above with edits by the author
Sean Lee suffered a non-contact ACL tear in May. The yellow arrow highlights the relative inward motion of his knee compared to the rest of his body.

As shown, Lee plants his foot with this toes facing out. Mild contact to his upper body—importantly, not to his knee—causes him to rotate back and to the left, a movement for which he set himself up by his positioning and weight distribution. However, since his foot remained planted, his knee twisted inward relative to his body, hips and foot, likely causing the tear.

The more recent case of St. Louis Rams running back Isaiah Pead involved no contact at all—just a dramatic change in direction while on the run. This GIF from SB Nation—via Ryan Van Bibber—shows Pead's cut very clearly.

Credit: SB Nation

When Pead plants his left foot, he does so with his toes pointed outward to the left, somewhat analogous to Lee. He then attempts to quickly shift his leftward-directed momentum to the right, but his knee remains out of plane due to his planted foot and thus gives inward.

When an athlete's knee collapses inward like Pead's without any contact whatsoever, the ACL becomes an immediate concern. Certain physical-exam tests can also strongly suggest a tear long before the player reaches an MRI machine.

Credit: SB Nation GIF screenshot with edits by the author
In the above screenshot, Isaiah Pead plants his foot with his toes point out (orange arrow) and tries to cut back, resulting in an inward force on his knee (yellow arrow) large enough to tear his ACL.

Pro Football Talk's Darin Gantt later confirmed Pead's diagnosis.

Bottom Line

Precisely how Dockett's injury occurred remains at least somewhat unclear, but his looming lengthy recovery—NFL players usually require at least seven or eight months to rehab a reconstructed ACL—serves as a reminder that under the right circumstances, any player can go down.

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The fact that Dockett played so long in the NFL without suffering a serious injury is a testament to his lower-body strength and footwork. After all, not every plant and cut in the NFL tears an ACL. Biomechanical, genetic and training factors come into play, but they are the subject of another article.

If all goes well, Dockett should recover in time for Week 1 of the 2015 NFL season. Hopefully, the Cardinals' defensive front can make do until then.

Dr. Dave Siebert is a second-year resident physician at the University of Washington and a member of the Professional Football Writers of America. He plans to pursue fellowship training in Primary Care (non-operative) Sports Medicine.

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